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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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