netdev - Re: [PATCH 09/12 v2 RFC] skbuff: add zc_flags to ubuf

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Date:   Tue, 22 Dec 2020 10:00:37 -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 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.

Just being very explicit, as this is a fragile bit of code. Come to
think of it, this probably deserves a BUILD_BUG_ON.