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Date:   Thu, 20 Oct 2016 21:19:29 +0000
From:   Asbjoern Sloth Toennesen <asbjorn@...jorn.st>
To:     "David S . Miller" <davem@...emloft.net>
Cc:     R Parameswaran <parameswaran.r7@...il.com>
Subject: Re: [PATCH] net: l2tp_eth: fix max_mtu

On Thu, 20 Oct 2016 20:08:26 +0000 (UTC), ast@...jorn.biz wrote:
> diff --git a/net/l2tp/l2tp_eth.c b/net/l2tp/l2tp_eth.c
> index 965f7e3..ba82dcc 100644
> --- a/net/l2tp/l2tp_eth.c
> +++ b/net/l2tp/l2tp_eth.c
> @@ -259,6 +259,7 @@ static int l2tp_eth_create(struct net *net, u32 tunnel_id, u32 session_id, u32 p
>  		session->mtu = dev->mtu - session->hdr_len;
>  	dev->mtu = session->mtu;
>  	dev->needed_headroom += session->hdr_len;
> +	dev->max_mtu = ETH_MAX_MTU - dev->needed_headroom;

I forgot to add that subtracting dev->needed_headroom doesn't
give the exact maximum MTU, but one that is a bit too high.
It could also just be set to ETH_MAX_MTU, to indicate that
the encapsulation is not included.

Calculations like in R. Parameswaran's patch is needed for the
exact one.
https://lkml.org/lkml/2016/10/17/3

-- 
Best regards
Asbjørn Sloth Tønnesen

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