Re: [PATCH] x86: create array based interface to change page attribute

["Thomas Hellström" <thomas@tungstengraphics.com>]

Arjan van de Ven wrote:
> Thomas Hellström wrote:
>> Dave Airlie wrote:
>>> When cpa was refactored to the new set_memory_ interfaces, it removed
>>> a special case fast path for AGP systems, which did a lot of page by 
>>> page
>>> attribute changing and held the flushes until they were finished. The
>>> DRM memory manager also required this to get useable speeds.
>>>
>>> This introduces a new interface, which accepts an array of memory 
>>> addresses
>>> to have attributes changed on and to flush once finished.
>>>
>>> Further changes to the AGP stack to actually use this interface will be
>>> published later.
>>>
>>> Signed-off-by: Dave Airlie <airlied@redhat.com>
>>> ---
>>>  arch/x86/mm/pageattr-test.c  |   12 ++-
>>>  arch/x86/mm/pageattr.c       |  164 
>>> +++++++++++++++++++++++++++++++-----------
>>>  include/asm-x86/cacheflush.h |    3 +
>>>  3 files changed, 132 insertions(+), 47 deletions(-)
>>>   
>> ...
>> Dave,
>> Nice work, but how do we handle highmem pages? 
>
> Cache attributes fundamentally work on a mapping and not on physical 
> memory.
> (MTRR's are special there, they do work on physical memory, but that's 
> a special case and not relevant here)
>
> So to be honest, your question doesn't make sense;  because all I can 
> do is ask "which mapping of these pages".
>
> Even the old interfaces prior to 2.6.24 only managed to deal with SOME 
> of the mappings of a page.
> And if/when future CPUs don't require all mappings to be in sync, 
> clearly the kernel will only change
> the mapping that is requested as well.
>
>
>
>> Since this is an AGPgart and DRM fastpath, the interface should 
>> ideally be adapted to match the data structures used by those callers. 
>
> Actually, the interface has to make sense conceptually convey the 
> right information,
> the implementation should not have to second guess internals of 
> AGP/DRM because that
> would just be a recipe for disaster.
>> The AGPgart module uses an array of addresses, which effectively 
>> stops it from using pages beyond the DMA32 limit. The DRM memory 
>> manager uses an array of struct page pointers, but using that was, If 
>> I understand things correctly, rejected.
>
> yes because simply put, if you pass a struct page to such a function, 
> you're not telling it which
> mapping or mappings you want changed....
> (And if you say "only the 1:1 mapping, so why doesn't the other side 
> calculate that"... there's no speed gain in doing
> the calculation for that on the other side of an interface, and that 
> makes it zero reason to misdesign the interface
> to only have the "which mapping" information implicit)
>
Hmm. I get the point. There should be ways to do reasonably efficient 
workarounds in the drivers.

/Thomas