xen/netfront: improve truesize tracking

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)->pull_to so use that instead.

Fixes WARN_ON from skb_try_coalesce.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Sander Eikelenboom <linux@eikelenboom.it>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: annie li <annie.li@oracle.com>
Cc: xen-devel@lists.xen.org
Cc: netdev@vger.kernel.org
Cc: stable@kernel.org # 3.7.x only
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 23 deletions

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index c26e28b4bd9f..7ffa43bd7cf9 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -1015,29 +1015,10 @@ err:
                 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)