netdev - Re: [PATCH] IPv6: Set SIT tunnel hard_header

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Message-ID: <20201109151630.2a4579cb@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Mon, 9 Nov 2020 15:16:30 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Oliver Herms <oliver.peter.herms@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuznet@....inr.ac.ru,
        yoshfuji@...ux-ipv6.org
Subject: Re: [PATCH] IPv6: Set SIT tunnel hard_header_len to zero

On Tue, 3 Nov 2020 11:41:33 +0100 Oliver Herms wrote:
> Due to the legacy usage of hard_header_len for SIT tunnels while
> already using infrastructure from net/ipv4/ip_tunnel.c the
> calculation of the path MTU in tnl_update_pmtu is incorrect.
> This leads to unnecessary creation of MTU exceptions for any
> flow going over a SIT tunnel.
> 
> As SIT tunnels do not have a header themsevles other than their
> transport (L3, L2) headers we're leaving hard_header_len set to zero
> as tnl_update_pmtu is already taking care of the transport headers
> sizes.
> 
> This will also help avoiding unnecessary IPv6 GC runs and spinlock
> contention seen when using SIT tunnels and for more than
> net.ipv6.route.gc_thresh flows.
> 
> Fixes: c54419321455 ("GRE: Refactor GRE tunneling code.")
> Signed-off-by: Oliver Herms <oliver.peter.herms@...il.com>

Applied, thanks! Let's carry on the discussion about ipv6 tunnels,
though, it does look questionable.