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Message-ID: <20251003143233.1985150-1-gourry@gourry.net>
Date: Fri, 3 Oct 2025 10:32:32 -0400
From: Gregory Price <gourry@...rry.net>
To: linux-cxl@...r.kernel.org
Cc: 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,
gourry@...rry.net,
linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: [PATCH] Documentation/driver-api/cxl: remove page-allocator quirk section
The node/zone quirk section of the cxl documentation is incorrect.
The actual reason for fallback allocation misbehavior in the
described configuration is due to a kswapd/reclaim thrashing scenario
fixed by the linked patch. Remove this section.
Link: https://lore.kernel.org/linux-mm/20250919162134.1098208-1-hannes@cmpxchg.org/
Signed-off-by: Gregory Price <gourry@...rry.net>
---
.../cxl/allocation/page-allocator.rst | 31 -------------------
1 file changed, 31 deletions(-)
diff --git a/Documentation/driver-api/cxl/allocation/page-allocator.rst b/Documentation/driver-api/cxl/allocation/page-allocator.rst
index 7b8fe1b8d5bb..3fa584a248bd 100644
--- a/Documentation/driver-api/cxl/allocation/page-allocator.rst
+++ b/Documentation/driver-api/cxl/allocation/page-allocator.rst
@@ -41,37 +41,6 @@ To simplify this, the page allocator will prefer :code:`ZONE_MOVABLE` over
will fallback to allocate from :code:`ZONE_NORMAL`.
-Zone and Node Quirks
-====================
-Let's consider a configuration where the local DRAM capacity is largely onlined
-into :code:`ZONE_NORMAL`, with no :code:`ZONE_MOVABLE` capacity present. The
-CXL capacity has the opposite configuration - all onlined in
-:code:`ZONE_MOVABLE`.
-
-Under the default allocation policy, the page allocator will completely skip
-:code:`ZONE_MOVABLE` as a valid allocation target. This is because, as of
-Linux v6.15, the page allocator does (approximately) the following: ::
-
- for (each zone in local_node):
-
- for (each node in fallback_order):
-
- attempt_allocation(gfp_flags);
-
-Because the local node does not have :code:`ZONE_MOVABLE`, the CXL node is
-functionally unreachable for direct allocation. As a result, the only way
-for CXL capacity to be used is via `demotion` in the reclaim path.
-
-This configuration also means that if the DRAM ndoe has :code:`ZONE_MOVABLE`
-capacity - when that capacity is depleted, the page allocator will actually
-prefer CXL :code:`ZONE_MOVABLE` pages over DRAM :code:`ZONE_NORMAL` pages.
-
-We may wish to invert this priority in future Linux versions.
-
-If `demotion` and `swap` are disabled, Linux will begin to cause OOM crashes
-when the DRAM nodes are depleted. See the reclaim section for more details.
-
-
CGroups and CPUSets
===================
Finally, assuming CXL memory is reachable via the page allocation (i.e. onlined
--
2.51.0
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