Re: [RFC PATCH v2 11/18] cxl: docs/linux/memory-hotplug

[Randy Dunlap]

On 4/30/25 11:10 AM, Gregory Price wrote:
> Add documentation on how the CXL driver surfaces memory through the
> DAX driver and memory-hotplug.
> 
> Signed-off-by: Gregory Price <gourry@xxxxxxxxxx>
> ---
>  Documentation/driver-api/cxl/index.rst        |  1 +
>  .../driver-api/cxl/linux/memory-hotplug.rst   | 78 +++++++++++++++++++
>  2 files changed, 79 insertions(+)
>  create mode 100644 Documentation/driver-api/cxl/linux/memory-hotplug.rst
> 


> diff --git a/Documentation/driver-api/cxl/linux/memory-hotplug.rst b/Documentation/driver-api/cxl/linux/memory-hotplug.rst
> new file mode 100644
> index 000000000000..a757e0e4487e
> --- /dev/null
> +++ b/Documentation/driver-api/cxl/linux/memory-hotplug.rst
> @@ -0,0 +1,78 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +==============
> +Memory Hotplug
> +==============
> +The final phase of surfacing CXL memory to the kernel page allocator is for
> +the `DAX` driver to surface a `Driver Managed` memory region via the
> +memory-hotplug component.
> +
> +There are four major configurations to consider

Add ending ':' ?

> +
> +1) Default Online Behavior (on/off and zone)
> +2) Hotplug Memory Block size
> +3) Memory Map Resource location
> +4) Driver-Managed Memory Designation
> +
> +Default Online Behavior
> +=======================
> +The default-online behavior of hotplug memory is dictated by the following,
> +in order of precedence:
> +
> +- :code:`CONFIG_MHP_DEFAULT_ONLINE_TYPE` Build Configuration
> +- :code:`memhp_default_state` Boot parameters

                                      parameter

> +- :code:`/sys/devices/system/memory/auto_online_blocks` value
> +
> +These dictate whether hotplugged memory blocks arrive in one of three states:
> +
> +1) Offline
> +2) Online in :code:`ZONE_NORMAL`
> +3) Online in :code:`ZONE_MOVABLE`
> +
> +:code:`ZONE_NORMAL` implies this capacity may be used for almost any allocation,
> +while :code:`ZONE_MOVABLE` implies this capacity should only be used for
> +migratable allocations.
> +
> +:code:`ZONE_MOVABLE` attempts to retain the hotplug-ability of a memory block
> +so that it the entire region may be hot-unplugged at a later time.  Any capacity
> +onlined into :code:`ZONE_NORMAL` should be considered permanently attached to
> +the page allocator.
> +
> +Hotplug Memory Block Size
> +=========================
> +By default, on most architectures, the Hotplug Memory Block Size is either
> +128MB or 256MB.  On x86, the block size increases up to 2GB as total memory
> +capacity exceeds 64GB.  As of v6.15, Linux does not take into account the
> +size and alignment of the ACPI CEDT CFMWS regions (see Early Boot docs) when
> +deciding the Hotplug Memory Block Size.
> +
> +Memory Map
> +==========
> +The location of :code:`struct folio` allocations to represent the hotplugged
> +memory capacity are dicated by the following system settings:

                       dictated

> +
> +- :code:`/sys_module/memory_hotplug/parameters/memmap_on_memory`
> +- :code:`/sys/bus/dax/devices/daxN.Y/memmap_on_memory`
> +
> +If both of these parameters are set to true, :code:`struct folio` for this
> +capacity will be carved out of the memory block being onlined.  This has
> +performance implications if the memory is particularly high-latency and
> +its :code:`struct folio` becomes hotly contended.
> +
> +If either parameter is set to false, :code:`struct folio` for this capacity
> +will be allocated from the local node of the processor running the hotplug
> +procedure.  This capacity will be allocated from :code:`ZONE_NORMAL` on
> +that node, as it is a :code:`GFP_KERNEL` allocation.
> +
> +Systems with extremely large amounts of :code:`ZONE_MOVABLE` memory (e.g.
> +CXL memory pools) must ensure that there is sufficient local
> +:code:`ZONE_NORMAL` capacity to host the memory map for the hotplugged capacity.
> +
> +Driver Managed Memory
> +=====================
> +The DAX driver surfaces this memory to memory-hotplug as "Driver Managed". This
> +is not a configurable setting, but it's important to not that driver managed

                                                        note

> +memory is explicitly excluded from use during kexec.  This is required to ensure
> +any reset or out-of-band operations that the CXL device may be subject to during
> +a functional system-reboot (such as a reset-on-probe) will not cause portions of
> +the kexec kernel to be overwritten.

-- 
~Randy