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

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Message-ID: <57dc4929-268b-4f3f-a0f8-43d6ec85974f@linux.alibaba.com>
Date: Fri, 2 May 2025 09:02:46 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Daniel Gomez <da.gomez@...nel.org>,
 Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc: akpm@...ux-foundation.org, hughd@...gle.com, willy@...radead.org,
 david@...hat.com, 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 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?