linux-kernel - Re: [PATCH v3 3/7] dma-buf: Document RDMA non-ODP invalidate

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Message-ID: <20260121135948.GB961572@ziepe.ca>
Date: Wed, 21 Jan 2026 09:59:48 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Christian König <christian.koenig@....com>
Cc: Leon Romanovsky <leon@...nel.org>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Alex Deucher <alexander.deucher@....com>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Gerd Hoffmann <kraxel@...hat.com>,
	Dmitry Osipenko <dmitry.osipenko@...labora.com>,
	Gurchetan Singh <gurchetansingh@...omium.org>,
	Chia-I Wu <olvaffe@...il.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Lucas De Marchi <lucas.demarchi@...el.com>,
	Thomas Hellström <thomas.hellstrom@...ux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@...el.com>,
	Kevin Tian <kevin.tian@...el.com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	Felix Kuehling <Felix.Kuehling@....com>,
	Alex Williamson <alex@...zbot.org>,
	Ankit Agrawal <ankita@...dia.com>,
	Vivek Kasireddy <vivek.kasireddy@...el.com>,
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
	amd-gfx@...ts.freedesktop.org, virtualization@...ts.linux.dev,
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	iommu@...ts.linux.dev, kvm@...r.kernel.org
Subject: Re: [PATCH v3 3/7] dma-buf: Document RDMA non-ODP
 invalidate_mapping() special case

On Wed, Jan 21, 2026 at 02:52:53PM +0100, Christian König wrote:
> On 1/21/26 14:18, Jason Gunthorpe wrote:
> > On Wed, Jan 21, 2026 at 10:17:16AM +0100, Christian König wrote:
> >> The whole idea is to make invalidate_mappings truly optional.
> > 
> > But it's not really optional! It's absence means we are ignoring UAF
> > security issues when the exporters do their move_notify() and nothing
> > happens.
> 
> No that is unproblematic.
> 
> See the invalidate_mappings callback just tells the importer that
> the mapping in question can't be relied on any more.
> 
> But the mapping is truly freed only by the importer calling
> dma_buf_unmap_attachment().
> 
> In other words the invalidate_mappings give the signal to the
> importer to disable all operations and the
> dma_buf_unmap_attachment() is the signal from the importer that the
> housekeeping structures can be freed and the underlying address
> space or backing object re-used.

I see

Can we document this please, I haven't seen this scheme described
anyhwere.

And let's clarify what I said in my other email that this new revoke
semantic is not just a signal to maybe someday unmap but a hard
barrier that it must be done once the fences complete, similar to
non-pinned importers.

The cover letter should be clarified with this understanding too.

Jason