linux-kernel - Re: [PATCH 3/3] mm,migration: Remove straggling migration PTEs when page tables are being moved after the VMA has already moved

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Date:	Wed, 28 Apr 2010 09:39:48 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrea Arcangeli <aarcange@...hat.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>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/3] mm,migration: Remove straggling migration PTEs when
 page tables are being moved after the VMA has already moved

On Wed, 28 Apr 2010 00:58:52 +0200
Andrea Arcangeli <aarcange@...hat.com> wrote:

> On Wed, Apr 28, 2010 at 12:30:04AM +0200, Andrea Arcangeli wrote:
> > I'll now evaluate the fix and see if I can find any other
> > way to handle this.
> 
> 
> I think a better fix for bug mentioned in patch 3, is like below. This
> seems to work fine on aa.git with the old (stable) 2.6.33 anon-vma
> code. Not sure if this also works with the new anon-vma code in
> mainline but at first glance I think it should. At that point we
> should be single threaded so it shouldn't matter if anon_vma is
> temporary null.
> 
> Then you've to re-evaluate the vma_adjust fixes for mainline-only in
> patch 2 at the light of the below (I didn't check patch 2 in detail).
> 
> Please try to reproduce with the below applied.
> 
> ----
> Subject: fix race between shift_arg_pages and rmap_walk
> 
> From: Andrea Arcangeli <aarcange@...hat.com>
> 
> migrate.c requires rmap to be able to find all ptes mapping a page at
> all times, otherwise the migration entry can be instantiated, but it
> can't be removed if the second rmap_walk fails to find the page.
> 
> So shift_arg_pages must run atomically with respect of rmap_walk, and
> it's enough to run it under the anon_vma lock to make it atomic.
> 
> And split_huge_page() will have the same requirements as migrate.c
> already has.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>

Seems nice.

I'll test this but I think we need to take care of do_mremap(), too.
And it's more complicated....

Thanks,
-Kame

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