iov_iter: optimize page_copy_sane() (6daef95b) · Commits · 戴 / test

Avoid cache line miss dereferencing struct page if we can. page_copy_sane() mostly deals with order-0 pages. Extra cache line miss is visible on TCP recvmsg() calls dealing with GRO packets (typically 45 page frags are attached to one skb). Bringing the 45 struct pages into cpu cache while copying the data is not free, since the freeing of the skb (and associated page frags put_page()) can happen after cache lines have been evicted. Signed-off-by:

Eric Dumazet <edumazet@google.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by:

Al Viro <viro@zeniv.linux.org.uk>

lib/iov_iter.c

+15 −2

Original line number	Diff line number	Diff line
		@@ -861,8 +861,21 @@ EXPORT_SYMBOL(_copy_from_iter_full_nocache);

		static inline bool page_copy_sane(struct page *page, size_t offset, size_t n)
		{
		struct page *head = compound_head(page);
		size_t v = n + offset + page_address(page) - page_address(head);
		struct page *head;
		size_t v = n + offset;

		/*
		* The general case needs to access the page order in order
		* to compute the page size.
		* However, we mostly deal with order-0 pages and thus can
		* avoid a possible cache line miss for requests that fit all
		* page orders.
		*/
		if (n <= v && v <= PAGE_SIZE)
		return true;

		head = compound_head(page);
		v += (page - head) << PAGE_SHIFT;

		if (likely(n <= v && v <= (PAGE_SIZE << compound_order(head))))
		return true;

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