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FAILED: patch "[PATCH] mm/damon/vaddr: drop last same folio access check" failed to apply to 6.1-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 6.1-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-6.1.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 '2026090858-legislate-monogamy-2299@gregkh' --subject-prefix 'PATCH 6.1.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 | 42 +++++++++---------------------------------
 1 file changed, 9 insertions(+), 33 deletions(-)
mm/damon/vaddr.c +9 -33
index 708f281c1b6b4..4aa89347d754d 100644
@@ -424,7 +424,6 @@ static void damon_va_prepare_access_checks(struct damon_ctx *ctx)
 }
 
 struct damon_young_walk_private {
-	unsigned long *page_sz;
 	bool young;
 };
 

@@ -453,10 +452,8 @@ 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 = HPAGE_PMD_SIZE;
+						addr))
 			priv->young = true;
-		}
 		put_page(page);
 huge_out:
 		spin_unlock(ptl);

@@ -475,10 +472,8 @@ 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);

@@ -505,10 +500,8 @@ static int damon_young_hugetlb_entry(pte_t *pte, unsigned long hmask,
 	get_page(page);
 
 	if (pte_young(entry) || !page_is_idle(page) ||
-	    mmu_notifier_test_young(walk->mm, addr)) {
-		*priv->page_sz = huge_page_size(h);
+	    mmu_notifier_test_young(walk->mm, addr))
 		priv->young = true;
-	}
 
 	put_page(page);
 

@@ -525,11 +518,9 @@ static const struct mm_walk_ops damon_young_ops = {
 	.hugetlb_entry = damon_young_hugetlb_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,
 	};
 

@@ -546,25 +537,13 @@ 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_page_sz = PAGE_SIZE;
-	static bool last_accessed;
-
-	/* If the region is in the last checked page, reuse the result */
-	if (same_target && (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_addr = r->sampling_addr;
 }
 
 static unsigned int damon_va_check_accesses(struct damon_ctx *ctx)

@@ -573,17 +552,14 @@ 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);
 		if (!mm)
 			continue;
-		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;
 		}
 		mmput(mm);
 	}
Signature · 1 line
2.47.3
AI review
#2 lore raw

Re: [PATCH 6.1.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] The PMD is read directly via *pmd outside a lock, which can cause torn reads on architectures where PMD size is larger than the word size (e.g., 32-bit PAE). --

commit 3ab3595f2bebbbf09add4e61c160f37ee6aa2807
Author: SJ Park <[email protected]>

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

This commit removes the same-folio access check optimization from DAMON's virtual address monitoring. The optimization was found to race when multiple kdamonds are running, and its impact was deemed doubtful.

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 708f281c1b6b4..4aa89347d754d 100644

[ ... ]

mm/damon/vaddr.c static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr, · line 456

Quoted 8 lines
mm/damon/vaddr.c +1 -2
@@ -453,10 +452,8 @@ 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 = HPAGE_PMD_SIZE;
+						addr))
 			priv->young = true;

[Severity: High]
This is a pre-existing issue, but does reading the PMD directly via *pmd outside a lock risk torn reads on architectures where the PMD size is larger than the word size (like 32-bit PAE)?

Earlier in damon_young_pmd_entry(), the code does:

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

Since damon_young_pmd_entry() runs during DAMON's page table walk under mmap_read_lock(), concurrent THP modifications aren't prevented. If a torn read occurs on the lockless *pmd evaluation during a concurrent THP split or collapse, it could bypass both the pmd_trans_huge() and pmd_bad() checks.

If it falls through to the regular page path, pte_offset_map_lock() might map a bogus physical address from the torn value, allowing arbitrary memory to be treated as PTEs.

A similar lockless check appears to exist in damon_mkold_pmd_entry() as well. Could these paths use pmd_read_atomic() or pmdp_get_lockless() to safely evaluate the PMD?

Quoted 4 lines
mm/damon/vaddr.c +0 -1
-		}
 		put_page(page);
 huge_out:
 		spin_unlock(ptl);
Signature · 1 line
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=1
maintainer Applied
#3 lore raw

Re: [PATCH 6.1.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 6.1, thanks.

Signature · 2 lines
Thanks,
Sasha