netdev - Re: [PATCH net v2] tun: add missing rx stats accounting in tun_xdp

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Date: Tue, 16 Jan 2024 10:04:35 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Yunjian Wang <wangyunjian@...wei.com>, 
 willemdebruijn.kernel@...il.com, 
 jasowang@...hat.com, 
 kuba@...nel.org, 
 davem@...emloft.net
Cc: netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org, 
 xudingke@...wei.com, 
 Yunjian Wang <wangyunjian@...wei.com>
Subject: Re: [PATCH net v2] tun: add missing rx stats accounting in
 tun_xdp_act

Yunjian Wang wrote:
> There are few places on the receive path where packet receives and packet
> drops were not accounted for. This patch fixes that issue.
> 
> Fixes: 8ae1aff0b331 ("tuntap: split out XDP logic")

Before this commit this_cpu_inc(tun->pcpu_stats->rx_dropped) would
get called, so that is indeed a regression in that patch. Please add
that to the commit message.

This commit also adds new accounting of successfully received bytes
with dev_sw_netstats_rx_add. I don't know off the top of my head if
other devices account XDP_TX and XDP_REDIRECT in that counter. Either
way, good to be explicit about such subtle details. Oddly, before
8ae1aff0b331 those, too, would be attributed to rx_dropped, because
after the out: label.

> Signed-off-by: Yunjian Wang <wangyunjian@...wei.com>
> ---
> v2: add Fixes tag
> ---
>  drivers/net/tun.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index afa5497f7c35..232e5319ac77 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1626,17 +1626,14 @@ static int tun_xdp_act(struct tun_struct *tun, struct bpf_prog *xdp_prog,
>  		       struct xdp_buff *xdp, u32 act)
>  {
>  	int err;
> +	unsigned int datasize = xdp->data_end - xdp->data;
>  
>  	switch (act) {
>  	case XDP_REDIRECT:
>  		err = xdp_do_redirect(tun->dev, xdp, xdp_prog);
> -		if (err)
> -			return err;
>  		break;
>  	case XDP_TX:
>  		err = tun_xdp_tx(tun->dev, xdp);
> -		if (err < 0)
> -			return err;
>  		break;
>  	case XDP_PASS:
>  		break;

err is uninitialized in this case?

> @@ -1651,6 +1648,13 @@ static int tun_xdp_act(struct tun_struct *tun, struct bpf_prog *xdp_prog,
>  		break;
>  	}
>  
> +	if (err < 0) {
> +		act = err;
> +		dev_core_stats_rx_dropped_inc(tun->dev);
> +	} else if (act == XDP_REDIRECT || act == XDP_TX) {
> +		dev_sw_netstats_rx_add(tun->dev, datasize);
> +	}
> +
>  	return act;
>  }
>  
> -- 
> 2.41.0
>