linux-kernel - Re: [PATCH 0/3] Fix migration races in rmap

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Message-ID: <20100427223242.GG8860@random.random>
Date:	Wed, 28 Apr 2010 00:32:42 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Mel Gorman <mel@....ul.ie>, Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/3] Fix migration races in rmap_walk() V2

On Tue, Apr 27, 2010 at 05:27:36PM -0500, Christoph Lameter wrote:
> Can we simply wait like in the fault path?

There is no bug there, no need to wait either. I already audited it
before, and I didn't see any bug. Unless you can show a bug with CPU A
running the rmap_walk on process1 before process2, there is no bug to
fix there.

> 
> > Patch 3 notes that while a VMA is moved under the anon_vma lock, the page
> > 	tables are not similarly protected. Where migration PTEs are
> > 	encountered, they are cleaned up.
> 
> This means they are copied / moved etc and "cleaned" up in a state when
> the page was unlocked. Migration entries are not supposed to exist when
> a page is not locked.

patch 3 is real, and the first thought I had was to lock down the page
before running vma_adjust and unlock after move_page_tables. But these
are virtual addresses. Maybe there's a simpler way to keep migration
away while we run those two operations.
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