[RFC PATCH 01/14] mm/damon/core: safely handle no region case in damon_set_regions()

[SeongJae Park <sj@kernel.org>]

damon_set_regions() calls damon_first_region() regardless of the number
of DAMON regions in a given DAMON target.  damon_first_region()
internally uses list_first_entry(), which clearly documents the list is
expected to be not empty.  Due to the internal implementation of the
macro, damon_set_regions() is safe for now.  But the internal
implementation of the macro can be changed in future.  Refactor the
function to explicitly and safely handle the empty region list case
without depending on the internal implementation.

No behavioral change is intended.

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 0267faf216b95..40946a7f6f549 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -356,6 +356,19 @@ int damon_set_regions(struct damon_target *t, struct damon_addr_range *ranges,
 			damon_destroy_region(r, t);
 	}
 
+	if (!damon_nr_regions(t)) {
+		for (i = 0; i < nr_ranges; i++) {
+			r = damon_new_region(
+					ALIGN_DOWN(ranges[i].start,
+						min_region_sz),
+					ALIGN(ranges[i].end, min_region_sz));
+			if (!r)
+				return -ENOMEM;
+			damon_add_region(r, t);
+		}
+		return 0;
+	}
+
 	r = damon_first_region(t);
 	/* Add new regions or resize existing regions to fit in the ranges */
 	for (i = 0; i < nr_ranges; i++) {
-- 
2.47.3