netdev - Re: [Xen-devel] [PATCH] xen/netfront: handle compound page fragments on transmit

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Date:	Tue, 20 Nov 2012 15:06:54 +0000
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	Stefan Bader <stefan.bader@...onical.com>,
	Sander Eikelenboom <linux@...elenboom.it>,
	Eric Dumazet <edumazet@...gle.com>,
	KonradRzeszutekWilk <konrad@...nel.org>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	ANNIE LI <annie.li@...cle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen/netfront: handle compound page
 fragments on transmit

On Tue, 2012-11-20 at 14:32 +0000, Jan Beulich wrote:
> > @@ -517,15 +540,16 @@ static int xennet_start_xmit(struct sk_buff *skb, 
> > struct net_device *dev)
> >  	grant_ref_t ref;
> >  	unsigned long mfn;
> >  	int notify;
> > -	int frags = skb_shinfo(skb)->nr_frags;
> > +	int slots;
> >  	unsigned int offset = offset_in_page(data);
> >  	unsigned int len = skb_headlen(skb);
> >  	unsigned long flags;
> >  
> > -	frags += DIV_ROUND_UP(offset + len, PAGE_SIZE);
> > -	if (unlikely(frags > MAX_SKB_FRAGS + 1)) {
> > -		printk(KERN_ALERT "xennet: skb rides the rocket: %d frags\n",
> > -		       frags);
> > +	slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) +
> > +		xennet_count_skb_frag_slots(skb);
> > +	if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
> 
> But still - isn't this wrong now (i.e. can't it now validly exceed the
> boundary checked for)?

In practice no because of the property that the number of pages backing
the frags is <= MAX_SKB_FRAGS even if you are using compound pages as
the frags.

Ian.


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