damon_del_region() should be called for targets that have one or more
regions.  Add a sanity check for that under CONFIG_DAMON_DEBUG_SANITY.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 0c1353164ec81..92eac5eb96026 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -152,8 +152,21 @@ void damon_add_region(struct damon_region *r, struct damon_target *t)
 	t->nr_regions++;
 }
 
+#ifdef CONFIG_DAMON_DEBUG_SANITY
+static void damon_verify_del_region(struct damon_target *t)
+{
+	WARN_ONCE(t->nr_regions == 0, "t->nr_regions == 0\n");
+}
+#else
+static void damon_verify_del_region(struct damon_target *t)
+{
+}
+#endif
+
 static void damon_del_region(struct damon_region *r, struct damon_target *t)
 {
+	damon_verify_del_region(t);
+
 	list_del(&r->list);
 	t->nr_regions--;
 }
-- 
2.47.3