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

From Al Viro <viro@ZenIV.linux.org.uk>
Newsgroups linux.kernel
Subject Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn
Date 2017-01-12 23:40 +0100
Message-ID <sYWch-196-1@gated-at.bofh.it> (permalink)
References <sYUk9-8sU-5@gated-at.bofh.it> <sYW2B-15S-5@gated-at.bofh.it>
Organization linux.* mail to news gateway

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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?

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4.9.0 regression in pipe-backed iov_iter with systemd-nspawn "Alan J. Wylie" <alan@wylie.me.uk> - 2017-01-12 21:40 +0100
  Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Linus Torvalds <torvalds@linux-foundation.org> - 2017-01-12 23:30 +0100
    Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-12 23:40 +0100
      Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-12 23:50 +0100
        Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Linus Torvalds <torvalds@linux-foundation.org> - 2017-01-13 00:10 +0100
          Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Linus Torvalds <torvalds@linux-foundation.org> - 2017-01-13 00:20 +0100
            Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-13 00:30 +0100
          Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-13 00:20 +0100
    Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn "Alan J. Wylie" <alan@wylie.me.uk> - 2017-01-12 23:50 +0100
      Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-13 00:00 +0100
      Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Linus Torvalds <torvalds@linux-foundation.org> - 2017-01-13 00:30 +0100
        Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-13 05:10 +0100
          Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn "Alan J. Wylie" <alan@wylie.me.uk> - 2017-01-13 08:40 +0100
        Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn "Alan J. Wylie" <alan@wylie.me.uk> - 2017-01-13 08:40 +0100
          Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-13 10:40 +0100
            Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn "Alan J. Wylie" <alan@wylie.me.uk> - 2017-01-13 11:00 +0100
              Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-13 11:30 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn "Alan J. Wylie" <alan@wylie.me.uk> - 2017-01-13 11:40 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-13 12:30 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-13 12:20 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Linus Torvalds <torvalds@linux-foundation.org> - 2017-01-13 20:40 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-13 21:10 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-13 21:20 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Linus Torvalds <torvalds@linux-foundation.org> - 2017-01-13 21:40 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-13 21:50 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-13 23:00 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-13 23:00 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-13 23:20 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-14 00:00 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Linus Torvalds <torvalds@linux-foundation.org> - 2017-01-14 02:10 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-14 02:30 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-14 02:50 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Linus Torvalds <torvalds@linux-foundation.org> - 2017-01-14 02:50 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-14 03:00 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-15 02:00 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn "Alan J. Wylie" <alan@wylie.me.uk> - 2017-01-14 14:30 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn "Alan J. Wylie" <alan@wylie.me.uk> - 2017-01-14 17:40 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Linus Torvalds <torvalds@linux-foundation.org> - 2017-01-14 19:10 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Linus Torvalds <torvalds@linux-foundation.org> - 2017-01-13 21:10 +0100
                Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Al Viro <viro@ZenIV.linux.org.uk> - 2017-01-13 21:20 +0100

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