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Date:   Sat, 19 Feb 2022 21:39:52 -0800
From:   Dongli Zhang <dongli.zhang@...cle.com>
To:     David Ahern <dsahern@...il.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        rostedt@...dmis.org, mingo@...hat.com, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, imagedong@...cent.com,
        joao.m.martins@...cle.com, joe.jin@...cle.com, edumazet@...gle.com
Subject: Re: [PATCH v2 1/3] net: tap: track dropped skb via kfree_skb_reason()

Hi David,

On 2/19/22 2:46 PM, David Ahern wrote:
> On 2/19/22 12:12 PM, Dongli Zhang wrote:
>> The TAP can be used as vhost-net backend. E.g., the tap_handle_frame() is
>> the interface to forward the skb from TAP to vhost-net/virtio-net.
>>
>> However, there are many "goto drop" in the TAP driver. Therefore, the
>> kfree_skb_reason() is involved at each "goto drop" to help userspace
>> ftrace/ebpf to track the reason for the loss of packets.
>>
>> The below reasons are introduced:
>>
>> - SKB_DROP_REASON_SKB_CSUM
>> - SKB_DROP_REASON_SKB_COPY_DATA
>> - SKB_DROP_REASON_SKB_GSO_SEG
>> - SKB_DROP_REASON_DEV_HDR
>> - SKB_DROP_REASON_FULL_RING
>>
>> Cc: Joao Martins <joao.m.martins@...cle.com>
>> Cc: Joe Jin <joe.jin@...cle.com>
>> Signed-off-by: Dongli Zhang <dongli.zhang@...cle.com>
>> ---
>>  drivers/net/tap.c          | 30 ++++++++++++++++++++++--------
>>  include/linux/skbuff.h     |  9 +++++++++
>>  include/trace/events/skb.h |  5 +++++
>>  3 files changed, 36 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
>> index 8e3a28ba6b28..ab3592506ef8 100644
>> --- a/drivers/net/tap.c
>> +++ b/drivers/net/tap.c
>> @@ -322,6 +322,7 @@ rx_handler_result_t tap_handle_frame(struct sk_buff **pskb)
>>  	struct tap_dev *tap;
>>  	struct tap_queue *q;
>>  	netdev_features_t features = TAP_FEATURES;
>> +	int drop_reason;
> 
> enum skb_drop_reason drop_reason;

According to cscope, so far all 'drop_reason' are declared in type 'int' (e.g.,
ip_rcv_finish_core()).

I will change above to enum.

> 
>>  
>>  	tap = tap_dev_get_rcu(dev);
>>  	if (!tap)
>> @@ -343,12 +344,16 @@ rx_handler_result_t tap_handle_frame(struct sk_buff **pskb)
>>  		struct sk_buff *segs = __skb_gso_segment(skb, features, false);
>>  		struct sk_buff *next;
>>  
>> -		if (IS_ERR(segs))
>> +		if (IS_ERR(segs)) {
>> +			drop_reason = SKB_DROP_REASON_SKB_GSO_SEG;
>>  			goto drop;
>> +		}
>>  
>>  		if (!segs) {
>> -			if (ptr_ring_produce(&q->ring, skb))
>> +			if (ptr_ring_produce(&q->ring, skb)) {
>> +				drop_reason = SKB_DROP_REASON_FULL_RING;
>>  				goto drop;
>> +			}
>>  			goto wake_up;
>>  		}
> 
> you missed the walk of segs and calling ptr_ring_produce.

This call site of kfree_skb() and kfree_skb_list() is unique and there is not
any "goto drop" involved. From developers' view, we will be able to tell if the
'drop' is at here according to the instruction pointers in callstack.

 360                 consume_skb(skb);
 361                 skb_list_walk_safe(segs, skb, next) {
 362                         skb_mark_not_on_list(skb);
 363                         if (ptr_ring_produce(&q->ring, skb)) {
 364                                 kfree_skb(skb);
 365                                 kfree_skb_list(next);
 366                                 break;
 367                         }
 368                 }

I will add the reason to "walk of segs" case as well. About kfree_skb_list(), I
will introduce a kfree_skb_list_reason().

Thank you very much!

Dongli Zhang

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