[ofa-general] Re: [PATCH 08 of 11] anon-vma-rwsem
Linus Torvalds
torvalds at linux-foundation.org
Wed May 7 17:55:33 PDT 2008
On Wed, 7 May 2008, Robin Holt wrote:
>
> In order to invalidate the remote page table entries, we need to message
> (uses XPC) to the remote side. The remote side needs to acquire the
> importing process's mmap_sem and call zap_page_range(). Between the
> messaging and the acquiring a sleeping lock, I would argue this will
> require sleeping locks in the path prior to the mmu_notifier invalidate_*
> callouts().
You simply will *have* to do it without locally holding all the MM
spinlocks. Because quite frankly, slowing down all the normal VM stuff for
some really esoteric hardware simply isn't acceptable. We just don't do
it.
So what is it that actually triggers one of these events?
The most obvious solution is to just queue the affected pages while
holding the spinlocks (perhaps locking them locally), and then handling
all the stuff that can block after releasing things. That's how we
normally do these things, and it works beautifully, without making
everything slower.
Sometimes we go to extremes, and actually break the locks are restart
(ugh), and it gets ugly, but even that tends to be preferable to using the
wrong locking.
The thing is, spinlocks really kick ass. Yes, they restrict what you can
do within them, but if 99.99% of all work is non-blocking, then the really
odd rare blocking case is the one that needs to accomodate, not the rest.
Linus
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