linux-kernel - Re: [patch 11/21] Xen-paravirt: Add apply_to_page_range() which applies a function to a pte range.

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Message-ID: <45D55DC2.3070703@yahoo.com.au>
Date:	Fri, 16 Feb 2007 18:31:14 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>, Andi Kleen <ak@....de>,
	linux-kernel@...r.kernel.org, virtualization@...ts.osdl.org,
	xen-devel@...ts.xensource.com, Chris Wright <chrisw@...s-sol.org>,
	Zachary Amsden <zach@...are.com>,
	Ian Pratt <ian.pratt@...source.com>,
	Christian Limpach <Christian.Limpach@...cam.ac.uk>,
	Christoph Lameter <clameter@....com>,
	Linux Memory Management <linux-mm@...ck.org>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [patch 11/21] Xen-paravirt: Add apply_to_page_range() which applies
 a function to a pte range.

Andrew Morton wrote:
> On Thu, 15 Feb 2007 18:25:00 -0800 Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> 
> 
>>Add a new mm function apply_to_page_range() which applies a given
>>function to every pte in a given virtual address range in a given mm
>>structure. This is a generic alternative to cut-and-pasting the Linux
>>idiomatic pagetable walking code in every place that a sequence of
>>PTEs must be accessed.
>>
>>Although this interface is intended to be useful in a wide range of
>>situations, it is currently used specifically by several Xen
>>subsystems, for example: to ensure that pagetables have been allocated
>>for a virtual address range, and to construct batched special
>>pagetable update requests to map I/O memory (in ioremap()).
> 
> 
> There was some discussion about this sort of thing last week.  The
> consensus was that it's better to run the callback against a whole pmd's
> worth of ptes, mainly to amortise the callback's cost (a lot).
> 
> It was implemented in
> ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.20/2.6.20-mm1/broken-out/smaps-extract-pmd-walker-from-smaps-code.patch


Speaking of that patch, I missed the discussion, but I'd hope it doesn't
go upstream in its current form.

We now have one way of walking range of ptes. The code may be duplicated a
few times, but it is simple, we know how it works, and it is easy to get
right because everyone does the same thing.

We used to have about a dozen slightly different ways of doing this until
Hugh spent the effort to standardise it all. Isn't it nice?

If we want an ever-so-slightly lower performing interface for those paths
that don't care to count every cycle -- which I think is a fine idea BTW
-- it should be implemented in mm/memory.c and it should use our standard
form of pagetable walking.

-- 
SUSE Labs, Novell Inc.
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