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    SubjectRe: [PATCH 0 of 9] mmu notifier #v12
    On Tue, Apr 22, 2008 at 08:36:04AM -0500, Robin Holt wrote:
    > I am a little confused about the value of the seq_lock versus a simple
    > atomic, but I assumed there is a reason and left it at that.

    There's no value for anything but get_user_pages (get_user_pages takes
    its own lock internally though). I preferred to explain it as a
    seqlock because it was simpler for reading, but I totally agree in the
    final implementation it shouldn't be a seqlock. My code was meant to
    be pseudo-code only. It doesn't even need to be atomic ;).

    > I don't know what you mean by "it'd" run slower and what you mean by
    > "armed and disarmed".

    1) when armed the time-window where the kvm-page-fault would be
    blocked would be a bit larger without invalidate_page for no good

    2) if you were to remove invalidate_page when disarmed the VM could
    would need two branches instead of one in various places

    I don't want to waste cycles if not wasting them improves performance
    both when armed and disarmed.

    > For the sake of this discussion, I will assume "it'd" means the kernel in
    > general and not KVM. With the two call sites for range_begin/range_end,

    I actually meant for both.

    > By disarmed, I will assume you mean no notifiers registered for a
    > particular mm. In that case, the cache will make the second call
    > effectively free. So, for the disarmed case, I see no measurable
    > difference.

    For rmap is sure effective free, for do_wp_page it costs one branch
    for no good reason.

    > For the case where there is a notifier registered, I certainly can see
    > a difference. I am not certain how to quantify the difference as it


    > When I was discussing this difference with Jack, he reminded me that
    > the GRU, due to its hardware, does not have any race issues with the
    > invalidate_page callout simply doing the tlb shootdown and not modifying
    > any of its internal structures. He then put a caveat on the discussion
    > that _either_ method was acceptable as far as he was concerned. The real
    > issue is getting a patch in that satisfies all needs and not whether
    > there is a seperate invalidate_page callout.

    Sure, we have that patch now, I'll send it out in a minute, I was just
    trying to explain why it makes sense to have an invalidate_page too
    (which remains the only difference by now), removing it would be a
    regression on all sides, even if a minor one.

     \ /
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