[PATCH 7.2 v2 00/12] Remove read-only THP support for FSes without large folio support

[Zi Yan <ziy@nvidia.com>]

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:

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

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 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 3 patches enables the support, so that without READ_ONLY_THP_FOR_FS,
   read-only THP still works for large folio FSes,
2. Patch 4 removes READ_ONLY_THP_FOR_FS Kconfig,
3. the rest of patches remove code related to READ_ONLY_THP_FOR_FS.


The overview of the changes is:

1. collapse_file() checks for to-be-collapsed folio dirtiness after they
   are locked, unmapped, and corresponding mappings are flushed from
   TLBs to prevent writes to candidate folios. 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() no longer always returned true if
   CONFIG_READ_ONLY_THP_FOR_FS was enabaled. This affects whether
   khugepaged will be on or not. It now depends on anon and shmem
   configurations. Namely, if a user who has set
   /sys/kernel/mm/transparent_hugepage/enabled to always or madvise but
   explicitly disabled anon PMD THP and shmem THP via the per-order
   sysfs controls, read-only THP support will be disabled.

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.

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 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]

Zi Yan (12):
  mm/khugepaged: remove READ_ONLY_THP_FOR_FS check
  mm/khugepaged: add folio dirty check after try_to_unmap_flush()
  mm/huge_memory: remove READ_ONLY_THP_FOR_FS from file_thp_enabled()
  mm: remove READ_ONLY_THP_FOR_FS Kconfig option
  mm/khugepaged: remove READ_ONLY_THP_FOR_FS check in
    hugepage_pmd_enabled()
  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 from comments in
    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                    | 29 ----------
 mm/Kconfig                                 | 11 ----
 mm/filemap.c                               |  1 -
 mm/huge_memory.c                           | 37 ++----------
 mm/khugepaged.c                            | 65 ++++++++++------------
 mm/truncate.c                              |  8 +--
 tools/testing/selftests/mm/guard-regions.c |  9 +--
 tools/testing/selftests/mm/khugepaged.c    |  4 +-
 13 files changed, 49 insertions(+), 178 deletions(-)

-- 
2.43.0