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

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Date:   Thu, 23 Nov 2023 14:46:46 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Ryan Roberts <ryan.roberts@....com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        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>,
        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 Thu, Nov 23, 2023 at 07:11:19PM +0000, Ryan Roberts wrote:
> Hi,
> 
> I'm not sure I've 100% understood the crossover between this series and my work
> to support arm64's contpte mappings generally for anonymous and file-backed memory.

No worry, there's no confliction.  If you worked on that it's only be
something nice on top.  Also, I'm curious if you have performance numbers,
because I'm going to do some test for hugetlb cont_ptes (which is only the
current plan), and if you got those it'll be a great baseline for me,
because it should be similar in you case even though the goal is slightly
different.

> 
> My approach is to transparently use contpte mappings when core-mm request pte
> mappings that meet the requirements; and its all based around intercepting the
> normal (non-hugetlb) helpers (e.g. set_ptes(), ptep_get() and friends). There is
> no semantic change to the core-mm. See [1]. It relies on 1) the page cache using
> large folios and 2) my "small-sized THP" series which starts using arbitrary
> sized large folios for anonymous memory [2].
> 
> If I've understood this conversation correctly there is an object called hugepd,
> which today is only supported by powerpc, but which could allow the core-mm to
> control the mapping granularity? I can see some value in exposing that control
> to core-mm in the (very) long term.

For me it's needed immediately, because hugetlb_follow_page_mask() will be
gone after the last patch.

> 
> [1] https://lore.kernel.org/all/20231115163018.1303287-1-ryan.roberts@arm.com/
> [2] https://lore.kernel.org/linux-mm/20231115132734.931023-1-ryan.roberts@arm.com/

AFAICT you haven't yet worked on gup then, after I glimpsed the above
series.

It's a matter of whether one follow_page_mask() call can fetch more than
one page* for a cont_pte entry on aarch64 for a large non-hugetlb folio
(and if this series lands, it'll be the same to hugetlb or non-hugetlb).
Now the current code can only fetch one page I think.

Thanks,

-- 
Peter Xu