mm/damon/core-test: test max_nr_accesses overflow caused divide-by-zero

Commit 35f5d94 ("mm/damon: implement a function for max nr_accesses
safe calculation") has fixed an overflow bug that could cause
divide-by-zero.  Add a kunit test for the bug to ensure similar bugs are
not introduced again.

Link: https://lkml.kernel.org/r/20231213190338.54146-3-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: sjp38

mm/damon/core-test.h

@@ -302,6 +302,16 @@ static void damon_test_set_regions(struct kunit *test)
 	damon_destroy_target(t);
 }
 
+static void damon_test_nr_accesses_to_accesses_bp(struct kunit *test)
+{
+	struct damon_attrs attrs = {
+		.sample_interval = 10,
+		.aggr_interval = ((unsigned long)UINT_MAX + 1) * 10
+	};
+
+	KUNIT_EXPECT_EQ(test, damon_nr_accesses_to_accesses_bp(123, &attrs), 0);
+}
+
 static void damon_test_update_monitoring_result(struct kunit *test)
 {
 	struct damon_attrs old_attrs = {
@@ -487,6 +497,7 @@ static struct kunit_case damon_test_cases[] = {
 	KUNIT_CASE(damon_test_split_regions_of),
 	KUNIT_CASE(damon_test_ops_registration),
 	KUNIT_CASE(damon_test_set_regions),
+	KUNIT_CASE(damon_test_nr_accesses_to_accesses_bp),
 	KUNIT_CASE(damon_test_update_monitoring_result),
 	KUNIT_CASE(damon_test_set_attrs),
 	KUNIT_CASE(damon_test_moving_sum),