xen/netback: Calculate the number of SKB slots required correctly

When calculating the number of slots required for a packet header, the code
was reserving too many slots if the header crossed a page boundary. Since
netbk_gop_skb copies the header to the start of the page, the count of
slots required for the header should be based solely on the header size.

This problem is easy to reproduce if a VIF is bridged to a USB 3G modem
device as the skb->data value always starts near the end of the first page.

Signed-off-by: Simon Graham <simon.graham@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 1 insertions, 2 deletions

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index 2596401308a8..f4a6fcaeffb1 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -325,8 +325,7 @@ unsigned int xen_netbk_count_skb_slots(struct xenvif *vif, struct sk_buff *skb)
         unsigned int count;
         int i, copy_off;
 
-        count = DIV_ROUND_UP(
-                        offset_in_page(skb->data)+skb_headlen(skb), PAGE_SIZE);
+        count = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE);
 
         copy_off = skb_headlen(skb) % PAGE_SIZE;