linux-kernel - Re: [PATCH RFC 06/12] mm/gup: Drop folio_fast_pin

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Message-ID: <ZV21GCbG48nTLDzn@infradead.org>
Date:   Wed, 22 Nov 2023 00:00:24 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        James Houghton <jthoughton@...gle.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        Yang Shi <shy828301@...il.com>,
        Rik van Riel <riel@...riel.com>,
        Hugh Dickins <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org, Mike Rapoport <rppt@...nel.org>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH RFC 06/12] mm/gup: Drop folio_fast_pin_allowed() in
 hugepd processing

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.