Merge branch 'xen-netback'

Paul Durrant says:

====================
xen-netback: fix rx slot estimation

Sander Eikelenboom reported an issue with ring overflow in netback in
3.14-rc3. This turns outo be be because of a bug in the ring slot estimation
code. This patch series fixes the slot estimation, fixes the BUG_ON() that
was supposed to catch the issue that Sander ran into and also makes a small
fix to start_new_rx_buffer().

v3:
 - Added a cap of MAX_SKB_FRAGS to estimate in patch #2

v2:
 - Added BUG_ON() to patch #1
 - Added more explanation to patch #3
====================

Reported-By: Sander Eikelenboom <linux@eikelenboom.it>
Tested-By: Sander Eikelenboom <linux@eikelenboom.it>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 29 insertions, 4 deletions

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index 438d0c09b7e6..cd0bd95ccc14 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -192,8 +192,8 @@ static bool start_new_rx_buffer(int offset, unsigned long size, int head)
          * into multiple copies tend to give large frags their
          * own buffers as before.
          */
-        if ((offset + size > MAX_BUFFER_OFFSET) &&
-            (size <= MAX_BUFFER_OFFSET) && offset && !head)
+        BUG_ON(size > MAX_BUFFER_OFFSET);
+        if ((offset + size > MAX_BUFFER_OFFSET) && offset && !head)
                 return true;
 
         return false;
@@ -482,6 +482,8 @@ static void xenvif_rx_action(struct xenvif *vif)
 
         while ((skb = skb_dequeue(&vif->rx_queue)) != NULL) {
                 RING_IDX max_slots_needed;
+                RING_IDX old_req_cons;
+                RING_IDX ring_slots_used;
                 int i;
 
                 /* We need a cheap worse case estimate for the number of
@@ -493,9 +495,28 @@ 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);
                 }
+
+                /* To avoid the estimate becoming too pessimal for some
+                 * frontends that limit posted rx requests, cap the estimate
+                 * at MAX_SKB_FRAGS.
+                 */
+                if (max_slots_needed > MAX_SKB_FRAGS)
+                        max_slots_needed = MAX_SKB_FRAGS;
+
+                /* We may need one more slot for GSO metadata */
                 if (skb_is_gso(skb) &&
                    (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV4 ||
                     skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6))
@@ -511,8 +532,12 @@ static void xenvif_rx_action(struct xenvif *vif)
                         vif->rx_last_skb_slots = 0;
 
                 sco = (struct skb_cb_overlay *)skb->cb;
+
+                old_req_cons = vif->rx.req_cons;
                 sco->meta_slots_used = xenvif_gop_skb(skb, &npo);
-                BUG_ON(sco->meta_slots_used > max_slots_needed);
+                ring_slots_used = vif->rx.req_cons - old_req_cons;
+
+                BUG_ON(ring_slots_used > max_slots_needed);
 
                 __skb_queue_tail(&rxq, skb);
         }