Re: [PATCH 4/9] iommufd: Allow a DMABUF to be revoked

[Nicolin Chen]

On Fri, Nov 07, 2025 at 12:49:36PM -0400, Jason Gunthorpe wrote:
> When connected to VFIO, the only DMABUF exporter that is accepted, the
> move_notify callback will be made when VFIO wants to remove access to the
> MMIO. This is being called revoke.
> 
> Wire up revoke to go through all the iommu_domain's that have mapped the
> DMABUF and unmap them.
> 
> The locking here is unpleasant, since the existing locking scheme was
> designed to come from the iopt through the area to the pages we cannot use
> pages as starting point for the locking. There is no way to obtain the
> domains_rwsem before obtaining the pages mutex to reliably use the
> existing domains_itree.
> 
> Solve this problem by adding a new tracking structure just for DMABUF
> revoke. Record a linked list of areas and domains inside the pages
> mutex. Clean the entries on the list during revoke. The map/unmaps are now
> all done under a pages mutex while updating the tracking linked list so
> nothing can get out of sync. Only one lock is required for revoke
> processing.
> 
> Signed-off-by: Jason Gunthorpe <j...@nvidia.com>
 
Reviewed-by: Nicolin Chen <nicol...@nvidia.com>