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

From: Christoph Hellwig <[email-masked]>
Date: Wed, 22 Nov 2023 00:00:24 -0800

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?

> 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.