[PATCH v2] btrfs: fix BUG trying to resume balance without resume flag

[Anand Jain <anand.jain@oracle.com>]

We set the BTRFS_BALANCE_RESUME flag in the btrfs_recover_balance(),
which is not called during the remount. So when resuming from the
paused balance we hit 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 BTRFS.

  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()
instead of btrfs_recover_balance().

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
v1->v2: btrfs_resume_balance_async() can be called only from remount or
mount, we don't need to hold fs_info->balance_lock.

Strictly speaking we should rather keep the balance at the paused state
unless it is resumed by the user again, that means neither mount nor
remount-rw should resume the balance automatically, former case needs
writing balance status to the disk. Which needs compatibility
verification.  So for now just avoid BUG. 

 fs/btrfs/volumes.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 3e6983a169c4..64bcaf25908b 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4115,6 +4115,8 @@ int btrfs_resume_balance_async(struct btrfs_fs_info *fs_info)
 		return 0;
 	}
 
+	fs_info->balance_ctl->b_flags |= BTRFS_BALANCE_RESUME;
+
 	tsk = kthread_run(balance_kthread, fs_info, "btrfs-balance");
 	return PTR_ERR_OR_ZERO(tsk);
 }
@@ -4156,7 +4158,6 @@ int btrfs_recover_balance(struct btrfs_fs_info *fs_info)
 
 	bctl->fs_info = fs_info;
 	bctl->flags = btrfs_balance_flags(leaf, item);
-	bctl->flags |= BTRFS_BALANCE_RESUME;
 
 	btrfs_balance_data(leaf, item, &disk_bargs);
 	btrfs_disk_balance_args_to_cpu(&bctl->data, &disk_bargs);
-- 
2.7.0