nbd: enable replace socket if only one connection is configured

Nbd server with multiple connections could be upgraded since
560bc4b3 (nbd: handle dead connections). But if only one conncection
is configured, after we take down nbd server, all inflight IO
would finally timeout and return error. We could requeue them
like what we do with multiple connections and wait for new socket
in submit path.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Hou Pu <houpu@bytedance.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -395,16 +395,19 @@ static enum blk_eh_timer_return nbd_xmit_timeout(struct request *req,
 	}
 	config = nbd->config;
 
-	if (config->num_connections > 1) {
+	if (config->num_connections > 1 ||
+	    (config->num_connections == 1 && nbd->tag_set.timeout)) {
 		dev_err_ratelimited(nbd_to_dev(nbd),
 				    "Connection timed out, retrying (%d/%d alive)\n",
 				    atomic_read(&config->live_connections),
 				    config->num_connections);
 		/*
 		 * Hooray we have more connections, requeue this IO, the submit
-		 * path will put it on a real connection.
+		 * path will put it on a real connection. Or if only one
+		 * connection is configured, the submit path will wait util
+		 * a new connection is reconfigured or util dead timeout.
 		 */
-		if (config->socks && config->num_connections > 1) {
+		if (config->socks) {
 			if (cmd->index < config->num_connections) {
 				struct nbd_sock *nsock =
 					config->socks[cmd->index];
@@ -741,14 +744,12 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index)
 				dev_err(disk_to_dev(nbd->disk), "Receive data failed (result %d)\n",
 					result);
 				/*
-				 * If we've disconnected or we only have 1
-				 * connection then we need to make sure we
+				 * If we've disconnected, we need to make sure we
 				 * complete this request, otherwise error out
 				 * and let the timeout stuff handle resubmitting
 				 * this request onto another connection.
 				 */
-				if (nbd_disconnected(config) ||
-				    config->num_connections <= 1) {
+				if (nbd_disconnected(config)) {
 					cmd->status = BLK_STS_IOERR;
 					goto out;
 				}
@@ -825,7 +826,7 @@ static int find_fallback(struct nbd_device *nbd, int index)
 
 	if (config->num_connections <= 1) {
 		dev_err_ratelimited(disk_to_dev(nbd->disk),
-				    "Attempted send on invalid socket\n");
+				    "Dead connection, failed to find a fallback\n");
 		return new_index;
 	}