[PATCH 1/1] btrfs: btrfs_calc_avail_data_space cope with no read/write devices V2

[Andy Whitcroft <apw@canonical.com>]

When we mount a btrfs filesystem from read-only media there will be no
read/write devices; for example mounting an SD card with its lock enabled.
This triggers an immediate BUG during mount:

  kernel BUG at .../fs/btrfs/super.c:984!

This is triggered by statfs when calculating the free space in the
filesytem.  We bug if the number of read/write devices is 0.

This check seems spurious as the information collected is valid regardless
of whether the devices are read-only or not.  As the count is used to
size the sort array it seems more correct to switch it to the number of
open devices.

BugLink: http://bugs.launchpad.net/bugs/816770
Signed-off-by: Andy Whitcroft <apw@canonical.com>
---
 fs/btrfs/super.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 15634d4..ae4367a 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -980,7 +980,7 @@ static int btrfs_calc_avail_data_space(struct btrfs_root *root, u64 *free_bytes)
 	int i = 0, nr_devices;
 	int ret;
 
-	nr_devices = fs_info->fs_devices->rw_devices;
+	nr_devices = fs_info->fs_devices->open_devices;
 	BUG_ON(!nr_devices);
 
 	devices_info = kmalloc(sizeof(*devices_info) * nr_devices,
-- 
1.7.4.1