btrfs: add sanity check when resuming balance after mount

Replace a WARN_ON with a proper check and message in case something goes
really wrong and resumed balance cannot set up its exclusive status.
The check is a user friendly assertion, I don't expect to ever happen
under normal circumstances.

Also document that the paused balance starts here and owns the exclusive
op status.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 7e3656c9798b9005bb597519cb1bf3c38e8c0ae5..585f930132148523cb9c3dda4ed8aadd1b5ebb2b 100644 (file)
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4047,7 +4047,19 @@ int btrfs_recover_balance(struct btrfs_fs_info *fs_info)
        btrfs_balance_sys(leaf, item, &disk_bargs);
        btrfs_disk_balance_args_to_cpu(&bctl->sys, &disk_bargs);
 
-       WARN_ON(test_and_set_bit(BTRFS_FS_EXCL_OP, &fs_info->flags));
+       /*
+        * This should never happen, as the paused balance state is recovered
+        * during mount without any chance of other exclusive ops to collide.
+        *
+        * This gives the exclusive op status to balance and keeps in paused
+        * state until user intervention (cancel or umount). If the ownership
+        * cannot be assigned, show a message but do not fail. The balance
+        * is in a paused state and must have fs_info::balance_ctl properly
+        * set up.
+        */
+       if (test_and_set_bit(BTRFS_FS_EXCL_OP, &fs_info->flags))
+               btrfs_warn(fs_info,
+       "cannot set exclusive op status to balance, resume manually");
 
        mutex_lock(&fs_info->volume_mutex);
        mutex_lock(&fs_info->balance_mutex);