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 <bor...@mellanox.com>
Signed-off-by: Ben Ben-Ishay <benis...@mellanox.com>
Signed-off-by: Or Gerlitz <ogerl...@mellanox.com>
Signed-off-by: Yoray Zack <yor...@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 72d88566694e..05573d848ff5 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -282,4 +282,6 @@ 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 1635111c5bd2..206edb051135 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -15,6 +15,9 @@
 
 #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;                              \
@@ -476,7 +479,13 @@ 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

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