netdev - Re: SKB paged fragment lifecycle on receive

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Date:	Sat, 25 Jun 2011 12:58:41 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	David Miller <davem@...emloft.net>, eric.dumazet@...il.com,
	netdev@...r.kernel.org, xen-devel@...ts.xensource.com,
	rusty@...tcorp.com.au
Subject: Re: SKB paged fragment lifecycle on receive

On Fri, 2011-06-24 at 13:11 -0700, Jeremy Fitzhardinge wrote:
> On 06/24/2011 12:46 PM, David Miller wrote:
> > Pages get transferred between different SKBs all the time.
> >
> > For example, GRO makes extensive use of this technique.
> > See net/core/skbuff.c:skb_gro_receive().
> >
> > It is just one example.
> 
> I see, and the new skb doesn't get a destructor copied from the
> original, so there'd be no second callback.

What about if we were to have a per-shinfo destructor (called once for
each page as its refcount goes 1->0, from whichever skb ends up with the
last ref) as well as the skb-destructors. This already handles the
cloning case but when pages are moved between shinfo then would it make
sense for that to be propagated between skb's under these circumstances
and/or require them to be the same? Since in the case of something like
skb_gro_receive the skbs (and hence the frag array pages) are all from
the same 'owner' (even if the skb is actually created by the stack on
their behalf) I suspect this could work?

But I bet this assumption isn't valid in all cases.

In which case I end up wondering about a destructor per page in the frag
array. At which point we might as well consider it as a part of the core
mm stuff rather than something net specific?

Ian.

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