xen-netback: napi: fix abuse of budget

netback seems to be somewhat confused about the napi budget parameter. The
parameter is supposed to limit the number of skbs processed in each poll,
but netback has this confused with grant operations.

This patch fixes that, properly limiting the work done in each poll. Note
that this limit makes sure we do not process any more data from the shared
ring than we intend to pass back from the poll. This is important to
prevent tx_queue potentially growing without bound.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/netback.c

@@ -1445,14 +1445,15 @@ static bool tx_credit_exceeded(struct xenvif *vif, unsigned size)
 	return false;
 }
 
-static unsigned xenvif_tx_build_gops(struct xenvif *vif)
+static unsigned xenvif_tx_build_gops(struct xenvif *vif, int budget)
 {
 	struct gnttab_copy *gop = vif->tx_copy_ops, *request_gop;
 	struct sk_buff *skb;
 	int ret;
 
 	while ((nr_pending_reqs(vif) + XEN_NETBK_LEGACY_SLOTS_MAX
-		< MAX_PENDING_REQS)) {
+		< MAX_PENDING_REQS) &&
+	       (skb_queue_len(&vif->tx_queue) < budget)) {
 		struct xen_netif_tx_request txreq;
 		struct xen_netif_tx_request txfrags[XEN_NETBK_LEGACY_SLOTS_MAX];
 		struct page *page;
@@ -1614,14 +1615,13 @@ static unsigned xenvif_tx_build_gops(struct xenvif *vif)
 }
 
 
-static int xenvif_tx_submit(struct xenvif *vif, int budget)
+static int xenvif_tx_submit(struct xenvif *vif)
 {
 	struct gnttab_copy *gop = vif->tx_copy_ops;
 	struct sk_buff *skb;
 	int work_done = 0;
 
-	while (work_done < budget &&
-	       (skb = __skb_dequeue(&vif->tx_queue)) != NULL) {
+	while ((skb = __skb_dequeue(&vif->tx_queue)) != NULL) {
 		struct xen_netif_tx_request *txp;
 		u16 pending_idx;
 		unsigned data_len;
@@ -1696,14 +1696,14 @@ int xenvif_tx_action(struct xenvif *vif, int budget)
 	if (unlikely(!tx_work_todo(vif)))
 		return 0;
 
-	nr_gops = xenvif_tx_build_gops(vif);
+	nr_gops = xenvif_tx_build_gops(vif, budget);
 
 	if (nr_gops == 0)
 		return 0;
 
 	gnttab_batch_copy(vif->tx_copy_ops, nr_gops);
 
-	work_done = xenvif_tx_submit(vif, nr_gops);
+	work_done = xenvif_tx_submit(vif);
 
 	return work_done;
 }