linux-kernel - Re: task_numa_fault() && TASK

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Message-ID: <alpine.LSU.2.11.1409041210400.12218@eggly.anvils>
Date:	Thu, 4 Sep 2014 12:14:51 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Oleg Nesterov <oleg@...hat.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Kautuk Consul <consul.kautuk@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	Ionut Alexa <ionut.m.alexa@...il.com>,
	Guillaume Morin <guillaume@...infr.org>,
	linux-kernel@...r.kernel.org, Kirill Tkhai <tkhai@...dex.ru>
Subject: Re: task_numa_fault() && TASK_DEAD

On Thu, 4 Sep 2014, Oleg Nesterov wrote:
> On 09/04, Peter Zijlstra wrote:
> > On Wed, Sep 03, 2014 at 06:08:19PM +0200, Oleg Nesterov wrote:
> >
> > > And a stupid (really, I don't understand this code) question:
> > >
> > > 	/* for example, ksmd faulting in a user's mm */
> > > 	if (!p->mm)
> > > 		return;

I don't understand your difficulty with that, I thought the comment
was helpful enough.  Does the original commit comment help?

commit 2832bc19f6668fd00116f61f821105040599ef8b
Author: Hugh Dickins <hughd@...gle.com>
Date:   Wed Dec 19 17:42:16 2012 -0800

    sched: numa: ksm: fix oops in task_numa_placment()
    
    task_numa_placement() oopsed on NULL p->mm when task_numa_fault() got
    called in the handling of break_ksm() for ksmd.  That might be a
    peculiar case, which perhaps KSM could takes steps to avoid? but it's
    more robust if task_numa_placement() allows for such a possibility.
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