FAILED: patch "[PATCH] mm/damon/core: avoid infinite kdamond_merge_regions()" failed to apply to 5.15-stable tree

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 123e4619ab6c8ab1c4cb1d7a58311a2af13929cd
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026090834-entourage-unwound-48d4@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

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

From 123e4619ab6c8ab1c4cb1d7a58311a2af13929cd Mon Sep 17 00:00:00 2001
From: SJ Park <[email protected]>
Date: Tue, 14 Jul 2026 20:09:56 -0700
Subject: [PATCH] mm/damon/core: avoid infinite kdamond_merge_regions()
internal loop

Patch series "mm/damon: unurgent fixes for infinite loop, NULL de-ref and races", v1.1.

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.

This patch (of 6):

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.

index 6c4215cc809e..603b102ff80f 100644
@@ -3372,7 +3372,7 @@ 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,

@@ -3380,9 +3380,14 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold,
 			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);
+		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

Patch series "mm/damon: unurgent fixes for infinite loop, NULL de-ref and races", v1.1.

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.

This patch (of 6):

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.

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

index 4f031412f65cc..3a50ced2efc0f 100644
@@ -523,15 +523,20 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold,
 
 	max_thres = c->aggr_interval /
 		(c->sample_interval ?  c->sample_interval : 1);
-	do {
+	while (true) {
 		nr_regions = 0;
 		damon_for_each_target(t, c) {
 			damon_merge_regions_of(t, threshold, sz_limit);
 			nr_regions += damon_nr_regions(t);
 		}
-		threshold = max(1, threshold * 2);
-	} while (nr_regions > c->max_nr_regions &&
-			threshold / 2 < max_thres);
+		if (nr_regions <= c->max_nr_regions ||
+				max_thres <= threshold)
+			break;
+		if (threshold < max_thres / 2)
+			threshold = max(1, threshold * 2);
+		else
+			threshold = max_thres;
+	}
 }
 
 /*

2.47.3