xen/netfront: improve truesize tracking

Ian Campbell · davem330 · commit d9a58a782e39 · 2013-01-07T19:51:19.000-08:00
Using RX_COPY_THRESHOLD is incorrect if the SKB is actually smaller
than that. We have already accounted for this in
NETFRONT_SKB_CB(skb)-&gt;pull_to so use that instead.

Fixes WARN_ON from skb_try_coalesce.

Signed-off-by: Ian Campbell &lt;ian.campbell@citrix.com&gt;
Cc: Sander Eikelenboom &lt;linux@eikelenboom.it&gt;
Cc: Konrad Rzeszutek Wilk &lt;konrad.wilk@oracle.com&gt;
Cc: annie li &lt;annie.li@oracle.com&gt;
Cc: xen-devel@lists.xen.org
Cc: netdev@vger.kernel.org
Cc: stable@kernel.org # 3.7.x only
Acked-by: Eric Dumazet &lt;edumazet@google.com&gt;
Signed-off-by: David S. Miller &lt;davem@davemloft.net&gt;
diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
@@ -1015,29 +1015,10 @@ static int xennet_poll(struct napi_struct *napi, int budget)
 		i = xennet_fill_frags(np, skb, &tmpq);
 
 		/*
-		 * Truesize approximates the size of true data plus
-		 * any supervisor overheads. Adding hypervisor
-		 * overheads has been shown to significantly reduce
-		 * achievable bandwidth with the default receive
-		 * buffer size. It is therefore not wise to account
-		 * for it here.
-		 *
-		 * After alloc_skb(RX_COPY_THRESHOLD), truesize is set
-		 * to RX_COPY_THRESHOLD + the supervisor
-		 * overheads. Here, we add the size of the data pulled
-		 * in xennet_fill_frags().
-		 *
-		 * We also adjust for any unused space in the main
-		 * data area by subtracting (RX_COPY_THRESHOLD -
-		 * len). This is especially important with drivers
-		 * which split incoming packets into header and data,
-		 * using only 66 bytes of the main data area (see the
-		 * e1000 driver for example.)  On such systems,
-		 * without this last adjustement, our achievable
-		 * receive throughout using the standard receive
-		 * buffer size was cut by 25%(!!!).
-		 */
-		skb->truesize += skb->data_len - RX_COPY_THRESHOLD;
+                 * Truesize is the actual allocation size, even if the
+                 * allocation is only partially used.
+                 */
+		skb->truesize += PAGE_SIZE * skb_shinfo(skb)->nr_frags;
 		skb->len += skb->data_len;
 
 		if (rx->flags & XEN_NETRXF_csum_blank)
