Re: [PATCH v3] docs: update THP admin guide about non-tmpfs filesystem support

[Luis Chamberlain <mcgrof@kernel.org>]

On Fri, Apr 04, 2025 at 06:18:12PM +0200, David Hildenbrand wrote:
> On 04.04.25 17:32, Luis Chamberlain wrote:
> > On Fri, Apr 04, 2025 at 04:06:57PM +0200, Pankaj Raghav (Samsung) wrote:
> > > From: Pankaj Raghav <p.raghav@samsung.com>
> > > 
> > > THP support for non-tmpfs filesystem has been around for some time now.
> > > Update the admin guide to reflect it.
> > > 
> > > While we are at it, move FilePmdMapped to previous paragraph for clarity,
> > > and clarify ShmemPmdMapped & ShmemHugePage.
> > > 
> > > Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> > > Acked-by: David Hildenbrand <david@redhat.com>
> > > ---
> > > 
> > > Changes since v2:
> > > - Address comment from Bagas Sanjaya
> > > - Squash commits and Ack from David
> > > 
> > >   Documentation/admin-guide/mm/transhuge.rst | 22 +++++++++++++++-------
> > >   1 file changed, 15 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> > > index dff8d5985f0f..f8aae64e38d0 100644
> > > --- a/Documentation/admin-guide/mm/transhuge.rst
> > > +++ b/Documentation/admin-guide/mm/transhuge.rst
> > > @@ -12,8 +12,8 @@ using huge pages for the backing of virtual memory with huge pages
> > >   that supports the automatic promotion and demotion of page sizes and
> > >   without the shortcomings of hugetlbfs.
> > > -Currently THP only works for anonymous memory mappings and tmpfs/shmem.
> > > -But in the future it can expand to other filesystems.
> > > +Currently, THP only works for anonymous memory mappings, tmpfs/shmem and
> > > +filesystems that support large folios.
> > 
> > That seems to allude that THP can be supported on filesystems
> > that suppor large folios. I don't think we want to call that THP
> > and that can confuse folks. Leaving "currently" also seems to
> > indicate that there is more work to be done for THP for filesystems
> > but that's not true as well. So how about something like:
> > 
> > THP only works for anonymous memory mappings, and the tmpfs/shmem is the only
> > filesystem to support it. The alternative to THP for other filesystems is to
> > support large folios and with it you can end up using huge pages
> 
> That makes things more complicated without a good reason.
> 
> See CONFIG_READ_ONLY_THP_FOR_FS as an early usage of the term "THP" for
> stuff we have in the pagecache. 

OK.

> (with large folios we now properly implement
> this concept, and support more than only PMD size)

Do we really want to call large folio support on filesystems THP?

  Luis