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FAILED: patch "[PATCH] mm/damon/vaddr: drop last same folio access check" failed to apply to 5.15-stable tree

PATCH Applied [email protected] 4 participants 4 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 5.15-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-5.15.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 '2026090859-trouble-fanatic-b990@gregkh' --subject-prefix 'PATCH 5.15.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 | 33 ++++++---------------------------
 1 file changed, 6 insertions(+), 27 deletions(-)
mm/damon/vaddr.c +6 -27
index dbb0f0fb2e598..bc0034b235d83 100644
@@ -516,7 +516,6 @@ void damon_va_prepare_access_checks(struct damon_ctx *ctx)
 }
 
 struct damon_young_walk_private {
-	unsigned long *page_sz;
 	bool young;
 };
 

@@ -545,10 +544,8 @@ static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr,
 			goto huge_out;
 		if (pmd_young(*pmd) || !page_is_idle(page) ||
 					mmu_notifier_test_young(walk->mm,
-						addr)) {
-			*priv->page_sz = ((1UL) << HPAGE_PMD_SHIFT);
+						addr))
 			priv->young = true;
-		}
 		put_page(page);
 huge_out:
 		spin_unlock(ptl);

@@ -567,10 +564,8 @@ static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr,
 	if (!page)
 		goto out;
 	if (pte_young(*pte) || !page_is_idle(page) ||
-			mmu_notifier_test_young(walk->mm, addr)) {
-		*priv->page_sz = PAGE_SIZE;
+			mmu_notifier_test_young(walk->mm, addr))
 		priv->young = true;
-	}
 	put_page(page);
 out:
 	pte_unmap_unlock(pte, ptl);

@@ -581,11 +576,9 @@ static struct mm_walk_ops damon_young_ops = {
 	.pmd_entry = damon_young_pmd_entry,
 };
 
-static bool damon_va_young(struct mm_struct *mm, unsigned long addr,
-		unsigned long *page_sz)
+static bool damon_va_young(struct mm_struct *mm, unsigned long addr)
 {
 	struct damon_young_walk_private arg = {
-		.page_sz = page_sz,
 		.young = false,
 	};
 

@@ -604,25 +597,11 @@ static bool damon_va_young(struct mm_struct *mm, unsigned long addr,
 static void damon_va_check_access(struct damon_ctx *ctx,
 			       struct mm_struct *mm, struct damon_region *r)
 {
-	static struct mm_struct *last_mm;
-	static unsigned long last_addr;
-	static unsigned long last_page_sz = PAGE_SIZE;
-	static bool last_accessed;
-
-	/* If the region is in the last checked page, reuse the result */
-	if (mm == last_mm && (ALIGN_DOWN(last_addr, last_page_sz) ==
-				ALIGN_DOWN(r->sampling_addr, last_page_sz))) {
-		if (last_accessed)
-			r->nr_accesses++;
-		return;
-	}
+	bool accessed;
 
-	last_accessed = damon_va_young(mm, r->sampling_addr, &last_page_sz);
-	if (last_accessed)
+	accessed = damon_va_young(mm, r->sampling_addr);
+	if (accessed)
 		r->nr_accesses++;
-
-	last_mm = mm;
-	last_addr = r->sampling_addr;
 }
 
 unsigned int damon_va_check_accesses(struct damon_ctx *ctx)
Signature · 1 line
2.47.3
AI review
#2 lore raw

Re: [PATCH 5.15.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] Lockless PMD read without pmdp_get_lockless() causes torn reads on architectures with non-atomic PMD accesses. --

commit 9322a55f3a2ea66cbeb4b3b0525204fccdd0b8a4
Author: SJ Park <[email protected]>
Subject: mm/damon/vaddr: drop last same folio access check optimization

This commit drops an optimization in DAMON's virtual memory access check that reused results if the region fell within the last checked page. The optimization was removed because it could race when multiple kdamonds are running and its actual impact was questionable.

Link https://lore.kernel.org/[email protected] open here
Link https://lore.kernel.org/[email protected] [1] open here
Quoted 4 lines
mm/damon/vaddr.c +0 -0
index dbb0f0fb2e598..bc0034b235d83 100644

[ ... ]

mm/damon/vaddr.c static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr, · line 546

Quoted 4 lines
mm/damon/vaddr.c +0 -0
@@ -545,10 +544,8 @@ static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr,
 			goto huge_out;
 		if (pmd_young(*pmd) || !page_is_idle(page) ||
 					mmu_notifier_test_young(walk->mm,

[Severity: High]
This is a pre-existing issue, but does the lockless PMD read earlier in damon_young_pmd_entry() risk torn reads on architectures with non-atomic PMD accesses?

In the THP check, the code performs a direct dereference:

	if (pmd_huge(*pmd)) {
		ptl = pmd_lock(walk->mm, pmd);

If a torn read occurs (e.g., on 32-bit architectures with 64-bit page table entries like ARM LPAE or i386 PAE) while concurrent huge page operations like THP collapse or split are happening, pmd_huge() might misidentify a huge PMD as a regular PMD.

mm/damon/vaddr.c static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr, · line 566

Quoted 12 lines
mm/damon/vaddr.c +1 -3
-						addr)) {
-			*priv->page_sz = ((1UL) << HPAGE_PMD_SHIFT);
+						addr))
 			priv->young = true;
-		}
 		put_page(page);
 huge_out:
 		spin_unlock(ptl);

@@ -567,10 +564,8 @@ static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr,
 	if (!page)
 		goto out;
 	if (pte_young(*pte) || !page_is_idle(page) ||

[Severity: High]
This is also a pre-existing issue, but does the regular page fallback path have the same torn read vulnerability?

Just before this block, the code directly dereferences the PMD again:

	if (pmd_none(*pmd) || unlikely(pmd_bad(*pmd)))
		return -EINVAL;
	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);

If the execution falls through to pte_offset_map_lock() due to a torn read misidentifying a huge PMD as regular, it would attempt to map the huge page's physical memory as a page table. Dereferencing this could trigger memory corruption or a machine check exception on non-existent memory.

Should these accesses be using pmdp_get_lockless() to ensure atomic reads?

Quoted 7 lines
mm/damon/vaddr.c +1 -3
-			mmu_notifier_test_young(walk->mm, addr)) {
-		*priv->page_sz = PAGE_SIZE;
+			mmu_notifier_test_young(walk->mm, addr))
 		priv->young = true;
-	}
 	put_page(page);
 out:
Signature · 1 line
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=1
maintainer Applied
#3 lore raw

Re: [PATCH 5.15.y] mm/damon/vaddr: drop last same folio access check optimization

In reply to SJ Park
Quoted 2 lines

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

Queued for 5.15, thanks.

Signature · 2 lines
Thanks,
Sasha