netdev - Re: [PATCH 05/12 v2 RFC] skbuff: replace sock_zerocopy_put() with skb_zcopy

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Date:   Tue, 22 Dec 2020 17:36:44 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jonathan Lemon <jonathan.lemon@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH 05/12 v2 RFC] skbuff: replace sock_zerocopy_put() with skb_zcopy_put()

On Tue, Dec 22, 2020 at 12:19 PM Jonathan Lemon
<jonathan.lemon@...il.com> wrote:
>
> On Tue, Dec 22, 2020 at 09:42:40AM -0500, Willem de Bruijn wrote:
> > On Mon, Dec 21, 2020 at 7:09 PM Jonathan Lemon <jonathan.lemon@...il.com> wrote:
> > >
> > > From: Jonathan Lemon <bsd@...com>
> > >
> > > Replace sock_zerocopy_put with the generic skb_zcopy_put()
> > > function.  Pass 'true' as the success argument, as this
> > > is identical to no change.
> > >
> > > Signed-off-by: Jonathan Lemon <jonathan.lemon@...il.com>
> >
> > uarg->zerocopy may be false if sock_zerocopy_put_abort is called from
> > tcp_sendmsg_locked
>
> Yes, it may well be false.  The original logic goes:
>
>    sock_zerocopy_put_abort()
>    sock_zerocopy_put()
>    sock_zerocopy_callback(..., success = uarg->zerocopy)
>      if (success)
>
> The new logic is now:
>
>    sock_zerocopy_put_abort()
>    sock_zerocopy_callback(..., success = true)
>      uarg->zerocopy = uarg->zerocopy & success
>      if (uarg->zerocopy)
>
> The success value ls latched into uarg->zerocopy, and any failure
> is persistent.

Good point.

It's not entirely obvious, and a bit unintuitive to pass success in an
abort statement. But it works.

Abort does not schedule a notification if no skb associated with the
uarg was sent. And if it was, the zerocopy state will reflect the & of
all those packets. On which note, if renaming the currently
inconsistent name, maybe renaming to zerocopy_success is the more
descriptive one, then.