linux-kernel - Re: [PATCH 1/2] oom: don't assume that a coredumping thread will exit soon

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Date:	Wed, 3 Dec 2014 14:07:51 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Cong Wang <xiyou.wangcong@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] oom: don't assume that a coredumping thread will
 exit soon

On Tue 02-12-14 20:16:22, Oleg Nesterov wrote:
> On 12/02, Michal Hocko wrote:
> >
> > On Tue 02-12-14 18:50:41, Oleg Nesterov wrote:
> > > On 12/02, Michal Hocko wrote:
> > > >
> > > > I guess the patch as is makes sense and it is an improvement. We need
> > > > to call the helper in mem_cgroup_out_of_memory as well, though.
> > >
> > > Yes, but can't we do this in a separate patch?
> >
> > I would prefer if it was in the same patch because we might be facing
> > the same problem in memcg as with the global case. And worse, smaller
> > limit tend to trigger corner cases more often than the global case.
> 
> OK, I'll do V2...
> 
> But let me explain why I thought about another patch. I do not want
> to export task_will_free_mem(). If nothing else, its name matches the
> current "quickly exit and free its memory" comments but not the reality.

Yes, the name suggests much more than what it does.

> An exiting thread won't free the memory (ignoring task_struct/etc) if
> the process is multithreaded.
> 
> I'd rather add another helper for oom_kill.c and memcontrol.c which does
> 
> 	if (fatal_signal_pending(current) || task_will_free_mem(current)) {
> 		set_thread_flag(TIF_MEMDIE);
> 		return true;
> 	}
> 
> 	return false;
> 
> This way the patch could document that fatal_signal_pending() is not
> exactly right as we discussed, and then we can improve this helper.
> 
> But OK, probably this helper doesn't really make sense, and I can not
> invent the good name for it ;)

I've failed to come up with a better name as well. The code duplication
is not nice either so I guess it would be better to keep the helper
localt to mm/oom_kill.c and have it open coded in mm/memcontro.c. Git
blame will still tell us all the motivation if they are in the single
patch.

> > > try_charge() plays with TIF_MEMDIE/PF_EXITING too, but probably this
> > > is fine.
> >
> > try_charge is OK because this is from the time when the allocation has
> > been already done and we just decide to bypass the charge.
> 
> Yes, thanks, this was my vague understanding but I wasn't sure. However,
> I am not sure that PF_EXITING check is 100% right (again, this can only
> mean that a single thread from a thread group exits), but I do not
> understand this code and I agree this is another story in any case.

See d8dc595ce390 "memcg: do not hang on OOM when killed by userspace OOM
access to memory reserves" for more details.

-- 
Michal Hocko
SUSE Labs
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