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

[Tetsuo Handa]

On 2018/10/22 16:13, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> 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@xxxxxxxxxxxxxxxxxxx
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  mm/memcontrol.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e79cb59552d9..a9dfed29967b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1380,10 +1380,22 @@ 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;
> +

This is not wrong but is strange. We can use mutex_lock_killable(&oom_lock)
so that any killed threads no longer wait for oom_lock.

Also, closing this race for only memcg OOM path is strange. Global OOM path
(which are CLONE_VM without CLONE_THREAD) is still suffering this race
(though frequency is lower than memcg OOM due to use of mutex_trylock()). Either
checking before calling out_of_memory() or checking task_will_free_mem(current)
inside out_of_memory() will close this race for both paths.

>  	ret = out_of_memory(&oc);
> +
> +unlock:
>  	mutex_unlock(&oom_lock);
>  	return ret;
>  }
>