netdev - Re: [PATCH 0/9 v1 RFC] Generic zcopy

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Message-ID: <20201218211648.rh5ktnkm333sw4hf@bsd-mbp.dhcp.thefacebook.com>
Date:   Fri, 18 Dec 2020 13:16:48 -0800
From:   Jonathan Lemon <jonathan.lemon@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH 0/9 v1 RFC] Generic zcopy_* functions

On Fri, Dec 18, 2020 at 03:49:44PM -0500, Willem de Bruijn wrote:
> On Fri, Dec 18, 2020 at 3:23 PM Jonathan Lemon <jonathan.lemon@...il.com> wrote:
> >
> > From: Jonathan Lemon <bsd@...com>
> >
> > This is set of cleanup patches for zerocopy which are intended
> > to allow a introduction of a different zerocopy implementation.
> 
> Can you describe in more detail what exactly is lacking in the current
> zerocopy interface for this this different implementation? Or point to
> a github tree with the feature patches attached, perhaps.

I'll get the zctap features up into a github tree.

Essentially, I need different behavior from ubuf_info:
  - no refcounts on RX packets (static ubuf)
  - access to the skb on RX skb free (for page handling)
  - no page pinning on TX/tx completion
  - marking the skb data as inaccessible so skb_condense()
    and skb_zeroocopy_clone() leave it alone.

> I think it's good to split into multiple smaller patchsets, starting
> with core stack support. But find it hard to understand which of these
> changes are truly needed to support a new use case.

Agree - kind of hard to see why this is done without a use case.
These patches are purely restructuring, and don't introduce any
new features.


> If anything, eating up the last 8 bits in skb_shared_info should be last resort.

I would like to add 2 more bits in the future, which is why I
moved them.  Is there a compelling reason to leave the bits alone?
--
Jonathan