linux-kernel - [PATCH mm-unstable] mm/damon/core: ensure max threshold attempt for max_nr

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Date: Thu, 27 Jun 2024 09:31:52 -0700
From: SeongJae Park <sj@...nel.org>
To: SeongJae Park <sj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc: damon@...ts.linux.dev,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH mm-unstable] mm/damon/core: ensure max threshold attempt for max_nr_regions violation

Since commit ed6cb9d022c8 ("mm/damon/core: increase regions merge
aggressiveness while respecting min_nr_regions") of mm-unstable,
kdamond_merge_regions() stops the repeated merge attempts for meeting
max_nr_regions if the merge threshold that increased for next iteration
is higher than the possible maximum threshold.  Hence, it can skip a
merge attempt with the maximum threshold depending on the amount of the
threshold increase, the maximum threshold, and the last-used threshold
value.  In extreme situations (e.g., region 1 has 100% access frequency,
region 2 has 0% access frequency, so on), this can degrade the
max_nr_regions violation recovery.

Fix this by stopping the loop by comparing the last-used threshold
instead of the to-be-used threshold, and if the last-used threshold is
same to or higher than the maximum possible threshold.

Fixes: ed6cb9d022c8 ("mm/damon/core: increase regions merge aggressiveness while respecting min_nr_regions") # mm-unstable
Closes: https://lore.kernel.org/20240626214954.46143-1-sj@kernel.org
Signed-off-by: SeongJae Park <sj@...nel.org>
---
 mm/damon/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index dac27b949403..7a87628b76ab 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1718,7 +1718,7 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold,
 		}
 		threshold = max(1, threshold * 2);
 	} while (nr_regions > c->attrs.max_nr_regions &&
-			threshold <= max_thres);
+			threshold / 2 < max_thres);
 }

 /*
-- 
2.39.2