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Date:   Thu, 2 Aug 2018 14:14:46 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Roman Gushchin <guro@fb.com>, linux-mm@kvack.org,
        Johannes Weiner <hannes@cmpxchg.org>,
        David Rientjes <rientjes@google.com>,
        Tejun Heo <tj@kernel.org>, kernel-team@fb.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 3/3] mm, oom: introduce memory.oom.group
Message-ID: <20180802121446.GK10808@dhcp22.suse.cz>
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On Thu 02-08-18 20:53:14, Tetsuo Handa wrote:
> On 2018/08/02 20:21, Michal Hocko wrote:
> > On Thu 02-08-18 19:53:13, Tetsuo Handa wrote:
> >> On 2018/08/02 9:32, Roman Gushchin wrote:
> > [...]
> >>> +struct mem_cgroup *mem_cgroup_get_oom_group(struct task_struct *victim,
> >>> +					    struct mem_cgroup *oom_domain)
> >>> +{
> >>> +	struct mem_cgroup *oom_group = NULL;
> >>> +	struct mem_cgroup *memcg;
> >>> +
> >>> +	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
> >>> +		return NULL;
> >>> +
> >>> +	if (!oom_domain)
> >>> +		oom_domain = root_mem_cgroup;
> >>> +
> >>> +	rcu_read_lock();
> >>> +
> >>> +	memcg = mem_cgroup_from_task(victim);
> >>
> >> Isn't this racy? I guess that memcg of this "victim" can change to
> >> somewhere else from the one as of determining the final candidate.
> > 
> > How is this any different from the existing code? We select a victim and
> > then kill it. The victim might move away and won't be part of the oom
> > memcg anymore but we will still kill it. I do not remember this ever
> > being a problem. Migration is a privileged operation. If you loose this
> > restriction you shouldn't allow to move outside of the oom domain.
> 
> The existing code kills one process (plus other processes sharing mm if any).
> But oom_cgroup kills multiple processes. Thus, whether we made decision based
> on correct memcg becomes important.

Yes but a proper configuration should already mitigate the harm because
you shouldn't be able to migrate the task outside of the oom domain.
	A (oom.group = 1)
       / \
      B   C

moving task between B and C should be harmless while moving it out of A
subtree completely is a dubious configuration.

> >> This "victim" might have already passed exit_mm()/cgroup_exit() from do_exit().
> > 
> > Why does this matter? The victim hasn't been killed yet so if it exists
> > by its own I do not think we really have to tear the whole cgroup down.
> 
> The existing code does not send SIGKILL if find_lock_task_mm() failed. Who can
> guarantee that the victim is not inside do_exit() yet when this code is executed?

I do not follow. Why does this matter at all?

-- 
Michal Hocko
SUSE Labs