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

Jason Gunthorpe
2026-01-23T16:31:34Z

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