[Xen-devel] [PATCH net] xen-netback: Fix slot estimation

[Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>]

A recent commit (a02eb4 "xen-netback: worse-case estimate in xenvif_rx_action is
underestimating") capped the slot estimation to MAX_SKB_FRAGS, but that triggers
the next BUG_ON a few lines down, as the packet consumes more slots than
estimated.
This patch remove that cap, and if the frontend doesn't provide enough slot,
put back the skb to the top of the queue and caps rx_last_skb_slots. When the
next try also fails, it drops the packet.
Capping rx_last_skb_slots is needed because if the frontend never gives enough
slots, the ring gets stalled.

Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
Cc: Paul Durrant <paul.durrant@xxxxxxxxxx>
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
Cc: David Vrabel <david.vrabel@xxxxxxxxxx>

diff --git a/drivers/net/xen-netback/netback.c 
b/drivers/net/xen-netback/netback.c
index da85ffb..7164157 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -600,13 +600,6 @@ static void xenvif_rx_action(struct xenvif *vif)
                                                         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 ||
@@ -615,9 +608,27 @@ static void xenvif_rx_action(struct xenvif *vif)
 
                /* If the skb may not fit then bail out now */
                if (!xenvif_rx_ring_slots_available(vif, max_slots_needed)) {
+                       /* If the skb needs more than MAX_SKB_FRAGS slots, it
+                        * can happen that the frontend never gives us enough.
+                        * To avoid spining on that packet, first we put it back
+                        * to the top of the queue, but if the next try fail,
+                        * we drop it.
+                        */
+                       if (max_slots_needed > MAX_SKB_FRAGS &&
+                           vif->rx_last_skb_slots == MAX_SKB_FRAGS) {
+                               kfree_skb(skb);
+                               vif->rx_last_skb_slots = 0;
+                               continue;
+                       }
                        skb_queue_head(&vif->rx_queue, skb);
                        need_to_notify = true;
-                       vif->rx_last_skb_slots = max_slots_needed;
+                       /* Cap this otherwise if the guest never gives us
+                        * enough slot, rx_work_todo will spin
+                        */
+                       vif->rx_last_skb_slots =
+                               max_slots_needed > MAX_SKB_FRAGS ?
+                               MAX_SKB_FRAGS :
+                               max_slots_needed;
                        break;
                } else
                        vif->rx_last_skb_slots = 0;

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