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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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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