AI Trajectory

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Seq	Timestamp	Type	Name	Duration
0/0	2026/10/14 14:53	flow	patch-triage	11m Args: null Results: { "EnableConfigs": [ "NUMA", "NUMA_BALANCING", "SMP" ], "FocusSymbols": [ "task_numa_fault", "should_numa_migrate_memory", "numa_migrate_check", "update_task_scan_period", "task_numa_placement", "numa_migrate_preferred", "migrate_degrades_locality" ], "KMSANReasoning": "The patch series modifies NUMA balancing and memory tiering logic:\n1. It introduces a new sysctl (`numa_balancing_promote_free_space_bypass`) to control whether memory tiering promotion can bypass hotness checks when fast-tier nodes have free space.\n2. It refines NUMA fault locality statistics by distinguishing remote-private faults, expanding `numa_faults_locality` in `struct task_struct` from 3 to `NR_NUMA_FAULT_LOCALITY` (4) elements and properly clearing it.\n3. It distinguishes slow-tier access-time metadata from real `cpupid` by introducing `LAST_CPUPID_ACCESS_TIME` and `LAST_CPUPID_UNSET`.\n\nNone of these changes introduce or expose uninitialized memory. All variables, including the `last_cpupid` out-parameter in `numa_migrate_check`, are deterministically initialized before use. The modified structures and counters are internal scheduler accounting fields that are zero-initialized (via memset/kzalloc) and never copied or leaked to user space. There are no changes to user-kernel copying interfaces, padding exposure, or uninitialized buffer reads. Therefore, a dedicated KMSAN fuzzing session is not warranted, and standard KASAN fuzzing is sufficient.", "NeedsKMSAN": false, "Reasoning": "The patch series modifies core scheduler and memory management code implementing NUMA balancing and memory tiering. Specifically, it changes NUMA hint fault handling, scan period calculations, memory tiering promotion bypass decisions, and NUMA-aware task placement and load balancing. These changes are in reachable kernel core paths and warrant fuzzing to verify stability across various NUMA layouts, sysctl configurations, and memory access patterns.", "WorthFuzzing": true }
1/1	2026/10/14 14:53	action	read-patch-diff	0m Args: null Results: { "PatchDiff": "commit e10a96482a628c719d4f99a8b017d64b7c5da549\nAuthor: syz-cluster \u003ctriage@syzkaller.com\u003e\nDate: Wed Oct 14 14:53:44 2026 +0000\n\n syz-cluster: applied patch under review\n\ndiff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst\nindex ffea61d448ebb..4a035b18aff43 100644\n--- a/Documentation/admin-guide/sysctl/kernel.rst\n+++ b/Documentation/admin-guide/sysctl/kernel.rst\n@@ -765,6 +765,22 @@ will be limited to be no more than the set value.\n A rule of thumb is to set this to less than 1/10 of the PMEM node\n write bandwidth.\n \n+numa_balancing_promote_free_space_bypass\n+========================================\n+\n+Controls whether memory tiering promotion may bypass the normal\n+hint-fault hotness check and promotion rate limit when the target\n+fast-tier node has enough free space.\n+\n+The default value is 1. In this mode, if the fast tier has enough free\n+space, recently accessed slow-tier pages can be promoted without the\n+hotness check or the promotion rate limit, so the kernel can fill\n+available fast-tier capacity.\n+\n+When set to 0, enough free space no longer bypasses these controls.\n+A slow-tier page must pass the normal hotness and rate-limit checks before\n+promotion.\n+\n oops_all_cpu_backtrace\n ======================\n \ndiff --git a/include/linux/mm.h b/include/linux/mm.h\nindex dd09c438fa23e..bb0c726401529 100644\n--- a/include/linux/mm.h\n+++ b/include/linux/mm.h\n@@ -2420,6 +2420,9 @@ static inline int folio_nid(const struct folio folio)\n #define PAGE_ACCESS_TIME_MASK\t\t\t\t\\\n \t(LAST_CPUPID_MASK \u003c\u003c PAGE_ACCESS_TIME_BUCKETS)\n \n+#define LAST_CPUPID_UNSET\t\t(-1 \u0026 LAST_CPUPID_MASK)\n+#define LAST_CPUPID_ACCESS_TIME\t\t(-2 \u0026 LAST_CPUPID_MASK)\n+\n static inline int cpu_pid_to_cpupid(int cpu, int pid)\n {\n \treturn ((cpu \u0026 LAST__CPU_MASK) \u003c\u003c LAST__PID_SHIFT) \| (pid \u0026 LAST__PID_MASK);\n@@ -2468,7 +2471,7 @@ static inline int folio_last_cpupid(struct folio folio)\n }\n static inline void page_cpupid_reset_last(struct page page)\n {\n-\tpage-\u003e_last_cpupid = -1 \u0026 LAST_CPUPID_MASK;\n+\tpage-\u003e_last_cpupid = LAST_CPUPID_UNSET;\n }\n #else\n static inline int folio_last_cpupid(struct folio folio)\ndiff --git a/include/linux/mmzone.h b/include/linux/mmzone.h\nindex 94f9c3ff54160..5d583b17dfa29 100644\n--- a/include/linux/mmzone.h\n+++ b/include/linux/mmzone.h\n@@ -297,9 +297,8 @@ enum node_stat_item {\n \t /\n \tPGPROMOTE_CANDIDATE,\n \t/\n-\t Not rate-limited (NRL) candidate pages for those can be promoted\n-\t * without considering hot threshold because of enough free pages in\n-\t * fast-tier node. These promotions bypass the regular hotness checks\n+\t * Not rate-limited (NRL) candidate pages that are promoted through the\n+\t * free-space bypass. These promotions bypass the regular hotness checks\n \t * and do NOT influence the promotion rate-limiter or\n \t * threshold-adjustment logic.\n \t * This is for statistics/monitoring purposes.\ndiff --git a/include/linux/sched.h b/include/linux/sched.h\nindex d35ae49a991f7..1be7a9ba75539 100644\n--- a/include/linux/sched.h\n+++ b/include/linux/sched.h\n@@ -832,6 +832,16 @@ struct task_ipi_mask {\n struct task_ipi_mask { };\n #endif\n \n+#ifdef CONFIG_NUMA_BALANCING\n+enum numa_faults_locality {\n+\tNUMA_FAULT_REMOTE,\n+\tNUMA_FAULT_LOCAL,\n+\tNUMA_FAULT_MIGRATE_FAIL,\n+\tNUMA_FAULT_REMOTE_PRIVATE,\n+\tNR_NUMA_FAULT_LOCALITY,\n+};\n+#endif\n+\n struct task_struct {\n #ifdef CONFIG_THREAD_INFO_IN_TASK\n \t/\n@@ -1422,11 +1432,10 @@ struct task_struct {\n \n \t/\n \t * numa_faults_locality tracks if faults recorded during the last\n-\t * scan window were remote/local or failed to migrate. The task scan\n-\t * period is adapted based on the locality of the faults with different\n-\t * weights depending on whether they were shared or private faults\n+\t * scan window were remote/local, remote private, or failed to migrate.\n+\t * The task scan period is adapted based on those fault patterns.\n \t /\n-\tunsigned long\t\t\tnuma_faults_locality[3];\n+\tunsigned long\t\t\tnuma_faults_locality[NR_NUMA_FAULT_LOCALITY];\n \n \tunsigned long\t\t\tnuma_pages_migrated;\n #endif / CONFIG_NUMA_BALANCING /\ndiff --git a/kernel/sched/fair.c b/kernel/sched/fair.c\nindex 57360f5cdde4f..11b54bb353adf 100644\n--- a/kernel/sched/fair.c\n+++ b/kernel/sched/fair.c\n@@ -134,6 +134,8 @@ static unsigned int sysctl_sched_cfs_bandwidth_slice\t\t= 5000UL;\n #ifdef CONFIG_NUMA_BALANCING\n / Restrict the NUMA promotion throughput (MB/s) for each target node. /\n static unsigned int sysctl_numa_balancing_promote_rate_limit = 65536;\n+/ Preserve the existing default of using free fast-tier space eagerly. /\n+static unsigned int sysctl_numa_balancing_promote_free_space_bypass = 1;\n #endif\n \n #ifdef CONFIG_SYSCTL\n@@ -157,6 +159,15 @@ static const struct ctl_table sched_fair_sysctls[] = {\n \t\t.proc_handler\t= proc_dointvec_minmax,\n \t\t.extra1\t\t= SYSCTL_ZERO,\n \t},\n+\t{\n+\t\t.procname\t= \"numa_balancing_promote_free_space_bypass\",\n+\t\t.data\t\t= \u0026sysctl_numa_balancing_promote_free_space_bypass,\n+\t\t.maxlen\t\t= sizeof(unsigned int),\n+\t\t.mode\t\t= 0644,\n+\t\t.proc_handler\t= proc_douintvec_minmax,\n+\t\t.extra1\t\t= SYSCTL_ZERO,\n+\t\t.extra2\t\t= SYSCTL_ONE,\n+\t},\n #endif / CONFIG_NUMA_BALANCING /\n };\n \n@@ -1317,6 +1328,21 @@ static int select_idle_sibling(struct task_struct p, int prev_cpu, int cpu);\n static unsigned long task_h_load(struct task_struct p);\n static unsigned long capacity_of(int cpu);\n \n+#ifdef CONFIG_NUMA_BALANCING\n+static inline bool numa_balancing_normal_enabled(void)\n+{\n+\treturn sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_NORMAL;\n+}\n+\n+static inline int task_numa_preferred_nid(struct task_struct p)\n+{\n+\tif (!numa_balancing_normal_enabled())\n+\t\treturn NUMA_NO_NODE;\n+\n+\treturn p-\u003enuma_preferred_nid;\n+}\n+#endif\n+\n /* Give new sched_entity start runnable values to heavy its load in infant time /\n void init_entity_runnable_average(struct sched_entity se)\n {\n@@ -1879,10 +1905,12 @@ static int get_pref_llc(struct task_struct p, struct sched_cache_group grp)\n \t\t * than sched_setnuma() at least -- and thus the\n \t\t * conflict only exists for a short period of time.\n \t\t /\n-\t\tif (static_branch_likely(\u0026sched_numa_balancing) \u0026\u0026\n-\t\t p-\u003enuma_preferred_nid \u003e= 0 \u0026\u0026\n-\t\t cpu_to_node(mm_sched_cpu) != p-\u003enuma_preferred_nid)\n-\t\t\tmm_sched_llc = -1;\n+\t\tif (static_branch_likely(\u0026sched_numa_balancing)) {\n+\t\t\tint pref_nid = task_numa_preferred_nid(p);\n+\n+\t\t\tif (pref_nid \u003e= 0 \u0026\u0026 cpu_to_node(mm_sched_cpu) != pref_nid)\n+\t\t\t\tmm_sched_llc = -1;\n+\t\t}\n #endif\n \t}\n \n@@ -1998,7 +2026,7 @@ static void get_scan_cpumasks(cpumask_var_t cpus, struct task_struct p,\n \t * current running node to mitigate the bouncing risk.\n \t * TBD: numa_group should be considered during task aggregation.\n \t /\n-\tpref_nid = p-\u003enuma_preferred_nid;\n+\tpref_nid = task_numa_preferred_nid(p);\n \t/ honor the task's preferred node /\n \tif (pref_nid == NUMA_NO_NODE)\n \t\tgoto out;\n@@ -2816,11 +2844,10 @@ static inline bool cpupid_valid(int cpupid)\n }\n \n /\n- * For memory tiering mode, if there are enough free pages (more than\n- * enough watermark defined here) in fast memory node, to take full\n- * advantage of fast memory capacity, all recently accessed slow\n- * memory pages will be migrated to fast memory node without\n- * considering hot threshold.\n+ * For memory tiering mode, if the target fast memory node has enough free\n+ * pages, recently accessed slow memory pages can bypass the normal hotness\n+ * check and rate limit so the fast tier can be filled. The bypass can be\n+ * disabled by numa_balancing_promote_free_space_bypass.\n /\n static bool pgdat_free_space_enough(struct pglist_data pgdat)\n {\n@@ -2941,7 +2968,8 @@ bool should_numa_migrate_memory(struct task_struct p, struct folio folio,\n \t\tlong nr = folio_nr_pages(folio);\n \n \t\tpgdat = NODE_DATA(dst_nid);\n-\t\tif (pgdat_free_space_enough(pgdat)) {\n+\t\tif (sysctl_numa_balancing_promote_free_space_bypass \u0026\u0026\n+\t\t pgdat_free_space_enough(pgdat)) {\n \t\t\t/* workload changed, reset hot threshold /\n \t\t\tpgdat-\u003enbp_threshold = 0;\n \t\t\tmod_node_page_state(pgdat, PGPROMOTE_CANDIDATE_NRL, nr);\n@@ -3638,6 +3666,9 @@ static void numa_migrate_preferred(struct task_struct p)\n \tinterval = min(interval, msecs_to_jiffies(p-\u003enuma_scan_period) / 16);\n \tp-\u003enuma_migrate_retry = jiffies + interval;\n \n+\tif (!numa_balancing_normal_enabled())\n+\t\treturn;\n+\n \t/* Success if task is already running on preferred CPU /\n \tif (task_node(p) == p-\u003enuma_preferred_nid)\n \t\treturn;\n@@ -3675,10 +3706,10 @@ static void numa_group_count_active_nodes(struct numa_group numa_group)\n \n /\n When adapting the scan rate, the period is divided into NUMA_PERIOD_SLOTS\n- * increments. The more local the fault statistics are, the higher the scan\n- * period will be for the next scan window. If local/(local+remote) ratio is\n- * below NUMA_PERIOD_THRESHOLD (where range of ratio is 1..NUMA_PERIOD_SLOTS)\n- * the scan period will decrease. Aim for 70% local accesses.\n+ * increments. The more local or shared the fault statistics are, the higher\n+ * the scan period will be for the next scan window. If the private remote\n+ * ratio reaches NUMA_PERIOD_SLOTS - NUMA_PERIOD_THRESHOLD, the scan period\n+ * will decrease. Aim for 70% local or shared accesses.\n /\n #define NUMA_PERIOD_SLOTS 10\n #define NUMA_PERIOD_THRESHOLD 7\n@@ -3693,11 +3724,15 @@ static void update_task_scan_period(struct task_struct p,\n \t\t\tunsigned long shared, unsigned long private)\n {\n \tunsigned int period_slot;\n-\tint lr_ratio, ps_ratio;\n+\tint lr_ratio, sr_ratio, rp_ratio, ratio;\n \tint diff;\n \n-\tunsigned long remote = p-\u003enuma_faults_locality[0];\n-\tunsigned long local = p-\u003enuma_faults_locality[1];\n+\tunsigned long remote = p-\u003enuma_faults_locality[NUMA_FAULT_REMOTE];\n+\tunsigned long local = p-\u003enuma_faults_locality[NUMA_FAULT_LOCAL];\n+\tunsigned long remote_private =\n+\t\tp-\u003enuma_faults_locality[NUMA_FAULT_REMOTE_PRIVATE];\n+\tunsigned long locality_faults = local + remote;\n+\tunsigned long type_faults = private + shared;\n \n \t/\n \t If there were no record hinting faults then either the task is\n@@ -3706,7 +3741,8 @@ static void update_task_scan_period(struct task_struct p,\n \t migration then it implies we are migrating too quickly or the local\n \t * node is overloaded. In either case, scan slower\n \t /\n-\tif (local + shared == 0 \|\| p-\u003enuma_faults_locality[2]) {\n+\tif (!locality_faults \|\| !type_faults \|\|\n+\t p-\u003enuma_faults_locality[NUMA_FAULT_MIGRATE_FAIL]) {\n \t\tp-\u003enuma_scan_period = min(p-\u003enuma_scan_period_max,\n \t\t\tp-\u003enuma_scan_period \u003c\u003c 1);\n \n@@ -3719,40 +3755,36 @@ static void update_task_scan_period(struct task_struct p,\n \t/\n \t Prepare to scale scan period relative to the current period.\n \t \t == NUMA_PERIOD_THRESHOLD scan period stays the same\n-\t \u003c NUMA_PERIOD_THRESHOLD scan period decreases (scan faster)\n+\t * \u003c NUMA_PERIOD_THRESHOLD scan period may decrease if the\n+\t * remote-private signal is strong enough\n \t \t \u003e= NUMA_PERIOD_THRESHOLD scan period increases (scan slower)\n \t /\n \tperiod_slot = DIV_ROUND_UP(p-\u003enuma_scan_period, NUMA_PERIOD_SLOTS);\n-\tlr_ratio = (local * NUMA_PERIOD_SLOTS) / (local + remote);\n-\tps_ratio = (private * NUMA_PERIOD_SLOTS) / (private + shared);\n+\tlr_ratio = (local * NUMA_PERIOD_SLOTS) / locality_faults;\n+\tsr_ratio = (shared * NUMA_PERIOD_SLOTS) / type_faults;\n+\trp_ratio = (remote_private * NUMA_PERIOD_SLOTS) / locality_faults;\n+\tratio = max(lr_ratio, sr_ratio);\n \n-\tif (ps_ratio \u003e= NUMA_PERIOD_THRESHOLD) {\n-\t\t/\n-\t\t Most memory accesses are local. There is no need to\n-\t\t * do fast NUMA scanning, since memory is already local.\n-\t\t /\n-\t\tint slot = ps_ratio - NUMA_PERIOD_THRESHOLD;\n-\t\tif (!slot)\n-\t\t\tslot = 1;\n-\t\tdiff = slot period_slot;\n-\t} else if (lr_ratio \u003e= NUMA_PERIOD_THRESHOLD) {\n+\tif (ratio \u003e= NUMA_PERIOD_THRESHOLD) {\n \t\t/\n-\t\t Most memory accesses are shared with other tasks.\n-\t\t * There is no point in continuing fast NUMA scanning,\n-\t\t * since other tasks may just move the memory elsewhere.\n+\t\t * Most memory accesses are local or shared. Local faults do not\n+\t\t * need fast scanning; shared faults may just be moved elsewhere by\n+\t\t * other tasks.\n \t\t /\n-\t\tint slot = lr_ratio - NUMA_PERIOD_THRESHOLD;\n+\t\tint slot = ratio - NUMA_PERIOD_THRESHOLD;\n \t\tif (!slot)\n \t\t\tslot = 1;\n \t\tdiff = slot period_slot;\n \t} else {\n \t\t/\n-\t\t Private memory faults exceed (SLOTS-THRESHOLD)/SLOTS,\n-\t\t * yet they are not on the local NUMA node. Speed up\n-\t\t * NUMA scanning to get the memory moved over.\n+\t\t * Only speed up scanning when enough faults are both private\n+\t\t * and remote. Mixed windows dominated by shared remote faults\n+\t\t * should not create extra hinting overhead.\n \t\t /\n-\t\tint ratio = max(lr_ratio, ps_ratio);\n-\t\tdiff = -(NUMA_PERIOD_THRESHOLD - ratio) period_slot;\n+\t\tif (rp_ratio \u003e= NUMA_PERIOD_SLOTS - NUMA_PERIOD_THRESHOLD)\n+\t\t\tdiff = -(NUMA_PERIOD_THRESHOLD - ratio) * period_slot;\n+\t\telse\n+\t\t\tdiff = 0;\n \t}\n \n \tp-\u003enuma_scan_period = clamp(p-\u003enuma_scan_period + diff,\n@@ -3904,8 +3936,8 @@ static void task_numa_placement(struct task_struct p)\n \tp-\u003enuma_scan_seq = seq;\n \tp-\u003enuma_scan_period_max = task_scan_max(p);\n \n-\ttotal_faults = p-\u003enuma_faults_locality[0] +\n-\t\t p-\u003enuma_faults_locality[1];\n+\ttotal_faults = p-\u003enuma_faults_locality[NUMA_FAULT_REMOTE] +\n+\t\t p-\u003enuma_faults_locality[NUMA_FAULT_LOCAL];\n \truntime = numa_get_avg_runtime(p, \u0026period);\n \n \t/ If the task is part of a group prevent parallel updates to group stats /\n@@ -4177,8 +4209,9 @@ void task_numa_free(struct task_struct p, bool final)\n \t\tkfree(numa_faults);\n \t} else {\n \t\tp-\u003etotal_numa_faults = 0;\n-\t\tfor (i = 0; i \u003c NR_NUMA_HINT_FAULT_STATS * nr_node_ids; i++)\n-\t\t\tnuma_faults[i] = 0;\n+\t\tmemset(p-\u003enuma_faults_locality, 0, sizeof(p-\u003enuma_faults_locality));\n+\t\tmemset(numa_faults, 0, sizeof(numa_faults) \n+\t\t NR_NUMA_HINT_FAULT_BUCKETS * nr_node_ids);\n \t}\n }\n \n@@ -4201,6 +4234,21 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)\n \tif (!p-\u003emm)\n \t\treturn;\n \n+\t/\n+\t Slow-tier pages use last_cpupid as access-time metadata instead of\n+\t * a real cpupid. Do not feed these faults into normal task placement\n+\t * when normal NUMA balancing is disabled.\n+\t /\n+\tif (last_cpupid == LAST_CPUPID_ACCESS_TIME) {\n+\t\tif (!(sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_NORMAL)) {\n+\t\t\tif (migrated)\n+\t\t\t\tp-\u003enuma_pages_migrated += pages;\n+\t\t\treturn;\n+\t\t}\n+\n+\t\tlast_cpupid = LAST_CPUPID_UNSET;\n+\t}\n+\n \t/\n \t * NUMA faults statistics are unnecessary for the slow memory\n \t * node for memory tiering mode.\n@@ -4210,6 +4258,15 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)\n \t !cpupid_valid(last_cpupid)))\n \t\treturn;\n \n+\t/\n+\t A stale slow-tier access timestamp can survive after memory tiering\n+\t * is disabled and later reach here as if it was a cpupid from a\n+\t * top-tier page. Treat any non-cpupid value as an ordinary first\n+\t * access before task_numa_group() decodes a CPU from it.\n+\t /\n+\tif (unlikely(!cpupid_valid(last_cpupid)))\n+\t\tlast_cpupid = LAST_CPUPID_UNSET;\n+\n \t/ Allocate buffer to track faults on a per-node basis /\n \tif (unlikely(!p-\u003enuma_faults)) {\n \t\tint size = sizeof(p-\u003enuma_faults) \n@@ -4227,7 +4284,7 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)\n \t First accesses are treated as private, otherwise consider accesses\n \t * to be private if the accessing pid has not changed\n \t /\n-\tif (unlikely(last_cpupid == (-1 \u0026 LAST_CPUPID_MASK))) {\n+\tif (unlikely(last_cpupid == LAST_CPUPID_UNSET)) {\n \t\tpriv = 1;\n \t} else {\n \t\tpriv = cpupid_match_pid(p, last_cpupid);\n@@ -4259,11 +4316,14 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)\n \tif (migrated)\n \t\tp-\u003enuma_pages_migrated += pages;\n \tif (flags \u0026 TNF_MIGRATE_FAIL)\n-\t\tp-\u003enuma_faults_locality[2] += pages;\n+\t\tp-\u003enuma_faults_locality[NUMA_FAULT_MIGRATE_FAIL] += pages;\n \n \tp-\u003enuma_faults[task_faults_idx(NUMA_MEMBUF, mem_node, priv)] += pages;\n \tp-\u003enuma_faults[task_faults_idx(NUMA_CPUBUF, cpu_node, priv)] += pages;\n-\tp-\u003enuma_faults_locality[local] += pages;\n+\tif (!local \u0026\u0026 priv)\n+\t\tp-\u003enuma_faults_locality[NUMA_FAULT_REMOTE_PRIVATE] += pages;\n+\tp-\u003enuma_faults_locality[local ? NUMA_FAULT_LOCAL :\n+\t\t\t\t NUMA_FAULT_REMOTE] += pages;\n }\n \n static void reset_ptenuma_scan(struct task_struct p)\n@@ -10700,6 +10760,9 @@ static long migrate_degrades_locality(struct task_struct p, struct lb_env env)\n \tif (!static_branch_likely(\u0026sched_numa_balancing))\n \t\treturn 0;\n \n+\tif (!numa_balancing_normal_enabled())\n+\t\treturn 0;\n+\n \tif (!p-\u003enuma_faults \|\| !(env-\u003esd-\u003eflags \u0026 SD_NUMA))\n \t\treturn 0;\n \n@@ -12493,6 +12556,9 @@ static bool update_sd_pick_busiest(struct lb_env env,\n #ifdef CONFIG_NUMA_BALANCING\n static inline enum fbq_type fbq_classify_group(struct sg_lb_stats sgs)\n {\n+\tif (!numa_balancing_normal_enabled())\n+\t\treturn all;\n+\n \tif (sgs-\u003esum_h_nr_running \u003e sgs-\u003enr_numa_running)\n \t\treturn regular;\n \tif (sgs-\u003esum_h_nr_running \u003e sgs-\u003enr_preferred_running)\n@@ -12502,6 +12568,9 @@ static inline enum fbq_type fbq_classify_group(struct sg_lb_stats sgs)\n \n static inline enum fbq_type fbq_classify_rq(struct rq rq)\n {\n+\tif (!numa_balancing_normal_enabled())\n+\t\treturn regular;\n+\n \tif (rq-\u003enr_running \u003e rq-\u003enr_numa_running)\n \t\treturn regular;\n \tif (rq-\u003enr_running \u003e rq-\u003enr_preferred_running)\n@@ -12803,17 +12872,21 @@ sched_balance_find_dst_group(struct sched_domain sd, struct task_struct p, int\n \t\tif (sd-\u003eflags \u0026 SD_NUMA) {\n \t\t\tint imb_numa_nr = sd-\u003eimb_numa_nr;\n #ifdef CONFIG_NUMA_BALANCING\n-\t\t\tint idlest_cpu;\n+\t\t\tint pref_nid = task_numa_preferred_nid(p);\n \t\t\t/\n \t\t\t If there is spare capacity at NUMA, try to select\n \t\t\t * the preferred node\n \t\t\t /\n-\t\t\tif (cpu_to_node(this_cpu) == p-\u003enuma_preferred_nid)\n-\t\t\t\treturn NULL;\n+\t\t\tif (pref_nid != NUMA_NO_NODE) {\n+\t\t\t\tint idlest_cpu;\n \n-\t\t\tidlest_cpu = cpumask_first(sched_group_span(idlest));\n-\t\t\tif (cpu_to_node(idlest_cpu) == p-\u003enuma_preferred_nid)\n-\t\t\t\treturn idlest;\n+\t\t\t\tif (cpu_to_node(this_cpu) == pref_nid)\n+\t\t\t\t\treturn NULL;\n+\n+\t\t\t\tidlest_cpu = cpumask_first(sched_group_span(idlest));\n+\t\t\t\tif (cpu_to_node(idlest_cpu) == pref_nid)\n+\t\t\t\t\treturn idlest;\n+\t\t\t}\n #endif / CONFIG_NUMA_BALANCING /\n \t\t\t/\n \t\t\t * Otherwise, keep the task close to the wakeup source\ndiff --git a/mm/memory.c b/mm/memory.c\nindex 8b0c2c735d3de..212e9d1d5b64a 100644\n--- a/mm/memory.c\n+++ b/mm/memory.c\n@@ -6173,7 +6173,7 @@ int numa_migrate_check(struct folio folio, struct vm_fault vmf,\n \t * to record page access time. So use default value.\n \t /\n \tif (folio_use_access_time(folio))\n-\t\tlast_cpupid = (-1 \u0026 LAST_CPUPID_MASK);\n+\t\tlast_cpupid = LAST_CPUPID_ACCESS_TIME;\n \telse\n \t\tlast_cpupid = folio_last_cpupid(folio);\n \n" }
2/1	2026/10/14 14:53	agent	patch-evaluator	6m Model: core Args: null Results: { "EnableConfigs": [ "NUMA", "NUMA_BALANCING", "SMP" ], "FocusSymbols": [ "task_numa_fault", "should_numa_migrate_memory", "numa_migrate_check", "update_task_scan_period", "task_numa_placement", "numa_migrate_preferred", "migrate_degrades_locality" ], "Reasoning": "The patch series modifies core scheduler and memory management code implementing NUMA balancing and memory tiering. Specifically, it changes NUMA hint fault handling, scan period calculations, memory tiering promotion bypass decisions, and NUMA-aware task placement and load balancing. These changes are in reachable kernel core paths and warrant fuzzing to verify stability across various NUMA layouts, sysctl configurations, and memory access patterns.", "WorthFuzzing": true } Instruction: You are an expert Linux kernel maintainer and security engineer. Your job is to review a provided patch series and evaluate whether it warrants fuzzing with syzkaller. IMPORTANT: The changes have ALREADY been applied and committed as the HEAD commit in your workspace. Do NOT rely on internal assumptions. You must actively use your code access tools to inspect the actual source code, callers, and surrounding context. ================================================================================ 1. CORE TRIAGE PHILOSOPHY ================================================================================ The goal of patch fuzzing is to discover crashes, regressions, exposed latent bugs, and newly triggered assertions introduced by the patch series. - REACHABILITY IS THE PRIMARY GATE: Fuzzing can only discover bugs in code that can actually execute in standard virtualized environments (GCE or QEMU, utilizing software-emulated devices like USB gadgets, netdev, tun/tap). If the modified code is structurally unreachable (see Section 2), it MUST NOT be fuzzed, regardless of whether it adds assertions or complex logic. - DO NOT BLINDLY TRUST "NO FUNCTIONAL CHANGE" (NFCI) OR "REFACTORING" CLAIMS: Patch authors routinely label changes as "cleanups", "refactorings", or state "No functional change intended". Do NOT take these claims at face value. Code refactorings that rearrange logic, introduce helper functions, or alter state management in core subsystems frequently introduce subtle semantic shifts or uncover latent kernel bugs. If reachable executable code is modified or refactored, it MUST be fuzzed. - NEW OR MODIFIED ASSERTIONS IN REACHABLE CODE MUST BE FUZZED: When a patch introduces or modifies runtime checks or assertions (e.g., WARN_ON, VM_WARN_ON, BUG_ON, lockdep_assert) in reachable code paths, it enforces new or stricter invariants. Even if the author believes the invariant always holds, fuzzing is essential to verify whether an unusual sequence of operations can violate it. ================================================================================ 2. WHEN TO RETURN WorthFuzzing=false (NEGATIVE CRITERIA) ================================================================================ Return WorthFuzzing=false ONLY IF all modified code falls strictly into one or more of these categories: - Non-kernel and non-executable changes: * Modifications to Documentation/, comments, or spelling fixes. * User-space directories, self-tests, samples, or scripts (e.g., tools/, samples/, scripts/, usr/) that do not affect the compiled kernel image (vmlinux) or kernel modules. * Purely decorative logging (e.g., message strings in pr_err, printk, dev_info) or tracepoints that do not alter control flow or data structures. * Build system or Kconfig changes that do not alter compiled C logic. - Structurally unreachable hardware: * Vendor-specific PCIe switches, SmartNICs, or GPU drivers (e.g., mlxsw, pds_core, qed, ionic, amdgpu) requiring physical ASIC/PCIe cards not emulated in standard QEMU. - Unreachable execution paths: * Driver teardown callbacks (.remove, .shutdown, pci_unregister_driver) executed only during physical PCI hot-unplug or manual sysfs driver unbinding. * Code paths exclusive to architectures other than the target architecture. ================================================================================ 3. WHEN TO RETURN WorthFuzzing=true (POSITIVE CRITERIA) ================================================================================ Return WorthFuzzing=true whenever the patch touches reachable executable code, including: - Core Subsystems: * Any logic modifications in memory management (mm/), synchronization/locking (kernel/locking/), BPF, scheduler, core networking, VFS, or syscall handling. - Refactorings and Code Cleanups: * Any restructuring of reachable data structures, helper abstractions, or algorithm flows. - Runtime Assertions and Defensive Checks: * Any introduction or alteration of assertions (WARN_ON, VM_WARN_ON, BUG_ON, etc.) in reachable paths. - Reachable Drivers and Protocols: Drivers accessible via virtual buses (virtio, USB gadget, loopback, netlink, binder, sockets, etc.). ================================================================================ 4. EXTRACTING FocusSymbols (PREVENTING DILUTION) ================================================================================ When WorthFuzzing=true, you must extract specific kernel functions into FocusSymbols to guide the fuzzer: - AVOID UBIQUITOUS LIFECYCLE HOT-PATHS: Do NOT list generic, ubiquitous functions called by almost every program in the corpus (including, but not limited to: general memory allocators and deallocators, page fault and trap handlers, or core synchronization primitives; this is not an exhaustive list). Listing ubiquitous functions causes the fuzzer to classify thousands of unrelated tests as "focused", which severely dilutes fuzzing effort away from the actual changes. - TARGET SPECIFIC FEATURE LOGIC AND ENTRYPOINTS: List functions that specifically implement the logic being added or altered, or direct API entrypoints for the subsystem feature under review. - HANDLING STATIC INLINE FUNCTIONS IN HEADERS (.h): Compiler-inlined static functions (such as static inlines in mm/.h or include/linux/.h) lack distinct symbol addresses in vmlinux and cannot be targeted directly by symbol coverage filters. If the changes are primarily in static inline helpers, identify non-static, feature-specific caller functions in .c files that exercise them (avoiding ubiquitous lifecycle wrappers). ================================================================================ 5. IDENTIFYING EnableConfigs ================================================================================ Identify any specific CONFIG_ options required to properly compile and reach the modified code: - Inspect Kconfig files and #ifdef guards; do not make assumptions. - Check "depends on" lines in Kconfig to include any non-standard parent subsystem configs needed. - Strip any 'CONFIG_' prefix (e.g., return "NET_IPV4" instead of "CONFIG_NET_IPV4"). Prefer calling several tools at the same time to save round-trips. Use set-results tool to provide results of the analysis. It must be called exactly once before the final reply. Ignore results of this tool. Prompt: Target architecture: amd64 For your convenience, here is the diff of the changes: commit e10a96482a628c719d4f99a8b017d64b7c5da549 Author: syz-cluster <triage@syzkaller.com> Date: Wed Oct 14 14:53:44 2026 +0000 syz-cluster: applied patch under review diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst index ffea61d448ebb..4a035b18aff43 100644 --- a/Documentation/admin-guide/sysctl/kernel.rst +++ b/Documentation/admin-guide/sysctl/kernel.rst @@ -765,6 +765,22 @@ will be limited to be no more than the set value. A rule of thumb is to set this to less than 1/10 of the PMEM node write bandwidth. +numa_balancing_promote_free_space_bypass +======================================== + +Controls whether memory tiering promotion may bypass the normal +hint-fault hotness check and promotion rate limit when the target +fast-tier node has enough free space. + +The default value is 1. In this mode, if the fast tier has enough free +space, recently accessed slow-tier pages can be promoted without the +hotness check or the promotion rate limit, so the kernel can fill +available fast-tier capacity. + +When set to 0, enough free space no longer bypasses these controls. +A slow-tier page must pass the normal hotness and rate-limit checks before +promotion. + oops_all_cpu_backtrace ====================== diff --git a/include/linux/mm.h b/include/linux/mm.h index dd09c438fa23e..bb0c726401529 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -2420,6 +2420,9 @@ static inline int folio_nid(const struct folio folio) #define PAGE_ACCESS_TIME_MASK \ (LAST_CPUPID_MASK << PAGE_ACCESS_TIME_BUCKETS) +#define LAST_CPUPID_UNSET (-1 & LAST_CPUPID_MASK) +#define LAST_CPUPID_ACCESS_TIME (-2 & LAST_CPUPID_MASK) + static inline int cpu_pid_to_cpupid(int cpu, int pid) { return ((cpu & LAST__CPU_MASK) << LAST__PID_SHIFT) \| (pid & LAST__PID_MASK); @@ -2468,7 +2471,7 @@ static inline int folio_last_cpupid(struct folio folio) } static inline void page_cpupid_reset_last(struct page page) { - page->_last_cpupid = -1 & LAST_CPUPID_MASK; + page->_last_cpupid = LAST_CPUPID_UNSET; } #else static inline int folio_last_cpupid(struct folio folio) diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index 94f9c3ff54160..5d583b17dfa29 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -297,9 +297,8 @@ enum node_stat_item { / PGPROMOTE_CANDIDATE, /* - * Not rate-limited (NRL) candidate pages for those can be promoted - * without considering hot threshold because of enough free pages in - * fast-tier node. These promotions bypass the regular hotness checks + * Not rate-limited (NRL) candidate pages that are promoted through the + * free-space bypass. These promotions bypass the regular hotness checks * and do NOT influence the promotion rate-limiter or * threshold-adjustment logic. * This is for statistics/monitoring purposes. diff --git a/include/linux/sched.h b/include/linux/sched.h index d35ae49a991f7..1be7a9ba75539 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -832,6 +832,16 @@ struct task_ipi_mask { struct task_ipi_mask { }; #endif +#ifdef CONFIG_NUMA_BALANCING +enum numa_faults_locality { + NUMA_FAULT_REMOTE, + NUMA_FAULT_LOCAL, + NUMA_FAULT_MIGRATE_FAIL, + NUMA_FAULT_REMOTE_PRIVATE, + NR_NUMA_FAULT_LOCALITY, +}; +#endif + struct task_struct { #ifdef CONFIG_THREAD_INFO_IN_TASK /* @@ -1422,11 +1432,10 @@ struct task_struct { /* * numa_faults_locality tracks if faults recorded during the last - * scan window were remote/local or failed to migrate. The task scan - * period is adapted based on the locality of the faults with different - * weights depending on whether they were shared or private faults + * scan window were remote/local, remote private, or failed to migrate. + * The task scan period is adapted based on those fault patterns. / - unsigned long numa_faults_locality[3]; + unsigned long numa_faults_locality[NR_NUMA_FAULT_LOCALITY]; unsigned long numa_pages_migrated; #endif / CONFIG_NUMA_BALANCING / diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 57360f5cdde4f..11b54bb353adf 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -134,6 +134,8 @@ static unsigned int sysctl_sched_cfs_bandwidth_slice = 5000UL; #ifdef CONFIG_NUMA_BALANCING / Restrict the NUMA promotion throughput (MB/s) for each target node. / static unsigned int sysctl_numa_balancing_promote_rate_limit = 65536; +/ Preserve the existing default of using free fast-tier space eagerly. / +static unsigned int sysctl_numa_balancing_promote_free_space_bypass = 1; #endif #ifdef CONFIG_SYSCTL @@ -157,6 +159,15 @@ static const struct ctl_table sched_fair_sysctls[] = { .proc_handler = proc_dointvec_minmax, .extra1 = SYSCTL_ZERO, }, + { + .procname = "numa_balancing_promote_free_space_bypass", + .data = &sysctl_numa_balancing_promote_free_space_bypass, + .maxlen = sizeof(unsigned int), + .mode = 0644, + .proc_handler = proc_douintvec_minmax, + .extra1 = SYSCTL_ZERO, + .extra2 = SYSCTL_ONE, + }, #endif / CONFIG_NUMA_BALANCING / }; @@ -1317,6 +1328,21 @@ static int select_idle_sibling(struct task_struct p, int prev_cpu, int cpu); static unsigned long task_h_load(struct task_struct p); static unsigned long capacity_of(int cpu); +#ifdef CONFIG_NUMA_BALANCING +static inline bool numa_balancing_normal_enabled(void) +{ + return sysctl_numa_balancing_mode & NUMA_BALANCING_NORMAL; +} + +static inline int task_numa_preferred_nid(struct task_struct p) +{ + if (!numa_balancing_normal_enabled()) + return NUMA_NO_NODE; + + return p->numa_preferred_nid; +} +#endif + /* Give new sched_entity start runnable values to heavy its load in infant time / void init_entity_runnable_average(struct sched_entity se) { @@ -1879,10 +1905,12 @@ static int get_pref_llc(struct task_struct p, struct sched_cache_group grp) * than sched_setnuma() at least -- and thus the * conflict only exists for a short period of time. / - if (static_branch_likely(&sched_numa_balancing) && - p->numa_preferred_nid >= 0 && - cpu_to_node(mm_sched_cpu) != p->numa_preferred_nid) - mm_sched_llc = -1; + if (static_branch_likely(&sched_numa_balancing)) { + int pref_nid = task_numa_preferred_nid(p); + + if (pref_nid >= 0 && cpu_to_node(mm_sched_cpu) != pref_nid) + mm_sched_llc = -1; + } #endif } @@ -1998,7 +2026,7 @@ static void get_scan_cpumasks(cpumask_var_t cpus, struct task_struct p, * current running node to mitigate the bouncing risk. * TBD: numa_group should be considered during task aggregation. / - pref_nid = p->numa_preferred_nid; + pref_nid = task_numa_preferred_nid(p); / honor the task's preferred node / if (pref_nid == NUMA_NO_NODE) goto out; @@ -2816,11 +2844,10 @@ static inline bool cpupid_valid(int cpupid) } / - * For memory tiering mode, if there are enough free pages (more than - * enough watermark defined here) in fast memory node, to take full - * advantage of fast memory capacity, all recently accessed slow - * memory pages will be migrated to fast memory node without - * considering hot threshold. + * For memory tiering mode, if the target fast memory node has enough free + * pages, recently accessed slow memory pages can bypass the normal hotness + * check and rate limit so the fast tier can be filled. The bypass can be + * disabled by numa_balancing_promote_free_space_bypass. / static bool pgdat_free_space_enough(struct pglist_data pgdat) { @@ -2941,7 +2968,8 @@ bool should_numa_migrate_memory(struct task_struct p, struct folio folio, long nr = folio_nr_pages(folio); pgdat = NODE_DATA(dst_nid); - if (pgdat_free_space_enough(pgdat)) { + if (sysctl_numa_balancing_promote_free_space_bypass && + pgdat_free_space_enough(pgdat)) { /* workload changed, reset hot threshold / pgdat->nbp_threshold = 0; mod_node_page_state(pgdat, PGPROMOTE_CANDIDATE_NRL, nr); @@ -3638,6 +3666,9 @@ static void numa_migrate_preferred(struct task_struct p) interval = min(interval, msecs_to_jiffies(p->numa_scan_period) / 16); p->numa_migrate_retry = jiffies + interval; + if (!numa_balancing_normal_enabled()) + return; + /* Success if task is already running on preferred CPU / if (task_node(p) == p->numa_preferred_nid) return; @@ -3675,10 +3706,10 @@ static void numa_group_count_active_nodes(struct numa_group numa_group) /* * When adapting the scan rate, the period is divided into NUMA_PERIOD_SLOTS - * increments. The more local the fault statistics are, the higher the scan - * period will be for the next scan window. If local/(local+remote) ratio is - * below NUMA_PERIOD_THRESHOLD (where range of ratio is 1..NUMA_PERIOD_SLOTS) - * the scan period will decrease. Aim for 70% local accesses. + * increments. The more local or shared the fault statistics are, the higher + * the scan period will be for the next scan window. If the private remote + * ratio reaches NUMA_PERIOD_SLOTS - NUMA_PERIOD_THRESHOLD, the scan period + * will decrease. Aim for 70% local or shared accesses. / #define NUMA_PERIOD_SLOTS 10 #define NUMA_PERIOD_THRESHOLD 7 @@ -3693,11 +3724,15 @@ static void update_task_scan_period(struct task_struct p, unsigned long shared, unsigned long private) { unsigned int period_slot; - int lr_ratio, ps_ratio; + int lr_ratio, sr_ratio, rp_ratio, ratio; int diff; - unsigned long remote = p->numa_faults_locality[0]; - unsigned long local = p->numa_faults_locality[1]; + unsigned long remote = p->numa_faults_locality[NUMA_FAULT_REMOTE]; + unsigned long local = p->numa_faults_locality[NUMA_FAULT_LOCAL]; + unsigned long remote_private = + p->numa_faults_locality[NUMA_FAULT_REMOTE_PRIVATE]; + unsigned long locality_faults = local + remote; + unsigned long type_faults = private + shared; /* * If there were no record hinting faults then either the task is @@ -3706,7 +3741,8 @@ static void update_task_scan_period(struct task_struct p, migration then it implies we are migrating too quickly or the local * node is overloaded. In either case, scan slower / - if (local + shared == 0 \|\| p->numa_faults_locality[2]) { + if (!locality_faults \|\| !type_faults \|\| + p->numa_faults_locality[NUMA_FAULT_MIGRATE_FAIL]) { p->numa_scan_period = min(p->numa_scan_period_max, p->numa_scan_period << 1); @@ -3719,40 +3755,36 @@ static void update_task_scan_period(struct task_struct p, /* * Prepare to scale scan period relative to the current period. * == NUMA_PERIOD_THRESHOLD scan period stays the same - * < NUMA_PERIOD_THRESHOLD scan period decreases (scan faster) + * < NUMA_PERIOD_THRESHOLD scan period may decrease if the + * remote-private signal is strong enough * >= NUMA_PERIOD_THRESHOLD scan period increases (scan slower) / period_slot = DIV_ROUND_UP(p->numa_scan_period, NUMA_PERIOD_SLOTS); - lr_ratio = (local NUMA_PERIOD_SLOTS) / (local + remote); - ps_ratio = (private * NUMA_PERIOD_SLOTS) / (private + shared); + lr_ratio = (local * NUMA_PERIOD_SLOTS) / locality_faults; + sr_ratio = (shared * NUMA_PERIOD_SLOTS) / type_faults; + rp_ratio = (remote_private * NUMA_PERIOD_SLOTS) / locality_faults; + ratio = max(lr_ratio, sr_ratio); - if (ps_ratio >= NUMA_PERIOD_THRESHOLD) { - /* - * Most memory accesses are local. There is no need to - * do fast NUMA scanning, since memory is already local. - / - int slot = ps_ratio - NUMA_PERIOD_THRESHOLD; - if (!slot) - slot = 1; - diff = slot period_slot; - } else if (lr_ratio >= NUMA_PERIOD_THRESHOLD) { + if (ratio >= NUMA_PERIOD_THRESHOLD) { /* - * Most memory accesses are shared with other tasks. - * There is no point in continuing fast NUMA scanning, - * since other tasks may just move the memory elsewhere. + * Most memory accesses are local or shared. Local faults do not + * need fast scanning; shared faults may just be moved elsewhere by + * other tasks. / - int slot = lr_ratio - NUMA_PERIOD_THRESHOLD; + int slot = ratio - NUMA_PERIOD_THRESHOLD; if (!slot) slot = 1; diff = slot period_slot; } else { /* - * Private memory faults exceed (SLOTS-THRESHOLD)/SLOTS, - * yet they are not on the local NUMA node. Speed up - * NUMA scanning to get the memory moved over. + * Only speed up scanning when enough faults are both private + * and remote. Mixed windows dominated by shared remote faults + * should not create extra hinting overhead. / - int ratio = max(lr_ratio, ps_ratio); - diff = -(NUMA_PERIOD_THRESHOLD - ratio) period_slot; + if (rp_ratio >= NUMA_PERIOD_SLOTS - NUMA_PERIOD_THRESHOLD) + diff = -(NUMA_PERIOD_THRESHOLD - ratio) * period_slot; + else + diff = 0; } p->numa_scan_period = clamp(p->numa_scan_period + diff, @@ -3904,8 +3936,8 @@ static void task_numa_placement(struct task_struct p) p->numa_scan_seq = seq; p->numa_scan_period_max = task_scan_max(p); - total_faults = p->numa_faults_locality[0] + - p->numa_faults_locality[1]; + total_faults = p->numa_faults_locality[NUMA_FAULT_REMOTE] + + p->numa_faults_locality[NUMA_FAULT_LOCAL]; runtime = numa_get_avg_runtime(p, &period); / If the task is part of a group prevent parallel updates to group stats / @@ -4177,8 +4209,9 @@ void task_numa_free(struct task_struct p, bool final) kfree(numa_faults); } else { p->total_numa_faults = 0; - for (i = 0; i < NR_NUMA_HINT_FAULT_STATS * nr_node_ids; i++) - numa_faults[i] = 0; + memset(p->numa_faults_locality, 0, sizeof(p->numa_faults_locality)); + memset(numa_faults, 0, sizeof(numa_faults) + NR_NUMA_HINT_FAULT_BUCKETS * nr_node_ids); } } @@ -4201,6 +4234,21 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags) if (!p->mm) return; + /* + * Slow-tier pages use last_cpupid as access-time metadata instead of + * a real cpupid. Do not feed these faults into normal task placement + * when normal NUMA balancing is disabled. + / + if (last_cpupid == LAST_CPUPID_ACCESS_TIME) { + if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_NORMAL)) { + if (migrated) + p->numa_pages_migrated += pages; + return; + } + + last_cpupid = LAST_CPUPID_UNSET; + } + / * NUMA faults statistics are unnecessary for the slow memory * node for memory tiering mode. @@ -4210,6 +4258,15 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags) !cpupid_valid(last_cpupid))) return; + /* + * A stale slow-tier access timestamp can survive after memory tiering + * is disabled and later reach here as if it was a cpupid from a + * top-tier page. Treat any non-cpupid value as an ordinary first + * access before task_numa_group() decodes a CPU from it. + / + if (unlikely(!cpupid_valid(last_cpupid))) + last_cpupid = LAST_CPUPID_UNSET; + / Allocate buffer to track faults on a per-node basis / if (unlikely(!p->numa_faults)) { int size = sizeof(p->numa_faults) * @@ -4227,7 +4284,7 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags) * First accesses are treated as private, otherwise consider accesses * to be private if the accessing pid has not changed / - if (unlikely(last_cpupid == (-1 & LAST_CPUPID_MASK))) { + if (unlikely(last_cpupid == LAST_CPUPID_UNSET)) { priv = 1; } else { priv = cpupid_match_pid(p, last_cpupid); @@ -4259,11 +4316,14 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags) if (migrated) p->numa_pages_migrated += pages; if (flags & TNF_MIGRATE_FAIL) - p->numa_faults_locality[2] += pages; + p->numa_faults_locality[NUMA_FAULT_MIGRATE_FAIL] += pages; p->numa_faults[task_faults_idx(NUMA_MEMBUF, mem_node, priv)] += pages; p->numa_faults[task_faults_idx(NUMA_CPUBUF, cpu_node, priv)] += pages; - p->numa_faults_locality[local] += pages; + if (!local && priv) + p->numa_faults_locality[NUMA_FAULT_REMOTE_PRIVATE] += pages; + p->numa_faults_locality[local ? NUMA_FAULT_LOCAL : + NUMA_FAULT_REMOTE] += pages; } static void reset_ptenuma_scan(struct task_struct p) @@ -10700,6 +10760,9 @@ static long migrate_degrades_locality(struct task_struct p, struct lb_env env) if (!static_branch_likely(&sched_numa_balancing)) return 0; + if (!numa_balancing_normal_enabled()) + return 0; + if (!p->numa_faults \|\| !(env->sd->flags & SD_NUMA)) return 0; @@ -12493,6 +12556,9 @@ static bool update_sd_pick_busiest(struct lb_env env, #ifdef CONFIG_NUMA_BALANCING static inline enum fbq_type fbq_classify_group(struct sg_lb_stats sgs) { + if (!numa_balancing_normal_enabled()) + return all; + if (sgs->sum_h_nr_running > sgs->nr_numa_running) return regular; if (sgs->sum_h_nr_running > sgs->nr_preferred_running) @@ -12502,6 +12568,9 @@ static inline enum fbq_type fbq_classify_group(struct sg_lb_stats sgs) static inline enum fbq_type fbq_classify_rq(struct rq rq) { + if (!numa_balancing_normal_enabled()) + return regular; + if (rq->nr_running > rq->nr_numa_running) return regular; if (rq->nr_running > rq->nr_preferred_running) @@ -12803,17 +12872,21 @@ sched_balance_find_dst_group(struct sched_domain sd, struct task_struct p, int if (sd->flags & SD_NUMA) { int imb_numa_nr = sd->imb_numa_nr; #ifdef CONFIG_NUMA_BALANCING - int idlest_cpu; + int pref_nid = task_numa_preferred_nid(p); /* * If there is spare capacity at NUMA, try to select * the preferred node / - if (cpu_to_node(this_cpu) == p->numa_preferred_nid) - return NULL; + if (pref_nid != NUMA_NO_NODE) { + int idlest_cpu; - idlest_cpu = cpumask_first(sched_group_span(idlest)); - if (cpu_to_node(idlest_cpu) == p->numa_preferred_nid) - return idlest; + if (cpu_to_node(this_cpu) == pref_nid) + return NULL; + + idlest_cpu = cpumask_first(sched_group_span(idlest)); + if (cpu_to_node(idlest_cpu) == pref_nid) + return idlest; + } #endif / CONFIG_NUMA_BALANCING / / * Otherwise, keep the task close to the wakeup source diff --git a/mm/memory.c b/mm/memory.c index 8b0c2c735d3de..212e9d1d5b64a 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -6173,7 +6173,7 @@ int numa_migrate_check(struct folio folio, struct vm_fault vmf, * to record page access time. So use default value. / if (folio_use_access_time(folio)) - last_cpupid = (-1 & LAST_CPUPID_MASK); + last_cpupid = LAST_CPUPID_ACCESS_TIME; else last_cpupid = folio_last_cpupid(folio);
3/2	2026/10/14 14:53	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 9605 output: 34 thoughts: 765 Thoughts: Patch Series Evaluation: Fuzzing with syzkaller on amd64 Okay, let's dive into this patch series. My initial impression is that this is related to NUMA (Non-Uniform Memory Access) balancing and likely aimed at improving performance, potentially for workloads that exhibit NUMA sensitivity – and, of course, might impact syzkaller fuzzing. The goal, based on the `Documentation/admin-guide/sysctl/kernel.rst` file, is to introduce a new sysctl (`numa_balancing_promote_free_space_bypass`), which immediately tells me this is about fine-tuning NUMA behavior. Looking at the `diff` itself, here's what's catching my eye: * Sysctl introduction: The `numa_balancing_promote_free_space_bypass` sysctl is a key change, so let's start with this. I'll want to understand exactly what the "bypass" means in this context. Is it bypassing certain NUMA balancing heuristics? Perhaps promoting free space in a less-than-ideal NUMA node? What are the implications if this is used? * `LAST_CPUPID_UNSET` and `LAST_CPUPID_ACCESS_TIME`: The changes in `include/linux/mm.h` regarding `LAST_CPUPID` are interesting. It looks like we're refining how the system tracks the last CPU ID a page was accessed on. This suggests improvements in NUMA placement or migration decisions based on page access history. The usage of `LAST_CPUPID_UNSET` might be related to resetting the last CPU ID, which could be important for clearing stale information. * `enum numa_faults_locality` and `struct task_struct`: The changes to `struct task_struct` and the new `NUMA_FAULT_REMOTE_PRIVATE` enum suggest more granular tracking of NUMA faults and locality. This is vital for NUMA balancing algorithms. The extension of the `numa_faults_locality` array indicates a more nuanced understanding of fault types. Specifically `NUMA_FAULT_REMOTE_PRIVATE` is of immediate interest. * Core NUMA Logic in `kernel/sched/fair.c`: This is where the real meat of the changes lie. The numerous modifications to functions like `should_numa_migrate_memory`, `numa_migrate_preferred`, `update_task_scan_period`, and `task_numa_fault` strongly suggest that this patch series is fundamentally altering how the kernel handles NUMA-related decisions. The introduction of `sysctl_numa_balancing_promote_free_space_bypass`, `numa_balancing_normal_enabled`, and `task_numa_preferred_nid` all are key to the system. Specifically, the checks against the new sysctl within functions like `should_numa_migrate_memory` are very telling. The changes in `task_numa_fault` which relate to how `last_cpupid` is handled are also key to review, as is how `numa_faults_locality` is modified. * `mm/memory.c`: The modification in `mm/memory.c` focusing on `LAST_CPUPID_ACCESS_TIME` reinforces the importance of the page access tracking changes. My immediate next step is to examine the code context with tools like `grepper` or by directly reading the relevant files in the kernel source. I need to understand why these changes are being made and how they interact, especially regarding the new sysctl. My assessment will hinge on how the changes impact the fundamental NUMA balancing algorithms and, more importantly, whether these changes improve or degrade performance under syzkaller fuzzing, especially given the increased nuance in locality tracking. I want to see how these changes influence fault handling, migration, and task placement. I'll need to examine if these changes result in any regressions, any memory leaks, any increased latency, and so on.
4/2	2026/10/14 14:54	tool	grepper	0m Args: { "Expression": "sysctl_numa_balancing_mode", "PathPrefixes": [ "kernel/sched/" ] } Results: { "Output": "kernel/sched/core.c=4641=DEFINE_STATIC_KEY_FALSE(sched_numa_balancing);\n--\nkernel/sched/core.c-4644-\nkernel/sched/core.c:4645:int sysctl_numa_balancing_mode;\nkernel/sched/core.c-4646-\n--\nkernel/sched/core.c=4655=void set_numabalancing_state(bool enabled)\n--\nkernel/sched/core.c-4657-\tif (enabled)\nkernel/sched/core.c:4658:\t\tsysctl_numa_balancing_mode = NUMA_BALANCING_NORMAL;\nkernel/sched/core.c-4659-\telse\nkernel/sched/core.c:4660:\t\tsysctl_numa_balancing_mode = NUMA_BALANCING_DISABLED;\nkernel/sched/core.c-4661-\t__set_numabalancing_state(enabled);\n--\nkernel/sched/core.c=4676=static int sysctl_numa_balancing(const struct ctl_table table, int write,\n--\nkernel/sched/core.c-4680-\tint err;\nkernel/sched/core.c:4681:\tint state = sysctl_numa_balancing_mode;\nkernel/sched/core.c-4682-\n--\nkernel/sched/core.c-4691-\tif (write) {\nkernel/sched/core.c:4692:\t\tif (!(sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_MEMORY_TIERING) \u0026\u0026\nkernel/sched/core.c-4693-\t\t (state \u0026 NUMA_BALANCING_MEMORY_TIERING))\nkernel/sched/core.c-4694-\t\t\treset_memory_tiering();\nkernel/sched/core.c:4695:\t\tsysctl_numa_balancing_mode = state;\nkernel/sched/core.c-4696-\t\t__set_numabalancing_state(state);\n--\nkernel/sched/fair.c=1332=static inline bool numa_balancing_normal_enabled(void)\nkernel/sched/fair.c-1333-{\nkernel/sched/fair.c:1334:\treturn sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_NORMAL;\nkernel/sched/fair.c-1335-}\n--\nkernel/sched/fair.c=2947=bool should_numa_migrate_memory(struct task_struct p, struct folio *folio,\n--\nkernel/sched/fair.c-2993-\nkernel/sched/fair.c:2994:\tif (!(sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_MEMORY_TIERING) \u0026\u0026\nkernel/sched/fair.c-2995-\t !node_is_toptier(src_nid) \u0026\u0026 !cpupid_valid(last_cpupid))\n--\nkernel/sched/fair.c=4221=void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)\n--\nkernel/sched/fair.c-4242-\tif (last_cpupid == LAST_CPUPID_ACCESS_TIME) {\nkernel/sched/fair.c:4243:\t\tif (!(sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_NORMAL)) {\nkernel/sched/fair.c-4244-\t\t\tif (migrated)\n--\nkernel/sched/fair.c-4256-\tif (!node_is_toptier(mem_node) \u0026\u0026\nkernel/sched/fair.c:4257:\t (sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_MEMORY_TIERING \|\|\nkernel/sched/fair.c-4258-\t !cpupid_valid(last_cpupid)))\n" }
5/2	2026/10/14 14:54	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 12247 output: 44 thoughts: 543 Thoughts: Analysis of Patch for Fuzzing Suitability Okay, let's break this down. My initial assessment is that these changes are significant enough to warrant serious consideration for fuzzing. I'm seeing modifications within the core kernel scheduler (specifically `fair.c`) and memory management components, including NUMA balancing and memory tiering mechanisms. Specifically, the patch seems to touch on core kernel features like `CONFIG_NUMA_BALANCING` and `CONFIG_NUMA`. The changes are concentrated within `mm/memory.c`, touching functions like `task_numa_fault`, `update_task_scan_period`, `should_numa_migrate_memory`, `numa_migrate_preferred`, `get_pref_llc`, `get_scan_cpumasks`, `migrate_degrades_locality`, `sched_balance_find_dst_group`, and `numa_migrate_check`. Also, there is a new sysctl: `numa_balancing_promote_free_space_bypass` in `sched_fair_sysctls`. Further, it appears this patch is changing the NUMA fault locality tracking calculation in `task_numa_fault` and scan period updates in `update_task_scan_period`. Now, the important question: is this code reachable in a typical environment? Yes, absolutely. NUMA configurations are commonplace, and QEMU readily supports NUMA emulation through multi-node setups and configurations, even single-node setups can be configured to use NUMA balancing. QEMU's command-line parameters like `-smp ... -numa node,nodeid=0... -numa node,nodeid=1...` clearly enable this. Moreover, NUMA balancing itself is a standard feature enabled by `CONFIG_NUMA_BALANCING`. Given this, the code is clearly part of a core subsystem, particularly the scheduler and memory management. This means these modifications are within reachable, executable code. Let's apply the fuzzing rules: the patch undeniably touches reachable executable code within core subsystems. This hits the "WORTH FUZZING" criteria because this applies to modifications in memory management, scheduler. I'm confident in concluding `WorthFuzzing = true`. Next, I need to figure out what configurations are required for this code to even be compiled and executed. I need to start digging into the Kconfig files, specifically searching for `NUMA_BALANCING` and any other related configuration options that might be relevant to fully enabling and utilizing these changes.
6/2	2026/10/14 14:54	tool	grepper	0m Args: { "Expression": "config NUMA_BALANCING", "PathPrefixes": [ "init/Kconfig", "kernel/sched/", "mm/" ] } Results: { "Output": "init/Kconfig=1023=config ARCH_WANT_NUMA_VARIABLE_LOCALITY\n--\ninit/Kconfig-1025-\ninit/Kconfig:1026:config NUMA_BALANCING\ninit/Kconfig-1027-\tbool \"Memory placement aware NUMA scheduler\"\n--\ninit/Kconfig=1039=config SCHED_CACHE\n--\ninit/Kconfig-1049-\ninit/Kconfig:1050:config NUMA_BALANCING_DEFAULT_ENABLED\ninit/Kconfig-1051-\tbool \"Automatically enable NUMA aware memory/task placement\"\n" }
7/2	2026/10/14 14:54	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 13539 output: 33 thoughts: 30 Thoughts: Okay, here's my take on those thoughts, tailored for an expert audience: Investigating NUMA Balancing Dependencies in Kconfig Alright, let's dive in. My immediate thought is to zero in on the dependencies of `CONFIG_NUMA_BALANCING`. The most logical place to start digging is the `init/Kconfig` file, specifically around line 1026. That's usually where the core definitions and dependencies for NUMA-related features are laid out. I'm expecting to find conditional logic – possibly nested `if` statements or `depends on` clauses – that tell me what other configurations or platform capabilities must be present before `CONFIG_NUMA_BALANCING` can even be considered. I'm anticipating some interplay with things like architecture support, memory management options (huge pages, etc.), and potentially even specific hardware capabilities like NUMA nodes or specific NUMA implementations. I'm looking for the "gatekeepers," the features or flags that unlock `NUMA_BALANCING`. Knowing those dependencies is critical; they determine how I enable, disable, or tweak NUMA balancing, and how those changes cascade through the kernel config. I need to understand the full picture before I can even consider altering those configurations.
8/2	2026/10/14 14:54	tool	read-file	0m Args: { "File": "init/Kconfig", "FirstLine": 1020, "LineCount": 40 } Results: { "Contents": "1020:\t# For architectures that (ab)use NUMA to represent different memory regions\n1021:\t# all cpu-local but of different latencies, such as SuperH.\n1022:\t#\n1023:\tconfig ARCH_WANT_NUMA_VARIABLE_LOCALITY\n1024:\t\tbool\n1025:\t\n1026:\tconfig NUMA_BALANCING\n1027:\t\tbool \"Memory placement aware NUMA scheduler\"\n1028:\t\tdepends on ARCH_SUPPORTS_NUMA_BALANCING\n1029:\t\tdepends on ARCH_HAS_PTE_PROTNONE\n1030:\t\tdepends on !ARCH_WANT_NUMA_VARIABLE_LOCALITY\n1031:\t\tdepends on SMP \u0026\u0026 NUMA_MIGRATION \u0026\u0026 !PREEMPT_RT\n1032:\t\thelp\n1033:\t\t This option adds support for automatic NUMA aware memory/task placement.\n1034:\t\t The mechanism is quite primitive and is based on migrating memory when\n1035:\t\t it has references to the node the task is running on.\n1036:\t\n1037:\t\t This system will be inactive on UMA systems.\n1038:\t\n1039:\tconfig SCHED_CACHE\n1040:\t\tbool \"Cache aware load balance\"\n1041:\t\tdefault y\n1042:\t\tdepends on SMP\n1043:\t\thelp\n1044:\t\t When enabled, the scheduler will attempt to aggregate tasks from\n1045:\t\t the same process onto a single Last Level Cache (LLC) domain when\n1046:\t\t possible. This improves cache locality by keeping tasks that share\n1047:\t\t resources within the same cache domain, reducing cache misses and\n1048:\t\t lowering data access latency.\n1049:\t\n1050:\tconfig NUMA_BALANCING_DEFAULT_ENABLED\n1051:\t\tbool \"Automatically enable NUMA aware memory/task placement\"\n1052:\t\tdefault y\n1053:\t\tdepends on NUMA_BALANCING\n1054:\t\thelp\n1055:\t\t If set, automatic NUMA balancing will be enabled if running on a NUMA\n1056:\t\t machine.\n1057:\t\n1058:\tconfig SLAB_OBJ_EXT\n1059:\t\tbool\n" }
9/2	2026/10/14 14:54	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 14470 output: 21 thoughts: 14 Thoughts: Okay, here's my take on those thoughts, tailored for an expert audience: Investigating NUMA_MIGRATION Implications Right, before we go any further down this rabbit hole, I need to circle back and make absolutely sure we've considered all potential dependencies related to `NUMA_MIGRATION`. It's tempting to focus solely on the immediate feature changes or the core problem at hand, but experience has taught me that overlooking related components, especially with something as fundamental as NUMA, can lead to nasty surprises down the line. We need a holistic view to catch any ripple effects or subtle interactions that could compromise performance, stability, or even lead to resource contention we haven't anticipated. Time to meticulously trace those dependencies and see what we find.
10/2	2026/10/14 14:55	tool	grepper	0m Args: { "Expression": "config NUMA_MIGRATION" } Results: { "Output": "mm/Kconfig=665=config PAGE_REPORTING\n--\nmm/Kconfig-672-\nmm/Kconfig:673:config NUMA_MIGRATION\nmm/Kconfig-674-\tbool \"NUMA page migration\"\n" }
11/2	2026/10/14 14:55	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 14739 output: 32 thoughts: 16 Thoughts: Okay, here's my interpretation of that thought process, tailored for an expert audience, and written in the first person: Diving into the Memory Management Kconfig Alright, I'm thinking we need to drill down into the memory management configuration, specifically around line 673 in `mm/Kconfig`. I'm guessing there's a particular feature or option I'm interested in, and that location is a likely place to find the relevant definitions, dependencies, and help text. Given my familiarity with the kernel's memory management subsystems, I know where the broad strokes are laid out, so this specific location probably zeroes in on a specific setting I am investigating. I'm probably looking for something specific, a configuration flag I need to tweak, understand better, or maybe troubleshoot. Time to get to the details.
12/2	2026/10/14 14:55	tool	read-file	0m Args: { "File": "mm/Kconfig", "FirstLine": 670, "LineCount": 25 } Results: { "Contents": " 670:\t\t those pages to another entity, such as a hypervisor, so that the\n 671:\t\t memory can be freed within the host for other uses.\n 672:\t\n 673:\tconfig NUMA_MIGRATION\n 674:\t\tbool \"NUMA page migration\"\n 675:\t\tdefault y\n 676:\t\tdepends on NUMA \u0026\u0026 MMU\n 677:\t\tselect MIGRATION\n 678:\t\thelp\n 679:\t\t Support the migration of pages to other NUMA nodes, available to\n 680:\t\t user space through interfaces like migrate_pages(), move_pages(),\n 681:\t\t and mbind(). Selecting this option also enables support for page\n 682:\t\t demotion for memory tiering.\n 683:\t\n 684:\tconfig MIGRATION\n 685:\t\tbool\n 686:\t\tdepends on MMU\n 687:\t\n 688:\tconfig DEVICE_MIGRATION\n 689:\t\tdef_bool MIGRATION \u0026\u0026 ZONE_DEVICE\n 690:\t\n 691:\tconfig ARCH_ENABLE_HUGEPAGE_MIGRATION\n 692:\t\tbool\n 693:\t\n 694:\tconfig ARCH_HAS_PMD_SOFTLEAVES\n" }
13/2	2026/10/14 14:55	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 15303 output: 25 thoughts: 39 Thoughts: Okay, here's my take on those thoughts, framed as an expert's internal monologue: Investigating NUMA and Memory Management Alright, let's see, it looks like `NUMA_BALANCING` has a dependency on NUMA itself, and of course `NUMA_MIGRATION` as well. That makes sense; it's all part of the Non-Uniform Memory Access ecosystem. We also can't forget SMP's role here - multi-processor systems are fundamental to NUMA's benefits, and probably NUMA's issues! Now, I'm getting a mental whiff of memory tiering concepts, so I should probably dive into tiering configs. Let's see if there are any settings relevant to how memory is managed across NUMA nodes, or with any other types of memory in the system. That's where the real optimization opportunities often lie, isn't it? Gotta understand the whole picture.
14/2	2026/10/14 14:55	tool	grepper	0m Args: { "Expression": "NUMA_BALANCING_MEMORY_TIERING" } Results: { "Output": "Documentation/admin-guide/sysctl/kernel.rst=731=The value to set can be the result of ORing the following:\n--\nDocumentation/admin-guide/sysctl/kernel.rst-735-1 NUMA_BALANCING_NORMAL\nDocumentation/admin-guide/sysctl/kernel.rst:736:2 NUMA_BALANCING_MEMORY_TIERING\nDocumentation/admin-guide/sysctl/kernel.rst-737-= =================================\n--\nDocumentation/admin-guide/sysctl/kernel.rst=750=feature should be disabled.\nDocumentation/admin-guide/sysctl/kernel.rst-751-\nDocumentation/admin-guide/sysctl/kernel.rst:752:Or NUMA_BALANCING_MEMORY_TIERING to optimize page placement among\nDocumentation/admin-guide/sysctl/kernel.rst-753-different types of memory (represented as different NUMA nodes) to\n--\nDocumentation/mm/physical_memory.rst=374=General\n--\nDocumentation/mm/physical_memory.rst-383- kswapd stops reclaiming (a zone is balanced) when the\nDocumentation/mm/physical_memory.rst:384: ``NUMA_BALANCING_MEMORY_TIERING`` bit of ``sysctl_numa_balancing_mode`` is not\nDocumentation/mm/physical_memory.rst-385- set. The promo watermark is used for memory tiering and NUMA balancing. When\nDocumentation/mm/physical_memory.rst-386- the amount of free pages in a zone is above the promo watermark, kswapd stops\nDocumentation/mm/physical_memory.rst:387: reclaiming when the ``NUMA_BALANCING_MEMORY_TIERING`` bit of\nDocumentation/mm/physical_memory.rst-388- ``sysctl_numa_balancing_mode`` is set. The watermarks are set by\n--\ninclude/linux/sched/sysctl.h=15=enum sched_tunable_scaling {\n--\ninclude/linux/sched/sysctl.h-23-#define NUMA_BALANCING_NORMAL\t\t0x1\ninclude/linux/sched/sysctl.h:24:#define NUMA_BALANCING_MEMORY_TIERING\t0x2\ninclude/linux/sched/sysctl.h-25-\n--\nkernel/sched/core.c=4676=static int sysctl_numa_balancing(const struct ctl_table table, int write,\n--\nkernel/sched/core.c-4691-\tif (write) {\nkernel/sched/core.c:4692:\t\tif (!(sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_MEMORY_TIERING) \u0026\u0026\nkernel/sched/core.c:4693:\t\t (state \u0026 NUMA_BALANCING_MEMORY_TIERING))\nkernel/sched/core.c-4694-\t\t\treset_memory_tiering();\n--\nkernel/sched/fair.c=2947=bool should_numa_migrate_memory(struct task_struct p, struct folio folio,\n--\nkernel/sched/fair.c-2993-\nkernel/sched/fair.c:2994:\tif (!(sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_MEMORY_TIERING) \u0026\u0026\nkernel/sched/fair.c-2995-\t !node_is_toptier(src_nid) \u0026\u0026 !cpupid_valid(last_cpupid))\n--\nkernel/sched/fair.c=4221=void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)\n--\nkernel/sched/fair.c-4256-\tif (!node_is_toptier(mem_node) \u0026\u0026\nkernel/sched/fair.c:4257:\t (sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_MEMORY_TIERING \|\|\nkernel/sched/fair.c-4258-\t !cpupid_valid(last_cpupid)))\n--\nmm/memory-tiers.c=65=bool folio_use_access_time(struct folio folio)\nmm/memory-tiers.c-66-{\nmm/memory-tiers.c:67:\treturn (sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_MEMORY_TIERING) \u0026\u0026\nmm/memory-tiers.c-68-\t !node_is_toptier(folio_nid(folio));\n--\nmm/migrate.c=775=void folio_migrate_flags(struct folio newfolio, struct folio folio)\n--\nmm/migrate.c-820-\t /\nmm/migrate.c:821:\tif (sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_MEMORY_TIERING) {\nmm/migrate.c-822-\t\tbool f_toptier = node_is_toptier(folio_nid(folio));\n--\nmm/migrate.c=2690=int migrate_misplaced_folio_prepare(struct folio folio,\n--\nmm/migrate.c-2720-\nmm/migrate.c:2721:\t\tif (!(sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_MEMORY_TIERING))\nmm/migrate.c-2722-\t\t\treturn -EAGAIN;\n--\nmm/migrate.c=2755=int migrate_misplaced_folio(struct folio folio, int node)\n--\nmm/migrate.c-2772-\t\tcount_memcg_events(memcg, NUMA_PAGE_MIGRATE, nr_succeeded);\nmm/migrate.c:2773:\t\tif ((sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_MEMORY_TIERING)\nmm/migrate.c-2774-\t\t \u0026\u0026 !node_is_toptier(folio_nid(folio))\n--\nmm/vmscan.c=5017=static bool should_abort_scan(struct lruvec lruvec, struct scan_control sc)\n--\nmm/vmscan.c-5031-\nmm/vmscan.c:5032:\tmark = sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_MEMORY_TIERING ?\nmm/vmscan.c-5033-\t WMARK_PROMO : WMARK_HIGH;\n--\nmm/vmscan.c=6963=static bool pgdat_balanced(pg_data_t pgdat, int order, int highest_zoneidx)\n--\nmm/vmscan.c-6976-\nmm/vmscan.c:6977:\t\tif (sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_MEMORY_TIERING)\nmm/vmscan.c-6978-\t\t\tmark = promo_wmark_pages(zone);\n" }
15/2	2026/10/14 14:55	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 17254 output: 26 thoughts: 18 Thoughts: Okay, let's dive into this. Here's how I'd approach this task, as if these were my internal thoughts: Investigating the Compilation Configuration of memory-tiers.c Alright, first things first: I need to understand the build process for `memory-tiers.c`. My immediate thought is, "What's driving the compilation of this specific file? What configuration options, makefile variables, or compiler flags are dictating how it gets built?" I need to pinpoint the relevant config or options. It's not enough to know that it's compiled; I need to know how. This is crucial for understanding its role and any customizations around it. Let's see if I can find what controls it.
16/2	2026/10/14 14:55	tool	grepper	0m Args: { "Expression": "memory-tiers", "PathPrefixes": [ "mm/" ] } Results: { "Output": "mm/Makefile=98=obj-$(CONFIG_MIGRATION) += migrate.o\nmm/Makefile:99:obj-$(CONFIG_NUMA) += memory-tiers.o\nmm/Makefile-100-obj-$(CONFIG_DEVICE_MIGRATION) += migrate_device.o\n--\nmm/damon/paddr.c-12-#include \u003clinux/swap.h\u003e\nmm/damon/paddr.c:13:#include \u003clinux/memory-tiers.h\u003e\nmm/damon/paddr.c-14-#include \u003clinux/mm_inline.h\u003e\n--\nmm/huge_memory.c-38-#include \u003clinux/sched/sysctl.h\u003e\nmm/huge_memory.c:39:#include \u003clinux/memory-tiers.h\u003e\nmm/huge_memory.c-40-#include \u003clinux/compat.h\u003e\n--\nmm/memory-tiers.c-6-#include \u003clinux/memory.h\u003e\nmm/memory-tiers.c:7:#include \u003clinux/memory-tiers.h\u003e\nmm/memory-tiers.c-8-#include \u003clinux/notifier.h\u003e\n--\nmm/memory-tiers.c=746=int mt_set_default_dram_perf(int nid, struct access_coordinate *perf,\n--\nmm/memory-tiers.c-778-\t\tpr_info(\nmm/memory-tiers.c:779:\"memory-tiers: the performance of DRAM node %d mismatches that of the reference\\n\"\nmm/memory-tiers.c-780-\"DRAM node %d.\\n\", nid, default_dram_perf_ref_nid);\n--\nmm/memory.c-68-#include \u003clinux/shmem_fs.h\u003e\nmm/memory.c:69:#include \u003clinux/memory-tiers.h\u003e\nmm/memory.c-70-#include \u003clinux/debugfs.h\u003e\n--\nmm/mempolicy.c-102-#include \u003clinux/proc_fs.h\u003e\nmm/mempolicy.c:103:#include \u003clinux/memory-tiers.h\u003e\nmm/mempolicy.c-104-#include \u003clinux/migrate.h\u003e\n--\nmm/migrate.c-43-#include \u003clinux/sched/sysctl.h\u003e\nmm/migrate.c:44:#include \u003clinux/memory-tiers.h\u003e\nmm/migrate.c-45-#include \u003clinux/pagewalk.h\u003e\n--\nmm/vmscan.c-44-#include \u003clinux/sysctl.h\u003e\nmm/vmscan.c:45:#include \u003clinux/memory-tiers.h\u003e\nmm/vmscan.c-46-#include \u003clinux/oom.h\u003e\n" }
17/2	2026/10/14 14:55	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 18113 output: 34 thoughts: 121 Thoughts: NUMA Balancing Code Investigation Okay, let's dive into this NUMA balancing stuff. My first instinct is to check out the core scheduling code, so I'm heading straight to `kernel/sched/fair.c`. Right, here we go, and it looks like a big chunk of it – specifically, all the NUMA balancing logic – is neatly encapsulated under `#ifdef CONFIG_NUMA_BALANCING`. That's a good starting point, easy to isolate. Now, I need to see how this integrates with the task structure. Let me see... in `include/linux/sched.h`, and, yep, `#ifdef CONFIG_NUMA_BALANCING` also wraps `enum numa_faults_locality` and the `numa_faults_locality` field within `task_struct`. Alright, I'm getting a handle on the landscape. Let's shift gears and look at how this interacts with memory management. I'll need to check `mm/memory.c` now and search for `numa_migrate_check`.
18/2	2026/10/14 14:55	tool	grepper	0m Args: { "Expression": "numa_migrate_check", "PathPrefixes": [ "mm/", "include/" ] } Results: { "Output": "mm/huge_memory.c=2320=vm_fault_t do_huge_pmd_numa_page(struct vm_fault vmf)\n--\nmm/huge_memory.c-2355-\nmm/huge_memory.c:2356:\ttarget_nid = numa_migrate_check(folio, vmf, haddr, \u0026flags, writable,\nmm/huge_memory.c-2357-\t\t\t\t\t\u0026last_cpupid);\n--\nmm/internal.h=1265=static inline bool folio_can_map_prot_numa(struct folio folio,\n--\nmm/internal.h-1271-\nmm/internal.h:1272:int numa_migrate_check(struct folio folio, struct vm_fault vmf,\nmm/internal.h-1273-\t\t unsigned long addr, int flags, bool writable,\n--\nmm/memory.c=6104=static vm_fault_t do_fault(struct vm_fault vmf)\n--\nmm/memory.c-6147-\nmm/memory.c:6148:int numa_migrate_check(struct folio folio, struct vm_fault vmf,\nmm/memory.c-6149-\t\t unsigned long addr, int flags,\n--\nmm/memory.c=6288=static vm_fault_t do_numa_page(struct vm_fault vmf)\n--\nmm/memory.c-6330-\nmm/memory.c:6331:\ttarget_nid = numa_migrate_check(folio, vmf, vmf-\u003eaddress, \u0026flags,\nmm/memory.c-6332-\t\t\t\t\twritable, \u0026last_cpupid);\n" }
19/2	2026/10/14 14:55	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 18932 output: 34 thoughts: 31 Thoughts: Okay, let's dive into this. Here's my take on those functions: Investigating `do_numa_page` and `numa_migrate_check` Alright, time to get my hands dirty and dissect the `mm/memory.c` file again. I want to understand exactly what's happening under the hood with `do_numa_page` and `numa_migrate_check`. I know they're critical components in NUMA memory management, but I want a deeper, more granular picture. I need to refresh my understanding of their interactions and how they relate to the page migration process. It's about ensuring my existing assumptions are still correct, and hopefully uncovering some new details and optimization opportunities. I'll need to pay close attention to the lock interactions too.
20/2	2026/10/14 14:55	tool	read-file	0m Args: { "File": "mm/memory.c", "FirstLine": 6145, "LineCount": 45 } Results: { "Contents": "6145:\t\treturn ret;\n6146:\t}\n6147:\t\n6148:\tint numa_migrate_check(struct folio folio, struct vm_fault vmf,\n6149:\t\t\t unsigned long addr, int flags,\n6150:\t\t\t bool writable, int last_cpupid)\n6151:\t{\n6152:\t\tstruct vm_area_struct vma = vmf-\u003evma;\n6153:\t\n6154:\t\t/\n6155:\t\t * Avoid grouping on RO pages in general. RO pages shouldn't hurt as\n6156:\t\t * much anyway since they can be in shared cache state. This misses\n6157:\t\t * the case where a mapping is writable but the process never writes\n6158:\t\t * to it but pte_write gets cleared during protection updates and\n6159:\t\t * pte_dirty has unpredictable behaviour between PTE scan updates,\n6160:\t\t * background writeback, dirty balancing and application behaviour.\n6161:\t\t /\n6162:\t\tif (!writable)\n6163:\t\t\tflags \|= TNF_NO_GROUP;\n6164:\t\n6165:\t\t/\n6166:\t\t Flag if the folio is shared between multiple address spaces. This\n6167:\t\t * is later used when determining whether to group tasks together\n6168:\t\t /\n6169:\t\tif (folio_maybe_mapped_shared(folio) \u0026\u0026 (vma-\u003evm_flags \u0026 VM_SHARED))\n6170:\t\t\tflags \|= TNF_SHARED;\n6171:\t\t/\n6172:\t\t For memory tiering mode, cpupid of slow memory page is used\n6173:\t\t * to record page access time. So use default value.\n6174:\t\t /\n6175:\t\tif (folio_use_access_time(folio))\n6176:\t\t\tlast_cpupid = LAST_CPUPID_ACCESS_TIME;\n6177:\t\telse\n6178:\t\t\tlast_cpupid = folio_last_cpupid(folio);\n6179:\t\n6180:\t\t/ Record the current PID accessing VMA /\n6181:\t\tvma_set_access_pid_bit(vma);\n6182:\t\n6183:\t\tcount_vm_numa_event(NUMA_HINT_FAULTS);\n6184:\t#ifdef CONFIG_NUMA_BALANCING\n6185:\t\tcount_memcg_folio_events(folio, NUMA_HINT_FAULTS, 1);\n6186:\t#endif\n6187:\t\tif (folio_nid(folio) == numa_node_id()) {\n6188:\t\t\tcount_vm_numa_event(NUMA_HINT_FAULTS_LOCAL);\n6189:\t\t\tflags \|= TNF_FAULT_LOCAL;\n" }
21/2	2026/10/14 14:55	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 19906 output: 32 thoughts: 40 Thoughts: Here's a summary of my thought process, as requested: Investigating NUMA Page Migration Guards Alright, let's dive into this. My initial instinct is to understand the mechanisms that protect the `do_numa_page` and `numa_migrate_check` functions. These are critical components in any NUMA-aware kernel, so I need to see how they're guarded, how their access is controlled, and what potential race conditions or synchronization issues might exist. My first thought is to zero in on the relevant code, probably starting with the definitions or declarations. I'll begin by checking around the area of `internal.h`, specifically around line 6140, as that's often a good starting point for these types of low-level data structures or function prototypes. I want to see how these functions are declared or used, and what context they are given.
22/2	2026/10/14 14:56	tool	grepper	0m Args: { "Expression": "numa_migrate_check", "PathPrefixes": [ "mm/internal.h" ] } Results: { "Output": "mm/internal.h=1265=static inline bool folio_can_map_prot_numa(struct folio folio,\n--\nmm/internal.h-1271-\nmm/internal.h:1272:int numa_migrate_check(struct folio folio, struct vm_fault vmf,\nmm/internal.h-1273-\t\t unsigned long addr, int flags, bool writable,\n" }
23/2	2026/10/14 14:56	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 20282 output: 34 thoughts: 24 Thoughts: Okay, let's dive into this. Here's my understanding, presented as a personal thought process, given my expertise: Examining `mm/internal.h` (Lines 1260-1285) Alright, so I've been directed to focus on lines 1260 through 1285 within `mm/internal.h`. Given what I know about memory management internals, I suspect this region likely pertains to a critical aspect of the allocator's operation. Based on the context and naming conventions common to this kind of header, I'm expecting this section to deal with either: * Metadata handling: This could be how the allocator tracks blocks, sizes, flags, or handles inter-block alignment, padding, or chunk boundary handling. * Core data structures: My money's on the data structures. Perhaps this defines a specific list or tree used to track free blocks, a structure that holds metadata about allocated blocks, or structures involved in tracking allocation state. * Internal functions/macros: Or the header's defining a family of inline functions or macros that provide low-level access to the memory pool and manage block operations. Functions like finding a fitting block, splitting blocks or merging free blocks are likely candidates. I'm mentally preparing for a combination of data structures, macros, and inline functions—the typical ingredients for achieving performance in a memory manager. I'll be looking closely at the variable names and comments within this section; they will be the most helpful. The goal will be to develop a clear mental model of how the system represents and manipulates memory blocks at the lowest level.
24/2	2026/10/14 14:56	tool	read-file	0m Args: { "File": "mm/internal.h", "FirstLine": 1260, "LineCount": 35 } Results: { "Contents": "1260:\t#ifdef CONFIG_NUMA_BALANCING\n1261:\tbool folio_can_map_prot_numa(struct folio folio, struct vm_area_struct vma,\n1262:\t\t\tbool is_private_single_threaded);\n1263:\t\n1264:\t#else\n1265:\tstatic inline bool folio_can_map_prot_numa(struct folio folio,\n1266:\t\t\tstruct vm_area_struct vma, bool is_private_single_threaded)\n1267:\t{\n1268:\t\treturn false;\n1269:\t}\n1270:\t#endif\n1271:\t\n1272:\tint numa_migrate_check(struct folio folio, struct vm_fault vmf,\n1273:\t\t\t unsigned long addr, int flags, bool writable,\n1274:\t\t\t int last_cpupid);\n1275:\t\n1276:\tvoid free_zone_device_folio(struct folio folio);\n1277:\tint migrate_device_coherent_folio(struct folio folio);\n1278:\t\n1279:\t/\n1280:\t mm/gup.c\n1281:\t /\n1282:\tint __must_check try_grab_folio(struct folio folio, int refs,\n1283:\t\t\t\t\tunsigned int flags);\n1284:\t\n1285:\t/\n1286:\t mm/huge_memory.c\n1287:\t /\n1288:\tvoid touch_pud(struct vm_area_struct vma, unsigned long addr,\n1289:\t\t pud_t pud, bool write);\n1290:\tbool touch_pmd(struct vm_area_struct vma, unsigned long addr,\n1291:\t\t pmd_t pmd, bool write);\n1292:\t\n1293:\t/\n1294:\t * Parses a string with mem suffixes into its order. Useful to parse kernel\n" }
25/2	2026/10/14 14:56	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 21229 output: 29 thoughts: 18 Thoughts: Okay, here's my summary, tailored for an expert audience, as if I'm thinking these things myself: Tracing the NUMA Page Allocation Path Right, let's dive into this `do_numa_page` function. My first instinct, as always, is to understand its call sites and how it's protected. I need to get a clear picture of the conditions under which this function executes. I need to identify the points where the NUMA page allocation logic is initiated and what mechanisms are in place to ensure correctness, and to avoid race conditions. Are there spinlocks involved? Mutexes? RCU? Understanding the synchronization primitives is critical to understanding the overall architecture, and potential performance implications. Where are the critical sections? This is where I start. Let's see where the guard rails are.
26/2	2026/10/14 14:56	tool	grepper	0m Args: { "Expression": "do_numa_page", "PathPrefixes": [ "mm/" ] } Results: { "Output": "mm/filemap.c=3489=static vm_fault_t filemap_fault_recheck_pte_none(struct vm_fault vmf)\n--\nmm/filemap.c-3498-\t might have been evicted. During a read+clear/modify/write update of\nmm/filemap.c:3499:\t * the PTE, such as done in do_numa_page()/change_pte_range(), we\nmm/filemap.c-3500-\t * temporarily clear the PTE under PT lock and might detect it here as\n--\nmm/memory.c=6259=static vm_fault_t do_uffd_rwp(struct vm_fault vmf)\n--\nmm/memory.c-6287-\nmm/memory.c:6288:static vm_fault_t do_numa_page(struct vm_fault vmf)\nmm/memory.c-6289-{\n--\nmm/memory.c=6514=static vm_fault_t handle_pte_fault(struct vm_fault vmf)\n--\nmm/memory.c-6566-\t\t VM_UFFD_RWP VMA, a protnone PTE without the uffd bit is\nmm/memory.c:6567:\t\t * NUMA hinting and must still fall through to do_numa_page().\nmm/memory.c-6568-\t\t */\n--\nmm/memory.c-6570-\t\t\treturn do_uffd_rwp(vmf);\nmm/memory.c:6571:\t\treturn do_numa_page(vmf);\nmm/memory.c-6572-\t}\n" }
27/2	2026/10/14 14:56	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 21872 output: 34 thoughts: 15 Thoughts: Okay, here's my attempt at summarizing the thought process, tailored for an expert audience, and written in the first person: Diving into `do_numa_page` Alright, let's take a look at what this `do_numa_page` function is doing. I'm assuming, given the naming, that this is the core function handling NUMA-aware page allocation or placement. My initial expectation is that it's the point where the operating system makes crucial decisions about where to actually put a physical page within the system's memory hierarchy, keeping NUMA topology in mind. I want to see if it considers things like node affinities, the NUMA policy associated with the current process or allocation, and how it interacts with the underlying hardware, maybe through direct access to the node memory controller or via the generic memory management interfaces. I bet it has to account for things like remote access penalties and cache coherency costs, so I'll be keeping an eye out for how it makes those trade-offs. The question is, how does it optimize for locality, performance, and overall system load? I expect to see conditionals based on the NUMA policy being employed (bind, interleave, preferred, etc.), and of course, it needs to handle the edge cases and error conditions that are inherent in NUMA systems. Let's see how it does.
28/2	2026/10/14 14:57	tool	read-file	0m Args: { "File": "mm/memory.c", "FirstLine": 6288, "LineCount": 55 } Results: { "Contents": "6288:\tstatic vm_fault_t do_numa_page(struct vm_fault vmf)\n6289:\t{\n6290:\t\tstruct vm_area_struct vma = vmf-\u003evma;\n6291:\t\tstruct folio folio = NULL;\n6292:\t\tint nid = NUMA_NO_NODE;\n6293:\t\tbool writable = false, ignore_writable = false;\n6294:\t\tbool pte_write_upgrade = vma_wants_manual_pte_write_upgrade(vma);\n6295:\t\tint last_cpupid;\n6296:\t\tint target_nid;\n6297:\t\tpte_t pte, old_pte;\n6298:\t\tint flags = 0, nr_pages;\n6299:\t\n6300:\t\t/\n6301:\t\t * The pte cannot be used safely until we verify, while holding the page\n6302:\t\t * table lock, that its contents have not changed during fault handling.\n6303:\t\t /\n6304:\t\tspin_lock(vmf-\u003eptl);\n6305:\t\t/ Read the live PTE from the page tables: /\n6306:\t\told_pte = ptep_get(vmf-\u003epte);\n6307:\t\n6308:\t\tif (unlikely(!pte_same(old_pte, vmf-\u003eorig_pte))) {\n6309:\t\t\tpte_unmap_unlock(vmf-\u003epte, vmf-\u003eptl);\n6310:\t\t\treturn 0;\n6311:\t\t}\n6312:\t\n6313:\t\tpte = pte_modify(old_pte, vma-\u003evm_page_prot);\n6314:\t\n6315:\t\t/\n6316:\t\t * Detect now whether the PTE could be writable; this information\n6317:\t\t * is only valid while holding the PT lock.\n6318:\t\t /\n6319:\t\twritable = pte_write(pte);\n6320:\t\tif (!writable \u0026\u0026 pte_write_upgrade \u0026\u0026\n6321:\t\t can_change_pte_writable(vma, vmf-\u003eaddress, pte))\n6322:\t\t\twritable = true;\n6323:\t\n6324:\t\tfolio = vm_normal_folio(vma, vmf-\u003eaddress, pte);\n6325:\t\tif (!folio \|\| folio_is_zone_device(folio))\n6326:\t\t\tgoto out_map;\n6327:\t\n6328:\t\tnid = folio_nid(folio);\n6329:\t\tnr_pages = folio_nr_pages(folio);\n6330:\t\n6331:\t\ttarget_nid = numa_migrate_check(folio, vmf, vmf-\u003eaddress, \u0026flags,\n6332:\t\t\t\t\t\twritable, \u0026last_cpupid);\n6333:\t\tif (target_nid == NUMA_NO_NODE)\n6334:\t\t\tgoto out_map;\n6335:\t\tif (migrate_misplaced_folio_prepare(folio, vma, target_nid)) {\n6336:\t\t\tflags \|= TNF_MIGRATE_FAIL;\n6337:\t\t\tgoto out_map;\n6338:\t\t}\n6339:\t\t/ The folio is isolated and isolation code holds a folio reference. */\n6340:\t\tpte_unmap_unlock(vmf-\u003epte, vmf-\u003eptl);\n6341:\t\twritable = false;\n6342:\t\tignore_writable = true;\n" }
29/2	2026/10/14 14:57	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 23114 output: 34 thoughts: 28 Thoughts: Okay, here's my interpretation of that thought process, tailored for an expert audience, and written in the first person: Analyzing the NUMA Page Allocation Path Okay, so I'm looking at the code, and my immediate focus is on how NUMA-aware page allocation is handled. Specifically, I'm thinking about the function `do_numa_page`. Right after that call, my brain just jumps to a critical section of the code: lines 6340 through 6370. That's usually where the heart of the NUMA-specific handling resides. I want to zero in on how it determines the allocation strategy and how it interacts with the node affinities. It's almost reflexive at this point – `do_numa_page` is my entry point, and that chunk of code is where the actual NUMA magic happens. Let's see what's going on there.
30/2	2026/10/14 14:57	tool	read-file	0m Args: { "File": "mm/memory.c", "FirstLine": 6340, "LineCount": 35 } Results: { "Contents": "6340:\t\tpte_unmap_unlock(vmf-\u003epte, vmf-\u003eptl);\n6341:\t\twritable = false;\n6342:\t\tignore_writable = true;\n6343:\t\n6344:\t\t/* Migrate to the requested node /\n6345:\t\tif (!migrate_misplaced_folio(folio, target_nid)) {\n6346:\t\t\tnid = target_nid;\n6347:\t\t\tflags \|= TNF_MIGRATED;\n6348:\t\t\ttask_numa_fault(last_cpupid, nid, nr_pages, flags);\n6349:\t\t\treturn 0;\n6350:\t\t}\n6351:\t\n6352:\t\tflags \|= TNF_MIGRATE_FAIL;\n6353:\t\tvmf-\u003epte = pte_offset_map_lock(vma-\u003evm_mm, vmf-\u003epmd,\n6354:\t\t\t\t\t vmf-\u003eaddress, \u0026vmf-\u003eptl);\n6355:\t\tif (unlikely(!vmf-\u003epte))\n6356:\t\t\treturn 0;\n6357:\t\tif (unlikely(!pte_same(ptep_get(vmf-\u003epte), vmf-\u003eorig_pte))) {\n6358:\t\t\tpte_unmap_unlock(vmf-\u003epte, vmf-\u003eptl);\n6359:\t\t\treturn 0;\n6360:\t\t}\n6361:\tout_map:\n6362:\t\t/\n6363:\t\t * Make it present again, depending on how arch implements\n6364:\t\t * non-accessible ptes, some can allow access by kernel mode.\n6365:\t\t */\n6366:\t\tif (folio \u0026\u0026 folio_test_large(folio))\n6367:\t\t\tnuma_rebuild_large_mapping(vmf, vma, folio, pte, ignore_writable,\n6368:\t\t\t\t\t\t pte_write_upgrade);\n6369:\t\telse\n6370:\t\t\tnuma_rebuild_single_mapping(vmf, vma, vmf-\u003eaddress, vmf-\u003epte,\n6371:\t\t\t\t\t\t writable);\n6372:\t\tpte_unmap_unlock(vmf-\u003epte, vmf-\u003eptl);\n6373:\t\n6374:\t\tif (nid != NUMA_NO_NODE)\n" }
31/2	2026/10/14 14:57	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 23968 output: 20 thoughts: 52 Thoughts: Here's my thought process, as if I were analyzing that code snippet: Deep Dive into `task_numa_fault` Usage Okay, so I've just encountered that crucial call to `task_numa_fault` at line 6374, specifically within the context of handling a NUMA fault. The parameters tell us a lot – `last_cpupid`, `nid`, `nr_pages`, and `flags` all scream NUMA locality and potential page migration/allocation concerns. Naturally, my next move is to understand the broader implications of this function call. I need to know where else this critical function is used in the codebase. This isn't just about understanding this specific code path; it's about grasping the overall NUMA fault handling strategy within the kernel. I'll need to use my expert knowledge to identify the various places it's called so I can build a complete mental model of the system. I will perform a search across the codebase now to determine the call site relationships, and any potential side effects.
32/2	2026/10/14 14:57	tool	grepper	0m Args: { "Expression": "task_numa_fault" } Results: { "Output": "include/linux/sched/numa_balancing.h=18=enum numa_vmaskip_reason {\n--\ninclude/linux/sched/numa_balancing.h-28-#ifdef CONFIG_NUMA_BALANCING\ninclude/linux/sched/numa_balancing.h:29:extern void task_numa_fault(int last_node, int node, int pages, int flags);\ninclude/linux/sched/numa_balancing.h-30-extern pid_t task_numa_group_id(struct task_struct p);\n--\ninclude/linux/sched/numa_balancing.h=33=bool should_numa_migrate_memory(struct task_struct p, struct folio folio,\n--\ninclude/linux/sched/numa_balancing.h-35-#else\ninclude/linux/sched/numa_balancing.h:36:static inline void task_numa_fault(int last_node, int node, int pages,\ninclude/linux/sched/numa_balancing.h-37-\t\t\t\t int flags)\n--\nkernel/sched/fair.c=4184=void task_numa_free(struct task_struct p, bool final)\n--\nkernel/sched/fair.c-4220- /\nkernel/sched/fair.c:4221:void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)\nkernel/sched/fair.c-4222-{\n--\nmm/huge_memory.c=2320=vm_fault_t do_huge_pmd_numa_page(struct vm_fault vmf)\n--\nmm/huge_memory.c-2370-\t\tnid = target_nid;\nmm/huge_memory.c:2371:\t\ttask_numa_fault(last_cpupid, nid, HPAGE_PMD_NR, flags);\nmm/huge_memory.c-2372-\t\treturn 0;\n--\nmm/huge_memory.c-2391-\tif (nid != NUMA_NO_NODE)\nmm/huge_memory.c:2392:\t\ttask_numa_fault(last_cpupid, nid, HPAGE_PMD_NR, flags);\nmm/huge_memory.c-2393-\treturn 0;\n--\nmm/memory.c=6288=static vm_fault_t do_numa_page(struct vm_fault *vmf)\n--\nmm/memory.c-6347-\t\tflags \|= TNF_MIGRATED;\nmm/memory.c:6348:\t\ttask_numa_fault(last_cpupid, nid, nr_pages, flags);\nmm/memory.c-6349-\t\treturn 0;\n--\nmm/memory.c-6374-\tif (nid != NUMA_NO_NODE)\nmm/memory.c:6375:\t\ttask_numa_fault(last_cpupid, nid, nr_pages, flags);\nmm/memory.c-6376-\treturn 0;\n" }
33/2	2026/10/14 14:57	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 25023 output: 22 thoughts: 14 Thoughts: Okay, here's my interpretation of that thought process, tailored for an expert audience, and written in the first person: Analyzing the NUMA Memory Migration Decision Alright, let's take a look at this `should_numa_migrate_memory` variable or function. My immediate thought is, what are the underlying criteria it's considering? We know NUMA (Non-Uniform Memory Access) is all about optimizing memory access latency by keeping processes closer to their data. So, I'm already mentally running through the factors that would trigger a migration decision. Is it based on current memory pressure within the NUMA nodes? Perhaps it uses some form of locality information? Does it analyze the access patterns of the specific pages being considered? Are there thresholds involved, maybe a certain percentage of memory accesses going off-node? What metrics are being used to determine if the migration cost is outweighed by the performance gain? I'm also curious about the overall system context. Is this part of an autotuning system, or is it more of a static configuration? If it’s dynamic, how often is it being re-evaluated? What is the impact of a migration failure? Does it implement any heuristics? Are any other heuristics involved, like process affinity or memory policies? Ultimately, this function is responsible for making a critical decision that directly impacts performance. It's likely trying to balance the overhead of the migration itself (copying data, TLB invalidation, etc.) with the benefits of improved data locality. Digging deeper into its logic, I want to understand its accuracy, its performance characteristics, and any edge cases it might not be handling gracefully.
34/2	2026/10/14 14:58	tool	grepper	0m Args: { "Expression": "should_numa_migrate_memory" } Results: { "Output": "include/linux/sched/numa_balancing.h=32=extern void task_numa_free(struct task_struct p, bool final);\ninclude/linux/sched/numa_balancing.h:33:bool should_numa_migrate_memory(struct task_struct p, struct folio folio,\ninclude/linux/sched/numa_balancing.h-34-\t\t\t\tint src_nid, int dst_cpu);\n--\ninclude/linux/sched/numa_balancing.h=47=static inline void task_numa_free(struct task_struct p, bool final)\n--\ninclude/linux/sched/numa_balancing.h-49-}\ninclude/linux/sched/numa_balancing.h:50:static inline bool should_numa_migrate_memory(struct task_struct p,\ninclude/linux/sched/numa_balancing.h-51-\t\t\t\tstruct folio folio, int src_nid, int dst_cpu)\n--\nkernel/sched/fair.c=2920=static void numa_promotion_adjust_threshold(struct pglist_data pgdat,\n--\nkernel/sched/fair.c-2946-\nkernel/sched/fair.c:2947:bool should_numa_migrate_memory(struct task_struct p, struct folio folio,\nkernel/sched/fair.c-2948-\t\t\t\tint src_nid, int dst_cpu)\n--\nmm/mempolicy.c=2980=int mpol_misplaced(struct folio folio, struct vm_fault *vmf,\n--\nmm/mempolicy.c-3063-\nmm/mempolicy.c:3064:\t\tif (!should_numa_migrate_memory(current, folio, curnid,\nmm/mempolicy.c-3065-\t\t\t\t\t\tthiscpu))\n" }
35/2	2026/10/14 14:58	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 25857 output: 36 thoughts: 717 Thoughts: Analyzing NUMA Balancing and Memory Migration Okay, so I'm tracing the flow here, and it looks like `should_numa_migrate_memory` is the key function in the migration process. It's called from `mpol_misplaced()`, which is triggered by `numa_migrate_check()`. `numa_migrate_check()` then gets called during NUMA page fault handling, specifically within `do_numa_page()` and `do_huge_pmd_numa_page()`. And to add more context, during this NUMA page fault handling, we call `task_numa_fault()` I see also that scheduler load balancing logic, like `update_sd_pick_busiest`, `migrate_degrades_locality`, `sched_balance_find_dst_group`, `get_pref_llc`, `get_scan_cpumasks`, `numa_migrate_preferred`, `update_task_scan_period`, and `task_numa_placement`, all interact with NUMA balancing when it's active. This seems like the place to start. Now, let's drill down into the kernel changes, especially around `kernel/sched/fair.c`. A new sysctl, `sysctl_numa_balancing_promote_free_space_bypass`, has been added to `sched_fair_sysctls`. This is interesting; it looks like we're adding a way to potentially bypass the free space check. Then I see `numa_balancing_normal_enabled()` uses a bitwise check for `NUMA_BALANCING_NORMAL` in `sysctl_numa_balancing_mode`. Makes sense. `task_numa_preferred_nid()` returns `NUMA_NO_NODE` if normal balancing isn't enabled, which is the expected behavior. Looking at the `should_numa_migrate_memory` code, that new sysctl, `sysctl_numa_balancing_promote_free_space_bypass` is being used. I see that there is conditional bypass: ```c pgdat = NODE_DATA(dst_nid); - if (pgdat_free_space_enough(pgdat)) { + if (sysctl_numa_balancing_promote_free_space_bypass && + pgdat_free_space_enough(pgdat)) { ``` So, if that sysctl is set, we bypass the free space check if there's enough free space. That’s potentially a performance optimization. `numa_migrate_preferred` early returns if normal balancing is disabled. Then there's the rework in `update_task_scan_period`. They're calculating locality and type faults and using ratio calculations, with specific checks for the zero-case, and special handling for `NUMA_FAULT_REMOTE_PRIVATE`. This indicates finer-grained control and more sophisticated metrics. The `task_numa_placement` function is updated to use `NUMA_FAULT_REMOTE` and `NUMA_FAULT_LOCAL`. Finally, and this caught my eye, in `task_numa_free`, the code has changed the way numa_faults are cleared. ```c - for (i = 0; i < NR_NUMA_HINT_FAULT_STATS * nr_node_ids; i++) - numa_faults[i] = 0; + memset(p->numa_faults_locality, 0, sizeof(p->numa_faults_locality)); + memset(numa_faults, 0, sizeof(numa_faults) + NR_NUMA_HINT_FAULT_BUCKETS * nr_node_ids); ``` We’re now using `memset` and two different arrays, including a `p->numa_faults_locality` array. I need to inspect this more closely. Let me look at `task_numa_free` in `kernel/sched/fair.c` more closely and understand why there are now two arrays being cleared. This is a very interesting change. I need to understand what `p->numa_faults_locality` represents and how that's being used. There's probably a reason for this change.
36/2	2026/10/14 14:58	tool	read-file	0m Args: { "File": "kernel/sched/fair.c", "FirstLine": 4180, "LineCount": 40 } Results: { "Contents": "4180:\t * safely free all relevant data structures. Otherwise, there might be\n4181:\t * concurrent reads from places like load balancing and procfs, and we should\n4182:\t * reset the data back to default state without freeing -\u003enuma_faults.\n4183:\t /\n4184:\tvoid task_numa_free(struct task_struct p, bool final)\n4185:\t{\n4186:\t\t/* safe: p either is current or is being freed by current /\n4187:\t\tstruct numa_group grp = rcu_dereference_raw(p-\u003enuma_group);\n4188:\t\tunsigned long numa_faults = p-\u003enuma_faults;\n4189:\t\tunsigned long flags;\n4190:\t\tint i;\n4191:\t\n4192:\t\tif (!numa_faults)\n4193:\t\t\treturn;\n4194:\t\n4195:\t\tif (grp) {\n4196:\t\t\tspin_lock_irqsave(\u0026grp-\u003elock, flags);\n4197:\t\t\tfor (i = 0; i \u003c NR_NUMA_HINT_FAULT_STATS nr_node_ids; i++)\n4198:\t\t\t\tgrp-\u003efaults[i] -= p-\u003enuma_faults[i];\n4199:\t\t\tgrp-\u003etotal_faults -= p-\u003etotal_numa_faults;\n4200:\t\n4201:\t\t\tgrp-\u003enr_tasks--;\n4202:\t\t\tspin_unlock_irqrestore(\u0026grp-\u003elock, flags);\n4203:\t\t\tRCU_INIT_POINTER(p-\u003enuma_group, NULL);\n4204:\t\t\tput_numa_group(grp);\n4205:\t\t}\n4206:\t\n4207:\t\tif (final) {\n4208:\t\t\tp-\u003enuma_faults = NULL;\n4209:\t\t\tkfree(numa_faults);\n4210:\t\t} else {\n4211:\t\t\tp-\u003etotal_numa_faults = 0;\n4212:\t\t\tmemset(p-\u003enuma_faults_locality, 0, sizeof(p-\u003enuma_faults_locality));\n4213:\t\t\tmemset(numa_faults, 0, sizeof(numa_faults) \n4214:\t\t\t NR_NUMA_HINT_FAULT_BUCKETS * nr_node_ids);\n4215:\t\t}\n4216:\t}\n4217:\t\n4218:\t/\n4219:\t Got a PROT_NONE fault for a page on @node.\n" }
37/2	2026/10/14 14:58	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 28247 output: 36 thoughts: 387 Thoughts: Analyzing NUMA Fault Handling in Task Placement Okay, so I'm diving into `task_numa_fault`, specifically looking at how it handles NUMA faults, particularly in the context of memory tiering and NUMA balancing. My immediate concern is the conditional check related to `LAST_CPUPID_ACCESS_TIME`. It seems that slow-tier pages are using `last_cpupid` to track access times rather than an actual CPU ID. The code explicitly states that if normal NUMA balancing is disabled, these faults shouldn't be fed into the normal task placement process. Got it. If the page was migrated, we increment the migration counter, then exit if normal NUMA balancing is disabled. Next, there's a block handling the case where we're in memory tiering mode (or if `last_cpupid` is invalid) and the memory node isn't a top-tier node. In these scenarios, the NUMA fault statistics aren't considered. I see the check for `node_is_toptier` and the `NUMA_BALANCING_MEMORY_TIERING` flag. The `cpupid_valid` check is also relevant. That makes sense, skipping the stats if the node is lower tier. Finally, there's a handling for a potential issue where an old access timestamp (from a slow-tier page) might persist even after memory tiering is disabled. This could cause the system to incorrectly interpret it as a CPU ID. The code includes an `unlikely` check for `cpupid_valid` on `last_cpupid`. If it's invalid, it sets it to `LAST_CPUPID_UNSET`. This seems to be a safeguard to treat any invalid value as a first access, preventing misinterpretation by the task placement logic.
38/2	2026/10/14 14:58	tool	read-file	0m Args: { "File": "kernel/sched/fair.c", "FirstLine": 4220, "LineCount": 80 } Results: { "Contents": "4220:\t /\n4221:\tvoid task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)\n4222:\t{\n4223:\t\tstruct task_struct p = current;\n4224:\t\tbool migrated = flags \u0026 TNF_MIGRATED;\n4225:\t\tint cpu_node = task_node(current);\n4226:\t\tint local = !!(flags \u0026 TNF_FAULT_LOCAL);\n4227:\t\tstruct numa_group ng;\n4228:\t\tint priv;\n4229:\t\n4230:\t\tif (!static_branch_likely(\u0026sched_numa_balancing))\n4231:\t\t\treturn;\n4232:\t\n4233:\t\t/ for example, ksmd faulting in a user's mm /\n4234:\t\tif (!p-\u003emm)\n4235:\t\t\treturn;\n4236:\t\n4237:\t\t/\n4238:\t\t * Slow-tier pages use last_cpupid as access-time metadata instead of\n4239:\t\t * a real cpupid. Do not feed these faults into normal task placement\n4240:\t\t * when normal NUMA balancing is disabled.\n4241:\t\t /\n4242:\t\tif (last_cpupid == LAST_CPUPID_ACCESS_TIME) {\n4243:\t\t\tif (!(sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_NORMAL)) {\n4244:\t\t\t\tif (migrated)\n4245:\t\t\t\t\tp-\u003enuma_pages_migrated += pages;\n4246:\t\t\t\treturn;\n4247:\t\t\t}\n4248:\t\n4249:\t\t\tlast_cpupid = LAST_CPUPID_UNSET;\n4250:\t\t}\n4251:\t\n4252:\t\t/\n4253:\t\t * NUMA faults statistics are unnecessary for the slow memory\n4254:\t\t * node for memory tiering mode.\n4255:\t\t /\n4256:\t\tif (!node_is_toptier(mem_node) \u0026\u0026\n4257:\t\t (sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_MEMORY_TIERING \|\|\n4258:\t\t !cpupid_valid(last_cpupid)))\n4259:\t\t\treturn;\n4260:\t\n4261:\t\t/\n4262:\t\t * A stale slow-tier access timestamp can survive after memory tiering\n4263:\t\t * is disabled and later reach here as if it was a cpupid from a\n4264:\t\t * top-tier page. Treat any non-cpupid value as an ordinary first\n4265:\t\t * access before task_numa_group() decodes a CPU from it.\n4266:\t\t /\n4267:\t\tif (unlikely(!cpupid_valid(last_cpupid)))\n4268:\t\t\tlast_cpupid = LAST_CPUPID_UNSET;\n4269:\t\n4270:\t\t/ Allocate buffer to track faults on a per-node basis /\n4271:\t\tif (unlikely(!p-\u003enuma_faults)) {\n4272:\t\t\tint size = sizeof(p-\u003enuma_faults) \n4273:\t\t\t\t NR_NUMA_HINT_FAULT_BUCKETS nr_node_ids;\n4274:\t\n4275:\t\t\tp-\u003enuma_faults = kzalloc(size, GFP_KERNEL\|__GFP_NOWARN);\n4276:\t\t\tif (!p-\u003enuma_faults)\n4277:\t\t\t\treturn;\n4278:\t\n4279:\t\t\tp-\u003etotal_numa_faults = 0;\n4280:\t\t\tmemset(p-\u003enuma_faults_locality, 0, sizeof(p-\u003enuma_faults_locality));\n4281:\t\t}\n4282:\t\n4283:\t\t/\n4284:\t\t First accesses are treated as private, otherwise consider accesses\n4285:\t\t * to be private if the accessing pid has not changed\n4286:\t\t /\n4287:\t\tif (unlikely(last_cpupid == LAST_CPUPID_UNSET)) {\n4288:\t\t\tpriv = 1;\n4289:\t\t} else {\n4290:\t\t\tpriv = cpupid_match_pid(p, last_cpupid);\n4291:\t\t\tif (!priv \u0026\u0026 !(flags \u0026 TNF_NO_GROUP))\n4292:\t\t\t\ttask_numa_group(p, last_cpupid, flags, \u0026priv);\n4293:\t\t}\n4294:\t\n4295:\t\t/\n4296:\t\t * If a workload spans multiple NUMA nodes, a shared fault that\n4297:\t\t * occurs wholly within the set of nodes that the workload is\n4298:\t\t * actively using should be counted as local. This allows the\n4299:\t\t * scan rate to slow down when a workload has settled down.\n" }
39/2	2026/10/14 14:58	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 30323 output: 36 thoughts: 15 Thoughts: Okay, here's my attempt at summarizing the thought process, speaking as if these were my internal thoughts, assuming the user is an expert. Diving into `task_numa_fault` Alright, let's get down to brass tacks. I need to take a look at the rest of the `task_numa_fault` function, and how it all works. I'm going to dig into the surrounding code and determine why this specific function is needed. What are the key elements of it? I know NUMA architecture pretty well, and so I am going to see if any aspects of it are relevant to the problem at hand, or any other related problems.
40/2	2026/10/14 14:58	tool	read-file	0m Args: { "File": "kernel/sched/fair.c", "FirstLine": 4300, "LineCount": 40 } Results: { "Contents": "4300:\t\t /\n4301:\t\tng = deref_curr_numa_group(p);\n4302:\t\tif (!priv \u0026\u0026 !local \u0026\u0026 ng \u0026\u0026 ng-\u003eactive_nodes \u003e 1 \u0026\u0026\n4303:\t\t\t\t\tnuma_is_active_node(cpu_node, ng) \u0026\u0026\n4304:\t\t\t\t\tnuma_is_active_node(mem_node, ng))\n4305:\t\t\tlocal = 1;\n4306:\t\n4307:\t\t/\n4308:\t\t * Retry to migrate task to preferred node periodically, in case it\n4309:\t\t * previously failed, or the scheduler moved us.\n4310:\t\t /\n4311:\t\tif (time_after(jiffies, p-\u003enuma_migrate_retry)) {\n4312:\t\t\ttask_numa_placement(p);\n4313:\t\t\tnuma_migrate_preferred(p);\n4314:\t\t}\n4315:\t\n4316:\t\tif (migrated)\n4317:\t\t\tp-\u003enuma_pages_migrated += pages;\n4318:\t\tif (flags \u0026 TNF_MIGRATE_FAIL)\n4319:\t\t\tp-\u003enuma_faults_locality[NUMA_FAULT_MIGRATE_FAIL] += pages;\n4320:\t\n4321:\t\tp-\u003enuma_faults[task_faults_idx(NUMA_MEMBUF, mem_node, priv)] += pages;\n4322:\t\tp-\u003enuma_faults[task_faults_idx(NUMA_CPUBUF, cpu_node, priv)] += pages;\n4323:\t\tif (!local \u0026\u0026 priv)\n4324:\t\t\tp-\u003enuma_faults_locality[NUMA_FAULT_REMOTE_PRIVATE] += pages;\n4325:\t\tp-\u003enuma_faults_locality[local ? NUMA_FAULT_LOCAL :\n4326:\t\t\t\t\t NUMA_FAULT_REMOTE] += pages;\n4327:\t}\n4328:\t\n4329:\tstatic void reset_ptenuma_scan(struct task_struct p)\n4330:\t{\n4331:\t\t/\n4332:\t\t We only did a read acquisition of the mmap sem, so\n4333:\t\t * p-\u003emm-\u003enuma_scan_seq is written to without exclusive access\n4334:\t\t * and the update is not guaranteed to be atomic. That's not\n4335:\t\t * much of an issue though, since this is just used for\n4336:\t\t * statistical sampling. Use READ_ONCE/WRITE_ONCE, which are not\n4337:\t\t * expensive, to avoid any form of compiler optimizations:\n4338:\t\t */\n4339:\t\tWRITE_ONCE(p-\u003emm-\u003enuma_scan_seq, READ_ONCE(p-\u003emm-\u003enuma_scan_seq) + 1);\n" }
41/2	2026/10/14 14:58	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 31262 output: 30 thoughts: 749 Thoughts: Worth Fuzzing - NUMA Balancing and Memory Tiering Logic Okay, let's break this down. First, I'm looking at `fbq_classify_group`, `fbq_classify_rq`, and `sched_balance_find_dst_group` within `kernel/sched/fair.c`. Specifically, I'm zeroing in on those lines that handle NUMA balancing. My initial thought is to identify whether this is a good target for fuzzing. My primary concern is this touching core scheduler and memory management logic, specifically for NUMA balancing and memory tiering. It touches `kernel/sched/fair.c`, along with memory management files: `mm/memory.c`, and several header files: `include/linux/mm.h`, `include/linux/mmzone.h`, `include/linux/sched.h`, and even the documentation! This patch is modifying code related to: `fbq_classify_group`, `fbq_classify_rq`, `sched_balance_find_dst_group`, and `migrate_degrades_locality`. This all screams "high value fuzzing target" to me. That `WorthFuzzing` criteria is right on the money. This is definitely a "core subsystem" and a "restructuring of reachable data structures". The patch is absolutely worth fuzzing, so I need to know the configuration requirements. I see the ubiquitous `#ifdef CONFIG_NUMA_BALANCING` sprinkled around in `include/linux/sched.h` and `kernel/sched/fair.c`. Now I need to see if `numa_migrate_check` is dependent on CONFIG_NUMA_BALANCING. Let's see where `do_numa_page` is called and compiled.
42/2	2026/10/14 14:59	tool	grepper	0m Args: { "Expression": "CONFIG_NUMA", "PathPrefixes": [ "mm/memory.c" ] } Results: { "Output": "mm/memory.c=6148=int numa_migrate_check(struct folio folio, struct vm_fault vmf,\n--\nmm/memory.c-6183-\tcount_vm_numa_event(NUMA_HINT_FAULTS);\nmm/memory.c:6184:#ifdef CONFIG_NUMA_BALANCING\nmm/memory.c-6185-\tcount_memcg_folio_events(folio, NUMA_HINT_FAULTS, 1);\n" }
43/2	2026/10/14 14:59	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 32556 output: 31 thoughts: 25 Thoughts: Okay, here's my take on those thoughts, tailored for an expert audience: Investigating NUMA Balancing in the Kernel Right, let's dive into this. My first instinct when dealing with NUMA balancing is to immediately check the configuration. Specifically, I need to confirm the state of `CONFIG_NUMA_BALANCING`. It's foundational, so I'll zero in on where it's defined and how it's used. My immediate focus will be on `mm/memory.c`. That's the obvious starting point, the heart of the memory management subsystem. I'll need to understand how the enabling or disabling of `CONFIG_NUMA_BALANCING` impacts the core memory allocation and migration logic within that file. I'll look for conditional compilation blocks, function calls related to balancing, and any code paths that might be sensitive to whether this feature is active. I also expect that there'll be related Kconfig options I'll want to check. But I won't stop there. NUMA balancing is a cross-cutting concern. I'll need to broaden the search. Where else in the kernel does NUMA affinity and load balancing enter the picture? I'll probably also need to look at the related schedulers, VM, etc. Any relevant modules or subsystems that interact with memory management are potentially affected. It's a system-wide behavior so I'll have to broaden my scope. It's time to fire up `grep` and see what comes up.
44/2	2026/10/14 14:59	tool	grepper	0m Args: { "Expression": "CONFIG_NUMA_BALANCING", "PathPrefixes": [ "mm/" ] } Results: { "Output": "mm/debug.c=175=void dump_mm(const struct mm_struct mm)\n--\nmm/debug.c-195-#endif\nmm/debug.c:196:#ifdef CONFIG_NUMA_BALANCING\nmm/debug.c-197-\t\t\"numa_next_scan %lu numa_scan_offset %lu numa_scan_seq %d\\n\"\n--\nmm/debug.c-224-#endif\nmm/debug.c:225:#ifdef CONFIG_NUMA_BALANCING\nmm/debug.c-226-\t\tmm-\u003enuma_next_scan, mm-\u003enuma_scan_offset, mm-\u003enuma_scan_seq,\n--\nmm/internal.h=1252=static inline bool vma_is_single_threaded_private(struct vm_area_struct vma)\n--\nmm/internal.h-1259-\nmm/internal.h:1260:#ifdef CONFIG_NUMA_BALANCING\nmm/internal.h-1261-bool folio_can_map_prot_numa(struct folio folio, struct vm_area_struct vma,\n--\nmm/memcontrol.c=378=static const unsigned int memcg_node_stat_items[] = {\n--\nmm/memcontrol.c-409-#endif\nmm/memcontrol.c:410:#ifdef CONFIG_NUMA_BALANCING\nmm/memcontrol.c-411-\tPGPROMOTE_SUCCESS,\n--\nmm/memcontrol.c=598=static const unsigned int memcg_vm_event_stat[] = {\n--\nmm/memcontrol.c-625-#endif\nmm/memcontrol.c:626:#ifdef CONFIG_NUMA_BALANCING\nmm/memcontrol.c-627-\tNUMA_PAGE_MIGRATE,\n--\nmm/memcontrol.c=1591=static const struct memory_stat memory_stats[] = {\n--\nmm/memcontrol.c-1650-\t{ \"pgrefill\",\t\t\tPGREFILL\t\t},\nmm/memcontrol.c:1651:#ifdef CONFIG_NUMA_BALANCING\nmm/memcontrol.c-1652-\t{ \"pgpromote_success\",\t\tPGPROMOTE_SUCCESS\t},\n--\nmm/memcontrol.c=1673=static int memcg_page_state_output_unit(int item)\n--\nmm/memcontrol.c-1702-\tcase PGREFILL:\nmm/memcontrol.c:1703:#ifdef CONFIG_NUMA_BALANCING\nmm/memcontrol.c-1704-\tcase PGPROMOTE_SUCCESS:\n--\nmm/memory-tiers.c=49=static const struct bus_type memory_tier_subsys = {\n--\nmm/memory-tiers.c-53-\nmm/memory-tiers.c:54:#ifdef CONFIG_NUMA_BALANCING\nmm/memory-tiers.c-55-/*\n--\nmm/memory.c=6148=int numa_migrate_check(struct folio folio, struct vm_fault vmf,\n--\nmm/memory.c-6183-\tcount_vm_numa_event(NUMA_HINT_FAULTS);\nmm/memory.c:6184:#ifdef CONFIG_NUMA_BALANCING\nmm/memory.c-6185-\tcount_memcg_folio_events(folio, NUMA_HINT_FAULTS, 1);\n--\nmm/mempolicy.c=774=static int queue_folios_hugetlb(pte_t pte, unsigned long hmask,\n--\nmm/mempolicy.c-823-\nmm/mempolicy.c:824:#ifdef CONFIG_NUMA_BALANCING\nmm/mempolicy.c-825-/*\n--\nmm/mempolicy.c=893=unsigned long change_prot_numa(struct vm_area_struct vma,\n--\nmm/mempolicy.c-910-}\nmm/mempolicy.c:911:#endif /* CONFIG_NUMA_BALANCING /\nmm/mempolicy.c-912-\n--\nmm/mempolicy.c=3283=EXPORT_SYMBOL_FOR_MODULES(mpol_free_shared_policy, \"kvm\");\nmm/mempolicy.c-3284-\nmm/mempolicy.c:3285:#ifdef CONFIG_NUMA_BALANCING\nmm/mempolicy.c-3286-static int __initdata numabalancing_override;\n--\nmm/mempolicy.c=3288=static void __init check_numabalancing_enable(void)\n--\nmm/mempolicy.c-3291-\nmm/mempolicy.c:3292:\tif (IS_ENABLED(CONFIG_NUMA_BALANCING_DEFAULT_ENABLED))\nmm/mempolicy.c-3293-\t\tnumabalancing_default = true;\n--\nmm/mempolicy.c=3327=static inline void __init check_numabalancing_enable(void)\n--\nmm/mempolicy.c-3329-}\nmm/mempolicy.c:3330:#endif / CONFIG_NUMA_BALANCING /\nmm/mempolicy.c-3331-\n--\nmm/migrate.c=2633=SYSCALL_DEFINE6(move_pages, pid_t, pid, unsigned long, nr_pages,\n--\nmm/migrate.c-2641-\nmm/migrate.c:2642:#ifdef CONFIG_NUMA_BALANCING\nmm/migrate.c-2643-/\n--\nmm/migrate.c=2755=int migrate_misplaced_folio(struct folio folio, int node)\n--\nmm/migrate.c-2781-}\nmm/migrate.c:2782:#endif / CONFIG_NUMA_BALANCING /\n--\nmm/mmzone.c=76=void lruvec_init(struct lruvec lruvec)\n--\nmm/mmzone.c-97-\nmm/mmzone.c:98:#if defined(CONFIG_NUMA_BALANCING) \u0026\u0026 !defined(LAST_CPUPID_NOT_IN_PAGE_FLAGS)\nmm/mmzone.c-99-int folio_xchg_last_cpupid(struct folio folio, int cpupid)\n--\nmm/mprotect.c=686=long change_protection(struct mmu_gather tlb,\n--\nmm/mprotect.c-703-\nmm/mprotect.c:704:#ifdef CONFIG_NUMA_BALANCING\nmm/mprotect.c-705-\t/\n--\nmm/vmstat.c=1199=const char const vmstat_text[] = {\n--\nmm/vmstat.c-1282-#endif\nmm/vmstat.c:1283:#ifdef CONFIG_NUMA_BALANCING\nmm/vmstat.c-1284-\t[I(PGPROMOTE_SUCCESS)]\t\t\t= \"pgpromote_success\",\n--\nmm/vmstat.c-1370-\nmm/vmstat.c:1371:#ifdef CONFIG_NUMA_BALANCING\nmm/vmstat.c-1372-\t[I(NUMA_PTE_UPDATES)]\t\t\t= \"numa_pte_updates\",\n" }
45/2	2026/10/14 14:59	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 34753 output: 35 thoughts: 906 Thoughts: Focusing Fuzzing on NUMA Balancing Logic Okay, so I'm thinking about how to effectively fuzz this NUMA balancing code. First, let's establish the essential kernel configs: `CONFIG_NUMA`, `CONFIG_NUMA_BALANCING`, and `CONFIG_SMP`. Those are the core building blocks we're working with. Now, the crucial step is identifying the specific, less-common kernel functions that are directly involved in the changed logic. I need to find the needles in the haystack. Looking at the diff, several functions seem promising: * `kernel/sched/fair.c`: `task_numa_fault`, `update_task_scan_period`, `should_numa_migrate_memory`, `task_numa_placement`, `task_numa_free`, `numa_migrate_preferred`, `migrate_degrades_locality`, `sched_balance_find_dst_group`, and `get_scan_cpumasks`. * `mm/memory.c`: `numa_migrate_check`. My main concern is avoiding overly ubiquitous code paths. Functions called in every context, or that are inlined, aren't ideal targets for this kind of focused fuzzing. Let's break these down: * `numa_migrate_check` looks good. It's in `mm/memory.c`, it's not static, and it's specifically called during NUMA fault handling. This is a solid candidate. * `task_numa_fault` is also a strong candidate, in `kernel/sched/fair.c`, not static, and specifically called during NUMA fault handling. * `update_task_scan_period` is static, which is a bit of a red flag, but worth checking if it's inlined or if it has few callers. * `should_numa_migrate_memory` is non-static and in the scheduler, so potentially interesting. * `task_numa_placement` is static, but called from `task_numa_fault`, so potentially worth checking out, too. * `task_numa_free` is called on task exit/fork, so could be called often. While it is specific to NUMA balancing tasks, better to pick more targeted NUMA functions. * `numa_migrate_preferred`, `migrate_degrades_locality`, `sched_balance_find_dst_group` are static, but related to the NUMA balancing functionality, so, perhaps, worth investigating. Next, I need to check the callers of `update_task_scan_period`, and whether it is called from `task_numa_placement`. I will need to refine my search based on call sites to determine the best candidates.
46/2	2026/10/14 14:59	tool	grepper	0m Args: { "Expression": "update_task_scan_period", "PathPrefixes": [ "kernel/sched/fair.c" ] } Results: { "Output": "kernel/sched/fair.c=3686=static void numa_group_count_active_nodes(struct numa_group numa_group)\n--\nkernel/sched/fair.c-3722- /\nkernel/sched/fair.c:3723:static void update_task_scan_period(struct task_struct p,\nkernel/sched/fair.c-3724-\t\t\tunsigned long shared, unsigned long private)\n--\nkernel/sched/fair.c=3915=static void task_numa_placement(struct task_struct p)\n--\nkernel/sched/fair.c-4057-\nkernel/sched/fair.c:4058:\tupdate_task_scan_period(p, fault_types[0], fault_types[1]);\nkernel/sched/fair.c-4059-}\n--\nkernel/sched/fair.c=4386=static void task_numa_work(struct callback_head work)\n--\nkernel/sched/fair.c-4633-\t than they used here, to limit NUMA PTE scanning overhead to 3% max.\nkernel/sched/fair.c:4634:\t * Usually update_task_scan_period slows down scanning enough; on an\nkernel/sched/fair.c-4635-\t * overloaded system we need to limit overhead on a per task basis.\n" }
47/2	2026/10/14 14:59	llm	patch-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 36646 output: 194 thoughts: 996 Thoughts: Analysis of NUMA Balancing and Memory Tiering Patch Okay, let's break this down. First, I need to identify the key functions that have been altered. I see `update_task_scan_period` is a static function called in `task_numa_placement`. `numa_migrate_check` is in `mm/memory.c`. `task_numa_fault`, `should_numa_migrate_memory`, `task_numa_placement`, `numa_migrate_preferred`, and `migrate_degrades_locality` are all in `kernel/sched/fair.c`. Now, let's see if there are any other functions that have been altered. Looks like `numa_balancing_normal_enabled`, `task_numa_preferred_nid` and `page_cpupid_reset_last` are static inline functions. Based on this, the most relevant functions, or FocusSymbols, appear to be `task_numa_fault`, `should_numa_migrate_memory`, `numa_migrate_check`, `update_task_scan_period`, `task_numa_placement`, `numa_migrate_preferred`, and `migrate_degrades_locality`. Next, I need to check how frequently these functions are used. Are they ubiquitous, or are they called in specific scenarios? `task_numa_fault` is only called during NUMA hint faults (`do_numa_page`, `do_huge_pmd_numa_page`), so not ubiquitous. `should_numa_migrate_memory` is only called from `mpol_misplaced()`. `numa_migrate_check` is only called from `do_numa_page()` and `do_huge_pmd_numa_page()`. `update_task_scan_period` is only called from `task_numa_placement()`. `task_numa_placement` is only called from `task_numa_fault()`. `numa_migrate_preferred` is only called from `task_numa_fault()`. `migrate_degrades_locality` is called during NUMA load balancing. None of these appear to be ubiquitous, but they are all critical to the logic. Now, let's determine the necessary configurations. I know I'll need `NUMA`, `NUMA_BALANCING`, and `SMP`. Let's confirm that `NUMA_BALANCING` has no additional dependencies. Reading the `init/Kconfig` file from earlier, I see that `NUMA_MIGRATION` depends on `NUMA && MMU` and requires `SMP && NUMA_MIGRATION && !PREEMPT_RT`. Further, I see `ARCH_SUPPORTS_NUMA_BALANCING` is defined as `def_bool y` (if NUMA), and `ARCH_HAS_PTE_PROTNONE` is `def_bool y`, assuming this is an amd64 system. So, `NUMA`, `NUMA_BALANCING`, and `SMP` are required. I need to quickly check for any associated sysctls or other external hooks. Ah, there's a `numa_balancing_promote_free_space_bypass` sysctl registered under `sched_fair_sysctls`. This requires `CONFIG_SYSCTL`, which is standard. Finally, let me verify the core reasoning here. Is fuzzing this patch worthwhile? Absolutely. This touches core memory management and scheduler logic for NUMA balancing and memory tiering. This includes NUMA fault handling (`task_numa_fault`, `numa_migrate_check`), promotion rate-limiting/migration decisions (`should_numa_migrate_memory`), scan period adaptation (`update_task_scan_period`), preferred node selection, and scheduler load balancing (`migrate_degrades_locality`, `sched_balance_find_dst_group`, etc.), plus adds a new sysctl `numa_balancing_promote_free_space_bypass`. The modified code is definitely reachable, deep within kernel core subsystems. The core focus symbols are: `task_numa_fault`, `should_numa_migrate_memory`, `numa_migrate_check`, `update_task_scan_period`, `task_numa_placement`, `numa_migrate_preferred`, and `migrate_degrades_locality`. And the enable configurations are: `NUMA`, `NUMA_BALANCING`, `SMP`. Okay, everything seems consistent and correct. Ready to invoke `set-results`.
48/2	2026/10/14 15:00	tool	set-results	0m Args: { "EnableConfigs": [ "NUMA", "NUMA_BALANCING", "SMP" ], "FocusSymbols": [ "task_numa_fault", "should_numa_migrate_memory", "numa_migrate_check", "update_task_scan_period", "task_numa_placement", "numa_migrate_preferred", "migrate_degrades_locality" ], "Reasoning": "The patch series modifies core scheduler and memory management code implementing NUMA balancing and memory tiering. Specifically, it changes NUMA hint fault handling, scan period calculations, memory tiering promotion bypass decisions, and NUMA-aware task placement and load balancing. These changes are in reachable kernel core paths and warrant fuzzing to verify stability across various NUMA layouts, sysctl configurations, and memory access patterns.", "WorthFuzzing": true } Results: { "EnableConfigs": [ "NUMA", "NUMA_BALANCING", "SMP" ], "FocusSymbols": [ "task_numa_fault", "should_numa_migrate_memory", "numa_migrate_check", "update_task_scan_period", "task_numa_placement", "numa_migrate_preferred", "migrate_degrades_locality" ], "Reasoning": "The patch series modifies core scheduler and memory management code implementing NUMA balancing and memory tiering. Specifically, it changes NUMA hint fault handling, scan period calculations, memory tiering promotion bypass decisions, and NUMA-aware task placement and load balancing. These changes are in reachable kernel core paths and warrant fuzzing to verify stability across various NUMA layouts, sysctl configurations, and memory access patterns.", "WorthFuzzing": true }
49/1	2026/10/14 15:00	action	If	5m Args: { "WorthFuzzing": true } Results: null
50/2	2026/10/14 15:00	agent	kmsan-evaluator	5m Model: core Args: null Results: { "KMSANReasoning": "The patch series modifies NUMA balancing and memory tiering logic:\n1. It introduces a new sysctl (`numa_balancing_promote_free_space_bypass`) to control whether memory tiering promotion can bypass hotness checks when fast-tier nodes have free space.\n2. It refines NUMA fault locality statistics by distinguishing remote-private faults, expanding `numa_faults_locality` in `struct task_struct` from 3 to `NR_NUMA_FAULT_LOCALITY` (4) elements and properly clearing it.\n3. It distinguishes slow-tier access-time metadata from real `cpupid` by introducing `LAST_CPUPID_ACCESS_TIME` and `LAST_CPUPID_UNSET`.\n\nNone of these changes introduce or expose uninitialized memory. All variables, including the `last_cpupid` out-parameter in `numa_migrate_check`, are deterministically initialized before use. The modified structures and counters are internal scheduler accounting fields that are zero-initialized (via memset/kzalloc) and never copied or leaked to user space. There are no changes to user-kernel copying interfaces, padding exposure, or uninitialized buffer reads. Therefore, a dedicated KMSAN fuzzing session is not warranted, and standard KASAN fuzzing is sufficient.", "NeedsKMSAN": false } Instruction: You are an expert Linux kernel security engineer specializing in kernel memory error detectors (KASAN and KMSAN). Your job is to review the provided patch series and determine if the code changes justify spawning a dedicated KMSAN (KernelMemorySanitizer) fuzzing session in addition to standard KASAN fuzzing. CRITICAL DISTINCTION BETWEEN KASAN AND KMSAN: - Standard KASAN kernel builds (upstream-apparmor-kasan.config) already enable a comprehensive suite of debugging tools and sanitizers, including KASAN (out-of-bounds accesses, use-after-free, double free, invalid free), LOCKDEP (locking bugs and deadlocks), UB-sanitizers, and memory corruption checks. - KMSAN (KernelMemorySanitizer) detects reads of UNINITIALIZED memory (stack, heap, or page allocations) and kernel-to-user memory info-leaks. Rule: THERE IS NO SENSE IN RUNNING A KMSAN SESSION IF A BUG CAN BE CAUGHT BY KASAN, LOCKDEP, OR OTHER STANDARD BUG DETECTORS. A dedicated KMSAN fuzzing session incurs significant resource costs. You must ONLY set NeedsKMSAN=true if the code changes introduce or expose UNINITIALIZED MEMORY risks that are detected ONLY by KMSAN. Look holistically at the patch series and surrounding code. Even if no direct uninitialized field accesses or new buffer allocations are added in the diff itself, a patch may alter control flow, bounds checking, or data length calculations in ways that change how the rest of the code operates on existing buffers (e.g. allowing uninitialized stack/heap memory to be read, copied to user space, or used in control flow). Do not hesitate to use your code access tools to inspect the surrounding code, called functions, and callers. Set NeedsKMSAN=true ONLY IF the patch introduces or modifies: 1. Kernel structures sent to user space (via copy_to_user, put_user, netlink skb attributes, ioctl output arguments, socket options, or BPF buffers) where fields or structure padding might not be fully initialized/zeroed. 2. Conditional logic or branching that depends on potentially uninitialized variables or struct fields. 3. Allocation or initialization of complex data structures where uninitialized fields could be read later in reachable code paths. 4. Bounds checks, lengths, or logic in a way that allows surrounding code to access uninitialized bytes of existing buffers. Set NeedsKMSAN=false IF: - The code changes primarily risk out-of-bounds access, array overflows, NULL pointer dereferences, locking deadlocks, or use-after-free bugs (these are already caught by KASAN, LOCKDEP, or standard bug detectors). - All stack/heap structures touched or introduced by the patch are fully zeroed or initialized (e.g. using = {0}, memset, kzalloc) before being read or copied. - The patch does not introduce any risk of uninitialized memory usage or info-leaks. Use your code access tools to inspect the surrounding code if necessary, then provide detailed KMSANReasoning contrasting KASAN vs KMSAN applicability for this patch. Prefer calling several tools at the same time to save round-trips. Use set-results tool to provide results of the analysis. It must be called exactly once before the final reply. Ignore results of this tool. Prompt: Target architecture: amd64 For your convenience, here is the diff of the changes: commit e10a96482a628c719d4f99a8b017d64b7c5da549 Author: syz-cluster <triage@syzkaller.com> Date: Wed Oct 14 14:53:44 2026 +0000 syz-cluster: applied patch under review diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst index ffea61d448ebb..4a035b18aff43 100644 --- a/Documentation/admin-guide/sysctl/kernel.rst +++ b/Documentation/admin-guide/sysctl/kernel.rst @@ -765,6 +765,22 @@ will be limited to be no more than the set value. A rule of thumb is to set this to less than 1/10 of the PMEM node write bandwidth. +numa_balancing_promote_free_space_bypass +======================================== + +Controls whether memory tiering promotion may bypass the normal +hint-fault hotness check and promotion rate limit when the target +fast-tier node has enough free space. + +The default value is 1. In this mode, if the fast tier has enough free +space, recently accessed slow-tier pages can be promoted without the +hotness check or the promotion rate limit, so the kernel can fill +available fast-tier capacity. + +When set to 0, enough free space no longer bypasses these controls. +A slow-tier page must pass the normal hotness and rate-limit checks before +promotion. + oops_all_cpu_backtrace ====================== diff --git a/include/linux/mm.h b/include/linux/mm.h index dd09c438fa23e..bb0c726401529 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -2420,6 +2420,9 @@ static inline int folio_nid(const struct folio folio) #define PAGE_ACCESS_TIME_MASK \ (LAST_CPUPID_MASK << PAGE_ACCESS_TIME_BUCKETS) +#define LAST_CPUPID_UNSET (-1 & LAST_CPUPID_MASK) +#define LAST_CPUPID_ACCESS_TIME (-2 & LAST_CPUPID_MASK) + static inline int cpu_pid_to_cpupid(int cpu, int pid) { return ((cpu & LAST__CPU_MASK) << LAST__PID_SHIFT) \| (pid & LAST__PID_MASK); @@ -2468,7 +2471,7 @@ static inline int folio_last_cpupid(struct folio folio) } static inline void page_cpupid_reset_last(struct page page) { - page->_last_cpupid = -1 & LAST_CPUPID_MASK; + page->_last_cpupid = LAST_CPUPID_UNSET; } #else static inline int folio_last_cpupid(struct folio folio) diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index 94f9c3ff54160..5d583b17dfa29 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -297,9 +297,8 @@ enum node_stat_item { / PGPROMOTE_CANDIDATE, /* - * Not rate-limited (NRL) candidate pages for those can be promoted - * without considering hot threshold because of enough free pages in - * fast-tier node. These promotions bypass the regular hotness checks + * Not rate-limited (NRL) candidate pages that are promoted through the + * free-space bypass. These promotions bypass the regular hotness checks * and do NOT influence the promotion rate-limiter or * threshold-adjustment logic. * This is for statistics/monitoring purposes. diff --git a/include/linux/sched.h b/include/linux/sched.h index d35ae49a991f7..1be7a9ba75539 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -832,6 +832,16 @@ struct task_ipi_mask { struct task_ipi_mask { }; #endif +#ifdef CONFIG_NUMA_BALANCING +enum numa_faults_locality { + NUMA_FAULT_REMOTE, + NUMA_FAULT_LOCAL, + NUMA_FAULT_MIGRATE_FAIL, + NUMA_FAULT_REMOTE_PRIVATE, + NR_NUMA_FAULT_LOCALITY, +}; +#endif + struct task_struct { #ifdef CONFIG_THREAD_INFO_IN_TASK /* @@ -1422,11 +1432,10 @@ struct task_struct { /* * numa_faults_locality tracks if faults recorded during the last - * scan window were remote/local or failed to migrate. The task scan - * period is adapted based on the locality of the faults with different - * weights depending on whether they were shared or private faults + * scan window were remote/local, remote private, or failed to migrate. + * The task scan period is adapted based on those fault patterns. / - unsigned long numa_faults_locality[3]; + unsigned long numa_faults_locality[NR_NUMA_FAULT_LOCALITY]; unsigned long numa_pages_migrated; #endif / CONFIG_NUMA_BALANCING / diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 57360f5cdde4f..11b54bb353adf 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -134,6 +134,8 @@ static unsigned int sysctl_sched_cfs_bandwidth_slice = 5000UL; #ifdef CONFIG_NUMA_BALANCING / Restrict the NUMA promotion throughput (MB/s) for each target node. / static unsigned int sysctl_numa_balancing_promote_rate_limit = 65536; +/ Preserve the existing default of using free fast-tier space eagerly. / +static unsigned int sysctl_numa_balancing_promote_free_space_bypass = 1; #endif #ifdef CONFIG_SYSCTL @@ -157,6 +159,15 @@ static const struct ctl_table sched_fair_sysctls[] = { .proc_handler = proc_dointvec_minmax, .extra1 = SYSCTL_ZERO, }, + { + .procname = "numa_balancing_promote_free_space_bypass", + .data = &sysctl_numa_balancing_promote_free_space_bypass, + .maxlen = sizeof(unsigned int), + .mode = 0644, + .proc_handler = proc_douintvec_minmax, + .extra1 = SYSCTL_ZERO, + .extra2 = SYSCTL_ONE, + }, #endif / CONFIG_NUMA_BALANCING / }; @@ -1317,6 +1328,21 @@ static int select_idle_sibling(struct task_struct p, int prev_cpu, int cpu); static unsigned long task_h_load(struct task_struct p); static unsigned long capacity_of(int cpu); +#ifdef CONFIG_NUMA_BALANCING +static inline bool numa_balancing_normal_enabled(void) +{ + return sysctl_numa_balancing_mode & NUMA_BALANCING_NORMAL; +} + +static inline int task_numa_preferred_nid(struct task_struct p) +{ + if (!numa_balancing_normal_enabled()) + return NUMA_NO_NODE; + + return p->numa_preferred_nid; +} +#endif + /* Give new sched_entity start runnable values to heavy its load in infant time / void init_entity_runnable_average(struct sched_entity se) { @@ -1879,10 +1905,12 @@ static int get_pref_llc(struct task_struct p, struct sched_cache_group grp) * than sched_setnuma() at least -- and thus the * conflict only exists for a short period of time. / - if (static_branch_likely(&sched_numa_balancing) && - p->numa_preferred_nid >= 0 && - cpu_to_node(mm_sched_cpu) != p->numa_preferred_nid) - mm_sched_llc = -1; + if (static_branch_likely(&sched_numa_balancing)) { + int pref_nid = task_numa_preferred_nid(p); + + if (pref_nid >= 0 && cpu_to_node(mm_sched_cpu) != pref_nid) + mm_sched_llc = -1; + } #endif } @@ -1998,7 +2026,7 @@ static void get_scan_cpumasks(cpumask_var_t cpus, struct task_struct p, * current running node to mitigate the bouncing risk. * TBD: numa_group should be considered during task aggregation. / - pref_nid = p->numa_preferred_nid; + pref_nid = task_numa_preferred_nid(p); / honor the task's preferred node / if (pref_nid == NUMA_NO_NODE) goto out; @@ -2816,11 +2844,10 @@ static inline bool cpupid_valid(int cpupid) } / - * For memory tiering mode, if there are enough free pages (more than - * enough watermark defined here) in fast memory node, to take full - * advantage of fast memory capacity, all recently accessed slow - * memory pages will be migrated to fast memory node without - * considering hot threshold. + * For memory tiering mode, if the target fast memory node has enough free + * pages, recently accessed slow memory pages can bypass the normal hotness + * check and rate limit so the fast tier can be filled. The bypass can be + * disabled by numa_balancing_promote_free_space_bypass. / static bool pgdat_free_space_enough(struct pglist_data pgdat) { @@ -2941,7 +2968,8 @@ bool should_numa_migrate_memory(struct task_struct p, struct folio folio, long nr = folio_nr_pages(folio); pgdat = NODE_DATA(dst_nid); - if (pgdat_free_space_enough(pgdat)) { + if (sysctl_numa_balancing_promote_free_space_bypass && + pgdat_free_space_enough(pgdat)) { /* workload changed, reset hot threshold / pgdat->nbp_threshold = 0; mod_node_page_state(pgdat, PGPROMOTE_CANDIDATE_NRL, nr); @@ -3638,6 +3666,9 @@ static void numa_migrate_preferred(struct task_struct p) interval = min(interval, msecs_to_jiffies(p->numa_scan_period) / 16); p->numa_migrate_retry = jiffies + interval; + if (!numa_balancing_normal_enabled()) + return; + /* Success if task is already running on preferred CPU / if (task_node(p) == p->numa_preferred_nid) return; @@ -3675,10 +3706,10 @@ static void numa_group_count_active_nodes(struct numa_group numa_group) /* * When adapting the scan rate, the period is divided into NUMA_PERIOD_SLOTS - * increments. The more local the fault statistics are, the higher the scan - * period will be for the next scan window. If local/(local+remote) ratio is - * below NUMA_PERIOD_THRESHOLD (where range of ratio is 1..NUMA_PERIOD_SLOTS) - * the scan period will decrease. Aim for 70% local accesses. + * increments. The more local or shared the fault statistics are, the higher + * the scan period will be for the next scan window. If the private remote + * ratio reaches NUMA_PERIOD_SLOTS - NUMA_PERIOD_THRESHOLD, the scan period + * will decrease. Aim for 70% local or shared accesses. / #define NUMA_PERIOD_SLOTS 10 #define NUMA_PERIOD_THRESHOLD 7 @@ -3693,11 +3724,15 @@ static void update_task_scan_period(struct task_struct p, unsigned long shared, unsigned long private) { unsigned int period_slot; - int lr_ratio, ps_ratio; + int lr_ratio, sr_ratio, rp_ratio, ratio; int diff; - unsigned long remote = p->numa_faults_locality[0]; - unsigned long local = p->numa_faults_locality[1]; + unsigned long remote = p->numa_faults_locality[NUMA_FAULT_REMOTE]; + unsigned long local = p->numa_faults_locality[NUMA_FAULT_LOCAL]; + unsigned long remote_private = + p->numa_faults_locality[NUMA_FAULT_REMOTE_PRIVATE]; + unsigned long locality_faults = local + remote; + unsigned long type_faults = private + shared; /* * If there were no record hinting faults then either the task is @@ -3706,7 +3741,8 @@ static void update_task_scan_period(struct task_struct p, migration then it implies we are migrating too quickly or the local * node is overloaded. In either case, scan slower / - if (local + shared == 0 \|\| p->numa_faults_locality[2]) { + if (!locality_faults \|\| !type_faults \|\| + p->numa_faults_locality[NUMA_FAULT_MIGRATE_FAIL]) { p->numa_scan_period = min(p->numa_scan_period_max, p->numa_scan_period << 1); @@ -3719,40 +3755,36 @@ static void update_task_scan_period(struct task_struct p, /* * Prepare to scale scan period relative to the current period. * == NUMA_PERIOD_THRESHOLD scan period stays the same - * < NUMA_PERIOD_THRESHOLD scan period decreases (scan faster) + * < NUMA_PERIOD_THRESHOLD scan period may decrease if the + * remote-private signal is strong enough * >= NUMA_PERIOD_THRESHOLD scan period increases (scan slower) / period_slot = DIV_ROUND_UP(p->numa_scan_period, NUMA_PERIOD_SLOTS); - lr_ratio = (local NUMA_PERIOD_SLOTS) / (local + remote); - ps_ratio = (private * NUMA_PERIOD_SLOTS) / (private + shared); + lr_ratio = (local * NUMA_PERIOD_SLOTS) / locality_faults; + sr_ratio = (shared * NUMA_PERIOD_SLOTS) / type_faults; + rp_ratio = (remote_private * NUMA_PERIOD_SLOTS) / locality_faults; + ratio = max(lr_ratio, sr_ratio); - if (ps_ratio >= NUMA_PERIOD_THRESHOLD) { - /* - * Most memory accesses are local. There is no need to - * do fast NUMA scanning, since memory is already local. - / - int slot = ps_ratio - NUMA_PERIOD_THRESHOLD; - if (!slot) - slot = 1; - diff = slot period_slot; - } else if (lr_ratio >= NUMA_PERIOD_THRESHOLD) { + if (ratio >= NUMA_PERIOD_THRESHOLD) { /* - * Most memory accesses are shared with other tasks. - * There is no point in continuing fast NUMA scanning, - * since other tasks may just move the memory elsewhere. + * Most memory accesses are local or shared. Local faults do not + * need fast scanning; shared faults may just be moved elsewhere by + * other tasks. / - int slot = lr_ratio - NUMA_PERIOD_THRESHOLD; + int slot = ratio - NUMA_PERIOD_THRESHOLD; if (!slot) slot = 1; diff = slot period_slot; } else { /* - * Private memory faults exceed (SLOTS-THRESHOLD)/SLOTS, - * yet they are not on the local NUMA node. Speed up - * NUMA scanning to get the memory moved over. + * Only speed up scanning when enough faults are both private + * and remote. Mixed windows dominated by shared remote faults + * should not create extra hinting overhead. / - int ratio = max(lr_ratio, ps_ratio); - diff = -(NUMA_PERIOD_THRESHOLD - ratio) period_slot; + if (rp_ratio >= NUMA_PERIOD_SLOTS - NUMA_PERIOD_THRESHOLD) + diff = -(NUMA_PERIOD_THRESHOLD - ratio) * period_slot; + else + diff = 0; } p->numa_scan_period = clamp(p->numa_scan_period + diff, @@ -3904,8 +3936,8 @@ static void task_numa_placement(struct task_struct p) p->numa_scan_seq = seq; p->numa_scan_period_max = task_scan_max(p); - total_faults = p->numa_faults_locality[0] + - p->numa_faults_locality[1]; + total_faults = p->numa_faults_locality[NUMA_FAULT_REMOTE] + + p->numa_faults_locality[NUMA_FAULT_LOCAL]; runtime = numa_get_avg_runtime(p, &period); / If the task is part of a group prevent parallel updates to group stats / @@ -4177,8 +4209,9 @@ void task_numa_free(struct task_struct p, bool final) kfree(numa_faults); } else { p->total_numa_faults = 0; - for (i = 0; i < NR_NUMA_HINT_FAULT_STATS * nr_node_ids; i++) - numa_faults[i] = 0; + memset(p->numa_faults_locality, 0, sizeof(p->numa_faults_locality)); + memset(numa_faults, 0, sizeof(numa_faults) + NR_NUMA_HINT_FAULT_BUCKETS * nr_node_ids); } } @@ -4201,6 +4234,21 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags) if (!p->mm) return; + /* + * Slow-tier pages use last_cpupid as access-time metadata instead of + * a real cpupid. Do not feed these faults into normal task placement + * when normal NUMA balancing is disabled. + / + if (last_cpupid == LAST_CPUPID_ACCESS_TIME) { + if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_NORMAL)) { + if (migrated) + p->numa_pages_migrated += pages; + return; + } + + last_cpupid = LAST_CPUPID_UNSET; + } + / * NUMA faults statistics are unnecessary for the slow memory * node for memory tiering mode. @@ -4210,6 +4258,15 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags) !cpupid_valid(last_cpupid))) return; + /* + * A stale slow-tier access timestamp can survive after memory tiering + * is disabled and later reach here as if it was a cpupid from a + * top-tier page. Treat any non-cpupid value as an ordinary first + * access before task_numa_group() decodes a CPU from it. + / + if (unlikely(!cpupid_valid(last_cpupid))) + last_cpupid = LAST_CPUPID_UNSET; + / Allocate buffer to track faults on a per-node basis / if (unlikely(!p->numa_faults)) { int size = sizeof(p->numa_faults) * @@ -4227,7 +4284,7 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags) * First accesses are treated as private, otherwise consider accesses * to be private if the accessing pid has not changed / - if (unlikely(last_cpupid == (-1 & LAST_CPUPID_MASK))) { + if (unlikely(last_cpupid == LAST_CPUPID_UNSET)) { priv = 1; } else { priv = cpupid_match_pid(p, last_cpupid); @@ -4259,11 +4316,14 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags) if (migrated) p->numa_pages_migrated += pages; if (flags & TNF_MIGRATE_FAIL) - p->numa_faults_locality[2] += pages; + p->numa_faults_locality[NUMA_FAULT_MIGRATE_FAIL] += pages; p->numa_faults[task_faults_idx(NUMA_MEMBUF, mem_node, priv)] += pages; p->numa_faults[task_faults_idx(NUMA_CPUBUF, cpu_node, priv)] += pages; - p->numa_faults_locality[local] += pages; + if (!local && priv) + p->numa_faults_locality[NUMA_FAULT_REMOTE_PRIVATE] += pages; + p->numa_faults_locality[local ? NUMA_FAULT_LOCAL : + NUMA_FAULT_REMOTE] += pages; } static void reset_ptenuma_scan(struct task_struct p) @@ -10700,6 +10760,9 @@ static long migrate_degrades_locality(struct task_struct p, struct lb_env env) if (!static_branch_likely(&sched_numa_balancing)) return 0; + if (!numa_balancing_normal_enabled()) + return 0; + if (!p->numa_faults \|\| !(env->sd->flags & SD_NUMA)) return 0; @@ -12493,6 +12556,9 @@ static bool update_sd_pick_busiest(struct lb_env env, #ifdef CONFIG_NUMA_BALANCING static inline enum fbq_type fbq_classify_group(struct sg_lb_stats sgs) { + if (!numa_balancing_normal_enabled()) + return all; + if (sgs->sum_h_nr_running > sgs->nr_numa_running) return regular; if (sgs->sum_h_nr_running > sgs->nr_preferred_running) @@ -12502,6 +12568,9 @@ static inline enum fbq_type fbq_classify_group(struct sg_lb_stats sgs) static inline enum fbq_type fbq_classify_rq(struct rq rq) { + if (!numa_balancing_normal_enabled()) + return regular; + if (rq->nr_running > rq->nr_numa_running) return regular; if (rq->nr_running > rq->nr_preferred_running) @@ -12803,17 +12872,21 @@ sched_balance_find_dst_group(struct sched_domain sd, struct task_struct p, int if (sd->flags & SD_NUMA) { int imb_numa_nr = sd->imb_numa_nr; #ifdef CONFIG_NUMA_BALANCING - int idlest_cpu; + int pref_nid = task_numa_preferred_nid(p); /* * If there is spare capacity at NUMA, try to select * the preferred node / - if (cpu_to_node(this_cpu) == p->numa_preferred_nid) - return NULL; + if (pref_nid != NUMA_NO_NODE) { + int idlest_cpu; - idlest_cpu = cpumask_first(sched_group_span(idlest)); - if (cpu_to_node(idlest_cpu) == p->numa_preferred_nid) - return idlest; + if (cpu_to_node(this_cpu) == pref_nid) + return NULL; + + idlest_cpu = cpumask_first(sched_group_span(idlest)); + if (cpu_to_node(idlest_cpu) == pref_nid) + return idlest; + } #endif / CONFIG_NUMA_BALANCING / / * Otherwise, keep the task close to the wakeup source diff --git a/mm/memory.c b/mm/memory.c index 8b0c2c735d3de..212e9d1d5b64a 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -6173,7 +6173,7 @@ int numa_migrate_check(struct folio folio, struct vm_fault vmf, * to record page access time. So use default value. / if (folio_use_access_time(folio)) - last_cpupid = (-1 & LAST_CPUPID_MASK); + last_cpupid = LAST_CPUPID_ACCESS_TIME; else last_cpupid = folio_last_cpupid(folio);
51/3	2026/10/14 15:00	llm	kmsan-evaluator	3m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 8817 output: 21 thoughts: 859 Thoughts: KMSAN Fuzzing Justification: A Deep Dive Okay, here's the deal. I'm looking at this patch series and the primary question is whether these changes necessitate firing up a dedicated KMSAN fuzzing session, beyond the standard KASAN checks we're already running. Time to break it down. First, I've got a good handle on what the patch touches. We have changes across several files. It's not a massive rewrite, but the spread is noteworthy: * Documentation: Updates to the system control documentation. * Include Files: New defines are added to `mm.h`, specifically for tracking CPU IDs. There's also a modification in `mmzone.h` with some comment changes. `sched.h` has a more substantial update, with a new enum `numa_faults_locality` and an increase in the size of `numa_faults_locality` within `task_struct`. This bump from 3 to 4 elements is a flag. * kernel/sched/fair.c: This is where the bulk of the logic changes reside. A new sysctl (`sysctl_numa_balancing_promote_free_space_bypass`) is introduced. We get new functions, `numa_balancing_normal_enabled()` and `task_numa_preferred_nid()`. The existing functions like `get_pref_llc`, `get_scan_cpumasks`, and `sched_balance_find_dst_group` are being modified to use the new `task_numa_preferred_nid()`. It also appears to be using the new enum elements, with checks for `sysctl_numa_balancing_promote_free_space_bypass` in some places. There's a lot of NUMA balancing logic getting touched. There are several instances of `memset` using the new size and new enum, and checks for `last_cpupid == LAST_CPUPID_ACCESS_TIME`. There are some uses of the new enum in `task_numa_fault`, tracking `NUMA_FAULT_REMOTE_PRIVATE`. The new functions also appear in `migrate_degrades_locality`, `fbq_classify_group`, and `fbq_classify_rq`. * mm/memory.c: There's a change in `numa_migrate_check` to set `*last_cpupid = LAST_CPUPID_ACCESS_TIME;` under certain conditions. Now, a critical point of concern: the `numa_faults_locality` array in `task_struct`. This smells like potential memory access issues, especially since the size changed. I'm focusing on its initialization. Where is this array being initialized when a task is created or cloned? That's the key. I'll need to use grepper to track down all the instances of `numa_faults_locality` initialization and make sure that we're covering all relevant code paths. The change in its size and the new enum additions are a big red flag, and my gut says this needs a closer look, especially for data races. Time to dig in and ensure that all accesses are correctly synchronized and that the new size isn't causing any off-by-one errors.
52/3	2026/10/14 15:03	tool	grepper	0m Args: { "Expression": "numa_faults_locality" } Results: { "Output": "include/linux/sched.h=832=struct task_ipi_mask { };\n--\ninclude/linux/sched.h-835-#ifdef CONFIG_NUMA_BALANCING\ninclude/linux/sched.h:836:enum numa_faults_locality {\ninclude/linux/sched.h-837-\tNUMA_FAULT_REMOTE,\n--\ninclude/linux/sched.h=845=struct task_struct {\n--\ninclude/linux/sched.h-1433-\t/\ninclude/linux/sched.h:1434:\t numa_faults_locality tracks if faults recorded during the last\ninclude/linux/sched.h-1435-\t * scan window were remote/local, remote private, or failed to migrate.\n--\ninclude/linux/sched.h-1437-\t /\ninclude/linux/sched.h:1438:\tunsigned long\t\t\tnuma_faults_locality[NR_NUMA_FAULT_LOCALITY];\ninclude/linux/sched.h-1439-\n--\nkernel/sched/fair.c=3723=static void update_task_scan_period(struct task_struct p,\n--\nkernel/sched/fair.c-3729-\nkernel/sched/fair.c:3730:\tunsigned long remote = p-\u003enuma_faults_locality[NUMA_FAULT_REMOTE];\nkernel/sched/fair.c:3731:\tunsigned long local = p-\u003enuma_faults_locality[NUMA_FAULT_LOCAL];\nkernel/sched/fair.c-3732-\tunsigned long remote_private =\nkernel/sched/fair.c:3733:\t\tp-\u003enuma_faults_locality[NUMA_FAULT_REMOTE_PRIVATE];\nkernel/sched/fair.c-3734-\tunsigned long locality_faults = local + remote;\n--\nkernel/sched/fair.c-3744-\tif (!locality_faults \|\| !type_faults \|\|\nkernel/sched/fair.c:3745:\t p-\u003enuma_faults_locality[NUMA_FAULT_MIGRATE_FAIL]) {\nkernel/sched/fair.c-3746-\t\tp-\u003enuma_scan_period = min(p-\u003enuma_scan_period_max,\n--\nkernel/sched/fair.c-3791-\t\t\ttask_scan_min(p), task_scan_max(p));\nkernel/sched/fair.c:3792:\tmemset(p-\u003enuma_faults_locality, 0, sizeof(p-\u003enuma_faults_locality));\nkernel/sched/fair.c-3793-}\n--\nkernel/sched/fair.c=3915=static void task_numa_placement(struct task_struct p)\n--\nkernel/sched/fair.c-3938-\nkernel/sched/fair.c:3939:\ttotal_faults = p-\u003enuma_faults_locality[NUMA_FAULT_REMOTE] +\nkernel/sched/fair.c:3940:\t\t p-\u003enuma_faults_locality[NUMA_FAULT_LOCAL];\nkernel/sched/fair.c-3941-\truntime = numa_get_avg_runtime(p, \u0026period);\n--\nkernel/sched/fair.c=4184=void task_numa_free(struct task_struct p, bool final)\n--\nkernel/sched/fair.c-4211-\t\tp-\u003etotal_numa_faults = 0;\nkernel/sched/fair.c:4212:\t\tmemset(p-\u003enuma_faults_locality, 0, sizeof(p-\u003enuma_faults_locality));\nkernel/sched/fair.c-4213-\t\tmemset(numa_faults, 0, sizeof(numa_faults) \n--\nkernel/sched/fair.c=4221=void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)\n--\nkernel/sched/fair.c-4279-\t\tp-\u003etotal_numa_faults = 0;\nkernel/sched/fair.c:4280:\t\tmemset(p-\u003enuma_faults_locality, 0, sizeof(p-\u003enuma_faults_locality));\nkernel/sched/fair.c-4281-\t}\n--\nkernel/sched/fair.c-4318-\tif (flags \u0026 TNF_MIGRATE_FAIL)\nkernel/sched/fair.c:4319:\t\tp-\u003enuma_faults_locality[NUMA_FAULT_MIGRATE_FAIL] += pages;\nkernel/sched/fair.c-4320-\n--\nkernel/sched/fair.c-4323-\tif (!local \u0026\u0026 priv)\nkernel/sched/fair.c:4324:\t\tp-\u003enuma_faults_locality[NUMA_FAULT_REMOTE_PRIVATE] += pages;\nkernel/sched/fair.c:4325:\tp-\u003enuma_faults_locality[local ? NUMA_FAULT_LOCAL :\nkernel/sched/fair.c-4326-\t\t\t\t NUMA_FAULT_REMOTE] += pages;\n" }
53/3	2026/10/14 15:03	llm	kmsan-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 11687 output: 33 thoughts: 34 Thoughts: Okay, let's dive into this. I need to refresh my memory on the kernel's task creation process. My immediate thought is to examine `dup_task_struct` and, more specifically, the `copy_process` function. I want to see how the kernel handles the memory allocation and initialization of the `task_struct` structure itself. It’s been a while since I've scrutinized this low-level aspect, so understanding the specifics of how a new task gets its foundation – the `task_struct` – is crucial for a complete understanding. I want to see the exact mechanics, where the memory for that structure comes from, and what gets populated during that initial setup phase. Knowing this will give me a solid base to understand how to approach the particular issue at hand.
54/3	2026/10/14 15:03	tool	grepper	0m Args: { "Expression": "alloc_task_struct_node", "PathPrefixes": [ "kernel/fork.c" ] } Results: { "Output": "kernel/fork.c=184=static struct kmem_cache task_struct_cachep;\nkernel/fork.c-185-\nkernel/fork.c:186:static inline struct task_struct alloc_task_struct_node(int node)\nkernel/fork.c-187-{\n--\nkernel/fork.c=915=static struct task_struct dup_task_struct(struct task_struct orig, int node)\n--\nkernel/fork.c-921-\t\tnode = tsk_fork_get_node(orig);\nkernel/fork.c:922:\ttsk = alloc_task_struct_node(node);\nkernel/fork.c-923-\tif (!tsk)\n" }
55/3	2026/10/14 15:03	llm	kmsan-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 12108 output: 27 thoughts: 27 Thoughts: Okay, here's my take on those thoughts, tailored for an expert audience and presented in the first person: Investigating NUMA Faults in Fork Operations Alright, let's dive into this. My immediate thought is, when a process forks, I need to understand what's actually happening with the `numa_faults` and `numa_faults_locality` counters. They're critical metrics for gauging NUMA-related performance impacts, and understanding their behavior during a fork is key to building a performant system. I'm wondering if a deep dive into the relevant kernel code is required. How are these counters propagated, reset, or inherited across the new process? Does the kernel handle this in an optimal way, or is there room for tuning? I'm thinking about the implications of the page table copy on these counters. A fork creates a copy-on-write page table setup, so the initial faults might be minimal. But what happens as the child process starts accessing its own memory? Are the faults properly attributed, or could there be a skew due to the initial shared state? Is there a subtle performance hit here? I'll probably also want to think about the different possible NUMA configurations and the impact each might have on these numbers. Are there particular NUMA node affinities that can amplify or reduce these faults during the fork, and will these be consistent across systems? Ultimately, I want to see if this fork operation is creating a substantial difference in NUMA faulting, compared to not forking, and where any performance problems might show up.
56/3	2026/10/14 15:03	tool	grepper	0m Args: { "Expression": "task_numa", "PathPrefixes": [ "kernel/" ] } Results: { "Output": "kernel/fork.c=782=void __put_task_struct(struct task_struct tsk)\n--\nkernel/fork.c-790-\tcgroup_task_free(tsk);\nkernel/fork.c:791:\ttask_numa_free(tsk, true);\nkernel/fork.c-792-\tsecurity_task_free(tsk);\n--\nkernel/sched/debug.c=971=print_task(struct seq_file m, struct rq rq, struct task_struct p)\n--\nkernel/sched/debug.c-995-#ifdef CONFIG_NUMA_BALANCING\nkernel/sched/debug.c:996:\tSEQ_printf(m, \" %d %d\", task_node(p), task_numa_group_id(p));\nkernel/sched/debug.c-997-#endif\n--\nkernel/sched/debug.c=1391=static void sched_show_numa(struct task_struct p, struct seq_file m)\n--\nkernel/sched/debug.c-1400-\tSEQ_printf(m, \"current_node=%d, numa_group_id=%d\\n\",\nkernel/sched/debug.c:1401:\t\t\ttask_node(p), task_numa_group_id(p));\nkernel/sched/debug.c-1402-\tshow_numa_stats(p, m);\n--\nkernel/sched/fair.c=1332=static inline bool numa_balancing_normal_enabled(void)\n--\nkernel/sched/fair.c-1336-\nkernel/sched/fair.c:1337:static inline int task_numa_preferred_nid(struct task_struct p)\nkernel/sched/fair.c-1338-{\n--\nkernel/sched/fair.c=1886=static int get_pref_llc(struct task_struct p, struct sched_cache_group grp)\n--\nkernel/sched/fair.c-1908-\t\tif (static_branch_likely(\u0026sched_numa_balancing)) {\nkernel/sched/fair.c:1909:\t\t\tint pref_nid = task_numa_preferred_nid(p);\nkernel/sched/fair.c-1910-\n--\nkernel/sched/fair.c=2005=static void get_scan_cpumasks(cpumask_var_t cpus, struct task_struct p,\n--\nkernel/sched/fair.c-2028-\t /\nkernel/sched/fair.c:2029:\tpref_nid = task_numa_preferred_nid(p);\nkernel/sched/fair.c-2030-\t/ honor the task's preferred node /\n--\nkernel/sched/fair.c=2492=struct numa_group {\n--\nkernel/sched/fair.c-2516- /\nkernel/sched/fair.c:2517:static struct numa_group deref_task_numa_group(struct task_struct p)\nkernel/sched/fair.c-2518-{\n--\nkernel/sched/fair.c=2621=static void account_numa_dequeue(struct rq rq, struct task_struct p)\n--\nkernel/sched/fair.c-2635-\nkernel/sched/fair.c:2636:pid_t task_numa_group_id(struct task_struct p)\nkernel/sched/fair.c-2637-{\n--\nkernel/sched/fair.c-2653- array is for current counters, which are averaged into the\nkernel/sched/fair.c:2654: * first set by task_numa_placement.\nkernel/sched/fair.c-2655- /\n--\nkernel/sched/fair.c=2670=static inline unsigned long group_faults(struct task_struct p, int nid)\nkernel/sched/fair.c-2671-{\nkernel/sched/fair.c:2672:\tstruct numa_group ng = deref_task_numa_group(p);\nkernel/sched/fair.c-2673-\n--\nkernel/sched/fair.c=2814=static inline unsigned long group_weight(struct task_struct p, int nid,\n--\nkernel/sched/fair.c-2816-{\nkernel/sched/fair.c:2817:\tstruct numa_group ng = deref_task_numa_group(p);\nkernel/sched/fair.c-2818-\tunsigned long faults, total_faults;\n--\nkernel/sched/fair.c=3076=struct numa_stats {\n--\nkernel/sched/fair.c-3087-\nkernel/sched/fair.c:3088:struct task_numa_env {\nkernel/sched/fair.c-3089-\tstruct task_struct p;\n--\nkernel/sched/fair.c=3127=static inline int numa_idle_core(int idle_core, int cpu)\n--\nkernel/sched/fair.c-3148- /\nkernel/sched/fair.c:3149:static void update_numa_stats(struct task_numa_env env,\nkernel/sched/fair.c-3150-\t\t\t struct numa_stats ns, int nid,\n--\nkernel/sched/fair.c-3188-\nkernel/sched/fair.c:3189:static void task_numa_assign(struct task_numa_env env,\nkernel/sched/fair.c-3190-\t\t\t struct task_struct p, long imp)\n--\nkernel/sched/fair.c=3236=static bool load_too_imbalanced(long src_load, long dst_load,\nkernel/sched/fair.c:3237:\t\t\t\tstruct task_numa_env env)\nkernel/sched/fair.c-3238-{\n--\nkernel/sched/fair.c-3276- /\nkernel/sched/fair.c:3277:static bool task_numa_compare(struct task_numa_env env,\nkernel/sched/fair.c-3278-\t\t\t long taskimp, long groupimp, bool maymove)\n--\nkernel/sched/fair.c-3441-\nkernel/sched/fair.c:3442:\ttask_numa_assign(env, cur, imp);\nkernel/sched/fair.c-3443-\n--\nkernel/sched/fair.c-3465-\nkernel/sched/fair.c:3466:static void task_numa_find_cpu(struct task_numa_env env,\nkernel/sched/fair.c-3467-\t\t\t\tlong taskimp, long groupimp)\n--\nkernel/sched/fair.c-3496-\t\t\t\tenv-\u003edst_cpu = env-\u003edst_stats.idle_cpu;\nkernel/sched/fair.c:3497:\t\t\t\ttask_numa_assign(env, NULL, 0);\nkernel/sched/fair.c-3498-\t\t\t\treturn;\n--\nkernel/sched/fair.c-3515-\t\tenv-\u003edst_cpu = cpu;\nkernel/sched/fair.c:3516:\t\tif (task_numa_compare(env, taskimp, groupimp, maymove))\nkernel/sched/fair.c-3517-\t\t\tbreak;\n--\nkernel/sched/fair.c-3520-\nkernel/sched/fair.c:3521:static int task_numa_migrate(struct task_struct p)\nkernel/sched/fair.c-3522-{\nkernel/sched/fair.c:3523:\tstruct task_numa_env env = {\nkernel/sched/fair.c-3524-\t\t.p = p,\n--\nkernel/sched/fair.c-3578-\t/* Try to find a spot on the preferred nid. /\nkernel/sched/fair.c:3579:\ttask_numa_find_cpu(\u0026env, taskimp, groupimp);\nkernel/sched/fair.c-3580-\n--\nkernel/sched/fair.c-3609-\t\t\tupdate_numa_stats(\u0026env, \u0026env.dst_stats, env.dst_nid, true);\nkernel/sched/fair.c:3610:\t\t\ttask_numa_find_cpu(\u0026env, taskimp, groupimp);\nkernel/sched/fair.c-3611-\t\t}\n--\nkernel/sched/fair.c=3657=static void numa_migrate_preferred(struct task_struct p)\n--\nkernel/sched/fair.c-3676-\t/* Otherwise, try migrate to a CPU on the preferred node /\nkernel/sched/fair.c:3677:\ttask_numa_migrate(p);\nkernel/sched/fair.c-3678-}\n--\nkernel/sched/fair.c=3802=static u64 numa_get_avg_runtime(struct task_struct p, u64 period)\n--\nkernel/sched/fair.c-3808-\nkernel/sched/fair.c:3809:\tif (p-\u003elast_task_numa_placement) {\nkernel/sched/fair.c-3810-\t\tdelta = runtime - p-\u003elast_sum_exec_runtime;\nkernel/sched/fair.c:3811:\t\tperiod = now - p-\u003elast_task_numa_placement;\nkernel/sched/fair.c-3812-\n--\nkernel/sched/fair.c-3821-\tp-\u003elast_sum_exec_runtime = runtime;\nkernel/sched/fair.c:3822:\tp-\u003elast_task_numa_placement = now;\nkernel/sched/fair.c-3823-\n--\nkernel/sched/fair.c=3832=static int preferred_group_nid(struct task_struct p, int nid)\n--\nkernel/sched/fair.c-3914-\nkernel/sched/fair.c:3915:static void task_numa_placement(struct task_struct p)\nkernel/sched/fair.c-3916-\t__context_unsafe(/* conditional locking /)\n--\nkernel/sched/fair.c=4066=static inline void put_numa_group(struct numa_group grp)\n--\nkernel/sched/fair.c-4071-\nkernel/sched/fair.c:4072:static void task_numa_group(struct task_struct p, int cpupid, int flags,\nkernel/sched/fair.c-4073-\t\t\tint priv)\n--\nkernel/sched/fair.c-4183- /\nkernel/sched/fair.c:4184:void task_numa_free(struct task_struct p, bool final)\nkernel/sched/fair.c-4185-{\n--\nkernel/sched/fair.c-4220- /\nkernel/sched/fair.c:4221:void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)\nkernel/sched/fair.c-4222-{\n--\nkernel/sched/fair.c-4264-\t top-tier page. Treat any non-cpupid value as an ordinary first\nkernel/sched/fair.c:4265:\t * access before task_numa_group() decodes a CPU from it.\nkernel/sched/fair.c-4266-\t /\n--\nkernel/sched/fair.c-4291-\t\tif (!priv \u0026\u0026 !(flags \u0026 TNF_NO_GROUP))\nkernel/sched/fair.c:4292:\t\t\ttask_numa_group(p, last_cpupid, flags, \u0026priv);\nkernel/sched/fair.c-4293-\t}\n--\nkernel/sched/fair.c-4311-\tif (time_after(jiffies, p-\u003enuma_migrate_retry)) {\nkernel/sched/fair.c:4312:\t\ttask_numa_placement(p);\nkernel/sched/fair.c-4313-\t\tnuma_migrate_preferred(p);\n--\nkernel/sched/fair.c=4343=static bool vma_is_accessed(struct mm_struct mm, struct vm_area_struct vma)\n--\nkernel/sched/fair.c-4385- /\nkernel/sched/fair.c:4386:static void task_numa_work(struct callback_head work)\nkernel/sched/fair.c-4387-{\n--\nkernel/sched/fair.c=4643=void init_numa_balancing(u64 clone_flags, struct task_struct p)\n--\nkernel/sched/fair.c-4658-\tp-\u003enuma_migrate_retry\t\t= 0;\nkernel/sched/fair.c:4659:\t/* Protect against double add, see task_tick_numa and task_numa_work /\nkernel/sched/fair.c-4660-\tp-\u003enuma_work.next\t\t= \u0026p-\u003enuma_work;\n--\nkernel/sched/fair.c-4664-\tRCU_INIT_POINTER(p-\u003enuma_group, NULL);\nkernel/sched/fair.c:4665:\tp-\u003elast_task_numa_placement\t= 0;\nkernel/sched/fair.c-4666-\tp-\u003elast_sum_exec_runtime\t= 0;\nkernel/sched/fair.c-4667-\nkernel/sched/fair.c:4668:\tinit_task_work(\u0026p-\u003enuma_work, task_numa_work);\nkernel/sched/fair.c-4669-\n--\nkernel/sched/fair.c=12760=sched_balance_find_dst_group(struct sched_domain sd, struct task_struct p, int this_cpu)\n--\nkernel/sched/fair.c-12874-#ifdef CONFIG_NUMA_BALANCING\nkernel/sched/fair.c:12875:\t\t\tint pref_nid = task_numa_preferred_nid(p);\nkernel/sched/fair.c-12876-\t\t\t/\n" }
57/3	2026/10/14 15:03	llm	kmsan-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 16053 output: 36 thoughts: 22 Thoughts: Here's my summary, as if I were thinking it: Diving into `init_numa_balancing` Okay, time to get my hands dirty. I need to take a look at how NUMA balancing is initialized within the scheduler. My gut feeling tells me to start in `kernel/sched/fair.c`. That's where all the core CFS goodness lives. Let's start with `init_numa_balancing`. I'm expecting this function to set up the data structures and possibly configure some initial parameters related to NUMA node affinity and memory placement policies. I anticipate it might involve initialization of data structures relevant to per-CPU or per-task NUMA statistics, or maybe even setting up some initial thresholds or heuristics for the load balancing logic. Let's see what they've done!
58/3	2026/10/14 15:03	tool	read-file	0m Args: { "File": "kernel/sched/fair.c", "FirstLine": 4640, "LineCount": 40 } Results: { "Contents": "4640:\t\t}\n4641:\t}\n4642:\t\n4643:\tvoid init_numa_balancing(u64 clone_flags, struct task_struct p)\n4644:\t{\n4645:\t\tint mm_users = 0;\n4646:\t\tstruct mm_struct mm = p-\u003emm;\n4647:\t\n4648:\t\tif (mm) {\n4649:\t\t\tmm_users = atomic_read(\u0026mm-\u003emm_users);\n4650:\t\t\tif (mm_users == 1) {\n4651:\t\t\t\tmm-\u003enuma_next_scan = jiffies + msecs_to_jiffies(sysctl_numa_balancing_scan_delay);\n4652:\t\t\t\tmm-\u003enuma_scan_seq = 0;\n4653:\t\t\t}\n4654:\t\t}\n4655:\t\tp-\u003enode_stamp\t\t\t= 0;\n4656:\t\tp-\u003enuma_scan_seq\t\t= mm ? mm-\u003enuma_scan_seq : 0;\n4657:\t\tp-\u003enuma_scan_period\t\t= sysctl_numa_balancing_scan_delay;\n4658:\t\tp-\u003enuma_migrate_retry\t\t= 0;\n4659:\t\t/* Protect against double add, see task_tick_numa and task_numa_work /\n4660:\t\tp-\u003enuma_work.next\t\t= \u0026p-\u003enuma_work;\n4661:\t\tp-\u003enuma_faults\t\t\t= NULL;\n4662:\t\tp-\u003enuma_pages_migrated\t\t= 0;\n4663:\t\tp-\u003etotal_numa_faults\t\t= 0;\n4664:\t\tRCU_INIT_POINTER(p-\u003enuma_group, NULL);\n4665:\t\tp-\u003elast_task_numa_placement\t= 0;\n4666:\t\tp-\u003elast_sum_exec_runtime\t= 0;\n4667:\t\n4668:\t\tinit_task_work(\u0026p-\u003enuma_work, task_numa_work);\n4669:\t\n4670:\t\t/ New address space, reset the preferred nid /\n4671:\t\tif (!(clone_flags \u0026 CLONE_VM)) {\n4672:\t\t\tp-\u003enuma_preferred_nid = NUMA_NO_NODE;\n4673:\t\t\treturn;\n4674:\t\t}\n4675:\t\n4676:\t\t/\n4677:\t\t * New thread, keep existing numa_preferred_nid which should be copied\n4678:\t\t * already by arch_dup_task_struct but stagger when scans start.\n4679:\t\t */\n" }
59/3	2026/10/14 15:03	llm	kmsan-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 16985 output: 36 thoughts: 529 Thoughts: Analyzing NUMA Fault Tracking Okay, let's trace this. I need to understand what's happening with `numa_faults_locality` related to `numa_faults` during NUMA balancing, specifically in `init_numa_balancing` and `task_numa_fault`. I'm starting with `task_numa_fault`. I see that `task_numa_fault` allocates a buffer, `p->numa_faults`, to track per-node faults only if it's currently `NULL`. The code allocates memory and initializes it using `kzalloc`. Critically, it then zeroes out `p->numa_faults_locality` using `memset` after the allocation. That's the key. Now, considering task creation with `dup_task_struct`, the structure is copied. However, `init_numa_balancing(clone_flags, p)` sets `p->numa_faults` to `NULL`. The question is, is `numa_faults_locality` also cleared? Ah, no, `arch_dup_task_struct` copies the parent struct to `tsk` so it stays initialized. So, it's copied from the parent, the same initialization is present. Alright, so if `p->numa_faults` is NULL, then `task_numa_fault` handles the allocation and zeroing of `numa_faults_locality`. But what if `p->numa_faults` was already allocated? No, I have to go back to what I just said. `init_numa_balancing` sets `p->numa_faults` to `NULL`. That means it's always NULL at the point where `task_numa_fault` is called for the first time after a new process is created. The implication is, no matter what, `memset` is always called when `task_numa_fault` is executed. Finally, I need to look at `task_numa_free`. I have to check how the allocated memory and the locality information is freed, and there it is: `task_numa_free` sets `p->numa_faults` to NULL, and the rest is handled by the regular freeing mechanisms. That should cover the lifetime of these data structures, making sure they are initialized and freed correctly. Okay, I think I've got it. The flow seems sound; `numa_faults_locality` is correctly initialized to zero whenever `task_numa_fault` is called, because the code always starts with a NULL `p->numa_faults`. The lifetime is also controlled, and it seems safe.
60/3	2026/10/14 15:03	tool	read-file	0m Args: { "File": "kernel/sched/fair.c", "FirstLine": 4200, "LineCount": 40 } Results: { "Contents": "4200:\t\n4201:\t\t\tgrp-\u003enr_tasks--;\n4202:\t\t\tspin_unlock_irqrestore(\u0026grp-\u003elock, flags);\n4203:\t\t\tRCU_INIT_POINTER(p-\u003enuma_group, NULL);\n4204:\t\t\tput_numa_group(grp);\n4205:\t\t}\n4206:\t\n4207:\t\tif (final) {\n4208:\t\t\tp-\u003enuma_faults = NULL;\n4209:\t\t\tkfree(numa_faults);\n4210:\t\t} else {\n4211:\t\t\tp-\u003etotal_numa_faults = 0;\n4212:\t\t\tmemset(p-\u003enuma_faults_locality, 0, sizeof(p-\u003enuma_faults_locality));\n4213:\t\t\tmemset(numa_faults, 0, sizeof(numa_faults) \n4214:\t\t\t NR_NUMA_HINT_FAULT_BUCKETS * nr_node_ids);\n4215:\t\t}\n4216:\t}\n4217:\t\n4218:\t/\n4219:\t Got a PROT_NONE fault for a page on @node.\n4220:\t /\n4221:\tvoid task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)\n4222:\t{\n4223:\t\tstruct task_struct p = current;\n4224:\t\tbool migrated = flags \u0026 TNF_MIGRATED;\n4225:\t\tint cpu_node = task_node(current);\n4226:\t\tint local = !!(flags \u0026 TNF_FAULT_LOCAL);\n4227:\t\tstruct numa_group ng;\n4228:\t\tint priv;\n4229:\t\n4230:\t\tif (!static_branch_likely(\u0026sched_numa_balancing))\n4231:\t\t\treturn;\n4232:\t\n4233:\t\t/ for example, ksmd faulting in a user's mm /\n4234:\t\tif (!p-\u003emm)\n4235:\t\t\treturn;\n4236:\t\n4237:\t\t/\n4238:\t\t * Slow-tier pages use last_cpupid as access-time metadata instead of\n4239:\t\t * a real cpupid. Do not feed these faults into normal task placement\n" }
61/3	2026/10/14 15:03	llm	kmsan-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 18754 output: 20 thoughts: 2922 Thoughts: KMSAN Assessment of NUMA Balancing Patch Alright, let's break this down. My primary concern here is uninitialized memory and potential information leaks, given the KMSAN context. The patch `task_numa_free` now zeroes `p->numa_faults_locality`, and the goal here is to determine if this change, or any of the others, requires `NeedsKMSAN=true`. Remember, if KASAN or standard debuggers can catch it, we don't need KMSAN. First, the documentation and constant definitions are benign; no risk there. The `task_struct` change, expanding `numa_faults_locality` from 3 to 4 elements, is interesting. The code before used an index of 0, 1, and 2 directly. Now it's using an enum. The new element (MIGRATE_FAIL) is initialized to zero in `task_numa_free`. It's a straight expansion; there's no immediate KMSAN risk here, unless there's a subsequent use before initialization in a code path. The sysctl additions and modifications don't look immediately suspect. Static unsigned int values initialized to safe defaults, handled by the `proc_douintvec_minmax` handler, are unlikely to be sources of uninitialized reads. Now, looking deeper, the changes in `fair.c` are where it gets interesting. We see the `numa_faults_locality` array indexes are now using the enum values, rather than hardcoded 0, 1, and 2. There's a new `remote_private` counter being used. Specifically, the new `remote_private` counter in `task_numa_fault` is incremented based on conditional logic but isn't explicitly initialized anywhere. The usage of `LAST_CPUPID_ACCESS_TIME` and `LAST_CPUPID_UNSET` is also relevant. The code ensures these are properly handled to avoid issues with slow-tier pages when normal NUMA balancing is disabled. The update to `mm/memory.c` suggests a careful consideration of how `last_cpupid` is handled, especially when accessing the page. Looking at `task_numa_free` again, that `memset` is crucial. It zeroes the `numa_faults_locality` array before the task is reused, mitigating the KMSAN risk introduced by `remote_private`. My analysis is that the changes are reasonably safe. The expansion of `numa_faults_locality` and the introduction of new counters (like `remote_private`) could have been a risk, but the explicit zeroing in `task_numa_free` effectively handles any potential uninitialized memory issues. The other changes mostly involve refactoring with existing logic and sysctl variables, which are safe. Conclusion: Based on the code changes and the explicit zeroing, I believe that this patch series does not introduce a KMSAN vulnerability and does not warrant `NeedsKMSAN=true`. KASAN and standard debuggers should be sufficient to catch any remaining issues. The zeroing in `task_numa_free` has effectively neutralized the risk, and the other changes are either safe or well-handled within the existing system.
62/3	2026/10/14 15:04	tool	grepper	0m Args: { "Expression": "numa_migrate_check" } Results: { "Output": "mm/huge_memory.c=2320=vm_fault_t do_huge_pmd_numa_page(struct vm_fault vmf)\n--\nmm/huge_memory.c-2355-\nmm/huge_memory.c:2356:\ttarget_nid = numa_migrate_check(folio, vmf, haddr, \u0026flags, writable,\nmm/huge_memory.c-2357-\t\t\t\t\t\u0026last_cpupid);\n--\nmm/internal.h=1265=static inline bool folio_can_map_prot_numa(struct folio folio,\n--\nmm/internal.h-1271-\nmm/internal.h:1272:int numa_migrate_check(struct folio folio, struct vm_fault vmf,\nmm/internal.h-1273-\t\t unsigned long addr, int flags, bool writable,\n--\nmm/memory.c=6104=static vm_fault_t do_fault(struct vm_fault vmf)\n--\nmm/memory.c-6147-\nmm/memory.c:6148:int numa_migrate_check(struct folio folio, struct vm_fault vmf,\nmm/memory.c-6149-\t\t unsigned long addr, int flags,\n--\nmm/memory.c=6288=static vm_fault_t do_numa_page(struct vm_fault vmf)\n--\nmm/memory.c-6330-\nmm/memory.c:6331:\ttarget_nid = numa_migrate_check(folio, vmf, vmf-\u003eaddress, \u0026flags,\nmm/memory.c-6332-\t\t\t\t\twritable, \u0026last_cpupid);\n" }
63/3	2026/10/14 15:04	llm	kmsan-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 22851 output: 34 thoughts: 40 Thoughts: Here's my analysis of the provided snippet, framed as my own thought process: Diving into NUMA Page Allocation Alright, I need to understand the interaction with NUMA and page allocation. My focus right now is on how `last_cpupid` gets passed and utilized within the context of NUMA-aware page allocation. Specifically, I want to trace the flow through `do_numa_page` and `do_huge_pmd_numa_page` to see the exact mechanism. I'm assuming there's some interesting logic there, likely related to CPU affinity and placement decisions. I want to see how the system is making these choices and if there are any opportunities to improve or optimize the existing code in this area. Knowing these details will help me understand how a system could benefit from NUMA in my field.
64/3	2026/10/14 15:04	tool	read-file	0m Args: { "File": "mm/memory.c", "FirstLine": 6320, "LineCount": 50 } Results: { "Contents": "6320:\t\tif (!writable \u0026\u0026 pte_write_upgrade \u0026\u0026\n6321:\t\t can_change_pte_writable(vma, vmf-\u003eaddress, pte))\n6322:\t\t\twritable = true;\n6323:\t\n6324:\t\tfolio = vm_normal_folio(vma, vmf-\u003eaddress, pte);\n6325:\t\tif (!folio \|\| folio_is_zone_device(folio))\n6326:\t\t\tgoto out_map;\n6327:\t\n6328:\t\tnid = folio_nid(folio);\n6329:\t\tnr_pages = folio_nr_pages(folio);\n6330:\t\n6331:\t\ttarget_nid = numa_migrate_check(folio, vmf, vmf-\u003eaddress, \u0026flags,\n6332:\t\t\t\t\t\twritable, \u0026last_cpupid);\n6333:\t\tif (target_nid == NUMA_NO_NODE)\n6334:\t\t\tgoto out_map;\n6335:\t\tif (migrate_misplaced_folio_prepare(folio, vma, target_nid)) {\n6336:\t\t\tflags \|= TNF_MIGRATE_FAIL;\n6337:\t\t\tgoto out_map;\n6338:\t\t}\n6339:\t\t/* The folio is isolated and isolation code holds a folio reference. /\n6340:\t\tpte_unmap_unlock(vmf-\u003epte, vmf-\u003eptl);\n6341:\t\twritable = false;\n6342:\t\tignore_writable = true;\n6343:\t\n6344:\t\t/ Migrate to the requested node /\n6345:\t\tif (!migrate_misplaced_folio(folio, target_nid)) {\n6346:\t\t\tnid = target_nid;\n6347:\t\t\tflags \|= TNF_MIGRATED;\n6348:\t\t\ttask_numa_fault(last_cpupid, nid, nr_pages, flags);\n6349:\t\t\treturn 0;\n6350:\t\t}\n6351:\t\n6352:\t\tflags \|= TNF_MIGRATE_FAIL;\n6353:\t\tvmf-\u003epte = pte_offset_map_lock(vma-\u003evm_mm, vmf-\u003epmd,\n6354:\t\t\t\t\t vmf-\u003eaddress, \u0026vmf-\u003eptl);\n6355:\t\tif (unlikely(!vmf-\u003epte))\n6356:\t\t\treturn 0;\n6357:\t\tif (unlikely(!pte_same(ptep_get(vmf-\u003epte), vmf-\u003eorig_pte))) {\n6358:\t\t\tpte_unmap_unlock(vmf-\u003epte, vmf-\u003eptl);\n6359:\t\t\treturn 0;\n6360:\t\t}\n6361:\tout_map:\n6362:\t\t/\n6363:\t\t * Make it present again, depending on how arch implements\n6364:\t\t * non-accessible ptes, some can allow access by kernel mode.\n6365:\t\t */\n6366:\t\tif (folio \u0026\u0026 folio_test_large(folio))\n6367:\t\t\tnuma_rebuild_large_mapping(vmf, vma, folio, pte, ignore_writable,\n6368:\t\t\t\t\t\t pte_write_upgrade);\n6369:\t\telse\n" }
65/3	2026/10/14 15:04	llm	kmsan-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 23939 output: 36 thoughts: 204 Thoughts: Here's my analysis of the provided code snippet: Unconditional Assignment of `last_cpupid` in NUMA Migration Okay, so I'm looking at how NUMA migration works. Specifically, within `do_numa_page`, I see a call to `numa_migrate_check`. The critical thing is that `numa_migrate_check` is responsible for assigning a value to `last_cpupid`. Drilling into `numa_migrate_check`, I see this assignment happens unconditionally. Whether the folio uses access time or not, we get an assignment to `last_cpupid`. Either `LAST_CPUPID_ACCESS_TIME` or the result of `folio_last_cpupid` is assigned. This means the value of `last_cpupid` is always updated. This is interesting and a point to keep in mind, and the next thing to do is investigate `do_huge_pmd_numa_page` to see how it's used there, too. I need to understand the implications of this constant update to the `last_cpupid` variable in the context of the larger NUMA migration strategy.
66/3	2026/10/14 15:04	tool	read-file	0m Args: { "File": "mm/huge_memory.c", "FirstLine": 2345, "LineCount": 40 } Results: { "Contents": "2345:\t\twritable = pmd_write(pmd);\n2346:\t\tif (!writable \u0026\u0026 vma_wants_manual_pte_write_upgrade(vma) \u0026\u0026\n2347:\t\t can_change_pmd_writable(vma, vmf-\u003eaddress, pmd))\n2348:\t\t\twritable = true;\n2349:\t\n2350:\t\tfolio = vm_normal_folio_pmd(vma, haddr, pmd);\n2351:\t\tif (!folio)\n2352:\t\t\tgoto out_map;\n2353:\t\n2354:\t\tnid = folio_nid(folio);\n2355:\t\n2356:\t\ttarget_nid = numa_migrate_check(folio, vmf, haddr, \u0026flags, writable,\n2357:\t\t\t\t\t\t\u0026last_cpupid);\n2358:\t\tif (target_nid == NUMA_NO_NODE)\n2359:\t\t\tgoto out_map;\n2360:\t\tif (migrate_misplaced_folio_prepare(folio, vma, target_nid)) {\n2361:\t\t\tflags \|= TNF_MIGRATE_FAIL;\n2362:\t\t\tgoto out_map;\n2363:\t\t}\n2364:\t\t/* The folio is isolated and isolation code holds a folio reference. /\n2365:\t\tspin_unlock(vmf-\u003eptl);\n2366:\t\twritable = false;\n2367:\t\n2368:\t\tif (!migrate_misplaced_folio(folio, target_nid)) {\n2369:\t\t\tflags \|= TNF_MIGRATED;\n2370:\t\t\tnid = target_nid;\n2371:\t\t\ttask_numa_fault(last_cpupid, nid, HPAGE_PMD_NR, flags);\n2372:\t\t\treturn 0;\n2373:\t\t}\n2374:\t\n2375:\t\tflags \|= TNF_MIGRATE_FAIL;\n2376:\t\tvmf-\u003eptl = pmd_lock(vma-\u003evm_mm, vmf-\u003epmd);\n2377:\t\tif (unlikely(!pmd_same(pmdp_get(vmf-\u003epmd), vmf-\u003eorig_pmd))) {\n2378:\t\t\tspin_unlock(vmf-\u003eptl);\n2379:\t\t\treturn 0;\n2380:\t\t}\n2381:\tout_map:\n2382:\t\t/ Restore the PMD */\n2383:\t\tpmd = pmd_modify(pmdp_get(vmf-\u003epmd), vma-\u003evm_page_prot);\n2384:\t\tpmd = pmd_mkyoung(pmd);\n" }
67/3	2026/10/14 15:04	llm	kmsan-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 25121 output: 20 thoughts: 68 Thoughts: Here's my take on the thought process: Diving into CPUID Handling Okay, so `last_cpupid` is being written to, no conditional checks, right before it's fed into `task_numa_fault`. That's the core focus of this. My gut tells me there's more to this than meets the eye, especially concerning NUMA and fault handling. The question now is: What else is going on with that variable? It's time to thoroughly vet this. I need to track down every single instance where `last_cpupid`, `LAST_CPUPID_ACCESS_TIME`, and `LAST_CPUPID_UNSET` are touched – read, written, modified, anything. I need a full picture to understand the implications of this particular sequence. This analysis will require a deeper dive, especially if the code is tightly intertwined with NUMA optimizations or power management, or even context switches. A systemic analysis is called for.
68/3	2026/10/14 15:04	tool	grepper	0m Args: { "Expression": "LAST_CPUPID_" } Results: { "Output": "include/linux/mm.h=2405=static inline int folio_nid(const struct folio folio)\n--\ninclude/linux/mm.h-2412-#define PAGE_ACCESS_TIME_MIN_BITS\t12\ninclude/linux/mm.h:2413:#if LAST_CPUPID_SHIFT \u003c PAGE_ACCESS_TIME_MIN_BITS\ninclude/linux/mm.h-2414-#define PAGE_ACCESS_TIME_BUCKETS\t\t\t\t\\\ninclude/linux/mm.h:2415:\t(PAGE_ACCESS_TIME_MIN_BITS - LAST_CPUPID_SHIFT)\ninclude/linux/mm.h-2416-#else\n--\ninclude/linux/mm.h-2420-#define PAGE_ACCESS_TIME_MASK\t\t\t\t\\\ninclude/linux/mm.h:2421:\t(LAST_CPUPID_MASK \u003c\u003c PAGE_ACCESS_TIME_BUCKETS)\ninclude/linux/mm.h-2422-\ninclude/linux/mm.h:2423:#define LAST_CPUPID_UNSET\t\t(-1 \u0026 LAST_CPUPID_MASK)\ninclude/linux/mm.h:2424:#define LAST_CPUPID_ACCESS_TIME\t\t(-2 \u0026 LAST_CPUPID_MASK)\ninclude/linux/mm.h-2425-\n--\ninclude/linux/mm.h=2456=static inline bool __cpupid_match_pid(pid_t task_pid, int cpupid)\n--\ninclude/linux/mm.h-2461-#define cpupid_match_pid(task, cpupid) __cpupid_match_pid(task-\u003epid, cpupid)\ninclude/linux/mm.h:2462:#ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS\ninclude/linux/mm.h-2463-static inline int folio_xchg_last_cpupid(struct folio folio, int cpupid)\ninclude/linux/mm.h-2464-{\ninclude/linux/mm.h:2465:\treturn xchg(\u0026folio-\u003e_last_cpupid, cpupid \u0026 LAST_CPUPID_MASK);\ninclude/linux/mm.h-2466-}\n--\ninclude/linux/mm.h=2472=static inline void page_cpupid_reset_last(struct page page)\ninclude/linux/mm.h-2473-{\ninclude/linux/mm.h:2474:\tpage-\u003e_last_cpupid = LAST_CPUPID_UNSET;\ninclude/linux/mm.h-2475-}\n--\ninclude/linux/mm.h=2477=static inline int folio_last_cpupid(struct folio folio)\ninclude/linux/mm.h-2478-{\ninclude/linux/mm.h:2479:\treturn (folio-\u003eflags.f \u003e\u003e LAST_CPUPID_PGSHIFT) \u0026 LAST_CPUPID_MASK;\ninclude/linux/mm.h-2480-}\n--\ninclude/linux/mm.h=2484=static inline void page_cpupid_reset_last(struct page page)\ninclude/linux/mm.h-2485-{\ninclude/linux/mm.h:2486:\tpage-\u003eflags.f \|= LAST_CPUPID_MASK \u003c\u003c LAST_CPUPID_PGSHIFT;\ninclude/linux/mm.h-2487-}\ninclude/linux/mm.h:2488:#endif / LAST_CPUPID_NOT_IN_PAGE_FLAGS /\ninclude/linux/mm.h-2489-\n--\ninclude/linux/mm_types.h=80=struct page {\n--\ninclude/linux/mm_types.h-207-\ninclude/linux/mm_types.h:208:#ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS\ninclude/linux/mm_types.h-209-\tint _last_cpupid;\n--\ninclude/linux/mm_types.h=404=struct folio {\n--\ninclude/linux/mm_types.h-443-#endif\ninclude/linux/mm_types.h:444:#ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS\ninclude/linux/mm_types.h-445-\t\t\tint _last_cpupid;\n--\ninclude/linux/mm_types.h=526=FOLIO_MATCH(virtual, virtual);\ninclude/linux/mm_types.h-527-#endif\ninclude/linux/mm_types.h:528:#ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS\ninclude/linux/mm_types.h-529-FOLIO_MATCH(_last_cpupid, _last_cpupid);\n--\ninclude/linux/mmzone.h=1234=static inline bool zone_is_empty(const struct zone zone)\n--\ninclude/linux/mmzone.h-1248-#define ZONES_PGOFF\t\t(NODES_PGOFF - ZONES_WIDTH)\ninclude/linux/mmzone.h:1249:#define LAST_CPUPID_PGOFF\t(ZONES_PGOFF - LAST_CPUPID_WIDTH)\ninclude/linux/mmzone.h:1250:#define KASAN_TAG_PGOFF\t\t(LAST_CPUPID_PGOFF - KASAN_TAG_WIDTH)\ninclude/linux/mmzone.h-1251-#define LRU_GEN_PGOFF\t\t(KASAN_TAG_PGOFF - LRU_GEN_WIDTH)\n--\ninclude/linux/mmzone.h-1261-#define ZONES_PGSHIFT\t\t(ZONES_PGOFF * (ZONES_WIDTH != 0))\ninclude/linux/mmzone.h:1262:#define LAST_CPUPID_PGSHIFT\t(LAST_CPUPID_PGOFF * (LAST_CPUPID_WIDTH != 0))\ninclude/linux/mmzone.h-1263-#define KASAN_TAG_PGSHIFT\t(KASAN_TAG_PGOFF * (KASAN_TAG_WIDTH != 0))\n--\ninclude/linux/mmzone.h-1280-#define SECTIONS_MASK\t\t((1UL \u003c\u003c SECTIONS_WIDTH) - 1)\ninclude/linux/mmzone.h:1281:#define LAST_CPUPID_MASK\t((1UL \u003c\u003c LAST_CPUPID_SHIFT) - 1)\ninclude/linux/mmzone.h-1282-#define KASAN_TAG_MASK\t\t((1UL \u003c\u003c KASAN_TAG_WIDTH) - 1)\n--\ninclude/linux/page-flags-layout.h-89-\ninclude/linux/page-flags-layout.h:90:#define LAST_CPUPID_SHIFT (LAST__PID_SHIFT+LAST__CPU_SHIFT)\ninclude/linux/page-flags-layout.h-91-#else\ninclude/linux/page-flags-layout.h:92:#define LAST_CPUPID_SHIFT 0\ninclude/linux/page-flags-layout.h-93-#endif\n--\ninclude/linux/page-flags-layout.h-95-#if ZONES_WIDTH + LRU_GEN_WIDTH + SECTIONS_WIDTH + NODES_WIDTH + \\\ninclude/linux/page-flags-layout.h:96:\tKASAN_TAG_WIDTH + LAST_CPUPID_SHIFT \u003c= BITS_PER_LONG - NR_PAGEFLAGS\ninclude/linux/page-flags-layout.h:97:#define LAST_CPUPID_WIDTH LAST_CPUPID_SHIFT\ninclude/linux/page-flags-layout.h-98-#else\ninclude/linux/page-flags-layout.h:99:#define LAST_CPUPID_WIDTH 0\ninclude/linux/page-flags-layout.h-100-#endif\ninclude/linux/page-flags-layout.h-101-\ninclude/linux/page-flags-layout.h:102:#if LAST_CPUPID_SHIFT != 0 \u0026\u0026 LAST_CPUPID_WIDTH == 0\ninclude/linux/page-flags-layout.h:103:#define LAST_CPUPID_NOT_IN_PAGE_FLAGS\ninclude/linux/page-flags-layout.h-104-#endif\n--\ninclude/linux/page-flags-layout.h-106-#if ZONES_WIDTH + LRU_GEN_WIDTH + SECTIONS_WIDTH + NODES_WIDTH + \\\ninclude/linux/page-flags-layout.h:107:\tKASAN_TAG_WIDTH + LAST_CPUPID_WIDTH \u003e BITS_PER_LONG - NR_PAGEFLAGS\ninclude/linux/page-flags-layout.h-108-#error \"Not enough bits in page flags\"\n--\ninclude/linux/page-flags-layout.h-113-\t\t\t ZONES_WIDTH - LRU_GEN_WIDTH - SECTIONS_WIDTH - \\\ninclude/linux/page-flags-layout.h:114:\t\t\t NODES_WIDTH - KASAN_TAG_WIDTH - LAST_CPUPID_WIDTH)\ninclude/linux/page-flags-layout.h-115-\ninclude/linux/page-flags-layout.h-116-#define NR_NON_PAGEFLAG_BITS\t(SECTIONS_WIDTH + NODES_WIDTH + ZONES_WIDTH + \\\ninclude/linux/page-flags-layout.h:117:\t\t\t\tLAST_CPUPID_SHIFT + KASAN_TAG_WIDTH + \\\ninclude/linux/page-flags-layout.h-118-\t\t\t\tLRU_GEN_WIDTH + LRU_REFS_WIDTH)\n--\nkernel/sched/fair.c=4221=void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)\n--\nkernel/sched/fair.c-4241-\t /\nkernel/sched/fair.c:4242:\tif (last_cpupid == LAST_CPUPID_ACCESS_TIME) {\nkernel/sched/fair.c-4243-\t\tif (!(sysctl_numa_balancing_mode \u0026 NUMA_BALANCING_NORMAL)) {\n--\nkernel/sched/fair.c-4248-\nkernel/sched/fair.c:4249:\t\tlast_cpupid = LAST_CPUPID_UNSET;\nkernel/sched/fair.c-4250-\t}\n--\nkernel/sched/fair.c-4267-\tif (unlikely(!cpupid_valid(last_cpupid)))\nkernel/sched/fair.c:4268:\t\tlast_cpupid = LAST_CPUPID_UNSET;\nkernel/sched/fair.c-4269-\n--\nkernel/sched/fair.c-4286-\t /\nkernel/sched/fair.c:4287:\tif (unlikely(last_cpupid == LAST_CPUPID_UNSET)) {\nkernel/sched/fair.c-4288-\t\tpriv = 1;\n--\nlib/tests/printf_kunit.c=632=static const struct page_flags_test pft[] = {\n--\nlib/tests/printf_kunit.c-638-\t \"%d\", \"zone\"},\nlib/tests/printf_kunit.c:639:\t{LAST_CPUPID_WIDTH, LAST_CPUPID_PGSHIFT, LAST_CPUPID_MASK,\nlib/tests/printf_kunit.c-640-\t \"%#x\", \"lastcpupid\"},\n--\nlib/vsprintf.c=2082=static const struct page_flags_fields pff[] = {\n--\nlib/vsprintf.c-2088-\t \u0026default_dec_spec, \"zone\"},\nlib/vsprintf.c:2089:\t{LAST_CPUPID_WIDTH, LAST_CPUPID_PGSHIFT, LAST_CPUPID_MASK,\nlib/vsprintf.c-2090-\t \u0026default_flag_spec, \"lastcpupid\"},\n--\nmm/memory.c-92-\nmm/memory.c:93:#if defined(LAST_CPUPID_NOT_IN_PAGE_FLAGS) \u0026\u0026 !defined(CONFIG_COMPILE_TEST)\nmm/memory.c-94-#warning Unfortunate NUMA and NUMA Balancing config, growing page-frame for last_cpupid.\n--\nmm/memory.c=3891=static inline void wp_page_reuse(struct vm_fault vmf, struct folio folio)\n--\nmm/memory.c-3907-\t\t /\nmm/memory.c:3908:\t\tfolio_xchg_last_cpupid(folio, (1 \u003c\u003c LAST_CPUPID_SHIFT) - 1);\nmm/memory.c-3909-\t}\n--\nmm/memory.c=6148=int numa_migrate_check(struct folio folio, struct vm_fault vmf,\n--\nmm/memory.c-6175-\tif (folio_use_access_time(folio))\nmm/memory.c:6176:\t\tlast_cpupid = LAST_CPUPID_ACCESS_TIME;\nmm/memory.c-6177-\telse\n--\nmm/mm_init.c=112=void __init mminit_verify_pageflags_layout(void)\n--\nmm/mm_init.c-123-\t\tZONES_WIDTH,\nmm/mm_init.c:124:\t\tLAST_CPUPID_WIDTH,\nmm/mm_init.c-125-\t\tKASAN_TAG_WIDTH,\n--\nmm/mm_init.c-133-\t\tZONES_SHIFT,\nmm/mm_init.c:134:\t\tLAST_CPUPID_SHIFT,\nmm/mm_init.c-135-\t\tKASAN_TAG_WIDTH);\n--\nmm/mm_init.c-140-\t\t(unsigned long)ZONES_PGSHIFT,\nmm/mm_init.c:141:\t\t(unsigned long)LAST_CPUPID_PGSHIFT,\nmm/mm_init.c-142-\t\t(unsigned long)KASAN_TAG_PGSHIFT);\n--\nmm/mm_init.c-153-#endif\nmm/mm_init.c:154:#ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS\nmm/mm_init.c-155-\tmminit_dprintk(MMINIT_TRACE, \"pageflags_layout_nodeflags\",\n--\nmm/mmzone.c=76=void lruvec_init(struct lruvec lruvec)\n--\nmm/mmzone.c-97-\nmm/mmzone.c:98:#if defined(CONFIG_NUMA_BALANCING) \u0026\u0026 !defined(LAST_CPUPID_NOT_IN_PAGE_FLAGS)\nmm/mmzone.c-99-int folio_xchg_last_cpupid(struct folio folio, int cpupid)\n--\nmm/mmzone.c-106-\t\tflags = old_flags;\nmm/mmzone.c:107:\t\tlast_cpupid = (flags \u003e\u003e LAST_CPUPID_PGSHIFT) \u0026 LAST_CPUPID_MASK;\nmm/mmzone.c-108-\nmm/mmzone.c:109:\t\tflags \u0026= ~(LAST_CPUPID_MASK \u003c\u003c LAST_CPUPID_PGSHIFT);\nmm/mmzone.c:110:\t\tflags \|= (cpupid \u0026 LAST_CPUPID_MASK) \u003c\u003c LAST_CPUPID_PGSHIFT;\nmm/mmzone.c-111-\t} while (unlikely(!try_cmpxchg(\u0026folio-\u003eflags.f, \u0026old_flags, flags)));\n" }
69/3	2026/10/14 15:04	llm	kmsan-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 29239 output: 35 thoughts: 266 Thoughts: Investigating Potential Uninitialized Read or Leak of `numa_faults_locality` Okay, let's dive into this. The question on my mind is, can `numa_faults_locality` ever be read before it's initialized, or, worse, leak sensitive data to userspace? That's the real worry here. Right, where exactly is this thing used? I seem to recall that I've already done some digging... Let me just pull that up. Ah, yes. The definition lives in `include/linux/sched.h`, part of the `task_struct`, of course. Now, the previous search revealed a few spots in `kernel/sched/fair.c`: * `update_task_scan_period`: This function is reading the array, accessing elements like `[NUMA_FAULT_REMOTE]`, `[NUMA_FAULT_LOCAL]`, `[NUMA_FAULT_REMOTE_PRIVATE]`, and `[NUMA_FAULT_MIGRATE_FAIL]`. Good. * `task_numa_placement`: Also reads `[NUMA_FAULT_REMOTE]` and `[NUMA_FAULT_LOCAL]`. * `task_numa_free`: This one resets it with `memset`, so that's good. Resetting on free sounds like good practice. * `task_numa_fault`: This increments the elements. That's a counter, so it shouldn't matter if it's uninitialized, it will just start at zero. But wait. This still leaves open the question: is it exported to userspace? Is it showing up in `/proc` or exposed in a `seq_file` or `debugfs`? That's the critical piece. We need to check those. So, let's do a quick sweep of `/proc` and look for any relevant files or entries, and, for good measure, we should check `debugfs` as well.
70/3	2026/10/14 15:04	tool	grepper	0m Args: { "Expression": "numa_faults_locality", "PathPrefixes": [ "fs/", "kernel/" ] } Results: { "Output": "kernel/sched/fair.c=3723=static void update_task_scan_period(struct task_struct p,\n--\nkernel/sched/fair.c-3729-\nkernel/sched/fair.c:3730:\tunsigned long remote = p-\u003enuma_faults_locality[NUMA_FAULT_REMOTE];\nkernel/sched/fair.c:3731:\tunsigned long local = p-\u003enuma_faults_locality[NUMA_FAULT_LOCAL];\nkernel/sched/fair.c-3732-\tunsigned long remote_private =\nkernel/sched/fair.c:3733:\t\tp-\u003enuma_faults_locality[NUMA_FAULT_REMOTE_PRIVATE];\nkernel/sched/fair.c-3734-\tunsigned long locality_faults = local + remote;\n--\nkernel/sched/fair.c-3744-\tif (!locality_faults \|\| !type_faults \|\|\nkernel/sched/fair.c:3745:\t p-\u003enuma_faults_locality[NUMA_FAULT_MIGRATE_FAIL]) {\nkernel/sched/fair.c-3746-\t\tp-\u003enuma_scan_period = min(p-\u003enuma_scan_period_max,\n--\nkernel/sched/fair.c-3791-\t\t\ttask_scan_min(p), task_scan_max(p));\nkernel/sched/fair.c:3792:\tmemset(p-\u003enuma_faults_locality, 0, sizeof(p-\u003enuma_faults_locality));\nkernel/sched/fair.c-3793-}\n--\nkernel/sched/fair.c=3915=static void task_numa_placement(struct task_struct p)\n--\nkernel/sched/fair.c-3938-\nkernel/sched/fair.c:3939:\ttotal_faults = p-\u003enuma_faults_locality[NUMA_FAULT_REMOTE] +\nkernel/sched/fair.c:3940:\t\t p-\u003enuma_faults_locality[NUMA_FAULT_LOCAL];\nkernel/sched/fair.c-3941-\truntime = numa_get_avg_runtime(p, \u0026period);\n--\nkernel/sched/fair.c=4184=void task_numa_free(struct task_struct p, bool final)\n--\nkernel/sched/fair.c-4211-\t\tp-\u003etotal_numa_faults = 0;\nkernel/sched/fair.c:4212:\t\tmemset(p-\u003enuma_faults_locality, 0, sizeof(p-\u003enuma_faults_locality));\nkernel/sched/fair.c-4213-\t\tmemset(numa_faults, 0, sizeof(numa_faults) *\n--\nkernel/sched/fair.c=4221=void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)\n--\nkernel/sched/fair.c-4279-\t\tp-\u003etotal_numa_faults = 0;\nkernel/sched/fair.c:4280:\t\tmemset(p-\u003enuma_faults_locality, 0, sizeof(p-\u003enuma_faults_locality));\nkernel/sched/fair.c-4281-\t}\n--\nkernel/sched/fair.c-4318-\tif (flags \u0026 TNF_MIGRATE_FAIL)\nkernel/sched/fair.c:4319:\t\tp-\u003enuma_faults_locality[NUMA_FAULT_MIGRATE_FAIL] += pages;\nkernel/sched/fair.c-4320-\n--\nkernel/sched/fair.c-4323-\tif (!local \u0026\u0026 priv)\nkernel/sched/fair.c:4324:\t\tp-\u003enuma_faults_locality[NUMA_FAULT_REMOTE_PRIVATE] += pages;\nkernel/sched/fair.c:4325:\tp-\u003enuma_faults_locality[local ? NUMA_FAULT_LOCAL :\nkernel/sched/fair.c-4326-\t\t\t\t NUMA_FAULT_REMOTE] += pages;\n" }
71/3	2026/10/14 15:04	llm	kmsan-evaluator	0m Model: gemini-3.8-flash Args: null Results: null Tokens: input: 31003 output: 300 thoughts: 1234 Thoughts: KMSAN Assessment: A Thorough Review Okay, let's break this down. My initial concern was whether `numa_faults_locality` was making its way into userspace, which would be a definite red flag. But a quick check reveals that it's not copied to userspace at all. Good. Now, digging deeper into `dup_task_struct`, I see the standard allocation and initialization pattern: `alloc_task_struct_node`, `arch_dup_task_struct` (a simple copy), and then the critical `memset` calls to zero out `numa_faults_locality` during task initialization and after scan periods. This pattern effectively eliminates any risk of uninitialized memory. Let's quickly scan the patch files I'm looking at: documentation, include files, and relevant kernel code for any obvious slip-ups. I'm focusing on the core areas: sysctl registration, NUMA balancing logic, and the access-time metadata handling in `numa_migrate_check`. I'm explicitly looking for any kernel structures being passed to userspace via the standard mechanisms – nope. No conditional logic or branching that relies on uninitialized variables – no dice. No complex data structures are allocated or initialized where uninitialized fields could be read later – nope. No bounds checks or logic that could lead to accessing uninitialized memory in existing buffers – still negative. Now, let's measure this against the KMSAN criteria. This is the crucial part. It's time to evaluate this patch series against the established KMSAN rules, specifically designed to identify situations where KMSAN is the only effective tool. 1. Kernel structures sent to userspace? Absolutely not. 2. Conditional logic on uninitialized variables? Nope. 3. Allocations with the potential for later uninitialized reads? No way. 4. Bounds checks or logic exposing uninitialized memory? No chance. So, from my perspective, the patch series seems to be a clear "NeedsKMSAN = false". The changes are focused on NUMA balancing scheduling heuristics, the memory tiering promotion bypass via sysctl, and management of the `_last_cpupid` access-time metadata. There's nothing here that introduces any risk of uninitialized memory usage, information leaks, or exposure of uninitialized data to userspace. But let's be absolutely certain. I'm going to double-check that I haven't missed anything. I'm explicitly confirming: * No `copy_to_user` calls or information leaks have been added or modified. * No uninitialized variables are read in conditional statements or passed as arguments to other functions. * The sysctl handler, `proc_douintvec_minmax`, interacts with `sysctl_numa_balancing_promote_free_space_bypass`, which is a statically initialized global variable. * Any bugs here are more likely to be in logic or scheduling, or potentially out-of-bounds errors if array bounds were violated (which KASAN/UBSAN would catch – and here, array sizes are verified). Therefore, I'm confident in my assessment. The patch series doesn't introduce any new risks that would necessitate KMSAN. I'll summarize my conclusions to be submitted: * `NeedsKMSAN`: false * `KMSANReasoning`: The patch series focuses on NUMA balancing heuristics and a memory tiering promotion bypass. It does not introduce any uninitialized memory risks or information leaks. Standard sanitizers like KASAN are sufficient. KMSAN is unnecessary here.
72/3	2026/10/14 15:05	tool	set-results	0m Args: { "KMSANReasoning": "The patch series modifies NUMA balancing and memory tiering logic:\n1. It introduces a new sysctl (`numa_balancing_promote_free_space_bypass`) to control whether memory tiering promotion can bypass hotness checks when fast-tier nodes have free space.\n2. It refines NUMA fault locality statistics by distinguishing remote-private faults, expanding `numa_faults_locality` in `struct task_struct` from 3 to `NR_NUMA_FAULT_LOCALITY` (4) elements and properly clearing it.\n3. It distinguishes slow-tier access-time metadata from real `cpupid` by introducing `LAST_CPUPID_ACCESS_TIME` and `LAST_CPUPID_UNSET`.\n\nNone of these changes introduce or expose uninitialized memory. All variables, including the `last_cpupid` out-parameter in `numa_migrate_check`, are deterministically initialized before use. The modified structures and counters are internal scheduler accounting fields that are zero-initialized (via memset/kzalloc) and never copied or leaked to user space. There are no changes to user-kernel copying interfaces, padding exposure, or uninitialized buffer reads. Therefore, a dedicated KMSAN fuzzing session is not warranted, and standard KASAN fuzzing is sufficient.", "NeedsKMSAN": false } Results: { "KMSANReasoning": "The patch series modifies NUMA balancing and memory tiering logic:\n1. It introduces a new sysctl (`numa_balancing_promote_free_space_bypass`) to control whether memory tiering promotion can bypass hotness checks when fast-tier nodes have free space.\n2. It refines NUMA fault locality statistics by distinguishing remote-private faults, expanding `numa_faults_locality` in `struct task_struct` from 3 to `NR_NUMA_FAULT_LOCALITY` (4) elements and properly clearing it.\n3. It distinguishes slow-tier access-time metadata from real `cpupid` by introducing `LAST_CPUPID_ACCESS_TIME` and `LAST_CPUPID_UNSET`.\n\nNone of these changes introduce or expose uninitialized memory. All variables, including the `last_cpupid` out-parameter in `numa_migrate_check`, are deterministically initialized before use. The modified structures and counters are internal scheduler accounting fields that are zero-initialized (via memset/kzalloc) and never copied or leaked to user space. There are no changes to user-kernel copying interfaces, padding exposure, or uninitialized buffer reads. Therefore, a dedicated KMSAN fuzzing session is not warranted, and standard KASAN fuzzing is sufficient.", "NeedsKMSAN": false }

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