linux-kernel - [RFC PATCH v2 15/18] cxl: docs/allocation/hugepages

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Message-ID: <20250430181048.1197475-16-gourry@gourry.net>
Date: Wed, 30 Apr 2025 14:10:45 -0400
From: Gregory Price <gourry@...rry.net>
To: linux-cxl@...r.kernel.org
Cc: linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	kernel-team@...a.com,
	dave@...olabs.net,
	jonathan.cameron@...wei.com,
	dave.jiang@...el.com,
	alison.schofield@...el.com,
	vishal.l.verma@...el.com,
	ira.weiny@...el.com,
	dan.j.williams@...el.com,
	corbet@....net
Subject: [RFC PATCH v2 15/18] cxl: docs/allocation/hugepages

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

Signed-off-by: Gregory Price <gourry@...rry.net>
---
 .../driver-api/cxl/allocation/hugepages.rst   | 34 +++++++++++++++++++
 Documentation/driver-api/cxl/index.rst        |  1 +
 2 files changed, 35 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..279970d73a28
--- /dev/null
+++ b/Documentation/driver-api/cxl/allocation/hugepages.rst
@@ -0,0 +1,34 @@
+.. 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 - which CMA carves out contiguous capacity.
+
+HugeTLB
+=======
+HugeTLB capacity is reserved from the page allocator. Only CXL memory onlined
+as SystemRAM during :code:`__init` and via memory-hotplug can be used via
+HugeTLB.
+
+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 a768144698e9..e58d057a16e3 100644
--- a/Documentation/driver-api/cxl/index.rst
+++ b/Documentation/driver-api/cxl/index.rst
@@ -48,5 +48,6 @@ 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