Re: [PATCH v3 01/12] drm: Add dummy page per device or GEM object

["Christian König" <christian.koenig@amd.com>]

Mhm, I'm not aware of any let over pointer between TTM and GEM and we 
worked quite hard on reducing the size of the amdgpu_bo, so another 
extra pointer just for that corner case would suck quite a bit.

Christian.

Am 08.01.21 um 15:46 schrieb Andrey Grodzovsky:
> Daniel had some objections to this (see bellow) and so I guess I need 
> you both to agree on the approach before I proceed.
>
> Andrey
>
> On 1/8/21 9:33 AM, Christian König wrote:
>> Am 08.01.21 um 15:26 schrieb Andrey Grodzovsky:
>>> Hey Christian, just a ping.
>>
>> Was there any question for me here?
>>
>> As far as I can see the best approach would still be to fill the VMA 
>> with a single dummy page and avoid pointers in the GEM object.
>>
>> Christian.
>>
>>>
>>> Andrey
>>>
>>> On 1/7/21 11:37 AM, Andrey Grodzovsky wrote:
>>>>
>>>> On 1/7/21 11:30 AM, Daniel Vetter wrote:
>>>>> On Thu, Jan 07, 2021 at 11:26:52AM -0500, Andrey Grodzovsky wrote:
>>>>>> On 1/7/21 11:21 AM, Daniel Vetter wrote:
>>>>>>> On Tue, Jan 05, 2021 at 04:04:16PM -0500, Andrey Grodzovsky wrote:
>>>>>>>> On 11/23/20 3:01 AM, Christian König wrote:
>>>>>>>>> Am 23.11.20 um 05:54 schrieb Andrey Grodzovsky:
>>>>>>>>>> On 11/21/20 9:15 AM, Christian König wrote:
>>>>>>>>>>> Am 21.11.20 um 06:21 schrieb Andrey Grodzovsky:
>>>>>>>>>>>> Will be used to reroute CPU mapped BO's page faults once
>>>>>>>>>>>> device is removed.
>>>>>>>>>>> Uff, one page for each exported DMA-buf? That's not 
>>>>>>>>>>> something we can do.
>>>>>>>>>>>
>>>>>>>>>>> We need to find a different approach here.
>>>>>>>>>>>
>>>>>>>>>>> Can't we call alloc_page() on each fault and link them together
>>>>>>>>>>> so they are freed when the device is finally reaped?
>>>>>>>>>> For sure better to optimize and allocate on demand when we reach
>>>>>>>>>> this corner case, but why the linking ?
>>>>>>>>>> Shouldn't drm_prime_gem_destroy be good enough place to free ?
>>>>>>>>> I want to avoid keeping the page in the GEM object.
>>>>>>>>>
>>>>>>>>> What we can do is to allocate a page on demand for each fault 
>>>>>>>>> and link
>>>>>>>>> the together in the bdev instead.
>>>>>>>>>
>>>>>>>>> And when the bdev is then finally destroyed after the last 
>>>>>>>>> application
>>>>>>>>> closed we can finally release all of them.
>>>>>>>>>
>>>>>>>>> Christian.
>>>>>>>> Hey, started to implement this and then realized that by 
>>>>>>>> allocating a page
>>>>>>>> for each fault indiscriminately
>>>>>>>> we will be allocating a new page for each faulting virtual 
>>>>>>>> address within a
>>>>>>>> VA range belonging the same BO
>>>>>>>> and this is obviously too much and not the intention. Should I 
>>>>>>>> instead use
>>>>>>>> let's say a hashtable with the hash
>>>>>>>> key being faulting BO address to actually keep allocating and 
>>>>>>>> reusing same
>>>>>>>> dummy zero page per GEM BO
>>>>>>>> (or for that matter DRM file object address for non imported 
>>>>>>>> BOs) ?
>>>>>>> Why do we need a hashtable? All the sw structures to track this 
>>>>>>> should
>>>>>>> still be around:
>>>>>>> - if gem_bo->dma_buf is set the buffer is currently exported as 
>>>>>>> a dma-buf,
>>>>>>>     so defensively allocate a per-bo page
>>>>>>> - otherwise allocate a per-file page
>>>>>>
>>>>>> That exactly what we have in current implementation
>>>>>>
>>>>>>
>>>>>>> Or is the idea to save the struct page * pointer? That feels a 
>>>>>>> bit like
>>>>>>> over-optimizing stuff. Better to have a simple implementation 
>>>>>>> first and
>>>>>>> then tune it if (and only if) any part of it becomes a problem 
>>>>>>> for normal
>>>>>>> usage.
>>>>>>
>>>>>> Exactly - the idea is to avoid adding extra pointer to 
>>>>>> drm_gem_object,
>>>>>> Christian suggested to instead keep a linked list of dummy pages 
>>>>>> to be
>>>>>> allocated on demand once we hit a vm_fault. I will then also 
>>>>>> prefault the entire
>>>>>> VA range from vma->vm_end - vma->vm_start to vma->vm_end and map 
>>>>>> them
>>>>>> to that single dummy page.
>>>>> This strongly feels like premature optimization. If you're worried 
>>>>> about
>>>>> the overhead on amdgpu, pay down the debt by removing one of the 
>>>>> redundant
>>>>> pointers between gem and ttm bo structs (I think we still have 
>>>>> some) :-)
>>>>>
>>>>> Until we've nuked these easy&obvious ones we shouldn't play "avoid 1
>>>>> pointer just because" games with hashtables.
>>>>> -Daniel
>>>>
>>>>
>>>> Well, if you and Christian can agree on this approach and suggest 
>>>> maybe what pointer is
>>>> redundant and can be removed from GEM struct so we can use the 
>>>> 'credit' to add the dummy page
>>>> to GEM I will be happy to follow through.
>>>>
>>>> P.S Hash table is off the table anyway and we are talking only 
>>>> about linked list here since by prefaulting
>>>> the entire VA range for a vmf->vma i will be avoiding redundant 
>>>> page faults to same VMA VA range and so
>>>> don't need to search and reuse an existing dummy page but simply 
>>>> create a new one for each next fault.
>>>>
>>>> Andrey
>>

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