Re: [PATCH v7 2/8] iov_iter: Add a function to extract a page list from an iterator

[Christoph Hellwig]

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