linux-kernel - Re: [PATCH 26/31] sched, numa, mm: Add fault driven placement and migration policy

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Message-ID: <20121026141430.GA12158@gmail.com>
Date:	Fri, 26 Oct 2012 16:14:30 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 26/31] sched, numa, mm: Add fault driven placement and
 migration policy


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Fri, 2012-10-26 at 15:50 +0200, Ingo Molnar wrote:
> > 
> > Oh, just found the reason:
> > 
> > the ptep_modify_prot_start()/modify()/commit() sequence is 
> > SMP-unsafe - it has to be done under the mmap_sem 
> > write-locked.
> > 
> > It is safe against *hardware* updates to the PTE, but not 
> > safe against itself.
> 
> Shouldn't the pte_lock serialize all that still? All sites 
> that modify PTE contents should hold the pte_lock (and do 
> afaict).

Hm, indeed.

Is there no code under down_read() (in the page fault path) that 
modifies the pte via just pure atomics?

Thanks,

	Ingo
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