[PATCH 1/6] mm/damon/core: warn and fix nr_accesses[_bp] corruption

[SeongJae Park <sj@kernel.org>]

For a bug such as double aggregation reset[1], ->nr_accesses and/or
->nr_accesses_bp of damon_region could be corrupted.  Such corruption
can make monitoring results pretty inaccurate, so the root causing bug
should be investigated.  Meanwhile, the corruption itself can easily be
fixed but silently fixing it will hide the bug.

Fix the corruption as soon as found, but WARN_ONCE() so that we can be
aware of the existence of the bug while keeping the system running in a
more sane way.

[1] https://lore.kernel.org/20250302214145.356806-1-sj@kernel.org

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 587fb9a4fef8..0bb71e2ab713 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1391,6 +1391,19 @@ int damos_walk(struct damon_ctx *ctx, struct damos_walk_control *control)
 	return 0;
 }
 
+/*
+ * Warn and fix corrupted ->nr_accesses[_bp] for investigations and preventing
+ * the problem being propagated.
+ */
+static void damon_warn_fix_nr_accesses_corruption(struct damon_region *r)
+{
+	if (r->nr_accesses_bp == r->nr_accesses * 10000)
+		return;
+	WARN_ONCE(true, "invalid nr_accesses_bp at reset: %u %u\n",
+			r->nr_accesses_bp, r->nr_accesses);
+	r->nr_accesses_bp = r->nr_accesses * 10000;
+}
+
 /*
  * Reset the aggregated monitoring results ('nr_accesses' of each region).
  */
@@ -1404,6 +1417,7 @@ static void kdamond_reset_aggregated(struct damon_ctx *c)
 
 		damon_for_each_region(r, t) {
 			trace_damon_aggregated(ti, r, damon_nr_regions(t));
+			damon_warn_fix_nr_accesses_corruption(r);
 			r->last_nr_accesses = r->nr_accesses;
 			r->nr_accesses = 0;
 		}
-- 
2.39.5