linux-kernel - Re: [PATCH v4 2/3] gup: introduce unpin_user_folio_dirty

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Message-ID: <20250618115622.GM1376515@ziepe.ca>
Date: Wed, 18 Jun 2025 08:56:22 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: David Hildenbrand <david@...hat.com>
Cc: lizhe.67@...edance.com, akpm@...ux-foundation.org,
	alex.williamson@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, peterx@...hat.com
Subject: Re: [PATCH v4 2/3] gup: introduce unpin_user_folio_dirty_locked()

On Wed, Jun 18, 2025 at 01:52:37PM +0200, David Hildenbrand wrote:

> I thought we also wanted to optimize out the
> is_invalid_reserved_pfn() check for each subpage of a folio.

VFIO keeps a tracking structure for the ranges, you can record there
if a reserved PFN was ever placed into this range and skip the check
entirely.

It would be very rare for reserved PFNs and non reserved will to be
mixed within the same range, userspace could cause this but nothing
should.

Jason