From: Leno Hou When the Multi-Gen LRU (MGLRU) state is toggled dynamically, a race condition exists between the state switching and the memory reclaim path. This can lead to unexpected cgroup OOM kills, even when plenty of reclaimable memory is available. Problem Description ================== The issue arises from a "reclaim vacuum" during the transition. 1. When disabling MGLRU, lru_gen_change_state() sets lrugen->enabled to false before the pages are drained from MGLRU lists back to traditional LRU lists. 2. Concurrent reclaimers in shrink_lruvec() see lrugen->enabled as false and skip the MGLRU path. 3. However, these pages might not have reached the traditional LRU lists yet, or the changes are not yet visible to all CPUs due to a lack of synchronization. 4. get_scan_count() subsequently finds traditional LRU lists empty, concludes there is no reclaimable memory, and triggers an OOM kill. A similar race can occur during enablement, where the reclaimer sees the new state but the MGLRU lists haven't been populated via fill_evictable() yet. Solution ======== Introduce a 'draining' state (`lru_drain_core`) to bridge the transition. When transitioning, the system enters this intermediate state where the reclaimer is forced to attempt both MGLRU and traditional reclaim paths sequentially. This ensures that folios remain visible to at least one reclaim mechanism until the transition is fully materialized across all CPUs. Changes ======= v4: - Fix Sashiko.dev's AI CodeReview comments - Remove the patch maintain workingset refault context across - Remove folio_lru_gen(folio) != -1 which involved in v2 patch v3: - Rebase onto mm-new branch for queue testing - Don't look around while draining - Fix Barry Song's comment v2: - Replace with a static branch `lru_drain_core` to track the transition state. - Ensures all LRU helpers correctly identify page state by checking folio_lru_gen(folio) != -1 instead of relying solely on global flags. - Maintain workingset refault context across MGLRU state transitions - Fix build error when CONFIG_LRU_GEN is disabled. v1: - Use smp_store_release() and smp_load_acquire() to ensure the visibility of 'enabled' and 'draining' flags across CPUs. - Modify shrink_lruvec() to allow a "joint reclaim" period. If an lruvec is in the 'draining' state, the reclaimer will attempt to scan MGLRU lists first, and then fall through to traditional LRU lists instead of returning early. This ensures that folios are visible to at least one reclaim path at any given time. Race & Mitigation ================ A race window exists between checking the 'draining' state and performing the actual list operations. For instance, a reclaimer might observe the draining state as false just before it changes, leading to a suboptimal reclaim path decision. However, this impact is effectively mitigated by the kernel's reclaim retry mechanism (e.g., in do_try_to_free_pages). If a reclaimer pass fails to find eligible folios due to a state transition race, subsequent retries in the loop will observe the updated state and correctly direct the scan to the appropriate LRU lists. This ensures the transient inconsistency does not escalate into a terminal OOM kill. This effectively reduce the race window that previously triggered OOMs under high memory pressure. This fix has been verified on v7.0.0-rc1; dynamic toggling of MGLRU functions correctly without triggering unexpected OOM kills. To: Andrew Morton To: Axel Rasmussen To: Yuanchu Xie To: Wei Xu To: Barry Song <21cnbao@gmail.com> To: Jialing Wang To: Yafang Shao To: Yu Zhao To: Kairui Song To: Bingfang Guo Cc: linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org Signed-off-by: Leno Hou --- When the Multi-Gen LRU (MGLRU) state is toggled dynamically, a race condition exists between the state switching and the memory reclaim path. This can lead to unexpected cgroup OOM kills, even when plenty of reclaimable memory is available. Problem Description ================== The issue arises from a "reclaim vacuum" during the transition. 1. When disabling MGLRU, lru_gen_change_state() sets lrugen->enabled to false before the pages are drained from MGLRU lists back to traditional LRU lists. 2. Concurrent reclaimers in shrink_lruvec() see lrugen->enabled as false and skip the MGLRU path. 3. However, these pages might not have reached the traditional LRU lists yet, or the changes are not yet visible to all CPUs due to a lack of synchronization. 4. get_scan_count() subsequently finds traditional LRU lists empty, concludes there is no reclaimable memory, and triggers an OOM kill. A similar race can occur during enablement, where the reclaimer sees the new state but the MGLRU lists haven't been populated via fill_evictable() yet. Solution ======== Introduce a 'draining' state (`lru_drain_core`) to bridge the transition. When transitioning, the system enters this intermediate state where the reclaimer is forced to attempt both MGLRU and traditional reclaim paths sequentially. This ensures that folios remain visible to at least one reclaim mechanism until the transition is fully materialized across all CPUs. Changes ======= v4: - Fix Sashiko.dev's AI CodeReview comments - Remove the patch maintain workingset refault context across - Remove folio_lru_gen(folio) != -1 which involved in v2 patch v3: - Rebase onto mm-new branch for queue testing - Don't look around while draining - Fix Barry Song's comment v2: - Replace with a static branch `lru_drain_core` to track the transition state. - Ensures all LRU helpers correctly identify page state by checking folio_lru_gen(folio) != -1 instead of relying solely on global flags. - Maintain workingset refault context across MGLRU state transitions - Fix build error when CONFIG_LRU_GEN is disabled. v1: - Use smp_store_release() and smp_load_acquire() to ensure the visibility of 'enabled' and 'draining' flags across CPUs. - Modify shrink_lruvec() to allow a "joint reclaim" period. If an lruvec is in the 'draining' state, the reclaimer will attempt to scan MGLRU lists first, and then fall through to traditional LRU lists instead of returning early. This ensures that folios are visible to at least one reclaim path at any given time. Race & Mitigation ================ A race window exists between checking the 'draining' state and performing the actual list operations. For instance, a reclaimer might observe the draining state as false just before it changes, leading to a suboptimal reclaim path decision. However, this impact is effectively mitigated by the kernel's reclaim retry mechanism (e.g., in do_try_to_free_pages). If a reclaimer pass fails to find eligible folios due to a state transition race, subsequent retries in the loop will observe the updated state and correctly direct the scan to the appropriate LRU lists. This ensures the transient inconsistency does not escalate into a terminal OOM kill. This effectively reduce the race window that previously triggered OOMs under high memory pressure. This fix has been verified on v7.0.0-rc1; dynamic toggling of MGLRU functions correctly without triggering unexpected OOM kills. Reproduction =========== The issue was consistently reproduced on v6.1.157 and v6.18.3 using a high-pressure memory cgroup (v1) environment. Reproduction steps: 1. Create a 16GB memcg and populate it with 10GB file cache (5GB active) and 8GB active anonymous memory. 2. Toggle MGLRU state while performing new memory allocations to force direct reclaim. Reproduction script =================== ```bash MGLRU_FILE="/sys/kernel/mm/lru_gen/enabled" CGROUP_PATH="/sys/fs/cgroup/memory/memcg_oom_test" switch_mglru() { local orig_val=$(cat "$MGLRU_FILE") if [[ "$orig_val" != "0x0000" ]]; then echo n > "$MGLRU_FILE" & else echo y > "$MGLRU_FILE" & fi } mkdir -p "$CGROUP_PATH" echo $((16 * 1024 * 1024 * 1024)) > "$CGROUP_PATH/memory.limit_in_bytes" echo $$ > "$CGROUP_PATH/cgroup.procs" dd if=/dev/urandom of=/tmp/test_file bs=1M count=10240 dd if=/tmp/test_file of=/dev/null bs=1M # Warm up cache stress-ng --vm 1 --vm-bytes 8G --vm-keep -t 600 & sleep 5 switch_mglru stress-ng --vm 1 --vm-bytes 2G --vm-populate --timeout 5s || \ echo "OOM Triggered" grep oom_kill "$CGROUP_PATH/memory.oom_control" ``` --- Changes in v4: - Fix Sashiko.dev's AI CodeReview comments Link: https://sashiko.dev/#/patchset/20260316-b4-switch-mglru-v2-v3-0-c846ce9a2321%40gmail.com - Remove the patch maintain workingset refault context across - Remove folio_lru_gen(folio) != -1 which involved in v2 patch - Link to v3: https://lore.kernel.org/r/20260316-b4-switch-mglru-v2-v3-0-c846ce9a2321@gmail.com --- include/linux/mm_inline.h | 11 +++++++++++ mm/rmap.c | 2 +- mm/vmscan.c | 33 ++++++++++++++++++++++++--------- 3 files changed, 36 insertions(+), 10 deletions(-) diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h index ad50688d89db..1f6b19bf365b 100644 --- a/include/linux/mm_inline.h +++ b/include/linux/mm_inline.h @@ -102,6 +102,12 @@ static __always_inline enum lru_list folio_lru_list(const struct folio *folio) #ifdef CONFIG_LRU_GEN +static inline bool lru_gen_draining(void) +{ + DECLARE_STATIC_KEY_FALSE(lru_drain_core); + + return static_branch_unlikely(&lru_drain_core); +} #ifdef CONFIG_LRU_GEN_ENABLED static inline bool lru_gen_enabled(void) { @@ -316,6 +322,11 @@ static inline bool lru_gen_enabled(void) return false; } +static inline bool lru_gen_draining(void) +{ + return false; +} + static inline bool lru_gen_in_fault(void) { return false; diff --git a/mm/rmap.c b/mm/rmap.c index 6398d7eef393..0b5f663f3062 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -966,7 +966,7 @@ static bool folio_referenced_one(struct folio *folio, nr = folio_pte_batch(folio, pvmw.pte, pteval, max_nr); } - if (lru_gen_enabled() && pvmw.pte) { + if (lru_gen_enabled() && !lru_gen_draining() && pvmw.pte) { if (lru_gen_look_around(&pvmw, nr)) referenced++; } else if (pvmw.pte) { diff --git a/mm/vmscan.c b/mm/vmscan.c index 33287ba4a500..88b9db06e331 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -886,7 +886,7 @@ static enum folio_references folio_check_references(struct folio *folio, if (referenced_ptes == -1) return FOLIOREF_KEEP; - if (lru_gen_enabled()) { + if (lru_gen_enabled() && !lru_gen_draining()) { if (!referenced_ptes) return FOLIOREF_RECLAIM; @@ -2286,7 +2286,7 @@ static void prepare_scan_control(pg_data_t *pgdat, struct scan_control *sc) unsigned long file; struct lruvec *target_lruvec; - if (lru_gen_enabled()) + if (lru_gen_enabled() && !lru_gen_draining()) return; target_lruvec = mem_cgroup_lruvec(sc->target_mem_cgroup, pgdat); @@ -2625,6 +2625,7 @@ static bool can_age_anon_pages(struct lruvec *lruvec, #ifdef CONFIG_LRU_GEN +DEFINE_STATIC_KEY_FALSE(lru_drain_core); #ifdef CONFIG_LRU_GEN_ENABLED DEFINE_STATIC_KEY_ARRAY_TRUE(lru_gen_caps, NR_LRU_GEN_CAPS); #define get_cap(cap) static_branch_likely(&lru_gen_caps[cap]) @@ -5318,6 +5319,8 @@ static void lru_gen_change_state(bool enabled) if (enabled == lru_gen_enabled()) goto unlock; + static_branch_enable_cpuslocked(&lru_drain_core); + if (enabled) static_branch_enable_cpuslocked(&lru_gen_caps[LRU_GEN_CORE]); else @@ -5348,6 +5351,9 @@ static void lru_gen_change_state(bool enabled) cond_resched(); } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL))); + + static_branch_disable_cpuslocked(&lru_drain_core); + unlock: mutex_unlock(&state_mutex); put_online_mems(); @@ -5920,9 +5926,12 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc) bool proportional_reclaim; struct blk_plug plug; - if (lru_gen_enabled() && !root_reclaim(sc)) { + if ((lru_gen_enabled() || lru_gen_draining()) && !root_reclaim(sc)) { lru_gen_shrink_lruvec(lruvec, sc); - return; + + if (!lru_gen_draining()) + return; + } get_scan_count(lruvec, sc, nr); @@ -6182,10 +6191,13 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc) struct lruvec *target_lruvec; bool reclaimable = false; - if (lru_gen_enabled() && root_reclaim(sc)) { + if ((lru_gen_enabled() || lru_gen_draining()) && root_reclaim(sc)) { memset(&sc->nr, 0, sizeof(sc->nr)); lru_gen_shrink_node(pgdat, sc); - return; + + if (!lru_gen_draining()) + return; + } target_lruvec = mem_cgroup_lruvec(sc->target_mem_cgroup, pgdat); @@ -6455,7 +6467,7 @@ static void snapshot_refaults(struct mem_cgroup *target_memcg, pg_data_t *pgdat) struct lruvec *target_lruvec; unsigned long refaults; - if (lru_gen_enabled()) + if (lru_gen_enabled() && !lru_gen_draining()) return; target_lruvec = mem_cgroup_lruvec(target_memcg, pgdat); @@ -6845,9 +6857,12 @@ static void kswapd_age_node(struct pglist_data *pgdat, struct scan_control *sc) struct mem_cgroup *memcg; struct lruvec *lruvec; - if (lru_gen_enabled()) { + if (lru_gen_enabled() || lru_gen_draining()) { lru_gen_age_node(pgdat, sc); - return; + + if (!lru_gen_draining()) + return; + } lruvec = mem_cgroup_lruvec(NULL, pgdat); --- base-commit: 39849a55738542a4cdef8394095ccfa98530e250 change-id: 20260311-b4-switch-mglru-v2-8b926a03843f Best regards, -- Leno Hou