Re: [PATCH 0/3] Fix migration races in rmap_walk() V2

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

On Wed, Apr 28, 2010 at 12:32:42AM +0200, Andrea Arcangeli wrote:
> On Tue, Apr 27, 2010 at 05:27:36PM -0500, Christoph Lameter wrote:
> > Can we simply wait like in the fault path?
> 
> There is no bug there, no need to wait either. I already audited it
> before, and I didn't see any bug. Unless you can show a bug with CPU A
> running the rmap_walk on process1 before process2, there is no bug to
> fix there.
> 

Yes, this patch is now dropped.

> > 
> > > Patch 3 notes that while a VMA is moved under the anon_vma lock, the page
> > > 	tables are not similarly protected. Where migration PTEs are
> > > 	encountered, they are cleaned up.
> > 
> > This means they are copied / moved etc and "cleaned" up in a state when
> > the page was unlocked. Migration entries are not supposed to exist when
> > a page is not locked.
> 
> patch 3 is real, and the first thought I had was to lock down the page
> before running vma_adjust and unlock after move_page_tables. But these
> are virtual addresses. Maybe there's a simpler way to keep migration
> away while we run those two operations.
> 

I see there is a large discussion on that patch so I'll read that rather
than commenting here.

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

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