avoid the ->make_request_fn indirect for blk-mq drivers

avoid the ->make_request_fn indirect for blk-mq drivers

[Christoph Hellwig]

Hi Jens,

this small series avoids an indirect call for every submitted bio that
eventually ends up being handled by blk-mq drivers.  Let me know what
you think.

[PATCH 1/3] bcache: remove a duplicate ->make_request_fn assignment

[Christoph Hellwig]

The make_request_fn pointer should only be assigned by blk_alloc_queue.
Fix a left over manual initialization.

Fixes: ff27668ce809 ("bcache: pass the make_request methods to blk_queue_make_request")
Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 drivers/md/bcache/request.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
index 71a90fbec314b..77d1a26975174 100644
--- a/drivers/md/bcache/request.c
+++ b/drivers/md/bcache/request.c
@@ -1372,7 +1372,6 @@ void bch_flash_dev_request_init(struct bcache_device *d)
 {
 	struct gendisk *g = d->disk;
 
-	g->queue->make_request_fn		= flash_dev_make_request;
 	g->queue->backing_dev_info->congested_fn = flash_dev_congested;
 	d->cache_miss				= flash_dev_cache_miss;
 	d->ioctl				= flash_dev_ioctl;
-- 
2.26.1

[PATCH 1/3] bcache: remove a duplicate ->make_request_fn assignment

[Christoph Hellwig]

The make_request_fn pointer should only be assigned by blk_alloc_queue.
Fix a left over manual initialization.

Fixes: ff27668ce809 ("bcache: pass the make_request methods to blk_queue_make_request")
Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 drivers/md/bcache/request.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
index 71a90fbec314b..77d1a26975174 100644
--- a/drivers/md/bcache/request.c
+++ b/drivers/md/bcache/request.c
@@ -1372,7 +1372,6 @@ void bch_flash_dev_request_init(struct bcache_device *d)
 {
 	struct gendisk *g = d->disk;
 
-	g->queue->make_request_fn		= flash_dev_make_request;
 	g->queue->backing_dev_info->congested_fn = flash_dev_congested;
 	d->cache_miss				= flash_dev_cache_miss;
 	d->ioctl				= flash_dev_ioctl;
-- 
2.26.1

Re: [PATCH 1/3] bcache: remove a duplicate ->make_request_fn assignment

[Coly Li]

On 2020/4/25 15:53, Christoph Hellwig wrote:
> The make_request_fn pointer should only be assigned by blk_alloc_queue.
> Fix a left over manual initialization.
> 
> Fixes: ff27668ce809 ("bcache: pass the make_request methods to blk_queue_make_request")
> Signed-off-by: Christoph Hellwig <hch@lst.de>

It looks good to me.

Reviewed-by: Coly Li <colyli@suse.de>

Thanks.

Coly Li

> ---
>  drivers/md/bcache/request.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
> index 71a90fbec314b..77d1a26975174 100644
> --- a/drivers/md/bcache/request.c
> +++ b/drivers/md/bcache/request.c
> @@ -1372,7 +1372,6 @@ void bch_flash_dev_request_init(struct bcache_device *d)
>  {
>  	struct gendisk *g = d->disk;
>  
> -	g->queue->make_request_fn		= flash_dev_make_request;
>  	g->queue->backing_dev_info->congested_fn = flash_dev_congested;
>  	d->cache_miss				= flash_dev_cache_miss;
>  	d->ioctl				= flash_dev_ioctl;
> 

[PATCH 2/3] dm: remove the make_request_fn check in device_area_is_invalid

[Christoph Hellwig]

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 drivers/md/dm-table.c | 17 -----------------
 1 file changed, 17 deletions(-)

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 0a2cc197f62b4..8277b959e00bd 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -279,7 +279,6 @@ static struct dm_dev_internal *find_device(struct list_head *l, dev_t dev)
 static int device_area_is_invalid(struct dm_target *ti, struct dm_dev *dev,
 				  sector_t start, sector_t len, void *data)
 {
-	struct request_queue *q;
 	struct queue_limits *limits = data;
 	struct block_device *bdev = dev->bdev;
 	sector_t dev_size =
@@ -288,22 +287,6 @@ static int device_area_is_invalid(struct dm_target *ti, struct dm_dev *dev,
 		limits->logical_block_size >> SECTOR_SHIFT;
 	char b[BDEVNAME_SIZE];
 
-	/*
-	 * Some devices exist without request functions,
-	 * such as loop devices not yet bound to backing files.
-	 * Forbid the use of such devices.
-	 */
-	q = bdev_get_queue(bdev);
-	if (!q || !q->make_request_fn) {
-		DMWARN("%s: %s is not yet initialised: "
-		       "start=%llu, len=%llu, dev_size=%llu",
-		       dm_device_name(ti->table->md), bdevname(bdev, b),
-		       (unsigned long long)start,
-		       (unsigned long long)len,
-		       (unsigned long long)dev_size);
-		return 1;
-	}
-
 	if (!dev_size)
 		return 0;
 
-- 
2.26.1

[PATCH 2/3] dm: remove the make_request_fn check in device_area_is_invalid

[Christoph Hellwig]

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 drivers/md/dm-table.c | 17 -----------------
 1 file changed, 17 deletions(-)

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 0a2cc197f62b4..8277b959e00bd 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -279,7 +279,6 @@ static struct dm_dev_internal *find_device(struct list_head *l, dev_t dev)
 static int device_area_is_invalid(struct dm_target *ti, struct dm_dev *dev,
 				  sector_t start, sector_t len, void *data)
 {
-	struct request_queue *q;
 	struct queue_limits *limits = data;
 	struct block_device *bdev = dev->bdev;
 	sector_t dev_size =
@@ -288,22 +287,6 @@ static int device_area_is_invalid(struct dm_target *ti, struct dm_dev *dev,
 		limits->logical_block_size >> SECTOR_SHIFT;
 	char b[BDEVNAME_SIZE];
 
-	/*
-	 * Some devices exist without request functions,
-	 * such as loop devices not yet bound to backing files.
-	 * Forbid the use of such devices.
-	 */
-	q = bdev_get_queue(bdev);
-	if (!q || !q->make_request_fn) {
-		DMWARN("%s: %s is not yet initialised: "
-		       "start=%llu, len=%llu, dev_size=%llu",
-		       dm_device_name(ti->table->md), bdevname(bdev, b),
-		       (unsigned long long)start,
-		       (unsigned long long)len,
-		       (unsigned long long)dev_size);
-		return 1;
-	}
-
 	if (!dev_size)
 		return 0;
 
-- 
2.26.1

Re: [PATCH 2/3] dm: remove the make_request_fn check in device_area_is_invalid

[Mike Snitzer]

On Sat, Apr 25 2020 at  3:53am -0400,
Christoph Hellwig <hch@lst.de> wrote:

> Signed-off-by: Christoph Hellwig <hch@lst.de>

Think it'd be useful to add a commit message like you did for revert
commit f01b411f41f91fc3196eae4317cf8b4d872830a6 , e.g.:

We can't have queues without a make_request_fn any more (and the loop
device uses blk-mq these days anyway..).

But that aside:

Acked-by: Mike Snitzer <snitzer@redhat.com>

Thanks.

> ---
>  drivers/md/dm-table.c | 17 -----------------
>  1 file changed, 17 deletions(-)
> 
> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> index 0a2cc197f62b4..8277b959e00bd 100644
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -279,7 +279,6 @@ static struct dm_dev_internal *find_device(struct list_head *l, dev_t dev)
>  static int device_area_is_invalid(struct dm_target *ti, struct dm_dev *dev,
>  				  sector_t start, sector_t len, void *data)
>  {
> -	struct request_queue *q;
>  	struct queue_limits *limits = data;
>  	struct block_device *bdev = dev->bdev;
>  	sector_t dev_size =
> @@ -288,22 +287,6 @@ static int device_area_is_invalid(struct dm_target *ti, struct dm_dev *dev,
>  		limits->logical_block_size >> SECTOR_SHIFT;
>  	char b[BDEVNAME_SIZE];
>  
> -	/*
> -	 * Some devices exist without request functions,
> -	 * such as loop devices not yet bound to backing files.
> -	 * Forbid the use of such devices.
> -	 */
> -	q = bdev_get_queue(bdev);
> -	if (!q || !q->make_request_fn) {
> -		DMWARN("%s: %s is not yet initialised: "
> -		       "start=%llu, len=%llu, dev_size=%llu",
> -		       dm_device_name(ti->table->md), bdevname(bdev, b),
> -		       (unsigned long long)start,
> -		       (unsigned long long)len,
> -		       (unsigned long long)dev_size);
> -		return 1;
> -	}
> -
>  	if (!dev_size)
>  		return 0;
>  
> -- 
> 2.26.1
> 
> 
> --
> dm-devel mailing list
> dm-devel@redhat.com
> https://www.redhat.com/mailman/listinfo/dm-devel

Re: [PATCH 2/3] dm: remove the make_request_fn check in device_area_is_invalid

[Mike Snitzer]

On Sat, Apr 25 2020 at  3:53am -0400,
Christoph Hellwig <hch@lst.de> wrote:

> Signed-off-by: Christoph Hellwig <hch@lst.de>

Think it'd be useful to add a commit message like you did for revert
commit f01b411f41f91fc3196eae4317cf8b4d872830a6 , e.g.:

We can't have queues without a make_request_fn any more (and the loop
device uses blk-mq these days anyway..).

But that aside:

Acked-by: Mike Snitzer <snitzer@redhat.com>

Thanks.

> ---
>  drivers/md/dm-table.c | 17 -----------------
>  1 file changed, 17 deletions(-)
> 
> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> index 0a2cc197f62b4..8277b959e00bd 100644
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -279,7 +279,6 @@ static struct dm_dev_internal *find_device(struct list_head *l, dev_t dev)
>  static int device_area_is_invalid(struct dm_target *ti, struct dm_dev *dev,
>  				  sector_t start, sector_t len, void *data)
>  {
> -	struct request_queue *q;
>  	struct queue_limits *limits = data;
>  	struct block_device *bdev = dev->bdev;
>  	sector_t dev_size =
> @@ -288,22 +287,6 @@ static int device_area_is_invalid(struct dm_target *ti, struct dm_dev *dev,
>  		limits->logical_block_size >> SECTOR_SHIFT;
>  	char b[BDEVNAME_SIZE];
>  
> -	/*
> -	 * Some devices exist without request functions,
> -	 * such as loop devices not yet bound to backing files.
> -	 * Forbid the use of such devices.
> -	 */
> -	q = bdev_get_queue(bdev);
> -	if (!q || !q->make_request_fn) {
> -		DMWARN("%s: %s is not yet initialised: "
> -		       "start=%llu, len=%llu, dev_size=%llu",
> -		       dm_device_name(ti->table->md), bdevname(bdev, b),
> -		       (unsigned long long)start,
> -		       (unsigned long long)len,
> -		       (unsigned long long)dev_size);
> -		return 1;
> -	}
> -
>  	if (!dev_size)
>  		return 0;
>  
> -- 
> 2.26.1
> 
> 
> --
> dm-devel mailing list
> dm-devel@redhat.com
> https://www.redhat.com/mailman/listinfo/dm-devel

[PATCH 3/3] block: bypass ->make_request_fn for blk-mq drivers

[Christoph Hellwig]

Call blk_mq_make_request when no ->make_request_fn is set.  This is
safe now that blk_alloc_queue always sets up the pointer for make_request
based drivers.  This avoids an indirect call in the blk-mq driver I/O
fast path, which is rather expensive due to spectre mitigations.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 block/blk-core.c       | 26 +++++++++++++++++---------
 block/blk-mq.c         |  4 ++--
 drivers/md/dm.c        |  3 +++
 include/linux/blk-mq.h |  2 ++
 4 files changed, 24 insertions(+), 11 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 311596d5dbc41..0e9e1c83e5e84 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1072,7 +1072,10 @@ blk_qc_t generic_make_request(struct bio *bio)
 			/* Create a fresh bio_list for all subordinate requests */
 			bio_list_on_stack[1] = bio_list_on_stack[0];
 			bio_list_init(&bio_list_on_stack[0]);
-			ret = q->make_request_fn(q, bio);
+			if (q->make_request_fn)
+				ret = q->make_request_fn(q, bio);
+			else
+				ret = blk_mq_make_request(q, bio);
 
 			blk_queue_exit(q);
 
@@ -1112,9 +1115,7 @@ EXPORT_SYMBOL(generic_make_request);
  *
  * This function behaves like generic_make_request(), but does not protect
  * against recursion.  Must only be used if the called driver is known
- * to not call generic_make_request (or direct_make_request) again from
- * its make_request function.  (Calling direct_make_request again from
- * a workqueue is perfectly fine as that doesn't recurse).
+ * to be blk-mq based.
  */
 blk_qc_t direct_make_request(struct bio *bio)
 {
@@ -1122,20 +1123,27 @@ blk_qc_t direct_make_request(struct bio *bio)
 	bool nowait = bio->bi_opf & REQ_NOWAIT;
 	blk_qc_t ret;
 
+	if (WARN_ON_ONCE(q->make_request_fn))
+		goto io_error;
 	if (!generic_make_request_checks(bio))
 		return BLK_QC_T_NONE;
 
 	if (unlikely(blk_queue_enter(q, nowait ? BLK_MQ_REQ_NOWAIT : 0))) {
 		if (nowait && !blk_queue_dying(q))
-			bio_wouldblock_error(bio);
-		else
-			bio_io_error(bio);
-		return BLK_QC_T_NONE;
+			goto would_block;
+		goto io_error;
 	}
 
-	ret = q->make_request_fn(q, bio);
+	ret = blk_mq_make_request(q, bio);
 	blk_queue_exit(q);
 	return ret;
+
+would_block:
+	bio_wouldblock_error(bio);
+	return BLK_QC_T_NONE;
+io_error:
+	bio_io_error(bio);
+	return BLK_QC_T_NONE;
 }
 EXPORT_SYMBOL_GPL(direct_make_request);
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 71d0894ce1c58..bcc3a2397d4ae 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1984,7 +1984,7 @@ static void blk_add_rq_to_plug(struct blk_plug *plug, struct request *rq)
  *
  * Returns: Request queue cookie.
  */
-static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
+blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 {
 	const int is_sync = op_is_sync(bio->bi_opf);
 	const int is_flush_fua = op_is_flush(bio->bi_opf);
@@ -2096,6 +2096,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 
 	return cookie;
 }
+EXPORT_SYMBOL_GPL(blk_mq_make_request); /* only for request based dm */
 
 void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
 		     unsigned int hctx_idx)
@@ -2955,7 +2956,6 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
 	INIT_LIST_HEAD(&q->requeue_list);
 	spin_lock_init(&q->requeue_lock);
 
-	q->make_request_fn = blk_mq_make_request;
 	q->nr_requests = set->queue_depth;
 
 	/*
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index db9e461146531..0eb93da44ea2a 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1788,6 +1788,9 @@ static blk_qc_t dm_make_request(struct request_queue *q, struct bio *bio)
 	int srcu_idx;
 	struct dm_table *map;
 
+	if (dm_get_md_type(md) == DM_TYPE_REQUEST_BASED)
+		return blk_mq_make_request(q, bio);
+
 	map = dm_get_live_table(md, &srcu_idx);
 
 	/* if we're suspended, we have to queue this io for later */
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 51fbf6f76593a..d7307795439a4 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -578,4 +578,6 @@ static inline void blk_mq_cleanup_rq(struct request *rq)
 		rq->q->mq_ops->cleanup_rq(rq);
 }
 
+blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio);
+
 #endif
-- 
2.26.1

[PATCH 3/3] block: bypass ->make_request_fn for blk-mq drivers

[Christoph Hellwig]

Call blk_mq_make_request when no ->make_request_fn is set.  This is
safe now that blk_alloc_queue always sets up the pointer for make_request
based drivers.  This avoids an indirect call in the blk-mq driver I/O
fast path, which is rather expensive due to spectre mitigations.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 block/blk-core.c       | 26 +++++++++++++++++---------
 block/blk-mq.c         |  4 ++--
 drivers/md/dm.c        |  3 +++
 include/linux/blk-mq.h |  2 ++
 4 files changed, 24 insertions(+), 11 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 311596d5dbc41..0e9e1c83e5e84 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1072,7 +1072,10 @@ blk_qc_t generic_make_request(struct bio *bio)
 			/* Create a fresh bio_list for all subordinate requests */
 			bio_list_on_stack[1] = bio_list_on_stack[0];
 			bio_list_init(&bio_list_on_stack[0]);
-			ret = q->make_request_fn(q, bio);
+			if (q->make_request_fn)
+				ret = q->make_request_fn(q, bio);
+			else
+				ret = blk_mq_make_request(q, bio);
 
 			blk_queue_exit(q);
 
@@ -1112,9 +1115,7 @@ EXPORT_SYMBOL(generic_make_request);
  *
  * This function behaves like generic_make_request(), but does not protect
  * against recursion.  Must only be used if the called driver is known
- * to not call generic_make_request (or direct_make_request) again from
- * its make_request function.  (Calling direct_make_request again from
- * a workqueue is perfectly fine as that doesn't recurse).
+ * to be blk-mq based.
  */
 blk_qc_t direct_make_request(struct bio *bio)
 {
@@ -1122,20 +1123,27 @@ blk_qc_t direct_make_request(struct bio *bio)
 	bool nowait = bio->bi_opf & REQ_NOWAIT;
 	blk_qc_t ret;
 
+	if (WARN_ON_ONCE(q->make_request_fn))
+		goto io_error;
 	if (!generic_make_request_checks(bio))
 		return BLK_QC_T_NONE;
 
 	if (unlikely(blk_queue_enter(q, nowait ? BLK_MQ_REQ_NOWAIT : 0))) {
 		if (nowait && !blk_queue_dying(q))
-			bio_wouldblock_error(bio);
-		else
-			bio_io_error(bio);
-		return BLK_QC_T_NONE;
+			goto would_block;
+		goto io_error;
 	}
 
-	ret = q->make_request_fn(q, bio);
+	ret = blk_mq_make_request(q, bio);
 	blk_queue_exit(q);
 	return ret;
+
+would_block:
+	bio_wouldblock_error(bio);
+	return BLK_QC_T_NONE;
+io_error:
+	bio_io_error(bio);
+	return BLK_QC_T_NONE;
 }
 EXPORT_SYMBOL_GPL(direct_make_request);
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 71d0894ce1c58..bcc3a2397d4ae 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1984,7 +1984,7 @@ static void blk_add_rq_to_plug(struct blk_plug *plug, struct request *rq)
  *
  * Returns: Request queue cookie.
  */
-static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
+blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 {
 	const int is_sync = op_is_sync(bio->bi_opf);
 	const int is_flush_fua = op_is_flush(bio->bi_opf);
@@ -2096,6 +2096,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 
 	return cookie;
 }
+EXPORT_SYMBOL_GPL(blk_mq_make_request); /* only for request based dm */
 
 void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
 		     unsigned int hctx_idx)
@@ -2955,7 +2956,6 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
 	INIT_LIST_HEAD(&q->requeue_list);
 	spin_lock_init(&q->requeue_lock);
 
-	q->make_request_fn = blk_mq_make_request;
 	q->nr_requests = set->queue_depth;
 
 	/*
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index db9e461146531..0eb93da44ea2a 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1788,6 +1788,9 @@ static blk_qc_t dm_make_request(struct request_queue *q, struct bio *bio)
 	int srcu_idx;
 	struct dm_table *map;
 
+	if (dm_get_md_type(md) == DM_TYPE_REQUEST_BASED)
+		return blk_mq_make_request(q, bio);
+
 	map = dm_get_live_table(md, &srcu_idx);
 
 	/* if we're suspended, we have to queue this io for later */
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 51fbf6f76593a..d7307795439a4 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -578,4 +578,6 @@ static inline void blk_mq_cleanup_rq(struct request *rq)
 		rq->q->mq_ops->cleanup_rq(rq);
 }
 
+blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio);
+
 #endif
-- 
2.26.1

Re: [PATCH 3/3] block: bypass ->make_request_fn for blk-mq drivers

[Mike Snitzer]

On Sat, Apr 25 2020 at  3:53am -0400,
Christoph Hellwig <hch@lst.de> wrote:

> Call blk_mq_make_request when no ->make_request_fn is set.  This is
> safe now that blk_alloc_queue always sets up the pointer for make_request
> based drivers.  This avoids an indirect call in the blk-mq driver I/O
> fast path, which is rather expensive due to spectre mitigations.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Makes me cringe thinking about all the indirect calls sprinkled
throughout DM...

Acked-by: Mike Snitzer <snitzer@redhat.com>

Re: [PATCH 3/3] block: bypass ->make_request_fn for blk-mq drivers

[Mike Snitzer]

On Sat, Apr 25 2020 at  3:53am -0400,
Christoph Hellwig <hch@lst.de> wrote:

> Call blk_mq_make_request when no ->make_request_fn is set.  This is
> safe now that blk_alloc_queue always sets up the pointer for make_request
> based drivers.  This avoids an indirect call in the blk-mq driver I/O
> fast path, which is rather expensive due to spectre mitigations.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Makes me cringe thinking about all the indirect calls sprinkled
throughout DM...

Acked-by: Mike Snitzer <snitzer@redhat.com>

Re: avoid the ->make_request_fn indirect for blk-mq drivers

[Jens Axboe]

On 4/25/20 1:53 AM, Christoph Hellwig wrote:
> Hi Jens,
> 
> this small series avoids an indirect call for every submitted bio that
> eventually ends up being handled by blk-mq drivers.  Let me know what
> you think.

I like it, I've been pondering something like this for a bit, but
I like the simplicity of this one and changing it so that only
non-regular make_request_fn is set.

I'll apply this.

-- 
Jens Axboe