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References: <20221029130957.1292060-1-imagedong@tencent.com> <20221029130957.1292060-3-imagedong@tencent.com>
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From:   Eric Dumazet <edumazet@google.com>
Date:   Sat, 29 Oct 2022 08:23:36 -0700
Message-ID: <CANn89i+mK2VG2VnxPduSREHp70gJLP7oNp3SUoMW-YaSd7jJRA@mail.gmail.com>
Subject: Re: [PATCH net-next 2/9] net: tcp: add 'drop_reason' field to struct tcp_skb_cb
To:     menglong8.dong@gmail.com
Cc:     kuba@kernel.org, davem@davemloft.net, pabeni@redhat.com,
        yoshfuji@linux-ipv6.org, dsahern@kernel.org, imagedong@tencent.com,
        kafai@fb.com, asml.silence@gmail.com, keescook@chromium.org,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org
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On Sat, Oct 29, 2022 at 6:11 AM <menglong8.dong@gmail.com> wrote:
>
> From: Menglong Dong <imagedong@tencent.com>
>
> Because of the long call chain on processing skb in TCP protocol, it's
> hard to pass the drop reason to the code where the skb is freed.
>
> Therefore, we can store the drop reason to skb->cb, and pass it to
> kfree_skb_reason(). I'm lucky, the struct tcp_skb_cb still has 4 bytes
> spare space for this purpose.

No, we have needs for this space for future use.

skb->cb[] purpose is to store semi-permanent info, for skbs that stay
in a queue.

Here, you need a state stored only in the context of the current context.
Stack variables are better.