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

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Message-ID: <20260123141140.GC1589888@ziepe.ca>
Date: Fri, 23 Jan 2026 10:11:40 -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,
	intel-xe@...ts.freedesktop.org, linux-rdma@...r.kernel.org,
	iommu@...ts.linux.dev, kvm@...r.kernel.org
Subject: Re: [PATCH v3 6/7] vfio: Wait for dma-buf invalidation to complete

On Thu, Jan 22, 2026 at 07:44:04PM -0400, Jason Gunthorpe wrote:
> On Thu, Jan 22, 2026 at 12:32:03PM +0100, Christian König wrote:
> > >> What roughly happens is that each DMA-buf mapping through a couple
> > >> of hoops keeps a reference on the device, so even after a hotplug
> > >> event the device can only fully go away after all housekeeping
> > >> structures are destroyed and buffers freed.
> > > 
> > > A simple reference on the device means nothing for these kinds of
> > > questions. It does not stop unloading and reloading a driver.
> > 
> > Well as far as I know it stops the PCIe address space from being re-used.
> > 
> > So when you do an "echo 1 > remove" and then an re-scan on the
> > upstream bridge that works, but you get different addresses for your
> > MMIO BARs!
> 
> That's pretty a niche scenario.. Most people don't rescan their PCI
> bus. If you just do rmmod/insmod then it will be re-used, there is no
> rescan to move the MMIO around on that case.

Ah I just remembered there is another important detail here.

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..

Jason