What is limiting my CMA?

[Eric Pilmore]

I'm running on CentOS7 on a 5.3 based kernel. I have CMA configured
into the kernel and attempting to manipulate the amount of memory
reserved via the "cma" boot parameter. I have one system with 32GB of
memory of which I'm able to reserve 16GB for CMA. However, on another
system that has over 512GB of memory, I can only reserve up to 8GB for
CMA.

My question is, why am I not able to reserve a larger CMA region on my
512GB memory system as compared to my other system where I could
effectively reserve half of the DRAM space?

Thanks in advance for any advice or pointers to where I might be able
to find an explanation.

Eric Pilmore