Re: [PATCH v3 1/2] mm/highmem: make kmap cache coloring aware
From: Andrew Morton
Date: Fri Aug 01 2014 - 16:10:55 EST
On Fri, 25 Jul 2014 23:43:46 +0400 Max Filippov <jcmvbkbc@xxxxxxxxx> wrote:
> VIPT cache with way size larger than MMU page size may suffer from
> aliasing problem: a single physical address accessed via different
> virtual addresses may end up in multiple locations in the cache.
> Virtual mappings of a physical address that always get cached in
> different cache locations are said to have different colors.
> L1 caching hardware usually doesn't handle this situation leaving it
> up to software. Software must avoid this situation as it leads to
> data corruption.
>
> One way to handle this is to flush and invalidate data cache every time
> page mapping changes color. The other way is to always map physical page
> at a virtual address with the same color. Low memory pages already have
> this property. Giving architecture a way to control color of high memory
> page mapping allows reusing of existing low memory cache alias handling
> code.
>
> Provide hooks that allow architectures with aliasing cache to align
> mapping address of high pages according to their color. Such architectures
> may enforce similar coloring of low- and high-memory page mappings and
> reuse existing cache management functions to support highmem.
>
> This code is based on the implementation of similar feature for MIPS by
> Leonid Yegoshin <Leonid.Yegoshin@xxxxxxxxxx>.
>
It's worth mentioning that xtensa needs this.
What is (still) missing from these changelogs is a clear description of
the end-user visible effects. Does it fix some bug? If so what? Is
it a performace optimisation? If so how much? This info is the
top-line reason for the patchset and should be presented as such.
> --- a/mm/highmem.c
> +++ b/mm/highmem.c
> @@ -28,6 +28,9 @@
> #include <linux/highmem.h>
> #include <linux/kgdb.h>
> #include <asm/tlbflush.h>
> +#ifdef CONFIG_HIGHMEM
> +#include <asm/highmem.h>
> +#endif
Should be unneeded - the linux/highmem.h inclusion already did this.
Apart from that it all looks OK to me. I'm assuming this is 3.17-rc1
material, but I am unsure because of the missing end-user-impact info.
If it's needed in earlier kernels then we can tag it for -stable
backporting but again, the -stable team (ie: Greg) will want so see the
justification for that backport.
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