netdev - Re: [PATCH net-next] udp: add drop count for packets in udp_prod

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Message-ID: <CANn89i+k+dB_9G+gvCpv+_8wWSxp71LxQTdbOZVm7LSugRK5+A@mail.gmail.com>
Date: Wed, 28 Jan 2026 13:37:22 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Mahdi Faramarzpour <mahdifrmx@...il.com>
Cc: netdev@...r.kernel.org, willemdebruijn.kernel@...il.com, 
	davem@...emloft.net, dsahern@...nel.org, kuba@...nel.org, pabeni@...hat.com, 
	horms@...nel.org, kshitiz.bartariya@...omail.in
Subject: Re: [PATCH net-next] udp: add drop count for packets in udp_prod_queue

On Wed, Jan 28, 2026 at 1:27 PM Mahdi Faramarzpour <mahdifrmx@...il.com> wrote:
>
> On Wed, Jan 28, 2026 at 3:14 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Wed, Jan 28, 2026 at 8:03 AM Mahdi Faramarzpour <mahdifrmx@...il.com> wrote:
> > >
> > > This commit adds SNMP drop count increment for the packets in
> > > per NUMA queues which were introduced in commit b650bf0977d3
> > > ("udp: remove busylock and add per NUMA queues"). note that SNMP
> > > counters are incremented currently by the caller for skb. And
> > > that these skbs on the intermediate queue cannot be counted
> > > there so need similar logic in their error path.
> > >
> > > Signed-off-by: Mahdi Faramarzpour <mahdifrmx@...il.com>
> > > ---
> > > v6:
> > >   - increasing a single counter based on socket family
> > > v5: https://lore.kernel.org/netdev/20260122185357.50922-1-mahdifrmx@gmail.com/
> > >   - check if drop counts are non-zero before increasing countrers
> > > v4: https://lore.kernel.org/netdev/20260108102950.49417-1-mahdifrmx@gmail.com/
> > >   - move all changes to unlikely(to_drop) branch
> > > v3: https://lore.kernel.org/netdev/20260105114732.140719-1-mahdifrmx@gmail.com/
> > >   - remove the unreachable UDP_MIB_RCVBUFERRORS code
> > > v2: https://lore.kernel.org/netdev/20260105071218.10785-1-mahdifrmx@gmail.com/
> > >   - change ENOMEM to ENOBUFS
> > > v1: https://lore.kernel.org/netdev/20260104105732.427691-1-mahdifrmx@gmail.com/
> > > ---
> > >  net/ipv4/udp.c | 5 ++++-
> > >  1 file changed, 4 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> > > index ffe074cb5..dd302f5fb 100644
> > > --- a/net/ipv4/udp.c
> > > +++ b/net/ipv4/udp.c
> > > @@ -1797,10 +1797,13 @@ int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb)
> > >                         skb = to_drop;
> > >                         to_drop = skb->next;
> > >                         skb_mark_not_on_list(skb);
> > > -                       /* TODO: update SNMP values. */
> > >                         sk_skb_reason_drop(sk, skb, SKB_DROP_REASON_PROTO_MEM);
> > >                 }
> > >                 numa_drop_add(&udp_sk(sk)->drop_counters, nb);
> > > +               SNMP_ADD_STATS(__UDPX_MIB(sk, (sk->sk_family == PF_INET)),
> > > +                              UDP_MIB_MEMERRORS, nb);
> > > +               SNMP_ADD_STATS(__UDPX_MIB(sk, (sk->sk_family == PF_INET)),
> > > +                              UDP_MIB_INERRORS, nb);
> > >         }
> >
> > Hmm.. I have not followed your prior attempts, sorry for coming late.
> >
> > Note that an UDP AF_INET6 socket for which ipv6_only_sock(sk) is false
> > can receive
> > IPv4 and IPv6 UDP packets, if bound to any addresses.
> >
> > The to_drop list could contain a mix of IPv4 and IPv6 packets.
> >
> > Ideally we should track SNMP values in the proper space (Ipv4 , IPv6).
> >
> > See IPV6_V6ONLY and /proc/sys/net/ipv6/bindv6only for reference.
> This is the exact discussion I had with Willem de Bruijn:
> https://lore.kernel.org/netdev/willemdebruijn.kernel.17f36003699cb@gmail.com/

I think Willem was wrong :)

__udp_enqueue_schedule_skb() is called either from
__udp_queue_rcv_skb() for IPv4 packets,
or __udpv6_queue_rcv_skb() for IPv6 packets, and they both can feed
packets to a single and shared UDP socket.

An IPv4 packet will not go through __udpv6_queue_rcv_skb().

It would be deceptive to increment IPV6 UDP counters if a host only
receives IPv4 UDP packets.