Date: Wed, 21 Apr 2010 10:46:45 -0500 (CDT)
From: Christoph Lameter <cl@linux-foundation.org>
To: Mel Gorman <mel@csn.ul.ie>
cc: 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>,
       Minchan Kim <minchan.kim@gmail.com>,
       KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.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
In-Reply-To: <20100421153421.GM30306@csn.ul.ie>
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On Wed, 21 Apr 2010, Mel Gorman wrote:

> > > 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.
> >
>
> It's not unmap_and_move() that the problem is occurring on but during a
> page fault - presumably in do_swap_page but I'm not 100% certain.

remove_migration_pte() calls migration_entry_to_page(). So it must do that
only if the page is still locked.

You need to ensure that the page is not unlocked in move_to_new_page() if
the migration ptes are kept.

move_to_new_page() only unlocks the new page not the original page. So that is safe.

And it seems that the old page is also unlocked in unmap_and_move() only
after the migration_ptes have been removed? So we are fine after all...?


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