quoted

Please note that this only works for addresses in the kernel address
space. For user space the full 64 bit address range (minus the top
page) can be used for user space applications.

I wonder what is the unsigned long size (stack entry size) of the
s390? is it 64bit?

The s390 kernel is 64 bit only. So unsigned long is 64 bit as well.

quoted

I'm just writing this
here, just in case something like this comes up for uprobes or
something similar as well.

I'm considering another solution if it doesn't work. Of course if
above works, it is the best compression ratio.

I'm think we are not talking about uprobes here, and everything ftrace
related would just work (tm) with the first four MSBs assumed to be
zero.