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FAILED: patch "[PATCH] mm/damon/vaddr: drop last same folio access check" failed to apply to 7.2-stable tree

PATCH New [email protected] 3 participants 3 messages Sep 8 – Sep 9

Subsystems (3)
Stable branch Memory management DAMON
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author maintainer
#0 lore raw

The patch below does not apply to the 7.2-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-7.2.y
git checkout FETCH_HEAD
git cherry-pick -x 831846078caa14b7d00b2ccca8b8fe522afe3204
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026090857-turtle-delirious-bcbc@gregkh' --subject-prefix 'PATCH 7.2.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 831846078caa14b7d00b2ccca8b8fe522afe3204 Mon Sep 17 00:00:00 2001
From: SJ Park <[email protected]>
Date: Tue, 14 Jul 2026 20:09:58 -0700
Subject: [PATCH] mm/damon/vaddr: drop last same folio access check
optimization

The optimization can race when multiple kdamonds are running. Meanwhile, the impact of the optimization is quite doubtful. Just remove it.

The user impact of the issue should be quite trivial. After all, the race can happen only when the user intentionally setup DAMON in the way. Even if it happens, it would be rare and only degrade the best-effort monitoring results. No critical consequences like kernel panic or memory corruption happen.

The race possibility was discovered [1] by Sashiko.

Link https://lore.kernel.org/[email protected] open here
Link https://lore.kernel.org/[email protected] [1] open here
Fixes 3f49584b262c ("mm/damon: implement primitives for the virtual memory address spaces")
Signed-off-by SJ Park <[email protected]>
Cc <[email protected]> # 5.15.x
Signed-off-by Andrew Morton <[email protected]>
mm/damon/vaddr.c +6 -27
index d10b8042adb5..d487b7a4a104 100644
@@ -383,8 +383,6 @@ static void damon_va_prepare_access_checks(struct damon_ctx *ctx)
 }
 
 struct damon_young_walk_private {
-	/* size of the folio for the access checked virtual memory address */
-	unsigned long *folio_sz;
 	bool young;
 };
 

@@ -411,7 +409,6 @@ static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr,
 					mmu_notifier_test_young(walk->mm,
 						addr))
 			priv->young = true;
-		*priv->folio_sz = HPAGE_PMD_SIZE;
 huge_out:
 		spin_unlock(ptl);
 		return 0;

@@ -430,7 +427,6 @@ static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr,
 	if (pte_young(ptent) || !folio_test_idle(folio) ||
 			mmu_notifier_test_young(walk->mm, addr))
 		priv->young = true;
-	*priv->folio_sz = folio_size(folio);
 out:
 	pte_unmap_unlock(pte, ptl);
 	return 0;

@@ -458,7 +454,6 @@ static int damon_young_hugetlb_entry(pte_t *pte, unsigned long hmask,
 	if (pte_young(entry) || !folio_test_idle(folio) ||
 	    mmu_notifier_test_young(walk->mm, addr))
 		priv->young = true;
-	*priv->folio_sz = huge_page_size(h);
 
 	folio_put(folio);
 

@@ -470,11 +465,9 @@ static int damon_young_hugetlb_entry(pte_t *pte, unsigned long hmask,
 #define damon_young_hugetlb_entry NULL
 #endif /* CONFIG_HUGETLB_PAGE */
 
-static bool damon_va_young(struct mm_struct *mm, unsigned long addr,
-		unsigned long *folio_sz)
+static bool damon_va_young(struct mm_struct *mm, unsigned long addr)
 {
 	struct damon_young_walk_private arg = {
-		.folio_sz = folio_sz,
 		.young = false,
 	};
 

@@ -494,28 +487,17 @@ static bool damon_va_young(struct mm_struct *mm, unsigned long addr,
  * r	the region to be checked
  */
 static void __damon_va_check_access(struct mm_struct *mm,
-				struct damon_region *r, bool same_target)
+				struct damon_region *r)
 {
-	static unsigned long last_addr;
-	static unsigned long last_folio_sz = PAGE_SIZE;
-	static bool last_accessed;
+	bool accessed;
 
 	if (!mm) {
 		damon_update_region_access_rate(r, false);
 		return;
 	}
 
-	/* If the region is in the last checked page, reuse the result */
-	if (same_target && (ALIGN_DOWN(last_addr, last_folio_sz) ==
-				ALIGN_DOWN(r->sampling_addr, last_folio_sz))) {
-		damon_update_region_access_rate(r, last_accessed);
-		return;
-	}
-
-	last_accessed = damon_va_young(mm, r->sampling_addr, &last_folio_sz);
-	damon_update_region_access_rate(r, last_accessed);
-
-	last_addr = r->sampling_addr;
+	accessed = damon_va_young(mm, r->sampling_addr);
+	damon_update_region_access_rate(r, accessed);
 }
 
 static unsigned int damon_va_check_accesses(struct damon_ctx *ctx)

@@ -524,15 +506,12 @@ static unsigned int damon_va_check_accesses(struct damon_ctx *ctx)
 	struct mm_struct *mm;
 	struct damon_region *r;
 	unsigned int max_nr_accesses = 0;
-	bool same_target;
 
 	damon_for_each_target(t, ctx) {
 		mm = damon_get_mm(t);
-		same_target = false;
 		damon_for_each_region(r, t) {
-			__damon_va_check_access(mm, r, same_target);
+			__damon_va_check_access(mm, r);
 			max_nr_accesses = max(r->nr_accesses, max_nr_accesses);
-			same_target = true;
 		}
 		if (mm)
 			mmput(mm);
maintainer
#1 lore raw

PATCH 1/1 mm/damon/vaddr: drop last same folio access check optimization

The optimization can race when multiple kdamonds are running. Meanwhile, the impact of the optimization is quite doubtful. Just remove it.

The user impact of the issue should be quite trivial. After all, the race can happen only when the user intentionally setup DAMON in the way. Even if it happens, it would be rare and only degrade the best-effort monitoring results. No critical consequences like kernel panic or memory corruption happen.

The race possibility was discovered [1] by Sashiko.

Link https://lore.kernel.org/[email protected] open here
Link https://lore.kernel.org/[email protected] [1] open here
Fixes 3f49584b262c ("mm/damon: implement primitives for the virtual memory address spaces")
Signed-off-by SJ Park <[email protected]>
Cc <[email protected]> # 5.15.x
Signed-off-by Andrew Morton <[email protected]>
(cherry picked from commit 831846078caa14b7d00b2ccca8b8fe522afe3204)
Signed-off-by SJ Park <[email protected]>
---
 mm/damon/vaddr.c | 34 ++++++----------------------------
 1 file changed, 6 insertions(+), 28 deletions(-)
mm/damon/vaddr.c +6 -28
index 2c1c1952c008d..404785128ffd7 100644
@@ -382,8 +382,6 @@ static void damon_va_prepare_access_checks(struct damon_ctx *ctx)
 }
 
 struct damon_young_walk_private {
-	/* size of the folio for the access checked virtual memory address */
-	unsigned long *folio_sz;
 	bool young;
 };
 

@@ -410,7 +408,6 @@ static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr,
 					mmu_notifier_test_young(walk->mm,
 						addr))
 			priv->young = true;
-		*priv->folio_sz = HPAGE_PMD_SIZE;
 huge_out:
 		spin_unlock(ptl);
 		return 0;

@@ -429,7 +426,6 @@ static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr,
 	if (pte_young(ptent) || !folio_test_idle(folio) ||
 			mmu_notifier_test_young(walk->mm, addr))
 		priv->young = true;
-	*priv->folio_sz = folio_size(folio);
 out:
 	pte_unmap_unlock(pte, ptl);
 	return 0;

@@ -457,7 +453,6 @@ static int damon_young_hugetlb_entry(pte_t *pte, unsigned long hmask,
 	if (pte_young(entry) || !folio_test_idle(folio) ||
 	    mmu_notifier_test_young(walk->mm, addr))
 		priv->young = true;
-	*priv->folio_sz = huge_page_size(h);
 
 	folio_put(folio);
 

@@ -469,11 +464,9 @@ static int damon_young_hugetlb_entry(pte_t *pte, unsigned long hmask,
 #define damon_young_hugetlb_entry NULL
 #endif /* CONFIG_HUGETLB_PAGE */
 
-static bool damon_va_young(struct mm_struct *mm, unsigned long addr,
-		unsigned long *folio_sz)
+static bool damon_va_young(struct mm_struct *mm, unsigned long addr)
 {
 	struct damon_young_walk_private arg = {
-		.folio_sz = folio_sz,
 		.young = false,
 	};
 

@@ -493,29 +486,18 @@ static bool damon_va_young(struct mm_struct *mm, unsigned long addr,
  * r	the region to be checked
  */
 static void __damon_va_check_access(struct mm_struct *mm,
-				struct damon_region *r, bool same_target,
+				struct damon_region *r,
 				struct damon_attrs *attrs)
 {
-	static unsigned long last_addr;
-	static unsigned long last_folio_sz = PAGE_SIZE;
-	static bool last_accessed;
+	bool accessed;
 
 	if (!mm) {
 		damon_update_region_access_rate(r, false, attrs);
 		return;
 	}
 
-	/* If the region is in the last checked page, reuse the result */
-	if (same_target && (ALIGN_DOWN(last_addr, last_folio_sz) ==
-				ALIGN_DOWN(r->sampling_addr, last_folio_sz))) {
-		damon_update_region_access_rate(r, last_accessed, attrs);
-		return;
-	}
-
-	last_accessed = damon_va_young(mm, r->sampling_addr, &last_folio_sz);
-	damon_update_region_access_rate(r, last_accessed, attrs);
-
-	last_addr = r->sampling_addr;
+	accessed = damon_va_young(mm, r->sampling_addr);
+	damon_update_region_access_rate(r, accessed, attrs);
 }
 
 static unsigned int damon_va_check_accesses(struct damon_ctx *ctx)

@@ -524,16 +506,12 @@ static unsigned int damon_va_check_accesses(struct damon_ctx *ctx)
 	struct mm_struct *mm;
 	struct damon_region *r;
 	unsigned int max_nr_accesses = 0;
-	bool same_target;
 
 	damon_for_each_target(t, ctx) {
 		mm = damon_get_mm(t);
-		same_target = false;
 		damon_for_each_region(r, t) {
-			__damon_va_check_access(mm, r, same_target,
-					&ctx->attrs);
+			__damon_va_check_access(mm, r, &ctx->attrs);
 			max_nr_accesses = max(r->nr_accesses, max_nr_accesses);
-			same_target = true;
 		}
 		if (mm)
 			mmput(mm);
Signature · 1 line
2.47.3
AI review
#2 lore raw

Re: [PATCH 7.2.y] mm/damon/vaddr: drop last same folio access check optimization

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] DAMOS PTE-level PMD iteration incorrectly advances past unrelated memory when encountering large folio tail pages. --

commit d786b1179a857b410a9db73cdc4ee0d7bdcbede5
Author: SJ Park <[email protected]>

mm/damon/vaddr: drop last same folio access check optimization

This commit drops the last same folio access check optimization in DAMON. The optimization can race when multiple kdamonds are running, and its impact is doubtful.

Link https://lore.kernel.org/[email protected] open here
Link https://lore.kernel.org/[email protected] [1] open here

mm/damon/vaddr.c

Quoted 4 lines
mm/damon/vaddr.c +0 -0
index 2c1c1952c008d..404785128ffd7 100644

[ ... ]

[Severity: High]
This is a pre-existing issue, but does this code incorrectly advance past unrelated memory when encountering large folio tail pages?

Looking at damos_va_migrate_pmd_entry() and damos_va_stat_pmd_entry() in mm/damon/vaddr.c, the PTE iterator unconditionally advances by the full folio size:

mm/damon/vaddr.c:damos_va_migrate_pmd_entry() {
    ...
	for (; addr < next; pte += nr, addr += nr * PAGE_SIZE) {
		nr = 1;
		ptent = ptep_get(pte);

		if (pte_none(ptent) || !pte_present(ptent))
			continue;
		folio = vm_normal_folio(walk->vma, addr, ptent);
    ...
		damos_va_migrate_dests_add(folio, walk->vma, addr, dests,
				migration_lists);
		nr = folio_nr_pages(folio);
	}
    ...
}

If the loop encounters a PTE pointing to a tail page of a large folio, folio_nr_pages(folio) returns the size of the entire large folio. Because the current PTE does not point to the head of the folio, advancing pte and addr by the full folio size will overshoot into unrelated page table entries, skipping memory regions during migration and stats scanning.

Could this lead to incorrect functional behavior where unrelated memory regions are silently skipped by DAMOS actions?

Signature · 1 line
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=1