Xen devel - May 2012 - [PATCH] 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>
---
 drivers/net/xen-netback/netback.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/drivers/net/xen-netback/netback.c
b/drivers/net/xen-netback/netback.c
index 2596401..f4a6fca 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;
 
-- 
1.7.9.1

From: Simon Graham <simon.graham@citrix.com>
Date: Thu, 24 May 2012 12:26:07 -0400
> 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>
Applied.

On Thu, 2012-05-24 at 21:21 +0100, David Miller wrote:
> From: Simon Graham <simon.graham@citrix.com>
> Date: Thu, 24 May 2012 12:26:07 -0400
> 
> > 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>
> 
> Applied.
Thanks both!

Ian.