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

[David Howells]

Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:

> 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.

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