dm table: fix discard support

Remove 'discards_supported' from the dm_table structure.  The same
information can be easily discovered from the table's target(s) in
dm_table_supports_discards().

Before this fix dm_table_supports_discards() would skip checking the
individual targets' 'discards_supported' flag if any one target in the
table didn't set num_discard_requests > 0.  Now the per-target
'discards_supported' flag is effective at insuring the final DM device
advertises discard support.  But, to be clear, targets that don't
support discards (!num_discard_requests) will not receive discard
requests.

Also DMWARN if a target sets 'discards_supported' override but forgets
to set 'num_discard_requests'.
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-table.c

@@ -54,7 +54,6 @@ struct dm_table {
 	sector_t *highs;
 	struct dm_target *targets;
 
-	unsigned discards_supported:1;
 	unsigned integrity_supported:1;
 
 	/*
@@ -209,7 +208,6 @@ int dm_table_create(struct dm_table **result, fmode_t mode,
 	INIT_LIST_HEAD(&t->devices);
 	INIT_LIST_HEAD(&t->target_callbacks);
 	atomic_set(&t->holders, 0);
-	t->discards_supported = 1;
 
 	if (!num_targets)
 		num_targets = KEYS_PER_NODE;
@@ -791,8 +789,9 @@ int dm_table_add_target(struct dm_table *t, const char *type,
 
 	t->highs[t->num_targets++] = tgt->begin + tgt->len - 1;
 
-	if (!tgt->num_discard_requests)
-		t->discards_supported = 0;
+	if (!tgt->num_discard_requests && tgt->discards_supported)
+		DMWARN("%s: %s: ignoring discards_supported because num_discard_requests is zero.",
+		       dm_device_name(t->md), type);
 
 	return 0;
 
@@ -1359,19 +1358,19 @@ bool dm_table_supports_discards(struct dm_table *t)
 	struct dm_target *ti;
 	unsigned i = 0;
 
-	if (!t->discards_supported)
-		return 0;
-
 	/*
 	 * Unless any target used by the table set discards_supported,
 	 * require at least one underlying device to support discards.
 	 * t->devices includes internal dm devices such as mirror logs
 	 * so we need to use iterate_devices here, which targets
-	 * supporting discard must provide.
+	 * supporting discard selectively must provide.
 	 */
 	while (i < dm_table_get_num_targets(t)) {
 		ti = dm_table_get_target(t, i++);
 
+		if (!ti->num_discard_requests)
+			continue;
+
 		if (ti->discards_supported)
 			return 1;
 

drivers/md/dm.c

@@ -1179,7 +1179,8 @@ static int __clone_and_map_discard(struct clone_info *ci)
 
 		/*
 		 * Even though the device advertised discard support,
-		 * reconfiguration might have changed that since the
+		 * that does not mean every target supports it, and
+		 * reconfiguration might also have changed that since the
 		 * check was performed.
 		 */
 		if (!ti->num_discard_requests)