[PATCH v2 1/1] selftests: cgroup: add tests to verify the zswap writeback path

[Usama Arif]

Initate writeback with the below steps and check using
memory.stat.zswpwb if zswap writeback occurred:
1. Allocate memory.
2. Reclaim memory equal to the amount that was allocated in step 1.
   This will move it into zswap.
3. Save current zswap usage.
4. Move the memory allocated in step 1 back in from zswap.
5. Set zswap.max to half the amount that was recorded in step 3.
6. Attempt to reclaim memory equal to the amount that was allocated,
   this will either trigger writeback if its enabled, or reclamation
   will fail if writeback is disabled as there isn't enough zswap
   space.

Suggested-by: Nhat Pham <nphamcs@xxxxxxxxx>
Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx>
---
 tools/testing/selftests/cgroup/test_zswap.c | 125 +++++++++++++++++++-
 1 file changed, 124 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
index f0e488ed90d8..cd864ab825d0 100644
--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c
@@ -50,7 +50,7 @@ static int get_zswap_stored_pages(size_t *value)
 	return read_int("/sys/kernel/debug/zswap/stored_pages", value);
 }
 
-static int get_cg_wb_count(const char *cg)
+static long get_cg_wb_count(const char *cg)
 {
 	return cg_read_key_long(cg, "memory.stat", "zswpwb");
 }
@@ -248,6 +248,127 @@ static int test_zswapin(const char *root)
 	return ret;
 }
 
+/*
+ * Initate writeback with the following steps:
+ * 1. Allocate memory.
+ * 2. Reclaim memory equal to the amount that was allocated in step 1.
+      This will move it into zswap.
+ * 3. Save current zswap usage.
+ * 4. Move the memory allocated in step 1 back in from zswap.
+ * 5. Set zswap.max to half the amount that was recorded in step 3.
+ * 6. Attempt to reclaim memory equal to the amount that was allocated,
+      this will either trigger writeback if its enabled, or reclamation
+      will fail if writeback is disabled as there isn't enough zswap space.
+ */
+static int initiate_writeback(const char *cgroup, void *arg)
+{
+	char *test_group = arg;
+	size_t memsize = MB(4);
+	int ret = -1;
+	bool wb_enabled = cg_read_long(test_group, "memory.zswap.writeback");
+	long zswap_usage;
+
+	if (cg_write(test_group, "memory.max", "8M"))
+		return ret;
+
+	/* Allocate random memory to enusre high zswap memory usage */
+	char *mem = (char *)malloc(memsize);
+
+	if (!mem)
+		return ret;
+	for (int i = 0; i < memsize; i++)
+		mem[i] = rand() % 128;
+
+	/* Try and reclaim allocated memory */
+	if (cg_write(test_group, "memory.reclaim", "4M")) {
+		ksft_print_msg("Failed to reclaim all of the requested memory\n");
+		goto out;
+	}
+
+	zswap_usage = cg_read_long(test_group, "memory.zswap.current");
+
+	/* zswpin */
+	for (int i = 0; i < memsize; i++) {
+		if (mem[i] < 0 || mem[i] > 127) {
+			ksft_print_msg("invalid memory\n");
+			ret = -1;
+		}
+	}
+
+	if (cg_write_numeric(test_group, "memory.zswap.max", zswap_usage/2))
+		goto out;
+
+	/*
+	 * If writeback is enabled, trying to reclaim memory now will trigger a
+	 * writeback as zswap.max is half of what was needed when reclaim ran the first time.
+	 * If writeback is disabled, memory reclaim will fail as zswap is limited and
+	 * it can't writeback to swap.
+	 */
+	ret = cg_write(test_group, "memory.reclaim", "4M");
+	if (!wb_enabled && ret)
+		ret = 0;
+out:
+	free(mem);
+	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.zswap.writeback", wb ? "1" : "0"))
+		return ret;
+
+	zswpwb_before = get_cg_wb_count(test_group);
+	if (zswpwb_before < 0) {
+		ksft_print_msg("failed to get zswpwb_before\n");
+		goto out;
+	}
+
+	if (cg_run(test_group, initiate_writeback, (void *) test_group))
+		goto out;
+
+	/* Verify that zswap writeback occurred only if writeback was enabled */
+	zswpwb_after = get_cg_wb_count(test_group);
+	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 +546,8 @@ 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