linux-kernel - 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
From:   David Howells <dhowells@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     dhowells@...hat.com, Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>, linux-mm@...ck.org
Subject: Re: [PATCH v7 2/8] iov_iter: Add a function to extract a page list from an iterator

Christoph Hellwig <hch@...radead.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