netdev - Re: [PATCH v2 1/3] net: tap: track dropped skb via kfree_skb

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Date:   Sat, 19 Feb 2022 15:46:59 -0700
From:   David Ahern <dsahern@...il.com>
To:     Dongli Zhang <dongli.zhang@...cle.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()

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;

>  
>  	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.

>