linux-kernel - Re: Regression with a0747a859ef6 ("bdi: add error handle for bdi_debug

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Date:   Thu, 21 Dec 2017 16:29:30 -0800
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Jens Axboe <axboe@...nel.dk>, Bruno Wolff III <bruno@...ff.to>,
        weiping zhang <zwp10758@...il.com>
Cc:     Laura Abbott <labbott@...hat.com>, Jan Kara <jack@...e.cz>,
        linux-mm@...ck.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        regressions@...mhuis.info,
        weiping zhang <zhangweiping@...ichuxing.com>,
        linux-block@...r.kernel.org
Subject: Re: Regression with a0747a859ef6 ("bdi: add error handle for
 bdi_debug_register")

On Thu, 2017-12-21 at 10:02 -0700, Jens Axboe wrote:
> On 12/21/17 9:42 AM, Bruno Wolff III wrote:
> > 
> > On Thu, Dec 21, 2017 at 23:48:19 +0800,
> >   weiping zhang <zwp10758@...il.com> wrote:
> > > 
> > > > 
> > > > output you want. I never saw it for any kernels I compiled
> > > > myself. Only when I test kernels built by Fedora do I see it.
> > > > see it every boot ?
> > 
> > I don't look every boot. The warning gets scrolled of the screen.
> > Once I see the CPU hang warnings I know the boot is failing. I
> > don't always look at journalctl later to see what's there.
> 
> I'm going to revert a0747a859ef6 for now, since we're now 8 days into
> this and no progress has been made on fixing it.

I think this is correct.  If you build the kernel with
CONFIG_DEBUG_FS=N, you're definitely going to get the same hang
(because the debugfs_ functions fail with -ENODEV and the bdi will
never get registered).  This alone leads me to suspect the commit is
bogus because it's a randconfig/test accident waiting to happen.

We should still root cause the debugfs failure in this case, but I
really think debugfs files should be treated as optional, so a failure
in setting them up should translate to some sort of warning not a
failure to set up the bdi.

James