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

[Baolin Wang]

On 2025/5/2 23:31, David Hildenbrand wrote:
> 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.

Right.

So I asked Ville how the shmem folios are allocated in the i915 driver, 
and to see if we can make some improvements.