| .. 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. |
