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

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Date:	Tue, 27 Apr 2010 18:09:49 +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 2/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing
 the wrong VMA information

On Tue, 27 Apr 2010 09:59:51 +0100
Mel Gorman <mel@....ul.ie> wrote:

> On Tue, Apr 27, 2010 at 12:50:40PM +0900, KAMEZAWA Hiroyuki 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.
> > 
> 
> Same here, reproduced after 18 hours.
> 
Hmm. It seems rmap_one() is called and the race is not in vma_address()
but in remap_migration_pte().
So, I added more hooks for debug..but not reproduced yet.
(But I doubt my debug code, too ;)

But it seems strange to have a race in remap_migration_pte(), so, I doubt
my debug code, too.

Thanks,
-Kame

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