linux-kernel - Re: [RFC 00/10] mm: thp: always enable mTHP support

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Message-ID: <54744fad-daf4-4248-8690-93b39fd6c97d@lucifer.local>
Date: Wed, 3 Dec 2025 13:58:38 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Luiz Capitulino <luizcap@...hat.com>
Cc: david@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        ryan.roberts@....com, akpm@...ux-foundation.org
Subject: Re: [RFC 00/10] mm: thp: always enable mTHP support

Sorry I didn't say at the time - thanks, I do plan to look through this at
some point :)

Cheers, Lorenzo

On Thu, Nov 06, 2025 at 04:28:47PM -0500, 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 support. 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.
>
> An important detail is that we need to do the same refactoring for
> has_transparent_pud_hugepage(). I actually have patches for this one
> too, I'm not including them here because I want to get some initial
> feedback on the general approach first (and maybe it's better to
> do that in a separate series).
>
> Thanks to David Hildenbrand for suggesting this improvement and for
> providing guidance (all bugs and misconcentpions are mine).
>
> Luiz Capitulino (10):
>   docs: tmpfs: remove implementation detail reference
>   mm: introduce pgtable_has_pmd_leaves()
>   drivers: dax: use pgtable_has_pmd_leaves()
>   drivers: i915 selftest: use pgtable_has_pmd_leaves()
>   drivers: nvdimm: use pgtable_has_pmd_leaves()
>   mm: debug_vm_pgtable: use pgtable_has_pmd_leaves()
>   treewide: rename has_transparent_hugepage() to arch_has_pmd_leaves()
>   mm: replace thp_disabled_by_hw() with pgtable_has_pmd_leaves()
>   mm: thp: always enable mTHP support
>   mm: thp: x86: cleanup PSE feature bit usage
>
>  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                |  6 ------
>  arch/x86/include/asm/pgtable_32.h             |  6 ++++++
>  drivers/dax/dax-private.h                     |  2 +-
>  .../gpu/drm/i915/gem/selftests/huge_pages.c   |  2 +-
>  drivers/nvdimm/pfn_devs.c                     |  4 ++--
>  include/linux/huge_mm.h                       |  7 -------
>  include/linux/pgtable.h                       | 14 +++++++++++--
>  mm/debug_vm_pgtable.c                         | 20 +++++++++----------
>  mm/huge_memory.c                              | 13 ++++++------
>  mm/memory.c                                   | 12 ++++++++++-
>  mm/shmem.c                                    |  8 ++++----
>  20 files changed, 72 insertions(+), 59 deletions(-)
>
> --
> 2.51.1
>