linux-kernel - Re: [PATCH v3 6/7] vfio: Wait for dma-buf invalidation to complete

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Message-ID: <20260123163132.GA1641016@ziepe.ca>
Date: Fri, 23 Jan 2026 12:31:32 -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>,
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	Alex Williamson <alex@...zbot.org>,
	Ankit Agrawal <ankita@...dia.com>,
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Subject: Re: [PATCH v3 6/7] vfio: Wait for dma-buf invalidation to complete

On Fri, Jan 23, 2026 at 05:23:34PM +0100, Christian König wrote:
> > It is illegal to call the DMA API after your driver is unprobed. The
> > kernel can oops. So if a driver is allowing remove() to complete
> > before all the dma_buf_unmaps have been called it is buggy and risks
> > an oops.
> > 
> > https://lore.kernel.org/lkml/8067f204-1380-4d37-8ffd-007fc6f26738@kernel.org/T/#m0c7dda0fb5981240879c5ca489176987d688844c
> > 
> > As calling a dma_buf_unmap() -> dma_unma_sg() after remove() returns
> > is not allowed..
> 
> That is not even in the hands of the driver. The DMA-buf framework
> itself does a module_get() on the exporter.

module_get() prevents the module from being unloaded. It does not
prevent the user from using /sys/../unbind or various other ways to
remove the driver from the device.

rmmod is a popular way to trigger remove() on a driver but not the
only way, and you can't point to a module_get() to dismiss issues with
driver remove() correctness.

> Revoking the DMA mappings won't change anything on that, the
> importer needs to stop using the DMA-buf and drop all their
> references.

And to be correct an exporting driver needs to wait in its remove
function until all the unmaps are done.

Jason