How do I directly access reserved memory with a kernel module?

I'm trying to limit the OS (Ubuntu Server 15.04) to a certain memory usage and reserve the rest but write a kernel module to read/write to the reserved memory. I figured out how to limit the usage/reserve memory using the kernel parameters "mem=4G memmap=4G@0 memmap=4G$4G" (4GB for OS and 4GB reserved, split at 4GB point) but I don't know how DMA to reserved memory works with kernel modules. I was thinking just create a proc file but I'm not sure if you can create one outside of the OS's allocated memory.

Any suggestions? Thanks!

Edit: This is for research so it doesn't need to be "nice"

Update: Maybe I don't need to write a kernel module. I just found this and I'm going to give it a shot: http://ift.tt/1HvU0rg

Update: I tried the link above but I segfault whenever I try to write. Here's my code:

    #include <fcntl.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <sys/mann.h>

    #define RESERVED_MEMORY_SIZE 0x100000000

    int main() {
            int fd;
            char *reserved_memory;

            fd = open("/dev/mem", O_RDWR | O_SYNC);
            reserved_memory = (char *) mmap(0, RESERVED_MEMORY_SIZE, PROT_READ | PROT_WRITE, MAP_FILE | MAP_SHARED, fd, 4096);
            reserved_memory[0] = 'a';
            return 0;
    }

dmesg shows:

    a.out[1167]: segfault at ffffffffffffffff ip 00000000004005d7 sp 00007ffeffccbd80 error 7 in a.out[400000+1000]

For kicks I tried reserved_memory[1]:

    a.out[1180]: segfault at 0 ip 00000000004005db sp 00007ffc388d77b0 error 6 in a.out[400000+1000]

I'll look into the format of those messages so I can figure out what it's telling me.

Update:

I found this question by somebody with the same issue as me however the only solution appears to be a kernel rebuild. I'm going to try to avoid this so maybe my best option is a custom kernel module again. accessing mmaped /dev/mem?