linux-kernel - Re: [PATCH 5/5] vfio-pci: Best-effort huge pfnmaps with !MAP

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Message-ID: <20250619135852.GC1643312@nvidia.com>
Date: Thu, 19 Jun 2025 10:58:52 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Peter Xu <peterx@...hat.com>
Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kvm@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	Zi Yan <ziy@...dia.com>, Alex Mastro <amastro@...com>,
	David Hildenbrand <david@...hat.com>,
	Nico Pache <npache@...hat.com>
Subject: Re: [PATCH 5/5] vfio-pci: Best-effort huge pfnmaps with !MAP_FIXED
 mappings

On Wed, Jun 18, 2025 at 03:15:50PM -0400, Peter Xu wrote:
> > > So I changed my mind, slightly.  I can still have the "order" parameter to
> > > make the API cleaner (even if it'll be a pure overhead.. because all
> > > existing caller will pass in PUD_SIZE as of now), 
> > 
> > That doesn't seem right, the callers should report the real value not
> > artifically cap it.. Like ARM does have page sizes greater than PUD
> > that might be interesting to enable someday for PFN users.
> 
> It needs to pass in PUD_SIZE to match what vfio-pci currently supports in
> its huge_fault().

Hm, OK that does make sense. I would add a small comment though as it
is not so intuitive and may not apply to something using ioremap..

> So this will introduce a new file operation that will only be used so far
> in VFIO, playing similar role until we start to convert many
> get_unmapped_area() to this one.

Yes, if someone wants to do a project here you can markup
memfds/shmem/hugetlbfs/etc/etc to define their internal folio orders
and hopefully ultimately remove some of that alignment logic from the
arch code.

Jason