netdev - Re: [PATCH net-next 01/19] net: tcp: introduce tcp_drop

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Date:   Tue, 15 Feb 2022 09:34:44 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Menglong Dong <menglong8.dong@...il.com>
Cc:     David Ahern <dsahern@...nel.org>, Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...emloft.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Menglong Dong <imagedong@...cent.com>,
        Talal Ahmad <talalahmad@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Alexander Lobakin <alobakin@...me>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Antoine Tenart <atenart@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Paolo Abeni <pabeni@...hat.com>,
        Yunsheng Lin <linyunsheng@...wei.com>,
        Arnd Bergmann <arnd@...db.de>,
        Yajun Deng <yajun.deng@...ux.dev>,
        Roopa Prabhu <roopa@...dia.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Vasily Averin <vvs@...tuozzo.com>,
        Cong Wang <cong.wang@...edance.com>,
        Luiz Augusto von Dentz <luiz.von.dentz@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        flyingpeng@...cent.com
Subject: Re: [PATCH net-next 01/19] net: tcp: introduce tcp_drop_reason()

On Tue, Feb 15, 2022 at 3:30 AM <menglong8.dong@...il.com> wrote:
>
> From: Menglong Dong <imagedong@...cent.com>
>
> For TCP protocol, tcp_drop() is used to free the skb when it needs
> to be dropped. To make use of kfree_skb_reason() and collect drop
> reasons, introduce the function tcp_drop_reason().
>
> tcp_drop_reason() will finally call kfree_skb_reason() and pass the
> drop reason to 'kfree_skb' tracepoint.
>
> PS: __kfree_skb() was used in tcp_drop(), I'm not sure if it's ok
> to replace it with kfree_skb_reason().
>
> Signed-off-by: Menglong Dong <imagedong@...cent.com>
> ---
>  net/ipv4/tcp_input.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index af94a6d22a9d..e3811afd1756 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -4684,10 +4684,19 @@ static bool tcp_ooo_try_coalesce(struct sock *sk,
>         return res;
>  }
>
> -static void tcp_drop(struct sock *sk, struct sk_buff *skb)
> +static void tcp_drop_reason(struct sock *sk, struct sk_buff *skb,
> +                           enum skb_drop_reason reason)
>  {
>         sk_drops_add(sk, skb);
> -       __kfree_skb(skb);
> +       /* why __kfree_skb() used here before, other than kfree_skb()?
> +        * confusing......

Do not add comments like that if you do not know the difference...

__kfree_skb() is used by TCP stack because it owns skb in receive
queues, and avoids touching skb->users
because it must be one already.

(We made sure not using skb_get() in TCP)

It seems fine to use kfree_skb() in tcp_drop(), it is hardly fast
path, and the added cost is pure noise.