linux-kernel - Re: [PATCH v5 3/4] mm/gup: allow FOLL_LONGTERM & FOLL_PCI

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Message-ID: <ZrmuGrDaJTZFrKrc@infradead.org>
Date: Sun, 11 Aug 2024 23:39:22 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Martin Oliveira <martin.oliveira@...eticom.com>
Cc: linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Artemy Kovalyov <artemyko@...dia.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>,
	Logan Gunthorpe <logang@...tatee.com>,
	Michael Guralnik <michaelgur@...dia.com>,
	Mike Marciniszyn <mike.marciniszyn@...el.com>,
	Shiraz Saleem <shiraz.saleem@...el.com>, Tejun Heo <tj@...nel.org>,
	John Hubbard <jhubbard@...dia.com>,
	Dan Williams <dan.j.williams@...el.com>,
	David Sloan <david.sloan@...eticom.com>
Subject: Re: [PATCH v5 3/4] mm/gup: allow FOLL_LONGTERM & FOLL_PCI_P2PDMA

On Thu, Aug 08, 2024 at 12:33:39PM -0600, Martin Oliveira wrote:
> This check existed originally due to concerns that P2PDMA needed to copy
> fsdax until pgmap refcounts were fixed (see [1]).
> 
> The P2PDMA infrastructure will only call unmap_mapping_range() when the
> underlying device is unbound, and immediately after unmapping it waits
> for the reference of all ZONE_DEVICE pages to be released before
> continuing. This does not allow for a page to be reused and no user
> access fault is therefore possible. It does not have the same problem as
> fsdax.
> 
> The one minor concern with FOLL_LONGTERM pins is they will block device
> unbind until userspace releases them all.

This is unfortunately not really minor unless we have a well documented
way to force this :(