Re: [PATCH 1 of 13] Add apply_to_page_range() which applies a function to a pte range

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On 8/1/06, Eric W. Biederman <[email protected]> wrote:
Jeremy Fitzhardinge <[email protected]> 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.

The proper trick for this is:

1. place you "for each page" code in a #define like so:

#define FOR_EACH_PAGE_INNER do{ ... code ... }while(0);

2. create your function in a separate .h file without the double-include guard

3. inside this code, exchange the indirect function call with your define name:

(*fn)(args); --> FOR_EACH_PAGE_INNER

4. document how the macro will receive certain variables from it's
outer scope, and should leave the "function result" in another one.

this in effect creates a different copy of the page walker for each
function, and inlines your code in it.. just like it would do with a
C++ template.

A place where you can see this technique working is the software
triangle filler from MESA.

The doubt is... is this acceptable regarding linux-kernel coding-style?


- 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.



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