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: <aH4_QaNtIJMrPqOw@casper.infradead.org>
Date: Mon, 21 Jul 2025 14:23:13 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: 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>,
	Alistair Popple <apopple@...dia.com>,
	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 18, 2025 at 11:44:42AM -0300, Jason Gunthorpe wrote:
> On Fri, Jul 18, 2025 at 03:17:00PM +0100, Matthew Wilcox wrote:
> > On Fri, Jul 18, 2025 at 02:51:08PM +0300, Yonatan Maman wrote:
> > > +++ b/include/linux/memremap.h
> > > @@ -89,6 +89,14 @@ struct dev_pagemap_ops {
> > >  	 */
> > >  	vm_fault_t (*migrate_to_ram)(struct vm_fault *vmf);
> > >  
> > > +	/*
> > > +	 * Used for private (un-addressable) device memory only. Return a
> > > +	 * corresponding PFN for a page that can be mapped to device
> > > +	 * (e.g using dma_map_page)
> > > +	 */
> > > +	int (*get_dma_pfn_for_device)(struct page *private_page,
> > > +				      unsigned long *dma_pfn);
> > 
> > This makes no sense.  If a page is addressable then it has a PFN.
> > If a page is not addressable then it doesn't have a PFN.
> 
> The DEVICE_PRIVATE pages have a PFN, but it is not usable for
> anything.

OK, then I don't understand what DEVICE PRIVATE means.

I thought it was for memory on a PCIe device that isn't even visible
through a BAR and so the CPU has no way of addressing it directly.
But now you say that it has a PFN, which means it has a physical
address, which means it's accessible to the CPU.

So what is it?

> This is effectively converting from a DEVICE_PRIVATE page to an actual
> DMA'able address of some kind. The DEVICE_PRIVATE is just a non-usable
> proxy, like a swap entry, for where the real data is sitting.
> 
> Jason
>