[RFC PATCH 3/3] mm/damon/core: move static variable rotation to damon_ctx

[SJ Park <sj@kernel.org>]

damon_split_some_reigons() uses a static variable, rotation, to remember
how many times it was called.  It is again used to determine fair
selection of regions to split.  Since it is a static variable, it can be
indeterministic and therefore less fair.

The user impact is not critical.  No crash, no leak.  DAMON's
best-effort and indeterministic monitoring accuracy could be slightly
more indeterministic when multiple kdamonds are used in parallel.

Fix the problem by moving the static variable into the damon_ctx struct.

Fixes: 1a582e6e0174 ("mm/damon/core: split a fraction of regions when nr_regions exceeds max/2")
Cc: <stable@vger.kernel.org> # 7.3.x
Closes: https://sashiko.dev/#/bug/linux-eebfcdde-2881-48ef-8df1-e2019d2a47f2
Signed-off-by: SJ Park <sj@kernel.org>
---
 include/linux/damon.h | 5 +++++
 mm/damon/core.c       | 5 +++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index cb6a50032a1eb..380f9cf6ddd82 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -963,6 +963,11 @@ struct damon_ctx {
 	 * for more details.
 	 */
 	unsigned long last_nr_regions;
+	/*
+	 * number of damon_split_some_regions() invocations for this context.
+	 * Read damon_split_some_regions() for more details.
+	 */
+	unsigned long region_split_rotation;
 };
 
 /* 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 09722257ce736..73b571b429cb3 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -3753,12 +3753,13 @@ static void damon_split_regions_of(struct damon_ctx *ctx,
 static void damon_split_some_regions(struct damon_ctx *ctx,
 				     unsigned long split_step)
 {
-	static unsigned long rotation;
 	struct damon_target *t;
 	struct damon_region *r, *next;
-	unsigned long offset = rotation++ % split_step;
+	unsigned long offset;
 	unsigned long idx = 0;
 
+	offset = ctx->region_split_rotation++ % split_step;
+
 	damon_for_each_target(t, ctx) {
 		damon_for_each_region_safe(r, next, t) {
 			unsigned long sz_region, sz_sub;
-- 
2.47.3