The ABI status of filesystem formats

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

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