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

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Date:	Wed, 21 Apr 2010 10:31:29 -0500 (CDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
cc:	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>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.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 Wed, 21 Apr 2010, Mel Gorman wrote:

> On Wed, Apr 21, 2010 at 10:05:21AM -0500, Christoph Lameter wrote:
> > On Wed, 21 Apr 2010, Mel Gorman wrote:
> >
> > > No, remap_swapcache could just be called "remap". If it's 0, it's
> > > considered unsafe to remap the page.
> >
> > Call this "can_remap"?
> >
>
> can_do - ba dum tisch.
>
> While you are looking though, maybe you can confirm something for me.
>
> 1. Is leaving a migration PTE like this behind reasonable? (I think yes
>    particularly as the page was already unmapped so it's not a new fault
>    incurred)

The design of page migration only allows for the existence of these as
long as the page is locked. Not sure what would happen if you leave this
hanging around. Paths that are not prepared for a migration_pte may
encounter one.

> 2. Is the BUG_ON check in
>    include/linux/swapops.h#migration_entry_to_page() now wrong? (I
>    think yes, but I'm not sure and I'm having trouble verifying it)

The bug check ensures that migration entries only occur when the page
is locked. This patch changes that behavior. This is going too oops
therefore in unmap_and_move() when you try to remove the migration_ptes
from an unlocked page.


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