Re: [LSF/MM TOPIC] Memory hotplug, ZONE_DEVICE, and the future of struct page

[Dan Williams <dan.j.williams@intel.com>]

On Thu, Jan 12, 2017 at 3:14 PM, Jerome Glisse <jglisse@redhat.com> wrote:
> On Thu, Jan 12, 2017 at 02:43:03PM -0800, Dan Williams wrote:
>> Back when we were first attempting to support DMA for DAX mappings of
>> persistent memory the plan was to forgo 'struct page' completely and
>> develop a pfn-to-scatterlist capability for the dma-mapping-api. That
>> effort died in this thread:
>>
>>     https://lkml.org/lkml/2015/8/14/3
>>
>> ...where we learned that the dependencies on struct page for dma
>> mapping are deeper than a PFN_PHYS() conversion for some
>> architectures. That was the moment we pivoted to ZONE_DEVICE and
>> arranged for a 'struct page' to be available for any persistent memory
>> range that needs to be the target of DMA. ZONE_DEVICE enables any
>> device-driver that can target "System RAM" to also be able to target
>> persistent memory through a DAX mapping.
>>
>> Since that time the "page-less" DAX path has continued to mature [1]
>> without growing new dependencies on struct page, but at the same time
>> continuing to rely on ZONE_DEVICE to satisfy get_user_pages().
>>
>> Peer-to-peer DMA appears to be evolving from a niche embedded use case
>> to something general purpose platforms will need to comprehend. The
>> "map_peer_resource" [2] approach looks to be headed to the same
>> destination as the pfn-to-scatterlist effort. It's difficult to avoid
>> 'struct page' for describing DMA operations without custom driver
>> code.
>>
>> With that background, a statement and a question to discuss at LSF/MM:
>>
>> General purpose DMA, i.e. any DMA setup through the dma-mapping-api,
>> requires pfn_to_page() support across the entire physical address
>> range mapped.
>
> Note that in my case it is even worse. The pfn of the page does not
> correspond to anything so it need to go through a special function
> to find if a page can be mapped for another device and to provide a
> valid pfn at which the page can be access by other device.

I still haven't quite wrapped my head about how these pfn ranges are
created. Would this be a use case for a new pfn_t flag? It doesn't
sound like something we'd want to risk describing with raw 'unsigned
long' pfns.

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