[$] process_madvise(), pidfd capabilities, and the revenge of the PIDs

Once upon a time, there were few ways for one process to operate uponanother after its creation; sending signals and ptrace() wereabout it. In recent years, interest inproviding ways for processes to control others has been on the increase,and the kernel's process-management API has been expanded accordingly.Along these lines, the process_madvise() system call has been proposed as a way for one process to influencehow memory management is done in another. There is a newprocess_madvise() series which is interesting in its own right,but this series has also raised a couple of questions about how processmanagement should be improved in general.