iov_iter: optimize page_copy_sane()

commit 6daef95b8c914866a46247232a048447fff97279 upstream.

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>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index a0d1cd88f903f42e6c128b3daa8f065bfda19269..b396d328a7643b7c1984b8e87df9da88c45e7470 100644 (file)
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -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;