* Thomas Hellstrom <thellst...@vmware.com> wrote:
> Ingo Molnar wrote:
>> * Thomas Hellstrom <thellst...@vmware.com> wrote:
>>
>>
>>> Dave Airlie wrote:
>>>
>>>>> hm, i'm missing a description about how this bug was triggered.
>>>>> How did you end up getting highmem pages to a cpa call?
>>>>>
>>>> GEM and TTM both allocate page arrays and just pass them to cpa,
>>>> we don't know what type of pages the allocator gives us back and we
>>>> really shouldn't have to, so having cpa ignore highmem pages is
>>>> certainly the right option.
>>>>
>>>> GEM just uses shmem code to alloc the pages and TTM has its own allocator.
>>>>
>>>>
>>> Yes, Dave is right.
>>>
>>> Although I'm not 100% sure the TTM code I was using that triggered
>>> this has made it into 2.6.31.
>>> Old AGP uses (GFP_KERNEL | GFP_DMA32 | __GFP_ZERO), which (correct me
>>> if I'm wrong) never hands back highmem pages. This means that
>>> Intel's GEM is the only likely user for 2.6.31.
>>>
>>
>> (please dont top-post) I'm not sure you folks noticed this bit of my
>> mail:
>>
>>
>>>>> ok, that's a bug introduced in .29 but which was latent until
>>>>> now: drivers/char/agp/generic.c now uses it plus (indirectly) a
>>>>> number of AGP drivers, since:
>>>>>
>>>>> commit 07613ba2f464f59949266f4337b75b91eb610795
>>>>> Author: Dave Airlie <airl...@redhat.com>
>>>>> Date: Fri Jun 12 14:11:41 2009 +1000
>>>>>
>>>>> agp: switch AGP to use page array instead of unsigned long array
>>>>>
>>>>> I dont see how it can end up with highmem pages though. All the
>>>>> graphics apperture allocations happen to lowmem AFAICS. Did GEM
>>>>> add the possibility for user pages (highmem amongst them) ending
>>>>> up in that pool? Which code does that?
>>>>>
>>
>> Ingo
>>
> Sorry, my bad.
>
> The TTM code I tested is not in yet, and after double-checking it
> looks like Intel's gem is not changing caching policy before
> binding to AGP.
>
> This means the highmem problems that I saw were triggered by a
> combination of the virtual->physical bugfix and code that's not in
> the kernel yet, and since it's an optimization of the current code
> it's not likely to land in 2.6.31. The highmem fixes could thus,
> AFAICT be stripped out of the patch, unless GFP_DMA32 on a highmem
> system can actually hand back highmem pages, in which case AGP
> will not work correctly.
>
> As for highmem use in the future, the TTM page arrays are
> populated using fault(), which means that there will be an
> overhead ordering the pages so that we can use the
> set_pages_array() interface instead of set_memory() that we use
> today. Therefore, if possible, I'd prefer if we could pass arrays
> containing highmem pages to the set_pages_array() interface.
>
> There are no aliased mappings since
>
> 1) Any user space mappings to these pages are killed before changing
> caching policy.
> 2) The pages are allocated and owned by the driver.
> 3) kmap_atomic_prot() and vmap() are used to map these pages in kernel
> space.
>
> Code is in ttm_tt.c ttm_bo.c and ttm_bo_util.c
ok - thanks for the explanation. Since you intentionally want to use
highmem pages (and your use is safe) i concur with your original
patch in its entirety - even if that planned highmem use is not
upstream yet. Will get it .31-wards ASAP.
Ingo
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