Subject: Re: CONFIG_EXT4_USE_FOR_EXT23:  rootfs shows as ext2 instead of ext4
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From: Theodore Tso <tytso@MIT.EDU>
In-Reply-To: <4DA5AEBE.6060705@teksavvy.com>
Date: Wed, 13 Apr 2011 17:00:34 -0400
Cc: Linux Kernel <linux-kernel@vger.kernel.org>, linux-ext4@vger.kernel.org
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References: <4DA48AF4.5080803@teksavvy.com> <20110413004938.GE3682@thunk.org> <4DA5AEBE.6060705@teksavvy.com>
To: Mark Lord <kernel@teksavvy.com>
Sender: linux-kernel-owner@vger.kernel.org
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On Apr 13, 2011, at 10:10 AM, Mark Lord wrote:

> 
> It is dangerous if it really has mounted an ext4 as ext2;
> the on-disk formats are not 100% compatible.
> 
> I don't know what it is doing, so it looks quite dangerous.
> Perhaps it really did mount as ext4 though, in which case not.


As the config option name: CONFIG_EXT4_USE_FOR_EXT23 
might mave suggested to you, what this means is that we are 
using the ext4 file system for ext2/3 file systems.   So yes, we are
using the ext4 file system driver when the ext2 file system is 
requested.   Since the kernel tries mounting the "ext2" file system
first, since we are using the ext4 file system driver, it succeeds,
and since it was mounted when the requested name was "ext2",
that is what is recorded in the mount table.

It's not dangerous at all.   Again, as you can tell if you were to actually
look at your .config carefully, the ext2 file system was not compiled
into your kernel at all.   Ext4 will only try masquerading as ext2 if
CONFIG_EXT2_FS is disabled.  So it couldn't have possibly been 
using ext2 code, if you thought about it for a second.

I can write up a patch which explicitly tests for feature flags that go
beyond ext2 as of a particular version, and if so, refuse the mount
when ext4 is masquerading as ext2, and do the same for ext3.   I
probably will do this to avoid user questions, when I have some 
spare time.

However, please note that this will have no actual effect on how
anything int he kernel will behave --- none ---- except for a one
character change in /proc/mounts: s/2/4/.   (This is because the
kernel will now try ext2, and fail, try ext3 and fail, and then succeed
when the exact same file system driver is used.  Oh, it might also
extend the boot time by a few milliseconds.)

Have I made it clear enough now to assuage your fears?

-- Ted


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