Re: [PATCH 1/2] net: store KCOV remote handle in sk_buff

[Aleksandr Nogikh <nogikh@google.com>]

On Thu, Oct 15, 2020 at 10:04 PM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> 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.

Thank you all for your comments. I'll use skb extensions in v3 of the series.