I would like to write a kernel module that uses some memory space for its own purpose.

Reserve memory space with kernel parameter

The memory space was reserved through linux kernel parameter by modifying /etc/default/grub as below.
sudo update-grub and rebooting should follow to apply the modification.
The reserved area was from 0x100000000 ~ 0x1ffffffff (4GB~8GB).

GRUB_CMDLINE_LINUX_DEFAULT="memmap=4G\\\$0x100000000"

After rebooting, you can confirm the reduced memory space with the command cat /proc/meminfo.

Access to the reserved memory area

I could access to the reserved memory space in my kernel module with ioremap_nocache() function of linux kernel.
Please refer to the source code(hello.c) in the repo.