[CAUSE]
This situation is caused by several factors:
- Fuzzed image
The extent tree of this fs missed one backref for extent tree root.
So we can allocated space from that slot.
- MIXED_BG feature
Super block has MIXED_BG flag.
- No mixed block groups exists
All block groups are just regular ones.
This makes data space_info->block_groups[] contains metadata block
groups. And when we reserve space for data, we can use space in
metadata block group.
Then we hit the following file operations:
- fallocate
We need to allocate data extents.
find_free_extent() choose to use the metadata block to allocate space
from, and choose the space of extent tree root, since its backref is
missing.
This generate one delayed ref head with is_data = 1.
- extent tree update
We need to update extent tree at run_delayed_ref time.
This generate one delayed ref head with is_data = 0, for the same
bytenr of old extent tree root.
Then we trigger the BUG_ON().
[FIX]
The quick fix here is to check block_group->flags before using it.
The problem can only happen for MIXED_GROUPS fs. Regular filesystems
won't have space_info with DATA|METADATA flag, and no way to hit the
bug.