[PATCH 2/2] memcg: do not report racy no-eligible OOM tasks

Michal Hocko
2019-01-07T14:39:44Z

From: Michal Hocko <mhocko@suse.com>

Tetsuo has reported [1] that a single process group memcg might easily
swamp the log with no-eligible oom victim reports due to race between
the memcg charge and oom_reaper

Thread 1 Thread2 oom_reaper
try_charge try_charge
mem_cgroup_out_of_memory
mutex_lock(oom_lock)
mem_cgroup_out_of_memory
mutex_lock(oom_lock)
out_of_memory
select_bad_process
oom_kill_process(current)
wake_oom_reaper
oom_reap_task
MMF_OOM_SKIP->victim
mutex_unlock(oom_lock)
out_of_memory
select_bad_process # no task

If Thread1 didn't race it would bail out from try_charge and force the
charge. We can achieve the same by checking tsk_is_oom_victim inside
the oom_lock and therefore close the race.

[1] http://lkml.kernel.org/r/bb2074c0-34fe-8c2c-1c7d-db71338f1e7f@i-love.sakura.ne.jp
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
mm/memcontrol.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index af7f18b32389..90eb2e2093e7 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1387,10 +1387,22 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
.gfp_mask = gfp_mask,
.order = order,
};
- bool ret;
+ bool ret = true;

mutex_lock(&oom_lock);
+
+ /*
+ * multi-threaded tasks might race with oom_reaper and gain
+ * MMF_OOM_SKIP before reaching out_of_memory which can lead
+ * to out_of_memory failure if the task is the last one in
+ * memcg which would be a false possitive failure reported
+ */
+ if (tsk_is_oom_victim(current))
+ goto unlock;
+
ret = out_of_memory(&oc);
+
+unlock:
mutex_unlock(&oom_lock);
return ret;
}
--
2.20.1