[$] The ABI status of filesystem formats

One of the key rules of Linux kernel development is that the ABI betweenthe kernel and user space cannot be broken; any change that breakspreviously working programs will, outside of exceptional circumstances, bereverted. The rule seems clear, but there are ambiguities when it comes todetermining just what constitutes the kernel ABI; tracepoints are a perennial example of this. A recentdiscussion has brought another one of those ambiguities to light: the on-disk format of Linuxfilesystems.