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

[Baolin Wang]

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').

> Even if these are enabled, the possible difference may be that before, i915 was
> using PMD pages (THP) always and now mTHP will be used, unless the file size is
> as big as the PMD page. I think the always mount option would also try to infer
> the size to actually give a proper order folio according to that size. Baolin,
> is that correct?

Right.

> And Ville, can you confirm if what i915 needs is to enable PMD-size allocations
> always?