[PATCH v3 5/7] block: Delay default elevator initialization

[Damien Le Moal <damien.lemoal@wdc.com>]

When elevator_init_mq() is called from blk_mq_init_allocated_queue(),
the only information known about the device is the number of hardware
queues as the block device scan by the device driver is not completed
yet. The device type and the device required features are not set yet,
preventing to correctly choose the default elevator most suitable for
the device.

This currently affects all multi-queue zoned block devices which default
to the "none" elevator instead of the required "mq-deadline" elevator.
These drives currently include host-managed SMR disks connected to a
smartpqi HBA and null_blk block devices with zoned mode enabled.
Upcoming NVMe Zoned Namespace devices will also be affected.

Fix this by moving the execution of elevator_init_mq() from
blk_mq_init_allocated_queue() into __device_add_disk() to allow for the
device driver to probe the device characteristics and set attributes
of the device request queue prior to the elevator initialization.

Also to make sure that the elevator initialization is never done while
requests are in-flight (there should be none when the device driver
calls device_add_disk()), freeze and quiesce the device request queue
before executing blk_mq_init_sched().

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
---
 block/blk-mq.c   | 2 --
 block/elevator.c | 7 +++++++
 block/genhd.c    | 8 ++++++++
 3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index ee4caf0c0807..a37503984206 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2902,8 +2902,6 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
 	blk_mq_add_queue_tag_set(set, q);
 	blk_mq_map_swqueue(q);
 
-	elevator_init_mq(q);
-
 	return q;
 
 err_hctxs:
diff --git a/block/elevator.c b/block/elevator.c
index 520d6b224b74..096a670d22d7 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -712,7 +712,14 @@ void elevator_init_mq(struct request_queue *q)
 	if (!e)
 		return;
 
+	blk_mq_freeze_queue(q);
+	blk_mq_quiesce_queue(q);
+
 	err = blk_mq_init_sched(q, e);
+
+	blk_mq_unquiesce_queue(q);
+	blk_mq_unfreeze_queue(q);
+
 	if (err) {
 		pr_warn("\"%s\" elevator initialization failed, "
 			"falling back to \"none\"\n", e->elevator_name);
diff --git a/block/genhd.c b/block/genhd.c
index 54f1f0d381f4..7380dd7b2257 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -695,6 +695,13 @@ static void __device_add_disk(struct device *parent, struct gendisk *disk,
 	dev_t devt;
 	int retval;
 
+	/*
+	 * The disk queue should now be all set with enough information about
+	 * the device for the elevator code to pick an adequate default
+	 * elevator.
+	 */
+	elevator_init_mq(disk->queue);
+
 	/* minors == 0 indicates to use ext devt from part0 and should
 	 * be accompanied with EXT_DEVT flag.  Make sure all
 	 * parameters make sense.
@@ -734,6 +741,7 @@ static void __device_add_disk(struct device *parent, struct gendisk *disk,
 				    exact_match, exact_lock, disk);
 	}
 	register_disk(parent, disk, groups);
+
 	if (register_queue)
 		blk_register_queue(disk);
 
-- 
2.21.0