[PATCH 6.12.y v2] 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>
---
Changes from v1
- v1: https://lore.kernel.org/20261110022924.115699-1-sj@kernel.org
- Mark last_nr_regions as a private field to avoid kernel-doc warning.

 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 4688a10eca89b..b05c626f6479a 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -641,6 +641,12 @@ struct damon_ctx {
 
 	struct list_head adaptive_targets;
 	struct list_head schemes;
+/* private: */
+	/*
+	 * number of regions after last merge.  Read kdamond_split_regions()
+	 * for more details.
+	 */
+	unsigned long last_nr_regions;
 };
 
 static inline struct damon_region *damon_next_region(struct damon_region *r)
diff --git a/mm/damon/core.c b/mm/damon/core.c
index 3142776e2e0ee..ec7f22ad33e1d 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1932,7 +1932,6 @@ 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)
@@ -1942,14 +1941,14 @@ 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->attrs.max_nr_regions / 3)
 		nr_subregions = 3;
 
 	damon_for_each_target(t, ctx)
 		damon_split_regions_of(t, nr_subregions);
 
-	last_nr_regions = nr_regions;
+	ctx->last_nr_regions = nr_regions;
 }
 
 /*

base-commit: a100533c838bab5a555d92e082e38e092f2b4ea6
-- 
2.47.3