Re: [PATCH v5 3/9] vfio/pci: Add a helper to look up PFNs for DMABUFs

[Jason Gunthorpe <jgg@nvidia.com>]

On Mon, Aug 03, 2026 at 07:22:06PM +0100, Matt Evans wrote:
> The second problem is this isn't just a limitation to not do an mmap()
> with offset approaching 1TB, because you could run into trouble just
> mapping, say, a 2TB BAR and then splitting it by unmapping a hole in the
> middle:  the second VMA now has a huge offset.
> 
> It seems a proper fix is easy though:
> 
>  unsigned long vma_off = (vma->vm_pgoff - priv->vma_pgoff_adjust) <<
> 	 		  PAGE_SHIFT;  /* No masking! */
> 
> Then in vfio_pci_core_mmap_prep_dmabuf(),
> 
>  priv->vma_pgoff_adjust = vma->vm_pgoff;
> 
> I.e., if vma_pgoff_adjust just includes the region index up high, it
> cancels out the (same) index in the vm_pgoff.  Thus regular mmap should
> work (up to the 1TB offset), and DMABUF VMAs can have arbitrarily large
> offsets.

This is what I was expecting when I pointed at this stuff in the first
place :)

Jason