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

[Thomas Hellström]

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@xxxxxxxxxx>
> > > ---
> > >  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



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