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/ ;) -- Best Regards, Yan, Zi
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