linux-kernel - Re: [PATCH] mmu notifier #v11

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Message-ID: <20080405002330.GF14784@duo.random>
Date:	Sat, 5 Apr 2008 02:23:30 +0200
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Hugh Dickins <hugh@...itas.com>, 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, Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH] mmu notifier #v11

On Fri, Apr 04, 2008 at 03:06:18PM -0700, Christoph Lameter wrote:
> Adds some comments. Still objectionable is the multiple ways of
> invalidating pages in #v11. Callout now has similar locking to emm.

range_begin exists because range_end is called after the page has
already been freed. invalidate_page is called _before_ the page is
freed but _after_ the pte has been zapped.

In short when working with single pages it's a waste to block the
secondary-mmu page fault, because it's zero cost to invalidate_page
before put_page. Not even GRU need to do that.

Instead for the multiple-pte-zapping we have to call range_end _after_
the page is already freed. This is so that there is a single range_end
call for an huge amount of address space. So we need a range_begin for
the subsystems not using page pinning for example. When working with
single pages (try_to_unmap_one, do_wp_page) invalidate_page avoids to
block the secondary mmu page fault, and it's in turn faster.

Besides avoiding need of serializing the secondary mmu page fault,
invalidate_page also reduces the overhead when the mmu notifiers are
disarmed (i.e. kvm not running).
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