netdev - Re: [PATCH 3/4 v2 net-next] net: make GRO aware of skb->head

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Date:	Tue, 01 May 2012 19:04:52 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	Alexander Duyck <alexander.duyck@...il.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Neal Cardwell <ncardwell@...gle.com>,
	Tom Herbert <therbert@...gle.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Michael Chan <mchan@...adcom.com>,
	Matt Carlson <mcarlson@...adcom.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Ben Hutchings <bhutchings@...arflare.com>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	Maciej Żenczykowski <maze@...gle.com>
Subject: Re: [PATCH 3/4 v2 net-next] net: make GRO aware of skb->head_frag

On Tue, 2012-05-01 at 09:17 -0700, Alexander Duyck wrote:
> On 04/30/2012 11:39 PM, Eric Dumazet wrote:
> > On Mon, 2012-04-30 at 22:33 -0700, Alexander Duyck wrote:
> >
> >> The question I had was more specific to GRO.  As long as we have
> >> skb->users == 1 and the skb isn't cloned we should be fine.   It just
> >> hadn't occurred to me before that napi_gro_receive had the extra
> >> requirement that the skb couldn't be cloned.
> >>
> > OK
> >
> > By the way, even if skb was cloned, we would be allowed to steal
> > skb->head.
> >
> > When we clone an oskb we :
> >
> > 1) allocate a struct nskb sk_buff (or use the shadow in case of TCP)
> > 2) increment dataref
> The problem I have is with this piece right here.  So you increment
> dataref.  Now you have an skb that is still pointing to the shared info
> on this page and dataref is 2.  What about the side that is stealing the
> head?  Is it going to be tracking the dataref as well and decrementing
> it before put_page or does it just assume that dataref is 1 and call
> put_page directly?  I am guessing the latter since I didn't see anything
> that allowed for tracking the dataref of stolen heads.

The only changed thing is the kfree() replaced by put_page()

This kfree() was done when last reference to dataref was released.

If we had a problem before, we have same problem after my patch.

Truth is : In TCP (coalesce and splice()) and GRO, we owns skbs.

(See the various __kfree_skb(skb) calls in net/ipv4/tcp_input.c
There is one exception in ipv6 / treq->pktopts ) but its for SYN packet
and this wont be merged with a previous packet.




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