btrfs: fix crash when trying to resume balance without the resume flag

We set the BTRFS_BALANCE_RESUME flag in the btrfs_recover_balance()
only, which isn't called during the remount. So when resuming from
the paused balance we hit the bug:

 kernel: kernel BUG at fs/btrfs/volumes.c:3890!
 ::
 kernel:  balance_kthread+0x51/0x60 [btrfs]
 kernel:  kthread+0x111/0x130
 ::
 kernel: RIP: btrfs_balance+0x12e1/0x1570 [btrfs] RSP: ffffba7d0090bde8

Reproducer:
  On a mounted filesystem:

  btrfs balance start --full-balance /btrfs
  btrfs balance pause /btrfs
  mount -o remount,ro /dev/sdb /btrfs
  mount -o remount,rw /dev/sdb /btrfs

To fix this set the BTRFS_BALANCE_RESUME flag in
btrfs_resume_balance_async().

CC: stable@vger.kernel.org # 4.4+
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 292266f6ab9c9d8dfa18422998ec1884ae57369d..be3fc701f38948e37e5d776ee93413e0d06e85ff 100644 (file)
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4052,6 +4052,15 @@ int btrfs_resume_balance_async(struct btrfs_fs_info *fs_info)
                return 0;
        }
 
+       /*
+        * A ro->rw remount sequence should continue with the paused balance
+        * regardless of who pauses it, system or the user as of now, so set
+        * the resume flag.
+        */
+       spin_lock(&fs_info->balance_lock);
+       fs_info->balance_ctl->flags |= BTRFS_BALANCE_RESUME;
+       spin_unlock(&fs_info->balance_lock);
+
        tsk = kthread_run(balance_kthread, fs_info, "btrfs-balance");
        return PTR_ERR_OR_ZERO(tsk);
 }