with IOMMU, why don't we have to pin the DMA buffer pages?

Question

I heard from somewhere, without IOMMU, the S/W should pin the DMA buffer until the DMA is complete because the physical page can be assigned to another processor in the mean time. And I heard that if we have IOMMU for the device, we don't need to pin the page (the DMA buffer). But I think even with IOMMU, the pages can still be allocated to another process before DMA is complete so problem can arise. Why don't we have to pin the pages when we have IOMMU? Does the iommu subsystem pin the page?