Re: [PATCH v4 0/3] vfio: handle DMA map/unmap up to the addressable limit

[Alex Mastro <amastro@fb.com>]

On Mon, Oct 27, 2025 at 07:57:32PM -0600, Alex Williamson wrote:
> On Mon, 27 Oct 2025 09:02:55 -0700
> Alex Mastro <amastro@fb.com> wrote:
> 
> > On Tue, Oct 21, 2025 at 09:25:55AM -0700, Alex Mastro wrote:
> > > On Mon, Oct 20, 2025 at 03:36:33PM -0600, Alex Williamson wrote:     
> > > > Along with the tag, it would probably be useful in that same commit to
> > > > expand on the scope of the issue in the commit log.  I believe we allow
> > > > mappings to be created at the top of the address space that cannot be
> > > > removed via ioctl, but such inconsistency should result in an
> > > > application error due to the failed ioctl and does not affect cleanup
> > > > on release.  
> > 
> > I want to make sure I understand the cleanup on release path. Is my supposition
> > below correct?
> > 
> > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > index 916cad80941c..7f8d23b06680 100644
> > --- a/drivers/vfio/vfio_iommu_type1.c
> > +++ b/drivers/vfio/vfio_iommu_type1.c
> > @@ -1127,6 +1127,7 @@ static size_t unmap_unpin_slow(struct vfio_domain *domain,
> >  static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
> >  			     bool do_accounting)
> >  {
> > +	// end == 0 due to overflow
> >  	dma_addr_t iova = dma->iova, end = dma->iova + dma->size;
> >  	struct vfio_domain *domain, *d;
> >  	LIST_HEAD(unmapped_region_list);
> > @@ -1156,6 +1157,7 @@ static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
> >  	}
> >  
> >  	iommu_iotlb_gather_init(&iotlb_gather);
> > +	// doesn't enter the loop, never calls iommu_unmap
> 
> If it were only that, I think the iommu_domain_free() would be
> sufficient, but it looks like we're also missing the unpin.  Freeing

Oh, right.

> the IOMMU domain isn't going to resolve that.  So it actually appears
> we're leaking those pinned pages and this isn't as self-resolving as I
> had thought.  I imagine if you ran your new unit test to the point where
> we'd pinned and failed to unpin the majority of memory you'd start to
> see system-wide problems.  Thanks,

Makes sense.

> Alex
> 
> >  	while (iova < end) {
> >  		size_t unmapped, len;
> >  		phys_addr_t phys, next;
> > @@ -1210,6 +1212,7 @@ static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
> >  static void vfio_remove_dma(struct vfio_iommu *iommu, struct vfio_dma *dma)
> >  {
> >  	WARN_ON(!RB_EMPTY_ROOT(&dma->pfn_list));
> > +	// go here
> >  	vfio_unmap_unpin(iommu, dma, true);
> >  	vfio_unlink_dma(iommu, dma);
> >  	put_task_struct(dma->task);
> > @@ -2394,6 +2397,8 @@ static void vfio_iommu_unmap_unpin_all(struct vfio_iommu *iommu)
> >  	struct rb_node *node;
> >  
> >  	while ((node = rb_first(&iommu->dma_list)))
> > +		// eventually, we attempt to remove the end of address space
> > +		// mapping
> >  		vfio_remove_dma(iommu, rb_entry(node, struct vfio_dma, node));
> >  }
> >  
> > @@ -2628,6 +2633,8 @@ static void vfio_release_domain(struct vfio_domain *domain)
> >  		kfree(group);
> >  	}
> >  
> > +	// Is this backstop what saves us? Even though we didn't do individual
> > +	// unmaps, the "leaked" end of address space mappings get freed here?
> >  	iommu_domain_free(domain->domain);
> >  }
> >  
> > @@ -2643,10 +2650,12 @@ static void vfio_iommu_type1_release(void *iommu_data)
> >  		kfree(group);
> >  	}
> >  
> > +	// start here
> >  	vfio_iommu_unmap_unpin_all(iommu);
> >  
> >  	list_for_each_entry_safe(domain, domain_tmp,
> >  				 &iommu->domain_list, next) {
> > +		// eventually...
> >  		vfio_release_domain(domain);
> >  		list_del(&domain->next);
> >  		kfree(domain);
>