mm: shmem: fix the strategy for the tmpfs 'huge=' options

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. This approach differs from the strategy for
large folio allocation used by other file systems, however, tmpfs is somewhat
different from other file systems, as quoted from David's opinion:

: There were opinions in the past that tmpfs should just behave like any
: other fs, and I think that's what we tried to satisfy here: use the write
: size as an indication.
: 
: I assume there will be workloads where either approach will be beneficial.
: I also assume that workloads that use ordinary fs'es could benefit from
: the same strategy (start with PMD), while others will clearly not.

Link: https://lkml.kernel.org/r/10e7ac6cebe6535c137c064d5c5a235643eebb4a.1756888965.git.baolin.wang@linux.alibaba.com
Link: https://lore.kernel.org/lkml/0d734549d5ed073c80b11601da3abdd5223e1889.1753689802.git.baolin.wang@linux.alibaba.com/ [1]
Fixes: acd7ccb284b8 ("mm: shmem: add large folio support for tmpfs")
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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