+ mm-damon-vaddr-avoid-hw-driven-pte-updates-during-damon_hugetlb_mkold.patch added to mm-new branch

+ mm-damon-vaddr-avoid-hw-driven-pte-updates-during-damon_hugetlb_mkold.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: mm/damon/vaddr: avoid hw-driven pte updates during damon_hugetlb_mkold()
has been added to the -mm mm-new branch.  Its filename is
     mm-damon-vaddr-avoid-hw-driven-pte-updates-during-damon_hugetlb_mkold.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-damon-vaddr-avoid-hw-driven-pte-updates-during-damon_hugetlb_mkold.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

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and is updated there most days

------------------------------------------------------
From: SJ Park <sj@kernel.org>
Subject: mm/damon/vaddr: avoid hw-driven pte updates during damon_hugetlb_mkold()
Date: Mon, 7 Sep 2026 10:03:56 -0700

damon_hugetlb_mkold() reads the page table entry into a local variable,
unsets the accessed bit in the variable, and updates the page table entry
with the updated variable value.  If hardware updates the same page table
entry in parallel, the hw updates could be lost.  For example,
hardware-updated dirty bits might be lost.

Avoid the parallel updates by clearing the page table entry when reading
it together, using huge_ptep_get_and_clear().  If a parallel write to the
memory is made after the clearing, the hw will see the page table entry is
cleared, trigger page fault and wait until it is handled.  The page fault
handling will wait for damon_hugetlb_mkold() due to the page table lock.

Because hugetlbfs is an in-memory file system and hugetlb pages cannot be
reclaimed, no critical issue is expected to my best knowledge.  But
definitely this is a nasty bug that should be fixed sooner rather than
later.

The issue was discovered [1] by Sashiko.

Link: https://lore.kernel.org/20260907170358.100168-1-sj@kernel.org
Link: https://lore.kernel.org/20260830160545.98969-1-sj@kernel.org [1]
Fixes: 49f4203aae06 ("mm/damon: add access checking for hugetlb pages")
Signed-off-by: SJ Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 5.17.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/damon/vaddr.c |   21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

--- a/mm/damon/vaddr.c~mm-damon-vaddr-avoid-hw-driven-pte-updates-during-damon_hugetlb_mkold
+++ a/mm/damon/vaddr.c
@@ -283,22 +283,29 @@ out:
 }
 
 #ifdef CONFIG_HUGETLB_PAGE
+static bool damon_hugetlb_ptep_mkold(pte_t *pte, struct mm_struct *mm,
+		struct vm_area_struct *vma, unsigned long addr, pte_t *entry)
+{
+	unsigned long psize = huge_page_size(hstate_vma(vma));
+
+	if (!pte_young(*entry))
+		return false;
+	*entry = huge_ptep_get_and_clear(mm, addr, pte, psize);
+	*entry = pte_mkold(*entry);
+	set_huge_pte_at(mm, addr, pte, *entry, psize);
+	return true;
+}
+
 static void damon_hugetlb_mkold(pte_t *pte, struct mm_struct *mm,
 				struct vm_area_struct *vma, unsigned long addr)
 {
 	bool referenced = false;
 	pte_t entry = huge_ptep_get(mm, addr, pte);
 	struct folio *folio = pfn_folio(pte_pfn(entry));
-	unsigned long psize = huge_page_size(hstate_vma(vma));
 
 	folio_get(folio);
 
-	if (pte_young(entry)) {
-		referenced = true;
-		entry = pte_mkold(entry);
-		set_huge_pte_at(mm, addr, pte, entry, psize);
-	}
-
+	referenced = damon_hugetlb_ptep_mkold(pte, mm, vma, addr, &entry);
 	if (mmu_notifier_clear_young(mm, addr,
 				     addr + huge_page_size(hstate_vma(vma))))
 		referenced = true;
_

Patches currently in -mm which might be from sj@kernel.org are

mm-damon-core-skip-applying-scheme-if-region-split-for-quota-fails.patch
mm-damon-paddr-respect-folio-end-for-damos_stat.patch
mm-damon-paddr-respect-folio-end-for-damos-actions-except-stat.patch
mm-damon-vaddr-respect-folio-end-for-damos_stat.patch
mm-damon-vaddr-respect-folio-end-for-damos_migrate_hotcold.patch
mm-damon-core-handle-extreme-memory-state-in-damon_get_node_mem_bp.patch
mm-damon-core-handle-extreme-memory-state-in-get_node_memcg_used_bp.patch
mm-damon-core-handle-extreme-memory-state-in-get_in_active_mem_bp.patch
mm-damon-core-introduce-damon_filter_type_pgidle_unset.patch
mm-damon-paddr-support-pgidle_unset-probe-filter-type.patch
mm-damon-sysfs-support-pgidle_unset-probe-filter-type.patch
docs-mm-damon-design-document-pgidle_unset-probe-filter-type.patch
mm-damon-core-introduce-damon_prep-struct.patch
mm-damon-core-commit-preps.patch
mm-damon-core-introduce-damon_operations-prep_probes.patch
mm-damon-paddr-support-damon_prep.patch
mm-damon-sysfs-implement-preps-directory.patch
mm-damon-sysfs-implement-preps-nr_preps-file.patch
mm-damon-sysfs-create-directories-for-nr_preps-writes.patch
mm-damon-sysfs-implement-prep_action-file.patch
mm-damon-sysfs-pass-preps-to-damon-core.patch
selftests-damon-sysfssh-test-probe-prep-sysfs-files.patch
docs-mm-damon-design-document-probe-preps.patch
docs-admin-guide-mm-damon-usage-document-probe-preps-sysfs-files.patch
docs-abi-damon-document-probe-prep-sysfs-files.patch
mm-damon-tests-core-kunit-test-committing-psi-goal-to-psi-goal.patch
mm-damon-core-handle-uninitialized-damos_quota_goal-last_psi_total.patch
mm-damon-core-copy-nid-for-eligible_mem_bp-damos-quota-goal-commit.patch
mm-damon-sysfs-set-next-refresh-jiffies-per-sysfs-context.patch
mm-damon-tests-core-kunit-test-damon_commit_filter.patch
mm-damon-tests-core-kunit-add-damon_commit_probes-test.patch
selftests-damon-_damon_sysfs-implement-damonprobes.patch
selftests-damon-drgn_dump_damon_status-dump-probes.patch
selftests-damon-sysfspy-extend-commit-assertion-function-for-probes.patch
selftests-damon-sysfspy-test-damon-probes.patch
mm-damon-core-use-damon_nr_samples_per_aggr-for-max-merge-threshold.patch
mm-damon-core-remove-debug-messages.patch
mm-damon-core-remove-debug-messages-fix.patch
mm-damon-vaddr-remove-a-debug-message.patch
mm-damon-core-validate-number-of-probes-in-valid_probe_params.patch
mm-damon-sysfs-remove-probes-number-validation.patch
mm-damon-tests-core-kunit-extend-set_regions-test-for-error-case.patch
mm-damon-tests-core-kunit-test-=0-size-damon_set_regions-inputs.patch
mm-damon-tests-core-kunit-test-overlapping-ranges-for-set_regions.patch
mm-damon-tests-core-kunit-test-damon_nr_samples_per_aggr.patch
selftests-damon-sysfssh-test-hugepage_mem_bp-quota-goal.patch
docs-mm-damon-maintainer-profile-update-ai-review-for-sashiko-replies.patch
docs-abi-damon-recommend-subsystem-doc-instead-of-admin-guide.patch
mm-damon-core-error-damos_commit_quota_goal-for-zero-target_value.patch
revert-mm-damon-lru_sort-error-out-for-10000-active_mem_bp.patch
revert-samples-damon-mtier-error-out-for-zero-quota-goal-target-values.patch
mm-damon-core-handle-null-ctx-parameter-in-damon_call.patch
mm-damon-core-set-ctx-call_controls_obsolete-in-damon_new_ctx.patch
mm-damon-reclaim-remove-unnecessary-damon_call-param-validation.patch
mm-damon-lru_sort-remove-unnecessary-damon_call-param-validation.patch
mm-damon-vaddr-avoid-hw-driven-pte-updates-during-damon_hugetlb_mkold.patch
mm-damon-move-damon_hugetlb_mkold-from-vaddr-to-ops-common.patch
mm-damon-ops-common-handle-hugetlb-folios-in-folio-mkold-young-rmap-walkers.patch
mm-damon-paddr-support-hugetlb-folios-in-access-monitoring.patch