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/[email protected]/ [1] > Link: https://lore.kernel.org/all/[email protected]/ [2] > Link: https://lore.kernel.org/all/[email protected]/ [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 >
