linux-kernel - Re: [PATCH 3/5] oom: clear TIF_MEMDIE after oom_reaper managed to unmap the address space

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Message-ID: <20160204144319.GD14425@dhcp22.suse.cz>
Date:	Thu, 4 Feb 2016 15:43:19 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	akpm@...ux-foundation.org, rientjes@...gle.com, mgorman@...e.de,
	oleg@...hat.com, torvalds@...ux-foundation.org, hughd@...gle.com,
	andrea@...nel.org, riel@...hat.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] oom: clear TIF_MEMDIE after oom_reaper managed to
 unmap the address space

On Thu 04-02-16 23:22:18, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > From: Michal Hocko <mhocko@...e.com>
> > 
> > When oom_reaper manages to unmap all the eligible vmas there shouldn't
> > be much of the freable memory held by the oom victim left anymore so it
> > makes sense to clear the TIF_MEMDIE flag for the victim and allow the
> > OOM killer to select another task.
> 
> Just a confirmation. Is it safe to clear TIF_MEMDIE without reaching do_exit()
> with regard to freezing_slow_path()? Since clearing TIF_MEMDIE from the OOM
> reaper confuses
> 
>     wait_event(oom_victims_wait, !atomic_read(&oom_victims));
> 
> in oom_killer_disable(), I'm worrying that the freezing operation continues
> before the OOM victim which escaped the __refrigerator() actually releases
> memory. Does this cause consistency problem?

This is a good question! At first sight it seems this is not safe and we
might need to make the oom_reaper freezable so that it doesn't wake up
during suspend and interfere. Let me think about that.

> > +	/*
> > +	 * Clear TIF_MEMDIE because the task shouldn't be sitting on a
> > +	 * reasonably reclaimable memory anymore. OOM killer can continue
> > +	 * by selecting other victim if unmapping hasn't led to any
> > +	 * improvements. This also means that selecting this task doesn't
> > +	 * make any sense.
> > +	 */
> > +	tsk->signal->oom_score_adj = OOM_SCORE_ADJ_MIN;
> > +	exit_oom_victim(tsk);
> 
> I noticed that updating only one thread group's oom_score_adj disables
> further wake_oom_reaper() calls due to rough-grained can_oom_reap check at
> 
>   p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN
> 
> in oom_kill_process(). I think we need to either update all thread groups'
> oom_score_adj using the reaped mm equally or use more fine-grained can_oom_reap
> check which ignores OOM_SCORE_ADJ_MIN if all threads in that thread group are
> dying or exiting.

I do not understand. Why would you want to reap the mm again when
this has been done already? The mm is shared, right?
-- 
Michal Hocko
SUSE Labs