Re: [PATCH v1 04/18] mm: track mapcount of large folios in single value

[Yin, Fengwei]

On 4/10/2024 3:22 AM, David Hildenbrand wrote:
> Let's track the mapcount of large folios in a single value. The mapcount of
> a large folio currently corresponds to the sum of the entire mapcount and
> all page mapcounts.
>
> This sum is what we actually want to know in folio_mapcount() and it is
> also sufficient for implementing folio_mapped().
>
> With PTE-mapped THP becoming more important and more widely used, we want
> to avoid looping over all pages of a folio just to obtain the mapcount
> of large folios. The comment "In the common case, avoid the loop when no
> pages mapped by PTE" in folio_total_mapcount() does no longer hold for
> mTHP that are always mapped by PTE.
>
> Further, we are planning on using folio_mapcount() more
> frequently, and might even want to remove page mapcounts for large
> folios in some kernel configs. Therefore, allow for reading the mapcount of
> large folios efficiently and atomically without looping over any pages.
>
> Maintain the mapcount also for hugetlb pages for simplicity. Use the new
> mapcount to implement folio_mapcount() and folio_mapped(). Make
> page_mapped() simply call folio_mapped(). We can now get rid of
> folio_large_is_mapped().
>
> _nr_pages_mapped is now only used in rmap code and for debugging
> purposes. Keep folio_nr_pages_mapped() around, but document that its use
> should be limited to rmap internals and debugging purposes.
>
> This change implies one additional atomic add/sub whenever
> mapping/unmapping (parts of) a large folio.
>
> As we now batch RMAP operations for PTE-mapped THP during fork(),
> during unmap/zap, and when PTE-remapping a PMD-mapped THP, and we adjust
> the large mapcount for a PTE batch only once, the added overhead in the
> common case is small. Only when unmapping individual pages of a large folio
> (e.g., during COW), the overhead might be bigger in comparison, but it's
> essentially one additional atomic operation.
>
> Note that before the new mapcount would overflow, already our refcount
> would overflow: each mapping requires a folio reference. Extend the
> focumentation of folio_mapcount().
>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Reviewed-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>