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

[Luiz Capitulino <luizcap@redhat.com>]

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
>>
>