[PATCH v2 2/7] block: Change elevator_init_mq() to always succeed

[Damien Le Moal]

If the default elevator chosen is mq-deadline, elevator_init_mq() may
return an error if mq-deadline initialization fails, leading to
blk_mq_init_allocated_queue() returning an error, which in turn will
cause the block device initialization to fail and the device not being
exposed.

Instead of taking such extreme measure, handle mq-deadline
initialization failures in the same manner as when mq-deadline is not
available (no module to load), that is, default to the "none" scheduler.
With this change, elevator_init_mq() return type can be changed to void.

Signed-off-by: Damien Le Moal <damien.lem...@wdc.com>
Reviewed-by: Johannes Thumshirn <jthumsh...@suse.de>
---
 block/blk-mq.c   |  8 +-------
 block/blk.h      |  2 +-
 block/elevator.c | 23 ++++++++++++-----------
 3 files changed, 14 insertions(+), 19 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index de3aeafe48eb..0c9b1f403db8 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2833,8 +2833,6 @@ static unsigned int nr_hw_queues(struct blk_mq_tag_set 
*set)
 struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
                                                  struct request_queue *q)
 {
-       int ret = -ENOMEM;
-
        /* mark the queue as mq asap */
        q->mq_ops = set->ops;
 
@@ -2895,14 +2893,10 @@ 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);
 
-       ret = elevator_init_mq(q);
-       if (ret)
-               goto err_tag_set;
+       elevator_init_mq(q);
 
        return q;
 
-err_tag_set:
-       blk_mq_del_queue_tag_set(q);
 err_hctxs:
        kfree(q->queue_hw_ctx);
        q->nr_hw_queues = 0;
diff --git a/block/blk.h b/block/blk.h
index e4619fc5c99a..ed347f7a97b1 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -184,7 +184,7 @@ void blk_account_io_done(struct request *req, u64 now);
 
 void blk_insert_flush(struct request *rq);
 
-int elevator_init_mq(struct request_queue *q);
+void elevator_init_mq(struct request_queue *q);
 int elevator_switch_mq(struct request_queue *q,
                              struct elevator_type *new_e);
 void __elevator_exit(struct request_queue *, struct elevator_queue *);
diff --git a/block/elevator.c b/block/elevator.c
index ab4d50c0ed43..06b70981a054 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -642,34 +642,35 @@ static inline bool elv_support_iosched(struct 
request_queue *q)
 
 /*
  * For blk-mq devices supporting IO scheduling, we default to using 
mq-deadline,
- * if available, for single queue devices. If deadline isn't available OR we
- * have multiple queues, default to "none".
+ * if available, for single queue devices. If deadline isn't available OR
+ * deadline initialization fails OR we have multiple queues, default to "none".
  */
-int elevator_init_mq(struct request_queue *q)
+void elevator_init_mq(struct request_queue *q)
 {
        struct elevator_type *e;
-       int err = 0;
+       int err;
 
        if (!elv_support_iosched(q))
-               return 0;
+               return;
 
        if (q->nr_hw_queues != 1)
-               return 0;
+               return;
 
        WARN_ON_ONCE(test_bit(QUEUE_FLAG_REGISTERED, &q->queue_flags));
 
        if (unlikely(q->elevator))
-               goto out;
+               return;
 
        e = elevator_get(q, "mq-deadline", false);
        if (!e)
-               goto out;
+               return;
 
        err = blk_mq_init_sched(q, e);
-       if (err)
+       if (err) {
+               pr_warn("\"%s\" elevator initialization failed, "
+                       "falling back to \"none\"\n", e->elevator_name);
                elevator_put(e);
-out:
-       return err;
+       }
 }
 
 
-- 
2.21.0