Today, if an architecture implements has_transparent_hugepage() and the CPU
lacks support for PMD-sized pages, the THP code disables all THP, including
mTHP. In addition, the kernel lacks a well defined API to check for
PMD-sized page support. It currently relies on has_transparent_hugepage()
and thp_disabled_by_hw(), but they are not well defined and are tied to
THP support.

This series addresses both issues by introducing a new well defined API
to query PMD-sized page support: pgtable_has_pmd_leaves(). Using this
new helper, we ensure that mTHP remains enabled even when the
architecture or CPU doesn't support PMD-sized pages.

Thanks to David Hildenbrand for suggesting this improvement and for
providing guidance (all bugs and misconceptions are mine).

This applies to Linus tree eb3f4b7426cf ("Merge tag 'nfsd-7.1-2'
of git://git.kernel.org/pub/scm/linux/kernel/git/cel/linux")

NOTE: I used Claude Code Opus 4.6 to review the series before
posting. No issues found.

v5
--
- Moved init_arch_has_pmd_leaves() to mm_core_init() (David)
- Renamed init_arch_has_pmd_leaves() to pgtable_leaf_support_init() (Lance)
- Added new patch changing shmem_getattr() to set blksize according to
  highest supported THP order (Baolin)
- Added new patches to move has_transparent_hugepage() out of
  CONFIG_TRANSPARENT_HUGEPAGE guards (Sashiko)
- shmem_allowable_huge_orders(): rename variable and only disable
  PMD_ORDER (David)
- Added <linux/jmp_label.h> to include/linux/pgtable.h (Sashiko)
- Added new tags and removed Reviewed-by from changed patches

v4
--
- Used static key for pgtable_has_pmd_leaves() API (Lance)
- Moved shmem pgtable_has_pmd_leaves() check to
  shmem_allowable_huge_orders() (Baolin)
- Default pgtable_has_pmd_leaves() implementation to
  IS_ENABLED(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE) (Zi)
- Dropped patch “mm: thp: x86: cleanup PSE feature bit usage” (Dave)

v3
--
- Rebased on top of latest Linus tree
- Removed i915 patch as driver dropped has_transparent_hugepage() usage
- Moved init_arch_has_pmd_leaves() call in start_kernel() to avoid conflict
  with early_param handlers clearing CPU feature flags
- Fixed build error with CONFIG_MMU=n (kernel test robot)
- Fixed huge_anon_orders_inherit default setting when !pgtable_pmd_leaves() (Baolin)
- Small commit changelog improvements

v2
--
- Added support for always enabling mTHPs for shmem (Baolin)
- Improved commits changelog & added reviewed-by

v1
--
- Call init_arch_has_pmd_leaves() from start_kernel()
- Keep pgtable_has_pmd_leaves() calls tied to CONFIG_TRANSPARENT_HUGEPAGE (David)
- Clear PUD_ORDER when clearing PMD_ORDER (David)
- Small changelog improvements (David)
- Rebased on top of latest mm-new

Luiz Capitulino (14):
  docs: tmpfs: remove implementation detail reference
  mm: shmem: shmem_getattr(): set blksize to highest supported THP order
  mm: introduce pgtable_has_pmd_leaves()
  drivers: dax: use pgtable_has_pmd_leaves()
  drivers: nvdimm: use pgtable_has_pmd_leaves()
  mm: debug_vm_pgtable: use pgtable_has_pmd_leaves()
  mm: shmem: allow THP support determination at folio allocation time
  s390: move has_transparent_hugepage() out of THP guard
  powerpc: move has_transparent_hugepage() out of THP guard
  mips: move has_transparent_hugepage() out of THP guard
  x86: move has_transparent_hugepage() out of THP guard
  treewide: introduce arch_has_pmd_leaves()
  mm: replace thp_disabled_by_hw() with pgtable_has_pmd_leaves()
  mm: thp: always enable mTHP support

 Documentation/filesystems/tmpfs.rst           |  5 ++--
 arch/mips/include/asm/pgtable.h               |  6 ++--
 arch/mips/mm/tlb-r4k.c                        |  8 ++---
 arch/powerpc/include/asm/book3s/64/hash-4k.h  |  2 +-
 arch/powerpc/include/asm/book3s/64/hash-64k.h |  2 +-
 arch/powerpc/include/asm/book3s/64/pgtable.h  | 18 ++++++------
 arch/powerpc/include/asm/book3s/64/radix.h    | 14 ++++-----
 arch/powerpc/mm/book3s64/hash_pgtable.c       |  8 ++---
 arch/s390/include/asm/pgtable.h               |  6 ++--
 arch/x86/include/asm/pgtable.h                | 12 ++++----
 drivers/dax/dax-private.h                     |  2 +-
 drivers/nvdimm/pfn_devs.c                     |  6 ++--
 include/linux/huge_mm.h                       |  7 -----
 include/linux/pgtable.h                       | 20 +++++++++++--
 mm/debug_vm_pgtable.c                         | 20 ++++++-------
 mm/huge_memory.c                              | 27 ++++++++++++-----
 mm/memory.c                                   | 11 ++++++-
 mm/mm_init.c                                  |  1 +
 mm/shmem.c                                    | 29 ++++++++++++-------
 19 files changed, 120 insertions(+), 84 deletions(-)

-- 
2.54.0