From 26ae50a0901a44f7d50053f87dc38abd9f9a269c Mon Sep 17 00:00:00 2001 From: SJ Park Date: Sun, 25 Oct 2026 09:48:05 -0700 Subject: [PATCH] mm/damon/core: move static variable last_nr_regions to damon_ctx 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: 75c1c2b53c78 ("mm/damon/dbgfs: support multiple contexts") Signed-off-by: SJ Park Signed-off-by: Andrew Morton Closes: https://sashiko.dev/#/bug/linux-eebfcdde-2881-48ef-8df1-e2019d2a47f2 Cc: Fernand Sieber Cc: SeongJae Park Cc: # 5.15.x --- include/linux/damon.h | 5 +++++ mm/damon/core.c | 5 ++--- 2 files changed, 7 insertions(+), 3 deletions(-) diff --git a/include/linux/damon.h b/include/linux/damon.h index 63050eb2206a..cb6a50032a1e 100644 --- a/include/linux/damon.h +++ b/include/linux/damon.h @@ -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. */ diff --git a/mm/damon/core.c b/mm/damon/core.c index eb1fcdabc007..09722257ce73 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -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; } /*