On 12/12/19 11:08 PM, Naohiro Aota wrote:
On/After degraded mount, we might have no writable metadata block group due to broken write pointers. If you e.g. balance the FS before writing any data, alloc_tree_block_no_bg_flush() (called from insert_balance_item()) fails to allocate a tree block for it, due to global reservation failure. We can reproduce this situation with xfstests btrfs/124. While we can workaround the failure if we write some data and, as a result of writing, let a new metadata block group allocated, it's a bad practice to apply. This commit avoids such failures by ensuring that read-write mounted volume has non-zero metadata space. If metadata space is empty, it forces new metadata block group allocation.
Ick, I hate this, especially since it doesn't take into account if we're mounted read only. No instead add something btrfs_start_transaction() or something similar that does this check to allocate a chunk. And alloc_tree_block_no_bg_flush() only means we won't create the pending bg's in that path, we're still able to allocate chunks. So I'm not super sure what you are actually hitting here, but this is the wrong way to go about fixing it. Thanks,
Josef
