Subject: Re: [PATCH RFC 06/12] mm/gup: Drop folio_fast_pin_allowed() in hugepd processing

From: Peter Xu <[email-masked]>
Date: Wed, 22 Nov 2023 10:22:11 -0500

On Wed, Nov 22, 2023 at 12:00:24AM -0800, Christoph Hellwig wrote:
> On Tue, Nov 21, 2023 at 10:59:35AM -0500, Peter Xu wrote:
> > > What prevents us from ever using hugepd with file mappings?  I think
> > > it would naturally fit in with how large folios for the pagecache work.
> > > 
> > > So keeping this check and generalizing it seems like the better idea to
> > > me.
> > 
> > But then it means we're still keeping that dead code for fast-gup even if
> > we know that fact..  Or do we have a plan to add that support very soon, so
> > this code will be destined to add back?
> 
> The question wasn't mean retorical - we support arbitrary power of two
> sized folios for the pagepage, what prevents us from using hugepd with
> them right now?

Ah, didn't catch that point previously.  Hugepd is just not used outside
hugetlb right now, afaiu.

For example, __hugepte_alloc() (and that's the only one calls
hugepd_populate()) should be the function to allocate a hugepd (ppc only),
and it's only called in huge_pte_alloc(), which is part of the current
arch-specific hugetlb api.

And generic mm paths don't normally have hugepd handling, afaics.  For
example, page_vma_mapped_walk() doesn't handle hugepd at all unless in
hugetlb specific path.

There're actually (only) two generic mm paths that can handle hugepd,
namely:

  - fast-gup
  - walk_page_*() apis (aka, __walk_page_range())

For fast-gup I think the hugepd code is in use, however for walk_page_*
apis hugepd code shouldn't be reached iiuc as we have the hugetlb specific
handling (walk_hugetlb_range()), so anything within walk_pgd_range() to hit
a hugepd can be dead code to me (but note that this "dead code" is good
stuff to me, if one would like to merge hugetlb instead into generic mm).

This series tries to add slow gup into that list too, so the 3rd one to
support it.  I plan to look more into this area (e.g., __walk_page_range()
can be another good candidate soon).  I'm not sure whether we should teach
the whole mm to understand hugepd yet, but slow gup and __walk_page_range()
does look like good candidates to already remove the hugetlb specific code
paths - slow-gup has average ~add/~del LOCs (which this series does), and
__walk_page_range() can remove some code logically, no harm I yet see.

Indeed above are based on only my code observations, so I'll be more than
happy to be corrected otherwise, as early as possible.

> 
> > The other option is I can always add a comment above gup_huge_pd()
> > explaining this special bit, so that when someone is adding hugepd support
> > to file large folios we'll hopefully not forget it?  But then that
> > generalization work will only happen when the code will be needed.
> 
> If dropping the check is the right thing for now (and I think the ppc
> maintainers and willy as the large folio guy might have a more useful
> opinions than I do), leaving a comment in would be very useful.

Willy is in the loop, and I just notice I didn't really copy ppc list, even
I planned to..  I am adding the list ([email-masked]) into
this reply.  I'll remember to do so as long as there's a new version.

The other reason I feel like hugepd may or may not be further developed for
new features like large folio is that I saw Power9 started to shift to
radix pgtables, and afaics hugepd is only supported in hash tables
(hugepd_ok()).  But again, I confess I know nothing about Power at all.

Thanks,

-- 
Peter Xu