netdev - Re: [PATCH v9 bpf-next 07/14] net: xdp: add multi-buff support to xdp_build_skb_from

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Date:   Mon, 28 Jun 2021 14:05:42 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     bpf <bpf@...r.kernel.org>, Netdev <netdev@...r.kernel.org>,
        lorenzo.bianconi@...hat.com, David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, shayagr@...zon.com,
        "Jubran, Samih" <sameehj@...zon.com>,
        John Fastabend <john.fastabend@...il.com>,
        David Ahern <dsahern@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Saeed Mahameed <saeed@...nel.org>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Tirthendu <tirthendu.sarkar@...el.com>
Subject: Re: [PATCH v9 bpf-next 07/14] net: xdp: add multi-buff support to xdp_build_skb_from_frame

On Mon, Jun 14, 2021 at 5:51 AM Lorenzo Bianconi <lorenzo@...nel.org> wrote:
>
> Introduce xdp multi-buff support to
> __xdp_build_skb_from_frame/xdp_build_skb_from_frame
> utility routines.
>
> Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> ---
>  net/core/xdp.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index f61c63115c95..71bedf6049a1 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -582,9 +582,15 @@ struct sk_buff *__xdp_build_skb_from_frame(struct xdp_frame *xdpf,
>                                            struct sk_buff *skb,
>                                            struct net_device *dev)
>  {
> +       struct skb_shared_info *sinfo = xdp_get_shared_info_from_frame(xdpf);
>         unsigned int headroom, frame_size;
> +       int i, num_frags = 0;
>         void *hard_start;
>
> +       /* xdp multi-buff frame */
> +       if (unlikely(xdp_frame_is_mb(xdpf)))
> +               num_frags = sinfo->nr_frags;
> +
>         /* Part of headroom was reserved to xdpf */
>         headroom = sizeof(*xdpf) + xdpf->headroom;
>
> @@ -603,6 +609,13 @@ struct sk_buff *__xdp_build_skb_from_frame(struct xdp_frame *xdpf,
>         if (xdpf->metasize)
>                 skb_metadata_set(skb, xdpf->metasize);
>
> +       for (i = 0; i < num_frags; i++)
> +               skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags,
> +                               skb_frag_page(&sinfo->frags[i]),
> +                               skb_frag_off(&sinfo->frags[i]),
> +                               skb_frag_size(&sinfo->frags[i]),
> +                               xdpf->frame_sz);
> +

So this is assuming the header frame and all of the frags are using
the same size. Rather than reading the frags out and then writing them
back, why not just directly rewrite the nr_frags, add the total size
to skb->len and skb->data_len, and then update the truesize?

Actually, I think you might need to store the truesize somewhere in
addition to the data_len that you were storing in the shared info.