[PATCH 6.1.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 0387d86b56697..0b63617106416 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -465,6 +465,12 @@ 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 f0da5cb2b88a0..a1db159b18611 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1075,7 +1075,6 @@ 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)
@@ -1085,14 +1084,14 @@ 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;
 }
 
 /*
-- 
2.47.3