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

[Zi Yan <ziy@nvidia.com>]

On 27 Mar 2026, at 10:27, Lorenzo Stoakes (Oracle) wrote:

> On Fri, Mar 27, 2026 at 02:46:43PM +0100, David Hildenbrand (Arm) wrote:
>> On 3/27/26 02:42, 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.
>>>
>>> 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.
>>
>> Hi! :)
>>
>> The patch set might be better structured by
>>
>> 1) Teaching code paths to not only respect READ_ONLY_THP_FOR_FS but also
>> filesystems with large folios. At that point, READ_ONLY_THP_FOR_FS would
>> have no effect.
>
> And also please do some cleaning up of the mess we have in the code base if at
> all possible :) I feel like we're constantly building on sand with this, and
> should treat every major change as a chance to do this.
>
> Or otherwise we constantly keep leaving this mess around to deal with...

Got it. Let me read through feedbacks from individual patches and come up with
a plan.

>
>>
>> 2) Removing READ_ONLY_THP_FOR_FS along with all the old cruft that is no
>> longer required
>>
>> MADV_COLLAPSE will keep working the whole time.
>
> Obviously everything should keep working throughout any version of this series.
>
Ack.


Best Regards,
Yan, Zi