netdev - Re: [PATCH 0/9 v1 RFC] Generic zcopy

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Message-ID: <CAF=yD-+bVFBHPfFB+E1s4Qae5PZGQJaiarAN9hwpP2aTs1f_jg@mail.gmail.com>
Date:   Mon, 21 Dec 2020 17:52:08 -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 0/9 v1 RFC] Generic zcopy_* functions

> > >   - 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.