[PATCH net-next RFC v1 01/10] iov_iter: Skip copy in memcpy_to_page if src==dst

[Boris Pismenny]

When using direct data placement the NIC writes some of the payload
directly to the destination buffer, and constructs the SKB such that it
points to this data. As a result, the skb_copy datagram_iter call will
attempt to copy data when it is not necessary.

This patch adds a check to avoid this copy, and a static_key to enabled
it when TCP direct data placement is possible.

Signed-off-by: Boris Pismenny <borisp at mellanox.com>
Signed-off-by: Ben Ben-Ishay <benishay at mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz at mellanox.com>
Signed-off-by: Yoray Zack <yorayz at mellanox.com>
---
 include/linux/uio.h |  2 ++
 lib/iov_iter.c      | 11 ++++++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 3835a8a8e9ea..036df4073737 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -284,4 +284,6 @@ int iov_iter_for_each_range(struct iov_iter *i, size_t bytes,
 			    int (*f)(struct kvec *vec, void *context),
 			    void *context);
 
+extern struct static_key_false skip_copy_enabled;
+
 #endif
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 5e40786c8f12..27856c3db28b 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -13,6 +13,9 @@
 
 #define PIPE_PARANOIA /* for now */
 
+DEFINE_STATIC_KEY_FALSE(skip_copy_enabled);
+EXPORT_SYMBOL_GPL(skip_copy_enabled);
+
 #define iterate_iovec(i, n, __v, __p, skip, STEP) {	\
 	size_t left;					\
 	size_t wanted = n;				\
@@ -470,7 +473,13 @@ static void memcpy_from_page(char *to, struct page *page, size_t offset, size_t
 static void memcpy_to_page(struct page *page, size_t offset, const char *from, size_t len)
 {
 	char *to = kmap_atomic(page);
-	memcpy(to + offset, from, len);
+
+	if (static_branch_unlikely(&skip_copy_enabled)) {
+		if (to + offset != from)
+			memcpy(to + offset, from, len);
+	} else {
+		memcpy(to + offset, from, len);
+	}
 	kunmap_atomic(to);
 }
 
-- 
2.24.1