Re: MMU notifiers review and some proposals

[Andrea Arcangeli <andrea@qumranet.com>]

Hi Nick,

On Thu, Jul 24, 2008 at 04:39:49PM +0200, Nick Piggin wrote:
> I think everybody is hoping to have a workable mmu notifier scheme
> merged in 2.6.27 (myself included). However I do have some concerns

Just to be clear, I'm waiting mmu notifiers to showup on Linus's tree
before commenting this as it was all partly covered in the past
discussions anyway, so there's nothing really urgent or new here (at
least for me ;).

It's a tradeoff, you pointed out the positive sides and negative point
of both approaches, and depending which kind of the trade you're
interested about, you'll prefer one or the other approach. Your
preference is the exact opposite of what SGI liked and what we
liked. But all works for us, and all works for GRU (though -mm is
faster for the fast path), but only -mm can be easily later extended
for XPMEM/IB if they ever decide to schedule in the mmu notifier
methods in the future (which may never happen and it's unrelated to
the current patches that don't contemplate sleeping at all and it's
pure luck they can be trivially extended to provide for it).

As your patch shown the changes are fairly small anyway if we later
decide to change in 2.6.28-rc, in the meantime current code in -mm was
heavily tested and all code including kvm and gru has been tested only
with this, and this combined with the fact -mm guarantees the fastest
fast path, I hope we leave any discussion to the 2.6.28-rc merge
window, now it's time to go productive finally!

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