Hung tasks due to low memory are a bug not a feature. Any time you put the Linux kernel under memory pressure you trigger its wealth of defects in error handling paths, none of which are tested and most of which are rarely exercised in practice. For example squashfs used to have a resource leak under memory pressure where it would exit a function without releasing a lock, after which all block operations system-wide would hang forever until reboot. Linux is absolutely crawling with that type of defect, but not uniformly. Some subsystems have more than others, and btrfs is unusually dense with them.