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

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Message-ID: <20190619192719.GO9374@mellanox.com>
Date:   Wed, 19 Jun 2019 19:27:25 +0000
From:   Jason Gunthorpe <jgg@...lanox.com>
To:     John Hubbard <jhubbard@...dia.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 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:
> >>>
> ...
> >> Hum, so the only thing this config does is short circuit here:
> >>
> >> static inline bool is_device_public_page(const struct page *page)
> >> {
> >>         return IS_ENABLED(CONFIG_DEV_PAGEMAP_OPS) &&
> >>                 IS_ENABLED(CONFIG_DEVICE_PUBLIC) &&
> >>                 is_zone_device_page(page) &&
> >>                 page->pgmap->type == MEMORY_DEVICE_PUBLIC;
> >> }
> >>
> >> Which is called all over the place.. 
> > 
> > <sigh>  yes but the earlier patch:
> > 
> > [PATCH 03/22] mm: remove hmm_devmem_add_resource
> > 
> > Removes the only place type is set to MEMORY_DEVICE_PUBLIC.
> > 
> > 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? :)

Jason