Re: [PATCH v8 00/14] mm: thp: always enable mTHP support

[Usama Arif <usama.arif@linux.dev>]

On 18/09/2026 02:45, Luiz Capitulino wrote:
> Introduction
> ============
> 
> 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.


Hi Luiz,

Sorry for asking this so late in the series, but which CPUs lack support for
PMD sized pages?

Is this series mainly for PowerPC?

Thanks,
Usama

> 
> This happens because the has_transparent_hugepage() helper is overloaded:
> its name implies it checks whether THP is enabled, but on some architectures
> it actually checks whether the CPU supports PMD-sized pages. In addition,
> the THP and shmem code have a big switch on has_transparent_hugepage().
> 
> This series solves this by decoupling THP availability checking from
> querying CPU support for PMD-sized pages. THP availability can be checked
> with IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE). For querying PMD-sized page
> support, this series introduces a new helper called pgtable_has_pmd_leaves(),
> which is independent of THP, has well defined semantics and can be used
> in fast paths.
> 
> Core THP code and shmem can use pgtable_has_pmd_leaves() to determine
> PMD-sized THP support at page fault time (or folio allocation time for
> shmem), leaving THP and shmem always enabled for other THP sizes. For
> architectures and CPUs that do support PMD-sized pages there's no
> intended change in behavior.
> 
> We also convert each user of has_transparent_hugepage(), and the related
> helper thp_disabled_by_hw(), to IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)
> and/or pgtable_has_pmd_leaves() according to the call site's check semantics.
> On s390, powerpc, mips and x86 the arch has_transparent_hugepage()
> implementation is moved out of the CONFIG_TRANSPARENT_HUGEPAGE guard and
> renamed to arch_has_pmd_leaves().
> 
> Thanks to David Hildenbrand for suggesting this improvement and for
> providing initial guidance (all bugs and misconceptions are mine).
> 
> This applies to mm-new 93f615a22169 ("mm/swap, PM: hibernate: atomically
> replace hibernation pin").
> 
> Reporting availability of PMD-sized pages to user-space
> =======================================================
> 
> Before this series, not having PMD pages available would shut down THP support
> meaning that /sys/kernel/mm/transparent_hugepage would not be available.
> After this series, /sys/kernel/mm/transparent_hugepage and hpage_pmd_size are
> always available but hugepages-<PMD-size>kB is only available if PMD-sized
> pages are supported.
> 
> To me this seems logical, as hugepages-<size>kB is only available if <size>
> is supported. If this is correct, then MM kselftests that depend on PMD-sized
> pages will need to be updated to check for it as they fail with this series
> applied when PMD-sized pages are not available.
> 
> The alternative is changing hpage_pmd_size: either don't create the file
> when PMD-sized pages are not available or set hpage_pmd_size=0. My concern
> is that this could be considered an ABI breakage as this file was never
> reported to be optional or report a zero value.
> 
> Testing
> =======
> 
> - Ran MM kselftests on x86_64 and s390
> - Tested all mTHP sizes allocation on x86_64 (with and without PMD-sized
>   pages available)
> - Tested shmem with within_size w/ mTHP on x86_64 (with and without
>   PMD-sized pages available)
> - Performed defconfig build on x86_64, s390, arm64, powerpc, and mips
> 
> NOTES:
>   * I'm forcing arch_has_pmd_leaves() off on x86 to simulate not having
>     PMD-sized pages available
> 
>   * Running the MM kselftests when PMD pages are not available causes some
>     tests that depend on PMD-sized THP pages to fail as noted earlier
> 
> Changelog
> =========
> 
> v8
> --
> - Fixed build breakage on MIPS (Lance)
> - Rebased on top of mm-new (fixed conflicts in include/linux/pgtable.h and
>   mm/shmem.c - dropped a Reviewed-by as a result)
> 
> v7
> --
> - Applied on top of latest mm-new
> - Improved various changelogs including cover-letter
> - Changed arch_has_pmd_leaves() default implementation to use IS_ENABLED()
>   instead of IS_BUILTIN()
> 
> v6
> --
> - Rebased on top of mm-unstable (required a minor conflict resolution)
> - Added more Reviewed-by and Acked-by tags
> 
> 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
> - shmem_allowable_huge_orders(): rename variable and only disable
>   PMD_ORDER (David)
> - Added <linux/jmp_label.h> to include/linux/pgtable.h
> - 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_has_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/pgtable.c                        | 25 ++++++++++++++++
>  arch/mips/mm/tlb-r4k.c                        | 22 --------------
>  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                       | 21 ++++++++++++--
>  mm/debug_vm_pgtable.c                         | 20 ++++++-------
>  mm/huge_memory.c                              | 27 ++++++++++++-----
>  mm/memory.c                                   | 11 ++++++-
>  mm/mm_init.c                                  |  1 +
>  mm/shmem.c                                    | 29 ++++++++++++-------
>  20 files changed, 144 insertions(+), 100 deletions(-)
>