linux-kernel - Re: [PATCH] exit: clear TIF_MEMDIE after exit_task

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Message-ID: <20160229184444.GT16930@dhcp22.suse.cz>
Date:	Mon, 29 Feb 2016 19:44:44 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] exit: clear TIF_MEMDIE after exit_task_work

On Mon 29-02-16 19:21:31, Michal Hocko wrote:
> On Mon 29-02-16 20:02:09, Vladimir Davydov wrote:
> > An mm_struct may be pinned by a file. An example is vhost-net device
> > created by a qemu/kvm (see vhost_net_ioctl -> vhost_net_set_owner ->
> > vhost_dev_set_owner). If such process gets OOM-killed, the reference to
> > its mm_struct will only be released from exit_task_work -> ____fput ->
> > __fput -> vhost_net_release -> vhost_dev_cleanup, which is called after
> > exit_mmap, where TIF_MEMDIE is cleared. As a result, we can start
> > selecting the next victim before giving the last one a chance to free
> > its memory. In practice, this leads to killing several VMs along with
> > the fattest one.
> 
> I am wondering why our PF_EXITING protection hasn't fired up.

OK, I guess I can see it. exit_mm has done tsk->mm = NULL and so we are
skipping over that task because oom_scan_process_thread hasn't checked
PF_EXITING. I will try to think about this some more tomorrow with a
fresh brain.
-- 
Michal Hocko
SUSE Labs