[Xen-devel] [PATCH] xen/netfront: improve truesize tracking

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

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@xxxxxxxxxx>
Cc: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: annie li <annie.li@xxxxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
Cc: netdev@xxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxx # 3.7.x only
---
 drivers/net/xen-netfront.c |   15 +++++----------
 1 files changed, 5 insertions(+), 10 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index caa0110..b06ef81 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -971,17 +971,12 @@ err:
                 * 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%(!!!).
+                * We also adjust for the __pskb_pull_tail done in
+                * handle_incoming_queue which pulls data from the
+                * frags into the head area, which is already
+                * accounted in RX_COPY_THRESHOLD.
                 */
-               skb->truesize += skb->data_len - RX_COPY_THRESHOLD;
+               skb->truesize += skb->data_len - NETFRONT_SKB_CB(skb)->pull_to;
                skb->len += skb->data_len;
 
                if (rx->flags & XEN_NETRXF_csum_blank)
-- 
1.7.2.5


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