Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Jan 12, 2017 at 10:37:18PM +0000, Al Viro wrote:
> On Thu, Jan 12, 2017 at 02:26:42PM -0800, Linus Torvalds wrote:
> > On Thu, Jan 12, 2017 at 12:26 PM, Alan J. Wylie <alan@wylie.me.uk> wrote:
> > >
> > > Strace shows that the processes are hanging in write() and read() calls.
> > 
> > If this is splice-related, I'm assuming that they aren't actually the
> > two ends of the same pipe, and there is somebody doing splice in the
> > middle.
> > 
> > I'm not seeing that process. I'm assuming it's systemd.  Can you try
> > to find it and strace that one too? Because that middle man is likely
> > the one that has problems (and is not able to splice from one pipe to
> > the other).
> > 
> > Ugh. That one commit has had a lot of bugs in it already. We do not
> > have good splice test coverage, because almost nobody uses it.
> 
> FWIW, I would really like to know what kind of files had been involved.
> There are two paths that can lead to default_file_splice_read():
> splice_direct_to_actor() -> do_splice_to() -> default_file_splice_read() and
> do_splice() -> do_splice_to() -> default_file_splice_read().
> 
> The former only gets there for regular files and block devices.  The latter
> is guaranteed that file is not a pipe.  So
> 	* not a socket (have ->splice_read() of their own)
> 	* not a pipe or FIFO (neither path allows those)
> 	* not a block device (have ->splice_read() of their own)
> 	* not a regular file on a normal local fs (ditto)
> 
> So what is it called for in that reproducer?

PS: what about the /proc/mounts contents?  If it's something 9p-backed kvm,
your bisect might have been caught on the bug I'd mentioned - if the breakage
you are seeing in 4.9.3 has started after that commit and before the
backport of the fix, your bisect could converge there.  Does the
reproducer trigger on 523ac9afc73a + cherry-pick of 8e54cadab447?