A data corruption could cause damon_merge_two_regions() creating zero
length DAMON regions.  Add a sanity check for that under
CONFIG_DAMON_DEBUG_SANITY.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index ede76e0789be3..291a847a8c9bb 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2507,6 +2507,25 @@ static void kdamond_apply_schemes(struct damon_ctx *c)
 	mutex_unlock(&c->walk_control_lock);
 }
 
+#ifdef CONFIG_DAMON_DEBUG_SANITY
+static void damon_verify_merge_two_regions(
+		struct damon_region *l, struct damon_region *r)
+{
+	if (l->ar.start < l->ar.end)
+		return;
+
+	pr_err("damn_merge_two_regions() created incorrect left region\n");
+	pr_err("l: %lu-%lu, r: %lu-%lu\n",
+			l->ar.start, l->ar.end, r->ar.start, r->ar.end);
+	WARN_ONCE();
+}
+#else
+static void damon_verify_merge_two_regions(
+		struct damon_region *l, struct damon_region *r)
+{
+}
+#endif
+
 /*
  * Merge two adjacent regions into one region
  */
@@ -2520,6 +2539,9 @@ static void damon_merge_two_regions(struct damon_target *t,
 	l->nr_accesses_bp = l->nr_accesses * 10000;
 	l->age = (l->age * sz_l + r->age * sz_r) / (sz_l + sz_r);
 	l->ar.end = r->ar.end;
+
+	damon_verify_merge_two_regions(l, r);
+
 	damon_destroy_region(r, t);
 }
 
-- 
2.47.3