dm snapshot: introduce account_start_copy() and account_end_copy()

[ Upstream commit a2f83e8b0c ]

This simple refactoring moves code for modifying the semaphore cow_count
into separate functions to prepare for changes that will extend these
methods to provide for a more sophisticated mechanism for COW
throttling.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reviewed-by: Nikos Tsironis <ntsironis@arrikto.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/md/dm-snap.c

@@ -1399,6 +1399,16 @@ static void snapshot_dtr(struct dm_target *ti)
 	kfree(s);
 }
 
+static void account_start_copy(struct dm_snapshot *s)
+{
+	down(&s->cow_count);
+}
+
+static void account_end_copy(struct dm_snapshot *s)
+{
+	up(&s->cow_count);
+}
+
 /*
  * Flush a list of buffers.
  */
@@ -1594,7 +1604,7 @@ static void copy_callback(int read_err, unsigned long write_err, void *context)
 		rb_link_node(&pe->out_of_order_node, parent, p);
 		rb_insert_color(&pe->out_of_order_node, &s->out_of_order_tree);
 	}
-	up(&s->cow_count);
+	account_end_copy(s);
 }
 
 /*
@@ -1618,7 +1628,7 @@ static void start_copy(struct dm_snap_pending_exception *pe)
 	dest.count = src.count;
 
 	/* Hand over to kcopyd */
-	down(&s->cow_count);
+	account_start_copy(s);
 	dm_kcopyd_copy(s->kcopyd_client, &src, 1, &dest, 0, copy_callback, pe);
 }
 
@@ -1638,7 +1648,7 @@ static void start_full_bio(struct dm_snap_pending_exception *pe,
 	pe->full_bio = bio;
 	pe->full_bio_end_io = bio->bi_end_io;
 
-	down(&s->cow_count);
+	account_start_copy(s);
 	callback_data = dm_kcopyd_prepare_callback(s->kcopyd_client,
 						   copy_callback, pe);