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

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Message-ID: <20080227234317.GM28483@v2.random>
Date:	Thu, 28 Feb 2008 00:43:17 +0100
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Nick Piggin <npiggin@...e.de>,
	Steve Wise <swise@...ngridcomputing.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, linux-mm@...ck.org,
	Kanoj Sarcar <kanojsarcar@...oo.com>,
	Roland Dreier <rdreier@...co.com>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org,
	Avi Kivity <avi@...ranet.com>, kvm-devel@...ts.sourceforge.net,
	daniel.blueman@...drics.com, Robin Holt <holt@....com>,
	general@...ts.openfabrics.org, akpm@...ux-foundation.org
Subject: Re: [kvm-devel] [PATCH] mmu notifiers #v7

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.
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