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] kdamond_merge_regions() can fall into an infinite loop due to integer overflow of threshold. --

--- Patch [1]: [PATCH] mm/damon/core: skip aging from repeated aggressive merging --- commit 1fa7392cc3fc19083e90cfb01dcfa4f03ebbc373
Author: SJ Park <[email protected]>

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

This commit modifies the DAMON core to skip aging when repeating region merges during corner cases where the number of regions exceeds the user-defined maximum limit. This prevents temporary degradation of monitoring quality caused by updating region ages multiple times during repeated merge operations.

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

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

Quoted 3 lines

mm/damon/core.c +0 -0

[ ... ]

Quoted 10 lines

mm/damon/core.c +1 -0

@@ -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;

mm/damon/core.c static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold, · line 3385

Quoted 10 lines

mm/damon/core.c +3 -1

 		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);

[Severity: High]
This is a pre-existing issue, but can kdamond_merge_regions() fall into an infinite loop under certain configurations?

If a user configures aggr_interval and sample_interval such that their quotient (max_thres) exceeds 0x40000000, threshold could overflow in the max() assignment above.

Since threshold is an unsigned int, doubling it once it reaches 0x80000000 causes an overflow to 0, which max(1, 0) clamps back to 1.

If regions cannot be merged further due to the sz_limit constraint, nr_regions > c->attrs.max_nr_regions remains true indefinitely.

Because threshold cycles and never exceeds 0xFFFFFFFF, threshold / 2 never reaches max_thres, causing this loop condition to evaluate to true endlessly.

Signature · 2 lines

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

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.