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

Zi Yan
2023-08-09T15:45:42Z

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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@linux-foundation.org>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Yin Fengwei <fengwei.yin@intel.com>
> Cc: Yang Shi <shy828301@gmail.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Signed-off-by: David Hildenbrand <david@redhat.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@nvidia.com>

--
Best Regards,
Yan, Zi

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