linux-kernel - Re: [PATCH 1/3 for 2.6.38] oom: oom_kill_process: don't set TIF

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Message-ID: <20110314203152.GA25080@redhat.com>
Date:	Mon, 14 Mar 2011 21:31:52 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrey Vagin <avagin@...nvz.org>,
	David Rientjes <rientjes@...gle.com>,
	Frantisek Hrbata <fhrbata@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3 for 2.6.38] oom: oom_kill_process: don't set
	TIF_MEMDIE if !p->mm

On 03/14, Linus Torvalds wrote:
>
> On Mon, Mar 14, 2011 at 12:04 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> > oom_kill_process() simply sets TIF_MEMDIE and returns if PF_EXITING.
> > This is very wrong by many reasons. In particular, this thread can
> > be the dead group leader. Check p->mm != NULL.
>
> Explain more, please. Maybe I'm missing some context because I wasn't
> cc'd on the original thread, but PF_EXITING gets set by exit_signal(),
> and exit_mm() is called almost immediately afterwards which will set
> p->mm to NULL.
>
> So afaik, this will basically just remove the whole point of the code
> entirely - so why not remove it then?

I am afraid I am going to lie... But iirc I tried to remove this code
before. Can't find the previous discussion, probably I am wrong.

Anyway. I never understood why do we have this special case.

> The combination of testing PF_EXITING and p->mm just doesn't seem to
> make any sense.

To me, it doesn't make too much sense even if we do not check ->mm.

But. I _think_ the intent was to wait until this "exiting" process
does exit_mm() and frees the memory. This is like the
"the process of releasing memory " code in select_bad_process(). Once
again, this is only my speculation.

In any case, this patch doesn't pretend to be the right fix.

Oleg.

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