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 13:56:06 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     wangyunjian <wangyunjian@...wei.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


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.

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)