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From: ebiederm@xmission.com (Eric W. Biederman)
To: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: akpm@osdl.org, linux-kernel@vger.kernel.org,
       Chris Wright <chrisw@sous-sol.org>,
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       Rusty Russell <rusty@rustcorp.com.au>, Zachary Amsden <zach@vmware.com>
Subject: Re: [PATCH 1 of 13] Add apply_to_page_range() which applies a function to a pte range
References: <79a98a10911fc4e77dce.1154421372@ezr.goop.org>
Date: Tue, 01 Aug 2006 06:08:43 -0600
In-Reply-To: <79a98a10911fc4e77dce.1154421372@ezr.goop.org> (Jeremy
	Fitzhardinge's message of "Tue, 01 Aug 2006 01:36:12 -0700")
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Jeremy Fitzhardinge <jeremy@xensource.com> writes:

> 2 files changed, 99 insertions(+)
> include/linux/mm.h |    5 ++
> mm/memory.c        |   94 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>
>
> 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()).

- You don't handle huge pages.  For a generic function
  that sounds like a problem.
- I believe there is a reason the kernel doesn't already have
  a function like this.  I seem to recall there being efficiency
  and fast path arguments.
- Placing this code in mm/memory.c without a common consumer is
  pure kernel bloat for everyone who doesn't use this function,
  which is just about everyone.

Eric