When a memory device, such as CXL1.1 type3 memory, is emulated as
normal memory (E820_TYPE_RAM), the memory device is indistinguishable
from normal DRAM in terms of memory tiering with the current implementation.
The current memory tiering assigns all detected normal memory nodes
to the same DRAM tier. This results in normal memory devices with
different attributions being unable to be assigned to the correct memory tier,
leading to the inability to migrate pages between different types of memory.
https://lore.kernel.org/linux-mm/PH0PR08MB7955E9F08CCB64F23963B5C3A860A@PH0PR08MB7955.namprd08.prod.outlook.com/T/

This patchset automatically resolves the issues. It delays the initialization
of memory tiers for CPUless NUMA nodes until they obtain HMAT information
and after all devices are initialized at boot time, eliminating the need
for user intervention. If no HMAT is specified, it falls back to
using `default_dram_type`.

Example usecase:
We have CXL memory on the host, and we create VMs with a new system memory
device backed by host CXL memory. We inject CXL memory performance attributes
through QEMU, and the guest now sees memory nodes with performance attributes
in HMAT. With this change, we enable the guest kernel to construct
the correct memory tiering for the memory nodes.

-v6:
 Thanks to Ying's comments,
 * Move `default_dram_perf_lock` to the function's beginning for clarity
 * Fix double unlocking at v5
-v5:
 Thanks to Ying's comments,
 * Add comments about what is protected by `default_dram_perf_lock`
 * Fix an uninitialized pointer mtype
 * Slightly shorten the time holding `default_dram_perf_lock`
 * Fix a deadlock bug in `mt_perf_to_adistance`
 * https://lore.kernel.org/lkml/20240327041646.3258110-1-horenchuang@bytedance.com/T/#u
-v4:
 Thanks to Ying's comments,
 * Remove redundant code
 * Reorganize patches accordingly
 * https://lore.kernel.org/lkml/20240322070356.315922-1-horenchuang@bytedance.com/T/#u
-v3:
 Thanks to Ying's comments,
 * Make the newly added code independent of HMAT
 * Upgrade set_node_memory_tier to support more cases
 * Put all non-driver-initialized memory types into default_memory_types
   instead of using hmat_memory_types
 * find_alloc_memory_type -> mt_find_alloc_memory_type
 * https://lore.kernel.org/lkml/20240320061041.3246828-1-horenchuang@bytedance.com/T/#u
-v2:
 Thanks to Ying's comments,
 * Rewrite cover letter & patch description
 * Rename functions, don't use _hmat
 * Abstract common functions into find_alloc_memory_type()
 * Use the expected way to use set_node_memory_tier instead of modifying it
 * https://lore.kernel.org/lkml/20240312061729.1997111-1-horenchuang@bytedance.com/T/#u
-v1:
 * https://lore.kernel.org/lkml/20240301082248.3456086-1-horenchuang@bytedance.com/T/#u

Ho-Ren (Jack) Chuang (2):
  memory tier: dax/kmem: introduce an abstract layer for finding,
    allocating, and putting memory types
  memory tier: create CPUless memory tiers after obtaining HMAT info

 drivers/dax/kmem.c           |  20 +-----
 include/linux/memory-tiers.h |  13 ++++
 mm/memory-tiers.c            | 126 ++++++++++++++++++++++++++++++-----
 3 files changed, 125 insertions(+), 34 deletions(-)

-- 
Ho-Ren (Jack) Chuang