loop: export module parameters

Export 'max_loop' and 'max_part' parameters to sysfs so user can know
that how many devices are allowed and how many partitions are supported.

If 'max_loop' is 0, there is no restriction on the number of loop devices.
User can create/use the devices as many as minor numbers available. If
'max_part' is 0, it means simply the device doesn't support partitioning.

Also note that 'max_part' can be adjusted to power of 2 minus 1 form if
needed. User should check this value after the module loading if he/she
want to use that number correctly (i.e. fdisk, mknod, etc.).

Signed-off-by: Namhyung Kim <[email protected]>
Cc: Laurent Vivier <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

drivers/block/loop.c

@@ -1540,9 +1540,9 @@ static const struct block_device_operations lo_fops = {
  * And now the modules code and kernel interface.
  */
 static int max_loop;
-module_param(max_loop, int, 0);
+module_param(max_loop, int, S_IRUGO);
 MODULE_PARM_DESC(max_loop, "Maximum number of loop devices");
-module_param(max_part, int, 0);
+module_param(max_part, int, S_IRUGO);
 MODULE_PARM_DESC(max_part, "Maximum number of partitions per loop device");
 MODULE_LICENSE("GPL");
 MODULE_ALIAS_BLOCKDEV_MAJOR(LOOP_MAJOR);
@@ -1688,9 +1688,20 @@ static int __init loop_init(void)
 	 */
 
 	part_shift = 0;
-	if (max_part > 0)
+	if (max_part > 0) {
 		part_shift = fls(max_part);
 
+		/*
+		 * Adjust max_part according to part_shift as it is exported
+		 * to user space so that user can decide correct minor number
+		 * if [s]he want to create more devices.
+		 *
+		 * Note that -1 is required because partition 0 is reserved
+		 * for the whole disk.
+		 */
+		max_part = (1UL << part_shift) - 1;
+	}
+
 	if ((1UL << part_shift) > DISK_MAX_PARTS)
 		return -EINVAL;