linux-kernel - [RFC 00/10] mm: thp: always enable mTHP support

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Message-ID: <cover.1762464515.git.luizcap@redhat.com>
Date: Thu,  6 Nov 2025 16:28:47 -0500
From: Luiz Capitulino <luizcap@...hat.com>
To: david@...nel.org,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Cc: ryan.roberts@....com,
	akpm@...ux-foundation.org,
	lorenzo.stoakes@...cle.com
Subject: [RFC 00/10] mm: thp: always enable mTHP support

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 support. 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.

An important detail is that we need to do the same refactoring for
has_transparent_pud_hugepage(). I actually have patches for this one
too, I'm not including them here because I want to get some initial
feedback on the general approach first (and maybe it's better to
do that in a separate series).

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

Luiz Capitulino (10):
  docs: tmpfs: remove implementation detail reference
  mm: introduce pgtable_has_pmd_leaves()
  drivers: dax: use pgtable_has_pmd_leaves()
  drivers: i915 selftest: use pgtable_has_pmd_leaves()
  drivers: nvdimm: use pgtable_has_pmd_leaves()
  mm: debug_vm_pgtable: use pgtable_has_pmd_leaves()
  treewide: rename has_transparent_hugepage() to arch_has_pmd_leaves()
  mm: replace thp_disabled_by_hw() with pgtable_has_pmd_leaves()
  mm: thp: always enable mTHP support
  mm: thp: x86: cleanup PSE feature bit usage

 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                |  6 ------
 arch/x86/include/asm/pgtable_32.h             |  6 ++++++
 drivers/dax/dax-private.h                     |  2 +-
 .../gpu/drm/i915/gem/selftests/huge_pages.c   |  2 +-
 drivers/nvdimm/pfn_devs.c                     |  4 ++--
 include/linux/huge_mm.h                       |  7 -------
 include/linux/pgtable.h                       | 14 +++++++++++--
 mm/debug_vm_pgtable.c                         | 20 +++++++++----------
 mm/huge_memory.c                              | 13 ++++++------
 mm/memory.c                                   | 12 ++++++++++-
 mm/shmem.c                                    |  8 ++++----
 20 files changed, 72 insertions(+), 59 deletions(-)

-- 
2.51.1