block: null_blk: only set set->nr_maps as 3 if active poll_queues is > 0

It isn't correct to set set->nr_maps as 3 if g_poll_queues is > 0 since
we can change it via configfs for null_blk device created there, so only
set it as 3 if active poll_queues is > 0.

Fixes divide zero exception reported by Shinichiro.

Fixes: 2bfdbe8b7ebd ("null_blk: allow zero poll queues")
Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Link: https://lore.kernel.org/r/20211224010831.1521805-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
index 6be6ccd..13004be 100644 (file)
--- a/drivers/block/null_blk/main.c
+++ b/drivers/block/null_blk/main.c
@@ -1892,7 +1892,7 @@ static int null_init_tag_set(struct nullb *nullb, struct blk_mq_tag_set *set)
        if (g_shared_tag_bitmap)
                set->flags |= BLK_MQ_F_TAG_HCTX_SHARED;
        set->driver_data = nullb;
-       if (g_poll_queues)
+       if (poll_queues)
                set->nr_maps = 3;
        else
                set->nr_maps = 1;