On Mon, Feb 22, 2021 at 06:40:47PM +0200, Nikolay Borisov wrote:

btrfs_block_rsv_add can return only ENOSPC since it's called with
NO_FLUSH modifier. This so simplify the logic in
btrfs_delayed_inode_reserve_metadata to exploit this invariant.

This seems quite fragile, it's not straightforward to see from the
context that the NO_FLUSH code will always return ENOSPC. I followed a
few calls down from btrfs_block_rsv_add and it's well hidden inside
__reserve_bytes. So in case it's an invariant I'd rather add an
assertion, ie. ASSERT(ret == 0 || ret == -ENOSPC) so at least we know
when this gets broken. Otherwise looks ok.