netdev - Re: [Patch bpf-next v3 4/5] skmsg: use skb ext instead of TCP_SKB

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Date:   Mon, 15 Feb 2021 16:28:03 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, duanxiongchun@...edance.com,
        Dongdong Wang <wangdongdong.6@...edance.com>,
        jiang.wang@...edance.com, Cong Wang <cong.wang@...edance.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Lorenz Bauer <lmb@...udflare.com>
Subject: Re: [Patch bpf-next v3 4/5] skmsg: use skb ext instead of TCP_SKB_CB

On Mon, Feb 15, 2021 at 3:57 PM John Fastabend <john.fastabend@...il.com> wrote:
>
> For TCP case we can continue to use CB and not pay the price. For UDP
> and AF_UNIX we can do the extra alloc.

I see your point, but specializing TCP case does not give much benefit
here, the skmsg code would have to check skb->protocol etc. to decide
whether to use TCP_SKB_CB() or skb_ext:

if (skb->protocol == ...)
  TCP_SKB_CB(skb) = ...;
else
  ext = skb_ext_find(skb);

which looks ugly to me. And I doubt skb->protocol alone is sufficient to
distinguish TCP, so we may end up having more checks above.

So do you really want to trade code readability with an extra alloc?

>
> The use in tcf_classify_ingress is a miss case so not the common path. If
> it is/was in the common path I would suggest we rip it out.
>

Excellent point, what about nf_bridge_unshare()? It is a common path
for bridge netfilter, which is also probably why skb ext was introduced
(IIRC). secpath_set() seems on a common path for XFRM too.

Are you suggesting to remove them all? ;)

Thanks.