[PATCH 0/7] dm: add full blk-mq support to request-based DM

[PATCH 0/7] dm: add full blk-mq support to request-based DM

[Mike Snitzer]

This patchset updates request-based DM (used exclussively by DM
multipath) to support blk-mq I/O path or the old ->request_fn I/O
path -- the default is still the old ->request_fn I/O path.

Like was done for SCSI, I've exposed a dm-mod.ko 'use_blk_mq' module
parameter and a CONFIG_DM_MQ_DEFAULT.  I've queued these patches in
linux-next via linux-dm.git's 'for-next' branch -- my goal is for
these changes to land upstream in 4.1.

I've tested all 4 supported permutations of rq-based queue stacking:
1) blk-mq mpath stacked on blk-mq device(s)
2) blk-mq mpath stacked on ->request_fn device(s)
3) old ->request_fn mpath stacked on blk-mq device(s)
4) old ->request_fn mpath stacked on old ->request_fn device(s)

There is definitely much more room for optimizing the blk-mq mpath
queue (via dm_mq_ops setup) to have more awareness about the
characteristics of the underlying blk-mq device(s) -- Jens offered
some suggestions that are worth pursuing.  Goal being to leverage the
ability to have multiple software submission queues that map to the
underlying paths' HW queue.  I expect Keith to be the most likely
person to pursue these follow-on optimizations given his immediate
access to NVMe devices, etc.

blk-mq allows for the removal of 2 of the 3 mempools and associated
allocations that were traditionally performed using old ->request_fn
I/O path.  The md->bs bioset still remains for cloning a request's
bios.

I'll be pursuing the possibility of removing the bio cloning that
happens when cloning a request (it was put in place for partial
completion of a request's bios but the prospect of avoiding those bio
clones warrants further review of how useful those partial completions
are, see: http://www.redhat.com/archives/dm-devel/2014-June/msg00030.html)

Jens and Keith, special thanks for your help answering more blk-mq
questions than I'd have hoped would be needed while at LSF/MM.

Keith Busch (1):
  blk-mq: don't wait in blk_mq_queue_enter() if __GFP_WAIT isn't set

Mike Snitzer (6):
  blk-mq: fix use of incorrect goto label in blk_mq_init_queue error path
  blk-mq: add blk_mq_init_allocated_queue and export blk_mq_register_disk
  blk-mq: export blk_mq_run_hw_queues
  dm: add full blk-mq support to request-based DM
  dm: optimize dm_mq_queue_rq to _not_ use kthread if using pure blk-mq
  dm: add 'use_blk_mq' module param and expose in per-device ro sysfs attr

 block/blk-mq-sysfs.c   |   1 +
 block/blk-mq.c         |  47 ++++--
 drivers/md/Kconfig     |  11 ++
 drivers/md/dm-mpath.c  |   4 +-
 drivers/md/dm-sysfs.c  |   9 ++
 drivers/md/dm-table.c  |  17 +-
 drivers/md/dm.c        | 414 +++++++++++++++++++++++++++++++++++++++----------
 drivers/md/dm.h        |   5 +-
 include/linux/blk-mq.h |   3 +
 9 files changed, 401 insertions(+), 110 deletions(-)

-- 
1.9.5 (Apple Git-50.3)

[PATCH 1/7] blk-mq: fix use of incorrect goto label in blk_mq_init_queue error path

[Mike Snitzer]

If percpu_ref_init() fails the 'err_hctxs' label should be used instead
of 'err_map'.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 block/blk-mq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 4f4bea2..459840c 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1938,7 +1938,7 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 	 */
 	if (percpu_ref_init(&q->mq_usage_counter, blk_mq_usage_counter_release,
 			    PERCPU_REF_INIT_ATOMIC, GFP_KERNEL))
-		goto err_map;
+		goto err_hctxs;
 
 	setup_timer(&q->timeout, blk_mq_rq_timer, (unsigned long) q);
 	blk_queue_rq_timeout(q, 30000);
-- 
1.9.5 (Apple Git-50.3)

Re: [PATCH 1/7] blk-mq: fix use of incorrect goto label in blk_mq_init_queue error path

[Ming Lei]

On Thu, Mar 12, 2015 at 11:56 AM, Mike Snitzer <snitzer@redhat.com> wrote:
> If percpu_ref_init() fails the 'err_hctxs' label should be used instead
> of 'err_map'.
>
> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> ---
>  block/blk-mq.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 4f4bea2..459840c 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1938,7 +1938,7 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
>          */
>         if (percpu_ref_init(&q->mq_usage_counter, blk_mq_usage_counter_release,
>                             PERCPU_REF_INIT_ATOMIC, GFP_KERNEL))
> -               goto err_map;
> +               goto err_hctxs;

If it is changed to 'goto err_hctxs', percpu_ref_init() need to
move before blk_alloc_queue_node(), otherwise just 'goto err_hw'
is enough, but the former is better.

Thanks,

Re: [PATCH 1/7] blk-mq: fix use of incorrect goto label in blk_mq_init_queue error path

[Mike Snitzer]

On Thu, Mar 12 2015 at  3:48am -0400,
Ming Lei <tom.leiming@gmail.com> wrote:

> On Thu, Mar 12, 2015 at 11:56 AM, Mike Snitzer <snitzer@redhat.com> wrote:
> > If percpu_ref_init() fails the 'err_hctxs' label should be used instead
> > of 'err_map'.
> >
> > Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> > ---
> >  block/blk-mq.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 4f4bea2..459840c 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -1938,7 +1938,7 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
> >          */
> >         if (percpu_ref_init(&q->mq_usage_counter, blk_mq_usage_counter_release,
> >                             PERCPU_REF_INIT_ATOMIC, GFP_KERNEL))
> > -               goto err_map;
> > +               goto err_hctxs;
> 
> If it is changed to 'goto err_hctxs', percpu_ref_init() need to
> move before blk_alloc_queue_node(), otherwise just 'goto err_hw'
> is enough, but the former is better.

Yes, you're correct.  Patch 2 happened to eliminate this problem.  But
I'll get it fixed up and post v2.

The reason this thinko happened is I noticed the problem after I moved
the blk_alloc_queue_node() out in patch 2 and when I rebased to reorder
the fix before that patch 2 context got lost in the shuffle.

Thanks for your review!

Re: [PATCH 1/7] blk-mq: fix use of incorrect goto label in blk_mq_init_queue error path

[Mike Snitzer]

On Thu, Mar 12 2015 at  9:51am -0400,
Mike Snitzer <snitzer@redhat.com> wrote:

> On Thu, Mar 12 2015 at  3:48am -0400,
> Ming Lei <tom.leiming@gmail.com> wrote:
> 
> > On Thu, Mar 12, 2015 at 11:56 AM, Mike Snitzer <snitzer@redhat.com> wrote:
> > > If percpu_ref_init() fails the 'err_hctxs' label should be used instead
> > > of 'err_map'.
> > >
> > > Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> > > ---
> > >  block/blk-mq.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > > index 4f4bea2..459840c 100644
> > > --- a/block/blk-mq.c
> > > +++ b/block/blk-mq.c
> > > @@ -1938,7 +1938,7 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
> > >          */
> > >         if (percpu_ref_init(&q->mq_usage_counter, blk_mq_usage_counter_release,
> > >                             PERCPU_REF_INIT_ATOMIC, GFP_KERNEL))
> > > -               goto err_map;
> > > +               goto err_hctxs;
> > 
> > If it is changed to 'goto err_hctxs', percpu_ref_init() need to
> > move before blk_alloc_queue_node(), otherwise just 'goto err_hw'
> > is enough, but the former is better.
> 
> Yes, you're correct.  Patch 2 happened to eliminate this problem.  But
> I'll get it fixed up and post v2.

Actually, percpu_ref_init clearly cannot go before blk_alloc_queue_node
since percpu_ref_init deferences q.  (buildbot just emailed complaining
about crashes with my v2 patch).  I didn't test this 1/7 patch in
isolation but should have -- when 2/7 is applied the problem goes away.

v3 of these first 2 patches will be inbound shortly.

[PATCH 1/7 v2] blk-mq: fix use of incorrect goto label in blk_mq_init_queue error path

[Mike Snitzer]

If percpu_ref_init() fails the 'err_hctxs' label should be used instead
of 'err_map'.

Rather than reuse 'err_hw', which was introduced if the later
blk_mq_init_hw_queues() call fails, move percpu_ref_init() before
blk_alloc_queue_node().

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Reviewed-by: Ming Lei <ming.lei@canonical.com>
---
 block/blk-mq.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 4f4bea2..0a877d2 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1928,17 +1928,17 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 		hctxs[i]->queue_num = i;
 	}
 
-	q = blk_alloc_queue_node(GFP_KERNEL, set->numa_node);
-	if (!q)
-		goto err_hctxs;
-
 	/*
 	 * Init percpu_ref in atomic mode so that it's faster to shutdown.
 	 * See blk_register_queue() for details.
 	 */
 	if (percpu_ref_init(&q->mq_usage_counter, blk_mq_usage_counter_release,
 			    PERCPU_REF_INIT_ATOMIC, GFP_KERNEL))
-		goto err_map;
+		goto err_hctxs;
+
+	q = blk_alloc_queue_node(GFP_KERNEL, set->numa_node);
+	if (!q)
+		goto err_hctxs;
 
 	setup_timer(&q->timeout, blk_mq_rq_timer, (unsigned long) q);
 	blk_queue_rq_timeout(q, 30000);
-- 
1.9.5 (Apple Git-50.3)

[PATCH 1/7 v3] blk-mq: fix use of incorrect goto label in blk_mq_init_queue error path

[Mike Snitzer]

If percpu_ref_init() fails the allocated q and hctxs must get cleaned
up; using 'err_map' doesn't allow that to happen.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Reviewed-by: Ming Lei <ming.lei@canonical.com>
---
 block/blk-mq.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 4f4bea2..b7b8933 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1938,7 +1938,7 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 	 */
 	if (percpu_ref_init(&q->mq_usage_counter, blk_mq_usage_counter_release,
 			    PERCPU_REF_INIT_ATOMIC, GFP_KERNEL))
-		goto err_map;
+		goto err_mq_usage;
 
 	setup_timer(&q->timeout, blk_mq_rq_timer, (unsigned long) q);
 	blk_queue_rq_timeout(q, 30000);
@@ -1981,7 +1981,7 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 	blk_mq_init_cpu_queues(q, set->nr_hw_queues);
 
 	if (blk_mq_init_hw_queues(q, set))
-		goto err_hw;
+		goto err_mq_usage;
 
 	mutex_lock(&all_q_mutex);
 	list_add_tail(&q->all_q_node, &all_q_list);
@@ -1993,7 +1993,7 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 
 	return q;
 
-err_hw:
+err_mq_usage:
 	blk_cleanup_queue(q);
 err_hctxs:
 	kfree(map);
-- 
1.9.5 (Apple Git-50.3)

[PATCH 2/7] blk-mq: add blk_mq_init_allocated_queue and export blk_mq_register_disk

[Mike Snitzer]

Add a variant of blk_mq_init_queue that allows a previously allocated
queue to be initialized.  blk_mq_init_allocated_queue models
blk_init_allocated_queue -- which was also created for DM's use.

DM's approach to device creation requires a placeholder request_queue be
allocated for use with alloc_dev() but the decision about what type of
request_queue will be ultimately created is deferred until all component
devices referenced in the DM table are processed to determine the table
type (request-based, blk-mq request-based, or bio-based).

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 block/blk-mq-sysfs.c   |  1 +
 block/blk-mq.c         | 28 +++++++++++++++++++---------
 include/linux/blk-mq.h |  2 ++
 3 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
index 1630a20..b79685e 100644
--- a/block/blk-mq-sysfs.c
+++ b/block/blk-mq-sysfs.c
@@ -436,6 +436,7 @@ int blk_mq_register_disk(struct gendisk *disk)
 
 	return 0;
 }
+EXPORT_SYMBOL_GPL(blk_mq_register_disk);
 
 void blk_mq_sysfs_unregister(struct request_queue *q)
 {
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 459840c..d576a78 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1891,9 +1891,25 @@ void blk_mq_release(struct request_queue *q)
 
 struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 {
+	struct request_queue *uninit_q, *q;
+
+	uninit_q = blk_alloc_queue_node(GFP_KERNEL, set->numa_node);
+	if (!uninit_q)
+		return ERR_PTR(-ENOMEM);
+
+	q = blk_mq_init_allocated_queue(set, uninit_q);
+	if (!q)
+		blk_cleanup_queue(uninit_q);
+
+	return q;
+}
+EXPORT_SYMBOL(blk_mq_init_queue);
+
+struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
+						  struct request_queue *q)
+{
 	struct blk_mq_hw_ctx **hctxs;
 	struct blk_mq_ctx __percpu *ctx;
-	struct request_queue *q;
 	unsigned int *map;
 	int i;
 
@@ -1928,10 +1944,6 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 		hctxs[i]->queue_num = i;
 	}
 
-	q = blk_alloc_queue_node(GFP_KERNEL, set->numa_node);
-	if (!q)
-		goto err_hctxs;
-
 	/*
 	 * Init percpu_ref in atomic mode so that it's faster to shutdown.
 	 * See blk_register_queue() for details.
@@ -1981,7 +1993,7 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 	blk_mq_init_cpu_queues(q, set->nr_hw_queues);
 
 	if (blk_mq_init_hw_queues(q, set))
-		goto err_hw;
+		goto err_hctxs;
 
 	mutex_lock(&all_q_mutex);
 	list_add_tail(&q->all_q_node, &all_q_list);
@@ -1993,8 +2005,6 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 
 	return q;
 
-err_hw:
-	blk_cleanup_queue(q);
 err_hctxs:
 	kfree(map);
 	for (i = 0; i < set->nr_hw_queues; i++) {
@@ -2009,7 +2019,7 @@ err_percpu:
 	free_percpu(ctx);
 	return ERR_PTR(-ENOMEM);
 }
-EXPORT_SYMBOL(blk_mq_init_queue);
+EXPORT_SYMBOL(blk_mq_init_allocated_queue);
 
 void blk_mq_free_queue(struct request_queue *q)
 {
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 7aec861..9a75c88 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -164,6 +164,8 @@ enum {
 		<< BLK_MQ_F_ALLOC_POLICY_START_BIT)
 
 struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *);
+struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
+						  struct request_queue *q);
 void blk_mq_finish_init(struct request_queue *q);
 int blk_mq_register_disk(struct gendisk *);
 void blk_mq_unregister_disk(struct gendisk *);
-- 
1.9.5 (Apple Git-50.3)

Re: [PATCH 2/7] blk-mq: add blk_mq_init_allocated_queue and export blk_mq_register_disk

[Ming Lei]

On Thu, Mar 12, 2015 at 11:56 AM, Mike Snitzer <snitzer@redhat.com> wrote:
> Add a variant of blk_mq_init_queue that allows a previously allocated
> queue to be initialized.  blk_mq_init_allocated_queue models
> blk_init_allocated_queue -- which was also created for DM's use.
>
> DM's approach to device creation requires a placeholder request_queue be
> allocated for use with alloc_dev() but the decision about what type of
> request_queue will be ultimately created is deferred until all component
> devices referenced in the DM table are processed to determine the table
> type (request-based, blk-mq request-based, or bio-based).
>
> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> ---
>  block/blk-mq-sysfs.c   |  1 +
>  block/blk-mq.c         | 28 +++++++++++++++++++---------
>  include/linux/blk-mq.h |  2 ++
>  3 files changed, 22 insertions(+), 9 deletions(-)
>
> diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
> index 1630a20..b79685e 100644
> --- a/block/blk-mq-sysfs.c
> +++ b/block/blk-mq-sysfs.c
> @@ -436,6 +436,7 @@ int blk_mq_register_disk(struct gendisk *disk)
>
>         return 0;
>  }
> +EXPORT_SYMBOL_GPL(blk_mq_register_disk);
>
>  void blk_mq_sysfs_unregister(struct request_queue *q)
>  {
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 459840c..d576a78 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1891,9 +1891,25 @@ void blk_mq_release(struct request_queue *q)
>
>  struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
>  {
> +       struct request_queue *uninit_q, *q;
> +
> +       uninit_q = blk_alloc_queue_node(GFP_KERNEL, set->numa_node);
> +       if (!uninit_q)
> +               return ERR_PTR(-ENOMEM);
> +
> +       q = blk_mq_init_allocated_queue(set, uninit_q);
> +       if (!q)
> +               blk_cleanup_queue(uninit_q);
> +
> +       return q;

Current drivers suppose that queue won't be returned as null, and
looks you need to change the check of !q as IS_ERR(q) and return
ERR_PTR() if it is true.

> +}
> +EXPORT_SYMBOL(blk_mq_init_queue);
> +
> +struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
> +                                                 struct request_queue *q)
> +{
>         struct blk_mq_hw_ctx **hctxs;
>         struct blk_mq_ctx __percpu *ctx;
> -       struct request_queue *q;
>         unsigned int *map;
>         int i;
>
> @@ -1928,10 +1944,6 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
>                 hctxs[i]->queue_num = i;
>         }
>
> -       q = blk_alloc_queue_node(GFP_KERNEL, set->numa_node);
> -       if (!q)
> -               goto err_hctxs;
> -
>         /*
>          * Init percpu_ref in atomic mode so that it's faster to shutdown.
>          * See blk_register_queue() for details.
> @@ -1981,7 +1993,7 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
>         blk_mq_init_cpu_queues(q, set->nr_hw_queues);
>
>         if (blk_mq_init_hw_queues(q, set))
> -               goto err_hw;
> +               goto err_hctxs;
>
>         mutex_lock(&all_q_mutex);
>         list_add_tail(&q->all_q_node, &all_q_list);
> @@ -1993,8 +2005,6 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
>
>         return q;
>
> -err_hw:
> -       blk_cleanup_queue(q);
>  err_hctxs:
>         kfree(map);
>         for (i = 0; i < set->nr_hw_queues; i++) {
> @@ -2009,7 +2019,7 @@ err_percpu:
>         free_percpu(ctx);
>         return ERR_PTR(-ENOMEM);
>  }
> -EXPORT_SYMBOL(blk_mq_init_queue);
> +EXPORT_SYMBOL(blk_mq_init_allocated_queue);
>
>  void blk_mq_free_queue(struct request_queue *q)
>  {
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index 7aec861..9a75c88 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -164,6 +164,8 @@ enum {
>                 << BLK_MQ_F_ALLOC_POLICY_START_BIT)
>
>  struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *);
> +struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
> +                                                 struct request_queue *q);
>  void blk_mq_finish_init(struct request_queue *q);
>  int blk_mq_register_disk(struct gendisk *);
>  void blk_mq_unregister_disk(struct gendisk *);
> --
> 1.9.5 (Apple Git-50.3)
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-scsi" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html



-- 
Ming Lei

[PATCH 2/7 v2] blk-mq: add blk_mq_init_allocated_queue and export blk_mq_register_disk

[Mike Snitzer]

Add a variant of blk_mq_init_queue that allows a previously allocated
queue to be initialized.  blk_mq_init_allocated_queue models
blk_init_allocated_queue -- which was also created for DM's use.

DM's approach to device creation requires a placeholder request_queue be
allocated for use with alloc_dev() but the decision about what type of
request_queue will be ultimately created is deferred until all component
devices referenced in the DM table are processed to determine the table
type (request-based, blk-mq request-based, or bio-based).

Also, because of DM's late finalization of the request_queue type
the call to blk_mq_register_disk() doesn't happen during alloc_dev().
Must export blk_mq_register_disk() so that DM can backfill the 'mq' dir
once the blk-mq queue is fully allocated.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Reviewed-by: Ming Lei <ming.lei@canonical.com>
---
 block/blk-mq-sysfs.c   |  1 +
 block/blk-mq.c         | 24 +++++++++++++++++++-----
 include/linux/blk-mq.h |  2 ++
 3 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
index 1630a20..b79685e 100644
--- a/block/blk-mq-sysfs.c
+++ b/block/blk-mq-sysfs.c
@@ -436,6 +436,7 @@ int blk_mq_register_disk(struct gendisk *disk)
 
 	return 0;
 }
+EXPORT_SYMBOL_GPL(blk_mq_register_disk);
 
 void blk_mq_sysfs_unregister(struct request_queue *q)
 {
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 0a877d2..1cab7831 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1891,9 +1891,25 @@ void blk_mq_release(struct request_queue *q)
 
 struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 {
+	struct request_queue *uninit_q, *q;
+
+	uninit_q = blk_alloc_queue_node(GFP_KERNEL, set->numa_node);
+	if (!uninit_q)
+		return ERR_PTR(-ENOMEM);
+
+	q = blk_mq_init_allocated_queue(set, uninit_q);
+	if (IS_ERR(q))
+		blk_cleanup_queue(uninit_q);
+
+	return q;
+}
+EXPORT_SYMBOL(blk_mq_init_queue);
+
+struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
+						  struct request_queue *q)
+{
 	struct blk_mq_hw_ctx **hctxs;
 	struct blk_mq_ctx __percpu *ctx;
-	struct request_queue *q;
 	unsigned int *map;
 	int i;
 
@@ -1981,7 +1997,7 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 	blk_mq_init_cpu_queues(q, set->nr_hw_queues);
 
 	if (blk_mq_init_hw_queues(q, set))
-		goto err_hw;
+		goto err_hctxs;
 
 	mutex_lock(&all_q_mutex);
 	list_add_tail(&q->all_q_node, &all_q_list);
@@ -1993,8 +2009,6 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 
 	return q;
 
-err_hw:
-	blk_cleanup_queue(q);
 err_hctxs:
 	kfree(map);
 	for (i = 0; i < set->nr_hw_queues; i++) {
@@ -2009,7 +2023,7 @@ err_percpu:
 	free_percpu(ctx);
 	return ERR_PTR(-ENOMEM);
 }
-EXPORT_SYMBOL(blk_mq_init_queue);
+EXPORT_SYMBOL(blk_mq_init_allocated_queue);
 
 void blk_mq_free_queue(struct request_queue *q)
 {
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 7aec861..9a75c88 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -164,6 +164,8 @@ enum {
 		<< BLK_MQ_F_ALLOC_POLICY_START_BIT)
 
 struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *);
+struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
+						  struct request_queue *q);
 void blk_mq_finish_init(struct request_queue *q);
 int blk_mq_register_disk(struct gendisk *);
 void blk_mq_unregister_disk(struct gendisk *);
-- 
1.9.5 (Apple Git-50.3)

[PATCH 2/7 v3] blk-mq: add blk_mq_init_allocated_queue and export blk_mq_register_disk

[Mike Snitzer]

Add a variant of blk_mq_init_queue that allows a previously allocated
queue to be initialized.  blk_mq_init_allocated_queue models
blk_init_allocated_queue -- which was also created for DM's use.

DM's approach to device creation requires a placeholder request_queue be
allocated for use with alloc_dev() but the decision about what type of
request_queue will be ultimately created is deferred until all component
devices referenced in the DM table are processed to determine the table
type (request-based, blk-mq request-based, or bio-based).

Also, because of DM's late finalization of the request_queue type
the call to blk_mq_register_disk() doesn't happen during alloc_dev().
Must export blk_mq_register_disk() so that DM can backfill the 'mq' dir
once the blk-mq queue is fully allocated.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Reviewed-by: Ming Lei <ming.lei@canonical.com>
---
 block/blk-mq-sysfs.c   |  1 +
 block/blk-mq.c         | 30 ++++++++++++++++++++----------
 include/linux/blk-mq.h |  2 ++
 3 files changed, 23 insertions(+), 10 deletions(-)

diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
index 1630a20..b79685e 100644
--- a/block/blk-mq-sysfs.c
+++ b/block/blk-mq-sysfs.c
@@ -436,6 +436,7 @@ int blk_mq_register_disk(struct gendisk *disk)
 
 	return 0;
 }
+EXPORT_SYMBOL_GPL(blk_mq_register_disk);
 
 void blk_mq_sysfs_unregister(struct request_queue *q)
 {
diff --git a/block/blk-mq.c b/block/blk-mq.c
index b7b8933..3000121 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1891,9 +1891,25 @@ void blk_mq_release(struct request_queue *q)
 
 struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 {
+	struct request_queue *uninit_q, *q;
+
+	uninit_q = blk_alloc_queue_node(GFP_KERNEL, set->numa_node);
+	if (!uninit_q)
+		return ERR_PTR(-ENOMEM);
+
+	q = blk_mq_init_allocated_queue(set, uninit_q);
+	if (IS_ERR(q))
+		blk_cleanup_queue(uninit_q);
+
+	return q;
+}
+EXPORT_SYMBOL(blk_mq_init_queue);
+
+struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
+						  struct request_queue *q)
+{
 	struct blk_mq_hw_ctx **hctxs;
 	struct blk_mq_ctx __percpu *ctx;
-	struct request_queue *q;
 	unsigned int *map;
 	int i;
 
@@ -1928,17 +1944,13 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 		hctxs[i]->queue_num = i;
 	}
 
-	q = blk_alloc_queue_node(GFP_KERNEL, set->numa_node);
-	if (!q)
-		goto err_hctxs;
-
 	/*
 	 * Init percpu_ref in atomic mode so that it's faster to shutdown.
 	 * See blk_register_queue() for details.
 	 */
 	if (percpu_ref_init(&q->mq_usage_counter, blk_mq_usage_counter_release,
 			    PERCPU_REF_INIT_ATOMIC, GFP_KERNEL))
-		goto err_mq_usage;
+		goto err_hctxs;
 
 	setup_timer(&q->timeout, blk_mq_rq_timer, (unsigned long) q);
 	blk_queue_rq_timeout(q, 30000);
@@ -1981,7 +1993,7 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 	blk_mq_init_cpu_queues(q, set->nr_hw_queues);
 
 	if (blk_mq_init_hw_queues(q, set))
-		goto err_mq_usage;
+		goto err_hctxs;
 
 	mutex_lock(&all_q_mutex);
 	list_add_tail(&q->all_q_node, &all_q_list);
@@ -1993,8 +2005,6 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 
 	return q;
 
-err_mq_usage:
-	blk_cleanup_queue(q);
 err_hctxs:
 	kfree(map);
 	for (i = 0; i < set->nr_hw_queues; i++) {
@@ -2009,7 +2019,7 @@ err_percpu:
 	free_percpu(ctx);
 	return ERR_PTR(-ENOMEM);
 }
-EXPORT_SYMBOL(blk_mq_init_queue);
+EXPORT_SYMBOL(blk_mq_init_allocated_queue);
 
 void blk_mq_free_queue(struct request_queue *q)
 {
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 7aec861..9a75c88 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -164,6 +164,8 @@ enum {
 		<< BLK_MQ_F_ALLOC_POLICY_START_BIT)
 
 struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *);
+struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
+						  struct request_queue *q);
 void blk_mq_finish_init(struct request_queue *q);
 int blk_mq_register_disk(struct gendisk *);
 void blk_mq_unregister_disk(struct gendisk *);
-- 
1.9.5 (Apple Git-50.3)

[PATCH 3/7] blk-mq: export blk_mq_run_hw_queues

[Mike Snitzer]

Rename blk_mq_run_queues to blk_mq_run_hw_queues, add async argument,
and export it.

DM's suspend support must be able to run the queue without starting
stopped hw queues.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 block/blk-mq.c         | 8 ++++----
 include/linux/blk-mq.h | 1 +
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index d576a78..5706dd9 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -33,7 +33,6 @@ static DEFINE_MUTEX(all_q_mutex);
 static LIST_HEAD(all_q_list);
 
 static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx);
-static void blk_mq_run_queues(struct request_queue *q);
 
 /*
  * Check if any of the ctx's have pending work in this hardware queue
@@ -118,7 +117,7 @@ void blk_mq_freeze_queue_start(struct request_queue *q)
 
 	if (freeze) {
 		percpu_ref_kill(&q->mq_usage_counter);
-		blk_mq_run_queues(q);
+		blk_mq_run_hw_queues(q, false);
 	}
 }
 EXPORT_SYMBOL_GPL(blk_mq_freeze_queue_start);
@@ -904,7 +903,7 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
 			&hctx->run_work, 0);
 }
 
-static void blk_mq_run_queues(struct request_queue *q)
+void blk_mq_run_hw_queues(struct request_queue *q, bool async)
 {
 	struct blk_mq_hw_ctx *hctx;
 	int i;
@@ -915,9 +914,10 @@ static void blk_mq_run_queues(struct request_queue *q)
 		    test_bit(BLK_MQ_S_STOPPED, &hctx->state))
 			continue;
 
-		blk_mq_run_hw_queue(hctx, false);
+		blk_mq_run_hw_queue(hctx, async);
 	}
 }
+EXPORT_SYMBOL(blk_mq_run_hw_queues);
 
 void blk_mq_stop_hw_queue(struct blk_mq_hw_ctx *hctx)
 {
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 9a75c88..ebfe707 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -220,6 +220,7 @@ void blk_mq_start_hw_queue(struct blk_mq_hw_ctx *hctx);
 void blk_mq_stop_hw_queues(struct request_queue *q);
 void blk_mq_start_hw_queues(struct request_queue *q);
 void blk_mq_start_stopped_hw_queues(struct request_queue *q, bool async);
+void blk_mq_run_hw_queues(struct request_queue *q, bool async);
 void blk_mq_delay_queue(struct blk_mq_hw_ctx *hctx, unsigned long msecs);
 void blk_mq_tag_busy_iter(struct blk_mq_hw_ctx *hctx, busy_iter_fn *fn,
 		void *priv);
-- 
1.9.5 (Apple Git-50.3)

[PATCH 4/7] blk-mq: don't wait in blk_mq_queue_enter() if __GFP_WAIT isn't set

[Mike Snitzer]

From: Keith Busch <keith.busch@intel.com>

Return -EBUSY if we're unable to enter a queue immediately when
allocating a blk-mq request without __GFP_WAIT.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 block/blk-mq.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 5706dd9..01b2912 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -77,7 +77,7 @@ static void blk_mq_hctx_clear_pending(struct blk_mq_hw_ctx *hctx,
 	clear_bit(CTX_TO_BIT(hctx, ctx), &bm->word);
 }
 
-static int blk_mq_queue_enter(struct request_queue *q)
+static int blk_mq_queue_enter(struct request_queue *q, gfp_t gfp)
 {
 	while (true) {
 		int ret;
@@ -85,6 +85,9 @@ static int blk_mq_queue_enter(struct request_queue *q)
 		if (percpu_ref_tryget_live(&q->mq_usage_counter))
 			return 0;
 
+		if (!(gfp & __GFP_WAIT))
+			return -EBUSY;
+
 		ret = wait_event_interruptible(q->mq_freeze_wq,
 				!q->mq_freeze_depth || blk_queue_dying(q));
 		if (blk_queue_dying(q))
@@ -256,7 +259,7 @@ struct request *blk_mq_alloc_request(struct request_queue *q, int rw, gfp_t gfp,
 	struct blk_mq_alloc_data alloc_data;
 	int ret;
 
-	ret = blk_mq_queue_enter(q);
+	ret = blk_mq_queue_enter(q, gfp);
 	if (ret)
 		return ERR_PTR(ret);
 
@@ -1186,7 +1189,7 @@ static struct request *blk_mq_map_request(struct request_queue *q,
 	int rw = bio_data_dir(bio);
 	struct blk_mq_alloc_data alloc_data;
 
-	if (unlikely(blk_mq_queue_enter(q))) {
+	if (unlikely(blk_mq_queue_enter(q, GFP_KERNEL))) {
 		bio_endio(bio, -EIO);
 		return NULL;
 	}
-- 
1.9.5 (Apple Git-50.3)

[PATCH 5/7] dm: add full blk-mq support to request-based DM

[Mike Snitzer]

Commit e5863d9ad ("dm: allocate requests in target when stacking on
blk-mq devices") served as the first step toward fully utilizing blk-mq
in request-based DM -- it enabled stacking a old-style (request_fn)
request_queue ontop of the underlying blk-mq device(s).  This first step
didn't improve performance of DM multipath ontop of fast blk-mq devices
(e.g. NVMe) because the top-level old-style request_queue was severely
limited by the queue_lock.

The second step offered here enables stacking a blk-mq request_queue
ontop of the underlying blk-mq device(s).  This unlocks significant
performance gains on fast blk-mq devices, Keith Busch tested on his NVMe
testbed and offered this really positive news:

 "Just providing a performance update. All my fio tests are getting
  roughly equal performance whether accessed through the raw block
  device or the multipath device mapper (~470k IOPS). I could only push
  ~20% of the raw iops through dm before this conversion, so this latest
  tree is looking really solid from a performance standpoint."

Tested-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 drivers/md/dm-mpath.c |   4 +-
 drivers/md/dm-table.c |  11 +-
 drivers/md/dm.c       | 326 +++++++++++++++++++++++++++++++++++++++-----------
 3 files changed, 269 insertions(+), 72 deletions(-)

diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
index add6391..2bb6599 100644
--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -428,9 +428,9 @@ static int __multipath_map(struct dm_target *ti, struct request *clone,
 	} else {
 		/* blk-mq request-based interface */
 		*__clone = blk_get_request(bdev_get_queue(bdev),
-					   rq_data_dir(rq), GFP_KERNEL);
+					   rq_data_dir(rq), GFP_ATOMIC);
 		if (IS_ERR(*__clone))
-			/* ENOMEM, requeue */
+			/* ENOMEM or EBUSY, requeue */
 			return r;
 		(*__clone)->bio = (*__clone)->biotail = NULL;
 		(*__clone)->rq_disk = bdev->bd_disk;
diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 0573120..66600ca 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -18,6 +18,7 @@
 #include <linux/mutex.h>
 #include <linux/delay.h>
 #include <linux/atomic.h>
+#include <linux/blk-mq.h>
 
 #define DM_MSG_PREFIX "table"
 
@@ -1695,9 +1696,13 @@ void dm_table_run_md_queue_async(struct dm_table *t)
 	md = dm_table_get_md(t);
 	queue = dm_get_md_queue(md);
 	if (queue) {
-		spin_lock_irqsave(queue->queue_lock, flags);
-		blk_run_queue_async(queue);
-		spin_unlock_irqrestore(queue->queue_lock, flags);
+		if (queue->mq_ops)
+			blk_mq_run_hw_queues(queue, true);
+		else {
+			spin_lock_irqsave(queue->queue_lock, flags);
+			blk_run_queue_async(queue);
+			spin_unlock_irqrestore(queue->queue_lock, flags);
+		}
 	}
 }
 EXPORT_SYMBOL(dm_table_run_md_queue_async);
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index b8cb2d5..b5409ac 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -23,6 +23,7 @@
 #include <linux/kthread.h>
 #include <linux/ktime.h>
 #include <linux/elevator.h> /* for rq_end_sector() */
+#include <linux/blk-mq.h>
 
 #include <trace/events/block.h>
 
@@ -224,6 +225,9 @@ struct mapped_device {
 	int last_rq_rw;
 	sector_t last_rq_pos;
 	ktime_t last_rq_start_time;
+
+	/* for blk-mq request-based DM support */
+	struct blk_mq_tag_set tag_set;
 };
 
 /*
@@ -1022,6 +1026,11 @@ static void end_clone_bio(struct bio *clone, int error)
 	blk_update_request(tio->orig, 0, nr_bytes);
 }
 
+static struct dm_rq_target_io *tio_from_request(struct request *rq)
+{
+	return (rq->q->mq_ops ? blk_mq_rq_to_pdu(rq) : rq->special);
+}
+
 /*
  * Don't touch any member of the md after calling this function because
  * the md may be freed in dm_put() at the end of this function.
@@ -1045,8 +1054,10 @@ static void rq_completed(struct mapped_device *md, int rw, bool run_queue)
 	 * queue lock again.
 	 */
 	if (run_queue) {
-		if (!nr_requests_pending ||
-		    (nr_requests_pending >= md->queue->nr_congestion_on))
+		if (md->queue->mq_ops)
+			blk_mq_run_hw_queues(md->queue, true);
+		else if (!nr_requests_pending ||
+			 (nr_requests_pending >= md->queue->nr_congestion_on))
 			blk_run_queue_async(md->queue);
 	}
 
@@ -1059,13 +1070,17 @@ static void rq_completed(struct mapped_device *md, int rw, bool run_queue)
 static void free_rq_clone(struct request *clone)
 {
 	struct dm_rq_target_io *tio = clone->end_io_data;
+	struct mapped_device *md = tio->md;
 
 	blk_rq_unprep_clone(clone);
+
 	if (clone->q && clone->q->mq_ops)
 		tio->ti->type->release_clone_rq(clone);
 	else
-		free_clone_request(tio->md, clone);
-	free_rq_tio(tio);
+		free_clone_request(md, clone);
+
+	if (!md->queue->mq_ops)
+		free_rq_tio(tio);
 }
 
 /*
@@ -1094,17 +1109,22 @@ static void dm_end_request(struct request *clone, int error)
 	}
 
 	free_rq_clone(clone);
-	blk_end_request_all(rq, error);
+	if (!rq->q->mq_ops)
+		blk_end_request_all(rq, error);
+	else
+		blk_mq_end_request(rq, error);
 	rq_completed(md, rw, true);
 }
 
 static void dm_unprep_request(struct request *rq)
 {
-	struct dm_rq_target_io *tio = rq->special;
+	struct dm_rq_target_io *tio = tio_from_request(rq);
 	struct request *clone = tio->clone;
 
-	rq->special = NULL;
-	rq->cmd_flags &= ~REQ_DONTPREP;
+	if (!rq->q->mq_ops) {
+		rq->special = NULL;
+		rq->cmd_flags &= ~REQ_DONTPREP;
+	}
 
 	if (clone)
 		free_rq_clone(clone);
@@ -1113,18 +1133,29 @@ static void dm_unprep_request(struct request *rq)
 /*
  * Requeue the original request of a clone.
  */
-static void dm_requeue_unmapped_original_request(struct mapped_device *md,
-						 struct request *rq)
+static void old_requeue_request(struct request *rq)
 {
-	int rw = rq_data_dir(rq);
 	struct request_queue *q = rq->q;
 	unsigned long flags;
 
-	dm_unprep_request(rq);
-
 	spin_lock_irqsave(q->queue_lock, flags);
 	blk_requeue_request(q, rq);
 	spin_unlock_irqrestore(q->queue_lock, flags);
+}
+
+static void dm_requeue_unmapped_original_request(struct mapped_device *md,
+						 struct request *rq)
+{
+	int rw = rq_data_dir(rq);
+
+	dm_unprep_request(rq);
+
+	if (!rq->q->mq_ops)
+		old_requeue_request(rq);
+	else {
+		blk_mq_requeue_request(rq);
+		blk_mq_kick_requeue_list(rq->q);
+	}
 
 	rq_completed(md, rw, false);
 }
@@ -1136,35 +1167,44 @@ static void dm_requeue_unmapped_request(struct request *clone)
 	dm_requeue_unmapped_original_request(tio->md, tio->orig);
 }
 
-static void __stop_queue(struct request_queue *q)
-{
-	blk_stop_queue(q);
-}
-
-static void stop_queue(struct request_queue *q)
+static void old_stop_queue(struct request_queue *q)
 {
 	unsigned long flags;
 
+	if (blk_queue_stopped(q))
+		return;
+
 	spin_lock_irqsave(q->queue_lock, flags);
-	__stop_queue(q);
+	blk_stop_queue(q);
 	spin_unlock_irqrestore(q->queue_lock, flags);
 }
 
-static void __start_queue(struct request_queue *q)
+static void stop_queue(struct request_queue *q)
 {
-	if (blk_queue_stopped(q))
-		blk_start_queue(q);
+	if (!q->mq_ops)
+		old_stop_queue(q);
+	else
+		blk_mq_stop_hw_queues(q);
 }
 
-static void start_queue(struct request_queue *q)
+static void old_start_queue(struct request_queue *q)
 {
 	unsigned long flags;
 
 	spin_lock_irqsave(q->queue_lock, flags);
-	__start_queue(q);
+	if (blk_queue_stopped(q))
+		blk_start_queue(q);
 	spin_unlock_irqrestore(q->queue_lock, flags);
 }
 
+static void start_queue(struct request_queue *q)
+{
+	if (!q->mq_ops)
+		old_start_queue(q);
+	else
+		blk_mq_start_stopped_hw_queues(q, true);
+}
+
 static void dm_done(struct request *clone, int error, bool mapped)
 {
 	int r = error;
@@ -1203,13 +1243,20 @@ static void dm_done(struct request *clone, int error, bool mapped)
 static void dm_softirq_done(struct request *rq)
 {
 	bool mapped = true;
-	struct dm_rq_target_io *tio = rq->special;
+	struct dm_rq_target_io *tio = tio_from_request(rq);
 	struct request *clone = tio->clone;
+	int rw;
 
 	if (!clone) {
-		blk_end_request_all(rq, tio->error);
-		rq_completed(tio->md, rq_data_dir(rq), false);
-		free_rq_tio(tio);
+		rw = rq_data_dir(rq);
+		if (!rq->q->mq_ops) {
+			blk_end_request_all(rq, tio->error);
+			rq_completed(tio->md, rw, false);
+			free_rq_tio(tio);
+		} else {
+			blk_mq_end_request(rq, tio->error);
+			rq_completed(tio->md, rw, false);
+		}
 		return;
 	}
 
@@ -1225,7 +1272,7 @@ static void dm_softirq_done(struct request *rq)
  */
 static void dm_complete_request(struct request *rq, int error)
 {
-	struct dm_rq_target_io *tio = rq->special;
+	struct dm_rq_target_io *tio = tio_from_request(rq);
 
 	tio->error = error;
 	blk_complete_request(rq);
@@ -1244,7 +1291,7 @@ static void dm_kill_unmapped_request(struct request *rq, int error)
 }
 
 /*
- * Called with the clone's queue lock held
+ * Called with the clone's queue lock held (for non-blk-mq)
  */
 static void end_clone_request(struct request *clone, int error)
 {
@@ -1805,6 +1852,18 @@ static struct request *clone_rq(struct request *rq, struct mapped_device *md,
 
 static void map_tio_request(struct kthread_work *work);
 
+static void init_tio(struct dm_rq_target_io *tio, struct request *rq,
+		     struct mapped_device *md)
+{
+	tio->md = md;
+	tio->ti = NULL;
+	tio->clone = NULL;
+	tio->orig = rq;
+	tio->error = 0;
+	memset(&tio->info, 0, sizeof(tio->info));
+	init_kthread_work(&tio->work, map_tio_request);
+}
+
 static struct dm_rq_target_io *prep_tio(struct request *rq,
 					struct mapped_device *md, gfp_t gfp_mask)
 {
@@ -1816,13 +1875,7 @@ static struct dm_rq_target_io *prep_tio(struct request *rq,
 	if (!tio)
 		return NULL;
 
-	tio->md = md;
-	tio->ti = NULL;
-	tio->clone = NULL;
-	tio->orig = rq;
-	tio->error = 0;
-	memset(&tio->info, 0, sizeof(tio->info));
-	init_kthread_work(&tio->work, map_tio_request);
+	init_tio(tio, rq, md);
 
 	table = dm_get_live_table(md, &srcu_idx);
 	if (!dm_table_mq_request_based(table)) {
@@ -1866,11 +1919,11 @@ static int dm_prep_fn(struct request_queue *q, struct request *rq)
  * DM_MAPIO_REQUEUE : the original request needs to be requeued
  * < 0              : the request was completed due to failure
  */
-static int map_request(struct dm_target *ti, struct request *rq,
+static int map_request(struct dm_rq_target_io *tio, struct request *rq,
 		       struct mapped_device *md)
 {
 	int r;
-	struct dm_rq_target_io *tio = rq->special;
+	struct dm_target *ti = tio->ti;
 	struct request *clone = NULL;
 
 	if (tio->clone) {
@@ -1885,7 +1938,7 @@ static int map_request(struct dm_target *ti, struct request *rq,
 		}
 		if (IS_ERR(clone))
 			return DM_MAPIO_REQUEUE;
-		if (setup_clone(clone, rq, tio, GFP_KERNEL)) {
+		if (setup_clone(clone, rq, tio, GFP_NOIO)) {
 			/* -ENOMEM */
 			ti->type->release_clone_rq(clone);
 			return DM_MAPIO_REQUEUE;
@@ -1926,13 +1979,16 @@ static void map_tio_request(struct kthread_work *work)
 	struct request *rq = tio->orig;
 	struct mapped_device *md = tio->md;
 
-	if (map_request(tio->ti, rq, md) == DM_MAPIO_REQUEUE)
+	if (map_request(tio, rq, md) == DM_MAPIO_REQUEUE)
 		dm_requeue_unmapped_original_request(md, rq);
 }
 
 static void dm_start_request(struct mapped_device *md, struct request *orig)
 {
-	blk_start_request(orig);
+	if (!orig->q->mq_ops)
+		blk_start_request(orig);
+	else
+		blk_mq_start_request(orig);
 	atomic_inc(&md->pending[rq_data_dir(orig)]);
 
 	if (md->seq_rq_merge_deadline_usecs) {
@@ -2042,7 +2098,7 @@ static void dm_request_fn(struct request_queue *q)
 
 		dm_start_request(md, rq);
 
-		tio = rq->special;
+		tio = tio_from_request(rq);
 		/* Establish tio->ti before queuing work (map_tio_request) */
 		tio->ti = ti;
 		queue_kthread_work(&md->kworker, &tio->work);
@@ -2139,7 +2195,7 @@ static void dm_init_md_queue(struct mapped_device *md)
 {
 	/*
 	 * 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.
+	 * devices.  The type of this dm device may not have been decided yet.
 	 * The type is decided at the first table loading time.
 	 * To prevent problematic device stacking, clear the queue flag
 	 * for request stacking support until then.
@@ -2147,7 +2203,15 @@ static void dm_init_md_queue(struct mapped_device *md)
 	 * This queue is new, so no concurrency on the queue_flags.
 	 */
 	queue_flag_clear_unlocked(QUEUE_FLAG_STACKABLE, md->queue);
+}
 
+static void dm_init_old_md_queue(struct mapped_device *md)
+{
+	dm_init_md_queue(md);
+
+	/*
+	 * Initialize aspects of queue that aren't relevant for blk-mq
+	 */
 	md->queue->queuedata = md;
 	md->queue->backing_dev_info.congested_fn = dm_any_congested;
 	md->queue->backing_dev_info.congested_data = md;
@@ -2270,6 +2334,7 @@ static void unlock_fs(struct mapped_device *md);
 static void free_dev(struct mapped_device *md)
 {
 	int minor = MINOR(disk_devt(md->disk));
+	bool using_blk_mq = !!md->queue->mq_ops;
 
 	unlock_fs(md);
 	bdput(md->bdev);
@@ -2295,6 +2360,8 @@ static void free_dev(struct mapped_device *md)
 
 	put_disk(md->disk);
 	blk_cleanup_queue(md->queue);
+	if (using_blk_mq)
+		blk_mq_free_tag_set(&md->tag_set);
 	dm_stats_cleanup(&md->stats);
 	module_put(THIS_MODULE);
 	kfree(md);
@@ -2452,7 +2519,7 @@ static struct dm_table *__bind(struct mapped_device *md, struct dm_table *t,
 	 * This must be done before setting the queue restrictions,
 	 * because request-based dm may be run just after the setting.
 	 */
-	if (dm_table_request_based(t) && !blk_queue_stopped(q))
+	if (dm_table_request_based(t))
 		stop_queue(q);
 
 	__bind_mempools(md, t);
@@ -2534,14 +2601,6 @@ unsigned dm_get_md_type(struct mapped_device *md)
 	return md->type;
 }
 
-static bool dm_md_type_request_based(struct mapped_device *md)
-{
-	unsigned table_type = dm_get_md_type(md);
-
-	return (table_type == DM_TYPE_REQUEST_BASED ||
-		table_type == DM_TYPE_MQ_REQUEST_BASED);
-}
-
 struct target_type *dm_get_immutable_target_type(struct mapped_device *md)
 {
 	return md->immutable_target_type;
@@ -2558,6 +2617,14 @@ struct queue_limits *dm_get_queue_limits(struct mapped_device *md)
 }
 EXPORT_SYMBOL_GPL(dm_get_queue_limits);
 
+static void init_rq_based_worker_thread(struct mapped_device *md)
+{
+	/* Initialize the request-based DM worker thread */
+	init_kthread_worker(&md->kworker);
+	md->kworker_task = kthread_run(kthread_worker_fn, &md->kworker,
+				       "kdmwork-%s", dm_device_name(md));
+}
+
 /*
  * Fully initialize a request-based queue (->elevator, ->request_fn, etc).
  */
@@ -2566,29 +2633,140 @@ static int dm_init_request_based_queue(struct mapped_device *md)
 	struct request_queue *q = NULL;
 
 	if (md->queue->elevator)
-		return 1;
+		return 0;
 
 	/* Fully initialize the queue */
 	q = blk_init_allocated_queue(md->queue, dm_request_fn, NULL);
 	if (!q)
-		return 0;
+		return -EINVAL;
 
 	/* disable dm_request_fn's merge heuristic by default */
 	md->seq_rq_merge_deadline_usecs = 0;
 
 	md->queue = q;
-	dm_init_md_queue(md);
+	dm_init_old_md_queue(md);
 	blk_queue_softirq_done(md->queue, dm_softirq_done);
 	blk_queue_prep_rq(md->queue, dm_prep_fn);
 
-	/* Also initialize the request-based DM worker thread */
-	init_kthread_worker(&md->kworker);
-	md->kworker_task = kthread_run(kthread_worker_fn, &md->kworker,
-				       "kdmwork-%s", dm_device_name(md));
+	init_rq_based_worker_thread(md);
 
 	elv_register_queue(md->queue);
 
-	return 1;
+	return 0;
+}
+
+static int dm_mq_init_request(void *data, struct request *rq,
+			      unsigned int hctx_idx, unsigned int request_idx,
+			      unsigned int numa_node)
+{
+	struct mapped_device *md = data;
+	struct dm_rq_target_io *tio = blk_mq_rq_to_pdu(rq);
+
+	/*
+	 * Must initialize md member of tio, otherwise it won't
+	 * be available in dm_mq_queue_rq.
+	 */
+	tio->md = md;
+
+	return 0;
+}
+
+static int dm_mq_queue_rq(struct blk_mq_hw_ctx *hctx,
+			  const struct blk_mq_queue_data *bd)
+{
+	struct request *rq = bd->rq;
+	struct dm_rq_target_io *tio = blk_mq_rq_to_pdu(rq);
+	struct mapped_device *md = tio->md;
+	int srcu_idx;
+	struct dm_table *map = dm_get_live_table(md, &srcu_idx);
+	struct dm_target *ti;
+	sector_t pos;
+
+	/* always use block 0 to find the target for flushes for now */
+	pos = 0;
+	if (!(rq->cmd_flags & REQ_FLUSH))
+		pos = blk_rq_pos(rq);
+
+	ti = dm_table_find_target(map, pos);
+	if (!dm_target_is_valid(ti)) {
+		dm_put_live_table(md, srcu_idx);
+		DMERR_LIMIT("request attempted access beyond the end of device");
+		/*
+		 * Must perform setup, that rq_completed() requires,
+		 * before returning BLK_MQ_RQ_QUEUE_ERROR
+		 */
+		dm_start_request(md, rq);
+		return BLK_MQ_RQ_QUEUE_ERROR;
+	}
+	dm_put_live_table(md, srcu_idx);
+
+	if (ti->type->busy && ti->type->busy(ti))
+		return BLK_MQ_RQ_QUEUE_BUSY;
+
+	dm_start_request(md, rq);
+
+	/* Init tio using md established in .init_request */
+	init_tio(tio, rq, md);
+
+	/* Clone the request if underlying devices aren't blk-mq */
+	if (dm_table_get_type(map) == DM_TYPE_REQUEST_BASED) {
+		// FIXME: make the memory for clone part of the pdu
+		if (!clone_rq(rq, md, tio, GFP_ATOMIC))
+			return BLK_MQ_RQ_QUEUE_BUSY;
+	}
+
+	/* Establish tio->ti before queuing work (map_tio_request) */
+	tio->ti = ti;
+	queue_kthread_work(&md->kworker, &tio->work);
+
+	return BLK_MQ_RQ_QUEUE_OK;
+}
+
+static struct blk_mq_ops dm_mq_ops = {
+	.queue_rq = dm_mq_queue_rq,
+	.map_queue = blk_mq_map_queue,
+	.complete = dm_softirq_done,
+	.init_request = dm_mq_init_request,
+};
+
+static int dm_init_request_based_blk_mq_queue(struct mapped_device *md)
+{
+	struct request_queue *q;
+	int err;
+
+	memset(&md->tag_set, 0, sizeof(md->tag_set));
+	md->tag_set.ops = &dm_mq_ops;
+	md->tag_set.queue_depth = BLKDEV_MAX_RQ;
+	md->tag_set.numa_node = NUMA_NO_NODE;
+	md->tag_set.flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_SG_MERGE;
+	md->tag_set.nr_hw_queues = 1;
+	// FIXME: make the memory for non-blk-mq clone part of the pdu
+	// would need to be done only if new 'use_blk_mq' is set in DM sysfs
+	md->tag_set.cmd_size = sizeof(struct dm_rq_target_io);
+	md->tag_set.driver_data = md;
+
+	err = blk_mq_alloc_tag_set(&md->tag_set);
+	if (err)
+		return err;
+
+	q = blk_mq_init_allocated_queue(&md->tag_set, md->queue);
+	if (IS_ERR(q)) {
+		err = PTR_ERR(q);
+		goto out_tag_set;
+	}
+	md->queue = q;
+	dm_init_md_queue(md);
+
+	/* backfill 'mq' sysfs registration normally done in blk_register_queue */
+	blk_mq_register_disk(md->disk);
+
+	init_rq_based_worker_thread(md);
+
+	return 0;
+
+out_tag_set:
+	blk_mq_free_tag_set(&md->tag_set);
+	return err;
 }
 
 /*
@@ -2596,15 +2774,29 @@ static int dm_init_request_based_queue(struct mapped_device *md)
  */
 int dm_setup_md_queue(struct mapped_device *md)
 {
-	if (dm_md_type_request_based(md)) {
-		if (!dm_init_request_based_queue(md)) {
+	int r;
+	unsigned md_type = dm_get_md_type(md);
+
+	switch (md_type) {
+	case DM_TYPE_REQUEST_BASED:
+		r = dm_init_request_based_queue(md);
+		if (r) {
 			DMWARN("Cannot initialize queue for request-based mapped device");
-			return -EINVAL;
+			return r;
 		}
-	} else {
-		/* bio-based specific initialization */
+		break;
+	case DM_TYPE_MQ_REQUEST_BASED:
+		r = dm_init_request_based_blk_mq_queue(md);
+		if (r) {
+			DMWARN("Cannot initialize queue for request-based blk-mq mapped device");
+			return r;
+		}
+		break;
+	case DM_TYPE_BIO_BASED:
+		dm_init_old_md_queue(md);
 		blk_queue_make_request(md->queue, dm_make_request);
 		blk_queue_merge_bvec(md->queue, dm_merge_bvec);
+		break;
 	}
 
 	return 0;
-- 
1.9.5 (Apple Git-50.3)

[PATCH 6/7] dm: optimize dm_mq_queue_rq to _not_ use kthread if using pure blk-mq

[Mike Snitzer]

dm_mq_queue_rq() is in atomic context so care must be taken to not
sleep -- as such GFP_ATOMIC is used for the md->bs bioset allocations
and dm-mpath's call to blk_get_request().  In the future the bioset
allocations will hopefully go away (by removing support for partial
completions of a request).

But the kthread will still be used to queue work if blk-mq is used ontop
of old-style request_fn device(s).  Also prepare for supporting DM
blk-mq ontop of old-style request_fn device(s) if a new dm-mod
'use_blk_mq' parameter is set.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 drivers/md/dm.c | 65 +++++++++++++++++++++++++++++++++++++++------------------
 1 file changed, 45 insertions(+), 20 deletions(-)

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index b5409ac..b0c965a 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1074,9 +1074,10 @@ static void free_rq_clone(struct request *clone)
 
 	blk_rq_unprep_clone(clone);
 
-	if (clone->q && clone->q->mq_ops)
+	if (clone->q->mq_ops)
 		tio->ti->type->release_clone_rq(clone);
-	else
+	else if (!md->queue->mq_ops)
+		/* request_fn queue stacked on request_fn queue(s) */
 		free_clone_request(md, clone);
 
 	if (!md->queue->mq_ops)
@@ -1835,15 +1836,25 @@ static int setup_clone(struct request *clone, struct request *rq,
 static struct request *clone_rq(struct request *rq, struct mapped_device *md,
 				struct dm_rq_target_io *tio, gfp_t gfp_mask)
 {
-	struct request *clone = alloc_clone_request(md, gfp_mask);
+	/*
+	 * Do not allocate a clone if tio->clone was already set
+	 * (see: dm_mq_queue_rq).
+	 */
+	bool alloc_clone = !tio->clone;
+	struct request *clone;
 
-	if (!clone)
-		return NULL;
+	if (alloc_clone) {
+		clone = alloc_clone_request(md, gfp_mask);
+		if (!clone)
+			return NULL;
+	} else
+		clone = tio->clone;
 
 	blk_rq_init(NULL, clone);
 	if (setup_clone(clone, rq, tio, gfp_mask)) {
 		/* -ENOMEM */
-		free_clone_request(md, clone);
+		if (alloc_clone)
+			free_clone_request(md, clone);
 		return NULL;
 	}
 
@@ -1861,7 +1872,8 @@ static void init_tio(struct dm_rq_target_io *tio, struct request *rq,
 	tio->orig = rq;
 	tio->error = 0;
 	memset(&tio->info, 0, sizeof(tio->info));
-	init_kthread_work(&tio->work, map_tio_request);
+	if (md->kworker_task)
+		init_kthread_work(&tio->work, map_tio_request);
 }
 
 static struct dm_rq_target_io *prep_tio(struct request *rq,
@@ -1938,7 +1950,7 @@ static int map_request(struct dm_rq_target_io *tio, struct request *rq,
 		}
 		if (IS_ERR(clone))
 			return DM_MAPIO_REQUEUE;
-		if (setup_clone(clone, rq, tio, GFP_NOIO)) {
+		if (setup_clone(clone, rq, tio, GFP_ATOMIC)) {
 			/* -ENOMEM */
 			ti->type->release_clone_rq(clone);
 			return DM_MAPIO_REQUEUE;
@@ -2403,7 +2415,7 @@ static void __bind_mempools(struct mapped_device *md, struct dm_table *t)
 	p->bs = NULL;
 
 out:
-	/* mempool bind completed, now no need any mempools in the table */
+	/* mempool bind completed, no longer need any mempools in the table */
 	dm_table_free_md_mempools(t);
 }
 
@@ -2708,17 +2720,25 @@ static int dm_mq_queue_rq(struct blk_mq_hw_ctx *hctx,
 	/* Init tio using md established in .init_request */
 	init_tio(tio, rq, md);
 
+	/*
+	 * Establish tio->ti before queuing work (map_tio_request)
+	 * or making direct call to map_request().
+	 */
+	tio->ti = ti;
+
 	/* Clone the request if underlying devices aren't blk-mq */
 	if (dm_table_get_type(map) == DM_TYPE_REQUEST_BASED) {
-		// FIXME: make the memory for clone part of the pdu
+		/* clone request is allocated at the end of the pdu */
+		tio->clone = (void *)blk_mq_rq_to_pdu(rq) + sizeof(struct dm_rq_target_io);
 		if (!clone_rq(rq, md, tio, GFP_ATOMIC))
 			return BLK_MQ_RQ_QUEUE_BUSY;
+		queue_kthread_work(&md->kworker, &tio->work);
+	} else {
+		/* Direct call is fine since .queue_rq allows allocations */
+		if (map_request(tio, rq, md) == DM_MAPIO_REQUEUE)
+			dm_requeue_unmapped_original_request(md, rq);
 	}
 
-	/* Establish tio->ti before queuing work (map_tio_request) */
-	tio->ti = ti;
-	queue_kthread_work(&md->kworker, &tio->work);
-
 	return BLK_MQ_RQ_QUEUE_OK;
 }
 
@@ -2731,6 +2751,7 @@ static struct blk_mq_ops dm_mq_ops = {
 
 static int dm_init_request_based_blk_mq_queue(struct mapped_device *md)
 {
+	unsigned md_type = dm_get_md_type(md);
 	struct request_queue *q;
 	int err;
 
@@ -2740,9 +2761,11 @@ static int dm_init_request_based_blk_mq_queue(struct mapped_device *md)
 	md->tag_set.numa_node = NUMA_NO_NODE;
 	md->tag_set.flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_SG_MERGE;
 	md->tag_set.nr_hw_queues = 1;
-	// FIXME: make the memory for non-blk-mq clone part of the pdu
-	// would need to be done only if new 'use_blk_mq' is set in DM sysfs
-	md->tag_set.cmd_size = sizeof(struct dm_rq_target_io);
+	if (md_type == DM_TYPE_REQUEST_BASED) {
+		/* make the memory for non-blk-mq clone part of the pdu */
+		md->tag_set.cmd_size = sizeof(struct dm_rq_target_io) + sizeof(struct request);
+	} else
+		md->tag_set.cmd_size = sizeof(struct dm_rq_target_io);
 	md->tag_set.driver_data = md;
 
 	err = blk_mq_alloc_tag_set(&md->tag_set);
@@ -2760,7 +2783,8 @@ static int dm_init_request_based_blk_mq_queue(struct mapped_device *md)
 	/* backfill 'mq' sysfs registration normally done in blk_register_queue */
 	blk_mq_register_disk(md->disk);
 
-	init_rq_based_worker_thread(md);
+	if (md_type == DM_TYPE_REQUEST_BASED)
+		init_rq_based_worker_thread(md);
 
 	return 0;
 
@@ -2879,7 +2903,7 @@ static void __dm_destroy(struct mapped_device *md, bool wait)
 	set_bit(DMF_FREEING, &md->flags);
 	spin_unlock(&_minor_lock);
 
-	if (dm_request_based(md))
+	if (dm_request_based(md) && md->kworker_task)
 		flush_kthread_worker(&md->kworker);
 
 	/*
@@ -3133,7 +3157,8 @@ static int __dm_suspend(struct mapped_device *md, struct dm_table *map,
 	 */
 	if (dm_request_based(md)) {
 		stop_queue(md->queue);
-		flush_kthread_worker(&md->kworker);
+		if (md->kworker_task)
+			flush_kthread_worker(&md->kworker);
 	}
 
 	flush_workqueue(md->wq);
-- 
1.9.5 (Apple Git-50.3)

[PATCH 7/7] dm: add 'use_blk_mq' module param and expose in per-device ro sysfs attr

[Mike Snitzer]

Request-based DM's blk-mq support defaults to off; but a user can easily
change the default using the dm_mod.use_blk_mq module/boot option.

Also, you can check what mode a given request-based DM device is using
with: cat /sys/block/dm-X/dm/use_blk_mq

This change enabled further cleanup and reduced work (e.g. the
md->io_pool and md->rq_pool isn't created if using blk-mq).

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 drivers/md/Kconfig    | 11 +++++++++++
 drivers/md/dm-sysfs.c |  9 +++++++++
 drivers/md/dm-table.c |  6 +++---
 drivers/md/dm.c       | 51 ++++++++++++++++++++++++++++++++++++++++-----------
 drivers/md/dm.h       |  5 ++++-
 5 files changed, 67 insertions(+), 15 deletions(-)

diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index 63e05e3..109f9dc 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -196,6 +196,17 @@ config BLK_DEV_DM
 
 	  If unsure, say N.
 
+config DM_MQ_DEFAULT
+	bool "request-based DM: use blk-mq I/O path by default"
+	depends on BLK_DEV_DM
+	---help---
+	  This option enables the blk-mq based I/O path for request-based
+	  DM devices by default.  With the option the dm_mod.use_blk_mq
+	  module/boot option defaults to Y, without it to N, but it can
+	  still be overriden either way.
+
+	  If unsure say N.
+
 config DM_DEBUG
 	bool "Device mapper debugging support"
 	depends on BLK_DEV_DM
diff --git a/drivers/md/dm-sysfs.c b/drivers/md/dm-sysfs.c
index f5bb394..7e818f5 100644
--- a/drivers/md/dm-sysfs.c
+++ b/drivers/md/dm-sysfs.c
@@ -89,15 +89,24 @@ static ssize_t dm_attr_suspended_show(struct mapped_device *md, char *buf)
 	return strlen(buf);
 }
 
+static ssize_t dm_attr_use_blk_mq_show(struct mapped_device *md, char *buf)
+{
+	sprintf(buf, "%d\n", dm_use_blk_mq(md));
+
+	return strlen(buf);
+}
+
 static DM_ATTR_RO(name);
 static DM_ATTR_RO(uuid);
 static DM_ATTR_RO(suspended);
+static DM_ATTR_RO(use_blk_mq);
 static DM_ATTR_RW(rq_based_seq_io_merge_deadline);
 
 static struct attribute *dm_attrs[] = {
 	&dm_attr_name.attr,
 	&dm_attr_uuid.attr,
 	&dm_attr_suspended.attr,
+	&dm_attr_use_blk_mq.attr,
 	&dm_attr_rq_based_seq_io_merge_deadline.attr,
 	NULL,
 };
diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 66600ca..8d025f3 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -940,7 +940,7 @@ bool dm_table_mq_request_based(struct dm_table *t)
 	return dm_table_get_type(t) == DM_TYPE_MQ_REQUEST_BASED;
 }
 
-static int dm_table_alloc_md_mempools(struct dm_table *t)
+static int dm_table_alloc_md_mempools(struct dm_table *t, struct mapped_device *md)
 {
 	unsigned type = dm_table_get_type(t);
 	unsigned per_bio_data_size = 0;
@@ -958,7 +958,7 @@ static int dm_table_alloc_md_mempools(struct dm_table *t)
 			per_bio_data_size = max(per_bio_data_size, tgt->per_bio_data_size);
 		}
 
-	t->mempools = dm_alloc_md_mempools(type, t->integrity_supported, per_bio_data_size);
+	t->mempools = dm_alloc_md_mempools(md, type, t->integrity_supported, per_bio_data_size);
 	if (!t->mempools)
 		return -ENOMEM;
 
@@ -1128,7 +1128,7 @@ int dm_table_complete(struct dm_table *t)
 		return r;
 	}
 
-	r = dm_table_alloc_md_mempools(t);
+	r = dm_table_alloc_md_mempools(t, t->md);
 	if (r)
 		DMERR("unable to allocate mempools");
 
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index b0c965a..3165b94 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -228,8 +228,20 @@ struct mapped_device {
 
 	/* for blk-mq request-based DM support */
 	struct blk_mq_tag_set tag_set;
+	bool use_blk_mq;
 };
 
+#ifdef CONFIG_DM_MQ_DEFAULT
+static bool use_blk_mq = true;
+#else
+static bool use_blk_mq = false;
+#endif
+
+bool dm_use_blk_mq(struct mapped_device *md)
+{
+	return md->use_blk_mq;
+}
+
 /*
  * For mempools pre-allocation at the table loading time.
  */
@@ -2219,6 +2231,7 @@ static void dm_init_md_queue(struct mapped_device *md)
 
 static void dm_init_old_md_queue(struct mapped_device *md)
 {
+	md->use_blk_mq = false;
 	dm_init_md_queue(md);
 
 	/*
@@ -2260,6 +2273,7 @@ static struct mapped_device *alloc_dev(int minor)
 	if (r < 0)
 		goto bad_io_barrier;
 
+	md->use_blk_mq = use_blk_mq;
 	md->type = DM_TYPE_NONE;
 	mutex_init(&md->suspend_lock);
 	mutex_init(&md->type_lock);
@@ -2346,7 +2360,6 @@ static void unlock_fs(struct mapped_device *md);
 static void free_dev(struct mapped_device *md)
 {
 	int minor = MINOR(disk_devt(md->disk));
-	bool using_blk_mq = !!md->queue->mq_ops;
 
 	unlock_fs(md);
 	bdput(md->bdev);
@@ -2372,7 +2385,7 @@ static void free_dev(struct mapped_device *md)
 
 	put_disk(md->disk);
 	blk_cleanup_queue(md->queue);
-	if (using_blk_mq)
+	if (md->use_blk_mq)
 		blk_mq_free_tag_set(&md->tag_set);
 	dm_stats_cleanup(&md->stats);
 	module_put(THIS_MODULE);
@@ -2383,7 +2396,7 @@ static void __bind_mempools(struct mapped_device *md, struct dm_table *t)
 {
 	struct dm_md_mempools *p = dm_table_get_md_mempools(t);
 
-	if (md->io_pool && md->bs) {
+	if (md->bs) {
 		/* The md already has necessary mempools. */
 		if (dm_table_get_type(t) == DM_TYPE_BIO_BASED) {
 			/*
@@ -2793,13 +2806,21 @@ out_tag_set:
 	return err;
 }
 
+static unsigned filter_md_type(unsigned type, struct mapped_device *md)
+{
+	if (type == DM_TYPE_BIO_BASED)
+		return type;
+
+	return !md->use_blk_mq ? DM_TYPE_REQUEST_BASED : DM_TYPE_MQ_REQUEST_BASED;
+}
+
 /*
  * Setup the DM device's queue based on md's type
  */
 int dm_setup_md_queue(struct mapped_device *md)
 {
 	int r;
-	unsigned md_type = dm_get_md_type(md);
+	unsigned md_type = filter_md_type(dm_get_md_type(md), md);
 
 	switch (md_type) {
 	case DM_TYPE_REQUEST_BASED:
@@ -3503,16 +3524,19 @@ int dm_noflush_suspending(struct dm_target *ti)
 }
 EXPORT_SYMBOL_GPL(dm_noflush_suspending);
 
-struct dm_md_mempools *dm_alloc_md_mempools(unsigned type, unsigned integrity, unsigned per_bio_data_size)
+struct dm_md_mempools *dm_alloc_md_mempools(struct mapped_device *md, unsigned type,
+					    unsigned integrity, unsigned per_bio_data_size)
 {
 	struct dm_md_mempools *pools = kzalloc(sizeof(*pools), GFP_KERNEL);
-	struct kmem_cache *cachep;
+	struct kmem_cache *cachep = NULL;
 	unsigned int pool_size = 0;
 	unsigned int front_pad;
 
 	if (!pools)
 		return NULL;
 
+	type = filter_md_type(type, md);
+
 	switch (type) {
 	case DM_TYPE_BIO_BASED:
 		cachep = _io_cache;
@@ -3520,13 +3544,13 @@ struct dm_md_mempools *dm_alloc_md_mempools(unsigned type, unsigned integrity, u
 		front_pad = roundup(per_bio_data_size, __alignof__(struct dm_target_io)) + offsetof(struct dm_target_io, clone);
 		break;
 	case DM_TYPE_REQUEST_BASED:
+		cachep = _rq_tio_cache;
 		pool_size = dm_get_reserved_rq_based_ios();
 		pools->rq_pool = mempool_create_slab_pool(pool_size, _rq_cache);
 		if (!pools->rq_pool)
 			goto out;
 		/* fall through to setup remaining rq-based pools */
 	case DM_TYPE_MQ_REQUEST_BASED:
-		cachep = _rq_tio_cache;
 		if (!pool_size)
 			pool_size = dm_get_reserved_rq_based_ios();
 		front_pad = offsetof(struct dm_rq_clone_bio_info, clone);
@@ -3534,12 +3558,14 @@ struct dm_md_mempools *dm_alloc_md_mempools(unsigned type, unsigned integrity, u
 		WARN_ON(per_bio_data_size != 0);
 		break;
 	default:
-		goto out;
+		BUG();
 	}
 
-	pools->io_pool = mempool_create_slab_pool(pool_size, cachep);
-	if (!pools->io_pool)
-		goto out;
+	if (cachep) {
+		pools->io_pool = mempool_create_slab_pool(pool_size, cachep);
+		if (!pools->io_pool)
+			goto out;
+	}
 
 	pools->bs = bioset_create_nobvec(pool_size, front_pad);
 	if (!pools->bs)
@@ -3596,6 +3622,9 @@ MODULE_PARM_DESC(reserved_bio_based_ios, "Reserved IOs in bio-based mempools");
 module_param(reserved_rq_based_ios, uint, S_IRUGO | S_IWUSR);
 MODULE_PARM_DESC(reserved_rq_based_ios, "Reserved IOs in request-based mempools");
 
+module_param(use_blk_mq, bool, S_IRUGO | S_IWUSR);
+MODULE_PARM_DESC(use_blk_mq, "Use block multiqueue for request-based DM devices");
+
 MODULE_DESCRIPTION(DM_NAME " driver");
 MODULE_AUTHOR("Joe Thornber <dm-devel@redhat.com>");
 MODULE_LICENSE("GPL");
diff --git a/drivers/md/dm.h b/drivers/md/dm.h
index 5522422..6123c2b 100644
--- a/drivers/md/dm.h
+++ b/drivers/md/dm.h
@@ -211,6 +211,8 @@ int dm_kobject_uevent(struct mapped_device *md, enum kobject_action action,
 void dm_internal_suspend(struct mapped_device *md);
 void dm_internal_resume(struct mapped_device *md);
 
+bool dm_use_blk_mq(struct mapped_device *md);
+
 int dm_io_init(void);
 void dm_io_exit(void);
 
@@ -220,7 +222,8 @@ void dm_kcopyd_exit(void);
 /*
  * Mempool operations
  */
-struct dm_md_mempools *dm_alloc_md_mempools(unsigned type, unsigned integrity, unsigned per_bio_data_size);
+struct dm_md_mempools *dm_alloc_md_mempools(struct mapped_device *md, unsigned type,
+					    unsigned integrity, unsigned per_bio_data_size);
 void dm_free_md_mempools(struct dm_md_mempools *pools);
 
 /*
-- 
1.9.5 (Apple Git-50.3)

Re: [PATCH 0/7] dm: add full blk-mq support to request-based DM

[Jens Axboe]

On 03/11/2015 09:56 PM, Mike Snitzer wrote:
> This patchset updates request-based DM (used exclussively by DM
> multipath) to support blk-mq I/O path or the old ->request_fn I/O
> path -- the default is still the old ->request_fn I/O path.
>
> Like was done for SCSI, I've exposed a dm-mod.ko 'use_blk_mq' module
> parameter and a CONFIG_DM_MQ_DEFAULT.  I've queued these patches in
> linux-next via linux-dm.git's 'for-next' branch -- my goal is for
> these changes to land upstream in 4.1.
>
> I've tested all 4 supported permutations of rq-based queue stacking:
> 1) blk-mq mpath stacked on blk-mq device(s)
> 2) blk-mq mpath stacked on ->request_fn device(s)
> 3) old ->request_fn mpath stacked on blk-mq device(s)
> 4) old ->request_fn mpath stacked on old ->request_fn device(s)
>
> There is definitely much more room for optimizing the blk-mq mpath
> queue (via dm_mq_ops setup) to have more awareness about the
> characteristics of the underlying blk-mq device(s) -- Jens offered
> some suggestions that are worth pursuing.  Goal being to leverage the
> ability to have multiple software submission queues that map to the
> underlying paths' HW queue.  I expect Keith to be the most likely
> person to pursue these follow-on optimizations given his immediate
> access to NVMe devices, etc.
>
> blk-mq allows for the removal of 2 of the 3 mempools and associated
> allocations that were traditionally performed using old ->request_fn
> I/O path.  The md->bs bioset still remains for cloning a request's
> bios.
>
> I'll be pursuing the possibility of removing the bio cloning that
> happens when cloning a request (it was put in place for partial
> completion of a request's bios but the prospect of avoiding those bio
> clones warrants further review of how useful those partial completions
> are, see: http://www.redhat.com/archives/dm-devel/2014-June/msg00030.html)
>
> Jens and Keith, special thanks for your help answering more blk-mq
> questions than I'd have hoped would be needed while at LSF/MM.
>
> Keith Busch (1):
>    blk-mq: don't wait in blk_mq_queue_enter() if __GFP_WAIT isn't set
>
> Mike Snitzer (6):
>    blk-mq: fix use of incorrect goto label in blk_mq_init_queue error path
>    blk-mq: add blk_mq_init_allocated_queue and export blk_mq_register_disk
>    blk-mq: export blk_mq_run_hw_queues
>    dm: add full blk-mq support to request-based DM
>    dm: optimize dm_mq_queue_rq to _not_ use kthread if using pure blk-mq
>    dm: add 'use_blk_mq' module param and expose in per-device ro sysfs attr

Applied patch 1 to current tree, marked for stable. 2-4 are applied for 
4.1, in for-4.1/core.

-- 
Jens Axboe