linux-kernel - Re: [PATCH net-next] tun: return NET_XMIT

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Date:	Wed, 19 Nov 2014 14:46:04 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	jasowang@...hat.com
Cc:	cwang@...pensource.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, mst@...hat.com
Subject: Re: [PATCH net-next] tun: return NET_XMIT_DROP for dropped packets

From: Jason Wang <jasowang@...hat.com>
Date: Wed, 19 Nov 2014 11:15:36 +0800

> On 11/19/2014 03:53 AM, Cong Wang wrote:
>> On Mon, Nov 17, 2014 at 9:20 PM, Jason Wang <jasowang@...hat.com> 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.
>> pktgen is suspicious, it sends out packets directly without going through
>> qdisc, so it should not care about NET_XMIT_* qdisc error code?
> 
> Well, NET_XMIT_DROP has been used by some devices. I don't see any side
> effect of using this especially consider that pktgen can recognize them.
>> Looks like NETDEV_TX_OK doesn't have to mean TX is successful,
>> the comment says driver takes care of the packet, can be either dropped
>> or sent out. We might need a new code to distinguish success or failure.
> 
> Most drivers only drop bad packets when they return NETDEV_TX_OK and
> they will stop the txq before tx ring is full. This is not the case of
> tun, it never stop txq and keep accepting packets and dropping them when
> socket receive queue is full.

Agreed.

Often the issue with TX return values is lack of clear documentation
and use cases.

I've applied this patch, thanks Jason.
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