linux-kernel - Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

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Date:   Wed, 09 Aug 2023 11:45:31 -0400
From:   Zi Yan <ziy@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-doc@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Hugh Dickins <hughd@...gle.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Ryan Roberts <ryan.roberts@....com>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yang Shi <shy828301@...il.com>
Subject: Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

On 9 Aug 2023, at 4:32, David Hildenbrand wrote:

> Let's track the total mapcount for all large folios in the first subpage.
>
> The total mapcount is what we actually want to know in folio_mapcount()
> and it is also sufficient for implementing folio_mapped(). This also
> gets rid of any "raceiness" concerns as expressed in
> folio_total_mapcount().
>
> With sub-PMD THP becoming more important and things looking promising
> that we will soon get support for such anon THP, we want to avoid looping
> over all pages of a folio just to calculate the total mapcount. Further,
> we may soon want to use the total mapcount in other context more
> frequently, so prepare for reading it efficiently and atomically.
>
> Make room for the total mapcount in page[1] of the folio by moving
> _nr_pages_mapped to page[2] of the folio: it is not applicable to hugetlb
> -- and with the total mapcount in place probably also not desirable even
> if PMD-mappable hugetlb pages could get PTE-mapped at some point -- so we
> can overlay the hugetlb fields.
>
> Note that we currently don't expect any order-1 compound pages / THP in
> rmap code, and that such support is not planned. If ever desired, we could
> move the compound mapcount to another page, because it only applies to
> PMD-mappable folios that are definitely larger. Let's avoid consuming
> more space elsewhere for now -- we might need more space for a different
> purpose in some subpages soon.
>
> Maintain the total mapcount also for hugetlb pages. Use the total mapcount
> to implement folio_mapcount(), total_mapcount(), folio_mapped() and
> page_mapped().
>
> We can now get rid of folio_total_mapcount() and
> folio_large_is_mapped(), by just inlining reading of the total mapcount.
>
> _nr_pages_mapped is now only used in rmap code, so not accidentially
> externally where it might be used on arbitrary order-1 pages. The remaining
> usage is:
>
> (1) Detect how to adjust stats: NR_ANON_MAPPED and NR_FILE_MAPPED
>  -> If we would account the total folio as mapped when mapping a
>     page (based on the total mapcount), we could remove that usage.
>
> (2) Detect when to add a folio to the deferred split queue
>  -> If we would apply a different heuristic, or scan using the rmap on
>     the memory reclaim path for partially mapped anon folios to
>     split them, we could remove that usage as well.
>
> So maybe, we can simplify things in the future and remove
> _nr_pages_mapped. For now, leave these things as they are, they need more
> thought. Hugh really did a nice job implementing that precise tracking
> after all.
>
> Note: Not adding order-1 sanity checks to the file_rmap functions for
>       now. For anon pages, they are certainly not required right now.
>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: Mike Kravetz <mike.kravetz@...cle.com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: Yin Fengwei <fengwei.yin@...el.com>
> Cc: Yang Shi <shy828301@...il.com>
> Cc: Zi Yan <ziy@...dia.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  Documentation/mm/transhuge.rst | 12 +++++-----
>  include/linux/mm.h             | 31 ++++++------------------
>  include/linux/mm_types.h       |  6 +++--
>  include/linux/rmap.h           |  7 ++++--
>  mm/debug.c                     |  4 ++--
>  mm/huge_memory.c               |  2 ++
>  mm/hugetlb.c                   |  4 ++--
>  mm/internal.h                  | 17 ++++---------
>  mm/page_alloc.c                |  4 ++--
>  mm/rmap.c                      | 44 ++++++++++++++--------------------
>  10 files changed, 52 insertions(+), 79 deletions(-)
>
LGTM. Thanks. Reviewed-by: Zi Yan <ziy@...dia.com>

--
Best Regards,
Yan, Zi

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