Re: [PATCH v5 00/14] mm: thp: always enable mTHP support

[Luiz Capitulino <luizcap@redhat.com>]

[ Top posting ]

Andrew,

I wonder if it's time to add this series to mm-unstable so that it gets
more testing?

I (quickly) went through Sashiko's review and most are minor things (OK,
there's one review comment that I want to better check, but still some
testing in mm-unstable has more value).

- Luiz

On 2026-05-29 10:55, 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 eb3f4b7426cf ("Merge tag 'nfsd-7.1-2'
> of git://git.kernel.org/pub/scm/linux/kernel/git/cel/linux")
> 
> NOTE: I used Claude Code Opus 4.6 to review the series before
> posting. No issues found.
> 
> v5
> --
> - Moved init_arch_has_pmd_leaves() to mm_core_init() (David)
> - Renamed init_arch_has_pmd_leaves() to pgtable_leaf_support_init() (Lance)
> - Added new patch changing shmem_getattr() to set blksize according to
>    highest supported THP order (Baolin)
> - Added new patches to move has_transparent_hugepage() out of
>    CONFIG_TRANSPARENT_HUGEPAGE guards (Sashiko)
> - shmem_allowable_huge_orders(): rename variable and only disable
>    PMD_ORDER (David)
> - Added <linux/jmp_label.h> to include/linux/pgtable.h (Sashiko)
> - Added new tags and removed Reviewed-by from changed patches
> 
> v4
> --
> - Used 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 (14):
>    docs: tmpfs: remove implementation detail reference
>    mm: shmem: shmem_getattr(): set blksize to highest supported THP order
>    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: allow THP support determination at folio allocation time
>    s390: move has_transparent_hugepage() out of THP guard
>    powerpc: move has_transparent_hugepage() out of THP guard
>    mips: move has_transparent_hugepage() out of THP guard
>    x86: move has_transparent_hugepage() out of THP guard
>    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               |  6 ++--
>   arch/mips/mm/tlb-r4k.c                        |  8 ++---
>   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  | 18 ++++++------
>   arch/powerpc/include/asm/book3s/64/radix.h    | 14 ++++-----
>   arch/powerpc/mm/book3s64/hash_pgtable.c       |  8 ++---
>   arch/s390/include/asm/pgtable.h               |  6 ++--
>   arch/x86/include/asm/pgtable.h                | 12 ++++----
>   drivers/dax/dax-private.h                     |  2 +-
>   drivers/nvdimm/pfn_devs.c                     |  6 ++--
>   include/linux/huge_mm.h                       |  7 -----
>   include/linux/pgtable.h                       | 20 +++++++++++--
>   mm/debug_vm_pgtable.c                         | 20 ++++++-------
>   mm/huge_memory.c                              | 27 ++++++++++++-----
>   mm/memory.c                                   | 11 ++++++-
>   mm/mm_init.c                                  |  1 +
>   mm/shmem.c                                    | 29 ++++++++++++-------
>   19 files changed, 120 insertions(+), 84 deletions(-)
>