FAILED: patch "[PATCH] mm/damon/core: skip aging from repeated aggressive merging" failed to apply to 6.18-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

2026-09-08 12:57 UTC #0 lore raw

The patch below does not apply to the 6.18-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-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 0250dbe08c730d003ef9f484da56ae09a1ea0c4c
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026090840-shifter-galvanize-450f@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

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

From 0250dbe08c730d003ef9f484da56ae09a1ea0c4c Mon Sep 17 00:00:00 2001
From: SJ Park <[email protected]>
Date: Sun, 12 Jul 2026 09:54:30 -0700
Subject: [PATCH] mm/damon/core: skip aging from repeated aggressive merging

The number of DAMON regions could temporarily exceed the user-defined maximum number of regions limit for corner cases. For example, users could lower the limit via runtime parameters update. For such a case, kdamond_merge_regions() repeats merging regions in the case doubling the merge threshold. The repeated merge operation could update the age of regions multiple times. This corrupts the monitoring results. Fix the issue by asking the merge operation to skip aging for the corner case.

The user impact is degradation of the monitoring quality. The impact should be mild, since the degradation is only temporal, and it is not common to happen in realistic setups.

The issue was discovered [1,2] by Sashiko.

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

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

Link https://lore.kernel.org/[email protected] [2] open here

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

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

Cc <[email protected]> # 6.10

Signed-off-by Andrew Morton <[email protected]>

mm/damon/core.c +13 -8

index 806a67d02a6e..6c4215cc809e 100644
@@ -3308,7 +3308,7 @@ static unsigned int damon_merge_score(struct damon_region *r, bool last,
  * sz_limit	size upper limit of each region
  */
 static void damon_merge_regions_of(struct damon_target *t, unsigned int thres,
-		unsigned long sz_limit, struct damon_ctx *ctx)
+		unsigned long sz_limit, struct damon_ctx *ctx, bool count_age)
 {
 	struct damon_region *r, *prev = NULL, *next;
 	bool use_probe_hits = damon_has_probe_weights(ctx);

@@ -3319,12 +3319,14 @@ static void damon_merge_regions_of(struct damon_target *t, unsigned int thres,
 		score = damon_merge_score(r, false, ctx, use_probe_hits);
 		last_score = damon_merge_score(r, true, ctx, use_probe_hits);
 
-		if (abs_diff(score, last_score) > thres)
-			r->age = 0;
-		else if ((score == 0) != (last_score == 0))
-			r->age = 0;
-		else
-			r->age++;
+		if (count_age) {
+			if (abs_diff(score, last_score) > thres)
+				r->age = 0;
+			else if ((score == 0) != (last_score == 0))
+				r->age = 0;
+			else
+				r->age++;
+		}
 
 		if (!prev)
 			goto set_prev_continue;

@@ -3366,15 +3368,18 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold,
 	struct damon_target *t;
 	unsigned int nr_regions;
 	unsigned int max_thres;
+	bool count_age = true;
 
 	max_thres = c->attrs.aggr_interval /
 		(c->attrs.sample_interval ?  c->attrs.sample_interval : 1);
 	do {
 		nr_regions = 0;
 		damon_for_each_target(t, c) {
-			damon_merge_regions_of(t, threshold, sz_limit, c);
+			damon_merge_regions_of(t, threshold, sz_limit, c,
+					count_age);
 			nr_regions += damon_nr_regions(t);
 		}
+		count_age = false;
 		threshold = max(1, threshold * 2);
 	} while (nr_regions > c->attrs.max_nr_regions &&
 			threshold / 2 < max_thres);

mm/damon/tests/core-kunit.h +1 -1

index 73cb8fcdf07e..485472ddebd1 100644
@@ -257,7 +257,7 @@ static void damon_test_merge_regions_of(struct kunit *test)
 		damon_add_region(r, t);
 	}
 
-	damon_merge_regions_of(t, 9, 9999, ctx);
+	damon_merge_regions_of(t, 9, 9999, ctx, true);
 	/* 0-112, 114-130, 130-156, 156-170, 170-230, 230-10170 */
 	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 6u);
 	for (i = 0; i < 6; i++) {

SJ Park

maintainer

2026-09-09 04:41 UTC #1 lore raw

PATCH 1/1 mm/damon/core: skip aging from repeated aggressive merging

The user impact is degradation of the monitoring quality. The impact should be mild, since the degradation is only temporal, and it is not common to happen in realistic setups.

The issue was discovered [1,2] by Sashiko.

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

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

Link https://lore.kernel.org/[email protected] [2] open here

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

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

Cc <[email protected]> # 6.10

Signed-off-by Andrew Morton <[email protected]>

(cherry picked from commit 0250dbe08c730d003ef9f484da56ae09a1ea0c4c)

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

---
 mm/damon/core.c             | 21 +++++++++++++--------
 mm/damon/tests/core-kunit.h |  2 +-
 2 files changed, 14 insertions(+), 9 deletions(-)

mm/damon/core.c +13 -8

index 70ac1f08753d1..c98539efd2a56 100644
@@ -2364,17 +2364,19 @@ static void damon_merge_two_regions(struct damon_target *t,
  * sz_limit	size upper limit of each region
  */
 static void damon_merge_regions_of(struct damon_target *t, unsigned int thres,
-				   unsigned long sz_limit)
+		unsigned long sz_limit, bool count_age)
 {
 	struct damon_region *r, *prev = NULL, *next;
 
 	damon_for_each_region_safe(r, next, t) {
-		if (abs(r->nr_accesses - r->last_nr_accesses) > thres)
-			r->age = 0;
-		else if ((r->nr_accesses == 0) != (r->last_nr_accesses == 0))
-			r->age = 0;
-		else
-			r->age++;
+		if (count_age) {
+			if (abs(r->nr_accesses - r->last_nr_accesses) > thres)
+				r->age = 0;
+			else if ((r->nr_accesses == 0) != (r->last_nr_accesses == 0))
+				r->age = 0;
+			else
+				r->age++;
+		}
 
 		if (prev && prev->ar.end == r->ar.start &&
 		    abs(prev->nr_accesses - r->nr_accesses) <= thres &&

@@ -2408,15 +2410,18 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold,
 	struct damon_target *t;
 	unsigned int nr_regions;
 	unsigned int max_thres;
+	bool count_age = true;
 
 	max_thres = c->attrs.aggr_interval /
 		(c->attrs.sample_interval ?  c->attrs.sample_interval : 1);
 	do {
 		nr_regions = 0;
 		damon_for_each_target(t, c) {
-			damon_merge_regions_of(t, threshold, sz_limit);
+			damon_merge_regions_of(t, threshold, sz_limit,
+					count_age);
 			nr_regions += damon_nr_regions(t);
 		}
+		count_age = false;
 		threshold = max(1, threshold * 2);
 	} while (nr_regions > c->attrs.max_nr_regions &&
 			threshold / 2 < max_thres);

mm/damon/tests/core-kunit.h +1 -1

index 6687778752728..80dcf1d641e07 100644
@@ -261,7 +261,7 @@ static void damon_test_merge_regions_of(struct kunit *test)
 		damon_add_region(r, t);
 	}
 
-	damon_merge_regions_of(t, 9, 9999);
+	damon_merge_regions_of(t, 9, 9999, true);
 	/* 0-112, 114-130, 130-156, 156-170 */
 	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 5u);
 	for (i = 0; i < 5; i++) {

Signature · 1 line

2.47.3

[email protected]

AI review

2026-09-09 04:51 UTC #2 lore raw

Re: [PATCH 6.18.y] mm/damon/core: skip aging from repeated aggressive merging

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

Pre-existing issues:
- [High] Infinite loop in kdamond_merge_regions due to threshold integer overflow --

commit af1455c64e5ba1df658a88dc7d8a3cbfe32c0c5b
Author: SJ Park <[email protected]>

mm/damon/core: skip aging from repeated aggressive merging

The number of DAMON regions could temporarily exceed the user-defined maximum number of regions limit. This patch modifies kdamond_merge_regions() to skip aging when the merge operation is repeated to meet the max regions limit, preventing corruption of the monitoring results.

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

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

Link https://lore.kernel.org/[email protected] [2] open here

Quoted 4 lines

mm/damon/core.c +0 -0

index 70ac1f08753d1..c98539efd2a56 100644

[ ... ]

Quoted 10 lines

mm/damon/core.c +1 -0

@@ -2408,15 +2410,18 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold,
 	struct damon_target *t;
 	unsigned int nr_regions;
 	unsigned int max_thres;
+	bool count_age = true;
 
 	max_thres = c->attrs.aggr_interval /
 		(c->attrs.sample_interval ?  c->attrs.sample_interval : 1);
 	do {
 		nr_regions = 0;

mm/damon/core.c static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold, · line 2427

Quoted 10 lines

mm/damon/core.c +3 -1

 		damon_for_each_target(t, c) {
-			damon_merge_regions_of(t, threshold, sz_limit);
+			damon_merge_regions_of(t, threshold, sz_limit,
+					count_age);
 			nr_regions += damon_nr_regions(t);
 		}
+		count_age = false;
 		threshold = max(1, threshold * 2);
 	} while (nr_regions > c->attrs.max_nr_regions &&
 			threshold / 2 < max_thres);

[Severity: High]
This is a pre-existing issue, but can this loop run infinitely due to an integer overflow?

If a user updates DAMON parameters while it is running, setting a large aggr_interval and a small sample_interval (for example, aggr_us=0x80000000, sample_us=1), max_thres will be greater than or equal to 0x80000000.

Concurrently, if max_nr_regions is dramatically lowered, kdamond_merge_regions() could be called with a stale, small sz_limit. This small sz_limit would prevent regions from merging enough to meet the new max_nr_regions.

Inside the loop, threshold (a 32-bit unsigned int) doubles on each iteration. If nr_regions > c->attrs.max_nr_regions remains true, threshold will eventually overflow (for example, 0x80000000 * 2 = 0) and reset back to 1 via max(1, threshold * 2).

Because threshold / 2 never exceeds 0x7FFFFFFF, the condition threshold / 2 < max_thres will always evaluate to true when max_thres >= 0x80000000, permanently hanging the kdamond thread.

Signature · 1 line

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