linux-kernel - Re: [PATCH] mmu notifiers #v6

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Message-ID: <20080221045430.GC15215@wotan.suse.de>
Date:	Thu, 21 Feb 2008 05:54:30 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Andrea Arcangeli <andrea@...ranet.com>
Cc:	akpm@...ux-foundation.org, Robin Holt <holt@....com>,
	Avi Kivity <avi@...ranet.com>, Izik Eidus <izike@...ranet.com>,
	kvm-devel@...ts.sourceforge.net,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	general@...ts.openfabrics.org,
	Steve Wise <swise@...ngridcomputing.com>,
	Roland Dreier <rdreier@...co.com>,
	Kanoj Sarcar <kanojsarcar@...oo.com>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com, Christoph Lameter <clameter@....com>
Subject: Re: [PATCH] mmu notifiers #v6

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.

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