[PATCH 6.12.y] mm/damon/vaddr: drop last same folio access check optimization

[PATCH 6.12.y] mm/damon/vaddr: drop last same folio access check optimization

[SJ Park]

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/20260715031002.108504-4-sj@kernel.org
Link: https://lore.kernel.org/20260621204050.10993-1-sj@kernel.org [1]
Fixes: 3f49584b262c ("mm/damon: implement primitives for the virtual memory address spaces")
Signed-off-by: SJ Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 5.15.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 831846078caa14b7d00b2ccca8b8fe522afe3204)
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/vaddr.c | 34 ++++++----------------------------
 1 file changed, 6 insertions(+), 28 deletions(-)

diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
index 52e5c244bd021..089ee23e07de7 100644
--- a/mm/damon/vaddr.c
+++ b/mm/damon/vaddr.c
@@ -429,8 +429,6 @@ 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;
 };
 
@@ -466,7 +464,6 @@ 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;
 		folio_put(folio);
 huge_out:
 		spin_unlock(ptl);
@@ -488,7 +485,6 @@ 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);
 	folio_put(folio);
 out:
 	pte_unmap_unlock(pte, ptl);
@@ -517,7 +513,6 @@ 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);
 
@@ -535,11 +530,9 @@ static const struct mm_walk_ops damon_young_ops = {
 	.walk_lock = PGWALK_RDLOCK,
 };
 
-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,
 	};
 
@@ -556,29 +549,18 @@ 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)
@@ -587,16 +569,12 @@ 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);
-- 
2.47.3

Re: [PATCH 6.12.y] mm/damon/vaddr: drop last same folio access check optimization

[sashiko-bot]

> 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/20260715031002.108504-4-sj@kernel.org
> Link: https://lore.kernel.org/20260621204050.10993-1-sj@kernel.org [1]
> Fixes: 3f49584b262c ("mm/damon: implement primitives for the virtual memory address spaces")
> Signed-off-by: SJ Park <sj@kernel.org>
> Cc: <stable@vger.kernel.org> # 5.15.x
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> (cherry picked from commit 831846078caa14b7d00b2ccca8b8fe522afe3204)
> Signed-off-by: SJ Park <sj@kernel.org>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260909060130.269798-1-sj@kernel.org?part=1

Re: [PATCH 6.12.y] mm/damon/vaddr: drop last same folio access check optimization

[Sasha Levin]

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

Queued for 6.12, thanks.

-- 
Thanks,
Sasha