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

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Message-ID: <f0187458-e576-4894-b728-5914d3d9ed36@redhat.com>
Date: Tue, 6 May 2025 16:36:23 +0200
From: David Hildenbrand <david@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Daniel Gomez <da.gomez@...nel.org>
Cc: Ville Syrjälä <ville.syrjala@...ux.intel.com>,
 akpm@...ux-foundation.org, hughd@...gle.com, willy@...radead.org,
 wangkefeng.wang@...wei.com, 21cnbao@...il.com, ryan.roberts@....com,
 ioworker0@...il.com, da.gomez@...sung.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, regressions@...ts.linux.dev,
 intel-gfx@...ts.freedesktop.org, Eero Tamminen <eero.t.tamminen@...el.com>
Subject: Re: [REGRESSION] Re: [PATCH v3 3/6] mm: shmem: add large folio
 support for tmpfs

On 06.05.25 05:33, Baolin Wang wrote:
> 
> 
> 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.

Maybe preallocation (using fallocate) might be reasonable for their use 
case: if they know they will consume all that memory either way. If it's 
sparse, it's more problematic.

-- 
Cheers,

David / dhildenb