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Documentation/driver-api/cxl: remove page-allocator quirk section

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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@gourry.net>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
This commit is contained in:
Gregory Price
2025-10-03 10:32:32 -04:00
committed by Dave Jiang
parent 6146a0f1df
commit 82b5d7e30b
1 changed files with 0 additions and 31 deletions
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31
Documentation/driver-api/cxl/allocation/page-allocator.rst
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@@ -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