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Date:	Thu, 3 Sep 2015 13:36:45 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Richard Yao <ryao@...too.org>, Jan Kara <jack@...e.cz>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Raymond Jennings <shentino@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Theodore Ts'o" <tytso@....edu>, Albino B Neto <bino@...eup.net>
Subject: Re: [GIT PULL] Ext3 removal, quota & udf fixes

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.  :)

> 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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