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

On Fri,  1 May 2026 15:18:42 -0400 Luiz Capitulino <luizcap@redhat.com> wrote:

> 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().

Thanks.

Sashiko found a few other things to ask about:
	https://sashiko.dev/#/patchset/cover.1777663129.git.luizcap@redhat.com

On 2026-05-03 11:02, Andrew Morton wrote:
> On Fri,  1 May 2026 15:18:42 -0400 Luiz Capitulino <luizcap@redhat.com> wrote:
> 
>> 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().
> 
> Thanks.
> 
> Sashiko found a few other things to ask about:
> 	https://sashiko.dev/#/patchset/cover.1777663129.git.luizcap@redhat.com

Thanks, I'll go over those soon.

-cc my previous kernel mail

(Minor annoying reminder to update my email to ljs@kernel.org :P)

On Fri, May 01, 2026 at 03:18:42PM -0400, Luiz Capitulino wrote:
> 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
>

On 2026-05-14 02:35, Lorenzo Stoakes wrote:
> -cc my previous kernel mail
> 
> (Minor annoying reminder to update my email to ljs@kernel.org :P)

Nah, my fault. I promise I updated your email now.

> 
> On Fri, May 01, 2026 at 03:18:42PM -0400, Luiz Capitulino wrote:
>> 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
>>
>