Fix would require backport of commit 6d4572a9d71d ("btrfs: allow btrfs_truncate_block() to fallback to nocow for data space reservation"). The v5.4 got its own backport but it does not apply into v4.15.
Fix would require backport of commit 6d4572a9d71d ("btrfs: allow btrfs_truncate_ block() to fallback to nocow for data space reservation"). The v5.4 got its own backport but it does not apply into v4.15.