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Date:   Sun, 28 Aug 2022 01:38:56 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Jens Axboe <axboe@kernel.dk>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Christoph Hellwig <hch@infradead.org>,
        "Darrick J . Wong" <djwong@kernel.org>,
        Trond Myklebust <trond.myklebust@hammerspace.com>,
        Anna Schumaker <anna@kernel.org>, Jan Kara <jack@suse.cz>,
        Logan Gunthorpe <logang@deltatee.com>,
        linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-xfs@vger.kernel.org, linux-nfs@vger.kernel.org,
        linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 5/6] NFS: direct-io: convert to FOLL_PIN pages
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On Sat, Aug 27, 2022 at 04:55:18PM -0700, John Hubbard wrote:
> On 8/27/22 15:48, Al Viro wrote:
> > On Sat, Aug 27, 2022 at 01:36:06AM -0700, John Hubbard wrote:
> >> Convert the NFS Direct IO layer to use pin_user_pages_fast() and
> >> unpin_user_page(), instead of get_user_pages_fast() and put_page().
> > 
> > Again, this stuff can be hit with ITER_BVEC iterators
> > 
> >> -		result = iov_iter_get_pages_alloc2(iter, &pagevec,
> >> +		result = dio_w_iov_iter_pin_pages_alloc(iter, &pagevec,
> >>  						  rsize, &pgbase);
> > 
> > and this will break on those.
> 
> If anyone has an example handy, of a user space program that leads
> to this situation (O_DIRECT with ITER_BVEC), it would really help
> me reach enlightenment a lot quicker in this area. :)

Er...  splice(2) to O_DIRECT-opened file on e.g. ext4?  Or
sendfile(2) to the same, for that matter...