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From:   David Howells <dhowells@redhat.com>
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References: <Y8vkOk68ZFWPr9vq@infradead.org> <20230120175556.3556978-1-dhowells@redhat.com> <20230120175556.3556978-3-dhowells@redhat.com>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     dhowells@redhat.com, Al Viro <viro@zeniv.linux.org.uk>,
        Matthew Wilcox <willy@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
        Jeff Layton <jlayton@kernel.org>,
        Logan Gunthorpe <logang@deltatee.com>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
        John Hubbard <jhubbard@nvidia.com>, linux-mm@kvack.org
Subject: Re: [PATCH v7 2/8] iov_iter: Add a function to extract a page list from an iterator
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Date:   Sat, 21 Jan 2023 13:30:47 +0000
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Christoph Hellwig <hch@infradead.org> 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