Re: [kvm-devel] [PATCH] mmu notifiers #v7

[Andrea Arcangeli <andrea@qumranet.com>]

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.