netdev - Re: [net-next PATCH 09/10] udp: Use uh->len instead of skb->len to compute checksum in segmentation

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Message-ID: <CALx6S34u5WUGqTs88YS0Oj7V=GhBybLcsT9vTSJ+SOP9pCHAOg@mail.gmail.com>
Date:	Sat, 6 Feb 2016 12:59:14 -0800
From:	Tom Herbert <tom@...bertland.com>
To:	Alexander Duyck <aduyck@...antis.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Alexander Duyck <alexander.duyck@...il.com>,
	Edward Cree <ecree@...arflare.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [net-next PATCH 09/10] udp: Use uh->len instead of skb->len to
 compute checksum in segmentation

On Fri, Feb 5, 2016 at 3:28 PM, Alexander Duyck <aduyck@...antis.com> wrote:
> The segmentation code was having to do a bunch of work to pull the
> skb->len and strip the udp header offset before the value could be used to
> adjust the checksum.  Instead of doing all this work we can just use the
> value that goes into uh->len since that is the correct value with the
> correct byte order that we need anyway.  By using this value we can save
> ourselves a bunch of pain as there is no need to do multiple byte swaps.
>
> Signed-off-by: Alexander Duyck <aduyck@...antis.com>

Acked-by: Tom Herbert <tom@...bertland.com>

> ---
>  net/ipv4/udp_offload.c |   28 +++++++++++++---------------
>  1 file changed, 13 insertions(+), 15 deletions(-)
>
> diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> index 9e4816fc9927..56c4c8b88b28 100644
> --- a/net/ipv4/udp_offload.c
> +++ b/net/ipv4/udp_offload.c
> @@ -32,20 +32,23 @@ static struct sk_buff *__skb_udp_tunnel_segment(struct sk_buff *skb,
>                                              netdev_features_t features),
>         __be16 new_protocol, bool is_ipv6)
>  {
> +       int tnl_hlen = skb_inner_mac_header(skb) - skb_transport_header(skb);
>         struct sk_buff *segs = ERR_PTR(-EINVAL);
>         bool remcsum, need_csum, offload_csum;
> +       struct udphdr *uh = udp_hdr(skb);
>         u16 mac_offset = skb->mac_header;
> -       int mac_len = skb->mac_len;
> -       int tnl_hlen = skb_inner_mac_header(skb) - skb_transport_header(skb);
>         __be16 protocol = skb->protocol;
> +       u16 mac_len = skb->mac_len;
>         int udp_offset, outer_hlen;
> -       unsigned int oldlen;
> -
> -       oldlen = (u16)~skb->len;
> +       u32 partial;
>
>         if (unlikely(!pskb_may_pull(skb, tnl_hlen)))
>                 goto out;
>
> +       /* adjust partial header checksum to negate old length */
> +       partial = (__force u32)uh->check + (__force u16)~uh->len;
> +
> +       /* setup inner skb. */
>         skb->encapsulation = 0;
>         __skb_pull(skb, tnl_hlen);
>         skb_reset_mac_header(skb);
> @@ -89,9 +92,7 @@ static struct sk_buff *__skb_udp_tunnel_segment(struct sk_buff *skb,
>         udp_offset = outer_hlen - tnl_hlen;
>         skb = segs;
>         do {
> -               struct udphdr *uh;
> -               int len;
> -               __be32 delta;
> +               __be16 len;
>
>                 if (remcsum)
>                         skb->ip_summed = CHECKSUM_NONE;
> @@ -105,22 +106,19 @@ static struct sk_buff *__skb_udp_tunnel_segment(struct sk_buff *skb,
>                 skb->mac_len = mac_len;
>                 skb->protocol = protocol;
>
> -               skb_push(skb, outer_hlen);
> +               __skb_push(skb, outer_hlen);
>                 skb_reset_mac_header(skb);
>                 skb_set_network_header(skb, mac_len);
>                 skb_set_transport_header(skb, udp_offset);
> -               len = skb->len - udp_offset;
> +               len = htons(skb->len - udp_offset);
>                 uh = udp_hdr(skb);
> -               uh->len = htons(len);
> +               uh->len = len;
>
>                 if (!need_csum)
>                         continue;
>
> -               delta = htonl(oldlen + len);
> -
>                 uh->check = ~csum_fold((__force __wsum)
> -                                      ((__force u32)uh->check +
> -                                       (__force u32)delta));
> +                                      ((__force u32)len + partial));
>
>                 if (skb->encapsulation || !offload_csum) {
>                         uh->check = gso_make_checksum(skb, ~uh->check);
>