> > > - marking the skb data as inaccessible so skb_condense()
> > > and skb_zeroocopy_clone() leave it alone.
> >
> > Yep. Skipping content access on the Rx path will be interesting. I
> > wonder if that should be a separate opaque skb feature, independent
> > from whether the data is owned by userspace, peripheral memory, the
> > page cache or anything else.
>
> Would that be indicated by a bit on the skb (like pfmemalloc), or
> a bit in the skb_shared structure, as I'm leaning towards doing here?
I would guide it in part by avoiding cold cacheline accesses. That
might be hard if using skb_shinfo. OTOH, you don't have to worry about
copying the bit during clone operations.
> > > > If anything, eating up the last 8 bits in skb_shared_info should be
> > > > last resort.
> > >
> > > I would like to add 2 more bits in the future, which is why I
> > > moved them. Is there a compelling reason to leave the bits alone?
> >
> > Opportunity cost.
> >
> > We cannot grow skb_shared_info due to colocation with MTU sized linear
> > skbuff's in half a page.
> >
> > It took me quite some effort to free up a few bytes in commit
> > 4d276eb6a478 ("net: remove deprecated syststamp timestamp").
> >
> > If we are very frugal, we could shadow some bits to have different
> > meaning in different paths. SKBTX_IN_PROGRESS is transmit only, I
> > think. But otherwise we'll have to just dedicate the byte to more
> > flags. Yours are likely not to be the last anyway.
>
> The zerocopy/enable flags could be encoded in one of the lower 3 bits
> in the destructor_arg, (similar to nouarg) but that seems messy.
Agreed :)
Let's just expand the flags for now. It may be better to have one
general purpose 16 bit flags bitmap, rather than reserving 8 bits
specifically to zerocopy features.
