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    SubjectRe: [kvm-devel] [PATCH] mmu notifiers #v7
    On Wed, Feb 27, 2008 at 03:06:10PM -0800, Christoph Lameter wrote:
    > Ok so it somehow works slowly with GRU and you are happy with it. What

    As far as GRU is concerned, performance is the same as with your patch
    (Jack can confirm).

    > about the RDMA folks etc etc?

    If RDMA/IB folks needed to block in invalidate_range, I guess they
    need to do so on top of tmpfs too, and that never worked with your
    patch anyway.

    > Would it not be better to have a solution that fits all instead of hacking
    > something in now and then having to modify it later?

    The whole point is that your solution fits only GRU and KVM too.

    XPMEM in your patch works in a hacked mode limited to anonymous memory
    only, Robin already received incoming mail asking to allow xpmem to
    work on more than anonymous memory, so your solution-that-fits-all
    doesn't actually fit some of Robin's customer needs. So if it doesn't
    even entirely satisfy xpmem users, imagine the other potential
    blocking-users of this code.

    > Hmmm.. There were earlier discussions of changing the anon vma lock to a
    > rw lock because of contention issues in large systems. Maybe we can just
    > generally switch the locks taken while walking rmaps to semaphores? That
    > would still require to put the invalidate outside of the pte lock.

    anon_vma lock can remain a spinlock unless you also want to schedule
    inside try_to_unmap.

    If converting the i_mmap_lock to a mutex is a big trouble, another way
    that might work to allow invalidate_range to block, would be to try to
    boost the mm_users to prevent the mmu_notifier_release to run in
    another cpu the moment after i_mmap_lock spinlock is unlocked. But
    even if that works, it'll run slower and the mmu notifiers RCU locking
    should be switched to a mutex, so it'd be nice to have it as a
    separate option.


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