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

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Date:	Tue, 2 Dec 2014 18:50:41 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Michal Hocko <mhocko@...e.cz>
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 12/02, Michal Hocko wrote:
>
> On Fri 28-11-14 00:04:05, Oleg Nesterov wrote:
>
> > Note: this is only the first step, this patch doesn't try to solve other
> > problems. For example it doesn't try to clear the wrongly set TIF_MEMDIE
> > (SIGNAL_GROUP_COREDUMP check is obviously racy),
>
> I am not sure I understand this. What do you mean by wrongly set
> TIF_MEMDIE? That we give a process access to reserves even though it is
> already done with the coredumping?

I meant that (say) oom_kill_process() can set TIF_MEMDIE because
PF_EXITING && !SIGNAL_GROUP_COREDUMP, and after that this task can
participate the coredumping. For example, this thread can exit on its
own, but before it calls exit_mm() another thread can start the coredump.

In this case TIF_MEMDIE can fool oom-killer the same way,
oom_scan_process_thread() returns OOM_SCAN_ABORT if TIF_MEMDIE is set.

> > fatal_signal_pending() can be false positive, etc.
>
> When can this happen?

I meant "if (fatal_signal_pending(current) || task_will_free_mem(current))"
in out_of_memory(). Yes, sorry, "false positive" looks confusing. I meant
that fatal_signal_pending() can be true because of SIGNAL_GROUP_COREDUMP.

> > Signed-off-by: Oleg Nesterov <oleg@...hat.com>
>
> 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? try_charge() plays with
TIF_MEMDIE/PF_EXITING too, but probably this is fine.

> With that feel free to add
> Acked-by: Michal Hocko <mhocko@...e.cz>

Thanks.

> Also the original fix for the coredumping (edd45544c6f0 "oom: avoid
> deferring oom killer if exiting task is being traced") doesn't work
> really as per http://marc.info/?l=linux-kernel&m=141711049013620 then
> this and the follow up patch should be marked for stable I guess.

Perhaps this makes sense. It looks simple enough.

Oleg.

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