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From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date:   Thu, 15 Oct 2020 15:04:11 -0400
Message-ID: <CA+FuTSf2kfvdYydXYJNCCfE62q9DXXOBMh_ZSO5W=L9GK478HA@mail.gmail.com>
Subject: Re: [PATCH 1/2] net: store KCOV remote handle in sk_buff
To:     Jakub Kicinski <kuba@kernel.org>
Cc:     Aleksandr Nogikh <a.nogikh@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        David Miller <davem@davemloft.net>,
        Johannes Berg <johannes@sipsolutions.net>,
        Eric Dumazet <edumazet@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Marco Elver <elver@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        linux-wireless <linux-wireless@vger.kernel.org>,
        Aleksandr Nogikh <nogikh@google.com>,
        Florian Westphal <fw@strlen.de>
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Precedence: bulk

On Tue, Oct 13, 2020 at 12:50 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Tue, 13 Oct 2020 18:59:28 +0300 Aleksandr Nogikh wrote:
> > On Mon, 12 Oct 2020 at 09:04, Dmitry Vyukov <dvyukov@google.com> wrote:
> > >
> > > On Sat, Oct 10, 2020 at 5:14 PM Jakub Kicinski <kuba@kernel.org> wrote:
> > > >
> > > > On Sat, 10 Oct 2020 09:54:57 +0200 Dmitry Vyukov wrote:
> > > > > On Sat, Oct 10, 2020 at 1:16 AM Jakub Kicinski <kuba@kernel.org> wrote:
> > [...]
> > > > > > Could you use skb_extensions for this?
> > > > >
> > > > > Why? If for space, this is already under a non-production ifdef.
> > > >
> > > > I understand, but the skb_ext infra is there for uncommon use cases
> > > > like this one. Any particular reason you don't want to use it?
> > > > The slight LoC increase?
> > > >
> > > > Is there any precedent for adding the kcov field to other performance
> > > > critical structures?
> >
> > It would be great to come to some conclusion on where exactly to store
> > kcov_handle. Technically, it is possible to use skb extensions for the
> > purpose, though it will indeed slightly increase the complexity.
> >
> > Jakub, you think that kcov_handle should be added as an skb extension,
> > right?
>
> That'd be preferable. I understand with current use cases it doesn't
> really matter, but history shows people come up with all sort of
> wonderful use cases down the line. And when they do they rarely go back
> and fix such fundamental minutiae.
>
> > Though I do not really object to moving the field, it still seems to
> > me that sk_buff itself is a better place. Right now skb extensions
> > store values that are local to specific protocols and that are only
> > meaningful in the context of these protocols (correct me if I'm
> > wrong). Although this patch only adds remote kcov coverage to the wifi
> > code, kcov_handle can be meaningful for other protocols as well - just
> > like the already existing sk_buff fields. So adding kcov_handle to skb
> > extensions will break this logical separation.
>
> It's not as much protocols as subsystems. The values are meaningful to
> a subsystem which inserts them, that doesn't mean single layer of the
> stack. If it was about storing layer's context we would just use
> skb->cb.
>
> So I think the kcov use matches pretty well.

skb_extensions was the first thing that came to mind when I read this
patchset too. It is not specific to protocols.

We have long stopped growing sk_buff size.