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

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Date:   Mon, 20 Nov 2023 00:26:24 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     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 Wed, Nov 15, 2023 at 08:29:02PM -0500, Peter Xu wrote:
> Hugepd format is only used in PowerPC with hugetlbfs.  In commit
> a6e79df92e4a ("mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
> file-backed mappings"), we added a check to fail gup-fast if there's
> potential risk of violating GUP over writeback file systems.  That should
> never apply to hugepd.
>
> Drop that check, not only because it'll never be true for hugepd, but also
> it paves way for reusing the function outside fast-gup.

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.