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mm/damon/core: move static variable last_nr_regions to damon_ctx

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kdamond_split_regions() uses a static variable last_nr_regions, to infer
if the region boundaries are too stable.  If it looks too stable, it
does a more aggressive region split because it might be a fake stability
coming from non-aggressive split.  Because the variable is static, the
inference can be racy and wrong when multiple kdamonds are running in
parallel.

The user impact is not critical.  No crash, no leak.  DAMON's
best-effort monitoring accuracy can be degraded when multiple kdamonds
are used in parallel and the workload has certain access patterns. Still
it is a bug that is better to fix.

Move the variable into the damon_ctx struct to fix the problem.

Link: https://lore.kernel.org/20261025164808.152048-3-sj@kernel.org
Fixes: 75c1c2b53c ("mm/damon/dbgfs: support multiple contexts")
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Closes: https://sashiko.dev/#/bug/linux-eebfcdde-2881-48ef-8df1-e2019d2a47f2
Cc: Fernand Sieber <sieberf@amazon.com>
Cc: SeongJae Park <sjpark@amazon.de>
Cc: <stable@vger.kernel.org> # 5.15.x
This commit is contained in:
SJ Park 2026-10-25 09:48:05 -07:00 committed by Andrew Morton
parent a76adcace3
commit 26ae50a090
2 changed files with 7 additions and 3 deletions
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5
include/linux/damon.h
View File

@ -958,6 +958,11 @@ struct damon_ctx {
/* @rnd_state: Per-ctx PRNG state for damon_rand(). */
struct rnd_state rnd_state;
/*
* number of regions after last merge. Read kdamond_split_regions()
* for more details.
*/
unsigned long last_nr_regions;
};
/* Get a random number in [@l, @r) using @ctx's lockless PRNG. */

5
mm/damon/core.c
View File

@ -3795,7 +3795,6 @@ static noinline_for_stack void kdamond_split_regions(struct damon_ctx *ctx)
struct damon_target *t;
unsigned long nr_regions = 0;
unsigned long max_nr_regions = ctx->attrs.max_nr_regions;
static unsigned long last_nr_regions;
int nr_subregions = 2;
damon_for_each_target(t, ctx)
@ -3811,7 +3810,7 @@ static noinline_for_stack void kdamond_split_regions(struct damon_ctx *ctx)
}
/* Maybe the middle of the region has different access frequency */
if (last_nr_regions == nr_regions &&
if (ctx->last_nr_regions == nr_regions &&
nr_regions < max_nr_regions / 3)
nr_subregions = 3;
@ -3820,7 +3819,7 @@ static noinline_for_stack void kdamond_split_regions(struct damon_ctx *ctx)
ctx->min_region_sz);
done:
last_nr_regions = nr_regions;
ctx->last_nr_regions = nr_regions;
}
/*