drbd: improve resync request throttling due to sendbuf size

If we throttle resync because the socket sendbuffer is filling up,
tell TCP about it, so it may expand the sendbuffer for us.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_worker.c

@@ -592,7 +592,7 @@ static int make_resync_request(struct drbd_device *const device, int cancel)
 	const sector_t capacity = drbd_get_capacity(device->this_bdev);
 	int max_bio_size;
 	int number, rollback_i, size;
-	int align, queued, sndbuf;
+	int align, requeue = 0;
 	int i = 0;
 
 	if (unlikely(cancel))
@@ -619,17 +619,22 @@ static int make_resync_request(struct drbd_device *const device, int cancel)
 		goto requeue;
 
 	for (i = 0; i < number; i++) {
-		/* Stop generating RS requests, when half of the send buffer is filled */
+		/* Stop generating RS requests when half of the send buffer is filled,
+		 * but notify TCP that we'd like to have more space. */
 		mutex_lock(&connection->data.mutex);
 		if (connection->data.socket) {
-			queued = connection->data.socket->sk->sk_wmem_queued;
-			sndbuf = connection->data.socket->sk->sk_sndbuf;
-		} else {
-			queued = 1;
-			sndbuf = 0;
-		}
+			struct sock *sk = connection->data.socket->sk;
+			int queued = sk->sk_wmem_queued;
+			int sndbuf = sk->sk_sndbuf;
+			if (queued > sndbuf / 2) {
+				requeue = 1;
+				if (sk->sk_socket)
+					set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
+			}
+		} else
+			requeue = 1;
 		mutex_unlock(&connection->data.mutex);
-		if (queued > sndbuf / 2)
+		if (requeue)
 			goto requeue;
 
 next_sector: