Re: [PATCH 7/9] vfio/pci: Support mmap() of a VFIO DMABUF

[Matt Evans]

Hi Jason,

On 24/04/2026 19:30, Jason Gunthorpe wrote:
> On Thu, Apr 16, 2026 at 06:17:50AM -0700, Matt Evans wrote:
> > +
> > +	dma_resv_lock(priv->dmabuf->resv, NULL);
> >   	vdev = READ_ONCE(priv->vdev);
> >
> > +	if (READ_ONCE(priv->revoked) || !vdev) {
>
> Why is this read once? It is inside the resv lock so it is stable?
>
> > +		pr_debug_ratelimited("%s VA 0x%lx, pgoff 0x%lx: DMABUF revoked/cleaned up\n",
> > +				     __func__, vmf->address, vma->vm_pgoff);
> > +		dma_resv_unlock(priv->dmabuf->resv);
> > +		return VM_FAULT_SIGBUS;
> > +	}
> > +	/* vdev is usable */
> > +
> > +	if (!vfio_device_try_get_registration(&vdev->vdev)) {
> > +		/*
> > +		 * If vdev != NULL (above), the registration should
> > +		 * already be >0 and so this try_get should never
> > +		 * fail.
> > +		 */
> > +		dev_warn(&vdev->pdev->dev, "%s: Unexpected registration failure\n",
> > +			 __func__);
> > +		dma_resv_unlock(priv->dmabuf->resv);
> > +		return VM_FAULT_SIGBUS;
> > +	}
> > +	dma_resv_unlock(priv->dmabuf->resv);
> > +
> >   	scoped_guard(rwsem_read, &vdev->memory_lock) {
> > -		if (!priv->revoked) {
> > +		if (!READ_ONCE(priv->revoked)) {
>
> Same here, it is not read once since you hold the memory_lock it is
> stable.

I used them more as an 'eyecatcher' to complement the comment.  Although 
they're not strictly required (compiler barriers at the lock/unlocks), 
they stand out to say "something's going on here".

Revoked/status is read first holding resv, and _must be read a second 
time_ once that's released and after memory_lock is taken, so two 
READ_ONCEs seemed appropriate to show this.

But if you feel strongly, sure, I can remove it (though I'd add, say, a 
/* Re-read status value */ comment).


Matt