linux-kernel - Re: [RFC PATCH 0/6] Remove device private pages from physical address space

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Message-ID: <0414eb13-77ae-4c41-b32f-3ae637e5da48@nvidia.com>
Date: Tue, 2 Dec 2025 12:31:34 +1100
From: Jordan Niethe <jniethe@...dia.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: linux-mm@...ck.org, balbirs@...dia.com, matthew.brost@...el.com,
 akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, david@...hat.com, ziy@...dia.com,
 apopple@...dia.com, lorenzo.stoakes@...cle.com, lyude@...hat.com,
 dakr@...nel.org, airlied@...il.com, simona@...ll.ch, rcampbell@...dia.com,
 mpenttil@...hat.com, jgg@...dia.com
Subject: Re: [RFC PATCH 0/6] Remove device private pages from physical address
 space

Hi,

On 29/11/25 02:09, Matthew Wilcox wrote:
> On Fri, Nov 28, 2025 at 03:41:40PM +1100, Jordan Niethe wrote:
>> A consequence of placing the device private pages outside of the
>> physical address space is that they no longer have a PFN. However, it is
>> still necessary to be able to look up a corresponding device private
>> page from a device private PTE entry, which means that we still require
>> some way to index into this device private address space. This leads to
>> the idea of a device private PFN. This is like a PFN but instead of
> 
> Don't call it a "device private PFN".  That's going to lead to
> confusion.  Device private index?  Device memory index?

Sure, I think 'device memory index' is fine. What I was trying to
express with 'device private PFN' here is that each index into device
memory still represents a PAGE_SIZE region, but I agree it leads to
further confusion.

Thanks,
Jordan.

> 
>> By removing the device private pages from the physical address space,
>> this RFC also opens up the possibility to moving away from tracking
>> device private memory using struct pages in the future. This is
>> desirable as on systems with large amounts of memory these device
>> private struct pages use a signifiant amount of memory and take a
>> significant amount of time to initialize.
> 
> I did tell Jerome he was making a huge mistake with his design, but
> he forced it in anyway.