linux-kernel - Re: [HMM-v25 19/19] mm/hmm: add new helper to hotplug CDM memory region v3

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Message-ID: <1726639990.10465990.1504805251676.JavaMail.zimbra@redhat.com>
Date:   Thu, 7 Sep 2017 13:27:31 -0400 (EDT)
From:   Jerome Glisse <jglisse@...hat.com>
To:     Bob Liu <liubo95@...wei.com>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, John Hubbard <jhubbard@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        David Nellans <dnellans@...dia.com>,
        Balbir Singh <bsingharora@...il.com>,
        majiuyue <majiuyue@...wei.com>,
        "xieyisheng (A)" <xieyisheng1@...wei.com>
Subject: Re: [HMM-v25 19/19] mm/hmm: add new helper to hotplug CDM memory
 region v3

> On 2017/9/6 10:12, Jerome Glisse wrote:
> > On Wed, Sep 06, 2017 at 09:25:36AM +0800, Bob Liu wrote:
> >> On 2017/9/6 2:54, Ross Zwisler wrote:
> >>> On Mon, Sep 04, 2017 at 10:38:27PM -0400, Jerome Glisse wrote:
> >>>> On Tue, Sep 05, 2017 at 09:13:24AM +0800, Bob Liu wrote:
> >>>>> On 2017/9/4 23:51, Jerome Glisse wrote:
> >>>>>> On Mon, Sep 04, 2017 at 11:09:14AM +0800, Bob Liu wrote:
> >>>>>>> On 2017/8/17 8:05, Jérôme Glisse wrote:

[...]

> > For HMM each process give hint (somewhat similar to mbind) for range of
> > virtual address to the device kernel driver (through some API like OpenCL
> > or CUDA for GPU for instance). All this being device driver specific ioctl.
> > 
> > The kernel device driver have an overall view of all the process that use
> > the device and each of the memory advise they gave. From that informations
> > the kernel device driver decide what part of each process address space to
> > migrate to device memory.
> 
> Oh, I mean CDM-HMM.  I'm fine with HMM.

They are one and the same really. In both cases HMM is just a set of helpers
for device driver.

> > This obviously dynamic and likely to change over the process lifetime.
> > 
> > My understanding is that HMAT want similar API to allow process to give
> > direction on
> > where each range of virtual address should be allocated. It is expected
> > that most
> 
> Right, but not clear who should manage the physical memory allocation and
> setup the pagetable mapping. An new driver or the kernel?

Physical device memory is manage by the kernel device driver as it is today
and has it will be tomorrow. HMM does not change that, nor does it requires
any change to that.

Migrating process memory to or from device is done by the kernel through
the regular page migration. HMM provides new helper for device driver to
initiate such migration. There is no mechanisms like auto numa migration
for the reasons i explain previously.

Kernel device driver use all knowledge it has to decide what to migrate to
device memory. Nothing new here either, it is what happens today for special
allocated device object and it will just happen all the same for regular
mmap memory (private anonymous or mmap of a regular file of a filesystem).

So every low level thing happen in the kernel. Userspace only provides
directive to the kernel device driver through device specific API. But the
kernel device driver can ignore or override those directive.

> > software can easily infer what part of its address will need more
> > bandwidth, smaller
> > latency versus what part is sparsely accessed ...
> > 
> > For HMAT i think first target is HBM and persistent memory and device
> > memory might
> > be added latter if that make sense.
> > 
> 
> Okay, so there are two potential ways for CPU-addressable cache-coherent
> device memory
> (or cpu-less numa memory or "target domain" memory in ACPI spec )?
> 1. CDM-HMM
> 2. HMAT

No this are 2 orthogonal thing, they do not conflict with each others quite
the contrary. HMM (the CDM part is no different) is a set of helpers, see
it as a toolbox, for device driver.

HMAT is a way for firmware to report memory resources with more informations
that just range of physical address. HMAT is specific to platform that rely
on ACPI. HMAT does not provide any helpers to manage these memory.

So a device driver can get informations about device memory from HMAT and then
use HMM to help in managing and using this memory.

Jérôme