Christoph Hellwig [off-list ref] wrote:

On Fri, Jan 20, 2023 at 05:55:50PM +0000, David Howells wrote:

quoted

 (3) Any other sort of iterator.

     No refs or pins are obtained on the page, the assumption is made that
     the caller will manage page retention.  ITER_ALLOW_P2PDMA is not
     permitted.

The ITER_ALLOW_P2PDMA check here is fundamentally wrong and will
break things like io_uring to fixed buffers on NVMe.  ITER_ALLOW_P2PDMA
should strictly increase the group of acceptable iters, it does must
not restrict anything.

So just drop the check?  Or do I actually need to do something to the pages to
make it work?  If I understand the code correctly, it's not actually operable
right now on any page that doesn't have an appropriately marked PTE.

David