Re: [PATCH] IPv6: Set SIT tunnel hard_header_len to zero

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To: Oliver Herms <oliver.peter.herms@gmail.com>
Cc: Network Development <netdev@vger.kernel.org>,
	David Miller <davem@davemloft.net>,
	Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	Jakub Kicinski <kuba@kernel.org>
Subject: Re: [PATCH] IPv6: Set SIT tunnel hard_header_len to zero
Date: Tue, 3 Nov 2020 13:42:17 -0500	[thread overview]
Message-ID: <CA+FuTSdf35EqALizep-65_sF46pk46_RqmEhS9gjw_5rF5cr1Q@mail.gmail.com> (raw)
In-Reply-To: <20201103104133.GA1573211@tws>

On Tue, Nov 3, 2020 at 5:41 AM Oliver Herms
<oliver.peter.herms@gmail.com> 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.

Thanks. Yes, this is long overdue.

The hard_header_len issue was also recently discussed in the context
of GRE in commit fdafed459998 ("ip_gre: set dev->hard_header_len and
dev->needed_headroom properly").

Question is whether we should reserve room in needed_headroom instead.
AFAIK this existing update logic in ip6_tnl_xmit is sufficient

"
        /* Calculate max headroom for all the headers and adjust
         * needed_headroom if necessary.
         */
        max_headroom = LL_RESERVED_SPACE(dst->dev) + sizeof(struct ipv6hdr)
                        + dst->header_len + t->hlen;
        if (max_headroom > dev->needed_headroom)
                dev->needed_headroom = max_headroom;
"

> Fixes: c54419321455 ("GRE: Refactor GRE tunneling code.")

How did you arrive at this SHA1?

> Signed-off-by: Oliver Herms <oliver.peter.herms@gmail.com>
> ---
>  net/ipv6/sit.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/net/ipv6/sit.c b/net/ipv6/sit.c
> index 5e2c34c0ac97..5e7983cb6154 100644
> --- a/net/ipv6/sit.c
> +++ b/net/ipv6/sit.c
> @@ -1128,7 +1128,6 @@ static void ipip6_tunnel_bind_dev(struct net_device *dev)
>         if (tdev && !netif_is_l3_master(tdev)) {
>                 int t_hlen = tunnel->hlen + sizeof(struct iphdr);
>
> -               dev->hard_header_len = tdev->hard_header_len + sizeof(struct iphdr);
>                 dev->mtu = tdev->mtu - t_hlen;
>                 if (dev->mtu < IPV6_MIN_MTU)
>                         dev->mtu = IPV6_MIN_MTU;
> @@ -1426,7 +1425,6 @@ static void ipip6_tunnel_setup(struct net_device *dev)
>         dev->priv_destructor    = ipip6_dev_free;
>
>         dev->type               = ARPHRD_SIT;
> -       dev->hard_header_len    = LL_MAX_HEADER + t_hlen;
>         dev->mtu                = ETH_DATA_LEN - t_hlen;
>         dev->min_mtu            = IPV6_MIN_MTU;
>         dev->max_mtu            = IP6_MAX_MTU - t_hlen;
> --
> 2.25.1
>