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    Subject[PATCH 3.15 025/139] memcg: do not hang on OOM when killed by userspace OOM access to memory reserves
    Date
    3.15-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Michal Hocko <mhocko@suse.cz>

    commit d8dc595ce3909fbc131bdf5ab8c9808fe624b18d upstream.

    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_control

    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 further 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.

    I am bringing this patch again (rebased on the current mmotm tree). I
    hope we can move forward finally. If there is still an opposition then
    I would really appreciate a concurrent approach so that we can discuss
    alternatives.

    http://comments.gmane.org/gmane.linux.kernel.stable/77650 is a reference
    to the followup discussion when the patch has been dropped from the mmotm
    last time.

    Reported-by: Eric W. Biederman <ebiederm@xmission.com>
    Signed-off-by: Michal Hocko <mhocko@suse.cz>
    Acked-by: David Rientjes <rientjes@google.com>
    Acked-by: Johannes Weiner <hannes@cmpxchg.org>
    Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    mm/memcontrol.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    --- a/mm/memcontrol.c
    +++ b/mm/memcontrol.c
    @@ -2684,7 +2684,8 @@ static int mem_cgroup_try_charge(struct
    * free their memory.
    */
    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)))



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