nvmet-tcp: optimize tcp stack TX when data digest is used

If we have a 4-byte data digest to send to the wire, but we
have more data to send, set MSG_MORE to tell the stack
that more is coming.
Reviewed-by: Mark Wunderlich <mark.wunderlich@intel.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>

drivers/nvme/target/tcp.c

@@ -626,7 +626,7 @@ static int nvmet_try_send_r2t(struct nvmet_tcp_cmd *cmd, bool last_in_batch)
 	return 1;
 }
 
-static int nvmet_try_send_ddgst(struct nvmet_tcp_cmd *cmd)
+static int nvmet_try_send_ddgst(struct nvmet_tcp_cmd *cmd, bool last_in_batch)
 {
 	struct nvmet_tcp_queue *queue = cmd->queue;
 	struct msghdr msg = { .msg_flags = MSG_DONTWAIT };
@@ -636,6 +636,9 @@ static int nvmet_try_send_ddgst(struct nvmet_tcp_cmd *cmd)
 	};
 	int ret;
 
+	if (!last_in_batch && cmd->queue->send_list_len)
+		msg.msg_flags |= MSG_MORE;
+
 	ret = kernel_sendmsg(queue->sock, &msg, &iov, 1, iov.iov_len);
 	if (unlikely(ret <= 0))
 		return ret;
@@ -676,7 +679,7 @@ static int nvmet_tcp_try_send_one(struct nvmet_tcp_queue *queue,
 	}
 
 	if (cmd->state == NVMET_TCP_SEND_DDGST) {
-		ret = nvmet_try_send_ddgst(cmd);
+		ret = nvmet_try_send_ddgst(cmd, last_in_batch);
 		if (ret <= 0)
 			goto done_send;
 	}