linux-kernel - Re: can't oom-kill zap the victim's memory?

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Message-ID: <20150921153252.GA21988@redhat.com>
Date:	Mon, 21 Sep 2015 17:32:52 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Kyle Walker <kwalker@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Vladimir Davydov <vdavydov@...allels.com>,
	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Stanislav Kozina <skozina@...hat.com>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Subject: Re: can't oom-kill zap the victim's memory?

On 09/21, Michal Hocko wrote:
>
> On Mon 21-09-15 15:44:14, Oleg Nesterov wrote:
> [...]
> > So yes, in general oom_kill_process() can't call oom_unmap_func() directly.
> > That is why the patch uses queue_work(oom_unmap_func). The workqueue thread
> > takes mmap_sem and frees the memory allocated by user space.
>
> OK, this might have been a bit confusing. I didn't mean you cannot use
> mmap_sem directly from the workqueue context. You _can_ AFAICS. But I've
> mentioned that you _shouldn't_ use workqueue context in the first place
> because all the workers might be blocked on locks and new workers cannot
> be created due to memory pressure.

Yes, yes, and I already tried to comment this part. We probably need a
dedicated kernel thread, but I still think (although I am not sure) that
initial change can use workueue. In the likely case system_unbound_wq pool
should have an idle thread, if not - OK, this change won't help in this
case. This is minor.

> So I think we probably need to do this in the OOM killer context (with
> try_lock)

Yes we should try to do this in the OOM killer context, and in this case
(of course) we need trylock. Let me quote my previous email:

	And we want to avoid using workqueues when the caller can do this
	directly. And in this case we certainly need trylock. But this needs
	some refactoring: we do not want to do this under oom_lock, otoh it
	makes sense to do this from mark_oom_victim() if current && killed,
	and a lot more details.

and probably this is another reason why do we need MMF_MEMDIE. But again,
I think the initial change should be simple.

Oleg.

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