xen-blkfront: correct maximum segment accounting

Making use of "max_indirect_segments=" has issues:
- blkfront_setup_indirect() may end up with zero psegs when PAGE_SIZE
  is sufficiently much larger than XEN_PAGE_SIZE
- the variable driven by the command line option
  (xen_blkif_max_segments) has a somewhat different purpose, and hence
  should namely never end up being zero
- as long as the specified value is lower than the legacy default,
  we better don't use indirect segments at all (or we'd in fact lower
  throughput)
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/block/xen-blkfront.c

@@ -2223,7 +2223,7 @@ static int blkfront_setup_indirect(struct blkfront_ring_info *rinfo)
 	}
 	else
 		grants = info->max_indirect_segments;
-	psegs = grants / GRANTS_PER_PSEG;
+	psegs = DIV_ROUND_UP(grants, GRANTS_PER_PSEG);
 
 	err = fill_grant_buffer(rinfo,
 				(grants + INDIRECT_GREFS(grants)) * BLK_RING_SIZE(info));
@@ -2328,8 +2328,11 @@ static void blkfront_gather_backend_features(struct blkfront_info *info)
 
 	indirect_segments = xenbus_read_unsigned(info->xbdev->otherend,
 					"feature-max-indirect-segments", 0);
-	info->max_indirect_segments = min(indirect_segments,
-					  xen_blkif_max_segments);
+	if (indirect_segments > xen_blkif_max_segments)
+		indirect_segments = xen_blkif_max_segments;
+	if (indirect_segments <= BLKIF_MAX_SEGMENTS_PER_REQUEST)
+		indirect_segments = 0;
+	info->max_indirect_segments = indirect_segments;
 }
 
 /*
@@ -2652,6 +2655,9 @@ static int __init xlblk_init(void)
 	if (!xen_domain())
 		return -ENODEV;
 
+	if (xen_blkif_max_segments < BLKIF_MAX_SEGMENTS_PER_REQUEST)
+		xen_blkif_max_segments = BLKIF_MAX_SEGMENTS_PER_REQUEST;
+
 	if (xen_blkif_max_ring_order > XENBUS_MAX_RING_GRANT_ORDER) {
 		pr_info("Invalid max_ring_order (%d), will use default max: %d.\n",
 			xen_blkif_max_ring_order, XENBUS_MAX_RING_GRANT_ORDER);