Date: Thu, 22 Apr 2010 19:31:06 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: Minchan Kim <minchan.kim@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>, Christoph Lameter <cl@linux-foundation.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Andrea Arcangeli <aarcange@redhat.com>, Adam Litke <agl@us.ibm.com>,
       Avi Kivity <avi@redhat.com>, David Rientjes <rientjes@google.com>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Rik van Riel <riel@redhat.com>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org
Subject: Re: [PATCH 04/14] mm,migration: Allow the migration of
 PageSwapCache  pages
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On Thu, 22 Apr 2010 19:13:12 +0900
Minchan Kim <minchan.kim@gmail.com> wrote:

> On Thu, Apr 22, 2010 at 6:46 PM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> > Hmm..in my test, the case was.
> >
> > Before try_to_unmap:
> >        mapcount=1, SwapCache, remap_swapcache=1
> > After remap
> >        mapcount=0, SwapCache, rc=0.
> >
> > So, I think there may be some race in rmap_walk() and vma handling or
> > anon_vma handling. migration_entry isn't found by rmap_walk.
> >
> > Hmm..it seems this kind patch will be required for debug.
> 
> I looked do_swap_page, again.
> lock_page is called long after migration_entry_wait.
> It means lock_page can't close the race.
> 
> So I think this BUG is possible.
> What do you think?
> 

I think it's not a problem.
When migration-entry-wait is called, enry_wait() does

	pte_lock();
	check migration_pte
	check it's locked.

And after wait_on_page_locked(), it just returns to user and cause
page fault again. 

Thanks,
-Kame

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