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

[Luiz Capitulino <luizcap@redhat.com>]

On 9/21/26 6:36 AM, Usama Arif wrote:
> On Fri, 18 Sep 2026 10:01:35 -0400 Luiz Capitulino <luizcap@redhat.com> wrote:
> 
>>
>>
>> On 9/18/26 6:37 AM, Usama Arif wrote:
>>>
>>>
>>> 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?
>>
>> No :)
>>
>> The architectures that have conditional PMD page support (ie. discovered
>> at run-time via has_transparent_hugepage()) are x86, s390, powerpc and
>> mips.
>>
>> For x86 for example, even on 64-bit there are cases where the PSE bit
>> may not be present (eg. some errata and by hypervisor CPUID masking).
>>
>> That being said, the main goal of the series is that
>> has_transparent_hugepage() is overloaded: it has different semantics on
>> those architectures and is used to gate all THP support (which is not
>> the right thing to do for mTHP).
> 
> Thanks for clearing that and the patches.
> 
> They all look good to me, apart from the the comment in the last patch.
> 
> Feel free to add
> 
> Acked-by: Usama Arif <usama.arif@linux.dev>
> 
> to all of them apart from last.
> 
> One nit: I would put the above goal of clean up at the start of the
> coverletter, instead of starting with adding mTHP support for CPUs
> that lack PMD.

Thanks Usama, I appreciate the review. I'll do this if I need to send a
new version.

> 
> Thanks,
> Usama
> 
>>
>>>
>>> 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(-)
>>>>
>>>
>>
>>
>