memcg: do not hang on OOM when killed by userspace OOM access to memory reserves

Eric has reported that he can see task(s) stuck in memcg OOM handler
regularly.  The only way out is to

	echo 0 > $GROUP/memory.oom_controll

His usecase is:

- Setup a hierarchy with memory and the freezer (disable kernel oom and
  have a process watch for oom).

- In that memory cgroup add a process with one thread per cpu.

- In one thread slowly allocate once per second I think it is 16M of ram
  and mlock and dirty it (just to force the pages into ram and stay
  there).

- When oom is achieved loop:
  * attempt to freeze all of the tasks.
  * if frozen send every task SIGKILL, unfreeze, remove the directory in
    cgroupfs.

Eric has then pinpointed the issue to be memcg specific.

All tasks are sitting on the memcg_oom_waitq when memcg oom is disabled.
Those that have received fatal signal will bypass the charge and should
continue on their way out.  The tricky part is that the exit path might
trigger a page fault (e.g.  exit_robust_list), thus the memcg charge,
while its memcg is still under OOM because nobody has released any charges
yet.

Unlike with the in-kernel OOM handler the exiting task doesn't get
TIF_MEMDIE set so it doesn't shortcut futher charges of the killed task
and falls to the memcg OOM again without any way out of it as there are no
fatal signals pending anymore.

This patch fixes the issue by checking PF_EXITING early in
__mem_cgroup_try_charge and bypass the charge same as if it had fatal
signal pending or TIF_MEMDIE set.

Normally exiting tasks (aka not killed) will bypass the charge now but
this should be OK as the task is leaving and will release memory and
increasing the memory pressure just to release it in a moment seems
dubious wasting of cycles.  Besides that charges after exit_signals should
be rare.

Reported-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: David Rientjes <rientjes@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 60bd919e6b35..e114da6b46ac 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2670,7 +2670,8 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
 	 * MEMDIE process.
 	 */
 	if (unlikely(test_thread_flag(TIF_MEMDIE)
-		     || fatal_signal_pending(current)))
+		     || fatal_signal_pending(current))
+		     || current->flags & PF_EXITING)
 		goto bypass;
 
 	if (unlikely(task_in_memcg_oom(current)))