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.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.
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));
--
Jonathan
