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

[Al Viro]

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@xxxxxxxxxxx> 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?