Subject: Re: [PATCH net-next] udp: add drop count for packets in udp_prod_queue

From: Willem de Bruijn <[email-masked]>
Date: Sat, 24 Jan 2026 10:36:22 -0500

Paolo Abeni wrote:
> On 1/23/26 3:41 PM, Willem de Bruijn wrote:
> > Paolo Abeni wrote:
> >> I think that doing the SNMP accounting in __udp_enqueue_schedule_skb()
> >> (for `to_drop`), __udp_queue_rcv_skb() and __udpv6_queue_rcv_skb() (for
> >> `skb`) is a little confusing and possible error prone in the long run.
> >>
> >> I'm wondering if something alike the following (completely untested, not
> >> even built! just to give the idea) would be better?
> > 
> > I don't see the error prone issue with the simpler patch.
> > But SGTM if you prefer this.
> It's not a big deal but if we need to update the UDP MIB someday having
> to look in different places could cause missing something.
> 
> The code I proposed could be made smaller with something alike the
> following (still completely untested), which in turn looks quite alike
> what this patch is heading, I guess. So really it's not a big deal.
> ---
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 1db63db7e5d4..431de8dda0d3 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -1793,6 +1795,7 @@ int __udp_enqueue_schedule_skb(struct sock *sk,
> struct sk_buff *skb)
>  		}
>  	}
> 
> +	atomic_sub(total_size, &udp_prod_queue->rmem_alloc);
>  	if (unlikely(to_drop)) {
>  		for (nb = 0; to_drop != NULL; nb++) {
>  			skb = to_drop;
> @@ -1802,10 +1805,9 @@ int __udp_enqueue_schedule_skb(struct sock *sk,
> struct sk_buff *skb)
>  			sk_skb_reason_drop(sk, skb, SKB_DROP_REASON_PROTO_MEM);
>  		}
>  		numa_drop_add(&udp_sk(sk)->drop_counters, nb);
> +		return nb;
>  	}
> 
> -	atomic_sub(total_size, &udp_prod_queue->rmem_alloc);
> -
>  	return 0;
> 
>  drop:
> @@ -2345,6 +2347,9 @@ static int __udp_queue_rcv_skb(struct sock *sk,
> struct sk_buff *skb)
>  	}
> 
>  	rc = __udp_enqueue_schedule_skb(sk, skb);
> +	if (likely(!rc))
> +		return 0;
> +
>  	if (rc < 0) {
>  		int is_udplite = IS_UDPLITE(sk);
>  		int drop_reason;
> @@ -2365,6 +2370,9 @@ static int __udp_queue_rcv_skb(struct sock *sk,
> struct sk_buff *skb)
>  		return -1;
>  	}
> 
> +	/* rc > 0, packets dropped after dequeueing from prod_queue */
> +	SNMP_ADD_STATS(__UDPX_MIB(sk, true), UDP_MIB_MEMERRORS, rc);
> +	SNMP_ADD_STATS(__UDPX_MIB(sk, true), UDP_MIB_INERRORS, rc);
>  	return 0;
>  }
> 
> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> index 010b909275dd..df895096669e 100644
> --- a/net/ipv6/udp.c
> +++ b/net/ipv6/udp.c
> @@ -793,6 +793,9 @@ static int __udpv6_queue_rcv_skb(struct sock *sk,
> struct sk_buff *skb)
>  	}
> 
>  	rc = __udp_enqueue_schedule_skb(sk, skb);
> +	if (likely(!rc))
> +		return 0;
> +
>  	if (rc < 0) {
>  		int is_udplite = IS_UDPLITE(sk);
>  		enum skb_drop_reason drop_reason;
> @@ -813,6 +816,9 @@ static int __udpv6_queue_rcv_skb(struct sock *sk,
> struct sk_buff *skb)
>  		return -1;
>  	}
> 
> +	/* rc > 0, packets dropped after dequeueing from prod_queue */
> +	SNMP_ADD_STATS(__UDPX_MIB(sk, false), UDP_MIB_MEMERRORS, rc);
> +	SNMP_ADD_STATS(__UDPX_MIB(sk, false), UDP_MIB_INERRORS, rc);
>  	return 0;
>  }
> 

Honestly, I don't see this plumbing of nb as simpler than incrementing
the counters right where nb is known. All it takes there is to make
the second arg to UDPX_MIB conditional on sk_family. But again I don't
care strongly, so SGTM if you do.