linux-ext4 - Re: ext4 confusion

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Date:   Tue, 6 Mar 2018 15:34:25 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     linux-ext4@...r.kernel.org,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: ext4 confusion

On 03/06/2018 02:34 PM, Theodore Y. Ts'o wrote:
> On Mon, Mar 05, 2018 at 08:17:10PM -0800, Randy Dunlap wrote:
>>
>> ext4_fill_super() tells me:
>>
>> [    3.033174] EXT4-fs (sda5): couldn't mount as ext3 due to feature incompatibilities
>> [    3.100186] EXT4-fs (sda5): mounted filesystem with ordered data mode. Opts: (null)
>> [    3.102683] VFS: Mounted root (ext4 filesystem) readonly on device 8:5.
>>
>> This is a new install, new filesystem. It has never been ext2 or ext3.
>>
>> After bootup and before I do anything else, I can remount /dev/sda5 on / as
>> rw and everything is OK.
> 
> What is the boot command-line that are you using?  What does
> /proc/cmdline say?  Mine says (for example):
> 
> % cat /proc/cmdline 
> BOOT_IMAGE=/vmlinuz-4.15.3-00026-g373ea7d39542 root=/dev/mapper/cwcc-root ro fbcon=font:sun12x22 quiet

Mine says:

BOOT_IMAGE=/boot/bzImage-416-3m32 root=/dev/sda5 resume=/dev/sda3 splash=native quiet showopts elevator=cfq selinux=0 apparmor=0 debug initcall_debug ignore_loglevel


> The ro mount option is usually what causes the root file system to be
> mounted read-only.
> 
> I would check and see whether you are using the same init script path
> for your custom kernel versus your distro kernel.  In particular, is
> the initramfs the same for both?  With Debian, there is an initial
> ramdisk which is used:
> 
> 	linux	/vmlinuz-4.15.3-00026-g373ea7d39542 root=/dev/mapper/cwcc-root ro fbcon=font:sun12x22 quiet
> 	echo	'Loading initial ramdisk ...'
> 	initrd	/initrd.img-4.15.3-00026-g373ea7d39542

My custom kernel does not use an initramfs at all.

> With the Debian initial ram disk, e2fsck is actually run on the file
> system *before* it is mounted, and then it is mounted under userspace
> control, and the kernel's default autoprobing isn't used at all.  So
> with Debian's initramfs, it *knows* it is an ext4 file system and it
> mounts it directly as ext4, so in my kernel logs I just see this:
> 
> Mar  5 18:26:11 cwcc kernel: [   15.073579] EXT4-fs (dm-1): mounted filesystem with ordered data mode. Opts: (null)
> 
> So I don't see the "VFS: mounting root..." message at all.  I don't
> know what your distribution is doing, but you might want to check and
> see if the "VFS: mounting root" is showing up when you are booting the
> distro-kernel.  This really smells like a problem with how the
> initramfs for your custom kernel was set up...

OK, I don't see the "VFS: mounting root" message with the distro kernel.
With the distro kernel, I see:

[   10.678700] EXT4-fs (sda5): re-mounted. Opts: data=ordered,acl,user_xattr

and with my custom kernel (no initramfs) I see:

[    3.146701] EXT4-fs (sda5): mounted filesystem with ordered data mode. Opts: (null)
[    3.149212] VFS: Mounted root (ext4 filesystem) readonly on device 8:5.


so maybe when I'm not using an initramfs, the fs type probing is confused.

I'll keep looking into it.  Thanks for your time(s).  I'll just continue to remount
it R/W for now.

-- 
~Randy