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

David Hildenbrand
2024-04-10T08:20:52Z

On 09.04.24 22:13, Zi Yan wrote:
> On 9 Apr 2024, at 15:22, 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().
>
> s/focumentation/documentation/ ;)

Thanks! :)

--
Cheers,

David / dhildenb