virtio_net: fix oom handling on tx

virtio net will never try to overflow the TX ring, so the only reason
add_buf may fail is out of memory. Thus, we can not stop the
device until some request completes - there's no guarantee anything
at all is outstanding.

Make the error message clearer as well: error here does not
indicate queue full.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (...and avoid TX_BUSY)
Cc: stable@kernel.org  # .34.x (s/virtqueue_/vi->svq->vq_ops->/)
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -562,7 +562,6 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 	struct virtnet_info *vi = netdev_priv(dev);
 	int capacity;
 
-again:
 	/* Free up any pending old buffers before queueing new ones. */
 	free_old_xmit_skbs(vi);
 
@@ -571,14 +570,20 @@ again:
 
 	/* This can happen with OOM and indirect buffers. */
 	if (unlikely(capacity < 0)) {
-		netif_stop_queue(dev);
-		dev_warn(&dev->dev, "Unexpected full queue\n");
-		if (unlikely(!virtqueue_enable_cb(vi->svq))) {
-			virtqueue_disable_cb(vi->svq);
-			netif_start_queue(dev);
-			goto again;
+		if (net_ratelimit()) {
+			if (likely(capacity == -ENOMEM)) {
+				dev_warn(&dev->dev,
+					 "TX queue failure: out of memory\n");
+			} else {
+				dev->stats.tx_fifo_errors++;
+				dev_warn(&dev->dev,
+					 "Unexpected TX queue failure: %d\n",
+					 capacity);
+			}
 		}
-		return NETDEV_TX_BUSY;
+		dev->stats.tx_dropped++;
+		kfree_skb(skb);
+		return NETDEV_TX_OK;
 	}
 	virtqueue_kick(vi->svq);