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 10:18:58 +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 03:05:43 +0200
Andrea Arcangeli <aarcange@...hat.com> wrote:

> On Wed, Apr 28, 2010 at 09:39:48AM +0900, KAMEZAWA Hiroyuki wrote:
> > Seems nice.
> 
> What did you mean with objrmap inconsistency? I think this is single
> threaded there, userland didn't run yet and I don't think page faults
> could run. Maybe it's safer to add a VM_BUG_ON(vma->anon_vma) just
> before vma->anon_vma = anon_vma to be sure nothing run in between.
> 
I mean following relationship.

	vma_address(vma, page1) <-> address <-> pte <-> page2
	
	If page1 == page2, objrmap is consistent.
	If page1 != page2, objrmap is inconsistent.





> > I'll test this but I think we need to take care of do_mremap(), too.
> > And it's more complicated....
> 
> do_mremap has to be safe by:
> 
> 1) adjusting page->index atomically with the pte updates inside pt
>    lock (while it moves from one pte to another)
> 
I reviewed do_mremap again and am thinking it's safe.


	new_vma = copy_vma();
	....                   ----------(*)
	move_ptes().
	munmap unnecessary range.

At (*), if page1 != page2, rmap_walk will not run correctly.
But it seems copy_vma() keeps page1==page2 

As I reported, when there is a problem, vma_address() returns an address but
that address doesn't contain migration_pte.

BTW, page->index is not updated, we just keep [start_address, pgoff] to be
sane value.

Thanks,
-Kame


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