[PATCH] drm/ttm: stop warning on TT shrinker failure

[Thomas Hellström (Intel)]

On 3/23/21 4:45 PM, Christian König wrote:
> Am 23.03.21 um 16:13 schrieb Michal Hocko:
>> On Tue 23-03-21 14:56:54, Christian König wrote:
>>> Am 23.03.21 um 14:41 schrieb Michal Hocko:
>> [...]
>>>> Anyway, I am wondering whether the overall approach is sound. Why 
>>>> don't
>>>> you simply use shmem as your backing storage from the beginning and 
>>>> pin
>>>> those pages if they are used by the device?
>>> Yeah, that is exactly what the Intel guys are doing for their 
>>> integrated
>>> GPUs :)
>>>
>>> Problem is for TTM I need to be able to handle dGPUs and those have all
>>> kinds of funny allocation restrictions. In other words I need to 
>>> guarantee
>>> that the allocated memory is coherent accessible to the GPU without 
>>> using
>>> SWIOTLB.
>>>
>>> The simple case is that the device can only do DMA32, but you also got
>>> device which can only do 40bits or 48bits.
>>>
>>> On top of that you also got AGP, CMA and stuff like CPU cache behavior
>>> changes (write back vs. write through, vs. uncached).
>> OK, so the underlying problem seems to be that gfp mask (thus
>> mapping_gfp_mask) cannot really reflect your requirements, right?  Would
>> it help if shmem would allow to provide an allocation callback to
>> override alloc_page_vma which is used currently? I am pretty sure there
>> will be more to handle but going through shmem for the whole life time
>> is just so much easier to reason about than some tricks to abuse shmem
>> just for the swapout path.
>
> Well it's a start, but the pages can have special CPU cache settings. 
> So direct IO from/to them usually doesn't work as expected.
>
> Additional to that for AGP and CMA I need to make sure that I give 
> those pages back to the relevant subsystems instead of just dropping 
> the page reference.
>
> So I would need to block for the swapio to be completed.
>
> Anyway I probably need to revert those patches for now since this 
> isn't working as we hoped it would.
>
> Thanks for the explanation how stuff works here.

Another alternative here that I've tried before without being successful 
would perhaps be to drop shmem completely and, if it's a normal page (no 
dma or funny caching attributes) just use add_to_swap_cache()? If it's 
something else, try alloc a page with relevant gfp attributes, copy and 
add_to_swap_cache()? Or perhaps that doesn't work well from a shrinker 
either?

/Thomas



>
> Christian.
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