[PATCH 2/5] drm/gem: Add a mountpoint parameter to drm_gem_object_init()

[Tvrtko Ursulin]

On 12/03/2024 10:37, Christian König wrote:
> Am 12.03.24 um 11:31 schrieb Tvrtko Ursulin:
>>
>> On 12/03/2024 10:23, Christian König wrote:
>>> Am 12.03.24 um 10:30 schrieb Tvrtko Ursulin:
>>>>
>>>> On 12/03/2024 08:59, Christian König wrote:
>>>>> Am 12.03.24 um 09:51 schrieb Tvrtko Ursulin:
>>>>>>
>>>>>> Hi Maira,
>>>>>>
>>>>>> On 11/03/2024 10:05, Maíra Canal wrote:
>>>>>>> For some applications, such as using huge pages, we might want to 
>>>>>>> have a
>>>>>>> different mountpoint, for which we pass in mount flags that 
>>>>>>> better match
>>>>>>> our usecase.
>>>>>>>
>>>>>>> Therefore, add a new parameter to drm_gem_object_init() that 
>>>>>>> allow us to
>>>>>>> define the tmpfs mountpoint where the GEM object will be created. If
>>>>>>> this parameter is NULL, then we fallback to shmem_file_setup().
>>>>>>
>>>>>> One strategy for reducing churn, and so the number of drivers this 
>>>>>> patch touches, could be to add a lower level drm_gem_object_init() 
>>>>>> (which takes vfsmount, call it __drm_gem_object_init(), or 
>>>>>> drm__gem_object_init_mnt(), and make drm_gem_object_init() call 
>>>>>> that one with a NULL argument.
>>>>>
>>>>> I would even go a step further into the other direction. The shmem 
>>>>> backed GEM object is just some special handling as far as I can see.
>>>>>
>>>>> So I would rather suggest to rename all drm_gem_* function which 
>>>>> only deal with the shmem backed GEM object into drm_gem_shmem_*.
>>>>
>>>> That makes sense although it would be very churny. I at least would 
>>>> be on the fence regarding the cost vs benefit.
>>>
>>> Yeah, it should clearly not be part of this patch here.
>>>
>>>>
>>>>> Also the explanation why a different mount point helps with 
>>>>> something isn't very satisfying.
>>>>
>>>> Not satisfying as you think it is not detailed enough to say driver 
>>>> wants to use huge pages for performance? Or not satisying as you 
>>>> question why huge pages would help?
>>>
>>> That huge pages are beneficial is clear to me, but I'm missing the 
>>> connection why a different mount point helps with using huge pages.
>>
>> Ah right, same as in i915, one needs to mount a tmpfs instance passing 
>> huge=within_size or huge=always option. Default is 'never', see man 5 
>> tmpfs.
> 
> Thanks for the explanation, I wasn't aware of that.
> 
> Mhm, shouldn't we always use huge pages? Is there a reason for a DRM 
> device to not use huge pages with the shmem backend?

AFAIU, according to b901bb89324a ("drm/i915/gemfs: enable THP"), back 
then the understanding was within_size may overallocate, meaning there 
would be some space wastage, until the memory pressure makes the thp 
code split the trailing huge page. I haven't checked if that still applies.

Other than that I don't know if some drivers/platforms could have 
problems if they have some limitations or hardcoded assumptions when 
they iterate the sg list.

Te Cc is plenty large so perhaps someone else will have additional 
information. :)

Regards,

Tvrtko

> 
> I mean it would make this patch here even smaller.
> 
> Regards,
> Christian.
> 
>>
>>
>> Regards,
>>
>> Tvrtko
>