The M:N threading stack cleanup machinery tries to call MADV_FREE on the native thread's stack, and calls rb_bug if it fails. Unfortunately, there's no way to distinguish between "You passed bad parameters to madvise" and "MADV_FREE is not supported on the kernel you are running on"; both cases just return EINVAL. This means that if you have a Ruby on a system that was built on a system with MADV_FREE and run it on a system without it, you get a crash in nt_free_stack. I ran into this because rr actually emulates MADV_FREE by just returning EINVAL and pretending it's not supported (since it can otherwise introduce nondeterministic behaviour). So if you run bootstraptest/test_ractor.rb under rr, you get this crash. I think we should just get rid of the error handling here; freeing memory like this is strictly optional anyway. [Bug #20632]