cxl: docs/allocation/reclaim

Document a bit about how reclaim interacts with various CXL
configurations.

Signed-off-by: Gregory Price <gourry@gourry.net>
Link: https://patch.msgid.link/20250512162134.3596150-16-gourry@gourry.net
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

Author: Gregory Price

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

@@ -0,0 +1,51 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=======
+Reclaim
+=======
+Another way CXL memory can be utilized *indirectly* is via the reclaim system
+in :code:`mm/vmscan.c`.  Reclaim is engaged when memory capacity on the system
+becomes pressured based on global and cgroup-local `watermark` settings.
+
+In this section we won't discuss the `watermark` configurations, just how CXL
+memory can be consumed by various pieces of reclaim system.
+
+Demotion
+========
+By default, the reclaim system will prefer swap (or zswap) when reclaiming
+memory.  Enabling :code:`kernel/mm/numa/demotion_enabled` will cause vmscan
+to opportunistically prefer distant NUMA nodes to swap or zswap, if capacity
+is available.
+
+Demotion engages the :code:`mm/memory_tier.c` component to determine the
+next demotion node.  The next demotion node is based on the :code:`HMAT`
+or :code:`CDAT` performance data.
+
+cpusets.mems_allowed quirk
+--------------------------
+In Linux v6.15 and below, demotion does not respect :code:`cpusets.mems_allowed`
+when migrating pages.  As a result, if demotion is enabled, vmscan cannot
+guarantee isolation of a container's memory from nodes not set in mems_allowed.
+
+In Linux v6.XX and up, demotion does attempt to respect
+:code:`cpusets.mems_allowed`; however, certain classes of shared memory
+originally instantiated by another cgroup (such as common libraries - e.g.
+libc) may still be demoted.  As a result, the mems_allowed interface still
+cannot provide perfect isolation from the remote nodes.
+
+ZSwap and Node Preference
+=========================
+In Linux v6.15 and below, ZSwap allocates memory from the local node of the
+processor for the new pages being compressed.  Since pages being compressed
+are typically cold, the result is a cold page becomes promoted - only to
+be later demoted as it ages off the LRU.
+
+In Linux v6.XX, ZSwap tries to prefer the node of the page being compressed
+as the allocation target for the compression page.  This helps prevent
+thrashing.
+
+Demotion with ZSwap
+===================
+When enabling both Demotion and ZSwap, you create a situation where ZSwap
+will prefer the slowest form of CXL memory by default until that tier of
+memory is exhausted.

Documentation/driver-api/cxl/index.rst

@@ -46,5 +46,6 @@ that have impacts on each other.  The docs here break up configurations steps.
 
    allocation/dax
    allocation/page-allocator
+   allocation/reclaim
 
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