[PATCH 2/5] cgroup: Account for memory_recursiveprot in test_memcg_low()

[David Vernet <void@manifault.com>]

The test_memcg_low() testcase in test_memcontrol.c verifies the expected
behavior of groups using the memory.low knob. Part of the testcase verifies
that a group with memory.low that experiences reclaim due to memory
pressure elsewhere in the system, observes memory.events.low events as a
result of that reclaim.

In commit 8a931f801340 ("mm: memcontrol: recursive memory.low protection"),
the memory controller was updated to propagate memory.low and memory.min
protection from a parent group to its children via a configurable
memory_recursiveprot mount option. This unfortunately broke the memcg
tests, which asserts that a sibling that experienced reclaim but had a
memory.low value of 0, would not observe any memory.low events. This patch
updates test_memcg_low() to account for the new behavior introduced by
memory_recursiveprot.

So as to make the test resilient to multiple configurations, the patch also
adds a new proc_mount_contains() helper that checks for a string in
/proc/mounts, and is used to toggle behavior based on whether the default
memory_recursiveprot was present.

Signed-off-by: David Vernet <void@manifault.com>
---
 tools/testing/selftests/cgroup/cgroup_util.c     | 12 ++++++++++++
 tools/testing/selftests/cgroup/cgroup_util.h     |  1 +
 tools/testing/selftests/cgroup/test_memcontrol.c | 16 +++++++++++++---
 3 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/cgroup/cgroup_util.c b/tools/testing/selftests/cgroup/cgroup_util.c
index dbaa7aabbb4a..e5d8d727bdcf 100644
--- a/tools/testing/selftests/cgroup/cgroup_util.c
+++ b/tools/testing/selftests/cgroup/cgroup_util.c
@@ -535,6 +535,18 @@ int set_oom_adj_score(int pid, int score)
 	return 0;
 }
 
+int proc_mount_contains(const char *option)
+{
+	char buf[4 * PAGE_SIZE];
+	ssize_t read;
+
+	read = read_text("/proc/mounts", buf, sizeof(buf));
+	if (read < 0)
+		return read;
+
+	return strstr(buf, option) != NULL;
+}
+
 ssize_t proc_read_text(int pid, bool thread, const char *item, char *buf, size_t size)
 {
 	char path[PATH_MAX];
diff --git a/tools/testing/selftests/cgroup/cgroup_util.h b/tools/testing/selftests/cgroup/cgroup_util.h
index 628738532ac9..756f76052b44 100644
--- a/tools/testing/selftests/cgroup/cgroup_util.h
+++ b/tools/testing/selftests/cgroup/cgroup_util.h
@@ -48,6 +48,7 @@ extern int is_swap_enabled(void);
 extern int set_oom_adj_score(int pid, int score);
 extern int cg_wait_for_proc_count(const char *cgroup, int count);
 extern int cg_killall(const char *cgroup);
+int proc_mount_contains(const char *option);
 extern ssize_t proc_read_text(int pid, bool thread, const char *item, char *buf, size_t size);
 extern int proc_read_strstr(int pid, bool thread, const char *item, const char *needle);
 extern pid_t clone_into_cgroup(int cgroup_fd);
diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
index aa50eaa8b157..ea2fd27e52df 100644
--- a/tools/testing/selftests/cgroup/test_memcontrol.c
+++ b/tools/testing/selftests/cgroup/test_memcontrol.c
@@ -21,6 +21,8 @@
 #include "../kselftest.h"
 #include "cgroup_util.h"
 
+static bool has_recursiveprot;
+
 /*
  * This test creates two nested cgroups with and without enabling
  * the memory controller.
@@ -521,15 +523,18 @@ static int test_memcg_low(const char *root)
 	}
 
 	for (i = 0; i < ARRAY_SIZE(children); i++) {
+		int no_low_events_index = has_recursiveprot ? 2 : 1;
+
 		oom = cg_read_key_long(children[i], "memory.events", "oom ");
 		low = cg_read_key_long(children[i], "memory.events", "low ");
 
 		if (oom)
 			goto cleanup;
-		if (i < 2 && low <= 0)
+		if (i <= no_low_events_index && low <= 0)
 			goto cleanup;
-		if (i >= 2 && low)
+		if (i > no_low_events_index && low)
 			goto cleanup;
+
 	}
 
 	ret = KSFT_PASS;
@@ -1272,7 +1277,7 @@ struct memcg_test {
 int main(int argc, char **argv)
 {
 	char root[PATH_MAX];
-	int i, ret = EXIT_SUCCESS;
+	int i, proc_status, ret = EXIT_SUCCESS;
 
 	if (cg_find_unified_root(root, sizeof(root)))
 		ksft_exit_skip("cgroup v2 isn't mounted\n");
@@ -1288,6 +1293,11 @@ int main(int argc, char **argv)
 		if (cg_write(root, "cgroup.subtree_control", "+memory"))
 			ksft_exit_skip("Failed to set memory controller\n");
 
+	proc_status = proc_mount_contains("memory_recursiveprot");
+	if (proc_status < 0)
+		ksft_exit_skip("Failed to query cgroup mount option\n");
+	has_recursiveprot = proc_status;
+
 	for (i = 0; i < ARRAY_SIZE(tests); i++) {
 		switch (tests[i].fn(root)) {
 		case KSFT_PASS:
-- 
2.30.2