On Tue, 12 Oct 2021 12:12:35 -0500 Alex Sierra [off-list ref] wrote:

This patch series introduces MEMORY_DEVICE_COHERENT, a type of memory
owned by a device that can be mapped into CPU page tables like
MEMORY_DEVICE_GENERIC and can also be migrated like MEMORY_DEVICE_PRIVATE.
With MEMORY_DEVICE_COHERENT, we isolate the new memory type from other
subsystems as far as possible, though there are some small changes to
other subsystems such as filesystem DAX, to handle the new memory type
appropriately.

We use ZONE_DEVICE for this instead of NUMA so that the amdgpu
allocator can manage it without conflicting with core mm for non-unified
memory use cases.

How it works: The system BIOS advertises the GPU device memory (aka VRAM)
as SPM (special purpose memory) in the UEFI system address map.
The amdgpu driver registers the memory with devmap as
MEMORY_DEVICE_COHERENT using devm_memremap_pages.

The initial user for this hardware page migration capability will be
the Frontier supercomputer project.

To what other uses will this infrastructure be put?

Because I must ask: if this feature is for one single computer which
presumably has a custom kernel, why add it to mainline Linux?

Our nodes in the lab have .5 TB of
system memory plus 256 GB of device memory split across 4 GPUs, all in
the same coherent address space. Page migration is expected to improve
application efficiency significantly. We will report empirical results
as they become available.

This includes patches originally by Ralph Campbell to change ZONE_DEVICE
reference counting as requested in previous reviews of this patch series
(see https://patchwork.freedesktop.org/series/90706/). We extended
hmm_test to cover migration of MEMORY_DEVICE_COHERENT. This patch set
builds on HMM and our SVM memory manager already merged in 5.14.
We would like to complete review and merge this migration patchset for
5.16.