netdev - Re: GSO and IPv4 forwarding

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Message-ID: <AANLkTik+s4CHgPuY6uhe-0aD=WJs=UHhoEdok4AyTdYJ@mail.gmail.com>
Date:	Tue, 26 Oct 2010 22:26:07 +0200
From:	Kevin Wilson <wkevils@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: GSO and IPv4 forwarding

Hi,
Thanks a lot for your quick answer, I appreciate it (and did not
expect it to be so quick!)

Can someone please explain in 2-3 short sentences Why GRO can be
forwarded and LRO cannot be forwarded ?
rgs,
Kevin

On Tue, Oct 26, 2010 at 10:15 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le mardi 26 octobre 2010 à 22:02 +0200, Kevin Wilson a écrit :
>> Hi,
>> When we set a netdevice to support forwarding, we disable LRO.
>> This is done because we don't want to forward an SKB which has been
>> processed by LRO.
>>
>> This is done in inet_forward_change() in net/ipv4/devinet.c:
>> We call dev_disable_lro(dev) in this method, when setting forwarding
>> for the device.
>> As a result, in ip_forward(), the packet will be dropped. (because
>> skb_warn_if_lro(), called by this method,  returns TRUE)
>>
>> My question is:
>> dev_disable_lro(dev) disable the LRO feature (NETIF_F_LRO) of the
>> device. But suppose I have a device where GRO is enabled (and LRO is
>> not). And let's say I set forwarding on this device.
>>
>> it seems to me that is such case, calling dev_disable_lro(dev)  in
>> net_forward_change() to disable the LRO feature of the device (which
>> is already disabled) is not enough, and in such case , GRO packets,
>> which want to be forwarded,  will **not** be dropped in ip_forward().
>> (since kb_warn_if_lro() will return false in this case)
>>
>> Is it so ? I am ready to send a patch fixing it, but I am a newbie in
>> kernel, so I want to ask first.
>
> GRO packets can be forwarded just fine.
>
>
>
>
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