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

Zi Yan
2026-04-13T19:20:50Z

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(-)

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2.43.0