Why you need the real_* thing with genkernel


Sven Vermeulen Sun 25 November 2012

Today it bit me. I rebooted my workstation, and all hell broke loose. Well, actually, it froze. Literally, if you consider my root file system. When the system tried to remount the root file system read-write, it gave me this:

mount: / not mounted or bad option

So I did the first thing that always helps me, and that is to disable the initramfs booting and boot straight from the kernel. Now for those wondering why I boot with an initramfs while it still works directly with a kernel: it's a safety measure. Ever since there are talks, rumours, fear, uncertainty and doubt about supporting a separate /usr file system I started supporting an initramfs on my system in case an update really breaks the regular boot cycle. Same because I use lvm on most file systems, and software RAID on all of them. If I wouldn't have an initramfs laying around, I would be screwed the moment userspace decides not to support this straight from a kernel boot. Luckily, this isn't the case (yet) so I could continue working without an initramfs. But I digress. Back to the situation.

Booting without initramfs worked without errors of any kind. Next thing is to investigate why it fails. I reboot back with the initramfs, get my read-only root file system and start looking around. In my dmesg output, I notice the following:

EXT4-fs (md3): Cannot change data mode on remount

So that's weird, not? What is this data mode? Well, the data mode tells the file system (ext4 for me) how to handle writing data to disk. As you are all aware, ext4 is a journaled file system, meaning it writes changes into a journal before applying, allowing changes to be replayed when the system suddenly crashes. By default, ext4 uses ordered mode, writing the metadata (information about files and such, like inode information, timestamps, block maps, extended attributes, ... but not the data itself) to the journal right after writing data to the disk, after which the metadata is then written to disk as well.

On my system though, I use data=journal so data too is written to the journal first. This gives a higher degree of protection in case of a system crash (or immediate powerdown - my laptop doesn't recognize batteries anymore and with a daughter playing around, I've had my share of sudden powerdowns). I do boot with the rootflags=data=journal and I have data=journal in my fstab.

But the above error tells me otherwise. It tells me that the mode is not what I want it to be. So after fiddling a bit with the options and (of course) using Google to find more information, I found out that my initramfs doesn't check the rootflags parameter, so it mounts the root file system with the standard (ordered) mode. Trying to remount it later will fail, as my fstab contains the data=journal tag, and running mount -o remount,rw,data=ordered for fun doesn't give many smiles.

The man page for genkernel however showed me that it uses real_rootflags. So I reboot with that parameter set to real_rootflags=data=journal and all is okay again.

Edit: I wrote that even changing the default mount options in the file system itself (using tune2fs /dev/md3 -o journal_data) didn't help. However, that seems to be an error on my part, I didn't reboot after toggling this, which is apparently required. Thanks to Xake for pointing that out.