Re: [RFC PATCH] mm: shmem: fix the strategy for the tmpfs 'huge=' options

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

On 2025/7/30 16:14, Baolin Wang wrote:
> After commit acd7ccb284b8 ("mm: shmem: add large folio support for tmpfs"),
> we have extended tmpfs to allow any sized large folios, rather than just
> PMD-sized large folios.
> 
> The strategy discussed previously was:
> 
> "
> 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 the 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 huge 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.
> "
> 
> This means that when tmpfs is mounted with 'huge=always' or 'huge=within_size',
> tmpfs will allow getting a highest order hint based on the size of write() and
> fallocate() paths. It will then try each allowable large order, rather than
> continually attempting to allocate PMD-sized large folios as before.
> 
> However, this might break some user scenarios for those who want to use
> PMD-sized large folios, such as the i915 driver which did not supply a write
> size hint when allocating shmem [1].
> 
> Moreover, Hugh also complained that this will cause a regression in userspace
> with 'huge=always' or 'huge=within_size'.
> 
> So, let's revisit the strategy for tmpfs large page allocation. A simple fix
> would be to always try PMD-sized large folios first, and if that fails, fall
> back to smaller large folios. However, this approach differs from the strategy
> for large folio allocation used by other file systems. Is this acceptable?
> 
> [1] https://lore.kernel.org/lkml/0d734549d5ed073c80b11601da3abdd5223e1889.1753689802.git.baolin.wang@linux.alibaba.com/
> Fixes: acd7ccb284b8 ("mm: shmem: add large folio support for tmpfs")
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
> Note: this is just an RFC patch. I would like to hear others' opinions or
> see if there is a better way to address Hugh's concern.
> ---

Hi Hugh,

If we use this approach to fix the PMD large folio regression, should we 
also change tmpfs mmap() to allow allocating any sized large folios, but 
always try to allocate PMD-sized large folios first? What do you think? 
Thanks.