linux-ext4 - Re: Bug In ext4 in kernels > 2.6.39 - Not mounting with arguments/options I specify in fstab on root remount

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Date:	Sun, 06 Nov 2011 21:26:57 -0800
From:	Matt Parnell <mparnell@...il.com>
To:	Ted Ts'o <tytso@....edu>
CC:	Eric Sandeen <sandeen@...hat.com>, linux-ext4@...r.kernel.org
Subject: Re: Bug In ext4 in kernels > 2.6.39 - Not mounting with arguments/options
 I specify in fstab on root remount

On 11/06/2011 06:24 PM, Matt Parnell wrote:
> On 10/22/2011 02:32 AM, Ted Ts'o wrote:
>> On Sat, Oct 22, 2011 at 02:51:35AM -0500, Matt Parnell wrote:
>>> That doesn't really help me at all, it's not
>>> rootflags=data=writeback causing this, it's starting to make me
>>> think that arch's init may be to blame, although I previously ruled
>>> it out...
>> Well, it looks like rootflags=data=writeback is not making it to the
>> file system.  That's why it's not showing up in /proc/mounts, from you
>> showed us.  Can you look at the kernel dmesg?
>>
>> You should see something like this:
>>
>> [    1.421146] EXT3-fs (vda): error: couldn't mount because of 
>> unsupported optional features (240)
>> [    1.434057] EXT4-fs (vda): couldn't mount as ext2 due to feature 
>> incompatibilities
>> [    1.454631] EXT4-fs (vda): mounted filesystem with writeback data 
>> mode. Opts: data=writeback
>> [    1.455966] VFS: Mounted root (ext4 filesystem) readonly on device 
>> 254:0.
>>
>>
>> The first line is the failure to mount the root file system as ext3.
>> The 2nd is the failure to mount the file system as ext2, using the
>> ext4 file system driver.  The last two lines show the options show the
>> mount as ext4.
>>
>> What do those two lines look to you.  If you don't see "Opts:
>> data=writeback", then somehow the rootflags option isn't getting passed
>> down to the file system.  Then when you try to remount the file system
>> read/write, the fact that you have "data=writeback" in your /etc/fstab
>> causes the failure to remount.
>>
>> If you simply remove that from /etc/fstab, things should work better.
>> The remount will preserve whatever data=journalling mode was in use
>> when the root file system was originallymounted as.  If rootflags is
>> non-functional, then the file system won't be mounted as
>> data=writeback, but at least the boot sequence will continue without
>> blowing out.
>>
>>                             - Ted
> Looks like the kernel isn't creating the /dev/root link/block device, 
> either all of the time or some of the time, I'm confused.
I figured it out. data=writeback isn't needed and is done by default if 
there's no journal, as per commit 
373cd5c53d5ea6622c319ecd84e29e2737d488bd 
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=373cd5c53d5ea6622c319ecd84e29e2737d488bd

...sorry about throwing you guys through loops over this, although I'm 
guessing there's still a bug in that the kernel should just warn the 
user about the option when it's used and otherwise ignore it instead of 
breaking if there's no journal.
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