A new memory zone for CXL

[KyungsanKim]

Historically, Linux Zone has been extended reflecting the evolution of HW and SW changes as belows.
Recently, the CXL, a standard open-connectivity technology, is promising and actively developing with different HW characteristics from conventional memory devices.
As memory expander, CXL has a different latency range from DDR memory on DIMM interface due to that it works on PCIe link. Also, CXL is designed to support composable architecture over single and multi-node fabrics so that it would support many usecases for dynamic memory adjustment.

On the understanding, we suggest a new memory zone, ZONE_EXMEM, for CXL memory expander.

Linux Zone	Trigger	Desc	Option
ZONE_NORMAL	Initial design	normal addressable memory	mandatory
ZONE_DMA	HW (io)	addressable memory for some peripheral devices supporting limited address resolution	configurable
ZONE_HIGHMEM	HW (cpu)	a memory area that is only addressable by the kernel through mapping portions into its own address space. This is for example used by i386 to allow the kernel to address the memory beyond 900MB.	configurable
ZONE_DEVICE	HW (io)	offer paging and mmap for device driver identified physical address ranges. e.g) pmem, hmm, p2pdma - kernel/Documentation/vm/memory-model.rst	configurable
ZONE_MOVABLE	SW	similar to ZONE_NORMAL, except that it contains movable pages. Main use cases for ZONE_MOVABLE are to make memory offlining/unplug more likely to succeed, and to locally limit unmovable allocations - kernel/Documentation/admin-guide/mm/memory-hotplug.rst	configurable
ZONE_EXMEM	HW (cxl)	SMDK suggestion, Extented memory for latency isolation, pluggability	configurable

Author - Kyungsan Kim, Wonjae Lee, Donghwa Kang