[quotemstr]

You could synchronously invalidate the PTEs corresponding to the mapper pages on chmod and let the soft fault handler re-evaluate pages when next accessed.

That said, Linux gets this right, and Tanenbaum gets it wrong. Preserving access rights requested on open makes the system easier to reason about.

[dataflow]

You could but it would induce corruption in programs. Programs expect I/O to fail much more often than they expect memory accesses to fail.

[quotemstr]

Programs using mmap have to be prepared for SIGBUS on mapped region access anyway --- consider the surprise removal (yanking out USB stick) case.

That few programs, especially native code ones, bother to make themselves robust against this failure mode of mmap doesn't make it any less part of the mmap contract between the kernel and userspace.

[dataflow]

Contracts are one thing, reality is something else though.