Subject: Re: [PATCH net-next] tun: return NET_XMIT_DROP for dropped packets

From: Jason Wang <[email-masked]>
Date: Wed, 19 Nov 2014 11:09:33 +0800

On 11/19/2014 12:53 AM, Amos Kong wrote:
> On Tue, Nov 18, 2014 at 01:20:41PM +0800, Jason Wang wrote:
>> After commit 5d097109257c03a71845729f8db6b5770c4bbedc
>> ("tun: only queue packets on device"), NETDEV_TX_OK was returned for
>> dropped packets. This will confuse pktgen since dropped packets were
>> counted as sent ones.
>>
>> Fixing this by returning NET_XMIT_DROP to let pktgen count it as error
>> packet.
>>
>> Cc: Michael S. Tsirkin <[email-masked]>
>> Signed-off-by: Jason Wang <[email-masked]>
>> ---
>>  drivers/net/tun.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index e3fa65a..ac53a73 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -819,7 +819,7 @@ drop:
>>  	skb_tx_error(skb);
>>  	kfree_skb(skb);
>>  	rcu_read_unlock();
>> -	return NETDEV_TX_OK;
>> +	return NET_XMIT_DROP;
> Quoted from linux/drivers/firewire/net.c:
>
>   /*
>    * FIXME: According to a patch from 2003-02-26, "returning non-zero
>    * causes serious problems" here, allegedly.  Before that patch,
>    * -ERRNO was returned which is not appropriate under Linux 2.6.
>    * Perhaps more needs to be done?  Stop the queue in serious
>    * conditions and restart it elsewhere?
>    */
>
> I saw many drivers return NETDEV_TX_OK in xmit for drop packets, eg: virtio_net.c

Well, I think you miss some thing:

- Virtio-net only drop packets when there's a bug in either driver or
hypervisor. Other drivers only drop the bad packets. For pktgen, we can
make sure the packet is good.
- Most of the drivers (included virtio-net but not tun) will stop txq
before the ring is full, this could be detected by pktgen
- Tun keep accepting packets and dropping them even if the socket
receive queue is full.

So we really need NET_XMIT_DROP here to let pktgen know the packets were
not sent correctly.