From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Alan J. Wylie" <alan@wylie.me.uk>,
Thorsten Leemhuis <regressions@leemhuis.info>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn
Date: Thu, 12 Jan 2017 23:14:02 +0000 [thread overview]
Message-ID: <20170112231402.GD1555@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CA+55aFwrsardsZpTn6MopNTPZnRUazyP5oRgU_3JLbLoWDYkpQ@mail.gmail.com>
On Thu, Jan 12, 2017 at 03:02:13PM -0800, Linus Torvalds wrote:
> splice(5, NULL, 1, NULL, 9223372036854775807, 0) = -1 EAGAIN
> (Resource temporarily unavailable)
>
> and note that the commit in question introduces that -EAGAIN error code.
>
> The old code never returned EAGAIN at all (well, it could do so later,
> if NONBLOCK was set, obviously, but that doesn't seem to be the case
> here).
>
> So that commit seems to have introduced a new error case, and I
> suspect systemd-nospawn simply doesn't handle it. It is expecting
> splice_to_pipe() to actually block.
>
> Ergo: I think we need to do a wait_for_space() somewhere, getting rid
> of the EAGAIN.
>
> Looking at the callers of "do_splice_to()", we already have the
> wait_for_space() in do_splice(), but we do *not* have it in the
> do_splice_from() case when both the input and output file descriptors
> are pipes.
>From the look of his strace, the source is /dev/ptmx. Pipe-to-pipe
splice goes into splice_pipe_to_pipe() anyway. do_splice_from() is
pipe-to-non-pipe, and it doesn't go anywhere near default_file_splice_read()...
do_splice_to() is the only thing that can call default_file_splice_read()
and there are only two callers - do_splice() (with its wait_for_space())
and splice_direct_to_actor(), which has internal pipe for destination.
That certainly shouldn't be calling wait_for_space() - there's no other
thread that could possibly read from the destination getting more space
in there.
IOW, it's do_splice() -> do_splice_to() -> default_file_splice_read().
next prev parent reply other threads:[~2017-01-12 23:14 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-12 20:26 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn Alan J. Wylie
2017-01-12 20:31 ` Al Viro
2017-01-12 20:38 ` Alan J. Wylie
2017-01-12 22:26 ` Linus Torvalds
2017-01-12 22:37 ` Al Viro
2017-01-12 22:46 ` Al Viro
2017-01-12 23:02 ` Linus Torvalds
2017-01-12 23:14 ` Al Viro [this message]
2017-01-12 23:14 ` Linus Torvalds
2017-01-12 23:27 ` Al Viro
2017-01-12 22:46 ` Alan J. Wylie
2017-01-12 22:58 ` Al Viro
2017-01-12 23:28 ` Linus Torvalds
2017-01-13 4:00 ` Al Viro
2017-01-13 7:38 ` Alan J. Wylie
2017-01-13 7:23 ` Alan J. Wylie
2017-01-13 9:33 ` Al Viro
2017-01-13 9:54 ` Alan J. Wylie
2017-01-13 10:20 ` Al Viro
2017-01-13 10:32 ` Alan J. Wylie
2017-01-13 11:25 ` Al Viro
2017-01-13 11:18 ` Al Viro
2017-01-13 19:33 ` Linus Torvalds
2017-01-13 20:08 ` Al Viro
2017-01-13 20:11 ` Al Viro
2017-01-13 20:32 ` Linus Torvalds
2017-01-13 20:47 ` Al Viro
2017-01-13 21:55 ` Al Viro
2017-01-13 21:59 ` Al Viro
2017-01-13 22:13 ` Al Viro
2017-01-13 22:50 ` Al Viro
2017-01-14 0:59 ` Linus Torvalds
2017-01-14 1:24 ` Al Viro
2017-01-14 1:43 ` Al Viro
2017-01-14 1:46 ` Linus Torvalds
2017-01-14 1:57 ` Al Viro
2017-01-15 0:53 ` Al Viro
2017-01-14 13:16 ` Alan J. Wylie
2017-01-14 16:29 ` Alan J. Wylie
2017-01-14 17:57 ` Linus Torvalds
2017-01-13 20:08 ` Linus Torvalds
2017-01-13 20:16 ` Al Viro
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