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

From: Michal Hocko <mhocko@kernel.org>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: linux-mm@kvack.org, Johannes Weiner <hannes@cmpxchg.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2] memcg: do not report racy no-eligible OOM tasks
Date: Tue, 8 Jan 2019 12:46:39 +0100	[thread overview]
Message-ID: <20190108114639.GR31793@dhcp22.suse.cz> (raw)
In-Reply-To: <3b105bba-3542-1d00-c6e2-52f6d125eff2@i-love.sakura.ne.jp>

On Tue 08-01-19 19:39:58, Tetsuo Handa wrote:
> On 2019/01/08 17:14, Michal Hocko wrote:
> >>> 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);
> >>
> >> And because of "[PATCH 1/2] mm, oom: marks all killed tasks as oom
> >> victims", mark_oom_victim() will be called on current thread even if
> >> we used mutex_lock_killable(&oom_lock) here, like you said
> >>
> >>   mutex_lock_killable would take care of exiting task already. I would
> >>   then still prefer to check for mark_oom_victim because that is not racy
> >>   with the exit path clearing signals. I can update my patch to use
> >>   _killable lock variant if we are really going with the memcg specific
> >>   fix.
> >>
> >> . If current thread is not yet killed by the OOM killer but can terminate
> >> without invoking the OOM killer, using mutex_lock_killable(&oom_lock) here
> >> saves some processes. What is the race you are referring by "racy with the
> >> exit path clearing signals" ?
> > 
> > This is unrelated to the patch.
> 
> Ultimately related! This is the reasoning why your patch should be preferred
> over my patch.

No! I've said I do not mind using mutex_lock_killable on top of this
patch. I just want to have this fix minimal.

-- 
Michal Hocko
SUSE Labs