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Date: Thu, 3 Sep 2015 19:13:10 +0000
From: Richard Yao <ryao@gentoo.org>
To: Eric Sandeen <sandeen@redhat.com>
Cc: Jan Kara <jack@suse.cz>, LKML <linux-kernel@vger.kernel.org>,
        "linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Raymond Jennings <shentino@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Theodore Ts'o" <tytso@mit.edu>, Albino B Neto <bino@riseup.net>
Subject: Re: [GIT PULL] Ext3 removal, quota & udf fixes
Message-ID: <20150903191310.GC7732@woodpecker.gentoo.org>
References: <20150903182225.GA7732@woodpecker.gentoo.org>
 <55E8933D.6010906@redhat.com>
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On Thu, Sep 03, 2015 at 01:36:45PM -0500, Eric Sandeen wrote:
> On 9/3/15 1:22 PM, Richard Yao wrote:
> > What happens with this patch if /dev/$DEVICE is ext4 formatted and someone runs
> > `mount -t ext3 /dev/$DEVICE $MNT`?
> > 
> > This should fail with the ext3 driver, but it looks like it will work fine with
> > CONFIG_EXT4_USE_FOR_EXT23 because ext3_fs_type maps to ext4_mount.
> 
> Did you test it?
> 
> # truncate --size=1g fsfile
> # mkfs.ext4 fsfile
> # mount -o loop -t ext3 fsfile
> mount: wrong fs type, <snip>
> # dmesg | tail
> # EXT4-fs: (loop0): couldn't mount as ext3 due to feature incompatibilities
> 
> > My system is
> > not built with CONFIG_EXT4_USE_FOR_EXT23 (long story short: it uses a RHEL6
> > derived config on Linux 4.1) and I do not have time to rebuild it to verify my
> > suspicion, but I imagine there are others on the list that could trivially check
> > this.
> 
> Well by all means, let me do that for you.  :)

I had vaguely recalled that working, but I am glad to see that my recollection
is wrong. Thanks for confirming it.

> > Also, new kernels are typically drop-in replacements on older userlands. An edge
> > case that no one appears to have mentioned is the possibility of using a newer
> > kernel on an older system where the initramfs generator might only include ext3,
> > which this would break. It might not be terrible to write a small dummy ext3
> > module whose only purpose is to depend on ext4 and load it into the kernel on
> > those systems. That way initramfs software that properly grabs module
> > dependencies will include the ext4 module and `modprobe ext3` will do what it
> > always did in terms of making ext3 file systems mountable. I suppose that we
> > could use aliases, but given that there is a compatibility shim for CONFIG_EXT3
> > to avoid surprises, a dummy module seems reasonable.
> 
> I'm not wise in the ways of initramfs generation, so I'll defer on this one.
> 
> -Eric
> 
> > These are the only two things that I see preventing ext4 from being a drop-in
> > replacement for ext3.
> > 
> > That said, my only connection with ext3/ext4 is that I am one of the genkernel
> > developers (Gentoo's  Linux initramfs/kernel genreation framework/scripts), so
> > my opinion might not matter much here, but I am also in favor of killing ext3 in
> > favor of CONFIG_EXT4_USE_FOR_EXT23.
> > --
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>