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

[Daniel Gomez <da.gomez@kernel.org>]

On Fri, Apr 04, 2025 at 11:02:46AM +0100, Luis Chamberlain wrote:
> On Fri, Apr 04, 2025 at 06:31:16PM +0200, Pankaj Raghav (Samsung) wrote:
> > On Fri, Apr 04, 2025 at 08:32:15AM -0700, 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
> > 
> > But we do allocate a THP in the page cache if we support large folios.
> > 
> > See [1] where THP was supported through page cache. From what I
> > understand, THP support was added first to the page cache and then large
> > folios (orders in between) support came later.
> 
> I see, yes Do we want to clarify this further?

According to this thread [1], large folios currently depends on
CONFIG_TRANSPARENT_HUGEPAGE, but that will be removed eventually.

https://lore.kernel.org/all/ZPINmXyTgy2wqLqr@casper.infradead.org/

I agree this needs to be clarified. THP for anonymous memory and tmpfs/shmem is
not the same for filesystems that support large folios.