netdev - Re: [PATCH][v3] dev : fix mtu check when TSO is enabled

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Date:	Wed, 16 Mar 2011 17:19:14 +0100
From:	Daniel Lezcano <daniel.lezcano@...e.fr>
To:	Stephen Hemminger <shemminger@...tta.com>
CC:	David Miller <davem@...emloft.net>, eric.dumazet@...il.com,
	kaber@...sh.net, nightnord@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH][v3] dev : fix mtu check when TSO is enabled

On 03/16/2011 04:35 PM, Stephen Hemminger wrote:
> On Wed, 16 Mar 2011 14:56:09 +0100
> Daniel Lezcano<daniel.lezcano@...e.fr>  wrote:
>
>> On 03/15/2011 07:17 PM, Stephen Hemminger wrote:
>>> On Tue, 15 Mar 2011 14:57:40 +0100
>>> Daniel Lezcano<daniel.lezcano@...e.fr>   wrote:
>>>
>>>> On 03/15/2011 12:59 AM, David Miller wrote:
>>>>> From: Daniel Lezcano<daniel.lezcano@...e.fr>
>>>>> Date: Mon, 14 Mar 2011 21:39:50 +0100
>>>>>
>>>>>> +	len = dev->mtu + dev->hard_header_len + VLAN_HLEN;
>>>>>> +	if (skb->len<    len)
>>>>>> +		return true;
>>>>> This is not a correct translation of the original test:
>>>>>
>>>>>> -		     (skb->len>    (dev->mtu + dev->hard_header_len + VLAN_HLEN)))) {
>>>>> You need to use "<=" in your version, which currently rejects all
>>>>> full sized frames. :-)
>>>> Right, thanks.
>>>>
>>>>>> +
>>>>>> +	/* if TSO is enabled, we don't care about the length as the packet
>>>>>> +	 * could be forwarded without being segmented before
>>>>>> +	 */
>>>>>> +	if (skb->dev&&    skb->dev->features&    NETIF_F_TSO)
>>>>>> +		return true;
>>>>> I am trying to understand why you aren't simply checking also if this
>>>>> is a segmented frame?  Perhaps skb_is_gso()&&    device has NETIF_F_TSO
>>>>> set?
>>>> Maybe I am misunderstanding but the packet was forwarded by another device.
>>>> In our case from macvlan:
>>>>
>>>> macvlan_start_xmit
>>>>        macvlan_queue_xmit
>>>>            dest->forward
>>>>                dev_skb_forward
>>>>
>>>> When we reached dev_skb_forward, that means we passed through
>>>> dev_hard_start_xmit where the packet was already segmented so we should
>>>> exit at the first test (skb->len<   len). I don't see the point of adding
>>>> the skb_is_gso.
>>>> But maybe I am missing something, can you explain ?
>>> The macvlan device only has one downstream device (slave).
>>> If kernel is working properly, macvlan device should have a subset
>>> of the features of the underlying device
>> Right, dev->features = lowerdev->features&  MACVLAN_FEATURES
>>
>>> and macvlan device should
>>> have same MTU as underlying device.
>> Right,
>>
>> ...
>>
>>    if (!tb[IFLA_MTU])
>>           dev->mtu = lowerdev->mtu;
>>
>> ...
>>> If the feature/MTU flags
>>> were correct, then the path calling macvlan should be respecting
>>> the MTU.
>> But if the TSO is enabled on the macvlan (inherited from eg e1000), the
>> packet won't be fragmented to the mtu size no ?
> That is the responsiblity of the hardware that receives the packet.
> Macvlan should be passing it through to the lowerdev and since the hardware
> supports TSO, it will fragment it.

Ok, but in the case the macvlan is in bridge mode, the dev_skb_forward 
function will forward the packet (which is not fragmented) to to another 
macvlan port without going through the hardware driver. In this 
function, the packet length is checked against the mtu size and of 
course the packet is dropped in case the lower device support the TSO 
(if the packet is larger than the mtu size). Dave suggested to check 
skb_is_gso and against the TSO feature of the macvlan but I don't 
understand why we should check skb_is_gso too.

	if (skb_is_gso(skb)&&  (skb->dev&&  skb->dev->features&  NETIF_F_TSO))
		return true;


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