[PATCH v4 0/9] mm: thp: always enable mTHP support

[Luiz Capitulino <luizcap@redhat.com>]

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 08d0d3466664 ("Merge tag 'net-7.1-rc2'
of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net")

NOTE: I used Claude Code Opus 4.6 to *review* the series before
posting. It did find one issue where a pgtable_has_pmd_leaves()
check was missing when assining huge_shmem_orders_inherit in
shmem_init().

v4
--
- Use 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 (9):
  docs: tmpfs: remove implementation detail reference
  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: drop has_transparent_hugepage() usage
  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               |  4 +--
 arch/mips/mm/tlb-r4k.c                        |  4 +--
 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  | 10 ++++----
 arch/powerpc/include/asm/book3s/64/radix.h    |  2 +-
 arch/powerpc/mm/book3s64/hash_pgtable.c       |  4 +--
 arch/s390/include/asm/pgtable.h               |  4 +--
 arch/x86/include/asm/pgtable.h                |  4 +--
 drivers/dax/dax-private.h                     |  2 +-
 drivers/nvdimm/pfn_devs.c                     |  6 +++--
 include/linux/huge_mm.h                       |  7 ------
 include/linux/pgtable.h                       | 19 ++++++++++++--
 init/main.c                                   |  1 +
 mm/debug_vm_pgtable.c                         | 20 +++++++--------
 mm/huge_memory.c                              | 25 +++++++++++++------
 mm/memory.c                                   | 11 +++++++-
 mm/shmem.c                                    | 21 +++++++++-------
 19 files changed, 93 insertions(+), 60 deletions(-)

-- 
2.53.0