dm zero: add discard support

Add zero_io_hints() and set discard limits so that the zero target
advertises support for discards.

The zero target will ignore discards.

This is useful when the user combines dm-zero with other
discard-supporting targets in the same table; without dm-zero support,
discards would be disabled for the whole combined device.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Tested-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-zero.c

@@ -27,6 +27,7 @@ static int zero_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 	 * Silently drop discards, avoiding -EOPNOTSUPP.
 	 */
 	ti->num_discard_bios = 1;
+	ti->discards_supported = true;
 
 	return 0;
 }
@@ -45,6 +46,7 @@ static int zero_map(struct dm_target *ti, struct bio *bio)
 		zero_fill_bio(bio);
 		break;
 	case REQ_OP_WRITE:
+	case REQ_OP_DISCARD:
 		/* writes get silently dropped */
 		break;
 	default:
@@ -57,13 +59,21 @@ static int zero_map(struct dm_target *ti, struct bio *bio)
 	return DM_MAPIO_SUBMITTED;
 }
 
+static void zero_io_hints(struct dm_target *ti, struct queue_limits *limits)
+{
+	limits->max_discard_sectors = UINT_MAX;
+	limits->max_hw_discard_sectors = UINT_MAX;
+	limits->discard_granularity = 512;
+}
+
 static struct target_type zero_target = {
 	.name   = "zero",
-	.version = {1, 1, 0},
+	.version = {1, 2, 0},
 	.features = DM_TARGET_NOWAIT,
 	.module = THIS_MODULE,
 	.ctr    = zero_ctr,
 	.map    = zero_map,
+	.io_hints = zero_io_hints,
 };
 
 static int __init dm_zero_init(void)