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: <20250801164058.GD26511@ziepe.ca>
Date: Fri, 1 Aug 2025 13:40:58 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Alistair Popple <apopple@...dia.com>
Cc: David Hildenbrand <david@...hat.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, 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?

Jason