ext4: fix largest free orders lists corruption on mb_optimize_scan switch

ext4: fix largest free orders lists corruption on mb_optimize_scan switch

The grp->bb_largest_free_order is updated regardless of whether
mb_optimize_scan is enabled. This can lead to inconsistencies between
grp->bb_largest_free_order and the actual s_mb_largest_free_orders list
index when mb_optimize_scan is repeatedly enabled and disabled via remount.

For example, if mb_optimize_scan is initially enabled, largest free
order is 3, and the group is in s_mb_largest_free_orders[3]. Then,
mb_optimize_scan is disabled via remount, block allocations occur,
updating largest free order to 2. Finally, mb_optimize_scan is re-enabled
via remount, more block allocations update largest free order to 1.

At this point, the group would be removed from s_mb_largest_free_orders[3]
under the protection of s_mb_largest_free_orders_locks[2]. This lock
mismatch can lead to list corruption.

To fix this, whenever grp->bb_largest_free_order changes, we now always
attempt to remove the group from its old order list. However, we only
insert the group into the new order list if `mb_optimize_scan` is enabled.
This approach helps prevent lock inconsistencies and ensures the data in
the order lists remains reliable.

Fixes: 196e402adf2e ("ext4: improve cr 0 / cr 1 group scanning")
CC: stable@vger.kernel.org
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20250714130327.1830534-12-libaokun1@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>