Re: [PATCH v1 00/10] Remove READ_ONLY_THP_FOR_FS Kconfig

[Zi Yan]

On 5 Apr 2026, at 13:38, Nico Pache wrote:

> On Thu, Mar 26, 2026 at 7:43 PM Zi Yan <ziy@xxxxxxxxxx> 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.
>
> Hi Zi,
>
> Thank you for tackling this :) Ill try to review the next version as
> I'm a little behind on this thread.

Sure. Thanks.

>
> Should we guard collapsing READ_ONLY_THPs with a sysctl? My fear is
> workloads that convert READ_ONLY THPs into writable pages (assuming
> this is common/possible; my understanding of FS is rather low),
> leading to storms of thp splitting. Do you think this is a real
> concern? I guess this is also true of read-only-->writable fs-THPS
> even without khugepaged, correct?

Why would a read-only THP need to be split when it becomes writable?
After this patchset, a read-only THP can only be created on a FS that
supports large folios (to be precise PMD THP). That means any write
to that read-only THP would just change it to a writable THP.

Let me know if I miss anything.

>
> Cheers,
> -- Nico
>
>>
>> The changes are:
>> 1. 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.
>> 2. file_thp_enabled() also checks mapping_max_folio_order() instead.
>> 3. 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.
>> 4. 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.
>> 5. folio_check_splittable() no longer checks READ_ONLY_THP_FOR_FS.
>> 6. Updated comments in various places.
>>
>> Changelog
>> ===
>> 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@xxxxxxxxxx/ [1]
>>
>> Zi Yan (10):
>>   mm: remove READ_ONLY_THP_FOR_FS Kconfig option
>>   mm/khugepaged: remove READ_ONLY_THP_FOR_FS check
>>   mm: fs: remove filemap_nr_thps*() functions and their users
>>   fs: remove nr_thps from struct address_space
>>   mm/huge_memory: remove READ_ONLY_THP_FOR_FS from file_thp_enabled()
>>   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                           | 29 ++---------------
>>  mm/khugepaged.c                            | 36 +++++-----------------
>>  mm/truncate.c                              |  8 ++---
>>  tools/testing/selftests/mm/guard-regions.c |  9 +++---
>>  tools/testing/selftests/mm/khugepaged.c    |  4 +--
>>  13 files changed, 23 insertions(+), 167 deletions(-)
>>
>> --
>> 2.43.0
>>


--
Best Regards,
Yan, Zi