ext3 compatibility between 2.4 and 2.6 kernels

Andreas Dilger adilger at clusterfs.com
Tue Feb 22 03:01:25 UTC 2005

On Feb 21, 2005  17:01 -0700, Howdy Pierce wrote:
> The problem we've encountered after upgrading to FC3 / kernel 2.6 on
> the central server is that the 2.4 kernel in the embedded system
> cannot read the root filesystem, and panics with a message about not
> being able to find init.

Having the specific messages from ext3 would help here.

> The scenario is -- running FC3:
>   1. "mkfs -t ext3 -O none <device>" on removable disk
>   2. "mount -Onoacl,oldalloc,nouser_xattr <device> <mountpoint>"
>   3. "tar xf" onto mountpoint
>   4. "umount <mountpoint>"

What does "dumpe2fs -h" say for the filesystem after umount?  The
"oldalloc" mount option shouldn't leave any permanent change to the
filesystem (it is just the inode allocation policy).  Also, using
"-O none" also disables sparse superblocks, which can consume a lot
of space for large filesystems.

> Furthermore, I've tried performing the mkfs on a RH9/kernel 2.4.20
> system, and then performing the mount/tar/unmount on a FC3/kernel
> 2.6.9 system.  Even this combination fails -- it seems the untarring
> on FC3 somehow creates an ext3 filesystem that the embedded 2.4 kernel
> cannot read.

Does your FC3 kernel have selinux enabled?

> Finally -- this may be a clue -- I've noticed that regardless of where
> the ext3 filesystem was created, symbolic links created on the FC3/2.6
> kernel cannot be read on my 2.4 kernel.

This sounds like there are EAs being placed on the symlinks, which can
cause problems for older kernels (it was an oversight during ext3
development that this was allowed).

> Can you recommend anything to fix the problem (aside from upgrading
> the embedded kernel to 2.6?)

I think there is a one-line fix to "ext3_inode_is_fast_symlink", adding
a check for "i_file_acl" to determine if it is a fast (internal to inode)
or slow (external block) symlink.  This should already be done in newer
2.4 kernels also.

Cheers, Andreas
Andreas Dilger
http://members.shaw.ca/adilger/             http://members.shaw.ca/golinux/

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