linux-kernel - Re: [PATCH v2 0/4] dma-buf: document revoke mechanism to invalidate shared buffers

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Message-ID: <20260119162027.GD961572@ziepe.ca>
Date: Mon, 19 Jan 2026 12:20:27 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
Cc: Leon Romanovsky <leon@...nel.org>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Christian König <christian.koenig@....com>,
	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>,
	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>,
	Alex Williamson <alex@...zbot.org>, 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, intel-xe@...ts.freedesktop.org,
	linux-rdma@...r.kernel.org, iommu@...ts.linux.dev,
	kvm@...r.kernel.org
Subject: Re: [PATCH v2 0/4] dma-buf: document revoke mechanism to invalidate
 shared buffers

On Sun, Jan 18, 2026 at 03:16:25PM +0100, Thomas Hellström wrote:
> > core
> > “revoked” state on the dma-buf object and a corresponding exporter-
> > triggered
> > revoke operation. Once a dma-buf is revoked, new access paths are
> > blocked so
> > that attempts to DMA-map, vmap, or mmap the buffer fail in a
> > consistent way.
> 
> This sounds like it does not match how many GPU-drivers use the
> move_notify() callback.
> 
> move_notify() would typically invalidate any device maps and any
> asynchronous part of that invalidation would be complete when the dma-
> buf's reservation object becomes idle WRT DMA_RESV_USAGE_BOOKKEEP
> fences.
> 
> However, the importer could, after obtaining the resv lock, obtain a
> new map using dma_buf_map_attachment(), and I'd assume the CPU maps
> work in the same way, I.E. move_notify() does not *permanently* revoke
> importer access.

I think this was explained a bit in this thread, but I wanted to
repeat the explanation to be really clear..

If the attachment is not pinned than calling move_notify() is as you
say. The importer should expect multiple move_notify() calls and
handle all of them. The exporter can move the location around and make
it revoked/unrevoked at will. If it is revoked then
dma_buf_map_attachment() fails, the importer could cache this and fail
DMAs until the next move_notify().

If the attachment is *pinned* then we propose to allow the importer to
revoke only and not require restoration. IOW a later move_notify()
that signals a previously failing dma_buf_map_attachment() is no
longer failing can be igmored by a pinned importer.

This at least matches what iommufd is able to do right now.

IOW, calling move_notify() on a pinned DMABUF is a special operationg
we are calling "revoke" and means that the exporter accepts that the
mapping is potentially gone from pinned importers forever. ie don't
use it lightly.

Jason