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

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 27 Apr 2010 09:07:06 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> On Mon, 26 Apr 2010 23:37:58 +0100
> Mel Gorman <mel@csn.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@csn.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@jp.fujitsu.com>
> 

Sorry. reproduced. It seems the same bug before patch. 
mapcount 1 -> unmap -> remap -> mapcount 0. And it was SwapCache.


Thanks,
-Kame

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