Subject: Re: [PATCH net v5 2/2] vhost_net: fix tx queue stuck when sendmsg fails

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 27 Dec 2020 06:20:50 -0500

On Fri, Dec 25, 2020 at 03:24:33PM +0800, wangyunjian wrote:
> From: Yunjian Wang <[email-masked]>
> 
> Currently the driver doesn't drop a packet which can't be sent by tun
> (e.g bad packet). In this case, the driver will always process the
> same packet lead to the tx queue stuck.

So not making progress on a bad packet has some advantages,
e.g. this is easier to debug.
When is it important to drop the packet and continue?

> To fix this issue:
> 1. in the case of persistent failure (e.g bad packet), the driver
>    can skip this descriptor by ignoring the error.
> 2. in the case of transient failure (e.g -ENOBUFS, -EAGAIN and -ENOMEM),
>    the driver schedules the worker to try again.
> 
> Fixes: 3a4d5c94e959 ("vhost_net: a kernel-level virtio server")

I'd just drop this tag, looks more like a feature than a bug ...

> Signed-off-by: Yunjian Wang <[email-masked]>
> Acked-by: Willem de Bruijn <[email-masked]>
> ---
>  drivers/vhost/net.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index c8784dfafdd7..01558fb2c552 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -827,14 +827,13 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
>  				msg.msg_flags &= ~MSG_MORE;
>  		}
>  
> -		/* TODO: Check specific error and bomb out unless ENOBUFS? */
>  		err = sock->ops->sendmsg(sock, &msg, len);
> -		if (unlikely(err < 0)) {
> +		if (unlikely(err == -EAGAIN || err == -ENOMEM || err == -ENOBUFS)) {
>  			vhost_discard_vq_desc(vq, 1);
>  			vhost_net_enable_vq(net, vq);
>  			break;
>  		}
> -		if (err != len)
> +		if (err >= 0 && err != len)
>  			pr_debug("Truncated TX packet: len %d != %zd\n",
>  				 err, len);
>  done:
> @@ -922,7 +921,6 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
>  			msg.msg_flags &= ~MSG_MORE;
>  		}
>  
> -		/* TODO: Check specific error and bomb out unless ENOBUFS? */
>  		err = sock->ops->sendmsg(sock, &msg, len);
>  		if (unlikely(err < 0)) {
>  			if (zcopy_used) {
> @@ -931,11 +929,13 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
>  				nvq->upend_idx = ((unsigned)nvq->upend_idx - 1)
>  					% UIO_MAXIOV;
>  			}
> -			vhost_discard_vq_desc(vq, 1);
> -			vhost_net_enable_vq(net, vq);
> -			break;
> +			if (err == -EAGAIN || err == -ENOMEM || err == -ENOBUFS) {
> +				vhost_discard_vq_desc(vq, 1);
> +				vhost_net_enable_vq(net, vq);
> +				break;
> +			}
>  		}
> -		if (err != len)
> +		if (err >= 0 && err != len)
>  			pr_debug("Truncated TX packet: "
>  				 " len %d != %zd\n", err, len);
>  		if (!zcopy_used)
> -- 
> 2.23.0