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

[Jason Gunthorpe]

On Tue, Jul 22, 2025 at 10:49:10AM +1000, Alistair Popple wrote:
> > So what is it?
> 
> IMHO a hack, because obviously we shouldn't require real physical addresses for
> something the CPU can't actually address anyway and this causes real
> problems

IMHO what DEVICE PRIVATE really boils down to is a way to have swap
entries that point to some kind of opaque driver managed memory.

We have alot of assumptions all over about pfn/phys to page
relationships so anything that has a struct page also has to come with
a fake PFN today..

> (eg. it doesn't actually work on anything other than x86_64). There's no reason
> the "PFN" we store in device-private entries couldn't instead just be an index
> into some data structure holding pointers to the struct pages. So instead of
> using pfn_to_page()/page_to_pfn() we would use device_private_index_to_page()
> and page_to_device_private_index().

It could work, but any of the pfn conversions would have to be tracked
down.. Could be troublesome.

Jason