DAX Driver Operation

The Direct Access Device driver was originally designed to provide a memory-like access mechanism to memory-like block-devices. It was extended to support CXL Memory Devices, which provide user-configured memory devices.

The CXL subsystem depends on the DAX subsystem to either:

• Generate a file-like interface to userland via /dev/daxN.Y, or

• Engage the memory-hotplug interface to add CXL memory to page allocator.

The DAX subsystem exposes this ability through the cxl_dax_region driver. A dax_region provides the translation between a CXL memory_region and a DAX Device.

DAX Device

A DAX Device is a file-like interface exposed in /dev/daxN.Y. A memory region exposed via dax device can be accessed via userland software via the mmap() system-call. The result is direct mappings to the CXL capacity in the task’s page tables.

Users wishing to manually handle allocation of CXL memory should use this interface.

kmem conversion

The dax_kmem driver converts a DAX Device into a series of hotplug memory blocks managed by kernel/memory-hotplug.c. This capacity will be exposed to the kernel page allocator in the user-selected memory zone.

The memmap_on_memory setting (both global and DAX device local) dictates where the kernell will allocate the struct folio descriptors for this memory will come from. If memmap_on_memory is set, memory hotplug will set aside a portion of the memory block capacity to allocate folios. If unset, the memory is allocated via a normal GFP_KERNEL allocation - and as a result will most likely land on the local NUM node of the CPU executing the hotplug operation.