[PATCH 2/2] mm, memcg: don't try to kill a process if memcg is not populated

[Yafang Shao <laoar.shao@gmail.com>]

Recently Shakeel reported a issue which also confused me serveral months
earlier. Bellow is his report -

Lowering memory.max can trigger an oom-kill if the reclaim does not
succeed. However if oom-killer does not find a process for killing, it
dumps a lot of warnings.

Deleting a memcg does not reclaim memory from it and the memory can
linger till there is a memory pressure. One normal way to proactively
reclaim such memory is to set memory.max to 0 just before deleting the
memcg. However if some of the memcg's memory is pinned by others, this
operation can trigger an oom-kill without any process and thus can log a
lot un-needed warnings. So, ignore all such warnings from memory.max.

[shakeelb@google.com: commit log above]

A better way to avoid this issue is to avoid trying to kill a process if
memcg is not populated.
Note that OOM is different with OOM kill. OOM is a status that the
system or memcg is out of memory, while OOM kill is a result that a
process inside this memcg is killed when this memcg is in OOM status.
That is the same reason why there're both MEMCG_OOM event and
MEMCG_OOM_KILL event. If we have already known that there's nothing to
kill, i.e. the memcg is not populated, then we don't need to have a try.

Cc: Shakeel Butt <shakeelb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Roman Gushchin <guro@fb.com>
Cc: Greg Thelen <gthelen@google.com>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 mm/memcontrol.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 985edce98491..29afe3df9d98 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6102,6 +6102,10 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
 		}
 
 		memcg_memory_event(memcg, MEMCG_OOM);
+
+		if (!cgroup_is_populated(memcg->css.cgroup))
+			break;
+
 		if (!mem_cgroup_oom_kill(memcg, GFP_KERNEL, 0))
 			break;
 	}
-- 
2.18.2