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

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Date:   Wed, 16 Feb 2022 10:38:28 +0800
From:   Menglong Dong <menglong8.dong@...il.com>
To:     David Ahern <dsahern@...nel.org>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        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 Wed, Feb 16, 2022 at 2:47 AM David Ahern <dsahern@...nel.org> wrote:
>
> On 2/15/22 10:34 AM, Eric Dumazet wrote:
> >> 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.
>
> and it bypasses kfree_skb tracepoint which seems by design.

Do you mean it shouldn't be traced here?
According to my understanding, __kfree_skb() was used in the
beginning as skb->users aren't touched by TCP. Later,
tcp_drop() was introduced to record drop count to the socket.

Considering the skb is indeed dropped and no other event is triggered,
is it ok to trigger the kfree_skb tracepoint?

Thanks!
Menglong Dong