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

["Lorenzo Stoakes (Oracle)" <ljs@kernel.org>]

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...

>
> 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.

>
> --
> Cheers,
>
> David

Cheers, Lorenzo