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

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Message-ID: <540742D1.3010102@redhat.com>
Date:	Wed, 03 Sep 2014 12:33:21 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>
CC:	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

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On 09/03/2014 12:08 PM, Oleg Nesterov wrote:
> On 09/02, Oleg Nesterov wrote:
>> 
>> The usage of TASK_DEAD in task_numa_fault() is wrong in any
>> case.
> 
> Rik, I can't understand why task_numa_fault() needs this check at
> all, but "if (p->state == TASK_DEAD)" looks certainly wrong. You
> could replace this check with BUG_ON(p->state == TASK_DEAD).
> Perhaps you meant PF_EXITING?

I do not know why that code is in there, either.

I suspect it was added after some conversation on irc, with
either Peter or Mel.

> And a stupid (really, I don't understand this code) question:
> 
> /* for example, ksmd faulting in a user's mm */ if (!p->mm) 
> return;
> 
> OK, but perhaps it make sense to pass "mm" as another argument and
> do
> 
> /* ksmd faulting in a user's mm, or debugger, or kthread use_mm()
> caller */ if (p->mm != mm) return;

I suppose that makes sense, since it would be possible for one task
to cause a page fault in another task's mm, with eg. ptrace peek/poke
or similar code paths.

Currently the numa code could end up accounting such a fault in the
wrong mm, when it would be better to not account the fault at all.

This is a bit of a corner case, and probably not the highest priority
thing to fix right now, but it would be fairly easy.

- -- 
All rights reversed
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