[PATCH 3/7] dm: add reserved_bio_based_ios module parameter

[Mike Snitzer <snitzer@redhat.com>]

Allow user to change the number of IOs that are reserved by
bio-based DM's mempools by writing to this file:
/sys/module/dm_mod/parameters/reserved_bio_based_ios

The default value is RESERVED_BIO_BASED_IOS (16).

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 drivers/md/dm.c | 42 ++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 40 insertions(+), 2 deletions(-)

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 8553d03..b44ae19 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -217,6 +217,17 @@ static struct kmem_cache *_io_cache;
 static struct kmem_cache *_rq_tio_cache;
 
 /*
+ * Bio-based DM's mempools' reserved IOs set by the user
+ */
+static unsigned reserved_bio_based_ios;
+
+/*
+ * A copy of reserved_bio_based_ios because it can change anytime.
+ * If values disagree, the user has changed reserved_bio_based_ios.
+ */
+static unsigned reserved_bio_based_ios_latch;
+
+/*
  * Request-based DM's mempools' reserved IOs set by the user
  */
 static unsigned reserved_rq_based_ios;
@@ -228,10 +239,26 @@ static unsigned reserved_rq_based_ios;
 static unsigned reserved_rq_based_ios_latch;
 
 /*
- * This mutex protects reserved_rq_based_ios_latch.
+ * This mutex protects reserved_bio_based_ios_latch and reserved_rq_based_ios_latch.
  */
 static DEFINE_MUTEX(dm_mempools_lock);
 
+static void __reserved_bio_based_ios_refresh(void)
+{
+	BUG_ON(!mutex_is_locked(&dm_mempools_lock));
+
+	reserved_bio_based_ios_latch = ACCESS_ONCE(reserved_bio_based_ios);
+
+	/*
+	 * If the user uses "0", it means default.  Modify
+	 * reserved_bio_based_ios to report the default to the user.
+	 */
+	if (!reserved_bio_based_ios_latch) {
+		(void)cmpxchg(&reserved_bio_based_ios, 0, RESERVED_BIO_BASED_IOS);
+		reserved_bio_based_ios_latch = reserved_bio_based_ios;
+	}
+}
+
 static void __reserved_request_based_ios_refresh(void)
 {
 	BUG_ON(!mutex_is_locked(&dm_mempools_lock));
@@ -274,6 +301,7 @@ static int __init local_init(void)
 		_major = r;
 
 	mutex_lock(&dm_mempools_lock);
+	__reserved_bio_based_ios_refresh();
 	__reserved_request_based_ios_refresh();
 	mutex_unlock(&dm_mempools_lock);
 
@@ -2899,7 +2927,14 @@ struct dm_md_mempools *dm_alloc_md_mempools(unsigned type, unsigned integrity, u
 
 	if (type == DM_TYPE_BIO_BASED) {
 		cachep = _io_cache;
-		pool_size = RESERVED_BIO_BASED_IOS;
+
+		mutex_lock(&dm_mempools_lock);
+		/* Check if reserved_bio_based_ios changed. */
+		if (reserved_bio_based_ios != reserved_bio_based_ios_latch)
+			__reserved_bio_based_ios_refresh();
+		pool_size = reserved_bio_based_ios_latch;
+		mutex_unlock(&dm_mempools_lock);
+
 		front_pad = roundup(per_bio_data_size, __alignof__(struct dm_target_io)) + offsetof(struct dm_target_io, clone);
 	} else if (type == DM_TYPE_REQUEST_BASED) {
 		cachep = _rq_tio_cache;
@@ -2969,6 +3004,9 @@ module_exit(dm_exit);
 module_param(major, uint, 0);
 MODULE_PARM_DESC(major, "The major number of the device mapper");
 
+module_param(reserved_bio_based_ios, uint, S_IRUGO | S_IWUSR);
+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");
 
-- 
1.8.1.4