dm: do not initialise full request queue when bio based

Change bio-based mapped devices no longer to have a fully initialized
request_queue (request_fn, elevator, etc).  This means bio-based DM
devices no longer register elevator sysfs attributes ('iosched/' tree
or 'scheduler' other than "none").

In contrast, a request-based DM device will continue to have a full
request_queue and will register elevator sysfs attributes.  Therefore
a user can determine a DM device's type by checking if elevator sysfs
attributes exist.

First allocate a minimalist request_queue structure for a DM device
(needed for both bio and request-based DM).

Initialization of a full request_queue is deferred until it is known
that the DM device is request-based, at the end of the table load
sequence.

Factor DM device's request_queue initialization:
- common to both request-based and bio-based into dm_init_md_queue().
- specific to request-based into dm_init_request_based_queue().

The md->type_lock mutex is used to protect md->queue, in addition to
md->type, during table_load().

A DM device's first table_load will establish the immutable md->type.
But md->queue initialization, based on md->type, may fail at that time
(because blk_init_allocated_queue cannot allocate memory).  Therefore
any subsequent table_load must (re)try dm_setup_md_queue independently of
establishing md->type.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
index 4702f380cb45..ed8585954a3a 100644
--- a/drivers/md/dm-ioctl.c
+++ b/drivers/md/dm-ioctl.c
@@ -1189,7 +1189,7 @@ static int table_load(struct dm_ioctl *param, size_t param_size)
 		goto out;
 	}
 
-	/* Protect md->type against concurrent table loads. */
+	/* Protect md->type and md->queue against concurrent table loads. */
 	dm_lock_md_type(md);
 	if (dm_get_md_type(md) == DM_TYPE_NONE)
 		/* Initial table load: acquire type of table. */
@@ -1201,6 +1201,15 @@ static int table_load(struct dm_ioctl *param, size_t param_size)
 		r = -EINVAL;
 		goto out;
 	}
+
+	/* setup md->queue to reflect md's type (may block) */
+	r = dm_setup_md_queue(md);
+	if (r) {
+		DMWARN("unable to set up device queue for new table.");
+		dm_table_destroy(t);
+		dm_unlock_md_type(md);
+		goto out;
+	}
 	dm_unlock_md_type(md);
 
 	/* stage inactive table */