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

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Message-ID: <ZV90JcnQ1RGud/0R@casper.infradead.org>
Date:   Thu, 23 Nov 2023 15:47:49 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Peter Xu <peterx@...hat.com>, 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>,
        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 22, 2023 at 12:00:24AM -0800, Christoph Hellwig wrote:
> > 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.

It looks like ARM (in the person of Ryan) are going to add support for
something equivalent to hugepd.  Insofar as I understand hugepd, anyway.
I've done my best to set up the generic code so that the arch code can
use whatever size TLB entries it supports.  I haven't been looking to the
hugetlb code as a reference for it, since it can assume natural alignment
and generic THP/large folio must be able to handle arbitrary alignment.

If powerpc want to join in on the fun, they're more than welcome, but I
get the feeling that investment in Linux-on-PPC is somewhat smaller than
Linux-on-ARM these days.  Even if we restrict that to the server space.