linux-kernel - Re: [PATCH 18/22] mm: mark DEVICE_PUBLIC as broken

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Message-ID: <d2b77ea1-7b27-e37d-c248-267a57441374@nvidia.com>
Date:   Thu, 13 Jun 2019 18:23:04 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Ira Weiny <ira.weiny@...el.com>, Jason Gunthorpe <jgg@...lanox.com>
CC:     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 6/13/19 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.

So maybe it will get re-added as part of a future patchset to use that
kind of memory, but yes, we should not hesitate to clean house at this
point, and delete unused code.

thanks,
-- 
John Hubbard
NVIDIA

> 
>>
>> So, yes, we really don't want any distro or something to turn this on
>> until it has a use.
>>
>> Reviewed-by: Jason Gunthorpe <jgg@...lanox.com>
>>
>> Jason
>> _______________________________________________
>> Linux-nvdimm mailing list
>> Linux-nvdimm@...ts.01.org
>> https://lists.01.org/mailman/listinfo/linux-nvdimm

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