cxl: docs/allocation/page-allocator

Document some interesting interactions that occur when exposing CXL
memory capacity to page allocator.

Signed-off-by: Gregory Price <gourry@gourry.net>
Link: https://patch.msgid.link/20250512162134.3596150-15-gourry@gourry.net
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

Documentation/driver-api/cxl/allocation/page-allocator.rst

@@ -0,0 +1,85 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==================
+The Page Allocator
+==================
+
+The kernel page allocator services all general page allocation requests, such
+as :code:`kmalloc`.  CXL configuration steps affect the behavior of the page
+allocator based on the selected `Memory Zone` and `NUMA node` the capacity is
+placed in.
+
+This section mostly focuses on how these configurations affect the page
+allocator (as of Linux v6.15) rather than the overall page allocator behavior.
+
+NUMA nodes and mempolicy
+========================
+Unless a task explicitly registers a mempolicy, the default memory policy
+of the linux kernel is to allocate memory from the `local NUMA node` first,
+and fall back to other nodes only if the local node is pressured.
+
+Generally, we expect to see local DRAM and CXL memory on separate NUMA nodes,
+with the CXL memory being non-local.  Technically, however, it is possible
+for a compute node to have no local DRAM, and for CXL memory to be the
+`local` capacity for that compute node.
+
+
+Memory Zones
+============
+CXL capacity may be onlined in :code:`ZONE_NORMAL` or :code:`ZONE_MOVABLE`.
+
+As of v6.15, the page allocator attempts to allocate from the highest
+available and compatible ZONE for an allocation from the local node first.
+
+An example of a `zone incompatibility` is attempting to service an allocation
+marked :code:`GFP_KERNEL` from :code:`ZONE_MOVABLE`.  Kernel allocations are
+typically not migratable, and as a result can only be serviced from
+:code:`ZONE_NORMAL` or lower.
+
+To simplify this, the page allocator will prefer :code:`ZONE_MOVABLE` over
+:code:`ZONE_NORMAL` by default, but if :code:`ZONE_MOVABLE` is depleted, it
+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
+in :code:`ZONE_NORMAL`), the :code:`cpusets.mems_allowed` may be used by
+containers to limit the accessibility of certain NUMA nodes for tasks in that
+container.  Users may wish to utilize this in multi-tenant systems where some
+tasks prefer not to use slower memory.
+
+In the reclaim section we'll discuss some limitations of this interface to
+prevent demotions of shared data to CXL memory (if demotions are enabled).
+

Documentation/driver-api/cxl/index.rst

@@ -45,5 +45,6 @@ that have impacts on each other.  The docs here break up configurations steps.
    :caption: Memory Allocation
 
    allocation/dax
+   allocation/page-allocator
 
 .. only::  subproject and html