loop: preallocate eight loop devices

The kernel on-demand loop device instantiation breaks several user space
tools as the tools are not ready to cope with the "on-demand feature".  Fix
it by instantiate default 8 loop devices and also reinstate max_loop module
parameter.
Signed-off-by: Ken Chen <kenchen@google.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/loop.c

@@ -1354,7 +1354,7 @@ static struct block_device_operations lo_fops = {
  */
 static int max_loop;
 module_param(max_loop, int, 0);
-MODULE_PARM_DESC(max_loop, "obsolete, loop device is created on-demand");
+MODULE_PARM_DESC(max_loop, "Maximum number of loop devices");
 MODULE_LICENSE("GPL");
 MODULE_ALIAS_BLOCKDEV_MAJOR(LOOP_MAJOR);
 
@@ -1394,16 +1394,11 @@ int loop_unregister_transfer(int number)
 EXPORT_SYMBOL(loop_register_transfer);
 EXPORT_SYMBOL(loop_unregister_transfer);
 
-static struct loop_device *loop_init_one(int i)
+static struct loop_device *loop_alloc(int i)
 {
 	struct loop_device *lo;
 	struct gendisk *disk;
 
-	list_for_each_entry(lo, &loop_devices, lo_list) {
-		if (lo->lo_number == i)
-			return lo;
-	}
-
 	lo = kzalloc(sizeof(*lo), GFP_KERNEL);
 	if (!lo)
 		goto out;
@@ -1427,8 +1422,6 @@ static struct loop_device *loop_init_one(int i)
 	disk->private_data	= lo;
 	disk->queue		= lo->lo_queue;
 	sprintf(disk->disk_name, "loop%d", i);
-	add_disk(disk);
-	list_add_tail(&lo->lo_list, &loop_devices);
 	return lo;
 
 out_free_queue:
@@ -1439,15 +1432,37 @@ static struct loop_device *loop_init_one(int i)
 	return NULL;
 }
 
-static void loop_del_one(struct loop_device *lo)
+static void loop_free(struct loop_device *lo)
 {
-	del_gendisk(lo->lo_disk);
 	blk_cleanup_queue(lo->lo_queue);
 	put_disk(lo->lo_disk);
 	list_del(&lo->lo_list);
 	kfree(lo);
 }
 
+static struct loop_device *loop_init_one(int i)
+{
+	struct loop_device *lo;
+
+	list_for_each_entry(lo, &loop_devices, lo_list) {
+		if (lo->lo_number == i)
+			return lo;
+	}
+
+	lo = loop_alloc(i);
+	if (lo) {
+		add_disk(lo->lo_disk);
+		list_add_tail(&lo->lo_list, &loop_devices);
+	}
+	return lo;
+}
+
+static void loop_del_one(struct loop_device *lo)
+{
+	del_gendisk(lo->lo_disk);
+	loop_free(lo);
+}
+
 static struct kobject *loop_probe(dev_t dev, int *part, void *data)
 {
 	struct loop_device *lo;
@@ -1464,28 +1479,77 @@ static struct kobject *loop_probe(dev_t dev, int *part, void *data)
 
 static int __init loop_init(void)
 {
-	if (register_blkdev(LOOP_MAJOR, "loop"))
-		return -EIO;
-	blk_register_region(MKDEV(LOOP_MAJOR, 0), 1UL << MINORBITS,
-				  THIS_MODULE, loop_probe, NULL, NULL);
+	int i, nr;
+	unsigned long range;
+	struct loop_device *lo, *next;
+
+	/*
+	 * loop module now has a feature to instantiate underlying device
+	 * structure on-demand, provided that there is an access dev node.
+	 * However, this will not work well with user space tool that doesn't
+	 * know about such "feature".  In order to not break any existing
+	 * tool, we do the following:
+	 *
+	 * (1) if max_loop is specified, create that many upfront, and this
+	 *     also becomes a hard limit.
+	 * (2) if max_loop is not specified, create 8 loop device on module
+	 *     load, user can further extend loop device by create dev node
+	 *     themselves and have kernel automatically instantiate actual
+	 *     device on-demand.
+	 */
+	if (max_loop > 1UL << MINORBITS)
+		return -EINVAL;
 
 	if (max_loop) {
-		printk(KERN_INFO "loop: the max_loop option is obsolete "
-				 "and will be removed in March 2008\n");
+		nr = max_loop;
+		range = max_loop;
+	} else {
+		nr = 8;
+		range = 1UL << MINORBITS;
+	}
+
+	if (register_blkdev(LOOP_MAJOR, "loop"))
+		return -EIO;
 
+	for (i = 0; i < nr; i++) {
+		lo = loop_alloc(i);
+		if (!lo)
+			goto Enomem;
+		list_add_tail(&lo->lo_list, &loop_devices);
 	}
+
+	/* point of no return */
+
+	list_for_each_entry(lo, &loop_devices, lo_list)
+		add_disk(lo->lo_disk);
+
+	blk_register_region(MKDEV(LOOP_MAJOR, 0), range,
+				  THIS_MODULE, loop_probe, NULL, NULL);
+
 	printk(KERN_INFO "loop: module loaded\n");
 	return 0;
+
+Enomem:
+	printk(KERN_INFO "loop: out of memory\n");
+
+	list_for_each_entry_safe(lo, next, &loop_devices, lo_list)
+		loop_free(lo);
+
+	unregister_blkdev(LOOP_MAJOR, "loop");
+	return -ENOMEM;
 }
 
 static void __exit loop_exit(void)
 {
+	unsigned long range;
 	struct loop_device *lo, *next;
 
+	range = max_loop ? max_loop :  1UL << MINORBITS;
+
 	list_for_each_entry_safe(lo, next, &loop_devices, lo_list)
 		loop_del_one(lo);
 
-	blk_unregister_region(MKDEV(LOOP_MAJOR, 0), 1UL << MINORBITS);
+	blk_unregister_region(MKDEV(LOOP_MAJOR, 0), range);
 	if (unregister_blkdev(LOOP_MAJOR, "loop"))
 		printk(KERN_WARNING "loop: cannot unregister blkdev\n");
 }