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Date:   Thu, 24 Dec 2020 09:09:36 +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 1:56 PM
> To: wangyunjian <wangyunjian@...wei.com>
> Cc: netdev@...r.kernel.org; mst@...hat.com;
> willemdebruijn.kernel@...il.com; 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 下午12:37, wangyunjian wrote:
> >> -----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;
> > }
> 
> 
> -EAGAIN and -ENOBFS are fine. But I don't see how -ENOMEM can be returned.

The tun_build_skb() and tun_napi_alloc_frags() return -ENOMEM when memory
allocation fails.

Thanks

> 
> Thanks
> 
> 
> >> 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)

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