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Subject: Re: [PATCH 18/22] mm: mark DEVICE_PUBLIC as broken
To:     Michal Hocko <mhocko@kernel.org>
CC:     Jason Gunthorpe <jgg@mellanox.com>,
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From:   John Hubbard <jhubbard@nvidia.com>
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On 6/25/19 10:45 PM, Michal Hocko wrote:
> On Tue 25-06-19 20:15:28, John Hubbard wrote:
>> On 6/19/19 12:27 PM, Jason Gunthorpe wrote:
>>> On Thu, Jun 13, 2019 at 06:23:04PM -0700, John Hubbard wrote:
>>>> On 6/13/19 5:43 PM, Ira Weiny wrote:
>>>>> On Thu, Jun 13, 2019 at 07:58:29PM +0000, Jason Gunthorpe wrote:
>>>>>> On Thu, Jun 13, 2019 at 12:53:02PM -0700, Ralph Campbell wrote:
>>>>>>>
>>>> ...
>>>>> So I think it is ok.  Frankly I was wondering if we should remove the public
>>>>> type altogether but conceptually it seems ok.  But I don't see any users of it
>>>>> so...  should we get rid of it in the code rather than turning the config off?
>>>>>
>>>>> Ira
>>>>
>>>> That seems reasonable. I recall that the hope was for those IBM Power 9
>>>> systems to use _PUBLIC, as they have hardware-based coherent device (GPU)
>>>> memory, and so the memory really is visible to the CPU. And the IBM team
>>>> was thinking of taking advantage of it. But I haven't seen anything on
>>>> that front for a while.
>>>
>>> Does anyone know who those people are and can we encourage them to
>>> send some patches? :)
>>>
>>
>> I asked about this, and it seems that the idea was: DEVICE_PUBLIC was there
>> in order to provide an alternative way to do things (such as migrate memory
>> to and from a device), in case the combination of existing and near-future
>> NUMA APIs was insufficient. This probably came as a follow-up to the early
>> 2017-ish conversations about NUMA, in which the linux-mm recommendation was
>> "try using HMM mechanisms, and if those are inadequate, then maybe we can
>> look at enhancing NUMA so that it has better handling of advanced (GPU-like)
>> devices".
> 
> Yes that was the original idea. It sounds so much better to use a common
> framework rather than awkward special cased cpuless NUMA nodes with
> a weird semantic. User of the neither of the two has shown up so I guess
> that the envisioned HW just didn't materialized. Or has there been a
> completely different approach chosen?

The HW showed up, alright: it's the IBM Power 9, which provides HW-based
memory coherency between its CPUs and GPUs. So on this system, the CPU is
allowed to access GPU memory, which *could* be modeled as DEVICE_PUBLIC.

However, what happened was that the system worked well enough with a combination
of the device driver, plus NUMA APIs, plus heaven knows what sort of /proc tuning
might have also gone on. :) No one saw the need to reach for the DEVICE_PUBLIC
functionality.

> 
>> In the end, however, _PUBLIC was never used, nor does anyone in the local
>> (NVIDIA + IBM) kernel vicinity seem to have plans to use it.  So it really
>> does seem safe to remove, although of course it's good to start with 
>> BROKEN and see if anyone pops up and complains.
> 
> Well, I do not really see much of a difference. Preserving an unused
> code which doesn't have any user in sight just adds a maintenance burden
> whether the code depends on BROKEN or not. We can always revert patches
> which remove the code once a real user shows up.

Sure, I don't see much difference either. Either way seems fine.

thanks,
-- 
John Hubbard
NVIDIA