[PATCH 5.15.y] mm/damon/core: move static variable last_nr_regions to damon_ctx

[SJ Park <sj@kernel.org>]

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 <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
(cherry picked from commit 26ae50a0901a44f7d50053f87dc38abd9f9a269c)
Signed-off-by: SJ Park <sj@kernel.org>
---
 include/linux/damon.h | 6 ++++++
 mm/damon/core.c       | 5 ++---
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index d68b67b8d458d..bc0cdd67e2473 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -203,6 +203,12 @@ struct damon_ctx {
 	unsigned long min_nr_regions;
 	unsigned long max_nr_regions;
 	struct list_head adaptive_targets;
+/* private: */
+	/*
+	 * number of regions after last merge.  Read kdamond_split_regions()
+	 * for more details.
+	 */
+	unsigned long last_nr_regions;
 };
 
 #define damon_next_region(r) \
diff --git a/mm/damon/core.c b/mm/damon/core.c
index 3a50ced2efc0f..1ce0438328b94 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -603,7 +603,6 @@ static void kdamond_split_regions(struct damon_ctx *ctx)
 {
 	struct damon_target *t;
 	unsigned int nr_regions = 0;
-	static unsigned int last_nr_regions;
 	int nr_subregions = 2;
 
 	damon_for_each_target(t, ctx)
@@ -613,14 +612,14 @@ static void kdamond_split_regions(struct damon_ctx *ctx)
 		return;
 
 	/* 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 < ctx->max_nr_regions / 3)
 		nr_subregions = 3;
 
 	damon_for_each_target(t, ctx)
 		damon_split_regions_of(ctx, t, nr_subregions);
 
-	last_nr_regions = nr_regions;
+	ctx->last_nr_regions = nr_regions;
 }
 
 /*
-- 
2.47.3