linux-kernel - Re: [PATCH v2 2/4] mm: Add file_operations.get_mapping

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Message-ID: <aUVrfs1w6Sg0jfRw@x1.local>
Date: Fri, 19 Dec 2025 10:13:02 -0500
From: Peter Xu <peterx@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: kvm@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Nico Pache <npache@...hat.com>, Zi Yan <ziy@...dia.com>,
	Alex Mastro <amastro@...com>, David Hildenbrand <david@...hat.com>,
	Alex Williamson <alex@...zbot.org>, Zhi Wang <zhiw@...dia.com>,
	David Laight <david.laight.linux@...il.com>,
	Yi Liu <yi.l.liu@...el.com>, Ankit Agrawal <ankita@...dia.com>,
	Kevin Tian <kevin.tian@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 2/4] mm: Add file_operations.get_mapping_order()

On Fri, Dec 19, 2025 at 10:59:57AM -0400, Jason Gunthorpe wrote:
> On Tue, Dec 16, 2025 at 02:44:29PM -0500, Peter Xu wrote:
> > > > Or maybe I misunderstood what you're suggesting to document?  If so, please
> > > > let me know; some example would be greatly helpful.
> > > 
> > > Just document the 'VA % order = pgoff % order' equation in the kdoc
> > > for the new op.
> > 
> > When it's "related to PTEs", it's talking about (2) above, so that's really
> > what I want to avoid mentioning.
> 
> You can't avoid it. Drivers must ensure that
> 
>   pgoff % order == physical % order
> 
> And that is something only drivers can do by knowing about this
> requirement.

This is a current limitation that above must be guaranteed, there's not
much the driver can do, IMHO.

If you could remember, that's the only reason why I used to suggest (while
we were discussing this in v1) to make it *pgoff instead of pgoff, so that
drivers can change *pgoff to make it relevant to HPA.

I didn't take that approach as I want to make this simple until it's
justified to be required.

It holds true for vfio-pci, I hope it holds true forever.  If not, this API
will stop working, afaict.

-- 
Peter Xu