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 09:03:02 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Christoph Lameter <cl@...ux.com>,
	Andrea Arcangeli <aarcange@...hat.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 Tue, 27 Apr 2010 22:30:52 +0100
Mel Gorman <mel@....ul.ie> wrote:

> During exec(), a temporary stack is setup and moved later to its final
> location. There is a race between migration and exec whereby a migration
> PTE can be placed in the temporary stack. When this VMA is moved under the
> lock, migration no longer knows where the PTE is, fails to remove the PTE
> and the migration PTE gets copied to the new location.  This later causes
> a bug when the migration PTE is discovered but the page is not locked.
> 
> This patch handles the situation by removing the migration PTE when page
> tables are being moved in case migration fails to find them. The alternative
> would require significant modification to vma_adjust() and the locks taken
> to ensure a VMA move and page table copy is atomic with respect to migration.
> 
> Signed-off-by: Mel Gorman <mel@....ul.ie>

Mel, I don't like this fix. Consider following,

 1. try_to_unmap(oldpage)
 2. copy and replace
 3. remove_migration_ptes(oldpage, newpage)

What this patch handles is "3: remove_migration_ptes fails to remap it and
migration_pte will remain there case....The fact "new page is not mapped" means
"get_page() is not called against the new page".
So, the new page have been able to be freed until we restart move_ptes.

I bet calling __get_user_pages_fast() before vma_adjust() is the way to go. 
When page_count(page) != page_mapcount(page) +1, migration skip it.

Thanks,
-Kame

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