[PATCH] selftests: cgroup: add tests to verify the zswap writeback path

[Usama Arif <usamaarif642@xxxxxxxxx>]

The condition for writeback can be triggered by allocating random
memory more than memory.high to push memory into zswap, more than
zswap.max to trigger writeback if enabled, but less than memory.max
so that OOM is not triggered. Both values of memory.zswap.writeback
are tested.

Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx>
---
 tools/testing/selftests/cgroup/test_zswap.c | 83 +++++++++++++++++++++
 1 file changed, 83 insertions(+)

diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
index f0e488ed90d8..fe0e7221525c 100644
--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c
@@ -94,6 +94,19 @@ static int allocate_bytes(const char *cgroup, void *arg)
 	return 0;
 }
 
+static int allocate_random_bytes(const char *cgroup, void *arg)
+{
+	size_t size = (size_t)arg;
+	char *mem = (char *)malloc(size);
+
+	if (!mem)
+		return -1;
+	for (int i = 0; i < size; i++)
+		mem[i] = rand() % 128;
+	free(mem);
+	return 0;
+}
+
 static char *setup_test_group_1M(const char *root, const char *name)
 {
 	char *group_name = cg_name(root, name);
@@ -248,6 +261,74 @@ static int test_zswapin(const char *root)
 	return ret;
 }
 
+/* Test to verify the zswap writeback path */
+static int test_zswap_writeback(const char *root, bool wb)
+{
+	int ret = KSFT_FAIL;
+	char *test_group;
+	long zswpwb_before, zswpwb_after;
+
+	test_group = cg_name(root,
+		wb ? "zswap_writeback_enabled_test" : "zswap_writeback_disabled_test");
+	if (!test_group)
+		goto out;
+	if (cg_create(test_group))
+		goto out;
+	if (cg_write(test_group, "memory.max", "8M"))
+		goto out;
+	if (cg_write(test_group, "memory.high", "2M"))
+		goto out;
+	if (cg_write(test_group, "memory.zswap.max", "2M"))
+		goto out;
+	if (cg_write(test_group, "memory.zswap.writeback", wb ? "1" : "0"))
+		goto out;
+
+	zswpwb_before = cg_read_key_long(test_group, "memory.stat", "zswpwb ");
+	if (zswpwb_before < 0) {
+		ksft_print_msg("failed to get zswpwb_before\n");
+		goto out;
+	}
+
+	/*
+	 * Allocate more than memory.high to push memory into zswap,
+	 * more than zswap.max to trigger writeback if enabled,
+	 * but less than memory.max so that OOM is not triggered
+	 */
+	if (cg_run(test_group, allocate_random_bytes, (void *)MB(3)))
+		goto out;
+
+	/* Verify that zswap writeback occurred only if writeback was enabled */
+	zswpwb_after = cg_read_key_long(test_group, "memory.stat", "zswpwb ");
+	if (wb) {
+		if (zswpwb_after <= zswpwb_before) {
+			ksft_print_msg("writeback enabled and zswpwb_after <= zswpwb_before\n");
+			goto out;
+		}
+	} else {
+		if (zswpwb_after != zswpwb_before) {
+			ksft_print_msg("writeback disabled and zswpwb_after != zswpwb_before\n");
+			goto out;
+		}
+	}
+
+	ret = KSFT_PASS;
+
+out:
+	cg_destroy(test_group);
+	free(test_group);
+	return ret;
+}
+
+static int test_zswap_writeback_enabled(const char *root)
+{
+	return test_zswap_writeback(root, true);
+}
+
+static int test_zswap_writeback_disabled(const char *root)
+{
+	return test_zswap_writeback(root, false);
+}
+
 /*
  * When trying to store a memcg page in zswap, if the memcg hits its memory
  * limit in zswap, writeback should affect only the zswapped pages of that
@@ -425,6 +506,8 @@ struct zswap_test {
 	T(test_zswap_usage),
 	T(test_swapin_nozswap),
 	T(test_zswapin),
+	T(test_zswap_writeback_enabled),
+	T(test_zswap_writeback_disabled),
 	T(test_no_kmem_bypass),
 	T(test_no_invasive_cgroup_shrink),
 };
-- 
2.43.0