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

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Message-ID: <4D7F7054.70409@free.fr>
Date:	Tue, 15 Mar 2011 14:57:40 +0100
From:	Daniel Lezcano <daniel.lezcano@...e.fr>
To:	David Miller <davem@...emloft.net>
CC:	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 03/15/2011 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 ?

Thanks
   -- Daniel
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