netdev - Re: [PATCH net-next v4 1/5] net: hdlc_fr: Simpify fr_rx by using "goto rx

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Date:   Fri, 30 Oct 2020 17:11:24 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Xie He <xie.he.0141@...il.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Krzysztof Halasa <khc@...waw.pl>
Subject: Re: [PATCH net-next v4 1/5] net: hdlc_fr: Simpify fr_rx by using
 "goto rx_drop" to drop frames

On Fri, Oct 30, 2020 at 4:02 PM Xie He <xie.he.0141@...il.com> wrote:
>
> On Fri, Oct 30, 2020 at 9:35 AM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > In general we try to avoid changing counter behavior like that, as
> > existing users
> > may depend on current behavior, e.g., in dashboards or automated monitoring.
> >
> > I don't know how realistic that is in this specific case, no strong
> > objections. Use
> > good judgment.
>
> Originally this function only increases stats.rx_dropped only when
> there's a memory squeeze. I don't know the specification for the
> meaning of stats.rx_dropped, but as I understand it indicates a frame
> is dropped. This is why I wanted to increase it whenever we drop a
> frame.

Jakub recently made stats behavior less ambiguous, in commit
0db0c34cfbc9 ("net: tighten the definition of interface statistics").

That said, it's not entirely clear whether rx_dropped would be allowed
to include rx_errors.

My hunch is that it shouldn't. A quick scan of devices did quickly
show at least one example where it does: macvlan. But I expect that to
be an outlier.

> Originally this function drops a frame silently if the PVC virtual
> device that corresponds to the DLCI number and the protocol type
> doesn't exist. I think we may at least need some way to note this.
> Originally this function drops a frame with a kernel info message
> printed if the protocol type is not supported. I think this is a bad
> way because if the other end continuously sends us a lot of frames
> with unsupported protocol types, our kernel message log will be
> overwhelmed.
>
> I don't know how important it is to keep backwards compatibility. I
> usually don't consider this too much. But I can drop this change if we
> really want to keep the counter behavior unchanged. I think changing
> it is better if we don't consider backwards compatibility.

Please do always consider backward compatibility. In this case, I
don't think that the behavioral change is needed for the core of the
patch (changing control flow).