netdev - RE: [PATCH net v4 2/2] vhost_net: fix tx queue stuck when sendmsg fails

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Date:   Thu, 24 Dec 2020 04:37:52 +0000
From:   wangyunjian <wangyunjian@...wei.com>
To:     Jason Wang <jasowang@...hat.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "mst@...hat.com" <mst@...hat.com>,
        "willemdebruijn.kernel@...il.com" <willemdebruijn.kernel@...il.com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "Lilijun (Jerry)" <jerry.lilijun@...wei.com>,
        chenchanghu <chenchanghu@...wei.com>,
        xudingke <xudingke@...wei.com>,
        "huangbin (J)" <brian.huangbin@...wei.com>
Subject: RE: [PATCH net v4 2/2] vhost_net: fix tx queue stuck when sendmsg
 fails

> -----Original Message-----
> From: Jason Wang [mailto:jasowang@...hat.com]
> Sent: Thursday, December 24, 2020 11:10 AM
> To: wangyunjian <wangyunjian@...wei.com>; netdev@...r.kernel.org;
> mst@...hat.com; willemdebruijn.kernel@...il.com
> Cc: virtualization@...ts.linux-foundation.org; Lilijun (Jerry)
> <jerry.lilijun@...wei.com>; chenchanghu <chenchanghu@...wei.com>;
> xudingke <xudingke@...wei.com>; huangbin (J)
> <brian.huangbin@...wei.com>
> Subject: Re: [PATCH net v4 2/2] vhost_net: fix tx queue stuck when sendmsg
> fails
> 
> 
> On 2020/12/24 上午10:25, wangyunjian wrote:
> > From: Yunjian Wang <wangyunjian@...wei.com>
> >
> > 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.
> >
> > 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 -EAGAIN and -ENOMEM), the
> > driver schedules the worker to try again.
> 
> 
> I might be wrong but looking at alloc_skb_with_frags(), it returns -ENOBUFS
> actually:
> 
>      *errcode = -ENOBUFS;
>      skb = alloc_skb(header_len, gfp_mask);
>      if (!skb)
>          return NULL;

Yes, but the sock_alloc_send_pskb() returns - EAGAIN when no sndbuf space.
So there is need to check return value which is -EAGAIN or -ENOMEM or - EAGAIN?

struct sk_buff *sock_alloc_send_pskb()
{
...
	for (;;) {
...
		sk_set_bit(SOCKWQ_ASYNC_NOSPACE, sk);
		set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
		err = -EAGAIN;
		if (!timeo)
			goto failure;
...
	}
	skb = alloc_skb_with_frags(header_len, data_len, max_page_order,
				   errcode, sk->sk_allocation);
	if (skb)
		skb_set_owner_w(skb, sk);
	return skb;
...
	*errcode = err;
	return NULL;
}
> 
> Thanks
> 
> 
> >
> > Fixes: 3a4d5c94e959 ("vhost_net: a kernel-level virtio server")
> > Signed-off-by: Yunjian Wang <wangyunjian@...wei.com>
> > ---
> >   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..e76245daa7f6 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)) {
> >   			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) {
> > +				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)