xen-blkfront: improve aproximation of required grants per request

Improve the calculation of required grants to process a request by
using nr_phys_segments instead of always assuming a request is going
to use all posible segments.

nr_phys_segments contains the number of scatter-gather DMA addr+len
pairs, which is basically what we put at every granted page.
for_each_sg iterates over the DMA addr+len pairs and uses a grant
page for each of them.
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/xen-blkfront.c

@@ -400,10 +400,13 @@ static int blkif_queue_request(struct request *req)
 	if (unlikely(info->connected != BLKIF_STATE_CONNECTED))
 		return 1;
 
-	max_grefs = info->max_indirect_segments ?
-		    info->max_indirect_segments +
-		    INDIRECT_GREFS(info->max_indirect_segments) :
-		    BLKIF_MAX_SEGMENTS_PER_REQUEST;
+	max_grefs = req->nr_phys_segments;
+	if (max_grefs > BLKIF_MAX_SEGMENTS_PER_REQUEST)
+		/*
+		 * If we are using indirect segments we need to account
+		 * for the indirect grefs used in the request.
+		 */
+		max_grefs += INDIRECT_GREFS(req->nr_phys_segments);
 
 	/* Check if we have enough grants to allocate a requests */
 	if (info->persistent_gnts_c < max_grefs) {