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Date:   Tue, 12 Oct 2021 11:39:57 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Alex Sierra <alex.sierra@....com>
Cc:     <Felix.Kuehling@....com>, <linux-mm@...ck.org>,
        <rcampbell@...dia.com>, <linux-ext4@...r.kernel.org>,
        <linux-xfs@...r.kernel.org>, <amd-gfx@...ts.freedesktop.org>,
        <dri-devel@...ts.freedesktop.org>, <hch@....de>, <jgg@...dia.com>,
        <jglisse@...hat.com>, <apopple@...dia.com>
Subject: Re: [PATCH v1 00/12] MEMORY_DEVICE_COHERENT for CPU-accessible
 coherent device memory

On Tue, 12 Oct 2021 12:12:35 -0500 Alex Sierra <alex.sierra@....com> 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.

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