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Subject[PATCH 3/2 v2] dm: bio-based device must not register elevator in sysfs
Properly clear io scheduler state from sysfs if transitioning from a
request-based table back to bio-based. Unregister the elevator from
sysfs (removes 'iosched/') and have the 'scheduler' sysfs attribute
properly report "none" for bio-based DM.

Adjust elv_iosched_show() to return "none" if !blk_queue_stackable.
This allows the block layer to take bio-based DM into consideration.

These changes address the following (albeit obscure) corner-case:
# dmsetup create --notable bio-based
# echo "0 100 multipath ..." | dmsetup load bio-based
# echo "0 100 linear ..." | dmsetup load bio-based
# dmsetup resume bio-based
# ls /sys/block/dm-0/queue/
... iosched ...
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
block/elevator.c | 2 +-
drivers/md/dm-table.c | 1 +
drivers/md/dm.c | 44 ++++++++++++++++++++++++++++++++++++--------
drivers/md/dm.h | 1 +
4 files changed, 39 insertions(+), 9 deletions(-)
v2 changes:
* switched to using BUG_ON in dm_{init,clear}_request_based_queue
* refined dm_clear_request_based_queue slightly
* introduced dm_bio_based_md
* cleaned up some comments
Index: linux-2.6/drivers/md/dm-table.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-table.c
+++ linux-2.6/drivers/md/dm-table.c
@@ -803,6 +803,7 @@ int dm_table_set_type(struct dm_table *t
if (bio_based) {
/* We must use this table as bio-based */
t->type = DM_TYPE_BIO_BASED;
+ dm_clear_request_based_queue(t->md);
return 0;
}

Index: linux-2.6/drivers/md/dm.c
===================================================================
--- linux-2.6.orig/drivers/md/dm.c
+++ linux-2.6/drivers/md/dm.c
@@ -1445,6 +1445,11 @@ static int dm_request_based(struct mappe
return blk_queue_stackable(md->queue);
}

+static int dm_bio_based_md(struct mapped_device *md)
+{
+ return (md->queue->request_fn) ? 0 : 1;
+}
+
static int dm_request(struct request_queue *q, struct bio *bio)
{
struct mapped_device *md = q->queuedata;
@@ -1952,19 +1957,16 @@ bad_module_get:
}

/*
- * Fully initialize the request_queue (elevator, ->request_fn, etc).
+ * Fully initialize a request-based queue (->elevator, ->request_fn, etc).
*/
int dm_init_request_based_queue(struct mapped_device *md)
{
struct request_queue *q = NULL;
+ int register_elevator = 0;

- if (!md->queue)
- return 0;
+ BUG_ON(!md->queue);

- /*
- * Avoid re-initializing the queue (and leaking the existing
- * elevator) if dm_init_request_based_queue() was already used.
- */
+ /* Avoid re-initializing the queue if already fully initialized */
if (!md->queue->elevator) {
/* Fully initialize the queue */
q = blk_init_allocated_queue(md->queue, dm_request_fn, NULL);
@@ -1973,9 +1975,21 @@ int dm_init_request_based_queue(struct m
md->queue = q;
md->saved_make_request_fn = md->queue->make_request_fn;
blk_queue_make_request(md->queue, dm_request);
- elv_register_queue(md->queue);
+ register_elevator = 1;
+ } else if (dm_bio_based_md(md)) {
+ /*
+ * Queue was fully initialized on behalf of a previous
+ * request-based table load. Table is now switching from
+ * bio-based back to request-based, e.g.: rq -> bio -> rq
+ */
+ md->queue->request_fn = dm_request_fn;
+ register_elevator = 1;
}

+ /* Avoid re-registering elevator if already registered */
+ if (register_elevator)
+ elv_register_queue(md->queue);
+
/*
* Request-based dm devices cannot be stacked on top of bio-based dm
* devices. The type of this dm device has not been decided yet.
@@ -1996,6 +2010,20 @@ int dm_init_request_based_queue(struct m
return 1;
}

+void dm_clear_request_based_queue(struct mapped_device *md)
+{
+ BUG_ON(!md->queue);
+
+ if (dm_bio_based_md(md))
+ return; /* already bio-based so return */
+
+ /* Clear ->request_fn, not used for bio-based */
+ md->queue->request_fn = NULL;
+
+ /* Unregister the request-based queue's elevator from sysfs */
+ elv_unregister_queue(md->queue);
+}
+
static void unlock_fs(struct mapped_device *md);

static void free_dev(struct mapped_device *md)
Index: linux-2.6/drivers/md/dm.h
===================================================================
--- linux-2.6.orig/drivers/md/dm.h
+++ linux-2.6/drivers/md/dm.h
@@ -114,6 +114,7 @@ struct kobject *dm_kobject(struct mapped
struct mapped_device *dm_get_from_kobject(struct kobject *kobj);

int dm_init_request_based_queue(struct mapped_device *md);
+void dm_clear_request_based_queue(struct mapped_device *md);

/*
* Targets for linear and striped mappings
Index: linux-2.6/block/elevator.c
===================================================================
--- linux-2.6.orig/block/elevator.c
+++ linux-2.6/block/elevator.c
@@ -1086,7 +1086,7 @@ ssize_t elv_iosched_show(struct request_
struct elevator_type *__e;
int len = 0;

- if (!q->elevator)
+ if (!q->elevator || !blk_queue_stackable(q))
return sprintf(name, "none\n");

elv = e->elevator_type;

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