Re: [PATCH 2/2] mm,migration: Fix race between shift_arg_pages and rmap_walk by guaranteeing rmap_walk finds PTEs created within the temporary stack

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

On Thu, May 06, 2010 at 06:40:39PM -0700, Linus Torvalds wrote:
> On Fri, 7 May 2010, Mel Gorman wrote:
> > 
> > Page migration requires rmap to be able to find all migration ptes
> > created by migration. If the second rmap_walk clearing migration PTEs
> > misses an entry, it is left dangling causing a BUG_ON to trigger during
> > fault. For example;
> 
> So I still absolutely detest this patch.
> 
> Why didn't the other - much simpler - patch work? The one Rik pointed to:
> 
> 	http://lkml.org/lkml/2010/4/30/198
> 

Oh, it works, but it depends on a magic check is_vma_temporary_stack().
Kamezawa had a variant that did not depend on magic but it increased the
size of vm_area_struct. That magic check was just something that would
be easy to break and not spot hence the temporary VMA instead.

> and didn't do that _disgusting_ temporary anon_vma?
> 
> Alternatively, why don't we just take the anon_vma lock over this region, 
> so that rmap can't _walk_ the damn thing?
> 

Because move_page_tables() calls into the page allocator. I'll create a
version that allocates the PMDs in advance and see what it looks like.

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