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: <20160204163113.GF14425@dhcp22.suse.cz>
Date:	Thu, 4 Feb 2016 17:31:13 +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 Fri 05-02-16 00:08:25, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > > > +	/*
> > > > +	 * 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?
> 
> The mm is shared between previous victim and next victim, but these victims
> are in different thread groups. The OOM killer selects next victim whose mm
> was already reaped due to sharing previous victim's memory.

OK, now I got your point. From your previous email it sounded like you
were talking about oom_reaper and its invocation which is was confusing.

> We don't want the OOM killer to select such next victim.

Yes, selecting such a task doesn't make much sense. It has been killed
so it has fatal_signal_pending. If it wanted to allocate it would get
TIF_MEMDIE already and it's address space has been reaped so there is
nothing to free left. These CLONE_VM without CLONE_SIGHAND is really
crazy combo, it is just causing troubles all over and I am not convinced
it is actually that helpful </rant>.


> Maybe set MMF_OOM_REAP_DONE on
> the previous victim's mm and check it instead of TIF_MEMDIE when selecting
> a victim? That will also avoid problems caused by clearing TIF_MEMDIE?

Hmm, it doesn't seem we are under MMF_ availabel bits pressure right now
so using the flag sounds like the easiest way to go. Then we even do not
have to play with OOM_SCORE_ADJ_MIN which might be updated from the
userspace after the oom reaper has done that. Care to send a patch?

Thanks!
-- 
Michal Hocko
SUSE Labs