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

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Date:   Thu, 14 Dec 2017 01:10:42 +0100
From:   Stefano Brivio <sbrivio@...hat.com>
To:     Matthias Schiffer <mschiffer@...verse-factory.net>
Cc:     "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 Thu, 14 Dec 2017 00:57:32 +0100
Matthias Schiffer <mschiffer@...verse-factory.net> wrote:

> As you note, there is another occurrence of this calculation in
> vxlan_config_apply():
> 
> 
> [...]
>         if (lowerdev) {
> [...]
>                 max_mtu = lowerdev->mtu - (use_ipv6 ? VXLAN6_HEADROOM :
>                                            VXLAN_HEADROOM);
>         }
> 
>         if (dev->mtu > max_mtu)
>                 dev->mtu = max_mtu;
> [...]
> 
> 
> Unless I'm overlooking something, this should already do the same thing and
> your patch is redundant.

The code above sets max_mtu, and only if dev->mtu exceeds that, the
latter is then clamped.

What my patch does is to actually set dev->mtu to that value, no matter
what's the previous value set by ether_setup() (only on creation, and
only if lowerdev is there), just like the previous behaviour used to be.

Let's consider these two cases, on the existing code:

1. lowerdev->mtu is 1500:
   - ether_setup(), called by vxlan_setup(), sets dev->mtu to 1500
   - here max_mtu is 1450
   - we enter the second if clause above (dev->mtu > max_mtu)
   - at the end of vxlan_config_apply(), dev->mtu will be 1450

which is consistent with the previous behaviour.

2. lowerdev->mtu is 9000:
   - ether_setup(), called by vxlan_setup(), sets dev->mtu to 1500
   - here max_mtu is 8950
   - we do not enter the second if clause above (dev->mtu < max_mtu)
   - at the end of vxlan_config_apply(), dev->mtu will still be 1500

which is not consistent with the previous behaviour, where it used to
be 8950 instead.

-- 
Stefano