[PATCH v3 00/10] mm: thp: always enable mTHP support

Luiz Capitulino
2026-04-08T20:33:21Z

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 bfe62a454542 ("Merge tag 'soc-fixes-7.0-2'
of git://git.kernel.org/pub/scm/linux/kernel/git/soc/soc").

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 (10):
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: 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 +-
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 | 15 +++++++-------
mm/memory.c | 10 +++++++++-
mm/shmem.c | 11 +++++-----
20 files changed, 80 insertions(+), 60 deletions(-)

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2.53.0