linux-kernel - Re: [PATCH RFC 07/12] mm/gup: Refactor record

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Message-ID: <ZVZwJSRYWInodpjO@x1n>
Date:   Thu, 16 Nov 2023 14:40:21 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Mike Kravetz <mike.kravetz@...cle.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Mike Rapoport <rppt@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Rik van Riel <riel@...riel.com>,
        James Houghton <jthoughton@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH RFC 07/12] mm/gup: Refactor record_subpages() to find 1st
 small page

On Thu, Nov 16, 2023 at 02:51:52PM +0000, Matthew Wilcox wrote:
> On Wed, Nov 15, 2023 at 08:29:03PM -0500, Peter Xu wrote:
> > All the fast-gup functions take a tail page to operate, always need to do
> > page mask calculations before feeding that into record_subpages().
> > 
> > Merge that logic into record_subpages(), so that we always take a head
> > page, and leave the rest calculation to record_subpages().
> 
> This is a bit fragile.  You're assuming that pmd_page() always returns
> a head page, and that's only true today because I looked at the work
> required vs the reward and decided to cap the large folio size at PMD
> size.  If we allowed 2*PMD_SIZE (eg 4MB on x86), pmd_page() would not
> return a head page.  There is a small amount of demand for > PMD size
> large folio support, so I suspect we will want to do this eventually.
> I'm not particularly trying to do these conversions, but it would be
> good to not add more assumptions that pmd_page() returns a head page.

Makes sense.  Actually, IIUC arm64's CONT_PMD pages can already make that
not a head page.

The code should still be correct, though.  AFAIU what I need to do then is
renaming the first field of record_subpages() (s/head/base/) in the next
version, or just keep it the old one ("page"), then update the commit
message.

Thanks,

-- 
Peter Xu