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SubjectRe: [PATCH v2] mm/highmem: make kmap cache coloring aware
On 07/24/2014 03:21 PM, Andrew Morton wrote:
> On Thu, 24 Jul 2014 04:38:01 +0400 Max Filippov <jcmvbkbc@gmail.com> wrote:
>
>> On Thu, Jul 24, 2014 at 1:17 AM, Andrew Morton
>> <akpm@linux-foundation.org> wrote:
>>> Fifthly, it would be very useful to publish the performance testing
>>> results for at least one architecture so that we can determine the
>>> patchset's desirability. And perhaps to motivate other architectures
>>> to implement this.
>> What sort of performance numbers would be relevant?
>> For xtensa this patch enables highmem use for cores with aliasing cache,
>> that is access to a gigabyte of memory (typical on KC705 FPGA board) vs.
>> only 128MBytes of low memory, which is highly desirable. But performance
>> comparison of these two configurations seems to make little sense.
>> OTOH performance comparison of highmem variants with and without
>> cache aliasing would show the quality of our cache flushing code.
> I'd assumed the patch was making cache coloring available as a
> performance tweak. But you appear to be saying that the (high) memory
> is simply unavailable for such cores without this change. I think.

>
> Please ensure that v3's changelog explains the full reason for the
> patch. Assume you're talking to all-the-worlds-an-x86 dummies, OK?
>

I am not sure that I will work on it again, we move to bigger pages and
non-aliasing cache, and I ask Steven Hill to help with MIPS variant.
So, I try to summarise an expanation here:

If cache line of some page in MIPS (and XTENSA?) is accessed via
multiple page virtual addresses (kernel or/and user) then it may be
located twice or more times in L1 cache which is an obvious coherency
bug. It is a trade-off for simple L1 access hardware. Two virtual
addresses of page which hits the same location in L1 cache are named as
"in the same page colour". Usually, colours are numbered and sequential
page colours looks like 0,1,0,1 or 0,1,2,3,0,1,2,3... It is usually
least one-two-or-three bits of PFN.

One simple way to hit this problem is using current HIGHMEM remapping
service because it doesn't take care of "page colouring". To prevent
coherency failure a current HIGHMEM code attempts to flush page from L1
cache each time before changing it's virtual address: flush cache each
PKMAP recycle and at each kunmap_atomic(), see arch/arm/mm/highmem.c -
MIPS code even doesn't have a flush here (BUG!).

However, kunmap_atomic() should do it locally to CPU without kmap_lock
by definition of kmap_atomic() and can't prevent a situation then a
second CPU hyper-thread accesses the same page via kmap() right after
kmap_atomic() got a page and uses a different page colour (different
virtual address set). Also, setting the whole cycle
kmap_atomic()...page...access...kunmap_atomic() under kmap_lock is
impractical.

This patch introduces some interface for architecture code to work with
coloured pages in PKMAP array which eliminates the kmap_atomic problem
and cancels cache flush requirements. It also can be consistent with
kmap_coherent() code which is required for some cache aliasing
architecture to handle aliasing between kernel virtual address and user
virtual address. The whole idea of this patch - force the same page
colour then page is assigned some PKMAP virtual address or kmap_atomic
address. Page colour is set by architecture code, usually it is a
physical address colour (which is usually == KVA colour).

- Leonid.



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