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

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On 20.11.12 at 16:06, Ian Campbell [off-list ref] wrote:

On Tue, 2012-11-20 at 14:32 +0000, Jan Beulich wrote:

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@@ -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.

So are you saying that there is something in the system
preventing up to MAX_SKB_FRAGS * SKB_FRAG_PAGE_ORDER
(or NETDEV_FRAG_PAGE_MAX_ORDER) skb-s to be created? I
didn't find any. I do notice that __netdev_alloc_frag() currently
never gets called with a size larger than PAGE_SIZE, but
considering that the function just recently got made capable of
that, I'm sure respective users will show up rather sooner than
later.

Jan