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SubjectRe: [PATCH RFC 34/39] mm/rmap: introduce folio_try_dup_anon_rmap_[pte|ptes|pmd]()
From
> -static inline void __page_dup_rmap(struct page *page, bool compound)
> +static inline int __folio_try_dup_anon_rmap(struct folio *folio,
> + struct page *page, unsigned int nr_pages,
> + struct vm_area_struct *src_vma, enum rmap_mode mode)
> {
> - VM_WARN_ON(folio_test_hugetlb(page_folio(page)));
> + int i;
>
> - if (compound) {
> - struct folio *folio = (struct folio *)page;
> + VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
>
> - VM_BUG_ON_PAGE(compound && !PageHead(page), page);
> - atomic_inc(&folio->_entire_mapcount);
> - } else {
> - atomic_inc(&page->_mapcount);
> + /*
> + * No need to check+clear for already shared PTEs/PMDs of the folio.
> + * This includes PTE mappings of (order-0) KSM folios.
> + */
> + if (likely(mode == RMAP_MODE_PTE)) {
> + for (i = 0; i < nr_pages; i++) {
> + if (PageAnonExclusive(page + i))
> + goto clear;
> + }
> + } else if (mode == RMAP_MODE_PMD) {
> + if (PageAnonExclusive(page))
> + goto clear;
> }
> + goto dup;
> +
> +clear:
> + /*
> + * If this folio may have been pinned by the parent process,
> + * don't allow to duplicate the mappings but instead require to e.g.,
> + * copy the subpage immediately for the child so that we'll always
> + * guarantee the pinned folio won't be randomly replaced in the
> + * future on write faults.
> + */
> + if (likely(!folio_is_device_private(folio) &&
> + unlikely(folio_needs_cow_for_dma(src_vma, folio))))
> + return -EBUSY;
> +
> + if (likely(mode == RMAP_MODE_PTE)) {
> + for (i = 0; i < nr_pages; i++)
> + ClearPageAnonExclusive(page + i);
> + } else if (mode == RMAP_MODE_PMD) {
> + ClearPageAnonExclusive(page);
> + }
> +
> +dup:
> + __folio_dup_rmap(folio, page, nr_pages, mode);
> + return 0;

Playing with this, I think it can be implemented more efficiently by
only looping once and optimizing for the common case that PAE is set.
Will have to do some more measurements.

--
Cheers,

David / dhildenb

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