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Reply-To: sandeen@redhat.com
Subject: Re: ext4 confusion
To: Randy Dunlap <rdunlap@infradead.org>, linux-ext4@vger.kernel.org,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>
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From: Eric Sandeen <esandeen@redhat.com>
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Date: Mon, 5 Mar 2018 22:34:55 -0600
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On 3/5/18 10:17 PM, Randy Dunlap wrote:
> Hi,
> 
> With the following kernel config entries on Linux 4.16-rc3:
> 
> CONFIG_EXT2_FS=m
> # CONFIG_EXT2_FS_XATTR is not set
> CONFIG_EXT3_FS=m
> # CONFIG_EXT3_FS_POSIX_ACL is not set
> # CONFIG_EXT3_FS_SECURITY is not set
> CONFIG_EXT4_FS=y
> CONFIG_EXT4_FS_POSIX_ACL=y
> CONFIG_EXT4_FS_SECURITY=y
> # CONFIG_EXT4_ENCRYPTION is not set
> # CONFIG_EXT4_DEBUG is not set
> CONFIG_JBD2=y
> # CONFIG_JBD2_DEBUG is not set
> 
> 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 am I doing wrong?

Hm you're the second person to report this in a week.  
(see EXT4-fs (mmcblk0p1): couldn't mount as ext3 due to feature incompatibilities)

It sounds like filesystem probing behavior; it tried to mount as ext3 and failed
(as it should fail) then moved on to ext4 and succeeded.

I sent:

[PATCH V2] ext4: don't complain about incorrect features when probing

which should fix it, but I really wonder why this is just now showing up for 
people; it could be a combination of /etc/filesystems, fstab entries, blkid
handling in mount, etc...

Is this a new OS/installer release where this is showing up, or did a simple
kernel upgrade trigger it?  If the latter, what kernel version didn't have
the above kernel message?

-Eric