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: <cn7hcxskr5prkc3jnd4vzzeau5weevzumcspzfayeiwdexkkfe@ovvgraqo7svh>
Date: Wed, 23 Jul 2025 14:10:01 +1000
From: Alistair Popple <apopple@...dia.com>
To: 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 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. The proof
will be writing the patches of course.

 - Alistair

> Jason