linux-kernel - Re: [PATCH RFC 1/4] net: skb: use line number to trace dropped skb

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Date:   Thu, 3 Feb 2022 09:14:50 -0800
From:   Dongli Zhang <dongli.zhang@...cle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Menglong Dong <imagedong@...cent.com>,
        Joao Martins <joao.m.martins@...cle.com>, joe.jin@...cle.com
Subject: Re: [PATCH RFC 1/4] net: skb: use line number to trace dropped skb

Hi Eric,

On 2/3/22 7:59 AM, Eric Dumazet wrote:
> On Thu, Feb 3, 2022 at 7:38 AM Dongli Zhang <dongli.zhang@...cle.com> wrote:
>>
>> Sometimes the kernel may not directly call kfree_skb() to drop the sk_buff.
>> Instead, it "goto drop" and call kfree_skb() at 'drop'. This make it
>> difficult to track the reason that the sk_buff is dropped.
>>
>> The commit c504e5c2f964 ("net: skb: introduce kfree_skb_reason()") has
>> introduced the kfree_skb_reason() to help track the reason. However, we may
>> need to define many reasons for each driver/subsystem.
>>
>> To avoid introducing so many new reasons, this is to use line number
>> ("__LINE__") to trace where the sk_buff is dropped. As a result, the reason
>> will be generated automatically.
>>
>> Cc: Joao Martins <joao.m.martins@...cle.com>
>> Cc: Joe Jin <joe.jin@...cle.com>
>> Signed-off-by: Dongli Zhang <dongli.zhang@...cle.com>
>> ---
>>  include/linux/skbuff.h     | 21 ++++-----------------
>>  include/trace/events/skb.h | 35 ++++++-----------------------------
>>  net/core/dev.c             |  2 +-
>>  net/core/skbuff.c          |  9 ++++-----
>>  net/ipv4/tcp_ipv4.c        | 14 +++++++-------
>>  net/ipv4/udp.c             | 14 +++++++-------
>>  6 files changed, 29 insertions(+), 66 deletions(-)
>>
>> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
>> index 8a636e678902..471268a4a497 100644
>> --- a/include/linux/skbuff.h
>> +++ b/include/linux/skbuff.h
>> @@ -307,21 +307,8 @@ struct sk_buff_head {
>>
>>  struct sk_buff;
>>
>> -/* The reason of skb drop, which is used in kfree_skb_reason().
>> - * en...maybe they should be splited by group?
>> - *
>> - * Each item here should also be in 'TRACE_SKB_DROP_REASON', which is
>> - * used to translate the reason to string.
>> - */
>> -enum skb_drop_reason {
>> -       SKB_DROP_REASON_NOT_SPECIFIED,
>> -       SKB_DROP_REASON_NO_SOCKET,
>> -       SKB_DROP_REASON_PKT_TOO_SMALL,
>> -       SKB_DROP_REASON_TCP_CSUM,
>> -       SKB_DROP_REASON_SOCKET_FILTER,
>> -       SKB_DROP_REASON_UDP_CSUM,
>> -       SKB_DROP_REASON_MAX,
>> -};
> 
> 
> Seriously, we have to stop messing with things like that.
> 
> Your patch comes too late, another approach has been taken.
> 
> Please continue this effort by providing patches that improve things,
> instead of throwing away effort already done.

Thank you very much for the suggestion!

I will introduce new reasons to TUN and TAP drivers, in order to track the
dropped sk_buff.

Dongli Zhang

> 
> I say no to this patch.
>