mm/demotion: Expose memory tier details via sysfs

This patch adds /sys/devices/virtual/memory_tiering/ where all memory tier related details can be found. All allocated memory tiers will be listed there as /sys/devices/virtual/memory_tiering/memory_tierN/

The nodes which are part of a specific memory tier can be listed via /sys/devices/virtual/memory_tiering/memory_tierN/nodes

The abstract distance range value of a specific memory tier can be listed via /sys/devices/virtual/memory_tiering/memory_tierN/abstract_distance

A directory hierarchy looks like

:/sys/devices/virtual/memory_tiering$ tree memory_tier4/
memory_tier4/
├── abstract_distance
├── nodes
├── subsystem -> ../../../../bus/memory_tiering
└── uevent

All toptier nodes are listed via
/sys/devices/virtual/memory_tiering/toptier_nodes

:/sys/devices/virtual/memory_tiering$ cat toptier_nodes
0,2
:/sys/devices/virtual/memory_tiering$ cat memory_tier4/nodes
0,2
:/sys/devices/virtual/memory_tiering$ cat memory_tier4/abstract_distance
512 - 639

---

Changes from v2:
* update macro to static inline
* Fix build error with CONFIG_MIGRATION disabled
* drop abstract_distance

 .../ABI/testing/sysfs-kernel-mm-memory-tiers  |  35 ++++
 mm/memory-tiers.c                             | 154 +++++++++++++++---
 2 files changed, 167 insertions(+), 22 deletions(-)
 create mode 100644 Documentation/ABI/testing/sysfs-kernel-mm-memory-tiers

new file mode 100644
index 000000000000..55051fcf5502
@@ -0,0 +1,35 @@
+What:		/sys/devices/virtual/memory_tiering/
+Date:		August 2022
+Contact:	Linux memory management mailing list <[email protected]>
+Description:	A collection of all the memory tiers allocated.
+
+		Individual memory tier details are contained in subdirectories
+		named by the abstract distance of the memory tier.
+
+		/sys/devices/virtual/memory_tiering/memory_tierN/
+
+
+What:		/sys/devices/virtual/memory_tiering/memory_tierN/
+		/sys/devices/virtual/memory_tiering/memory_tierN/nodes
+Date:		August 2022
+Contact:	Linux memory management mailing list <[email protected]>
+Description:	Directory with details of a specific memory tier
+
+		This is the directory containing information about a particular
+		memory tier, memtierN, where N is derived based on abstract distance.
+
+		A smaller value of N implies a higher (faster) memory tier in the
+		hierarchy.
+
+		nodes: NUMA nodes that are part of this memory tier.
+
+
+What:		/sys/devices/virtual/memory_tiering/toptier_nodes
+Date:		August 2022
+Contact:	Linux memory management mailing list <[email protected]>
+Description:	Toptier node mask
+
+		A toptier is defined as the memory tier from which memory promotion
+		is not done by the kernel.
+
+		toptier_nodes: Union of NUMA nodes that are part of each toptier.

2.37.2