damon_split_region_at() should be called with the correct address to
split on.  Add a sanity check for that under CONFIG_DAMON_DEBUG_SANITY.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 94be598cafef1..16f7994903d6d 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2629,6 +2629,23 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold,
 			threshold / 2 < max_thres);
 }
 
+#ifdef CONFIG_DAMON_DEBUG_SANITY
+static void damon_verify_split_region_at(struct damon_region *r,
+		unsigned long sz_r)
+{
+	if (sz_r > 0 && sz_r < damon_sz_region(r))
+		return;
+	pr_err("damon_split_region() call with r %lu-%lu (%lu), sz_r %lu\n",
+			r->ar.start, r->ar.end, damon_sz_region(r), sz_r);
+	WARN_ONCE();
+}
+#else
+static void damon_verify_split_region_at(struct damon_region *r,
+		unsigned long sz_r)
+{
+}
+#endif
+
 /*
  * Split a region in two
  *
@@ -2640,6 +2657,8 @@ static void damon_split_region_at(struct damon_target *t,
 {
 	struct damon_region *new;
 
+	damon_verify_split_region_at(r, sz_r);
+
 	new = damon_new_region(r->ar.start + sz_r, r->ar.end);
 	if (!new)
 		return;
-- 
2.47.3