Re: [PATCH -mm -repost] memcg: do not hang on OOM when killed by userspace OOM access to memory reserves

[David Rientjes]

On Wed, 23 Apr 2014, Michal Hocko wrote:

> 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 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.
> 
> Reported-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>

Acked-by: David Rientjes <rientjes@xxxxxxxxxx>

I think we should wait for a Tested-by from Eric if this is going to be 
backported to stable, though, to meet the criteria.
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