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

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Message-ID: <a3f1af02-ef3f-40f8-be79-4c3929a59bb7@redhat.com>
Date: Thu, 24 Jul 2025 10:52:54 +0200
From: David Hildenbrand <david@...hat.com>
To: Alistair Popple <apopple@...dia.com>, Jason Gunthorpe <jgg@...pe.ca>
Cc: Matthew Wilcox <willy@...radead.org>, Yonatan Maman <ymaman@...dia.com>,
 Jérôme Glisse <jglisse@...hat.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Leon Romanovsky
 <leon@...nel.org>, Lyude Paul <lyude@...hat.com>,
 Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Ben Skeggs <bskeggs@...dia.com>,
 Michael Guralnik <michaelgur@...dia.com>, Or Har-Toov <ohartoov@...dia.com>,
 Daisuke Matsuda <dskmtsd@...il.com>, Shay Drory <shayd@...dia.com>,
 linux-mm@...ck.org, linux-rdma@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, Gal Shalom <GalShalom@...dia.com>
Subject: Re: [PATCH v2 1/5] mm/hmm: HMM API to enable P2P DMA for device
 private pages

On 23.07.25 06:10, Alistair Popple wrote:
> On Wed, Jul 23, 2025 at 12:51:42AM -0300, Jason Gunthorpe wrote:
>> 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..
> 
> Hmm ... maybe. To get that PFN though we have to come from either a special
> swap entry which we already have special cases for, or a struct page (which is
> a device private page) which we mostly have to handle specially anyway. I'm not
> sure there's too many places that can sensibly handle a fake PFN without somehow
> already knowing it is device-private PFN.
> 
>>> (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.
> 
> I looked at this a while back and I'm reasonably optimistic that this is doable
> because we already have to treat these specially everywhere anyway.
How would that look like?

E.g., we have code like

if (is_device_private_entry(entry)) {
	page = pfn_swap_entry_to_page(entry);
	folio = page_folio(page);

	...
	folio_get(folio);
	...
}

We could easily stop allowing pfn_swap_entry_to_page(), turning these 
into non-pfn swap entries.

Would it then be something like

if (is_device_private_entry(entry)) {
	page = device_private_entry_to_page(entry);
	
	...
}

Whereby device_private_entry_to_page() obtains the "struct page" not via 
the PFN but some other magical (index) value?

-- 
Cheers,

David / dhildenb