Linux Virtualization - Jun 2010 - [PATCH for-2.6.35] 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 at redhat.com>
---
 drivers/net/virtio_net.c |   15 ++++++++-------
 1 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 85615a3..e48a06f 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -563,7 +563,6 @@ 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);
 
@@ -572,12 +571,14 @@ 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_warn(&dev->dev,
+					 "Unexpected TX queue failure: %d\n",
+					 capacity);
 		}
 		return NETDEV_TX_BUSY;
 	}
-- 
1.7.1.12.g42b7f

On Thu, 2010-06-10 at 18:20 +0300, Michael S. Tsirkin
wrote:
> 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 at redhat.com>
> ---
>  drivers/net/virtio_net.c |   15 ++++++++-------
>  1 files changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 85615a3..e48a06f 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -563,7 +563,6 @@ 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);
> 
> @@ -572,12 +571,14 @@ 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_warn(&dev->dev,
> +					 "Unexpected TX queue failure: %d\n",
> +					 capacity);
>  		}
>  		return NETDEV_TX_BUSY;
>  	}
It is not clear to me how xmit_skb() can return -ENOMEM.
xmit_skb() calls virtqueue_add_buf_gfp() which can return -ENOSPC.
Even vring_add_indirect() doesn't return -ENOMEM on kmalloc failure.

Thanks
Sridhar