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

[Nick Piggin <nickpiggin@xxxxxxxxxxxx>]

Andrew Morton wrote:
> On Thu, 15 Feb 2007 18:25:00 -0800 Jeremy Fitzhardinge <jeremy@xxxxxxxx> 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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