linux-kernel - Re: [PATCH 2/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information

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Message-Id: <20100427130319.248cb8e1.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Tue, 27 Apr 2010 13:03:19 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.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>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing
 the wrong VMA information

On Tue, 27 Apr 2010 12:50:40 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:

> On Tue, 27 Apr 2010 09:07:06 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> 
> > On Mon, 26 Apr 2010 23:37:58 +0100
> > Mel Gorman <mel@....ul.ie> wrote:
> > 
> > > vma_adjust() is updating anon VMA information without any locks taken.
> > > In contrast, file-backed mappings use the i_mmap_lock and this lack of
> > > locking can result in races with page migration. During rmap_walk(),
> > > vma_address() can return -EFAULT for an address that will soon be valid.
> > > This leaves a dangling migration PTE behind which can later cause a BUG_ON
> > > to trigger when the page is faulted in.
> > > 
> > > With the recent anon_vma changes, there can be more than one anon_vma->lock
> > > that can be taken in a anon_vma_chain but a second lock cannot be spinned
> > > upon in case of deadlock. Instead, the rmap walker tries to take locks of
> > > different anon_vma's. If the attempt fails, the operation is restarted.
> > > 
> > > Signed-off-by: Mel Gorman <mel@....ul.ie>
> > 
> > Ok, acquiring vma->anon_vma->spin_lock always sounds very safe.
> > (but slow.)
> > 
> > I'll test this, too.
> > 
> > Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > 
> 
> Sorry. reproduced. It seems the same bug before patch. 
> mapcount 1 -> unmap -> remap -> mapcount 0. And it was SwapCache.
> 
Sorry again. It was _not_ SwapCache. Just SwapBacked.

Thanks,
-Kame

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