netdev - Re: [PATCH net] vxlan: Restore initial MTU setting based on lower device

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Date:   Thu, 14 Dec 2017 19:26:20 +0300
From:   Alexey Kodanev <alexey.kodanev@...cle.com>
To:     Stefano Brivio <sbrivio@...hat.com>
Cc:     Matthias Schiffer <mschiffer@...verse-factory.net>,
        "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Junhan Yan <juyan@...hat.com>, Jiri Benc <jbenc@...hat.com>,
        Hangbin Liu <haliu@...hat.com>
Subject: Re: [PATCH net] vxlan: Restore initial MTU setting based on lower
 device

On 12/14/2017 03:36 PM, Stefano Brivio wrote:
> On Thu, 14 Dec 2017 14:23:36 +0300
> Alexey Kodanev <alexey.kodanev@...cle.com> wrote:
> 
>> On 12/14/2017 03:31 AM, Stefano Brivio wrote:
...
>>
>> if we move it up in "if (lowerdev) { ..." branch we will be checking the presence
>> of "lowerdev" and also not calculating it again. Also I would check max_mtu for
>> minimum as it might happen to be negative, though unlikely corner case...
> 
> Indeed it might happen to be negative (only for IPv6, down to -2), good
> catch.
> 
> For the benefit of others: it took me a few minutes to see how this is
> *not* unrelated, because we are introducing a direct assignment of
> dev->mtu to set max_mtu, whereas earlier it was just used in
> comparisons, so it didn't matter whether it was negative.
> 
>> diff --git a/drivers/net/vxlan.c b/drivers/net/vxlan.c
>> index 19b9cc5..1000b0e 100644
>> --- a/drivers/net/vxlan.c
>> +++ b/drivers/net/vxlan.c
>> @@ -3103,6 +3103,11 @@ static void vxlan_config_apply(struct net_device *dev,
>>
>>                 max_mtu = lowerdev->mtu - (use_ipv6 ? VXLAN6_HEADROOM :
>>                                            VXLAN_HEADROOM);
>> +               if (max_mtu < ETH_MIN_MTU)
>> +                       max_mtu = ETH_MIN_MTU;
>> +
>> +               if (!changelink && !conf->mtu)
>> +                       dev->mtu = max_mtu;
> 
> I don't really have a strong preference here. On one hand, you're
> hiding this a bit from the "device creation" path. On the other hand,
> it's a bit more compact. So I'm also fine with this.
> 
> Can you perhaps submit a formal patch?
> 

OK, I'll send a patch.

Thanks,
Alexey