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

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Message-ID: <ZVzT5_3Zn-Y-6xth@x1n>
Date:   Tue, 21 Nov 2023 10:59:35 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
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 Mon, Nov 20, 2023 at 12:26:24AM -0800, Christoph Hellwig wrote:
> 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.

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 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.

Thanks,

-- 
Peter Xu