sock: MSG_ZEROCOPY notification coalescing

In the simple case, each sendmsg() call generates data and eventually
a zerocopy ready notification N, where N indicates the Nth successful
invocation of sendmsg() with the MSG_ZEROCOPY flag on this socket.

TCP and corked sockets can cause send() calls to append new data to an
existing sk_buff and, thus, ubuf_info. In that case the notification
must hold a range. odify ubuf_info to store a inclusive range [N..N+m]
and add skb_zerocopy_realloc() to optionally extend an existing range.

Also coalesce notifications in this common case: if a notification
[1, 1] is about to be queued while [0, 0] is the queue tail, just modify
the head of the queue to read [0, 1].

Coalescing is limited to a few TSO frames worth of data to bound
notification latency.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -444,15 +444,26 @@ enum {
  */
 struct ubuf_info {
 	void (*callback)(struct ubuf_info *, bool zerocopy_success);
-	void *ctx;
-	unsigned long desc;
-	u16 zerocopy:1;
+	union {
+		struct {
+			unsigned long desc;
+			void *ctx;
+		};
+		struct {
+			u32 id;
+			u16 len;
+			u16 zerocopy:1;
+			u32 bytelen;
+		};
+	};
 	atomic_t refcnt;
 };
 
 #define skb_uarg(SKB)	((struct ubuf_info *)(skb_shinfo(SKB)->destructor_arg))
 
 struct ubuf_info *sock_zerocopy_alloc(struct sock *sk, size_t size);
+struct ubuf_info *sock_zerocopy_realloc(struct sock *sk, size_t size,
+					struct ubuf_info *uarg);
 
 static inline void sock_zerocopy_get(struct ubuf_info *uarg)
 {