DAMOS filters' default reject behavior is not very simple.  Actually
there was a mistake[1] during the development.  Add a kunit test for
validating the behavior.

[1] https://lore.kernel.org/20250227002913.19359-1-sj@kernel.org

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/tests/core-kunit.h | 70 +++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)

diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
index be0fea9ee5fc..298c67557fae 100644
--- a/mm/damon/tests/core-kunit.h
+++ b/mm/damon/tests/core-kunit.h
@@ -510,6 +510,75 @@ static void damon_test_feed_loop_next_input(struct kunit *test)
 			damon_feed_loop_next_input(last_input, 2000));
 }
 
+static void damon_test_set_filters_default_reject(struct kunit *test)
+{
+	struct damos scheme;
+	struct damos_filter *target_filter, *anon_filter;
+
+	INIT_LIST_HEAD(&scheme.filters);
+	INIT_LIST_HEAD(&scheme.ops_filters);
+
+	damos_set_filters_default_reject(&scheme);
+	/*
+	 * No filter is installed.  Allow by default on both core and ops layer
+	 * filtering stages, since there are no filters at all.
+	 */
+	KUNIT_EXPECT_EQ(test, scheme.core_filters_default_reject, false);
+	KUNIT_EXPECT_EQ(test, scheme.ops_filters_default_reject, false);
+
+	target_filter = damos_new_filter(DAMOS_FILTER_TYPE_TARGET, true, true);
+	damos_add_filter(&scheme, target_filter);
+	damos_set_filters_default_reject(&scheme);
+	/*
+	 * A core-handled allow-filter is installed.
+	 * Rejct by default on core layer filtering stage due to the last
+	 * core-layer-filter's behavior.
+	 * Allow by default on ops layer filtering stage due to the absence of
+	 * ops layer filters.
+	 */
+	KUNIT_EXPECT_EQ(test, scheme.core_filters_default_reject, true);
+	KUNIT_EXPECT_EQ(test, scheme.ops_filters_default_reject, false);
+
+	target_filter->allow = false;
+	damos_set_filters_default_reject(&scheme);
+	/*
+	 * A core-handled reject-filter is installed.
+	 * Allow by default on core layer filtering stage due to the last
+	 * core-layer-filter's behavior.
+	 * Allow by default on ops layer filtering stage due to the absence of
+	 * ops layer filters.
+	 */
+	KUNIT_EXPECT_EQ(test, scheme.core_filters_default_reject, false);
+	KUNIT_EXPECT_EQ(test, scheme.ops_filters_default_reject, false);
+
+	anon_filter = damos_new_filter(DAMOS_FILTER_TYPE_ANON, true, true);
+	damos_add_filter(&scheme, anon_filter);
+
+	damos_set_filters_default_reject(&scheme);
+	/*
+	 * A core-handled reject-filter and ops-handled allow-filter are installed.
+	 * Allow by default on core layer filtering stage due to the existence
+	 * of the ops-handled filter.
+	 * Reject by default on ops layer filtering stage due to the last
+	 * ops-layer-filter's behavior.
+	 */
+	KUNIT_EXPECT_EQ(test, scheme.core_filters_default_reject, false);
+	KUNIT_EXPECT_EQ(test, scheme.ops_filters_default_reject, true);
+
+	target_filter->allow = true;
+	damos_set_filters_default_reject(&scheme);
+	/*
+	 * A core-handled allow-filter and ops-handled allow-filter are
+	 * installed.
+	 * Allow by default on core layer filtering stage due to the existence
+	 * of the ops-handled filter.
+	 * Reject by default on ops layer filtering stage due to the last
+	 * ops-layer-filter's behavior.
+	 */
+	KUNIT_EXPECT_EQ(test, scheme.core_filters_default_reject, false);
+	KUNIT_EXPECT_EQ(test, scheme.ops_filters_default_reject, true);
+}
+
 static struct kunit_case damon_test_cases[] = {
 	KUNIT_CASE(damon_test_target),
 	KUNIT_CASE(damon_test_regions),
@@ -527,6 +596,7 @@ static struct kunit_case damon_test_cases[] = {
 	KUNIT_CASE(damos_test_new_filter),
 	KUNIT_CASE(damos_test_filter_out),
 	KUNIT_CASE(damon_test_feed_loop_next_input),
+	KUNIT_CASE(damon_test_set_filters_default_reject),
 	{},
 };
 
-- 
2.39.5