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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 28 Dec 2020 07:19:45 -0500

On Mon, Dec 28, 2020 at 11:55:36AM +0000, wangyunjian wrote:
> > -----Original Message-----
> > From: Michael S. Tsirkin [mailto:[email-masked]]
> > Sent: Sunday, December 27, 2020 7:21 PM
> > To: wangyunjian <[email-masked]>
> > Cc: [email-masked]; [email-masked];
> > [email-masked]; [email-masked];
> > Lilijun (Jerry) <[email-masked]>; chenchanghu
> > <[email-masked]>; xudingke <[email-masked]>; huangbin (J)
> > <[email-masked]>
> > Subject: Re: [PATCH net v5 2/2] vhost_net: fix tx queue stuck when sendmsg
> > fails
> > 
> > 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?
> 
> In the case, the VM will not be able to send packets persistently. Services of VM
> are affected.
> 
> Thanks

Well VM can always harm itself right? Just halt the CPU, services will
be affected ;)

> > 
> > 
> > > 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 ...
> 
> OK
> 
> > 
> > 
> > > 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