Re: [PATCH 2/2] mm,migration: Avoid race between shift_arg_pages() and rmap_walk() during migration by not migrating temporary stacks

[Mel Gorman <mel@csn.ul.ie>]

On Mon, May 10, 2010 at 12:41:07PM -0500, Christoph Lameter wrote:
> On Tue, 4 May 2010, Mel Gorman wrote:
> 
> > On Sat, May 01, 2010 at 08:56:18PM -0500, Christoph Lameter wrote:
> > > On Thu, 29 Apr 2010, Mel Gorman wrote:
> > >
> > > > There is a race between shift_arg_pages and migration that triggers this bug.
> > > > A temporary stack is setup during exec and later moved. If migration moves
> > > > a page in the temporary stack and the VMA is then removed before migration
> > > > completes, the migration PTE may not be found leading to a BUG when the
> > > > stack is faulted.
> > >
> > > A simpler solution would be to not allow migration of the temporary stack?
> > >
> >
> > The patch's intention is to not migrate pages within the temporary
> > stack. What are you suggesting that is different?
> 
> A simple way to disallow migration of pages is to increment the refcount
> of a page.
> 

I guess it could be done by walking the page-tables in advance of the move
and elevating the page count of any pages faulted and then finding those
pages afterwards.  The fail path would be a bit of a pain though if the page
tables are partially moved though. It's unnecessarily complicated when the
temporary stack can be easily avoided.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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