This is just a heads-up to hopefully save other people some time. I'm running 2.6.28 with debian/unstable, finally converted the root to ext4, and discovered that Debian with an initrd can't boot it because the initrd scripts apparently decide it is an ext3 filesystem with unsupported features. Not that they report this: instead you get an ENODEV message that had me convinced ext4 wasn't registering itself or some other such weirdness. Works fine if you use a "rootfstype=ext4" kernel boot parameter, or no initrd.
I traced the problem thru the Debian initrd scripts to klibc/bin/fstype and have filed a Debian bug on it. If kernel-hackers are the ultimate upstream klibc source, please let me know and I'll refile it in other/klibc/kinit. # /usr/lib/klibc/bin/fstype /dev/sda5 //a 100% made-from-scratch ext4 fs FSTYPE=ext3 FSSIZE=12000649216
I submitted a patch to klibc@zytor.com mailing list on Sat, 11 Oct 2008 10:54:44 -0400. (Subject line: [PATCH] fstype: Fix ext4/ext4dev probing). No one ever got back to me. hpa? Do you need me to resend?
Created attachment 19648 [details] Patch to klibc to allow it to properly probe ext4/ext4dev filesystems
Applied, and a new klibc (1.5.15) has been pushed out.