Comment 28 for bug 1799237

To summarize: 4.15.0-20 is the last kernel where mprotect works, all kernels between 4.15.0-20 and 4.15.0-34 are not testable, 4.15.0-34 is the first kernel that boots correctly and mprotect fails.
You have a test program, instructions how to run it and here's a guide how to set up pmem emulation: http://pmem.io/2016/02/22/pm-emulation.html (just adding memmap=1G!4G to kernel command line should be enough).

We (me and Igor) are not kernel developers. The ball is on your side :).