On Mon, Dec 21, 2020 at 7:09 PM Jonathan Lemon <jonathan.le...@gmail.com> wrote:
>
> From: Jonathan Lemon <b...@fb.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.le...@gmail.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.
