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

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Message-ID: <20250702233244.GG1139770@nvidia.com>
Date: Wed, 2 Jul 2025 20:32:44 -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, Jul 02, 2025 at 04:58:46PM -0400, Peter Xu wrote:
> > So you have to do it the other way and pass the pgoff to the vmap so
> > the vmap ends up with the same colouring as a user VMa holding the
> > same pages..
> 
> Not sure if I get that point, but.. it'll be hard to achieve at least.
> 
> The vmap() happens (submit/complete queues initializes) when io_uring
> instance is created.  The mmap() happens later, and it can also happen
> multiple times, so that all of the VAs got mmap()ed need to share the same
> colouring with the vmap()..  In this case it sounds reasonable to me to
> have the alignment done at mmap(), against the vmap() results.

The way this usually works is the memory is bound to a mmap "cookie"
- the pgoff - which userspace can use as many times as it likes.

Usually you know the thing being allocated will be mmap'd and what
it's pgoff will be because it is 1:1 with the cookie/pgoff.

Didn't try to guess what io_uring has done here, but, IMHO, it would
be weird if the pgoffs are not 1:1 with the vmaps.

Since you said the pgoff was constant and not exchanged user/kernel
then presumably the vmap just needs to use that constant pgoff for its
colouring.

> > > The changes comparing to previous:
> > > 
> > >     (1) merged pgoff and *phys_pgoff parameters into one unsigned long, so
> > >     the hook can adjust the pgoff for the va allocator to be used.  The
> > >     adjustment will not be visible to future mmap() when VMA is created.
> > 
> > It seems functional, but the above is better, IMHO.
> 
> Do you mean we can start with no modification allowed on *pgoff?  I'd
> prefer having *pgoff modifiable from the start, as it'll not only work for
> io_uring / parisc above since the 1st day (so we don't need to introduce it
> on top, modifying existing users..), but it'll also be cleaner to be used
> in the current VFIO's use case.

I think modifiably pgoff is really a weird concept... Especially if it
is only modified for the alignment calculation.

But if it is the only way so be it

Jason