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: <b8009500-8b0b-4bb9-ae5e-6d2135adbfdd@redhat.com>
Date: Fri, 1 Aug 2025 18:50:18 +0200
From: David Hildenbrand <david@...hat.com>
To: Jason Gunthorpe <jgg@...pe.ca>, Alistair Popple <apopple@...dia.com>
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 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 ...

-- 
Cheers,

David / dhildenb