linux-kernel - Re: mm, oom_reaper: How to handle race with oom_killer

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Message-ID: <20160622065016.GD7520@dhcp22.suse.cz>
Date:	Wed, 22 Jun 2016 08:50:16 +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, mgorman@...hsingularity.net,
	hughd@...gle.com, riel@...hat.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: mm, oom_reaper: How to handle race with oom_killer_disable() ?

On Wed 22-06-16 08:40:15, Michal Hocko wrote:
> On Wed 22-06-16 06:47:48, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> > > On Wed 22-06-16 00:32:29, Tetsuo Handa wrote:
> > > > Michal Hocko wrote:
> > > [...]
> > > > > Hmm, what about the following instead. It is rather a workaround than a
> > > > > full flaged fix but it seems much more easier and shouldn't introduce
> > > > > new issues.
> > > > 
> > > > Yes, I think that will work. But I think below patch (marking signal_struct
> > > > to ignore TIF_MEMDIE instead of clearing TIF_MEMDIE from task_struct) on top of
> > > > current linux.git will implement no-lockup requirement. No race is possible unlike
> > > > "[PATCH 10/10] mm, oom: hide mm which is shared with kthread or global init".
> > > 
> > > Not really. Because without the exit_oom_victim from oom_reaper you have
> > > no guarantee that the oom_killer_disable will ever return. I have
> > > mentioned that in the changelog. There is simply no guarantee the oom
> > > victim will ever reach exit_mm->exit_oom_victim.
> > 
> > Why? Since any allocation after setting oom_killer_disabled = true will be
> > forced to fail, nobody will be blocked on waiting for memory allocation. Thus,
> > the TIF_MEMDIE tasks will eventually reach exit_mm->exit_oom_victim, won't it?
> 
> What if it gets blocked waiting for an operation which cannot make any
> forward progress because it cannot proceed with an allocation (e.g.
> an open coded allocation retry loop - not that uncommon when sending
> a bio)? I mean if we want to guarantee a forward progress then there has
> to be something to clear the flag no matter in what state the oom victim
> is or give up on oom_killer_disable.

That being said I guess the patch to try_to_freeze_tasks after
oom_killer_disable should be simple enough to go for now and stable
trees and we can come up with something less hackish later. I do not
like the fact that oom_killer_disable doesn't act as a full "barrier"
anymore.

What do you think?
-- 
Michal Hocko
SUSE Labs