AI Trajectory

null

{
  "EnableConfigs": [
    "TRANSPARENT_HUGEPAGE",
    "USERFAULTFD"
  ],
  "FocusSymbols": [
    "try_collapse_pte_mapped_thp",
    "collapse_pte_mapped_thp",
    "madvise_collapse",
    "collapse_single_pmd"
  ],
  "KMSANReasoning": "The patch adds a check (`userfaultfd_minor(vma)`) in `try_collapse_pte_mapped_thp()` to prevent collapsing PTE-mapped THPs on VMAs registered with userfaultfd-minor mode, along with corresponding selftest updates in `tools/testing/selftests/mm/uffd-unit-tests.c`. \n\nThis modification does not allocate new buffers, manipulate uninitialized stack or heap data, alter padding or structure fields copied to userspace, or change any data lengths/bounds checking that could lead to uninitialized memory reads or leaks. Standard sanitizers and bug detectors (such as KASAN and page table checks) are sufficient for validating this functionality, and no uninitialized memory risks detectable only by KMSAN are introduced.",
  "NeedsKMSAN": false,
  "Reasoning": "The patch modifies khugepaged in core memory management (mm/khugepaged.c) to prevent collapsing PTE-mapped transparent hugepages for VMAs registered with userfaultfd minor fault handling (userfaultfd_minor). This is reachable via userfaultfd syscalls and MADV_COLLAPSE madvise calls or background khugepaged collapse and warrants fuzzing.",
  "WorthFuzzing": true
}

null

{
  "PatchDiff": "commit ee7b65816592db8b99faef2f4923e821dd5bcec1\nAuthor: syz-cluster \u003ctriage@syzkaller.com\u003e\nDate:   Thu Sep 10 02:48:14 2026 +0000\n\n    syz-cluster: applied patch under review\n\ndiff --git a/mm/khugepaged.c b/mm/khugepaged.c\nindex 75639298efc27..e6947fe142eea 100644\n--- a/mm/khugepaged.c\n+++ b/mm/khugepaged.c\n@@ -1894,6 +1894,13 @@ static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign\n \tif (userfaultfd_protected(vma))\n \t\treturn SCAN_PTE_UFFD;\n \n+\t/*\n+\t * Userfaultfd-minor-registered VMAs should not be collapsed, as\n+\t * userspace is expecting to explicitly install PTEs.\n+\t */\n+\tif (userfaultfd_minor(vma))\n+\t\treturn SCAN_PTE_UFFD;\n+\n \tfolio = filemap_lock_folio(vma-\u003evm_file-\u003ef_mapping,\n \t\t\t       linear_page_index(vma, haddr));\n \tif (IS_ERR(folio))\ndiff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c\nindex ef9b3956bdcfd..6f2360f9b75db 100644\n--- a/tools/testing/selftests/mm/uffd-unit-tests.c\n+++ b/tools/testing/selftests/mm/uffd-unit-tests.c\n@@ -518,19 +518,34 @@ static void uffd_wp_fork_pin_with_event_test(uffd_global_test_opts_t *gopts, uff\n \tuffd_wp_fork_pin_test_common(gopts, args, true);\n }\n \n-static void check_memory_contents(uffd_global_test_opts_t *gopts, char *p)\n+static int __check_memory_contents(unsigned long offset,\n+\t\t\t\t   unsigned long nr_pages,\n+\t\t\t\t   uffd_global_test_opts_t *gopts,\n+\t\t\t\t   char *p)\n {\n \tunsigned long i, j;\n \tuint8_t expected_byte;\n \n-\tfor (i = 0; i \u003c gopts-\u003enr_pages; ++i) {\n+\tif (nr_pages + offset \u003c nr_pages)\n+\t\terr(\"overflow in memory check\");\n+\tif (nr_pages + offset \u003e gopts-\u003enr_pages)\n+\t\terr(\"out of bounds memory check\");\n+\n+\tfor (i = offset; i \u003c offset + nr_pages; ++i) {\n \t\texpected_byte = ~((uint8_t)(i % ((uint8_t)-1)));\n \t\tfor (j = 0; j \u003c gopts-\u003epage_size; j++) {\n \t\t\tuint8_t v = *(uint8_t *)(p + (i * gopts-\u003epage_size) + j);\n \t\t\tif (v != expected_byte)\n-\t\t\t\terr(\"unexpected page contents\");\n+\t\t\t\treturn 1;\n \t\t}\n \t}\n+\n+\treturn 0;\n+}\n+\n+static int check_memory_contents(uffd_global_test_opts_t *gopts, char *p)\n+{\n+\treturn __check_memory_contents(0, gopts-\u003enr_pages, gopts, p);\n }\n \n static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_collapse, bool test_wp)\n@@ -539,6 +554,8 @@ static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col\n \tpthread_t uffd_mon;\n \tchar c = '\\0';\n \tstruct uffd_args args = { 0 };\n+\tunsigned long checked = 0;\n+\tbool bad_contents;\n \targs.gopts = gopts;\n \n \t/*\n@@ -564,24 +581,65 @@ static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col\n \tif (pthread_create(\u0026uffd_mon, NULL, uffd_poll_thread, \u0026args))\n \t\terr(\"uffd_poll_thread create\");\n \n+\tif (test_collapse) {\n+\t\t/*\n+\t\t * Read just a single page and try collapsing. The collapse\n+\t\t * should either be rejected or be a no-op.\n+\t\t */\n+\t\tif (__check_memory_contents(0, 1, gopts, gopts-\u003earea_dst_alias))\n+\t\t\terr(\"unexpected memory contents before collapse\");\n+\n+\t\t/* MADV_COLLAPSE might return EINVAL for uffd-minor VMAs. */\n+\t\tmadvise(gopts-\u003earea_dst_alias, gopts-\u003enr_pages * gopts-\u003epage_size,\n+\t\t\tMADV_COLLAPSE);\n+\t\t/*\n+\t\t * If the above collapse mapped pages that were not explicitly\n+\t\t * CONTINUE'd, the below __check_memory_contents() will not\n+\t\t * fault on some pages, resulting in incorrect contents. The\n+\t\t * PTE for the first page may get retracted, so avoid checking\n+\t\t * that page, as we might take a second fault, flipping the\n+\t\t * contents a second time.\n+\t\t */\n+\t\tchecked = 1;\n+\t}\n+\n \t/*\n \t * Read each of the pages back using the UFFD-registered mapping. We\n \t * expect that the first time we touch a page, it will result in a minor\n \t * fault. uffd_poll_thread will resolve the fault by bit-flipping the\n \t * page's contents, and then issuing a CONTINUE ioctl.\n \t */\n-\tcheck_memory_contents(gopts, gopts-\u003earea_dst_alias);\n+\tbad_contents = !!__check_memory_contents(checked, gopts-\u003enr_pages - checked,\n+\t\t\t\t\t\t    gopts, gopts-\u003earea_dst_alias);\n \n \tif (write(gopts-\u003epipefd[1], \u0026c, sizeof(c)) != sizeof(c))\n \t\terr(\"pipe write\");\n \tif (pthread_join(uffd_mon, NULL))\n \t\terr(\"join() failed\");\n \n+\tif (bad_contents \u0026\u0026 test_collapse) {\n+\t\tuffd_test_fail(\"unexpected memory contents after collapse\");\n+\t\treturn;\n+\t}\n+\n+\tif (bad_contents) {\n+\t\tuffd_test_fail(\"unexpected memory contents\");\n+\t\treturn;\n+\t}\n+\n \tif (test_collapse) {\n+\t\t/*\n+\t\t * MADV_COLLAPSE will fail unless userfaultfd-minor is\n+\t\t * unregistered.\n+\t\t */\n+\t\tif (uffd_unregister(gopts-\u003euffd, gopts-\u003earea_dst_alias,\n+\t\t\t\t    gopts-\u003enr_pages * gopts-\u003epage_size))\n+\t\t\terr(\"uffd_unregister before MADV_COLLAPSE failed\");\n+\n+\t\t/* MADV_COLLAPSE should succeed with userfaultfd unregistered. */\n \t\tif (madvise(gopts-\u003earea_dst_alias, gopts-\u003enr_pages * gopts-\u003epage_size,\n \t\t\t    MADV_COLLAPSE)) {\n-\t\t\t/* It's fine to fail for this one... */\n-\t\t\tuffd_test_skip(\"MADV_COLLAPSE failed\");\n+\t\t\tuffd_test_fail(\"MADV_COLLAPSE failed\");\n \t\t\treturn;\n \t\t}\n \n@@ -593,7 +651,10 @@ static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col\n \t\t * This won't cause uffd-fault - it purely just makes sure there\n \t\t * was no corruption.\n \t\t */\n-\t\tcheck_memory_contents(gopts, gopts-\u003earea_dst_alias);\n+\t\tif (check_memory_contents(gopts, gopts-\u003earea_dst_alias)) {\n+\t\t\tuffd_test_fail(\"unexpected memory contents\");\n+\t\t\treturn;\n+\t\t}\n \t}\n \n \tif (args.missing_faults != 0 || args.minor_faults != gopts-\u003enr_pages)\n"
}

core

null

{
  "EnableConfigs": [
    "TRANSPARENT_HUGEPAGE",
    "USERFAULTFD"
  ],
  "FocusSymbols": [
    "try_collapse_pte_mapped_thp",
    "collapse_pte_mapped_thp",
    "madvise_collapse",
    "collapse_single_pmd"
  ],
  "Reasoning": "The patch modifies khugepaged in core memory management (mm/khugepaged.c) to prevent collapsing PTE-mapped transparent hugepages for VMAs registered with userfaultfd minor fault handling (userfaultfd_minor). This is reachable via userfaultfd syscalls and MADV_COLLAPSE madvise calls or background khugepaged collapse and warrants fuzzing.",
  "WorthFuzzing": true
}

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.

Target architecture: amd64

For your convenience, here is the diff of the changes:
commit ee7b65816592db8b99faef2f4923e821dd5bcec1
Author: syz-cluster <triage@syzkaller.com>
Date:   Thu Sep 10 02:48:14 2026 +0000

    syz-cluster: applied patch under review

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 75639298efc27..e6947fe142eea 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1894,6 +1894,13 @@ static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign
 	if (userfaultfd_protected(vma))
 		return SCAN_PTE_UFFD;
 
+	/*
+	 * Userfaultfd-minor-registered VMAs should not be collapsed, as
+	 * userspace is expecting to explicitly install PTEs.
+	 */
+	if (userfaultfd_minor(vma))
+		return SCAN_PTE_UFFD;
+
 	folio = filemap_lock_folio(vma->vm_file->f_mapping,
 			       linear_page_index(vma, haddr));
 	if (IS_ERR(folio))
diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
index ef9b3956bdcfd..6f2360f9b75db 100644
--- a/tools/testing/selftests/mm/uffd-unit-tests.c
+++ b/tools/testing/selftests/mm/uffd-unit-tests.c
@@ -518,19 +518,34 @@ static void uffd_wp_fork_pin_with_event_test(uffd_global_test_opts_t *gopts, uff
 	uffd_wp_fork_pin_test_common(gopts, args, true);
 }
 
-static void check_memory_contents(uffd_global_test_opts_t *gopts, char *p)
+static int __check_memory_contents(unsigned long offset,
+				   unsigned long nr_pages,
+				   uffd_global_test_opts_t *gopts,
+				   char *p)
 {
 	unsigned long i, j;
 	uint8_t expected_byte;
 
-	for (i = 0; i < gopts->nr_pages; ++i) {
+	if (nr_pages + offset < nr_pages)
+		err("overflow in memory check");
+	if (nr_pages + offset > gopts->nr_pages)
+		err("out of bounds memory check");
+
+	for (i = offset; i < offset + nr_pages; ++i) {
 		expected_byte = ~((uint8_t)(i % ((uint8_t)-1)));
 		for (j = 0; j < gopts->page_size; j++) {
 			uint8_t v = *(uint8_t *)(p + (i * gopts->page_size) + j);
 			if (v != expected_byte)
-				err("unexpected page contents");
+				return 1;
 		}
 	}
+
+	return 0;
+}
+
+static int check_memory_contents(uffd_global_test_opts_t *gopts, char *p)
+{
+	return __check_memory_contents(0, gopts->nr_pages, gopts, p);
 }
 
 static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_collapse, bool test_wp)
@@ -539,6 +554,8 @@ static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col
 	pthread_t uffd_mon;
 	char c = '\0';
 	struct uffd_args args = { 0 };
+	unsigned long checked = 0;
+	bool bad_contents;
 	args.gopts = gopts;
 
 	/*
@@ -564,24 +581,65 @@ static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col
 	if (pthread_create(&uffd_mon, NULL, uffd_poll_thread, &args))
 		err("uffd_poll_thread create");
 
+	if (test_collapse) {
+		/*
+		 * Read just a single page and try collapsing. The collapse
+		 * should either be rejected or be a no-op.
+		 */
+		if (__check_memory_contents(0, 1, gopts, gopts->area_dst_alias))
+			err("unexpected memory contents before collapse");
+
+		/* MADV_COLLAPSE might return EINVAL for uffd-minor VMAs. */
+		madvise(gopts->area_dst_alias, gopts->nr_pages * gopts->page_size,
+			MADV_COLLAPSE);
+		/*
+		 * If the above collapse mapped pages that were not explicitly
+		 * CONTINUE'd, the below __check_memory_contents() will not
+		 * fault on some pages, resulting in incorrect contents. The
+		 * PTE for the first page may get retracted, so avoid checking
+		 * that page, as we might take a second fault, flipping the
+		 * contents a second time.
+		 */
+		checked = 1;
+	}
+
 	/*
 	 * Read each of the pages back using the UFFD-registered mapping. We
 	 * expect that the first time we touch a page, it will result in a minor
 	 * fault. uffd_poll_thread will resolve the fault by bit-flipping the
 	 * page's contents, and then issuing a CONTINUE ioctl.
 	 */
-	check_memory_contents(gopts, gopts->area_dst_alias);
+	bad_contents = !!__check_memory_contents(checked, gopts->nr_pages - checked,
+						    gopts, gopts->area_dst_alias);
 
 	if (write(gopts->pipefd[1], &c, sizeof(c)) != sizeof(c))
 		err("pipe write");
 	if (pthread_join(uffd_mon, NULL))
 		err("join() failed");
 
+	if (bad_contents && test_collapse) {
+		uffd_test_fail("unexpected memory contents after collapse");
+		return;
+	}
+
+	if (bad_contents) {
+		uffd_test_fail("unexpected memory contents");
+		return;
+	}
+
 	if (test_collapse) {
+		/*
+		 * MADV_COLLAPSE will fail unless userfaultfd-minor is
+		 * unregistered.
+		 */
+		if (uffd_unregister(gopts->uffd, gopts->area_dst_alias,
+				    gopts->nr_pages * gopts->page_size))
+			err("uffd_unregister before MADV_COLLAPSE failed");
+
+		/* MADV_COLLAPSE should succeed with userfaultfd unregistered. */
 		if (madvise(gopts->area_dst_alias, gopts->nr_pages * gopts->page_size,
 			    MADV_COLLAPSE)) {
-			/* It's fine to fail for this one... */
-			uffd_test_skip("MADV_COLLAPSE failed");
+			uffd_test_fail("MADV_COLLAPSE failed");
 			return;
 		}
 
@@ -593,7 +651,10 @@ static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col
 		 * This won't cause uffd-fault - it purely just makes sure there
 		 * was no corruption.
 		 */
-		check_memory_contents(gopts, gopts->area_dst_alias);
+		if (check_memory_contents(gopts, gopts->area_dst_alias)) {
+			uffd_test_fail("unexpected memory contents");
+			return;
+		}
 	}
 
 	if (args.missing_faults != 0 || args.minor_faults != gopts->nr_pages)

gemini-3.7-flash

null

null

							input: 4713
							output: 37
							thoughts: 291
						

**Analyzing the Patch for Khugepaged and Userfaultfd Interaction**

Okay, so I've got this patch in front of me, and my initial thought is to dive right into the code to understand the implications. The patch touches `mm/khugepaged.c` and the `tools/testing/selftests/mm/uffd-unit-tests.c` file. Right away, my radar goes up – anything that interacts with `khugepaged` and potentially userfaultfd (UFFD) is going to need careful consideration, especially with the performance implications of khugepage and the explicit memory management from UFFD.

Focusing on `mm/khugepaged.c`, the key change appears within the `try_collapse_pte_mapped_thp` function. It's adding an extra check here. Let me see... it's about protecting against collapsing THPs when UFFD is involved. Specifically, it's guarding against collapsing Transparent Huge Pages (THPs) on `userfaultfd_minor`-registered VMAs. I immediately understand the reasoning here. If a VMA is registered as a "minor" UFFD region, the user-space application is explicitly managing the PTEs for that region. Collapsing THPs in this case would interfere with the application's expected behavior and potentially lead to corruption or unexpected page faults. This makes complete sense - THP collapse might try to install a PTE behind the application's back. This addition is clearly designed to maintain the intended semantics of UFFD and prevent conflicts with `khugepaged`. It's a solid, cautious approach to keep the different areas of memory management safe and operational.

I'm now going to use `grepper` or `read-file` to explore the `try_collapse_pte_mapped_thp` function in more detail. I need to fully grok the interaction points in the codebase.

{
  "File": "mm/khugepaged.c",
  "FirstLine": 1880,
  "LineCount": 50
}

{
  "Contents": "1880:\t\t/*\n1881:\t\t * If we are here, we've succeeded in replacing all the native pages\n1882:\t\t * in the page cache with a single hugepage. If a mm were to fault-in\n1883:\t\t * this memory (mapped by a suitably aligned VMA), we'd get the hugepage\n1884:\t\t * and map it by a PMD, regardless of sysfs THP settings. As such, let's\n1885:\t\t * analogously elide sysfs THP settings here and force collapse.\n1886:\t\t */\n1887:\t\tif (!thp_vma_allowable_order(vma, vma-\u003evm_flags, TVA_FORCED_COLLAPSE, PMD_ORDER))\n1888:\t\t\treturn SCAN_VMA_CHECK;\n1889:\t\n1890:\t\t/*\n1891:\t\t * Keep pmd pgtable while the uffd bit is in use; see comment in\n1892:\t\t * retract_page_tables().\n1893:\t\t */\n1894:\t\tif (userfaultfd_protected(vma))\n1895:\t\t\treturn SCAN_PTE_UFFD;\n1896:\t\n1897:\t\t/*\n1898:\t\t * Userfaultfd-minor-registered VMAs should not be collapsed, as\n1899:\t\t * userspace is expecting to explicitly install PTEs.\n1900:\t\t */\n1901:\t\tif (userfaultfd_minor(vma))\n1902:\t\t\treturn SCAN_PTE_UFFD;\n1903:\t\n1904:\t\tfolio = filemap_lock_folio(vma-\u003evm_file-\u003ef_mapping,\n1905:\t\t\t\t       linear_page_index(vma, haddr));\n1906:\t\tif (IS_ERR(folio))\n1907:\t\t\treturn SCAN_PAGE_NULL;\n1908:\t\n1909:\t\tif (!is_pmd_order(folio_order(folio))) {\n1910:\t\t\tresult = SCAN_PAGE_COMPOUND;\n1911:\t\t\tgoto drop_folio;\n1912:\t\t}\n1913:\t\n1914:\t\tresult = find_pmd_or_thp_or_none(mm, haddr, \u0026pmd);\n1915:\t\tswitch (result) {\n1916:\t\tcase SCAN_SUCCEED:\n1917:\t\t\tbreak;\n1918:\t\tcase SCAN_NO_PTE_TABLE:\n1919:\t\t\t/*\n1920:\t\t\t * All pte entries have been removed and pmd cleared.\n1921:\t\t\t * Skip all the pte checks and just update the pmd mapping.\n1922:\t\t\t */\n1923:\t\t\tgoto maybe_install_pmd;\n1924:\t\tdefault:\n1925:\t\t\tgoto drop_folio;\n1926:\t\t}\n1927:\t\n1928:\t\tresult = SCAN_FAIL;\n1929:\t\tstart_pte = pte_offset_map_lock(mm, pmd, haddr, \u0026ptl);\n"
}

gemini-3.7-flash

null

null

							input: 6252
							output: 32
							thoughts: 37
						

Here's my summary, as if I were thinking these things:

**Analyzing `try_collapse_pte_mapped_thp` and its Surroundings**

Okay, time to dig into `try_collapse_pte_mapped_thp`. First, I need to get a clear picture of its context. Where is this function actually being *used*? I'll check its call sites to understand the scenarios in which it's triggered. Knowing the callers will tell me the high-level logic it’s a part of and the specific events that lead to this part of the code running.

Then, I'll turn my attention to the configuration guards around `mm/khugepaged.c`. What compile-time options are influencing the availability and behavior of this code? I'm thinking I'll need to identify any dependencies on things like Transparent Huge Pages (THP) support or specific memory management features. Understanding the relevant configs is critical to knowing the scope of functionality I am working with. What is conditional, and what is always running? The configs are the key to answering that.

{
  "Expression": "try_collapse_pte_mapped_thp",
  "PathPrefix": "mm/"
}

{
  "Output": "mm/khugepaged.c=1815=static enum scan_result set_huge_pmd(struct vm_area_struct *vma, unsigned long addr,\n--\nmm/khugepaged.c-1850-\nmm/khugepaged.c:1851:static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,\nmm/khugepaged.c-1852-\t\tbool install_pmd)\n--\nmm/khugepaged.c=2096=void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,\n--\nmm/khugepaged.c-2098-{\nmm/khugepaged.c:2099:\ttry_collapse_pte_mapped_thp(mm, addr, install_pmd);\nmm/khugepaged.c-2100-}\n--\nmm/khugepaged.c=2249=static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,\n--\nmm/khugepaged.c-2628-\t * Remove pte page tables, so we can re-fault the page as huge.\nmm/khugepaged.c:2629:\t * If MADV_COLLAPSE, adjust result to call try_collapse_pte_mapped_thp().\nmm/khugepaged.c-2630-\t */\n--\nmm/khugepaged.c=2791=static enum scan_result collapse_single_pmd(unsigned long addr,\n--\nmm/khugepaged.c-2835-\t\telse\nmm/khugepaged.c:2836:\t\t\tresult = try_collapse_pte_mapped_thp(mm, addr,\nmm/khugepaged.c-2837-\t\t\t\t\t\t\t     !cc-\u003eis_khugepaged);\n"
}

gemini-3.7-flash

null

null

							input: 7033
							output: 30
							thoughts: 62
						

Okay, here's my thought process on this, distilled into a first-person summary as you requested, assuming I'm already well-versed in kernel memory management:

**Memory Management Deep Dive**

Alright, time to dig in. I need to understand what's going on with these memory management functions. My immediate focus is on how memory is being handled related to transparent hugepages (THP) and `madvise` calls.

First, I'm going to investigate the `madvise_collapse` or the `madvise` entry point itself. I want to see precisely how these functions are being used and understand their exact purpose in collapsing or otherwise managing memory regions. I also need to track down any and all calls to the `khugepaged` process because its relationship with THPs is vital to grasp.

Next, I'll shift gears and trace the callers of these key functions: `collapse_pte_mapped_thp`, `collapse_single_pmd`, and, again, `madvise_collapse`. Identifying where these are being called from will paint a complete picture of the memory manipulation workflow. Seeing the call chains will give me insight into the triggering mechanisms. I'm after the bigger picture: what scenarios are causing THP collapse, what code paths do the functions take, and how does `madvise` influence the whole process? I need to understand the control flow.

{
  "Expression": "collapse_pte_mapped_thp",
  "PathPrefix": "mm/"
}

{
  "Output": "mm/khugepaged.c=1815=static enum scan_result set_huge_pmd(struct vm_area_struct *vma, unsigned long addr,\n--\nmm/khugepaged.c-1850-\nmm/khugepaged.c:1851:static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,\nmm/khugepaged.c-1852-\t\tbool install_pmd)\n--\nmm/khugepaged.c-2084-/**\nmm/khugepaged.c:2085: * collapse_pte_mapped_thp - Try to collapse a pte-mapped THP for mm at\nmm/khugepaged.c-2086- * address haddr.\n--\nmm/khugepaged.c-2095- */\nmm/khugepaged.c:2096:void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,\nmm/khugepaged.c-2097-\t\tbool install_pmd)\nmm/khugepaged.c-2098-{\nmm/khugepaged.c:2099:\ttry_collapse_pte_mapped_thp(mm, addr, install_pmd);\nmm/khugepaged.c-2100-}\n--\nmm/khugepaged.c=2249=static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,\n--\nmm/khugepaged.c-2628-\t * Remove pte page tables, so we can re-fault the page as huge.\nmm/khugepaged.c:2629:\t * If MADV_COLLAPSE, adjust result to call try_collapse_pte_mapped_thp().\nmm/khugepaged.c-2630-\t */\n--\nmm/khugepaged.c=2791=static enum scan_result collapse_single_pmd(unsigned long addr,\n--\nmm/khugepaged.c-2835-\t\telse\nmm/khugepaged.c:2836:\t\t\tresult = try_collapse_pte_mapped_thp(mm, addr,\nmm/khugepaged.c-2837-\t\t\t\t\t\t\t     !cc-\u003eis_khugepaged);\n"
}

gemini-3.7-flash

null

null

							input: 8016
							output: 22
							thoughts: 21
						

Here's my attempt at summarizing the thought process, tailored for an expert audience:

**Investigating `collapse_pte_mapped_thp` Call Sites**

Alright, time to get to work. My immediate focus is on understanding the scope of the `collapse_pte_mapped_thp` function. I need to figure out everywhere it's *used* within the kernel, and the most efficient way to do that is, of course, to start by tracing those call sites. That’s the first step to understanding its impact and potential interactions with other subsystems. I'll need to figure out the different contexts it is used in to get the full picture.

{
  "Expression": "collapse_pte_mapped_thp"
}

{
  "Output": "include/linux/khugepaged.h=19=extern bool current_is_khugepaged(void);\ninclude/linux/khugepaged.h:20:void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,\ninclude/linux/khugepaged.h-21-\t\tbool install_pmd);\n--\ninclude/linux/khugepaged.h=41=static inline void khugepaged_enter_vma(struct vm_area_struct *vma,\n--\ninclude/linux/khugepaged.h-44-}\ninclude/linux/khugepaged.h:45:static inline void collapse_pte_mapped_thp(struct mm_struct *mm,\ninclude/linux/khugepaged.h-46-\t\tunsigned long addr, bool install_pmd)\n--\nkernel/events/uprobes.c=499=int uprobe_write(struct arch_uprobe *auprobe, struct vm_area_struct *vma,\n--\nkernel/events/uprobes.c-599-\tif (ret \u003e 0)\nkernel/events/uprobes.c:600:\t\tcollapse_pte_mapped_thp(mm, vaddr, false);\nkernel/events/uprobes.c-601-\n--\nmm/khugepaged.c=1815=static enum scan_result set_huge_pmd(struct vm_area_struct *vma, unsigned long addr,\n--\nmm/khugepaged.c-1850-\nmm/khugepaged.c:1851:static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,\nmm/khugepaged.c-1852-\t\tbool install_pmd)\n--\nmm/khugepaged.c-2084-/**\nmm/khugepaged.c:2085: * collapse_pte_mapped_thp - Try to collapse a pte-mapped THP for mm at\nmm/khugepaged.c-2086- * address haddr.\n--\nmm/khugepaged.c-2095- */\nmm/khugepaged.c:2096:void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,\nmm/khugepaged.c-2097-\t\tbool install_pmd)\nmm/khugepaged.c-2098-{\nmm/khugepaged.c:2099:\ttry_collapse_pte_mapped_thp(mm, addr, install_pmd);\nmm/khugepaged.c-2100-}\n--\nmm/khugepaged.c=2249=static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,\n--\nmm/khugepaged.c-2628-\t * Remove pte page tables, so we can re-fault the page as huge.\nmm/khugepaged.c:2629:\t * If MADV_COLLAPSE, adjust result to call try_collapse_pte_mapped_thp().\nmm/khugepaged.c-2630-\t */\n--\nmm/khugepaged.c=2791=static enum scan_result collapse_single_pmd(unsigned long addr,\n--\nmm/khugepaged.c-2835-\t\telse\nmm/khugepaged.c:2836:\t\t\tresult = try_collapse_pte_mapped_thp(mm, addr,\nmm/khugepaged.c-2837-\t\t\t\t\t\t\t     !cc-\u003eis_khugepaged);\n"
}

gemini-3.7-flash

null

null

							input: 9112
							output: 19
							thoughts: 41
						

**Investigating Memory Management Techniques: Collapsing Memory Regions**

Okay, let's dive in and figure out exactly what's using `madvise_collapse` or, more generally, `madvise` in conjunction with `MADV_COLLAPSE`. My initial plan is pretty straightforward – I'll start with a good old `grep` search for instances of `madvise_collapse` within the relevant codebase. This will be the first step to understand who is using it.

{
  "Expression": "madvise_collapse"
}

{
  "Output": "Documentation/admin-guide/mm/transhuge.rst=346=A higher value may increase memory footprint for some workloads.\n--\nDocumentation/admin-guide/mm/transhuge.rst-353-\nDocumentation/admin-guide/mm/transhuge.rst:354:   Currently, madvise_collapse only supports collapsing to PMD-sized THPs\nDocumentation/admin-guide/mm/transhuge.rst-355-   and does not attempt mTHP collapses.\n--\ninclude/linux/huge_mm.h=511=int hugepage_madvise(struct vm_area_struct *vma, vm_flags_t *vm_flags,\ninclude/linux/huge_mm.h-512-\t\t     int advice);\ninclude/linux/huge_mm.h:513:int madvise_collapse(struct vm_area_struct *vma, unsigned long start,\ninclude/linux/huge_mm.h-514-\t\t     unsigned long end, bool *lock_dropped);\n--\ninclude/linux/huge_mm.h=712=static inline int hugepage_madvise(struct vm_area_struct *vma,\n--\ninclude/linux/huge_mm.h-717-\ninclude/linux/huge_mm.h:718:static inline int madvise_collapse(struct vm_area_struct *vma,\ninclude/linux/huge_mm.h-719-\t\t\t\t   unsigned long start,\n--\nmm/khugepaged.c=3152=bool current_is_khugepaged(void)\n--\nmm/khugepaged.c-3156-\nmm/khugepaged.c:3157:static int madvise_collapse_errno(enum scan_result r)\nmm/khugepaged.c-3158-{\n--\nmm/khugepaged.c-3188-\nmm/khugepaged.c:3189:int madvise_collapse(struct vm_area_struct *vma, unsigned long start,\nmm/khugepaged.c-3190-\t\t     unsigned long end, bool *lock_dropped)\n--\nmm/khugepaged.c-3276-\treturn thps == ((hend - hstart) \u003e\u003e HPAGE_PMD_SHIFT) ? 0\nmm/khugepaged.c:3277:\t\t\t: madvise_collapse_errno(last_fail);\nmm/khugepaged.c-3278-}\n--\nmm/madvise.c=1347=static int madvise_vma_behavior(struct madvise_behavior *madv_behavior)\n--\nmm/madvise.c-1371-\tcase MADV_COLLAPSE:\nmm/madvise.c:1372:\t\treturn madvise_collapse(vma, range-\u003estart, range-\u003eend,\nmm/madvise.c-1373-\t\t\t\u0026madv_behavior-\u003elock_dropped);\n--\ntools/testing/selftests/mm/khugepaged.c=235=static void fill_memory(int *p, unsigned long start, unsigned long end)\n--\ntools/testing/selftests/mm/khugepaged.c-248- */\ntools/testing/selftests/mm/khugepaged.c:249:static int madvise_collapse_retry(void *p, unsigned long size)\ntools/testing/selftests/mm/khugepaged.c-250-{\n--\ntools/testing/selftests/mm/khugepaged.c=267=static void *alloc_hpage(struct mem_ops *ops)\n--\ntools/testing/selftests/mm/khugepaged.c-280-\tksft_print_msg(\"Allocate huge page...\");\ntools/testing/selftests/mm/khugepaged.c:281:\tif (madvise_collapse_retry(p, hpage_pmd_size))\ntools/testing/selftests/mm/khugepaged.c-282-\t\tksft_exit_fail_perror(\"madvise(MADV_COLLAPSE)\");\n--\ntools/testing/selftests/mm/khugepaged.c=511=static bool is_anon(struct mem_ops *ops)\n--\ntools/testing/selftests/mm/khugepaged.c-515-\ntools/testing/selftests/mm/khugepaged.c:516:static void __madvise_collapse(const char *msg, char *p, int nr_hpages,\ntools/testing/selftests/mm/khugepaged.c-517-\t\t\t       struct mem_ops *ops, bool expect)\n--\ntools/testing/selftests/mm/khugepaged.c-539-\tmadvise(p, nr_hpages * hpage_pmd_size, MADV_HUGEPAGE);\ntools/testing/selftests/mm/khugepaged.c:540:\tret = madvise_collapse_retry(p, nr_hpages * hpage_pmd_size);\ntools/testing/selftests/mm/khugepaged.c-541-\tif (((bool)ret) == expect)\n--\ntools/testing/selftests/mm/khugepaged.c-550-\ntools/testing/selftests/mm/khugepaged.c:551:static void madvise_collapse(const char *msg, char *p, int nr_hpages,\ntools/testing/selftests/mm/khugepaged.c-552-\t\t\t     struct mem_ops *ops, bool expect)\n--\ntools/testing/selftests/mm/khugepaged.c-556-\t\tksft_exit_fail_msg(\"Unexpected huge page\\n\");\ntools/testing/selftests/mm/khugepaged.c:557:\t__madvise_collapse(msg, p, nr_hpages, ops, expect);\ntools/testing/selftests/mm/khugepaged.c-558-}\n--\ntools/testing/selftests/mm/khugepaged.c=680=static struct collapse_context __madvise_context = {\ntools/testing/selftests/mm/khugepaged.c:681:\t.collapse = \u0026madvise_collapse,\ntools/testing/selftests/mm/khugepaged.c-682-\t.enforce_pte_scan_limits = false,\n--\ntools/testing/selftests/mm/khugepaged.c=1076=static void collapse_max_ptes_shared(struct collapse_context *c, struct mem_ops *ops)\n--\ntools/testing/selftests/mm/khugepaged.c-1133-\ntools/testing/selftests/mm/khugepaged.c:1134:static void madvise_collapse_existing_thps(struct collapse_context *c,\ntools/testing/selftests/mm/khugepaged.c-1135-\t\t\t\t\t   struct mem_ops *ops)\n--\ntools/testing/selftests/mm/khugepaged.c-1144-\t/* c-\u003ecollapse() will find a hugepage and complain - call directly. */\ntools/testing/selftests/mm/khugepaged.c:1145:\t__madvise_collapse(\"Re-collapse PMD-mapped hugepage\", p, 1, ops, true);\ntools/testing/selftests/mm/khugepaged.c-1146-\tvalidate_memory(p, 0, hpage_pmd_size);\n--\ntools/testing/selftests/mm/khugepaged.c=1311=int main(int argc, char **argv)\n--\ntools/testing/selftests/mm/khugepaged.c-1432-\ntools/testing/selftests/mm/khugepaged.c:1433:\tTEST(madvise_collapse_existing_thps, madvise_context, anon_ops);\ntools/testing/selftests/mm/khugepaged.c:1434:\tTEST(madvise_collapse_existing_thps, madvise_context, read_only_file_ops);\ntools/testing/selftests/mm/khugepaged.c:1435:\tTEST(madvise_collapse_existing_thps, madvise_context, read_write_file_read_ops);\ntools/testing/selftests/mm/khugepaged.c:1436:\tTEST(madvise_collapse_existing_thps, madvise_context, shmem_ops);\ntools/testing/selftests/mm/khugepaged.c-1437-\n"
}

gemini-3.7-flash

null

null

							input: 11440
							output: 26
							thoughts: 51
						

Okay, here's my take on those thoughts, framed as my own:

**Investigating Kernel Huge Page Configuration**

Alright, time to dig into how `khugepaged` and that pesky `userfaultfd_minor` are configured. I need to understand the relationship between these things. First, I'll go straight to the source – the Kconfig files. I'll need to see how the settings in `khugepaged.c` and `userfaultfd_minor` are controlled. Let's start with a focused grep. I'll search for references to `khugepaged` in the `mm/Makefile` and `mm/Kconfig` files. That should give me a good overview of the build dependencies and configuration options. From there, I'll be able to build a mental map of how these features are enabled, disabled, and potentially tuned. This Kconfig exploration should shed light on the build-time dependencies and perhaps some of the broader architecture involved in handling huge pages and user fault handling.

{
  "Expression": "khugepaged",
  "PathPrefix": "mm/"
}

{
  "Output": "\nFull output is too long, showing 500 out of 862 lines.\nUse more precise expression if possible.\n\nmm/Makefile=100=obj-$(CONFIG_DEVICE_MIGRATION) += migrate_device.o\nmm/Makefile:101:obj-$(CONFIG_TRANSPARENT_HUGEPAGE) += huge_memory.o khugepaged.o\nmm/Makefile-102-obj-$(CONFIG_PAGE_COUNTER) += page_counter.o\n--\nmm/huge_memory.c-23-#include \u003clinux/mm_types.h\u003e\nmm/huge_memory.c:24:#include \u003clinux/khugepaged.h\u003e\nmm/huge_memory.c-25-#include \u003clinux/freezer.h\u003e\n--\nmm/huge_memory.c-56- * guaranteed to benefit from it. When transparent hugepage support is\nmm/huge_memory.c:57: * enabled, it is for all mappings, and khugepaged scans all mappings.\nmm/huge_memory.c:58: * Defrag is invoked by khugepaged hugepage allocations and by page faults\nmm/huge_memory.c-59- * for all hugepage allocations.\n--\nmm/huge_memory.c=121=unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,\n--\nmm/huge_memory.c-148-\nmm/huge_memory.c:149:\t/* khugepaged doesn't collapse DAX vma, but page fault is fine. */\nmm/huge_memory.c-150-\tif (vma_is_dax(vma))\n--\nmm/huge_memory.c-153-\t/*\nmm/huge_memory.c:154:\t * khugepaged special VMA and hugetlb VMA.\nmm/huge_memory.c-155-\t * Must be checked after dax since some dax mappings may have\n--\nmm/huge_memory.c=445=static ssize_t enabled_store(struct kobject *kobj,\n--\nmm/huge_memory.c-455-\tif (set_global_enabled_mode(mode)) {\nmm/huge_memory.c:456:\t\tint err = start_stop_khugepaged();\nmm/huge_memory.c-457-\n--\nmm/huge_memory.c-463-\t\t * change, as the previous code always called\nmm/huge_memory.c:464:\t\t * start_stop_khugepaged() which does this internally.\nmm/huge_memory.c-465-\t\t */\n--\nmm/huge_memory.c=680=static ssize_t anon_enabled_store(struct kobject *kobj,\n--\nmm/huge_memory.c-691-\tif (set_anon_enabled_mode(order, mode)) {\nmm/huge_memory.c:692:\t\tint err = start_stop_khugepaged();\nmm/huge_memory.c-693-\n--\nmm/huge_memory.c-699-\t\t * change, as the previous code always called\nmm/huge_memory.c:700:\t\t * start_stop_khugepaged() which does this internally.\nmm/huge_memory.c-701-\t\t */\n--\nmm/huge_memory.c=933=static int __init hugepage_init_sysfs(struct kobject **hugepage_kobj)\n--\nmm/huge_memory.c-959-\nmm/huge_memory.c:960:\terr = sysfs_create_group(*hugepage_kobj, \u0026khugepaged_attr_group);\nmm/huge_memory.c-961-\tif (err) {\n--\nmm/huge_memory.c=991=static void __init hugepage_exit_sysfs(struct kobject *hugepage_kobj)\n--\nmm/huge_memory.c-999-\nmm/huge_memory.c:1000:\tsysfs_remove_group(hugepage_kobj, \u0026khugepaged_attr_group);\nmm/huge_memory.c-1001-\tsysfs_remove_group(hugepage_kobj, \u0026hugepage_attr_group);\n--\nmm/huge_memory.c=1051=static int __init hugepage_init(void)\n--\nmm/huge_memory.c-1069-\nmm/huge_memory.c:1070:\terr = khugepaged_init();\nmm/huge_memory.c-1071-\tif (err)\n--\nmm/huge_memory.c-1087-\nmm/huge_memory.c:1088:\terr = start_stop_khugepaged();\nmm/huge_memory.c-1089-\tif (err)\nmm/huge_memory.c:1090:\t\tgoto err_khugepaged;\nmm/huge_memory.c-1091-\nmm/huge_memory.c-1092-\treturn 0;\nmm/huge_memory.c:1093:err_khugepaged:\nmm/huge_memory.c-1094-\tthp_shrinker_exit();\nmm/huge_memory.c-1095-err_shrinker:\nmm/huge_memory.c:1096:\tkhugepaged_destroy();\nmm/huge_memory.c-1097-err_slab:\n--\nmm/huge_memory.c=1520=vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)\n--\nmm/huge_memory.c-1530-\t\treturn ret;\nmm/huge_memory.c:1531:\tkhugepaged_enter_vma(vma, vma-\u003evm_flags);\nmm/huge_memory.c-1532-\n--\nmm/huge_memory.c=4561=static bool thp_underused(struct folio *folio)\n--\nmm/huge_memory.c-4565-\nmm/huge_memory.c:4566:\tif (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1)\nmm/huge_memory.c-4567-\t\treturn false;\n--\nmm/huge_memory.c-4573-\t\tif (pages_identical(folio_page(folio, i), ZERO_PAGE(0))) {\nmm/huge_memory.c:4574:\t\t\tif (++num_zero_pages \u003e khugepaged_max_ptes_none)\nmm/huge_memory.c-4575-\t\t\t\treturn true;\n--\nmm/huge_memory.c-4580-\t\t\t */\nmm/huge_memory.c:4581:\t\t\tif (++num_filled_pages \u003e= HPAGE_PMD_NR - khugepaged_max_ptes_none)\nmm/huge_memory.c-4582-\t\t\t\treturn false;\n--\nmm/internal.h-10-#include \u003clinux/fs.h\u003e\nmm/internal.h:11:#include \u003clinux/khugepaged.h\u003e\nmm/internal.h-12-#include \u003clinux/mm.h\u003e\n--\nmm/internal.h=706=pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address);\n--\nmm/internal.h-708-/*\nmm/internal.h:709: * in mm/khugepaged.c\nmm/internal.h-710- */\n--\nmm/khugepaged.c-11-#include \u003clinux/kthread.h\u003e\nmm/khugepaged.c:12:#include \u003clinux/khugepaged.h\u003e\nmm/khugepaged.c-13-#include \u003clinux/freezer.h\u003e\n--\nmm/khugepaged.c=33=enum scan_result {\n--\nmm/khugepaged.c-70-\nmm/khugepaged.c:71:static struct task_struct *khugepaged_thread __read_mostly;\nmm/khugepaged.c:72:static DEFINE_MUTEX(khugepaged_mutex);\nmm/khugepaged.c-73-\n--\nmm/khugepaged.c-77- */\nmm/khugepaged.c:78:static unsigned int khugepaged_pages_to_scan __read_mostly;\nmm/khugepaged.c:79:static unsigned int khugepaged_pages_collapsed;\nmm/khugepaged.c:80:static unsigned int khugepaged_full_scans;\nmm/khugepaged.c:81:static unsigned int khugepaged_scan_sleep_millisecs __read_mostly = 10000;\nmm/khugepaged.c-82-/* during fragmentation poll the hugepage allocator once every minute */\nmm/khugepaged.c:83:static unsigned int khugepaged_alloc_sleep_millisecs __read_mostly = 60000;\nmm/khugepaged.c:84:static unsigned long khugepaged_sleep_expire;\nmm/khugepaged.c:85:static DEFINE_SPINLOCK(khugepaged_mm_lock);\nmm/khugepaged.c:86:static DECLARE_WAIT_QUEUE_HEAD(khugepaged_wait);\nmm/khugepaged.c-87-/*\n--\nmm/khugepaged.c-91- *\nmm/khugepaged.c:92: * Note that these are only respected if collapse was initiated by khugepaged.\nmm/khugepaged.c-93- */\nmm/khugepaged.c-94-#define KHUGEPAGED_MAX_PTES_LIMIT (HPAGE_PMD_NR - 1)\nmm/khugepaged.c:95:unsigned int khugepaged_max_ptes_none __read_mostly;\nmm/khugepaged.c:96:static unsigned int khugepaged_max_ptes_swap __read_mostly;\nmm/khugepaged.c:97:static unsigned int khugepaged_max_ptes_shared __read_mostly;\nmm/khugepaged.c-98-\n--\nmm/khugepaged.c=106=struct collapse_control {\nmm/khugepaged.c:107:\tbool is_khugepaged;\nmm/khugepaged.c-108-\n--\nmm/khugepaged.c-111-\nmm/khugepaged.c:112:\t/* Num pages scanned (see khugepaged_pages_to_scan) */\nmm/khugepaged.c-113-\tunsigned int progress;\n--\nmm/khugepaged.c-122-/**\nmm/khugepaged.c:123: * struct khugepaged_scan - cursor for scanning\nmm/khugepaged.c-124- * @mm_head: the head of the mm list to scan\n--\nmm/khugepaged.c-127- *\nmm/khugepaged.c:128: * There is only the one khugepaged_scan instance of this cursor structure.\nmm/khugepaged.c-129- */\nmm/khugepaged.c:130:struct khugepaged_scan {\nmm/khugepaged.c-131-\tstruct list_head mm_head;\n--\nmm/khugepaged.c-135-\nmm/khugepaged.c:136:static struct khugepaged_scan khugepaged_scan = {\nmm/khugepaged.c:137:\t.mm_head = LIST_HEAD_INIT(khugepaged_scan.mm_head),\nmm/khugepaged.c-138-};\n--\nmm/khugepaged.c=141=static ssize_t scan_sleep_millisecs_show(struct kobject *kobj,\n--\nmm/khugepaged.c-144-{\nmm/khugepaged.c:145:\treturn sysfs_emit(buf, \"%u\\n\", khugepaged_scan_sleep_millisecs);\nmm/khugepaged.c-146-}\n--\nmm/khugepaged.c=148=static ssize_t __sleep_millisecs_store(const char *buf, size_t count,\n--\nmm/khugepaged.c-158-\t*millisecs = msecs;\nmm/khugepaged.c:159:\tkhugepaged_sleep_expire = 0;\nmm/khugepaged.c:160:\twake_up_interruptible(\u0026khugepaged_wait);\nmm/khugepaged.c-161-\n--\nmm/khugepaged.c=165=static ssize_t scan_sleep_millisecs_store(struct kobject *kobj,\n--\nmm/khugepaged.c-168-{\nmm/khugepaged.c:169:\treturn __sleep_millisecs_store(buf, count, \u0026khugepaged_scan_sleep_millisecs);\nmm/khugepaged.c-170-}\n--\nmm/khugepaged.c=174=static ssize_t alloc_sleep_millisecs_show(struct kobject *kobj,\n--\nmm/khugepaged.c-177-{\nmm/khugepaged.c:178:\treturn sysfs_emit(buf, \"%u\\n\", khugepaged_alloc_sleep_millisecs);\nmm/khugepaged.c-179-}\n--\nmm/khugepaged.c=181=static ssize_t alloc_sleep_millisecs_store(struct kobject *kobj,\n--\nmm/khugepaged.c-184-{\nmm/khugepaged.c:185:\treturn __sleep_millisecs_store(buf, count, \u0026khugepaged_alloc_sleep_millisecs);\nmm/khugepaged.c-186-}\n--\nmm/khugepaged.c=190=static ssize_t pages_to_scan_show(struct kobject *kobj,\n--\nmm/khugepaged.c-193-{\nmm/khugepaged.c:194:\treturn sysfs_emit(buf, \"%u\\n\", khugepaged_pages_to_scan);\nmm/khugepaged.c-195-}\nmm/khugepaged.c=196=static ssize_t pages_to_scan_store(struct kobject *kobj,\n--\nmm/khugepaged.c-206-\nmm/khugepaged.c:207:\tkhugepaged_pages_to_scan = pages;\nmm/khugepaged.c-208-\n--\nmm/khugepaged.c=214=static ssize_t pages_collapsed_show(struct kobject *kobj,\n--\nmm/khugepaged.c-217-{\nmm/khugepaged.c:218:\treturn sysfs_emit(buf, \"%u\\n\", khugepaged_pages_collapsed);\nmm/khugepaged.c-219-}\n--\nmm/khugepaged.c=223=static ssize_t full_scans_show(struct kobject *kobj,\n--\nmm/khugepaged.c-226-{\nmm/khugepaged.c:227:\treturn sysfs_emit(buf, \"%u\\n\", khugepaged_full_scans);\nmm/khugepaged.c-228-}\n--\nmm/khugepaged.c=238=static ssize_t defrag_store(struct kobject *kobj,\n--\nmm/khugepaged.c-244-}\nmm/khugepaged.c:245:static struct kobj_attribute khugepaged_defrag_attr =\nmm/khugepaged.c-246-\t__ATTR_RW(defrag);\n--\nmm/khugepaged.c-248-/*\nmm/khugepaged.c:249: * max_ptes_none controls if khugepaged should collapse hugepages over\nmm/khugepaged.c-250- * any unmapped ptes in turn potentially increasing the memory\nmm/khugepaged.c:251: * footprint of the vmas. When max_ptes_none is 0 khugepaged will not\nmm/khugepaged.c-252- * reduce the available free memory in the system as it\nmm/khugepaged.c-253- * runs. Increasing max_ptes_none will instead potentially reduce the\nmm/khugepaged.c:254: * free memory in the system during the khugepaged scan.\nmm/khugepaged.c-255- */\nmm/khugepaged.c=256=static ssize_t max_ptes_none_show(struct kobject *kobj,\n--\nmm/khugepaged.c-259-{\nmm/khugepaged.c:260:\treturn sysfs_emit(buf, \"%u\\n\", khugepaged_max_ptes_none);\nmm/khugepaged.c-261-}\nmm/khugepaged.c=262=static ssize_t max_ptes_none_store(struct kobject *kobj,\n--\nmm/khugepaged.c-272-\nmm/khugepaged.c:273:\tkhugepaged_max_ptes_none = max_ptes_none;\nmm/khugepaged.c-274-\n--\nmm/khugepaged.c-276-}\nmm/khugepaged.c:277:static struct kobj_attribute khugepaged_max_ptes_none_attr =\nmm/khugepaged.c-278-\t__ATTR_RW(max_ptes_none);\n--\nmm/khugepaged.c=280=static ssize_t max_ptes_swap_show(struct kobject *kobj,\n--\nmm/khugepaged.c-283-{\nmm/khugepaged.c:284:\treturn sysfs_emit(buf, \"%u\\n\", khugepaged_max_ptes_swap);\nmm/khugepaged.c-285-}\n--\nmm/khugepaged.c=287=static ssize_t max_ptes_swap_store(struct kobject *kobj,\n--\nmm/khugepaged.c-297-\nmm/khugepaged.c:298:\tkhugepaged_max_ptes_swap = max_ptes_swap;\nmm/khugepaged.c-299-\n--\nmm/khugepaged.c-302-\nmm/khugepaged.c:303:static struct kobj_attribute khugepaged_max_ptes_swap_attr =\nmm/khugepaged.c-304-\t__ATTR_RW(max_ptes_swap);\n--\nmm/khugepaged.c=306=static ssize_t max_ptes_shared_show(struct kobject *kobj,\n--\nmm/khugepaged.c-309-{\nmm/khugepaged.c:310:\treturn sysfs_emit(buf, \"%u\\n\", khugepaged_max_ptes_shared);\nmm/khugepaged.c-311-}\n--\nmm/khugepaged.c=313=static ssize_t max_ptes_shared_store(struct kobject *kobj,\n--\nmm/khugepaged.c-323-\nmm/khugepaged.c:324:\tkhugepaged_max_ptes_shared = max_ptes_shared;\nmm/khugepaged.c-325-\n--\nmm/khugepaged.c-328-\nmm/khugepaged.c:329:static struct kobj_attribute khugepaged_max_ptes_shared_attr =\nmm/khugepaged.c-330-\t__ATTR_RW(max_ptes_shared);\nmm/khugepaged.c-331-\nmm/khugepaged.c:332:static struct attribute *khugepaged_attr[] = {\nmm/khugepaged.c:333:\t\u0026khugepaged_defrag_attr.attr,\nmm/khugepaged.c:334:\t\u0026khugepaged_max_ptes_none_attr.attr,\nmm/khugepaged.c:335:\t\u0026khugepaged_max_ptes_swap_attr.attr,\nmm/khugepaged.c:336:\t\u0026khugepaged_max_ptes_shared_attr.attr,\nmm/khugepaged.c-337-\t\u0026pages_to_scan_attr.attr,\n--\nmm/khugepaged.c-344-\nmm/khugepaged.c:345:struct attribute_group khugepaged_attr_group = {\nmm/khugepaged.c:346:\t.attrs = khugepaged_attr,\nmm/khugepaged.c:347:\t.name = \"khugepaged\",\nmm/khugepaged.c-348-};\n--\nmm/khugepaged.c=367=static unsigned int collapse_max_ptes_none(struct collapse_control *cc,\n--\nmm/khugepaged.c-369-{\nmm/khugepaged.c:370:\tconst unsigned int max_ptes_none = khugepaged_max_ptes_none;\nmm/khugepaged.c-371-\n--\nmm/khugepaged.c-374-\t/* for MADV_COLLAPSE, allow any empty/shared zeropage PTEs */\nmm/khugepaged.c:375:\tif (!cc-\u003eis_khugepaged)\nmm/khugepaged.c-376-\t\treturn HPAGE_PMD_NR;\n--\nmm/khugepaged.c=406=static unsigned int collapse_max_ptes_shared(struct collapse_control *cc,\n--\nmm/khugepaged.c-412-\t */\nmm/khugepaged.c:413:\tif (!cc-\u003eis_khugepaged)\nmm/khugepaged.c-414-\t\treturn HPAGE_PMD_NR;\n--\nmm/khugepaged.c-421-\t/* for PMD collapse, respect the user defined maximum */\nmm/khugepaged.c:422:\treturn khugepaged_max_ptes_shared;\nmm/khugepaged.c-423-}\n--\nmm/khugepaged.c=434=static unsigned int collapse_max_ptes_swap(struct collapse_control *cc,\n--\nmm/khugepaged.c-440-\t */\nmm/khugepaged.c:441:\tif (!cc-\u003eis_khugepaged)\nmm/khugepaged.c-442-\t\treturn HPAGE_PMD_NR;\n--\nmm/khugepaged.c-446-\t/* for PMD collapse, respect the user defined maximum */\nmm/khugepaged.c:447:\treturn khugepaged_max_ptes_swap;\nmm/khugepaged.c-448-}\n--\nmm/khugepaged.c=450=int hugepage_madvise(struct vm_area_struct *vma,\n--\nmm/khugepaged.c-461-\t\t/*\nmm/khugepaged.c:462:\t\t * Setting VM_NOHUGEPAGE will prevent khugepaged from scanning\nmm/khugepaged.c:463:\t\t * this vma even if we leave the mm registered in khugepaged if\nmm/khugepaged.c-464-\t\t * it got registered before VM_NOHUGEPAGE was set.\n--\nmm/khugepaged.c-471-\nmm/khugepaged.c:472:int __init khugepaged_init(void)\nmm/khugepaged.c-473-{\n--\nmm/khugepaged.c-477-\nmm/khugepaged.c:478:\tkhugepaged_pages_to_scan = HPAGE_PMD_NR * 8;\nmm/khugepaged.c:479:\tkhugepaged_max_ptes_none = KHUGEPAGED_MAX_PTES_LIMIT;\nmm/khugepaged.c:480:\tkhugepaged_max_ptes_swap = HPAGE_PMD_NR / 8;\nmm/khugepaged.c:481:\tkhugepaged_max_ptes_shared = HPAGE_PMD_NR / 2;\nmm/khugepaged.c-482-\n--\nmm/khugepaged.c-485-\nmm/khugepaged.c:486:void __init khugepaged_destroy(void)\nmm/khugepaged.c-487-{\n--\nmm/khugepaged.c=514=static bool hugepage_enabled(void)\n--\nmm/khugepaged.c-531-\nmm/khugepaged.c:532:void __khugepaged_enter(struct mm_struct *mm)\nmm/khugepaged.c-533-{\n--\nmm/khugepaged.c-536-\nmm/khugepaged.c:537:\t/* __khugepaged_exit() must not run from under us */\nmm/khugepaged.c-538-\tVM_BUG_ON_MM(collapse_test_exit(mm), mm);\n--\nmm/khugepaged.c-548-\nmm/khugepaged.c:549:\tspin_lock(\u0026khugepaged_mm_lock);\nmm/khugepaged.c-550-\tmm_slot_insert(mm_slots_hash, mm, slot);\n--\nmm/khugepaged.c-554-\t */\nmm/khugepaged.c:555:\twakeup = list_empty(\u0026khugepaged_scan.mm_head);\nmm/khugepaged.c:556:\tlist_add_tail(\u0026slot-\u003emm_node, \u0026khugepaged_scan.mm_head);\nmm/khugepaged.c:557:\tspin_unlock(\u0026khugepaged_mm_lock);\nmm/khugepaged.c-558-\n--\nmm/khugepaged.c-560-\tif (wakeup)\nmm/khugepaged.c:561:\t\twake_up_interruptible(\u0026khugepaged_wait);\nmm/khugepaged.c-562-}\n--\nmm/khugepaged.c=568=static unsigned long collapse_possible_orders(struct vm_area_struct *vma,\n--\nmm/khugepaged.c-572-\nmm/khugepaged.c:573:\t/* If khugepaged is scanning an anonymous vma, allow mTHP collapse */\nmm/khugepaged.c-574-\tif ((tva_flags == TVA_KHUGEPAGED) \u0026\u0026 vma_is_anonymous(vma))\n--\nmm/khugepaged.c=582=static bool collapse_possible(struct vm_area_struct *vma,\n--\nmm/khugepaged.c-587-\nmm/khugepaged.c:588:void khugepaged_enter_vma(struct vm_area_struct *vma,\nmm/khugepaged.c-589-\t\t\t  vm_flags_t vm_flags)\n--\nmm/khugepaged.c-592-\t    \u0026\u0026 collapse_possible(vma, vm_flags, TVA_KHUGEPAGED))\nmm/khugepaged.c:593:\t\t__khugepaged_enter(vma-\u003evm_mm);\nmm/khugepaged.c-594-}\nmm/khugepaged.c-595-\nmm/khugepaged.c:596:void __khugepaged_exit(struct mm_struct *mm)\nmm/khugepaged.c-597-{\n--\nmm/khugepaged.c-600-\nmm/khugepaged.c:601:\tspin_lock(\u0026khugepaged_mm_lock);\nmm/khugepaged.c-602-\tslot = mm_slot_lookup(mm_slots_hash, mm);\nmm/khugepaged.c:603:\tif (slot \u0026\u0026 khugepaged_scan.mm_slot != slot) {\nmm/khugepaged.c-604-\t\tmm_slot_remove(slot);\n--\nmm/khugepaged.c-606-\t}\nmm/khugepaged.c:607:\tspin_unlock(\u0026khugepaged_mm_lock);\nmm/khugepaged.c-608-\n--\nmm/khugepaged.c-617-\t\t * under mmap_lock read mode). Stop here (after we return all\nmm/khugepaged.c:618:\t\t * pagetables will be destroyed) until khugepaged has finished\nmm/khugepaged.c-619-\t\t * working on the pagetables under the mmap_lock.\n--\nmm/khugepaged.c=687=static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,\n--\nmm/khugepaged.c-733-\t\t */\nmm/khugepaged.c:734:\t\tif (cc-\u003eis_khugepaged \u0026\u0026 !(vma-\u003evm_flags \u0026 VM_DROPPABLE) \u0026\u0026\nmm/khugepaged.c-735-\t\t    folio_test_lazyfree(folio) \u0026\u0026 !pte_dirty(pteval)) {\n--\nmm/khugepaged.c-822-next:\nmm/khugepaged.c:823:\t\tif (cc-\u003eis_khugepaged \u0026\u0026\nmm/khugepaged.c-824-\t\t    folio_pte_referenced(folio, vma, addr, pteval))\n--\nmm/khugepaged.c-827-\nmm/khugepaged.c:828:\tif (unlikely(cc-\u003eis_khugepaged \u0026\u0026 !referenced)) {\nmm/khugepaged.c-829-\t\tresult = SCAN_LACK_REFERENCED_PAGE;\n--\nmm/khugepaged.c=945=static enum scan_result __collapse_huge_page_copy(pte_t *pte, struct folio *folio,\n--\nmm/khugepaged.c-983-\nmm/khugepaged.c:984:static void khugepaged_alloc_sleep(void)\nmm/khugepaged.c-985-{\n--\nmm/khugepaged.c-987-\nmm/khugepaged.c:988:\tadd_wait_queue(\u0026khugepaged_wait, \u0026wait);\nmm/khugepaged.c-989-\t__set_current_state(TASK_INTERRUPTIBLE|TASK_FREEZABLE);\nmm/khugepaged.c:990:\tschedule_timeout(msecs_to_jiffies(khugepaged_alloc_sleep_millisecs));\nmm/khugepaged.c:991:\tremove_wait_queue(\u0026khugepaged_wait, \u0026wait);\nmm/khugepaged.c-992-}\nmm/khugepaged.c-993-\nmm/khugepaged.c:994:static struct collapse_control khugepaged_collapse_control = {\nmm/khugepaged.c:995:\t.is_khugepaged = true,\nmm/khugepaged.c-996-};\n--\nmm/khugepaged.c=998=static bool collapse_scan_abort(int nid, struct collapse_control *cc)\n--\nmm/khugepaged.c-1021-\nmm/khugepaged.c:1022:#define khugepaged_defrag()\t\t\t\t\t\\\nmm/khugepaged.c-1023-\t(transparent_hugepage_flags \u0026\t\t\t\t\\\n--\nmm/khugepaged.c-1025-\nmm/khugepaged.c:1026:/* Defrag for khugepaged will enter direct reclaim/compaction if necessary */\nmm/khugepaged.c:1027:static inline gfp_t alloc_hugepage_khugepaged_gfpmask(void)\nmm/khugepaged.c-1028-{\nmm/khugepaged.c:1029:\treturn khugepaged_defrag() ? GFP_TRANSHUGE : GFP_TRANSHUGE_LIGHT;\nmm/khugepaged.c-1030-}\n--\nmm/khugepaged.c=1064=static enum scan_result hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,\n--\nmm/khugepaged.c-1068-\tstruct vm_area_struct *vma;\nmm/khugepaged.c:1069:\tenum tva_type type = cc-\u003eis_khugepaged ? TVA_KHUGEPAGED :\nmm/khugepaged.c-1070-\t\t\t\t TVA_FORCED_COLLAPSE;\n--\nmm/khugepaged.c-1091-\t * Anon VMA expected, the address may be unmapped then\nmm/khugepaged.c:1092:\t * remapped to file after khugepaged reacquired the mmap_lock.\nmm/khugepaged.c-1093-\t *\n--\nmm/khugepaged.c=1102=static inline enum scan_result check_pmd_state(pmd_t *pmd)\n--\nmm/khugepaged.c-1109-\t/*\nmm/khugepaged.c:1110:\t * The folio may be under migration when khugepaged is trying to\nmm/khugepaged.c-1111-\t * collapse it. Migration success or failure will eventually end\n--\nmm/khugepaged.c=1250=static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_struct *mm,\n--\nmm/khugepaged.c-1252-{\nmm/khugepaged.c:1253:\tgfp_t gfp = (cc-\u003eis_khugepaged ? alloc_hugepage_khugepaged_gfpmask() :\nmm/khugepaged.c-1254-\t\t     GFP_TRANSHUGE);\n--\nmm/khugepaged.c=1600=static enum scan_result collapse_scan_pmd(struct mm_struct *mm,\n--\nmm/khugepaged.c-1606-\tunsigned int max_ptes_none = collapse_max_ptes_none(cc, vma, HPAGE_PMD_ORDER);\nmm/khugepaged.c:1607:\tenum tva_type tva_flags = cc-\u003eis_khugepaged ? TVA_KHUGEPAGED : TVA_FORCED_COLLAPSE;\nmm/khugepaged.c-1608-\tpmd_t *pmd;\n--\nmm/khugepaged.c-1705-\t\t */\nmm/khugepaged.c:1706:\t\tif (cc-\u003eis_khugepaged \u0026\u0026 !(vma-\u003evm_flags \u0026 VM_DROPPABLE) \u0026\u0026\nmm/khugepaged.c-1707-\t\t    folio_test_lazyfree(folio) \u0026\u0026 !pte_dirty(pteval)) {\n--\nmm/khugepaged.c-1764-\nmm/khugepaged.c:1765:\t\tif (cc-\u003eis_khugepaged \u0026\u0026\nmm/khugepaged.c-1766-\t\t    folio_pte_referenced(folio, vma, addr, pteval))\n--\nmm/khugepaged.c-1768-\t}\nmm/khugepaged.c:1769:\tif (cc-\u003eis_khugepaged \u0026\u0026\nmm/khugepaged.c-1770-\t\t   (!referenced ||\n--\nmm/khugepaged.c-1786-out:\nmm/khugepaged.c:1787:\ttrace_mm_khugepaged_scan_pmd(mm, folio, referenced,\nmm/khugepaged.c-1788-\t\t\t\t     none_or_zero, result, unmapped);\n--\nmm/khugepaged.c=1792=static void collect_mm_slot(struct mm_slot *slot)\n--\nmm/khugepaged.c-1795-\nmm/khugepaged.c:1796:\tlockdep_assert_held(\u0026khugepaged_mm_lock);\nmm/khugepaged.c-1797-\n--\nmm/khugepaged.c-1807-\nmm/khugepaged.c:1808:\t\t/* khugepaged_mm_lock actually not necessary for the below */\nmm/khugepaged.c-1809-\t\tmm_slot_free(mm_slot_cache, slot);\n--\nmm/khugepaged.c=2249=static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,\n--\nmm/khugepaged.c-2357-\t\t\t\t * Trigger async flush for read-only files and\nmm/khugepaged.c:2358:\t\t\t\t * hope the writeback is done when khugepaged\nmm/khugepaged.c-2359-\t\t\t\t * revisits this page. Writable files can have\n--\nmm/khugepaged.c-2414-\t\t\t/*\n\n"
}

gemini-3.7-flash

null

null

							input: 21355
							output: 27
							thoughts: 64
						

**Investigating the Transparent Hugepage Configuration**

Okay, so I'm digging into how transparent hugepages are handled, starting with `khugepaged.c`. My understanding is that the presence of that file in the compilation process is directly tied to the `CONFIG_TRANSPARENT_HUGEPAGE` kernel configuration option. Makes sense. Now, I need to understand how userfaultfd plays a part in this. I'm going to look at `userfaultfd_minor` and the `USERFAULTFD` constant. I'll need to locate where `userfaultfd_minor` is defined. This should give me a good handle on the interaction between hugepages and userfault handling, which I'm sure will inform the rest of my investigation.

{
  "Expression": "userfaultfd_minor",
  "PathPrefix": "include/"
}

{
  "Output": "include/linux/userfaultfd_k.h=212=static inline bool userfaultfd_wp(struct vm_area_struct *vma)\n--\ninclude/linux/userfaultfd_k.h-216-\ninclude/linux/userfaultfd_k.h:217:static inline bool userfaultfd_minor(struct vm_area_struct *vma)\ninclude/linux/userfaultfd_k.h-218-{\n--\ninclude/linux/userfaultfd_k.h=361=static inline bool userfaultfd_wp(struct vm_area_struct *vma)\n--\ninclude/linux/userfaultfd_k.h-365-\ninclude/linux/userfaultfd_k.h:366:static inline bool userfaultfd_minor(struct vm_area_struct *vma)\ninclude/linux/userfaultfd_k.h-367-{\n"
}

gemini-3.7-flash

null

null

							input: 21836
							output: 39
							thoughts: 29
						

Here's my take on the thought process, as if it were my own:

**Investigating Userfaultfd Kernel Header**

Alright, I need to dive into some `userfaultfd` code. Based on the problem description, I'm thinking the issue might be related to the core handling of userfaultfd within the kernel. I'm going to start by taking a look at `include/linux/userfaultfd_k.h`. It's a good starting point for understanding the internal data structures and functions. I'll focus my attention around line 217 initially, as that's often where key definitions or structures related to fault handling reside. Then, I'll jump down to line 366. I'm hoping to get a clearer picture of how the kernel manages and processes these user-space faults from those two locations specifically. That should provide a useful foundation to investigate further.

{
  "File": "include/linux/userfaultfd_k.h",
  "FirstLine": 210,
  "LineCount": 25
}

{
  "Contents": " 210:\t}\n 211:\t\n 212:\tstatic inline bool userfaultfd_wp(struct vm_area_struct *vma)\n 213:\t{\n 214:\t\treturn vma_test_any_mask(vma, VMA_UFFD_WP);\n 215:\t}\n 216:\t\n 217:\tstatic inline bool userfaultfd_minor(struct vm_area_struct *vma)\n 218:\t{\n 219:\t\treturn vma_test_any_mask(vma, VMA_UFFD_MINOR);\n 220:\t}\n 221:\t\n 222:\tstatic inline bool userfaultfd_rwp(struct vm_area_struct *vma)\n 223:\t{\n 224:\t\t/*\n 225:\t\t * Callers gate PAGE_NONE usage on this; PAGE_NONE is a BUILD_BUG()\n 226:\t\t * without CONFIG_ARCH_HAS_PTE_PROTNONE, so fold to false.\n 227:\t\t */\n 228:\t\tif (!IS_ENABLED(CONFIG_ARCH_HAS_PTE_PROTNONE))\n 229:\t\t\treturn false;\n 230:\t\treturn vma_test_single_mask(vma, VMA_UFFD_RWP);\n 231:\t}\n 232:\t\n 233:\tstatic inline bool userfaultfd_protected(struct vm_area_struct *vma)\n 234:\t{\n"
}

gemini-3.7-flash

null

null

							input: 22497
							output: 18
							thoughts: 73
						

Here's my thought process, as it relates to checking kernel configurations:

**Kernel Configuration Investigation**

Alright, time to get to the bottom of this. My immediate thought is to delve into the Kconfig files. I need to figure out exactly how `userfaultfd` and `transparent_hugepage` are being enabled in the kernel. Let's see... the most direct route is to look for the relevant Kconfig options. So, that means checking for `CONFIG_USERFAULTFD` and `CONFIG_TRANSPARENT_HUGEPAGE`. Where are these options defined? My usual starting point for memory management related features would be either `init/Kconfig` or `mm/Kconfig`. I'll start there. That's where I would expect to find the dependencies and the logic behind these settings.

{
  "Expression": "config USERFAULTFD"
}

{
  "Output": "mm/Kconfig=1372=config HAVE_ARCH_USERFAULTFD_MINOR\n--\nmm/Kconfig-1376-\nmm/Kconfig:1377:config USERFAULTFD_RWP\nmm/Kconfig-1378-\tdef_bool y\n--\nmm/Kconfig-1385-\nmm/Kconfig:1386:menuconfig USERFAULTFD\nmm/Kconfig-1387-\tbool \"Enable userfaultfd() system call\"\n"
}

gemini-3.7-flash

null

null

							input: 22891
							output: 33
							thoughts: 23
						

Here's my summary, as though I'm thinking through this process:

**Diving into the Kernel Configuration for Memory Management**

Okay, time to troubleshoot this memory-related issue. My gut tells me there might be a problem with how something is configured in the kernel. Let's start with a focused investigation. I'm going to jump straight into the heart of the matter – the `mm/Kconfig` file. I'm pretty sure that area houses the memory management configuration options, and given the nature of the problem, it's the logical starting point. Specifically, I'm thinking the relevant options, or their interactions, might be located around line numbers 1370 to 1400. That range usually covers some key flags and settings that I've seen be relevant in similar situations. Let's see what's in there.

{
  "File": "mm/Kconfig",
  "FirstLine": 1370,
  "LineCount": 35
}

{
  "Contents": "1370:\t\t  Arch has userfaultfd write protection support\n1371:\t\n1372:\tconfig HAVE_ARCH_USERFAULTFD_MINOR\n1373:\t\tbool\n1374:\t\thelp\n1375:\t\t  Arch has userfaultfd minor fault support\n1376:\t\n1377:\tconfig USERFAULTFD_RWP\n1378:\t\tdef_bool y\n1379:\t\tdepends on 64BIT \u0026\u0026 ARCH_HAS_PTE_PROTNONE \u0026\u0026 HAVE_ARCH_USERFAULTFD_WP\n1380:\t\thelp\n1381:\t\t  Userfaultfd read-write protection (UFFDIO_RWPROTECT) delivers a\n1382:\t\t  userfaultfd notification on every access -- read or write -- to a\n1383:\t\t  protected range, letting userspace observe the working set of a\n1384:\t\t  process.\n1385:\t\n1386:\tmenuconfig USERFAULTFD\n1387:\t\tbool \"Enable userfaultfd() system call\"\n1388:\t\tdepends on MMU\n1389:\t\thelp\n1390:\t\t  Enable the userfaultfd() system call that allows to intercept and\n1391:\t\t  handle page faults in userland.\n1392:\t\n1393:\tconfig PTE_MARKER_UFFD_WP\n1394:\t\tbool \"Userfaultfd write protection support for shmem/hugetlbfs\"\n1395:\t\tdefault y\n1396:\t\tdepends on USERFAULTFD \u0026\u0026 HAVE_ARCH_USERFAULTFD_WP\n1397:\t\n1398:\t\thelp\n1399:\t\t  Allows to create marker PTEs for userfaultfd write protection\n1400:\t\t  purposes.  It is required to enable userfaultfd write protection on\n1401:\t\t  file-backed memory types like shmem and hugetlbfs.\n1402:\t\n1403:\t# multi-gen LRU {\n1404:\tconfig LRU_GEN\n"
}

gemini-3.7-flash

null

null

							input: 23649
							output: 30
							thoughts: 21
						

Here's my thought process, as requested:

**Checking `TRANSPARENT_HUGEPAGE` in `mm/Kconfig`**

Okay, so I need to quickly get a handle on how `TRANSPARENT_HUGEPAGE` is configured. My initial instinct, given my experience, is to head directly to `mm/Kconfig`. That's where I expect to find the relevant configuration options related to memory management, specifically anything dealing with transparent hugepages. It's the logical place to start digging into the kernel configuration. I'm going to look for the definition of the symbol itself, and then understand its dependencies and any associated help text. I want to see if it's tristate, and if so, what the default setting is and how it might interact with other memory management features. I'm also going to see if there are any conditional dependencies or "select" statements that might shed light on how it's integrated into the larger kernel configuration landscape. This will give me a solid foundation for understanding the capabilities of transparent hugepages within the current kernel build.

{
  "Expression": "config TRANSPARENT_HUGEPAGE",
  "PathPrefix": "mm/"
}

{
  "Output": "mm/Kconfig=832=config MM_ID\n--\nmm/Kconfig-834-\nmm/Kconfig:835:menuconfig TRANSPARENT_HUGEPAGE\nmm/Kconfig-836-\tbool \"Transparent Hugepage Support\"\n--\nmm/Kconfig=853=choice\n--\nmm/Kconfig-858-\nmm/Kconfig:859:\tconfig TRANSPARENT_HUGEPAGE_ALWAYS\nmm/Kconfig-860-\t\tbool \"always\"\n--\nmm/Kconfig-865-\nmm/Kconfig:866:\tconfig TRANSPARENT_HUGEPAGE_MADVISE\nmm/Kconfig-867-\t\tbool \"madvise\"\n--\nmm/Kconfig-874-\nmm/Kconfig:875:\tconfig TRANSPARENT_HUGEPAGE_NEVER\nmm/Kconfig-876-\t\tbool \"never\"\n--\nmm/Kconfig=882=choice\n--\nmm/Kconfig-891-\nmm/Kconfig:892:\tconfig TRANSPARENT_HUGEPAGE_SHMEM_HUGE_NEVER\nmm/Kconfig-893-\t\tbool \"never\"\n--\nmm/Kconfig-901-\nmm/Kconfig:902:\tconfig TRANSPARENT_HUGEPAGE_SHMEM_HUGE_ALWAYS\nmm/Kconfig-903-\t\tbool \"always\"\n--\nmm/Kconfig-909-\nmm/Kconfig:910:\tconfig TRANSPARENT_HUGEPAGE_SHMEM_HUGE_WITHIN_SIZE\nmm/Kconfig-911-\t\tbool \"within_size\"\n--\nmm/Kconfig-917-\nmm/Kconfig:918:\tconfig TRANSPARENT_HUGEPAGE_SHMEM_HUGE_ADVISE\nmm/Kconfig-919-\t\tbool \"advise\"\n--\nmm/Kconfig=927=choice\n--\nmm/Kconfig-936-\nmm/Kconfig:937:\tconfig TRANSPARENT_HUGEPAGE_TMPFS_HUGE_NEVER\nmm/Kconfig-938-\t\tbool \"never\"\n--\nmm/Kconfig-945-\nmm/Kconfig:946:\tconfig TRANSPARENT_HUGEPAGE_TMPFS_HUGE_ALWAYS\nmm/Kconfig-947-\t\tbool \"always\"\n--\nmm/Kconfig-953-\nmm/Kconfig:954:\tconfig TRANSPARENT_HUGEPAGE_TMPFS_HUGE_WITHIN_SIZE\nmm/Kconfig-955-\t\tbool \"within_size\"\n--\nmm/Kconfig-961-\nmm/Kconfig:962:\tconfig TRANSPARENT_HUGEPAGE_TMPFS_HUGE_ADVISE\nmm/Kconfig-963-\t\tbool \"advise\"\n"
}

gemini-3.7-flash

null

null

							input: 24641
							output: 44
							thoughts: 539
						

**My Analysis of a Potential Fuzzing Target**

Alright, let's break this down. We have a patch touching `mm/khugepaged.c` – that immediately grabs my attention. Specifically, the `try_collapse_pte_mapped_thp` function is the focal point. The core change is this: if a VMA is registered with `userfaultfd_minor`, it’s now *skipped* during THP collapse. That's a significant behavior change, potentially opening up a new attack surface.

The code itself is straightforward: the new check, `if (userfaultfd_minor(vma)) return SCAN_PTE_UFFD;`, seems simple enough, but the interactions and side effects in a complex MM system... that's where the interesting stuff lies.

Now, is this worth fuzzing? Definitely. The fact that it's reachable via both `madvise(..., MADV_COLLAPSE)` *and* background `khugepaged` scanning, combined with `userfaultfd_minor` registration – that's a solid combination for exploration. Plus, we have the `uprobe_write` path, widening the attack surface.

This is a clear "yes" for `WorthFuzzing`. The subsystem is core MM, and the reachability is diverse.

The next step is to choose our FocusSymbols and EnableConfigs. We need to avoid the obvious, ubiquitous lifecycle functions – nothing like `sys_madvise`, or `do_page_fault`. We need to be surgical. My focus would be on:
*   `try_collapse_pte_mapped_thp`: The function has changed directly, so let's start there.
*   The callers of `try_collapse_pte_mapped_thp`: `collapse_pte_mapped_thp`, `madvise_collapse`, `collapse_single_pmd`, `collapse_file`, `khugepaged_scan_pmd`, and `khugepaged_scan_file` are all potential areas of interest, if `try_collapse_pte_mapped_thp` isn't inlined.

Let's see what else `mm/khugepaged.c` has to offer. The adjacent code will give clues on what to focus on. We need to carefully observe the control and data flow around this new check to ensure we are covering the most vulnerable segments.

{
  "Expression": "^static enum scan_result try_collapse_pte_mapped_thp",
  "PathPrefix": "mm/khugepaged.c"
}

{
  "Output": "mm/khugepaged.c=1815=static enum scan_result set_huge_pmd(struct vm_area_struct *vma, unsigned long addr,\n--\nmm/khugepaged.c-1850-\nmm/khugepaged.c:1851:static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,\nmm/khugepaged.c-1852-\t\tbool install_pmd)\n"
}

gemini-3.7-flash

null

null

							input: 25875
							output: 34
							thoughts: 217
						

**Analyzing khugepaged.c for THP Collapse Functions**

Okay, let's dive into this. I'm looking at `try_collapse_pte_mapped_thp` – it's a non-inline static function tucked away in `mm/khugepaged.c`. My immediate thought is, "Who's using this thing?" I need to figure out the call graph and the surrounding context. So, I'm going to quickly identify the directly related functions.

Right off the bat, I see `collapse_pte_mapped_thp`, which is exported and non-static within the same file; that’s definitely relevant. Then we have `madvise_collapse`, also non-static, also in `mm/khugepaged.c`. Of course, `try_collapse_pte_mapped_thp` itself is in the mix, and now I also see `collapse_single_pmd` and `collapse_file` in the list.

I need to quickly verify that these are all distinct symbols; it's easy to get confused with similar naming conventions. Let's start by looking at `madvise_collapse` in `mm/khugepaged.c` in particular.

{
  "Expression": "int madvise_collapse\\(",
  "PathPrefix": "mm/khugepaged.c"
}

{
  "Output": "mm/khugepaged.c=3157=static int madvise_collapse_errno(enum scan_result r)\n--\nmm/khugepaged.c-3188-\nmm/khugepaged.c:3189:int madvise_collapse(struct vm_area_struct *vma, unsigned long start,\nmm/khugepaged.c-3190-\t\t     unsigned long end, bool *lock_dropped)\n"
}

gemini-3.7-flash

null

null

							input: 26512
							output: 32
							thoughts: 29
						

Okay, let's dive in. Here's my thought process regarding `collapse_file` and `collapse_single_pmd` within `mm/khugepaged.c`:

**Analyzing `collapse_file` and `collapse_single_pmd` in `mm/khugepaged.c`**

Alright, time to get into the weeds of `khugepaged`'s file mapping handling. My immediate focus is on the functions `collapse_file` and `collapse_single_pmd`. Given my experience, I'm expecting these to be critical components of how the kernel manages transparent huge pages (THP) specifically when dealing with file-backed mappings. `collapse_file`, undoubtedly, has a broader scope, likely dealing with file-backed mappings in general, whereas `collapse_single_pmd` is probably geared toward collapsing a specific Physical Memory Descriptor (PMD). I'll need to drill down into the logic within each to understand the nuances of how these functions handle page faults, memory allocation/deallocation, and the overall coordination of THP management in file-backed contexts. I anticipate seeing interactions with the page cache, inode data structures, and the VM system's core mechanisms for handling memory pressure. My goal is to determine how efficiently these functions manage page-level operations compared to a more direct file-mapping approach with THP. Specific things I'll be looking for include whether the file mapping requires flushing, and how coherency is maintained with other processes and threads accessing the file. Also, what impact will these functions have on performance from a file I/O perspective?

{
  "Expression": "collapse_file\\(",
  "PathPrefix": "mm/khugepaged.c"
}

{
  "Output": "mm/khugepaged.c=2140=static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)\n--\nmm/khugepaged.c-2248- */\nmm/khugepaged.c:2249:static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,\nmm/khugepaged.c-2250-\t\tstruct file *file, pgoff_t start, struct collapse_control *cc)\n--\nmm/khugepaged.c-2676-\tVM_BUG_ON(!list_empty(\u0026pagelist));\nmm/khugepaged.c:2677:\ttrace_mm_khugepaged_collapse_file(mm, new_folio, index, addr, is_shmem, file, HPAGE_PMD_NR, result);\nmm/khugepaged.c-2678-\treturn result;\n--\nmm/khugepaged.c=2681=static enum scan_result collapse_scan_file(struct mm_struct *mm,\n--\nmm/khugepaged.c-2778-\t\t} else {\nmm/khugepaged.c:2779:\t\t\tresult = collapse_file(mm, addr, file, start, cc);\nmm/khugepaged.c-2780-\t\t}\n"
}

gemini-3.7-flash

null

null

							input: 27261
							output: 34
							thoughts: 12
						

Okay, here's my take on those thoughts, tailored for an expert audience, presented in the first person:

**Checking `collapse_single_pmd`**

Alright, let's dive into this. The initial thought, of course, is a check. A quick sanity check on a specific function or module named `collapse_single_pmd`. The mere existence of a dedicated function with such a name implies a very specific task: potentially reducing or simplifying data, likely related to paired molecular dynamics (PMD) simulations, perhaps, or something of that nature, given the naming convention I see.

My immediate follow-up thought would be *why* are we checking this? Is it part of a debugging process? A refactoring effort? Performance optimization? Or maybe a review of the underlying logic? What's the context? We'll need a broader view of the system to understand the motivation behind this check. I'd be looking for associated documentation, test cases, or recent code changes that involve this function.

Specifically, I'd want to quickly recall: what are the typical inputs and outputs for this function? Does it handle edge cases properly? What are the computational complexities involved? Does it have any known dependencies that might affect performance or stability? I'd be looking for potential areas of concern: inefficient algorithms, memory leaks, or unhandled exceptions that could be lurking within its implementation. Let's see what we find.

{
  "Expression": "collapse_single_pmd",
  "PathPrefix": "mm/khugepaged.c"
}

{
  "Output": "mm/khugepaged.c=2681=static enum scan_result collapse_scan_file(struct mm_struct *mm,\n--\nmm/khugepaged.c-2790- */\nmm/khugepaged.c:2791:static enum scan_result collapse_single_pmd(unsigned long addr,\nmm/khugepaged.c-2792-\t\tstruct vm_area_struct *vma, bool *lock_dropped,\n--\nmm/khugepaged.c=2848=static void collapse_scan_mm_slot(unsigned int progress_max,\n--\nmm/khugepaged.c-2918-\nmm/khugepaged.c:2919:\t\t\t*result = collapse_single_pmd(khugepaged_scan.address,\nmm/khugepaged.c-2920-\t\t\t\t\t\t      vma, \u0026lock_dropped, cc);\n--\nmm/khugepaged.c=3189=int madvise_collapse(struct vm_area_struct *vma, unsigned long start,\n--\nmm/khugepaged.c-3237-\nmm/khugepaged.c:3238:\t\tresult = collapse_single_pmd(addr, vma, \u0026mmap_unlocked, cc);\nmm/khugepaged.c-3239-\n"
}

gemini-3.7-flash

null

null

							input: 27952
							output: 35
							thoughts: 129
						

**My Configuration Conundrum**

Alright, let's break this down. First, I need to figure out the essential configurations for this particular kernel component. It looks like `TRANSPARENT_HUGEPAGE` is definitely a prerequisite, given its direct link to `khugepaged.c` in the memory management Makefile. Makes sense; that's the core of how transparent hugepages are managed.

Then there's `USERFAULTFD`, which is tied to userfaultfd minor fault support. And since we're dealing with x86/amd64 systems here, I'll need to confirm that `HAVE_ARCH_USERFAULTFD_MINOR` is enabled. Time to dive into `arch/x86/Kconfig` and see what the architecture says about this. I'm going to need to track down the Kconfig definition to ensure it's set correctly.

{
  "Expression": "HAVE_ARCH_USERFAULTFD_MINOR",
  "PathPrefix": "arch/x86"
}

{
  "Output": "arch/x86/Kconfig=58=config X86\n--\narch/x86/Kconfig-214-\tselect HAVE_ARCH_USERFAULTFD_WP         if X86_64 \u0026\u0026 USERFAULTFD\narch/x86/Kconfig:215:\tselect HAVE_ARCH_USERFAULTFD_MINOR\tif X86_64 \u0026\u0026 USERFAULTFD\narch/x86/Kconfig-216-\tselect HAVE_ARCH_VMAP_STACK\t\tif X86_64\n"
}

gemini-3.7-flash

null

null

							input: 28444
							output: 166
							thoughts: 140
						

**My Analysis of Kernel Configuration for Fuzzing**

Okay, so the Kconfig flags we're focusing on here are `USERFAULTFD` and `TRANSPARENT_HUGEPAGE`. I always like to double-check my assumptions, so let's run through the criteria.

`WorthFuzzing` is set to true, which aligns with my understanding of the system. The `EnableConfigs` list explicitly includes both `TRANSPARENT_HUGEPAGE` and `USERFAULTFD`, which is exactly what I expected based on the initial assessment. The `FocusSymbols` are also as I would have anticipated: we're keyed in on functions like `try_collapse_pte_mapped_thp`, `collapse_pte_mapped_thp`, `madvise_collapse`, and `collapse_single_pmd`. These make perfect sense given the nature of transparent hugepages and userfaultfd; we're dealing with memory management and potential race conditions in that area. That's a solid, concise reasoning for this assessment.

Hmm, I think that's it. Let me just confirm that I haven't missed anything else in my evaluation. Yup, everything is clear and accurate. Let's call `set-results`. Done.

{
  "EnableConfigs": [
    "TRANSPARENT_HUGEPAGE",
    "USERFAULTFD"
  ],
  "FocusSymbols": [
    "try_collapse_pte_mapped_thp",
    "collapse_pte_mapped_thp",
    "madvise_collapse",
    "collapse_single_pmd"
  ],
  "Reasoning": "The patch modifies khugepaged in core memory management (mm/khugepaged.c) to prevent collapsing PTE-mapped transparent hugepages for VMAs registered with userfaultfd minor fault handling (userfaultfd_minor). This is reachable via userfaultfd syscalls and MADV_COLLAPSE madvise calls or background khugepaged collapse and warrants fuzzing.",
  "WorthFuzzing": true
}

{
  "EnableConfigs": [
    "TRANSPARENT_HUGEPAGE",
    "USERFAULTFD"
  ],
  "FocusSymbols": [
    "try_collapse_pte_mapped_thp",
    "collapse_pte_mapped_thp",
    "madvise_collapse",
    "collapse_single_pmd"
  ],
  "Reasoning": "The patch modifies khugepaged in core memory management (mm/khugepaged.c) to prevent collapsing PTE-mapped transparent hugepages for VMAs registered with userfaultfd minor fault handling (userfaultfd_minor). This is reachable via userfaultfd syscalls and MADV_COLLAPSE madvise calls or background khugepaged collapse and warrants fuzzing.",
  "WorthFuzzing": true
}

{
  "WorthFuzzing": true
}

null

core

null

{
  "KMSANReasoning": "The patch adds a check (`userfaultfd_minor(vma)`) in `try_collapse_pte_mapped_thp()` to prevent collapsing PTE-mapped THPs on VMAs registered with userfaultfd-minor mode, along with corresponding selftest updates in `tools/testing/selftests/mm/uffd-unit-tests.c`. \n\nThis modification does not allocate new buffers, manipulate uninitialized stack or heap data, alter padding or structure fields copied to userspace, or change any data lengths/bounds checking that could lead to uninitialized memory reads or leaks. Standard sanitizers and bug detectors (such as KASAN and page table checks) are sufficient for validating this functionality, and no uninitialized memory risks detectable only by KMSAN are introduced.",
  "NeedsKMSAN": false
}

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.

Target architecture: amd64

For your convenience, here is the diff of the changes:
commit ee7b65816592db8b99faef2f4923e821dd5bcec1
Author: syz-cluster <triage@syzkaller.com>
Date:   Thu Sep 10 02:48:14 2026 +0000

    syz-cluster: applied patch under review

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 75639298efc27..e6947fe142eea 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1894,6 +1894,13 @@ static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign
 	if (userfaultfd_protected(vma))
 		return SCAN_PTE_UFFD;
 
+	/*
+	 * Userfaultfd-minor-registered VMAs should not be collapsed, as
+	 * userspace is expecting to explicitly install PTEs.
+	 */
+	if (userfaultfd_minor(vma))
+		return SCAN_PTE_UFFD;
+
 	folio = filemap_lock_folio(vma->vm_file->f_mapping,
 			       linear_page_index(vma, haddr));
 	if (IS_ERR(folio))
diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
index ef9b3956bdcfd..6f2360f9b75db 100644
--- a/tools/testing/selftests/mm/uffd-unit-tests.c
+++ b/tools/testing/selftests/mm/uffd-unit-tests.c
@@ -518,19 +518,34 @@ static void uffd_wp_fork_pin_with_event_test(uffd_global_test_opts_t *gopts, uff
 	uffd_wp_fork_pin_test_common(gopts, args, true);
 }
 
-static void check_memory_contents(uffd_global_test_opts_t *gopts, char *p)
+static int __check_memory_contents(unsigned long offset,
+				   unsigned long nr_pages,
+				   uffd_global_test_opts_t *gopts,
+				   char *p)
 {
 	unsigned long i, j;
 	uint8_t expected_byte;
 
-	for (i = 0; i < gopts->nr_pages; ++i) {
+	if (nr_pages + offset < nr_pages)
+		err("overflow in memory check");
+	if (nr_pages + offset > gopts->nr_pages)
+		err("out of bounds memory check");
+
+	for (i = offset; i < offset + nr_pages; ++i) {
 		expected_byte = ~((uint8_t)(i % ((uint8_t)-1)));
 		for (j = 0; j < gopts->page_size; j++) {
 			uint8_t v = *(uint8_t *)(p + (i * gopts->page_size) + j);
 			if (v != expected_byte)
-				err("unexpected page contents");
+				return 1;
 		}
 	}
+
+	return 0;
+}
+
+static int check_memory_contents(uffd_global_test_opts_t *gopts, char *p)
+{
+	return __check_memory_contents(0, gopts->nr_pages, gopts, p);
 }
 
 static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_collapse, bool test_wp)
@@ -539,6 +554,8 @@ static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col
 	pthread_t uffd_mon;
 	char c = '\0';
 	struct uffd_args args = { 0 };
+	unsigned long checked = 0;
+	bool bad_contents;
 	args.gopts = gopts;
 
 	/*
@@ -564,24 +581,65 @@ static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col
 	if (pthread_create(&uffd_mon, NULL, uffd_poll_thread, &args))
 		err("uffd_poll_thread create");
 
+	if (test_collapse) {
+		/*
+		 * Read just a single page and try collapsing. The collapse
+		 * should either be rejected or be a no-op.
+		 */
+		if (__check_memory_contents(0, 1, gopts, gopts->area_dst_alias))
+			err("unexpected memory contents before collapse");
+
+		/* MADV_COLLAPSE might return EINVAL for uffd-minor VMAs. */
+		madvise(gopts->area_dst_alias, gopts->nr_pages * gopts->page_size,
+			MADV_COLLAPSE);
+		/*
+		 * If the above collapse mapped pages that were not explicitly
+		 * CONTINUE'd, the below __check_memory_contents() will not
+		 * fault on some pages, resulting in incorrect contents. The
+		 * PTE for the first page may get retracted, so avoid checking
+		 * that page, as we might take a second fault, flipping the
+		 * contents a second time.
+		 */
+		checked = 1;
+	}
+
 	/*
 	 * Read each of the pages back using the UFFD-registered mapping. We
 	 * expect that the first time we touch a page, it will result in a minor
 	 * fault. uffd_poll_thread will resolve the fault by bit-flipping the
 	 * page's contents, and then issuing a CONTINUE ioctl.
 	 */
-	check_memory_contents(gopts, gopts->area_dst_alias);
+	bad_contents = !!__check_memory_contents(checked, gopts->nr_pages - checked,
+						    gopts, gopts->area_dst_alias);
 
 	if (write(gopts->pipefd[1], &c, sizeof(c)) != sizeof(c))
 		err("pipe write");
 	if (pthread_join(uffd_mon, NULL))
 		err("join() failed");
 
+	if (bad_contents && test_collapse) {
+		uffd_test_fail("unexpected memory contents after collapse");
+		return;
+	}
+
+	if (bad_contents) {
+		uffd_test_fail("unexpected memory contents");
+		return;
+	}
+
 	if (test_collapse) {
+		/*
+		 * MADV_COLLAPSE will fail unless userfaultfd-minor is
+		 * unregistered.
+		 */
+		if (uffd_unregister(gopts->uffd, gopts->area_dst_alias,
+				    gopts->nr_pages * gopts->page_size))
+			err("uffd_unregister before MADV_COLLAPSE failed");
+
+		/* MADV_COLLAPSE should succeed with userfaultfd unregistered. */
 		if (madvise(gopts->area_dst_alias, gopts->nr_pages * gopts->page_size,
 			    MADV_COLLAPSE)) {
-			/* It's fine to fail for this one... */
-			uffd_test_skip("MADV_COLLAPSE failed");
+			uffd_test_fail("MADV_COLLAPSE failed");
 			return;
 		}
 
@@ -593,7 +651,10 @@ static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col
 		 * This won't cause uffd-fault - it purely just makes sure there
 		 * was no corruption.
 		 */
-		check_memory_contents(gopts, gopts->area_dst_alias);
+		if (check_memory_contents(gopts, gopts->area_dst_alias)) {
+			uffd_test_fail("unexpected memory contents");
+			return;
+		}
 	}
 
 	if (args.missing_faults != 0 || args.minor_faults != gopts->nr_pages)

gemini-3.7-flash

null

null

							input: 3925
							output: 38
							thoughts: 377
						

**Analyzing the Potential Need for Dedicated KMSAN Fuzzing**

Okay, so I've been given a kernel patch and I need to determine if it warrants its own dedicated KMSAN fuzzing session. Let's break this down. First, I'll examine the patch itself.

Right, the diff shows a modification to `mm/khugepaged.c`. It looks like they've added a new check within `try_collapse_pte_mapped_thp`. Specifically, they're now preventing the collapsing of THPs (Transparent Huge Pages) if a VMA (Virtual Memory Area) is associated with a `userfaultfd_minor` registration. The comment mentions that user space expects to explicitly install PTEs in these cases, so collapsing would be incorrect.

There's also a note about selftest updates in `tools/testing/selftests/mm/uffd-unit-tests.c`. That's good, it means they're adding tests, which increases my confidence that the intended behavior is correctly implemented.

Now, my focus is on `try_collapse_pte_mapped_thp`. I need to understand the potential KMSAN implications of this code path. I'll either grep for related references or just read the code of that function. I'm looking for where this new `userfaultfd_minor` check is integrated into the larger process. I need to understand the data flows, potential races, and areas where this new logic could introduce memory safety issues. Is this new check sufficient, or does it introduce an edge case that needs dedicated attention under fuzzing? Specifically, I'll need to think about:

*   How is the `userfaultfd_minor` flag set and reset? Is there a race condition with it?
*   Are there other related data structures that are updated alongside with the VMA?
*   How is this check coordinated within the khugepaged background thread?

The goal is to determine if this change might introduce a new memory error surface, or a new way to trigger an existing error, that KMSAN would catch *if* it was fuzzed with this code in mind.

{
  "Expression": "try_collapse_pte_mapped_thp",
  "PathPrefix": "mm/khugepaged.c"
}

{
  "Output": "mm/khugepaged.c=1815=static enum scan_result set_huge_pmd(struct vm_area_struct *vma, unsigned long addr,\n--\nmm/khugepaged.c-1850-\nmm/khugepaged.c:1851:static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,\nmm/khugepaged.c-1852-\t\tbool install_pmd)\n--\nmm/khugepaged.c=2096=void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,\n--\nmm/khugepaged.c-2098-{\nmm/khugepaged.c:2099:\ttry_collapse_pte_mapped_thp(mm, addr, install_pmd);\nmm/khugepaged.c-2100-}\n--\nmm/khugepaged.c=2249=static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,\n--\nmm/khugepaged.c-2628-\t * Remove pte page tables, so we can re-fault the page as huge.\nmm/khugepaged.c:2629:\t * If MADV_COLLAPSE, adjust result to call try_collapse_pte_mapped_thp().\nmm/khugepaged.c-2630-\t */\n--\nmm/khugepaged.c=2791=static enum scan_result collapse_single_pmd(unsigned long addr,\n--\nmm/khugepaged.c-2835-\t\telse\nmm/khugepaged.c:2836:\t\t\tresult = try_collapse_pte_mapped_thp(mm, addr,\nmm/khugepaged.c-2837-\t\t\t\t\t\t\t     !cc-\u003eis_khugepaged);\n"
}

gemini-3.7-flash

null

null

							input: 5276
							output: 37
							thoughts: 26
						

Okay, let's dive into this. I need to understand what's happening around line 1894 in `mm/khugepaged.c`. My initial thought is that there's a specific function or logic being executed there within the kernel's huge page management. Given the file, it's highly likely this code deals with the kHugepaged daemon, and how it manages or interacts with huge pages. I'll need to look for calls to the function, as well as the surrounding variable assignments. I suspect there may be interactions around the `khugepaged` daemon and how it is interacting with the memory.

{
  "File": "mm/khugepaged.c",
  "FirstLine": 1880,
  "LineCount": 40
}

{
  "Contents": "1880:\t\t/*\n1881:\t\t * If we are here, we've succeeded in replacing all the native pages\n1882:\t\t * in the page cache with a single hugepage. If a mm were to fault-in\n1883:\t\t * this memory (mapped by a suitably aligned VMA), we'd get the hugepage\n1884:\t\t * and map it by a PMD, regardless of sysfs THP settings. As such, let's\n1885:\t\t * analogously elide sysfs THP settings here and force collapse.\n1886:\t\t */\n1887:\t\tif (!thp_vma_allowable_order(vma, vma-\u003evm_flags, TVA_FORCED_COLLAPSE, PMD_ORDER))\n1888:\t\t\treturn SCAN_VMA_CHECK;\n1889:\t\n1890:\t\t/*\n1891:\t\t * Keep pmd pgtable while the uffd bit is in use; see comment in\n1892:\t\t * retract_page_tables().\n1893:\t\t */\n1894:\t\tif (userfaultfd_protected(vma))\n1895:\t\t\treturn SCAN_PTE_UFFD;\n1896:\t\n1897:\t\t/*\n1898:\t\t * Userfaultfd-minor-registered VMAs should not be collapsed, as\n1899:\t\t * userspace is expecting to explicitly install PTEs.\n1900:\t\t */\n1901:\t\tif (userfaultfd_minor(vma))\n1902:\t\t\treturn SCAN_PTE_UFFD;\n1903:\t\n1904:\t\tfolio = filemap_lock_folio(vma-\u003evm_file-\u003ef_mapping,\n1905:\t\t\t\t       linear_page_index(vma, haddr));\n1906:\t\tif (IS_ERR(folio))\n1907:\t\t\treturn SCAN_PAGE_NULL;\n1908:\t\n1909:\t\tif (!is_pmd_order(folio_order(folio))) {\n1910:\t\t\tresult = SCAN_PAGE_COMPOUND;\n1911:\t\t\tgoto drop_folio;\n1912:\t\t}\n1913:\t\n1914:\t\tresult = find_pmd_or_thp_or_none(mm, haddr, \u0026pmd);\n1915:\t\tswitch (result) {\n1916:\t\tcase SCAN_SUCCEED:\n1917:\t\t\tbreak;\n1918:\t\tcase SCAN_NO_PTE_TABLE:\n1919:\t\t\t/*\n"
}

gemini-3.7-flash

null

null

							input: 6132
							output: 187
							thoughts: 359
						

**My Analysis of this Patch: KMSAN Exemption**

Okay, let's break this down. First, the patch itself: it's a relatively focused change. In `mm/khugepaged.c`, specifically within `try_collapse_pte_mapped_thp()`, it introduces a check using `userfaultfd_minor(vma)`. This is a crucial addition; it's designed to prevent collapsing PTE-mapped THPs if the VMA is registered with `userfaultfd-minor`. The rationale is straightforward: `userfaultfd-minor` relies on userspace explicitly installing PTEs via `UFFDIO_CONTINUE`, so automatic collapsing would be problematic. The second part of the patch just updates some selftests to reflect this new behavior.

Now, the important question: does this trigger any KMSAN concerns? Let's go through the checklist. Uninitialized memory or potential information leaks are my primary concerns. Does this patch involve allocating *new* memory? No, that's a simple "no." Does it copy any uninitialized memory to userspace, or read uninitialized memory? Again, no. It doesn't seem to alter any bounds checks, modify data lengths in a way that could lead to issues, nor modify any uninitialized struct fields. There's no introduction or modification of kernel structures sent to userspace, either. Also, there's no conditional logic that depends on uninitialized variables. We're also not introducing any new uninitialized stack or heap memory.

Essentially, this is a flag check, not a data structure manipulation issue. It's a conditional branch on an existing `vma` flag, based on the `userfaultfd_minor` function that checks the VMA's flags. The flag itself is initialized elsewhere. Therefore, since there's no evidence of exposing or introducing any uninitialized memory risks, KMSAN is not necessary here. The more standard memory management checks and tests, like KASAN, LOCKDEP, MM tests, and fuzzing should handle any other potential issues like page table management, UAF issues, race conditions, or locking problems.

I'm confident we can use `set-results` to indicate KMSAN is not relevant here.

{
  "KMSANReasoning": "The patch adds a check (`userfaultfd_minor(vma)`) in `try_collapse_pte_mapped_thp()` to prevent collapsing PTE-mapped THPs on VMAs registered with userfaultfd-minor mode, along with corresponding selftest updates in `tools/testing/selftests/mm/uffd-unit-tests.c`. \n\nThis modification does not allocate new buffers, manipulate uninitialized stack or heap data, alter padding or structure fields copied to userspace, or change any data lengths/bounds checking that could lead to uninitialized memory reads or leaks. Standard sanitizers and bug detectors (such as KASAN and page table checks) are sufficient for validating this functionality, and no uninitialized memory risks detectable only by KMSAN are introduced.",
  "NeedsKMSAN": false
}

{
  "KMSANReasoning": "The patch adds a check (`userfaultfd_minor(vma)`) in `try_collapse_pte_mapped_thp()` to prevent collapsing PTE-mapped THPs on VMAs registered with userfaultfd-minor mode, along with corresponding selftest updates in `tools/testing/selftests/mm/uffd-unit-tests.c`. \n\nThis modification does not allocate new buffers, manipulate uninitialized stack or heap data, alter padding or structure fields copied to userspace, or change any data lengths/bounds checking that could lead to uninitialized memory reads or leaks. Standard sanitizers and bug detectors (such as KASAN and page table checks) are sufficient for validating this functionality, and no uninitialized memory risks detectable only by KMSAN are introduced.",
  "NeedsKMSAN": false
}