Re: [PATCH 7.2 v3 00/12] Remove read-only THP support for FSes without large folio support

Lorenzo Stoakes
2026-04-18T09:27:12Z

On Fri, Apr 17, 2026 at 10:44:17PM -0400, Zi Yan wrote:
> Hi all,
>
> This patchset removes READ_ONLY_THP_FOR_FS Kconfig and enables creating
> read-only THPs for FSes with large folio support (the supported orders
> need to include PMD_ORDER) by default.
>
> Before the patchset, the status of creating read-only THPs is below:

Good to specify the read-only bit up front!

>
> | PF | MADV_COLLAPSE | khugepaged |
> |-----------|---------------|------------|
> large folio FSes only | ✓ | x | x |
> READ_ONLY_THP_FOR_FS only | x | ✓ | ✓ |
> both | ✓ | ✓ | ✓ |

This diagrams seem familiar :P but very nice, thanks!

And since we include cover letter in series in mm this should be some nice
documentation in the commit msg also.

>
> where READ_ONLY_THP_FOR_FS implies no large folio FSes.
>
>
> Now without READ_ONLY_THP_FOR_FS:
>
> | PF | MADV_COLLAPSE | khugepaged |
> |-----------|---------------|------------|
> large folio FSes | ✓ | ✓ | ✓ |
> no large folio FSes | x | x | x |

This is really nice and clear thanks!

>
> This means no large folio FSes need to add large folio support (the
> supported orders need to include PMD_ORDER), so that they can leverage
> read-only THP creation function.
>
> To prevent breaking read-only THP support for large folio FSes,
> 1. first 4 patches enables the support, so that without READ_ONLY_THP_FOR_FS,
> read-only THP still works for large folio FSes,

I guess this introduces what was previously supported by
CONFIG_READ_ONLY_THP_FOR_FS to large folios as part of that before removal of
the config option?

> 2. Patch 5 removes READ_ONLY_THP_FOR_FS Kconfig,
> 3. the rest of patches remove code related to READ_ONLY_THP_FOR_FS.

Makes sense thanks!

>
>
> The overview of the changes is:
>
> 1. collapse_file() checks for to-be-collapsed folio dirtiness after they
> are locked, unmapped to make sure no new write happens. Before,
> mapping->nr_thps and inode->i_writecount are used to cause read-only
> THP truncation before a fd becomes writable.
>
> 2. hugepage_pmd_enabled() is true for anon, shmem, and file-backed cases
> if the global khugepaged control is on, otherwise, khugepaged for
> file-backed case is turned off and anon and shmem depend on per-size
> control knobs.
>
> 3. collapse_file() from mm/khugepaged.c, instead of checking
> CONFIG_READ_ONLY_THP_FOR_FS, makes sure the mapping_max_folio_order()
> of struct address_space of the file is at least PMD_ORDER.
>
> 4. file_thp_enabled() also checks mapping_max_folio_order() instead and
> no longer checks if the input file is opened as read-only (Change 1
> handles read-write files).
>
> 5. truncate_inode_partial_folio() calls folio_split() directly instead
> of the removed try_folio_split_to_order(), since large folios can
> only show up on a FS with large folio support.
>
> 6. nr_thps is removed from struct address_space, since it is no longer
> needed to drop all read-only THPs from a FS without large folio
> support when the fd becomes writable. Its related filemap_nr_thps*()
> are removed too.
>
> 7. folio_check_splittable() no longer checks READ_ONLY_THP_FOR_FS.
>
> 8. Updated comments in various places.
>
>
> Changelog
> ===
> From V2[3]:
> 1. removed unnecessary check in collapse_scan_file().
>
> 2. removed inode_is_open_for_write() check in file_thp_enabled().
>
> 3. changed hugepage_pmd_enabled() to return true if khugepaged global
> control is on instead of false. cleaned up anon and shmem code in the
> function.
>
> 4. moved folio dirtiness check after try_to_unmap() but before
> try_to_unmap_flush(), since that is sufficient to prevent new writes.
>
> 5. reordered patch 4 and 5, so that khugepaged behavior does not change
> after READ_ONLY_THP_FOR_FS is removed.
>
> 6. added read-write file test in khugepaged selftest.
>
> 7. removed the read-only file restriction from guard-region selftest.
>
> From V1[2]:
> 1. removed inode_is_open_for_write() check in collapse_file(), since the
> added folio dirtiness check after try_to_unmap_flush() should be
> sufficient to prevent writes to candidate folios.
>
> 2. removed READ_ONLY_THP_FOR_FS check in hugepage_pmd_enabled(), please
> see Patch 5 and item 2 in the overview for more details.
>
> 3. moved the patch removing READ_ONLY_THP_FOR_FS Kconfig after enabling
> khugepaged and MADV_COLLAPSE to create read-only THPs.
>
> 4. added mapping_pmd_thp_support() helper function.
>
> 5. used VM_WARN_ON_ONCE() in collapse_file() for mapping eligibility check
> and address alignment check instead of if + return error code. Always
> allow shmem, since MADV_COLLAPSE ignore shmem huge config.
>
> 6. added mapping eligibility check in collapse_scan_file().
>
> 7. removed trailing ; for folio_split() in the !CONFIG_TRANSPARENT_HUGEPAGE.
>
> 8. simplified code in folio_check_splittable() after removing
> READ_ONLY_THP_FOR_FS code.
>
> 9. clarified that read-only THP works for FSes with PMD THP support by
> default.
>
> From RFC[1]:
> 1. instead of removing READ_ONLY_THP_FOR_FS function entirely, turn it
> on by default for all FSes with large folio support and the supported
> orders includes PMD_ORDER.
>
> Suggestions and comments are welcome.
>
> Link: https://lore.kernel.org/all/20260323190644.1714379-1-ziy@nvidia.com/ [1]
> Link: https://lore.kernel.org/all/20260327014255.2058916-1-ziy@nvidia.com/ [2]
> Link: https://lore.kernel.org/all/20260413192030.3275825-1-ziy@nvidia.com/ [3]
>
> Zi Yan (12):
> mm/khugepaged: remove READ_ONLY_THP_FOR_FS check
> mm/khugepaged: add folio dirty check after try_to_unmap()
> mm/huge_memory: remove READ_ONLY_THP_FOR_FS from file_thp_enabled()
> mm/khugepaged: remove READ_ONLY_THP_FOR_FS check in
> hugepage_pmd_enabled()
> mm: remove READ_ONLY_THP_FOR_FS Kconfig option
> mm: fs: remove filemap_nr_thps*() functions and their users
> fs: remove nr_thps from struct address_space
> mm/huge_memory: remove folio split check for READ_ONLY_THP_FOR_FS
> mm/truncate: use folio_split() in truncate_inode_partial_folio()
> fs/btrfs: remove a comment referring to READ_ONLY_THP_FOR_FS
> selftests/mm: remove READ_ONLY_THP_FOR_FS in khugepaged
> selftests/mm: remove READ_ONLY_THP_FOR_FS code from guard-regions
>
> fs/btrfs/defrag.c | 3 -
> fs/inode.c | 3 -
> fs/open.c | 27 -----
> include/linux/fs.h | 5 -
> include/linux/huge_mm.h | 25 +----
> include/linux/pagemap.h | 35 ++-----
> include/linux/shmem_fs.h | 2 +-
> mm/Kconfig | 11 ---
> mm/filemap.c | 1 -
> mm/huge_memory.c | 39 ++------
> mm/khugepaged.c | 86 ++++++++--------
> mm/truncate.c | 8 +-
> tools/testing/selftests/mm/guard-regions.c | 18 +---
> tools/testing/selftests/mm/khugepaged.c | 110 +++++++++++++++------
> tools/testing/selftests/mm/run_vmtests.sh | 12 ++-
> 15 files changed, 156 insertions(+), 229 deletions(-)
>
> --
> 2.43.0
>