linux-kernel - Re: [PATCH 07/10] mm, oom: fortify task_will_free

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Message-ID: <20160613112746.GD6518@dhcp22.suse.cz>
Date:	Mon, 13 Jun 2016 13:27:46 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	linux-mm@...ck.org, rientjes@...gle.com, oleg@...hat.com,
	vdavydov@...allels.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/10] mm, oom: fortify task_will_free_mem

On Sat 11-06-16 17:10:03, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > > Also, I think setting TIF_MEMDIE on p when find_lock_task_mm(p) != p is
> > > wrong. While oom_reap_task() will anyway clear TIF_MEMDIE even if we set
> > > TIF_MEMDIE on p when p->mm == NULL, it is not true for CONFIG_MMU=n case.
> > 
> > Yes this would be racy for !CONFIG_MMU but does it actually matter?
> 
> I don't know because I've never used CONFIG_MMU=n kernels. But I think it
> actually matters. You fixed this race by commit 83363b917a2982dd ("oom:
> make sure that TIF_MEMDIE is set under task_lock").

Yes and that commit was trying to address a highly theoretical issue
reported by you. Let me quote:
:oom_kill_process is currently prone to a race condition when the OOM
:victim is already exiting and TIF_MEMDIE is set after the task releases
:its address space.  This might theoretically lead to OOM livelock if the
:OOM victim blocks on an allocation later during exiting because it
:wouldn't kill any other process and the exiting one won't be able to exit.
:The situation is highly unlikely because the OOM victim is expected to
:release some memory which should help to sort out OOM situation.

Even if such a race is possible it wouldn't be with the oom
reaper. Regarding CONFIG_MMU=n I am even less sure it is possible and
I would rather focus on CONFIG_MMU=y where we know that problems exist
rather than speculating about something as special as nommu which even
might not care at all.

-- 
Michal Hocko
SUSE Labs