linux-kernel - Re: iov_iter

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Date:   Thu, 7 Sep 2017 09:46:17 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Dave Jones <davej@...emonkey.org.uk>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Al Viro <viro@...IV.linux.org.uk>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        linux-xfs@...r.kernel.org
Subject: Re: iov_iter_pipe warning.

On Wed, Sep 06, 2017 at 04:03:37PM -0400, Dave Jones wrote:
> On Mon, Aug 28, 2017 at 09:25:42PM -0700, Darrick J. Wong wrote:
>  > On Mon, Aug 28, 2017 at 04:31:30PM -0400, Dave Jones wrote:
>  > > I'm still trying to narrow down an exact reproducer, but it seems having
>  > > trinity do a combination of sendfile & writev, with pipes and regular
>  > > files as fd's is the best repro.
>  > > 
>  > > Is this a real problem, or am I chasing ghosts ?  That it doesn't happen
>  > > on ext4 or btrfs is making me wonder...
>  > 
>  > <shrug> I haven't heard of any problems w/ directio xfs lately, but OTOH
>  > I think it's the only filesystem that uses iomap_dio_rw, which would
>  > explain why ext4/btrfs don't have this problem.
> 
> Another warning, from likely the same root cause.
> 
> WARNING: CPU: 3 PID: 572 at lib/iov_iter.c:962 iov_iter_pipe+0xe2/0xf0

	WARN_ON(pipe->nrbufs == pipe->buffers);

 *      @nrbufs: the number of non-empty pipe buffers in this pipe
 *      @buffers: total number of buffers (should be a power of 2)

So that's warning that the pipe buffer is already full before we
try to read from the filesystem?

That doesn't seem like an XFS problem - it indicates the pipe we are
filling in generic_file_splice_read() is not being emptied by
whatever we are splicing the file data to....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com