cxl: docs/allocation/hugepages

Add docs on how CXL capacity interacts with CMA and HugeTLB allocation
interfaces.

Signed-off-by: Gregory Price <gourry@gourry.net>
Link: https://patch.msgid.link/20250512162134.3596150-17-gourry@gourry.net
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

Author: Gregory Price

Documentation/driver-api/cxl/allocation/hugepages.rst

@@ -0,0 +1,32 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==========
+Huge Pages
+==========
+
+Contiguous Memory Allocator
+===========================
+CXL Memory onlined as SystemRAM during early boot is eligible for use by CMA,
+as the NUMA node hosting that capacity will be `Online` at the time CMA
+carves out contiguous capacity.
+
+CXL Memory deferred to the CXL Driver for configuration cannot have its
+capacity allocated by CMA - as the NUMA node hosting the capacity is `Offline`
+at :code:`__init` time - when CMA carves out contiguous capacity.
+
+HugeTLB
+=======
+Different huge page sizes allow different memory configurations.
+
+2MB Huge Pages
+--------------
+All CXL capacity regardless of configuration time or memory zone is eligible
+for use as 2MB huge pages.
+
+1GB Huge Pages
+--------------
+CXL capacity onlined in :code:`ZONE_NORMAL` is eligible for 1GB Gigantic Page
+allocation.
+
+CXL capacity onlined in :code:`ZONE_MOVABLE` is not eligible for 1GB Gigantic
+Page allocation.

Documentation/driver-api/cxl/index.rst

@@ -47,5 +47,6 @@ that have impacts on each other.  The docs here break up configurations steps.
    allocation/dax
    allocation/page-allocator
    allocation/reclaim
+   allocation/hugepages.rst
 
 .. only::  subproject and html