Re: [REGRESSION] Re: [PATCH v3 3/6] mm: shmem: add large folio support for tmpfs

[David Hildenbrand <david@redhat.com>]

On 02.05.25 15:10, Daniel Gomez wrote:
> On Fri, May 02, 2025 at 09:18:41AM +0100, David Hildenbrand wrote:
>> On 02.05.25 03:02, Baolin Wang wrote:
>>>
>>>
>>> On 2025/4/30 21:24, Daniel Gomez wrote:
>>>> On Wed, Apr 30, 2025 at 02:20:02PM +0100, Ville Syrjälä wrote:
>>>>> On Wed, Apr 30, 2025 at 02:32:39PM +0800, Baolin Wang wrote:
>>>>>> On 2025/4/30 01:44, Ville Syrjälä wrote:
>>>>>>> On Thu, Nov 28, 2024 at 03:40:41PM +0800, Baolin Wang wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> This causes a huge regression in Intel iGPU texturing performance.
>>>>>>
>>>>>> Unfortunately, I don't have such platform to test it.
>>>>>>
>>>>>>>
>>>>>>> I haven't had time to look at this in detail, but presumably the
>>>>>>> problem is that we're no longer getting huge pages from our
>>>>>>> private tmpfs mount (done in i915_gemfs_init()).
>>>>>>
>>>>>> IIUC, the i915 driver still limits the maximum write size to PAGE_SIZE
>>>>>> in the shmem_pwrite(),
>>>>>
>>>>> pwrite is just one random way to write to objects, and probably
>>>>> not something that's even used by current Mesa.
>>>>>
>>>>>> which prevents tmpfs from allocating large
>>>>>> folios. As mentioned in the comments below, tmpfs like other file
>>>>>> systems that support large folios, will allow getting a highest order
>>>>>> hint based on the size of the write and fallocate paths, and then will
>>>>>> attempt each allowable huge order.
>>>>>>
>>>>>> Therefore, I think the shmem_pwrite() function should be changed to
>>>>>> remove the limitation that the write size cannot exceed PAGE_SIZE.
>>>>
>>>> To enable mTHP on tmpfs, the necessary knobs must first be enabled in sysfs
>>>> as they are not enabled by default IIRC (only THP, PMD level). Ville, I
>>>> see i915_gemfs the huge=within_size mount option is passed. Can you confirm
>>>> if /sys/kernel/mm/transparent_hugepage/hugepages-*/enabled are also marked as
>>>> 'always' when the regression is found?
>>>
>>> The tmpfs mount will not be controlled by
>>> '/sys/kernel/mm/transparent_hugepage/hugepages-*Kb/enabled' (except for
>>> the debugging options 'deny' and 'force').
>>
>> Right, IIRC as requested by Willy, it should behave like other FSes where
>> there is no control over the folio size to be used.
> 
> Thanks for reminding me. I forgot we finally changed it.
> 
> Could the performance drop be due to the driver no longer using PMD-level pages?

I suspect that the faulting logic will now go to a smaller order first, 
indeed.

... trying to digest shmem_allowable_huge_orders() and 
shmem_huge_global_enabled(), having a hard time trying to isolate the 
tmpfs case: especially, if we run here into the vma vs. !vma case.

Without a VMA, I think we should have "mpfs will allow getting a highest 
order hint based on and fallocate paths, then will try each allowable 
order".

With a VMA (no access hint), "we still use PMD-sized order to locate 
huge pages due to lack of a write size hint."

So if we get a fallocate()/write() that is, say, 1 MiB, we'd now 
allocate an 1 MiB folio instead of a 2 MiB one.

-- 
Cheers,

David / dhildenb