linux-kernel - Re: [PATCH RFC 34/39] mm/rmap: introduce folio_try_dup_anon_rmap

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Date:   Mon, 4 Dec 2023 18:59:37 +0100
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        Ryan Roberts <ryan.roberts@....com>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH RFC 34/39] mm/rmap: introduce
 folio_try_dup_anon_rmap_[pte|ptes|pmd]()

> -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