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: <20250801165749.GF26511@ziepe.ca>
Date: Fri, 1 Aug 2025 13:57:49 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: David Hildenbrand <david@...hat.com>
Cc: Alistair Popple <apopple@...dia.com>,
	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 Fri, Aug 01, 2025 at 06:50:18PM +0200, David Hildenbrand wrote:
> 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 ...

Hm, so it would turn into another struct page split up where we get
ourselves a struct device_private and change all the places touching
its refcount and mapcount to use the new type.

If we could use some index scheme we could then divorce from struct
page and strink the struct size sooner.

Jason