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

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Date:	Mon, 21 Mar 2011 20:02:21 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jesse Gross <jesse@...ira.com>
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	Daniel Lezcano <daniel.lezcano@...e.fr>,
	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

Jesse Gross <jesse@...ira.com> writes:

> On Mon, Mar 21, 2011 at 3:58 PM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>> Stephen Hemminger <shemminger@...tta.com> writes:
>>
>>> On Wed, 16 Mar 2011 17:19:14 +0100
>>> Daniel Lezcano <daniel.lezcano@...e.fr> wrote:
>>>
>>>> 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;
>>>>
>>>>
>>>
>>> Then it is up to macvlan to do the same thing as bridge code.
>>>
>>> static inline unsigned packet_length(const struct sk_buff *skb)
>>> {
>>>       return skb->len - (skb->protocol == htons(ETH_P_8021Q) ? VLAN_HLEN : 0);
>>> }
>>
>> Which is incorrect in this context.  skb->len at this point in the code
>> includes the ethernet header, as we are down below dev_queue_xmit.
>>
>> The test really does need to be dev->mtu + dev->hard_header_len.
>>
>> As for conditionally include the VLAN_HLEN that is likely doable but I
>> think if we are being pedantic that case needs to deal with accelerated
>> vlan headers.
>
> If vlan acceleration is in use then the protocol is not ETH_P_8021Q
> and the tag is not included in skb->len.

Except for the case of double tagged packets.

Eric

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