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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20100428090302.5e69721f.kamezawa.hiroyu@jp.fujitsu.com>
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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/