[PATCH v2 1/5] mm/hmm: HMM API to enable P2P DMA for device private pages

[David Hildenbrand]

On 01.08.25 18:40, Jason Gunthorpe wrote:
> On Fri, Jul 25, 2025 at 10:31:25AM +1000, Alistair Popple wrote:
> 
>> The only issue would be if there were generic code paths that somehow have a
>> raw pfn obtained from neither a page-table walk or struct page. My assumption
>> (yet to be proven/tested) is that these paths don't exist.
> 
> hmm does it, it encodes the device private into a pfn and expects the
> caller to do pfn to page.
> 
> This isn't set in stone and could be changed..
> 
> But broadly, you'd want to entirely eliminate the ability to go from
> pfn to device private or from device private to pfn.
> 
> Instead you'd want to work on some (space #, space index) tuple, maybe
> encoded in a pfn_t, but absolutely and typesafely distinct. Each
> driver gets its own 0 based space for device private information, the
> space is effectively the pgmap.
> 
> And if you do this, maybe we don't need struct page (I mean the type!)
> backing device memory at all.... Which would be a very worthwhile
> project.
> 
> Do we ever even use anything in the device private struct page? Do we
> refcount it?

ref-counted and map-counted ...

-- 
Cheers,

David / dhildenb