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

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Date:   Sat, 19 Dec 2020 14:00:55 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jonathan Lemon <jonathan.lemon@...il.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        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 4:27 PM Jonathan Lemon <jonathan.lemon@...il.com> wrote:
>
> 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)

That is already the case for vhost and tpacket zerocopy use cases.

>   - access to the skb on RX skb free (for page handling)

To refers only to patch 9, moving the callback earlier in
skb_release_data, right?

>   - no page pinning on TX/tx completion

That is not part of the skb zerocopy infrastructure?

>   - marking the skb data as inaccessible so skb_condense()
>     and skb_zeroocopy_clone() leave it alone.

Yep. Skipping content access on the Rx path will be interesting. I
wonder if that should be a separate opaque skb feature, independent
from whether the data is owned by userspace, peripheral memory, the
page cache or anything else.

> > 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?

Opportunity cost.

We cannot grow skb_shared_info due to colocation with MTU sized linear
skbuff's in half a page.

It took me quite some effort to free up a few bytes in commit
4d276eb6a478 ("net: remove deprecated syststamp timestamp").

If we are very frugal, we could shadow some bits to have different
meaning in different paths. SKBTX_IN_PROGRESS is transmit only, I
think. But otherwise we'll have to just dedicate the byte to more
flags. Yours are likely not to be the last anyway.