[PATCH v3 16/17] cxl: docs/allocation/hugepages

Gregory Price
2025-05-12T16:22:07Z

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

Signed-off-by: Gregory Price <gourry@gourry.net>
---
.../driver-api/cxl/allocation/hugepages.rst | 32 +++++++++++++++++++
Documentation/driver-api/cxl/index.rst | 1 +
2 files changed, 33 insertions(+)
create mode 100644 Documentation/driver-api/cxl/allocation/hugepages.rst

diff --git a/Documentation/driver-api/cxl/allocation/hugepages.rst b/Documentation/driver-api/cxl/allocation/hugepages.rst
new file mode 100644
index 000000000000..1023c6922829
--- /dev/null
+++ b/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.
diff --git a/Documentation/driver-api/cxl/index.rst b/Documentation/driver-api/cxl/index.rst
index d3ab928d4d7c..366faf851fc7 100644
--- a/Documentation/driver-api/cxl/index.rst
+++ b/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
--
2.49.0