damon_merge_regions_of() should be called only after aggregation is
finished and therefore each region's nr_accesses and nr_accesses_bp
match.  There were bugs that broke the assumption, during development of
online DAMON parameter updates and monitoring results handling changes.
Add a sanity check for that under CONFIG_DAMON_DEBUG_SANITY.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 8a3738a17cdd9..93d1577b1ae07 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2505,6 +2505,20 @@ static void damon_merge_two_regions(struct damon_target *t,
 	damon_destroy_region(r, t);
 }
 
+#ifdef CONFIG_DAMON_DEBUG_SANITY
+static void damon_verify_merge_regions_of(struct damon_region *r)
+{
+	WARN_ONCE(r->nr_accesses != r->nr_accesses_bp / 10000,
+			"nr_accesses (%u) != nr_accesses_bp (%u)\n",
+			r->nr_accesses, r->nr_accesses_bp);
+}
+#else
+static void damon_verify_merge_regions_of(struct damon_region *r)
+{
+}
+#endif
+
+
 /*
  * Merge adjacent regions having similar access frequencies
  *
@@ -2518,6 +2532,7 @@ static void damon_merge_regions_of(struct damon_target *t, unsigned int thres,
 	struct damon_region *r, *prev = NULL, *next;
 
 	damon_for_each_region_safe(r, next, t) {
+		damon_verify_merge_regions_of(r);
 		if (abs(r->nr_accesses - r->last_nr_accesses) > thres)
 			r->age = 0;
 		else if ((r->nr_accesses == 0) != (r->last_nr_accesses == 0))
-- 
2.47.3