Re: [PATCH -mm 4/7] mm: make page colouring code generic

[Rik van Riel]

On 06/19/2012 07:27 PM, Andrew Morton wrote:
> On Mon, 18 Jun 2012 18:05:23 -0400
> Rik van Riel<riel@xxxxxxxxxx>  wrote:
>
> > From: Rik van Riel<riel@xxxxxxxxxxx>
> >
> > Fix the x86-64 page colouring code to take pgoff into account.
>
> Could we please have a full description of what's wrong with the
> current code?

Here is a copy of the text I added to the changelog:


The old x86 code will always align the mmap
to aliasing boundaries, even if the program mmaps
the file with a non-zero pgoff.

If program A mmaps the file with pgoff 0, and
program B mmaps the file with pgoff 1. The old
code would align the mmaps, resulting in misaligned
pages:

A:  0123
B:  123

After this patch, they are aligned so the pages
line up:

A: 0123
B:  123

> > Use the x86 and MIPS page colouring code as the basis for a generic
> > page colouring function.

Renamed to "cache alignment", by Andi's request.

> > Teach the generic arch_get_unmapped_area(_topdown) code to call the
> > page colouring code.
> >
> > Make sure that ALIGN_DOWN always aligns down, and ends up at the
> > right page colour.
>
> Some performance tests on the result would be interesting.  iirc, we've
> often had trouble demonstrating much or any benefit from coloring.

On AMD Bulldozer, I do not know what the benefits are.

On ARM, MIPS, SPARC and SH, the main benefit is avoiding
data corruption :)

These architectures have VIPT caches on some CPU models,
and MAP_SHARED read-write mappings have to be properly
aligned to guarantee data consistency.

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