[Xen-devel] [PATCH net v2 2/3] xen-netback: worse-case estimate in xenvif_rx_action is underestimating

[Paul Durrant <paul.durrant@xxxxxxxxxx>]

The worse-case estimate for skb ring slot usage in xenvif_rx_action()
fails to take fragment page_offset into account. The page_offset does,
however, affect the number of times the fragmentation code calls
start_new_rx_buffer() (i.e. consume another slot) and the worse-case
should assume that will always return true. This patch adds the page_offset
into the DIV_ROUND_UP for each frag.

Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
Cc: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
---
 drivers/net/xen-netback/netback.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/net/xen-netback/netback.c 
b/drivers/net/xen-netback/netback.c
index 72314c7..aa87ae8 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -493,8 +493,18 @@ static void xenvif_rx_action(struct xenvif *vif)
                                                PAGE_SIZE);
                for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
                        unsigned int size;
+                       unsigned int offset;
+
                        size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
-                       max_slots_needed += DIV_ROUND_UP(size, PAGE_SIZE);
+                       offset = skb_shinfo(skb)->frags[i].page_offset;
+
+                       /* For a worse-case estimate we need to factor in
+                        * the fragment page offset as this will affect the
+                        * number of times xenvif_gop_frag_copy() will
+                        * call start_new_rx_buffer().
+                        */
+                       max_slots_needed += DIV_ROUND_UP(offset + size,
+                                                        PAGE_SIZE);
                }
                if (skb_is_gso(skb) &&
                   (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV4 ||
-- 
1.7.10.4


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