[ofa-general] Re: [PATCH] mmu notifiers #v6

[Nick Piggin]

On Wed, Feb 20, 2008 at 11:39:42AM +0100, Andrea Arcangeli wrote:
> Given Nick's comments I ported my version of the mmu notifiers to
> latest mainline. There are no known bugs AFIK and it's obviously safe
> (nothing is allowed to schedule inside rcu_read_lock taken by
> mmu_notifier() with my patch).

Thanks! Yes the seqlock you are using now ends up looking similar
to what I did and I couldn't find a hole in that either. So I
think this is going to work.

I do prefer some parts of my patch, however for everyone's sanity,
I think you should be the maintainer of the mmu notifiers, and I
will send you incremental changes that can be discussed more easily
that way (nothing major, mainly style and minor things).


> XPMEM simply can't use RCU for the registration locking if it wants to
> schedule inside the mmu notifier calls. So I guess it's better to add
> the XPMEM invalidate_range_end/begin/external-rmap as a whole
> different subsystem that will have to use a mutex (not RCU) to
> serialize, and at the same time that CONFIG_XPMEM will also have to
> switch the i_mmap_lock to a mutex. I doubt xpmem fits inside a
> CONFIG_MMU_NOTIFIER anymore, or we'll all run a bit slower because of
> it. It's really a call of how much we want to optimize the MMU
> notifier, by keeping things like RCU for the registration.

I agree: your coherent, non-sleeping mmu notifiers are pretty simple
and unintrusive. The sleeping version is fundamentally going to either
need to change VM locks, or be non-coherent, so I don't think there is
a question of making one solution fit everybody. So the sleeping /
xrmap patch should be kept either completely independent, or as an
add-on to this one.

I will post some suggestions to you when I get a chance.