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Date:   Tue, 22 Dec 2020 13:29:23 -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 09/12 v2 RFC] skbuff: add zc_flags to ubuf_info for ubuf setup

On Tue, Dec 22, 2020 at 1:17 PM Jonathan Lemon <jonathan.lemon@...il.com> wrote:
>
> On Tue, Dec 22, 2020 at 10:00:37AM -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>
> > >
> > > Currently, an ubuf is attached to a new skb, the skb zc_flags
> > > is initialized to a fixed value.  Instead of doing this, set
> > > the default zc_flags in the ubuf, and have new skb's inherit
> > > from this default.
> > >
> > > This is needed when setting up different zerocopy types.
> > >
> > > Signed-off-by: Jonathan Lemon <jonathan.lemon@...il.com>
> > > ---
> > >  include/linux/skbuff.h | 3 ++-
> > >  net/core/skbuff.c      | 1 +
> > >  2 files changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > > index da0c1dddd0da..b90be4b0b2de 100644
> > > --- a/include/linux/skbuff.h
> > > +++ b/include/linux/skbuff.h
> > > @@ -478,6 +478,7 @@ struct ubuf_info {
> > >                 };
> > >         };
> > >         refcount_t refcnt;
> > > +       u8 zc_flags;
> > >
> > >         struct mmpin {
> > >                 struct user_struct *user;
> >
> > When allocating ubuf_info for msg_zerocopy, we actually allocate the
> > notification skb, to be sure that notifications won't be dropped due
> > to memory pressure at notification time. We actually allocate the skb
> > and place ubuf_info in skb->cb[].
> >
> > The struct is exactly 48 bytes on 64-bit platforms, filling all of cb.
> > This new field fills a 4B hole, so it should still be fine.
>
> Yes, I was careful not to increase the size.  I have future changes
> for this structure, moving 'struct mmpin' into a union.   Making the
> flags 16 bits shouldn't be a problem either.
>
>
> > Just being very explicit, as this is a fragile bit of code. Come to
> > think of it, this probably deserves a BUILD_BUG_ON.
>
> You mean like the one which exists in sock_zerocopy_alloc()?
>
>         BUILD_BUG_ON(sizeof(*uarg) > sizeof(skb->cb));

Ah good. Yes. I should have remembered that was there.

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