linux-kernel - Re: [PATCH 18/22] mm: mark DEVICE

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Date:   Tue, 25 Jun 2019 20:15:28 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Jason Gunthorpe <jgg@...lanox.com>
CC:     Ira Weiny <ira.weiny@...el.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 18/22] mm: mark DEVICE_PUBLIC as broken

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".

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.

thanks,
-- 
John Hubbard
NVIDIA