dm: make nr_requests tunable

Make it possible to change the value nr_requests for device mapper
devices.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 block/blk-sysfs.c |    9 +++++----
 drivers/md/dm.c   |   10 +++++++++-
 2 files changed, 14 insertions(+), 5 deletions(-)

Index: linux-4.13-rc5/drivers/md/dm.c
===================================================================
--- linux-4.13-rc5.orig/drivers/md/dm.c
+++ linux-4.13-rc5/drivers/md/dm.c
@@ -116,6 +116,11 @@ static struct kmem_cache *_rq_cache;
 #define RESERVED_BIO_BASED_IOS		16
 static unsigned reserved_bio_based_ios = RESERVED_BIO_BASED_IOS;
 
+ /*
+ * nr_requests limit for newly created devices.
+ */
+static unsigned default_nr_requests = DEFAULT_NR_REQUESTS;
+
 static int __dm_get_module_param_int(int *module_param, int min, int max)
 {
 	int param = ACCESS_ONCE(*module_param);
@@ -2126,7 +2131,7 @@ int dm_setup_md_queue(struct mapped_devi
 	case DM_TYPE_DAX_BIO_BASED:
 		dm_init_normal_md_queue(md);
 		blk_queue_make_request(md->queue, dm_make_request);
-		md->queue->nr_requests = DEFAULT_NR_REQUESTS;
+		md->queue->nr_requests = max(default_nr_requests, (unsigned)BLKDEV_MIN_RQ);
 		/*
 		 * DM handles splitting bios as needed.  Free the bio_split bioset
 		 * since it won't be used (saves 1 process per bio-based DM device).
@@ -3081,6 +3086,9 @@ MODULE_PARM_DESC(reserved_bio_based_ios,
 module_param(dm_numa_node, int, S_IRUGO | S_IWUSR);
 MODULE_PARM_DESC(dm_numa_node, "NUMA node for DM device memory allocations");
 
+module_param(default_nr_requests, uint, S_IRUGO | S_IWUSR);
+MODULE_PARM_DESC(default_nr_requests, "Default limit of in-flight I/Os");
+
 MODULE_DESCRIPTION(DM_NAME " driver");
 MODULE_AUTHOR("Joe Thornber <dm-devel@redhat.com>");
 MODULE_LICENSE("GPL");
Index: linux-4.13-rc5/block/blk-sysfs.c
===================================================================
--- linux-4.13-rc5.orig/block/blk-sysfs.c
+++ linux-4.13-rc5/block/blk-sysfs.c
@@ -67,9 +67,6 @@ queue_requests_store(struct request_queu
 	unsigned long nr;
 	int ret, err;
 
-	if (!q->request_fn && !q->mq_ops)
-		return -EINVAL;
-
 	ret = queue_var_store(&nr, page, count);
 	if (ret < 0)
 		return ret;
@@ -79,8 +76,12 @@ queue_requests_store(struct request_queu
 
 	if (q->request_fn)
 		err = blk_update_nr_requests(q, nr);
-	else
+	else if (q->mq_ops)
 		err = blk_mq_update_nr_requests(q, nr);
+	else {
+		q->nr_requests = nr;
+		err = 0;
+	}
 
 	if (err)
 		return err;