Re: regression - mm: shmem: add large folio support for tmpfs affect GPU performance.

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/7/25 12:47, Hugh Dickins wrote:
> On Fri, 25 Jul 2025, Baolin Wang wrote:
>>>
>>> I hope to correct the logic of i915 driver's shmem allocation, by extending
>>> the shmem write length in the i915 driver to allocate PMD- sized THPs. IIUC,
>>> some sample fix code is as follows (untested). Patryk, could you help test
>>> it to see if this resolves your issue? Thanks.
> 
> This patch cannot be the right fix.  It may be a very sensible workaround
> for some in-kernel drivers (I've not looked or tried); but unless I
> misunderstand, it does nothing to restore userspace behaviour on a
> huge=always tmpfs.

Yes. Initially, we wanted to maintain compatibility with the 'huge=' 
option, meaning that 'huge=always' tmpfs mount would still allocate 
PMD-sized THPs. However, the current implementation is the consensus we 
reached after much debate:

1. “When using tmpfs as a filesystem, it should behave like other 
filesystems. No more special mount options.” Per Matthew.
2. “Do not let the 'huge=' mount option mean 'PMD-sized' when other 
sizes exist.” Per David.

At the time, we should have sought your advice, but we failed. The long 
historical discussion is in this thread[1]. So now the strategy for 
tmpfs supporting large folios is:

"
Considering that tmpfs already has the 'huge=' option to control the 
PMD-sized large folios allocation, we can extend the 'huge=' option to 
allow any sized large folios. The semantics of the 'huge=' mount option are:
huge=never: no any sized large folios
huge=always: any sized large folios
huge=within_size: like 'always' but respect i_size
huge=advise: like 'always' if requested with madvise()

Note: For tmpfs mmap() faults, due to the lack of a write size hint, 
still allocate the PMD-sized large folios if 
huge=always/within_size/advise is set.

Moreover, the 'deny' and 'force' testing options controlled by 
'/sys/kernel/mm/transparent_hugepage/shmem_enabled' still retain the 
same semantics. The 'deny' can disable any sized large folios for tmpfs, 
while the 'force' can enable PMD sized large folios for tmpfs.
"

Currently, we have observed regression in the i915 driver but have not 
yet seen userspace regression on a huge=always tmpfs.

If you have better suggestions, please feel free to point them out. Thanks.

[1] https://lore.kernel.org/lkml/Zw_IT136rxW_KuhU@casper.infradead.org/

> Please reread my comment earlier in the thread, in particular,
> Passing a new SIGBUS xfstest does not excuse a regression: strict PAGE_SIZE
> SIGBUS behaviour is fine for the newly-featured mTHPs or large folios,
> but not for the long-established huge=always.