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SubjectRe: [PATCH 04/14] mm,migration: Allow the migration of PageSwapCache pages
On Sat, Apr 24, 2010 at 01:13:40PM +0200, Andrea Arcangeli wrote:
> On Sat, Apr 24, 2010 at 11:52:27AM +0100, Mel Gorman wrote:
> > I think you're right. This is a new bug introduced by the anon_vma changes. On
> > the plus side, it means we don't have to worry about -stable.
>
> Correct, no worry about -stable.
>
> > > vma_adjust already takes the anon_vma->lock and of course I also
> > > further verified that trying to apply your snippet to vma_adjust
> > > results in immediately deadlock as the very same lock is already taken
> > > in my tree as it's the same anon-vma (simpler).
> >
> > Yes, I expected that. Previously, there was only one anon_vma so if you
> > double-take the lock, bad things happen.
> >
> > > So aa.git will be
> > > immune from these bugs for now.
> > >
> >
> > It should be. I expect that's why you have never seen the bugon in
> > swapops.
>
> Correct, I never seen it, and I keep it under very great stress with
> swap storms of hugepages, lots of I/O and khugepaged at 100% cpu.
>

Well, to me this is also good because it shows it's not an existing bug in
migration or a new bug introduced by compaction either. Previously I hadn't
seen this bug either but until relatively recently, the bulk of the testing
was against 2.6.33.

> Also keep in mind expand_downwards which also adjusts
> vm_start/vm_pgoff the same way (and without mmap_sem write mode).
>

Will keep it in mind. It's taking the anon_vma lock but once again,
there might be more than one anon_vma to worry about and the proper
locking still isn't massively clear to me.

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


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