Re: [PATCH 5/6] NFS: direct-io: convert to FOLL_PIN pages

[Al Viro]

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...