blk-mq: prealloc tags when increase tagset nr_hw_queues

Just like blk_mq_alloc_tag_set(), it's better to prepare all tags before
using to map to queue ctxs in blk_mq_map_swqueue(), which now have to
consider empty set->tags[].

The good point is that we can fallback easily if increasing nr_hw_queues
fail, instead of just mapping to hctx[0] when fail in blk_mq_map_swqueue().

And the fallback path already has tags free & clean handling, so all
is good.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230821095602.70742-3-chengming.zhou@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -4397,6 +4397,16 @@ static int blk_mq_realloc_tag_set_tags(struct blk_mq_tag_set *set,
 		       sizeof(*set->tags));
 	kfree(set->tags);
 	set->tags = new_tags;
+
+	for (i = set->nr_hw_queues; i < new_nr_hw_queues; i++) {
+		if (!__blk_mq_alloc_map_and_rqs(set, i)) {
+			while (--i >= set->nr_hw_queues)
+				__blk_mq_free_map_and_rqs(set, i);
+			return -ENOMEM;
+		}
+		cond_resched();
+	}
+
 done:
 	set->nr_hw_queues = new_nr_hw_queues;
 	return 0;