linux-kernel - Re: [PATCH] oom killer: break from infinite loop

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Date:	Sun, 28 Mar 2010 18:28:21 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	anfei <anfei.zhou@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, rientjes@...gle.com,
	kosaki.motohiro@...fujitsu.com, nishimura@....nes.nec.co.jp,
	kamezawa.hiroyu@...fujitsu.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] oom killer: break from infinite loop

On 03/28, anfei wrote:
>
> On Fri, Mar 26, 2010 at 11:33:56PM +0100, Oleg Nesterov wrote:
>
> > Off-topic, but we shouldn't use force_sig(), SIGKILL doesn't
> > need "force" semantics.
> >
> This may need a dedicated patch, there are some other places to
> force_sig(SIGKILL, ...) too.

Yes, yes, sure.

> > I'd wish I could understand the changelog ;)
> >
> Assume thread A and B are in the same group.  If A runs into the oom,
> and selects B as the victim, B won't exit because at least in exit_mm(),
> it can not get the mm->mmap_sem semaphore which A has already got.

I see. But still I can't understand. To me, the problem is not that
B can't exit, the problem is that A doesn't know it should exit. All
threads should exit and free ->mm. Even if B could exit, this is not
enough. And, to some extent, it doesn't matter if it holds mmap_sem
or not.

Don't get me wrong. Even if I don't understand oom_kill.c the patch
looks obviously good to me, even from "common sense" pov. I am just
curious.

So, my understanding is: we are going to kill the whole thread group
but TIF_MEMDIE is per-thread. Mark the whole thread group as TIF_MEMDIE
so that any thread can notice this flag and (say, __alloc_pages_slowpath)
fail asap.

Is my understanding correct?

Oleg.

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