xen-netback: Fix handling of skbs requiring too many slots

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 introduces full_coalesce on the skb callback buffer, which is used in
start_new_rx_buffer() to decide whether netback needs coalescing more
aggresively. By doing that, no packet should need more than
(XEN_NETIF_MAX_TX_SIZE + 1) / PAGE_SIZE data slots (excluding the optional GSO
slot, it doesn't carry data, therefore irrelevant in this case), as the provided
buffers are fully utilized.

Signed-off-by: Zoltan Kiss <[e-mail]>
Cc: Paul Durrant <[e-mail]>
Cc: Wei Liu <[e-mail]>
Cc: Ian Campbell <[e-mail]>
Cc: David Vrabel <[e-mail]>
Reviewed-by: Paul Durrant <[e-mail]>
Acked-by: Wei Liu <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/xen-netback/netback.c

···
  * adding 'size' bytes to a buffer which currently contains 'offset'
  * bytes.
  */
-static bool start_new_rx_buffer(int offset, unsigned long size, int head)
+static bool start_new_rx_buffer(int offset, unsigned long size, int head,
+				bool full_coalesce)
 {
 	/* simple case: we have completely filled the current buffer. */
 	if (offset == MAX_BUFFER_OFFSET)
···
 	 *     (i)   this frag would fit completely in the next buffer
 	 * and (ii)  there is already some data in the current buffer
 	 * and (iii) this is not the head buffer.
+	 * and (iv)  there is no need to fully utilize the buffers
 	 *
 	 * Where:
 	 * - (i) stops us splitting a frag into two copies
···
 	 *   by (ii) but is explicitly checked because
 	 *   netfront relies on the first buffer being
 	 *   non-empty and can crash otherwise.
+	 * - (iv) is needed for skbs which can use up more than MAX_SKB_FRAGS
+	 *   slot
 	 *
 	 * This means we will effectively linearise small
 	 * frags but do not needlessly split large buffers
···
 	 * own buffers as before.
 	 */
 	BUG_ON(size > MAX_BUFFER_OFFSET);
-	if ((offset + size > MAX_BUFFER_OFFSET) && offset && !head)
+	if ((offset + size > MAX_BUFFER_OFFSET) && offset && !head &&
+	    !full_coalesce)
 		return true;
 
 	return false;
···
 
 	return meta;
 }
+
+struct xenvif_rx_cb {
+	int meta_slots_used;
+	bool full_coalesce;
+};
+
+#define XENVIF_RX_CB(skb) ((struct xenvif_rx_cb *)(skb)->cb)
 
 /*
  * Set up the grant operations for this fragment. If it's a flipping
···
 		if (bytes > size)
 			bytes = size;
 
-		if (start_new_rx_buffer(npo->copy_off, bytes, *head)) {
+		if (start_new_rx_buffer(npo->copy_off,
+					bytes,
+					*head,
+					XENVIF_RX_CB(skb)->full_coalesce)) {
 			/*
 			 * Netfront requires there to be some data in the head
 			 * buffer.
···
 	}
 }
 
-struct xenvif_rx_cb {
-	int meta_slots_used;
-};
-
-#define XENVIF_RX_CB(skb) ((struct xenvif_rx_cb *)(skb)->cb)
-
 void xenvif_kick_thread(struct xenvif *vif)
 {
 	wake_up(&vif->wq);
···
 
 		/* To avoid the estimate becoming too pessimal for some
 		 * frontends that limit posted rx requests, cap the estimate
-		 * at MAX_SKB_FRAGS.
+		 * at MAX_SKB_FRAGS. In this case netback will fully coalesce
+		 * the skb into the provided slots.
 		 */
-		if (max_slots_needed > MAX_SKB_FRAGS)
+		if (max_slots_needed > MAX_SKB_FRAGS) {
 			max_slots_needed = MAX_SKB_FRAGS;
+			XENVIF_RX_CB(skb)->full_coalesce = true;
+		} else {
+			XENVIF_RX_CB(skb)->full_coalesce = false;
+		}
 
 		/* We may need one more slot for GSO metadata */
 		if (skb_is_gso(skb) &&