linux-kernel - Re: [PATCH 1/5] oom: select_bad_process: check PF

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Message-ID: <20100601212023.GA24917@redhat.com>
Date:	Tue, 1 Jun 2010 23:20:23 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH 1/5] oom: select_bad_process: check PF_KTHREAD instead
	of !mm to skip kthreads

On 06/01, David Rientjes wrote:
>
> On Mon, 31 May 2010, KOSAKI Motohiro wrote:
>
> > select_bad_process() thinks a kernel thread can't have ->mm != NULL, this
> > is not true due to use_mm().
> >
> > Change the code to check PF_KTHREAD.
>
> This is already pushed in my oom killer rewrite as patch 14/18 "check
> PF_KTHREAD instead of !mm to skip kthreads".
>
> This does not need to be merged immediately since it's not vital: use_mm()
> is only temporary state and these kthreads will once again be excluded
> when they call unuse_mm().  The worst case scenario here is that the oom
> killer will erroneously select one of these kthreads which cannot die

It can't die but force_sig() does bad things which shouldn't be done
with workqueue thread. Note that it removes SIG_IGN, sets
SIGNAL_GROUP_EXIT, makes signal_pending/fatal_signal_pedning true, etc.

But yes, I agree, the problem is minor. But nevertheless it is bug,
the longstanding bug with the simple fix. Why should we "hide" this fix
inside the long series of non-trivial patches which rewrite oom-killer?
And it is completely orthogonal to other changes.

Oleg.

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