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

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Message-ID: <20110315111756.64b5f4b1@nehalam>
Date:	Tue, 15 Mar 2011 11:17:56 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Daniel Lezcano <daniel.lezcano@...e.fr>
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 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 and macvlan device should
have same MTU as underlying device. If the feature/MTU flags
were correct, then the path calling macvlan should be respecting
the MTU.



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