nbd: adjust 'max_part' according to part_shift

The 'max_part' parameter determines how many partitions are supported
on each nbd device. However the actual number can be changed to the
power of 2 minus 1 form during the module initialization as
alloc_disk() is called with (1 << part_shift) for some reason.

So adjust 'max_part' also at least for consistency with loop and brd.
It is exported via sysfs already, and a user should check this value
after module loading if [s]he wants to use that number correctly
(i.e. fdisk or something).

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Cc: Laurent Vivier <Laurent.Vivier@bull.net>
Cc: Paul Clements <Paul.Clements@steeleye.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 12 insertions, 1 deletions

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index fdee7567fd15..f533f3375e24 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -754,9 +754,20 @@ static int __init nbd_init(void)
                 return -ENOMEM;
 
         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 know the max number of
+                 * partition kernel should be able to manage.
+                 *
+                 * 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;