netdev - Re: [PATCH net-next 3/4] net: Support GRO/GSO fraglist chaining.

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Message-ID: <CA+FuTScnux23Gj1WTEXHmZkiFG3RQsgmSz19TOWdWByM4Rd15Q@mail.gmail.com>
Date:   Wed, 18 Dec 2019 11:02:39 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Steffen Klassert <steffen.klassert@...unet.com>
Cc:     David Miller <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 3/4] net: Support GRO/GSO fraglist chaining.

On Wed, Dec 18, 2019 at 8:35 AM Steffen Klassert
<steffen.klassert@...unet.com> wrote:
>
> This patch adds the core functions to chain/unchain
> GSO skbs at the frag_list pointer. This also adds
> a new GSO type SKB_GSO_FRAGLIST and a is_flist
> flag to napi_gro_cb which indicates that this
> flow will be GROed by fraglist chaining.
>
> Signed-off-by: Steffen Klassert <steffen.klassert@...unet.com>

> +struct sk_buff *skb_segment_list(struct sk_buff *skb,
> +                                netdev_features_t features,
> +                                unsigned int offset)
> +{
> +       struct sk_buff *list_skb = skb_shinfo(skb)->frag_list;
> +       unsigned int tnl_hlen = skb_tnl_header_len(skb);
> +       unsigned int delta_truesize = 0;
> +       unsigned int delta_len = 0;
> +       struct sk_buff *tail = NULL;
> +       struct sk_buff *nskb;
> +
> +       skb_push(skb, -skb_network_offset(skb) + offset);
> +
> +       skb_shinfo(skb)->frag_list = NULL;
> +
> +       do {
> +               nskb = list_skb;
> +               list_skb = list_skb->next;
> +
> +               if (!tail)
> +                       skb->next = nskb;
> +               else
> +                       tail->next = nskb;
> +
> +               tail = nskb;
> +
> +               delta_len += nskb->len;
> +               delta_truesize += nskb->truesize;
> +
> +               skb_push(nskb, -skb_network_offset(nskb) + offset);
> +
> +               if (!secpath_exists(nskb))
> +                       __skb_ext_copy(nskb, skb);

Of all the possible extensions, why is this only relevant to secpath?

More in general, this function open codes a variety of skb fields that
carry over from skb to nskb. How did you select this subset of fields?

> +
> +               memcpy(nskb->cb, skb->cb, sizeof(skb->cb));
> +
> +               nskb->tstamp = skb->tstamp;
> +               nskb->dev = skb->dev;
> +               nskb->queue_mapping = skb->queue_mapping;
> +
> +               nskb->mac_len = skb->mac_len;
> +               nskb->mac_header = skb->mac_header;
> +               nskb->transport_header = skb->transport_header;
> +               nskb->network_header = skb->network_header;
> +               skb_dst_copy(nskb, skb);
> +
> +               skb_headers_offset_update(nskb, skb_headroom(nskb) - skb_headroom(skb));
> +               skb_copy_from_linear_data_offset(skb, -tnl_hlen,
> +                                                nskb->data - tnl_hlen,
> +                                                offset + tnl_hlen);
> +
> +               if (skb_needs_linearize(nskb, features) &&
> +                   __skb_linearize(nskb))
> +                       goto err_linearize;
> +
> +       } while (list_skb);
> +
> +       skb->truesize = skb->truesize - delta_truesize;
> +       skb->data_len = skb->data_len - delta_len;
> +       skb->len = skb->len - delta_len;
> +       skb->ip_summed = nskb->ip_summed;
> +       skb->csum_level = nskb->csum_level;

This changed from the previous version, where nskb inherited ip_summed
and csum_level from skb. Why is that?