linux-kernel - Re: [HMM-CDM 0/5] Cache coherent device memory (CDM) with HMM

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Date:   Thu, 15 Jun 2017 09:40:43 +1000
From:   Balbir Singh <bsingharora@...il.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Jerome Glisse <jglisse@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        John Hubbard <jhubbard@...dia.com>,
        David Nellans <dnellans@...dia.com>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [HMM-CDM 0/5] Cache coherent device memory (CDM) with HMM

On Thu, Jun 15, 2017 at 7:58 AM, Dave Hansen <dave.hansen@...el.com> wrote:
> On 06/14/2017 02:38 PM, Jerome Glisse wrote:
>> On Wed, Jun 14, 2017 at 02:20:23PM -0700, Dave Hansen wrote:
>>> On 06/14/2017 01:11 PM, Jérôme Glisse wrote:
>>>> Cache coherent device memory apply to architecture with system bus
>>>> like CAPI or CCIX. Device connected to such system bus can expose
>>>> their memory to the system and allow cache coherent access to it
>>>> from the CPU.
>>> How does this interact with device memory that's enumerated in the new
>>> ACPI 6.2 HMAT?  That stuff is also in the normal e820 and, by default,
>>> treated as normal system RAM.  Would this mechanism be used for those
>>> devices as well?
>>>
>>> http://www.uefi.org/sites/default/files/resources/ACPI_6_2.pdf
>> It doesn't interact with that. HMM-CDM is a set of helper that don't
>> do anything unless instructed so. So for device memory to be presented
>> as HMM-CDM you need to hotplug it as ZONE_DEVICE(DEVICE_PUBLIC) which
>> can be done with the helper introduced in patch 2 of this patchset.
>

[Removing my cc'd email id and responding from a different address]

> I guess I'm asking whether we *should* instruct HMM-CDM to manage all
> coherent device memory.  If not, where do we draw the line for what we
> use HMM-CDM, and for what we use the core MM?
>

If you believe the memory is managed by the device (and owned by a device
driver) I'd suggest using HMM-CDM. The idea behind HMM-CDM was that
it enables transparent migration of pages and its preferred when
locality of computation
and locality of memory access is the preferred model.

The other model was N_COHERENT_MEMORY that used the core MM, but
there were objections to exposing device memory using that technology.

Balbir Singh.