[PATCH 3/3] nbd: adjust 'max_part' according to part_shift

[Namhyung Kim]

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@xxxxxxxxx>
Cc: Laurent Vivier <Laurent.Vivier@xxxxxxxx>
Cc: Paul Clements <Paul.Clements@xxxxxxxxxxxx>
---
 drivers/block/nbd.c |   13 ++++++++++++-
 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;
 
-- 
1.7.5.2

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