That's how mmap and mprotect work today. Sizes get rounded up to the next multiple of the page size. Many applications using mmap do not need to know about the page size.

I think it's even part of POSIX: “The system performs mapping operations over whole pages. Thus, while the parameter len need not meet a size or alignment constraint, the system shall include, in any mapping operation, any partial page specified by the address range starting at pa and continuing for len bytes.”

For JIT, you specifically need to be able to write some code to a page, then switch that page from writable to executable in place. Having the page move to a new location isn't acceptable, as the generated code will contain relative calls to other JIT generated code in other pages you've previously written to.