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

  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().

Signed-off-by: Anand Jain <anand.j...@oracle.com>
v2->v3: . Hold fs_info->balance_lock because in remount context there can
        be more the one thread accessing the balance_ctl.
        . Don't remove the BTRFS_BALANCE_RESUME set at recover_balance
        as in the original code.
        . Add comments.
v1->v2: btrfs_resume_balance_async() can be called only from remount or
        mount, we don't need to hold fs_info->balance_lock.

 fs/btrfs/volumes.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 40e4259bdd51..c68976856d87 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4112,6 +4112,15 @@ int btrfs_resume_balance_async(struct btrfs_fs_info 
                return 0;
+       /*
+        * A remount ro->rw sequence should continue with the
+        * paused balance irrespective 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);

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