mm/damon: unurgent fixes for infinite loop, NULL de-ref and races

RFC PATCH In review SJ Park 2 participants 8 messages Jul 12

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Stable branch Memory management Kselftest DAMON

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SJ Park

author maintainer

2026-07-12 17:03 UTC #0 lore raw

cover mm/damon: unurgent fixes for infinite loop, NULL de-ref and races

Sashiko found a few issues in DAMON that could cause infinite loop, NULL dereference and monitoring results degradation. The first two sounds scary but the infinite loop happens only under unreasonable user setup. The NULL dereference is only in a unit test. Monitoring results degradation is trivial since it is only best-effort, and those happens from only unlikely races. Still those are bugs that better to fix if possible. Fix those.

Changes from RFC
- RFC: https://lore.kernel.org/[email protected]
- Rebase to mm-new.

SJ Park (5):
mm/damon/core: avoid infinite kdamond_merge_regions() internal loop mm/damon/tests/core-kunit: catch test failure in
test_merge_regions_of()
mm/damon/vaddr: drop last same folio access check optimization mm/damon/paddr: drop last same folio access check reuse optimization mm/damon/sysfs: read ops_id only once

 mm/damon/core.c             | 13 +++++++++----
 mm/damon/paddr.c            | 20 ++++----------------
 mm/damon/sysfs.c            |  6 ++++--
 mm/damon/tests/core-kunit.h |  3 +++
 mm/damon/vaddr.c            | 26 ++++----------------------
 5 files changed, 24 insertions(+), 44 deletions(-)

base-commit: df701015aac3c13b1ecff311fda1a2ada2b9c1b2

Signature · 1 line

2.47.3

SJ Park

author maintainer

2026-07-12 17:03 UTC #1 lore raw

RFC PATCH 1/5 mm/damon/core: avoid infinite kdamond_merge_regions() internal loop

Due to online parameter update like events, the number of DAMON regions could be higher than the user-set upper limit. kdamond_merge_regions() repeats merge regions until the number meets the limit, while doubling the merge threshold up to the theoretical maximum threshold. It is tried only up to the theoretical maximum threshold because even the aggressive merging can fail from reducing the number of regions under the user-defined upper limit. For example, there could be many user-defined non-contiguous regions that cannot be merged.

The threshold based loop break condition is evaluated by comparing the threshold for the next merging try against the theoretical maximum threshold. If max_thres is larger than UINT_MAX / 2, doubling the threshold could make it overflow, and bypass the loop break condition. In the case, if the number of regions cannot be reduced under the upper limit like explained above, the loop will run infinitely.

Prevent the case by doing the break condition check before doubling the threshold. Also, prevent the threshold exceeding the maximum threshold, as it could overflow and apply the wrong merge threshold.

This issue is unlikely to occur in real world, since having the max_thres higher than UINT_MAX / 2 require unrealistically large aggregation intervals compared to the sampling interval. Also, it requires an unrealistically large number of uncontiguous regions setup. Nonetheless, the consequence is bad and the fix is simple.

The issue was discovered [1] by Sashiko.

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

Fixes 310d6c15e910 ("mm/damon/core: merge regions aggressively when max_nr_regions is unmet")

Cc <[email protected]> # 6.10.x

Signed-off-by SJ Park <[email protected]>

---
 mm/damon/core.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

mm/damon/core.c +9 -4

index 806a67d02a6e9..f3b6a46fdaabd 100644
@@ -3369,15 +3369,20 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold,
 
 	max_thres = c->attrs.aggr_interval /
 		(c->attrs.sample_interval ?  c->attrs.sample_interval : 1);
-	do {
+	while (true) {
 		nr_regions = 0;
 		damon_for_each_target(t, c) {
 			damon_merge_regions_of(t, threshold, sz_limit, c);
 			nr_regions += damon_nr_regions(t);
 		}
-		threshold = max(1, threshold * 2);
-	} while (nr_regions > c->attrs.max_nr_regions &&
-			threshold / 2 < max_thres);
+		if (nr_regions <= c->attrs.max_nr_regions ||
+				max_thres <= threshold)
+			break;
+		if (threshold < max_thres / 2)
+			threshold = max(1, threshold * 2);
+		else
+			threshold = max_thres;
+	}
 }
 
 #ifdef CONFIG_DAMON_DEBUG_SANITY

Signature · 1 line

2.47.3

SJ Park

author maintainer

2026-07-12 17:03 UTC #2 lore raw

RFC PATCH 2/5 mm/damon/tests/core-kunit: catch test failure in test_merge_regions_of()

In reply to SJ Park

KUNIT_EXPECT_EQ() does not abort the execution of test code when the expectation is not met. But damon_test_merge_regions_of() code after its initial KUNIT_EXPECT_EQ() call assumes the expectation is met. It does a per-region test with a hard-coded number of regions that is correct only if the expectation was met. As a result, __nth_region_of() could return NULL, and the test code can dereference NULL pointers. Fix the issue by catching the expectation failure and skip the per-region tests.

The user impact on realistic setups should be negligible, as it is a unit test.

The issue was discovered [1] by Sashiko.

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

Fixes 17ccae8bb5c9 ("mm/damon: add kunit tests")

Cc <[email protected]> # 5.15.x

Signed-off-by SJ Park <[email protected]>

---
 mm/damon/tests/core-kunit.h | 3 +++
 1 file changed, 3 insertions(+)

mm/damon/tests/core-kunit.h +3 -0

index 6ad73559dd8ea..a99363720e677 100644
@@ -260,11 +260,14 @@ static void damon_test_merge_regions_of(struct kunit *test)
 	damon_merge_regions_of(t, 9, 9999, ctx);
 	/* 0-112, 114-130, 130-156, 156-170, 170-230, 230-10170 */
 	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 6u);
+	if (damon_nr_regions(t) != 6)
+		goto out;
 	for (i = 0; i < 6; i++) {
 		r = __nth_region_of(t, i);
 		KUNIT_EXPECT_EQ(test, r->ar.start, saddrs[i]);
 		KUNIT_EXPECT_EQ(test, r->ar.end, eaddrs[i]);
 	}
+out:
 	damon_free_target(t);
 	damon_destroy_ctx(ctx);
 }

Signature · 1 line

2.47.3

SJ Park

author maintainer

2026-07-12 17:03 UTC #3 lore raw

RFC PATCH 3/5 mm/damon/vaddr: drop last same folio access check optimization

In reply to 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.

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

Fixes 3f49584b262c ("mm/damon: implement primitives for the virtual memory address spaces")

Cc <[email protected]> # 5.15.x

Signed-off-by SJ Park <[email protected]>

---
 mm/damon/vaddr.c | 26 ++++----------------------
 1 file changed, 4 insertions(+), 22 deletions(-)

mm/damon/vaddr.c +4 ~~-22~~

index d10b8042adb5b..16fe210d2042a 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,
 	};

@@ -496,26 +489,15 @@ static bool damon_va_young(struct mm_struct *mm, unsigned long addr,
 static void __damon_va_check_access(struct mm_struct *mm,
 				struct damon_region *r, bool same_target)
 {
-	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)

Signature · 1 line

2.47.3

[email protected]

AI review

2026-07-12 17:10 UTC #4 lore raw

Re: [RFC PATCH v1.1 3/5] 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: - [Low] Unused parameter and dead code tracking logic left behind after removing optimization. --

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] Unused parameter and dead code tracking logic left behind after removing optimization. --

--- Patch [3]: [RFC PATCH v1.1 3/5] mm/damon/vaddr: drop last same folio access check optimization --- commit 40e12defdea3b013644cc026283aa34cdd3ae2fb
Author: SJ Park <[email protected]>
mm/damon/vaddr: drop last same folio access check optimization

This commit removes the last same folio access check optimization from DAMON's virtual address space primitives. The optimization was found to cause race conditions when multiple kdamonds are running.

Quoted 4 lines

mm/damon/vaddr.c +0 -0
index d10b8042adb5b..16fe210d2042a 100644

[ ... ]

mm/damon/vaddr.c static bool damon_va_young(struct mm_struct *mm, unsigned long addr, · line 491

Quoted 4 lines

mm/damon/vaddr.c +0 -0

@@ -496,26 +489,15 @@ static bool damon_va_young(struct mm_struct *mm, unsigned long addr,
 static void __damon_va_check_access(struct mm_struct *mm,
 				struct damon_region *r, bool same_target)
 {

[Severity: Low]
Does this code still need the same_target parameter?

With the optimization removed below, the parameter is no longer used in __damon_va_check_access().

The caller, damon_va_check_accesses(), also still computes and tracks the local same_target variable on every iteration just to pass it here:

	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);
			max_nr_accesses = max(r->nr_accesses, max_nr_accesses);
			same_target = true;
		}
		if (mm)
			mmput(mm);
	}

Could we remove the parameter and the dead tracking code in the caller as well?

Quoted 26 lines

mm/damon/vaddr.c +3 ~~-14~~

-	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)

Signature · 1 line

Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=3

SJ Park

author maintainer

2026-07-12 17:24 UTC #5 lore raw

Re: [RFC PATCH v1.1 3/5] mm/damon/vaddr: drop last same folio access check optimization

Quoted 46 lines · On Sun, 12 Jul 2026 17:10:09 +0000 [email protected] wrote:

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] Unused parameter and dead code tracking logic left behind after removing optimization. --
--- Patch [3]: [RFC PATCH v1.1 3/5] mm/damon/vaddr: drop last same folio access check optimization --- commit 40e12defdea3b013644cc026283aa34cdd3ae2fb
Author: SJ Park <[email protected]>
mm/damon/vaddr: drop last same folio access check optimization
This commit removes the last same folio access check optimization from DAMON's virtual address space primitives. The optimization was found to cause race conditions when multiple kdamonds are running.
Quoted 4 lines
mm/damon/vaddr.c +0 -0
index d10b8042adb5b..16fe210d2042a 100644
[ ... ]
mm/damon/vaddr.c static bool damon_va_young(struct mm_struct *mm, unsigned long addr, · line 491
Quoted 4 lines
mm/damon/vaddr.c +0 -0
@@ -496,26 +489,15 @@ static bool damon_va_young(struct mm_struct *mm, unsigned long addr,
 static void __damon_va_check_access(struct mm_struct *mm,
 				struct damon_region *r, bool same_target)
 {
[Severity: Low]
Does this code still need the same_target parameter?
With the optimization removed below, the parameter is no longer used in __damon_va_check_access().
The caller, damon_va_check_accesses(), also still computes and tracks the local same_target variable on every iteration just to pass it here:
	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);
			max_nr_accesses = max(r->nr_accesses, max_nr_accesses);
			same_target = true;
		}
		if (mm)
			mmput(mm);
	}

Quoted 1 line

Could we remove the parameter and the dead tracking code in the caller as well?

Good finding. I will drop that in the next revision.

Thanks,
SJ

[...]

SJ Park

author maintainer

2026-07-12 17:03 UTC #6 lore raw

RFC PATCH 4/5 mm/damon/paddr: drop last same folio access check reuse optimization

In reply to SJ Park

It can race when multiple kdamonds are being used. The problem from the race is doubtful, but the gain from the optimization is also doubtful. Simply drop the optimization in favor of code simplicity.

The user impact is doubtfully trivial. After all, this kind of interference can happen only by intentional user setup. Even if it happens, it will be rare, and the consequence is degradation of the best-effort monitoring results. No critical consequences like kernel panic or memory corruption happen.

The race was discovered [1] by Sashiko.

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

Fixes a28397beb55b ("mm/damon: implement primitives for physical address space monitoring")

Cc <[email protected]> # 5.16.x

Signed-off-by SJ Park <[email protected]>

---
 mm/damon/paddr.c | 20 ++++----------------
 1 file changed, 4 insertions(+), 16 deletions(-)

mm/damon/paddr.c +4 ~~-16~~

index b85f88a7a38f4..e4f98d67461f5 100644
@@ -65,7 +65,7 @@ static void damon_pa_prepare_access_checks(struct damon_ctx *ctx)
 	}
 }
 
-static bool damon_pa_young(phys_addr_t paddr, unsigned long *folio_sz)
+static bool damon_pa_young(phys_addr_t paddr)
 {
 	struct folio *folio = damon_get_folio(PHYS_PFN(paddr));
 	bool accessed;

@@ -74,7 +74,6 @@ static bool damon_pa_young(phys_addr_t paddr, unsigned long *folio_sz)
 		return false;
 
 	accessed = damon_folio_young(folio);
-	*folio_sz = folio_size(folio);
 	folio_put(folio);
 	return accessed;
 }

@@ -82,23 +81,12 @@ static bool damon_pa_young(phys_addr_t paddr, unsigned long *folio_sz)
 static void __damon_pa_check_access(struct damon_region *r,
 		unsigned long addr_unit)
 {
-	static phys_addr_t last_addr;
-	static unsigned long last_folio_sz = PAGE_SIZE;
-	static bool last_accessed;
+	bool accessed;
 	phys_addr_t sampling_addr = damon_pa_phys_addr(
 			r->sampling_addr, addr_unit);
 
-	/* If the region is in the last checked page, reuse the result */
-	if (ALIGN_DOWN(last_addr, last_folio_sz) ==
-				ALIGN_DOWN(sampling_addr, last_folio_sz)) {
-		damon_update_region_access_rate(r, last_accessed);
-		return;
-	}
-
-	last_accessed = damon_pa_young(sampling_addr, &last_folio_sz);
-	damon_update_region_access_rate(r, last_accessed);
-
-	last_addr = sampling_addr;
+	accessed = damon_pa_young(sampling_addr);
+	damon_update_region_access_rate(r, accessed);
 }
 
 static unsigned int damon_pa_check_accesses(struct damon_ctx *ctx)

Signature · 1 line

2.47.3

SJ Park

author maintainer

2026-07-12 17:03 UTC #7 lore raw

RFC PATCH 5/5 mm/damon/sysfs: read ops_id only once

In reply to SJ Park

damon_sysfs_apply_inputs() reads ops_id twice. It could race with ops_id_store(). As a result, the min_region_sz could wrongly be set up. Read it once.

The user impact is trivial. Sane users ain't update the parameter in parallel. Even if it happens, only monitoring itself runs differently than expected. No critical consequences like kernel panic or memory corruption happen.

The issue was discovered [1] by Sashiko.

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

Signed-off-by SJ Park <[email protected]>

---
 mm/damon/sysfs.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

mm/damon/sysfs.c +4 -2

index b5fe036f78015..60a1a9e4ada34 100644
@@ -2094,14 +2094,16 @@ static inline bool damon_sysfs_kdamond_running(
 static int damon_sysfs_apply_inputs(struct damon_ctx *ctx,
 		struct damon_sysfs_context *sys_ctx)
 {
+	enum damon_ops_id ops_id;
 	int err;
 
-	err = damon_select_ops(ctx, sys_ctx->ops_id);
+	ops_id = READ_ONCE(sys_ctx->ops_id);
+	err = damon_select_ops(ctx, ops_id);
 	if (err)
 		return err;
 	ctx->addr_unit = sys_ctx->addr_unit;
 	/* addr_unit is respected by only DAMON_OPS_PADDR */
-	if (sys_ctx->ops_id == DAMON_OPS_PADDR)
+	if (ops_id == DAMON_OPS_PADDR)
 		ctx->min_region_sz = max(
 				DAMON_MIN_REGION_SZ / sys_ctx->addr_unit, 1);
 	ctx->pause = sys_ctx->pause;

Signature · 1 line

2.47.3