linux-kernel - Re: [GIT PULL] VFIO updates for v6.17-rc1

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Message-ID: <20250805114908.GE184255@nvidia.com>
Date: Tue, 5 Aug 2025 08:49:08 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: Alex Williamson <alex.williamson@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lizhe.67@...edance.com" <lizhe.67@...edance.com>
Subject: Re: [GIT PULL] VFIO updates for v6.17-rc1

On Tue, Aug 05, 2025 at 09:47:18AM +0200, David Hildenbrand wrote:
> > There was discussion here[1] where David Hildenbrand and Jason
> > Gunthorpe suggested this should be in common code and I believe there
> > was some intent that this would get reused.  I took this as
> > endorsement from mm folks.  This can certainly be pulled back into
> > subsystem code.
> 
> Yeah, we ended up here after trying to go the folio-way first, but then
> realizing that code that called GUP shouldn't have to worry about
> folios, just to detect consecutive pages+PFNs.
> 
> I think this helper will can come in handy even in folio context.
> I recall pointing Joanne at it in different fuse context.

The scatterlist code should use it also, it is doing the same logic.

> The concern is rather false positives, meaning, you want consecutive
> PFNs (just like within a folio), but -- because the stars aligned --
> you get consecutive "struct page" that do not translate to consecutive PFNs.

I wonder if we can address that from the other side and prevent the
memory code from creating a bogus contiguous struct page in the first
place so that struct page contiguity directly reflects physical
contiguity?

Jason