Re: [PATCH 1/2] net: tap: track dropped skb via kfree_skb_reason()

[Eric Dumazet]

On Mon, Feb 7, 2022 at 7:55 PM Dongli Zhang <dongli.zhang@xxxxxxxxxx> 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
>
> Cc: Joao Martins <joao.m.martins@xxxxxxxxxx>
> Cc: Joe Jin <joe.jin@xxxxxxxxxx>
> Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
> ---
>  drivers/net/tap.c          | 30 ++++++++++++++++++++++--------
>  include/linux/skbuff.h     |  5 +++++
>  include/trace/events/skb.h |  5 +++++
>  3 files changed, 32 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
> index 8e3a28ba6b28..232572289e63 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 = SKB_DROP_REASON_NOT_SPECIFIED;
>
>         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_SEGMENT;
>                         goto drop;
> +               }
>
>                 if (!segs) {
> -                       if (ptr_ring_produce(&q->ring, skb))
> +                       if (ptr_ring_produce(&q->ring, skb)) {
> +                               drop_reason = SKB_DROP_REASON_PTR_FULL;

PTR_FULL is strange .... How about FULL_RING, or FULL_QUEUE ?