Re: [PATCH] net: udp: increase UDP_MIB_RCVBUFERRORS when ENOBUFS

[Menglong Dong]

Hello~

On Mon, Oct 26, 2020 at 5:52 PM Paolo Abeni <pabeni@xxxxxxxxxx> wrote:
>
> Hello,
>
> On Mon, 2020-10-26 at 17:39 +0800, Menglong Dong wrote:
> > The error returned from __udp_enqueue_schedule_skb is ENOMEM or ENOBUFS.
> > For now, only ENOMEM is counted into UDP_MIB_RCVBUFERRORS in
> > __udp_queue_rcv_skb. UDP_MIB_RCVBUFERRORS should count all of the
> > failed skb because of memory errors during udp receiving, not just because of the limit of sock receive queue. We can see this
> > in __udp4_lib_mcast_deliver:
> >
> >               nskb = skb_clone(skb, GFP_ATOMIC);
> >
> >               if (unlikely(!nskb)) {
> >                       atomic_inc(&sk->sk_drops);
> >                       __UDP_INC_STATS(net, UDP_MIB_RCVBUFERRORS,
> >                                       IS_UDPLITE(sk));
> >                       __UDP_INC_STATS(net, UDP_MIB_INERRORS,
> >                                       IS_UDPLITE(sk));
> >                       continue;
> >               }
> >
> > See, UDP_MIB_RCVBUFERRORS is increased when skb clone failed. From this
> > point, ENOBUFS from __udp_enqueue_schedule_skb should be counted, too.
> > It means that the buffer used by all of the UDP sock is to the limit, and
> > it ought to be counted.
> >
> > Signed-off-by: Menglong Dong <menglong8.dong@xxxxxxxxx>
> > ---
> >  net/ipv4/udp.c | 4 +---
> >  net/ipv6/udp.c | 4 +---
> >  2 files changed, 2 insertions(+), 6 deletions(-)
> >
> > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> > index 09f0a23d1a01..49a69d8d55b3 100644
> > --- a/net/ipv4/udp.c
> > +++ b/net/ipv4/udp.c
> > @@ -2035,9 +2035,7 @@ static int __udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
> >               int is_udplite = IS_UDPLITE(sk);
> >
> >               /* Note that an ENOMEM error is charged twice */
> > -             if (rc == -ENOMEM)
> > -                     UDP_INC_STATS(sock_net(sk), UDP_MIB_RCVBUFERRORS,
> > -                                     is_udplite);
> > +             UDP_INC_STATS(sock_net(sk), UDP_MIB_RCVBUFERRORS, is_udplite);
> >               UDP_INC_STATS(sock_net(sk), UDP_MIB_INERRORS, is_udplite);
> >               kfree_skb(skb);
> >               trace_udp_fail_queue_rcv_skb(rc, sk);
> > diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> > index 29d9691359b9..d5e23b150fd9 100644
> > --- a/net/ipv6/udp.c
> > +++ b/net/ipv6/udp.c
> > @@ -634,9 +634,7 @@ static int __udpv6_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
> >               int is_udplite = IS_UDPLITE(sk);
> >
> >               /* Note that an ENOMEM error is charged twice */
> > -             if (rc == -ENOMEM)
> > -                     UDP6_INC_STATS(sock_net(sk),
> > -                                      UDP_MIB_RCVBUFERRORS, is_udplite);
> > +             UDP6_INC_STATS(sock_net(sk), UDP_MIB_RCVBUFERRORS, is_udplite);
> >               UDP6_INC_STATS(sock_net(sk), UDP_MIB_INERRORS, is_udplite);
> >               kfree_skb(skb);
> >               return -1;
>
> The diffstat is nice, but I'm unsure we can do this kind of change
> (well, I really think we should not do it): it will fool any kind of
> existing users (application, scripts, admin) currently reading the
> above counters and expecting UDP_MIB_RCVBUFERRORS being increased with
> the existing schema.
>
> Cheers,
>
> Paolo
>

Well, your words make sense, this change isn't friendly for the existing users.
It really puzzled me when this ENOBUFS happened, no counters were done and
I hardly figured out what happened.

So, is it a good idea to introduce a 'UDP_MIB_MEMERRORS'?

Cheers,

Menglong Dong