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 | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 291a847a8c9bb..94be598cafef1 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2545,6 +2545,22 @@ 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)
+{
+	if (r->nr_accesses == r->nr_accesses_bp / 10000)
+		return;
+	pr_err("nr_accesses (%u) != nr_accesses_bp (%u)\n",
+			r->nr_accesses, r->nr_accesses_bp);
+	WARN_ONCE();
+}
+#else
+static void damon_verify_merge_regions_of(struct damon_region *r)
+{
+}
+#endif
+
+
 /*
  * Merge adjacent regions having similar access frequencies
  *
@@ -2565,6 +2581,8 @@ static void damon_merge_regions_of(struct damon_target *t, unsigned int thres,
 		else
 			r->age++;
 
+		damon_verify_merge_regions_of(r);
+
 		if (prev && prev->ar.end == r->ar.start &&
 		    abs(prev->nr_accesses - r->nr_accesses) <= thres &&
 		    damon_sz_region(prev) + damon_sz_region(r) <= sz_limit)
-- 
2.47.3