netdev - Re: [PATCH 1/2] net: tap: track dropped skb via kfree_skb

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Message-ID: <CANn89i++X+QBtm=L19PBbGLUJb8ZiTib4AABL_WL+cUfgwVfOA@mail.gmail.com>
Date:   Mon, 7 Feb 2022 20:32:58 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Dongli Zhang <dongli.zhang@...cle.com>
Cc:     netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        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,
        David Ahern <dsahern@...il.com>
Subject: Re: [PATCH 1/2] net: tap: track dropped skb via kfree_skb_reason()

On Mon, Feb 7, 2022 at 7:55 PM Dongli Zhang <dongli.zhang@...cle.com> 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@...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     |  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 ?