linux-kernel - Re: iov_iter

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Message-ID: <20170410234830.tmqdhpjtfdveor3c@codemonkey.org.uk>
Date:   Mon, 10 Apr 2017 19:48:30 -0400
From:   Dave Jones <davej@...emonkey.org.uk>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: iov_iter_pipe warning.

On Mon, Apr 10, 2017 at 08:57:11PM +0100, Al Viro wrote:
 > On Mon, Apr 10, 2017 at 03:42:06PM -0400, Dave Jones wrote:
 > >  > Do you have a reliable reproducer?
 > > 
 > > Not reliable, but I'll see if I can find some time to narrow it down
 > > this week.  I've been working on better logging of "wtf just happened"
 > > the last few weeks, so it should get easier once I finish that work.
 > 
 > I would suggest
 > 	a) slapping WARN_ON(pipe->nr_bufs); right before the loop
 > in splice_direct_to_actor().  Internal pipe should be empty when we
 > enter this function.
 > 	b) the same WARN_ON() in the very end of the loop body.
 > 
 > We should have started with empty pipe.  We'd called ->splice_read()
 > and it had returned a positive number (in read_len).  That should be
 > the amount we'd pushed in there.  Then we call actor(), with
 > sd->total_len set to read_len.  Its return value is
 > 	* positive (or we would've buggered off)
 > 	* no less than read_len (ditto)
 > so it should have drained the pipe entirely, leaving it empty again.
 > 
 > Finding it not just non-empty, but full means that something's very
 > wrong.  The actor here is essentially ->splice_write(), and I'm really
 > curious which file is the target.  Actually, could you turn those
 > WARN_ON() into
 > 	if (WARN_ON(pipe->nr_bufs))
 > 		printk(KERN_ERR "->splice_write = %p",
 > 			sd->u.file->f_op->splice_write);
 > and see which function it is?

s/nr_bufs/nrbufs/ aside, I tried this, and it didn't trigger, even
though I hit the iov_iter_pipe WARN again.

	Dave