linux-kernel - Re: [PATCH] tap/tun: add stats accounting when failed to transfer data to user

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Message-ID: <688a4c3dd337_1cb79529426@willemb.c.googlers.com.notmuch>
Date: Wed, 30 Jul 2025 12:45:49 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: zhangyanjun@...tc.cn, 
 willemdebruijn.kernel@...il.com, 
 jasowang@...hat.com
Cc: netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org, 
 zhangyanjun@...tc.cn
Subject: Re: [PATCH] tap/tun: add stats accounting when failed to transfer
 data to user

zhangyanjun@ wrote:
> From: Yanjun Zhang <zhangyanjun@...tc.cn>
> 
> To more accurately detect packet dropped, we add the dropped packet
> counter with the device when kfree_skb is called because of failing
> to transfer data to user space.
> 
> Signed-off-by: Yanjun Zhang <zhangyanjun@...tc.cn>

Net-next is currently closed. For networking patch process, see also
Documentation/process/maintainer-netdev.rst.

> ---
>  drivers/net/tap.c | 14 +++++++++++---
>  drivers/net/tun.c |  9 ++++++---
>  2 files changed, 17 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
> index bdf0788d8..9d288a1ad 100644
> --- a/drivers/net/tap.c
> +++ b/drivers/net/tap.c
> @@ -759,6 +759,8 @@ static ssize_t tap_do_read(struct tap_queue *q,
>  {
>  	DEFINE_WAIT(wait);
>  	ssize_t ret = 0;
> +	struct tap_dev *tap;
> +	enum skb_drop_reason drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
>  
>  	if (!iov_iter_count(to)) {
>  		kfree_skb(skb);
> @@ -794,10 +796,16 @@ static ssize_t tap_do_read(struct tap_queue *q,
>  put:
>  	if (skb) {
>  		ret = tap_put_user(q, skb, to);
> -		if (unlikely(ret < 0))
> -			kfree_skb(skb);
> -		else
> +		if (unlikely(ret < 0)) {
> +			kfree_skb_reason(skb, drop_reason);

kfreee_skb_reason(skb, SKB_DROP_REASON_NOT_SPECIFIED) is equivalent to
kfree_skb().

> +			rcu_read_lock();
> +			tap = rcu_dereference(q->tap);
> +			if (tap && tap->count_rx_dropped)
> +				tap->count_rx_dropped(tap);
> +			rcu_read_unlock();
> +		} else {
>  			consume_skb(skb);
> +		}
>  	}
>  	return ret;
>  }
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index f8c5e2fd0..eb3c68e5f 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -2137,6 +2137,7 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
>  {
>  	ssize_t ret;
>  	int err;
> +	enum skb_drop_reason drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
>  
>  	if (!iov_iter_count(to)) {
>  		tun_ptr_free(ptr);
> @@ -2159,10 +2160,12 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
>  		struct sk_buff *skb = ptr;
>  
>  		ret = tun_put_user(tun, tfile, skb, to);
> -		if (unlikely(ret < 0))
> -			kfree_skb(skb);
> -		else
> +		if (unlikely(ret < 0)) {
> +			dev_core_stats_tx_dropped_inc(tun->dev);

I don't think counters need to be set in these recv syscall paths,
where an error is communicated through the return vaule.

> +			kfree_skb_reason(skb, drop_reason);
> +		} else {
>  			consume_skb(skb);
> +		}
>  	}
>  
>  	return ret;
> -- 
> 2.31.1
> 
> 
>