linux-kernel - Re: [PATCH 04/14] mm,migration: Allow the migration of PageSwapCache pages

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Message-Id: <20100422193106.9ffad4ec.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 22 Apr 2010 19:31:06 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Mel Gorman <mel@....ul.ie>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 04/14] mm,migration: Allow the migration of
 PageSwapCache  pages

On Thu, 22 Apr 2010 19:13:12 +0900
Minchan Kim <minchan.kim@...il.com> wrote:

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