Re: [PATCH v2 net-next 0/3] net: skb: introduce kfree_skb_with_reason() and use it for tcp and udp

[David Ahern]

On 1/3/22 6:47 PM, Cong Wang wrote:
> On Thu, Dec 30, 2021 at 05:32:37PM +0800, menglong8.dong@xxxxxxxxx wrote:
>> From: Menglong Dong <imagedong@xxxxxxxxxxx>
>>
>> In this series patch, the interface kfree_skb_with_reason() is
>> introduced(), which is used to collect skb drop reason, and pass
>> it to 'kfree_skb' tracepoint. Therefor, 'drop_monitor' or eBPF is
>> able to monitor abnormal skb with detail reason.
>>
> 
> We already something close, __dev_kfree_skb_any(). Can't we unify
> all of these?

Specifically?

The 'reason' passed around by those is either SKB_REASON_CONSUMED or
SKB_REASON_DROPPED and is used to call kfree_skb vs consume_skb. i.e.,
this is unrelated to this patch set and goal.

> 
> 
>> In fact, this series patches are out of the intelligence of David
>> and Steve, I'm just a truck man :/
>>
> 
> I think there was another discussion before yours, which I got involved
> as well.
> 
>> Previous discussion is here:
>>
>> https://lore.kernel.org/netdev/20211118105752.1d46e990@xxxxxxxxxxxxxxxxxx/
>> https://lore.kernel.org/netdev/67b36bd8-2477-88ac-83a0-35a1eeaf40c9@xxxxxxxxx/
>>
>> In the first patch, kfree_skb_with_reason() is introduced and
>> the 'reason' field is added to 'kfree_skb' tracepoint. In the
>> second patch, 'kfree_skb()' in replaced with 'kfree_skb_with_reason()'
>> in tcp_v4_rcv(). In the third patch, 'kfree_skb_with_reason()' is
>> used in __udp4_lib_rcv().
>>
> 
> I don't follow all the discussions here, but IIRC it would be nice
> if we can provide the SNMP stat code (for instance, TCP_MIB_CSUMERRORS) to
> user-space, because those stats are already exposed to user-space, so
> you don't have to invent new ones.

Those SNMP macros are not unique and can not be fed into a generic
kfree_skb_reason function.