[PATCH 2/3] mm/damon/core: move static variable last_nr_regions to damon_ctx

[SJ Park]

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.

Fixes: 75c1c2b53c78 ("mm/damon/dbgfs: support multiple contexts")
Cc: <stable@xxxxxxxxxxxxxxx> # 5.15.x
Closes: https://sashiko.dev/#/bug/linux-eebfcdde-2881-48ef-8df1-e2019d2a47f2
Signed-off-by: SJ Park <sj@xxxxxxxxxx>
---
 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 63050eb2206a0..cb6a50032a1eb 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 eb1fcdabc0075..09722257ce736 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;
 }
 
 /*
-- 
2.47.3