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

Jason Gunthorpe
2026-01-23T14:11:43Z

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