linux-kernel - Re: [PATCH v3 1/2] mm/highmem: make kmap cache coloring aware

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Date:	Sat, 2 Aug 2014 00:24:43 +0400
From:	Max Filippov <jcmvbkbc@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"linux-xtensa@...ux-xtensa.org" <linux-xtensa@...ux-xtensa.org>,
	Chris Zankel <chris@...kel.net>,
	Marc Gauthier <marc@...ence.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	"Linux/MIPS Mailing List" <linux-mips@...ux-mips.org>,
	LKML <linux-kernel@...r.kernel.org>,
	David Rientjes <rientjes@...gle.com>,
	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>,
	Steven Hill <Steven.Hill@...tec.com>
Subject: Re: [PATCH v3 1/2] mm/highmem: make kmap cache coloring aware

On Sat, Aug 2, 2014 at 12:10 AM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Fri, 25 Jul 2014 23:43:46 +0400 Max Filippov <jcmvbkbc@...il.com> 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@...tec.com>.
>>
>
> 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.

Ok, let me try again.

>> --- 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.

Ok, I'll drop it.

> 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.

It's not a fix, for xtensa it's a part of a new feature, so no need
for backporting.

-- 
Thanks.
-- Max
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