netdev - Re: [net-next 2/3] ip_gre: fix erspan tunnel mtu calculation

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Date:   Tue, 10 Oct 2017 13:27:03 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     William Tu <u9012063@...il.com>
Cc:     network dev <netdev@...r.kernel.org>, therbert@...gle.com,
        davem <davem@...emloft.net>
Subject: Re: [net-next 2/3] ip_gre: fix erspan tunnel mtu calculation

On Tue, Oct 10, 2017 at 4:47 AM, William Tu <u9012063@...il.com> wrote:
> Remove the unnecessary -4 and +4 bytes at mtu and headroom calculation.
> In addition, erspan uses fixed 8-byte gre header, so add ERSPAN_GREHDR_LEN
> macro for better readability.
>
> Now tunnel->hlen = grehdr(8) + erspanhdr(8) = 16 byte.
> The mtu should be ETH_DATA_LEN - 16 - iph(20) = 1464.
> After the ip_tunnel_bind_dev(), the mtu is adjusted to
> 1464 - 14 (dev->hard_header_len) = 1450.
> The maximum skb->len the erspan tunnel can carry without
> being truncated is 1450 + 14 = 1464 byte.
>
> Signed-off-by: William Tu <u9012063@...il.com>
> Cc: Xin Long <lucien.xin@...il.com>
> ---
>  include/net/erspan.h |  1 +
>  net/ipv4/ip_gre.c    | 11 +++++------
>  2 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/include/net/erspan.h b/include/net/erspan.h
> index ca94fc86865e..e28294e248d0 100644
> --- a/include/net/erspan.h
> +++ b/include/net/erspan.h
> @@ -28,6 +28,7 @@
>   */
>
>  #define ERSPAN_VERSION 0x1
> +#define ERSPAN_GREHDR_LEN 8    /* ERSPAN has fixed 8-byte GRE header */
>
>  #define VER_MASK       0xf000
>  #define VLAN_MASK      0x0fff
> diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
> index 286065c35959..6e6e4c4811cc 100644
> --- a/net/ipv4/ip_gre.c
> +++ b/net/ipv4/ip_gre.c
> @@ -569,8 +569,7 @@ static void erspan_fb_xmit(struct sk_buff *skb, struct net_device *dev,
>
>         key = &tun_info->key;
>
> -       /* ERSPAN has fixed 8 byte GRE header */
> -       tunnel_hlen = 8 + sizeof(struct erspanhdr);
> +       tunnel_hlen = ERSPAN_GREHDR_LEN + sizeof(struct erspanhdr);
>
>         rt = prepare_fb_xmit(skb, dev, &fl, tunnel_hlen);
>         if (!rt)
> @@ -591,7 +590,7 @@ static void erspan_fb_xmit(struct sk_buff *skb, struct net_device *dev,
>         erspan_build_header(skb, tunnel_id_to_key32(key->tun_id),
>                             ntohl(md->index), truncate);
>
> -       gre_build_header(skb, 8, TUNNEL_SEQ,
> +       gre_build_header(skb, ERSPAN_GREHDR_LEN, TUNNEL_SEQ,
>                          htons(ETH_P_ERSPAN), 0, htonl(tunnel->o_seqno++));
>
>         df = key->tun_flags & TUNNEL_DONT_FRAGMENT ?  htons(IP_DF) : 0;
> @@ -1242,14 +1241,14 @@ static int erspan_tunnel_init(struct net_device *dev)
>         struct ip_tunnel *tunnel = netdev_priv(dev);
>         int t_hlen;
>
> -       tunnel->tun_hlen = 8;
> +       tunnel->tun_hlen = ERSPAN_GREHDR_LEN;
>         tunnel->parms.iph.protocol = IPPROTO_GRE;
>         tunnel->hlen = tunnel->tun_hlen + tunnel->encap_hlen +
>                        sizeof(struct erspanhdr);
>         t_hlen = tunnel->hlen + sizeof(struct iphdr);
>
> -       dev->needed_headroom = LL_MAX_HEADER + t_hlen + 4;
> -       dev->mtu = ETH_DATA_LEN - t_hlen - 4;
> +       dev->needed_headroom = LL_MAX_HEADER + t_hlen;
> +       dev->mtu = ETH_DATA_LEN - t_hlen;
1. I guess '+4-4' stuff was copied from __gre_tunnel_init(), I'm thinking
it may be there for some reason.

2. 'dev->needed_headroom =' and 'dev->mtu =' are really needed ?
As I've seen both will be updated in .newlink:
ipgre_newlink() -> ip_tunnel_newlink() -> ip_tunnel_bind_dev()

>         dev->features           |= GRE_FEATURES;
>         dev->hw_features        |= GRE_FEATURES;
>         dev->priv_flags         |= IFF_LIVE_ADDR_CHANGE;
> --
> 2.7.4
>