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diff --git a/Documentation/driver-api/cxl/allocation/page-allocator.rst b/Documentation/driver-api/cxl/allocation/page-allocator.rst new file mode 100644 index 000000000000..3fa584a248bd --- /dev/null +++ b/Documentation/driver-api/cxl/allocation/page-allocator.rst @@ -0,0 +1,54 @@ +.. 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`. + + +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). + |