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

[John Hubbard]

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@xxxxxxxxxxxx>
>>
>> Jason
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