[PATCH] mm, oom: return error on access to memory.oom_group if groupoom is disabled

[Roman Gushchin]

Cgroup-aware OOM killer depends on cgroup mount option and is turned
off by default, despite the user interface (memory.oom_group file) is
always present. As it might be confusing to a user, let's return
-ENOTSUPP on an attempt to access to memory.oom_group if groupoom is not
enabled globally.

Example:
  $ cd /sys/fs/cgroup/user.slice/
  $ cat memory.oom_group
    cat: memory.oom_group: Unknown error 524
  $ echo 1 > memory.oom_group
    -bash: echo: write error: Unknown error 524
  $ mount -o remount,groupoom /sys/fs/cgroup
  $ echo 1 > memory.oom_group
  $ cat memory.oom_group
    1

Signed-off-by: Roman Gushchin <guro@xxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: kernel-team@xxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
 mm/memcontrol.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c76d5fb55c5c..b709ee4f914b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5482,6 +5482,9 @@ static int memory_oom_group_show(struct seq_file *m, void *v)
 	struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(m));
 	bool oom_group = memcg->oom_group;
 
+	if (!(cgrp_dfl_root.flags & CGRP_GROUP_OOM))
+		return -ENOTSUPP;
+
 	seq_printf(m, "%d\n", oom_group);
 
 	return 0;
@@ -5495,6 +5498,9 @@ static ssize_t memory_oom_group_write(struct kernfs_open_file *of,
 	int oom_group;
 	int err;
 
+	if (!(cgrp_dfl_root.flags & CGRP_GROUP_OOM))
+		return -ENOTSUPP;
+
 	err = kstrtoint(strstrip(buf), 0, &oom_group);
 	if (err)
 		return err;
-- 
2.14.3