linux-ext4 - Re: ext4 confusion

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Message-ID: <20180306064759.GI18129@dastard>
Date:   Tue, 6 Mar 2018 17:47:59 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Eric Sandeen <sandeen@...hat.com>, linux-ext4@...r.kernel.org,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: ext4 confusion

On Mon, Mar 05, 2018 at 09:12:03PM -0800, Randy Dunlap wrote:
> On 03/05/2018 08:45 PM, Eric Sandeen wrote:
> > On 3/5/18 10:42 PM, Randy Dunlap wrote:
> >>>> It's a new OS/installer.  OpenSUSE Tumbleweed, which is their bleeding edge
> >>>> rolling updates release.
> >>> Hrmph.  A lot of things go into this behavior, it may not be a kernel change at
> >>> all that has made it show up now...
> >> Yes, it could be that wonderful systemd or something else.
> > 
> > I think I'd pursue a parallel track of bugging SUSE about the issue... ;)
> > 
> > (I don't think the kernel will ever just downgrade an rw mount request to
> > ro, or skip an ro->rw transition silently... leaving it ro does seem
> > like an init bug, but *shrug* init long ago transitioned into deep magic.)
> 
> More info:  :(
> 
> This problem happens when booting my own custom 4.16-rc3 kernel.
> If I boot the OpenSUSE-supplied (4.15.7) kernel, the / fs is remounted rw later on.
> 
> So I'm more or less back to "what am I doing wrong"?

The filesystem probing order has probably changed. mount tries to
use blkid to determine the filesytem type to use, and if that
doesn't find a known type it will fall back to trying mounts with
explicit types as per the filesystem type order listed in
/proc/filesystems. (it's in the man mount page) Maybe the device
module hasn't been loaded when blkid runs to probe existing block
devices?

These sorts of whacky behaviours have occurred for me in the past
when either userspace behaviour changed, the order of filesystems
listed in /proc/filesystems or module load order changed. Typically
it's a difference in kernel config that causes such shenanigans.

Cheers,

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