How to find the range of MMAPable virtual addresses in a program?

Background information:

1. I'm using 64 bit Arch.

2. I'm writing an experimental custom allocator, so portable code is a stretch goal, not a requirement.

3. I'm not using C or C++. I'm doing my syscalls directly.

According to this SO post: https://stackoverflow.com/questions/17671423/where-is-the-stack-memory-allocated-from-for-a-linux-process A program's virtual address space is organized like this:

------------------ <--- Top of the process address space
Stack (grows down)
v v v v v v v v v
------------------

(unmapped)

------------------ <--- Maximum stack size.


(unmapped)


-------------------
mmap
-------------------


(unmapped)


-------------------
^ ^ ^ ^ ^ ^ ^ ^ ^ ^
brk (grows up)
-------------------
BSS
-------------------
Data
-------------------
Text
-------------------

------------------- <--- Bottom or process address space.

Presuming this is correct, I'm trying to find the exact range of MMAPable address space here. Is it sufficient to find the lower bound by doing sbrk(0), page aligning up, and then ensuring that brk and sbrk are never called again? For the upper bound is it sufficient to assume 008FFFFF FFFFFFFF - SIZE_OF_STACK - A_GIGABYTE_OF_PADDING, and then page aligning down, is safe address space? I'd prefer a more exact and robust answer for the upper bound, but I'll accept a reasonable approximation that's guaranteed safe.