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

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Date:	Fri, 5 Sep 2014 13:35:53 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Hugh Dickins <hughd@...gle.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 09/04, Hugh Dickins wrote:
>
> 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.

Yes, yes, sorry for confusion, the comment and the check itself look clear.

> 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

Yes. But I thought that even if task_numa_placment() didn't OOPS in case
when ->mm = NULL, it would be better to exclude ksmd. And other kthreads
which can have ->mm != NULL, and PTRACE_POKE or sys_process_vm_writev()
users:

	/* do nothing if this task accesses a foreign mm */
	if (p->mm != mm || (p->flags & PF_KTHREAD)
		return;

Please forget. This is minor, and my main question was the wrong usage
of TASK_DEAD.

Oleg.

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