Re: RO mount of ext4 filesystem causes writes

[Theodore Ts'o]

On Fri, Jun 23, 2023 at 08:06:02AM +0700, Bagas Sanjaya wrote:
> 
> No reply so far from the culprit author (Ye Bin) nor from Ted. Can
> you help in this case?

There's been no reply because I haven't been able to replicate it, and
I didn't have the time do enough work to convince myself the report
was bogus.  At this point, I have spent time trying to reproduce it,
and I've had no luck.

So, unless you can give me a simple set of reproduction instructions,
I'm going to have to treat this report is invalid.

Regards,

						- Ted

Note: this test was done using kvm-xfstests which can be found
https://github.com/tytso/xfstests-bld using the install-kconfig and
the kbuild script that can also be found in this report.  So if you
want to play along from home, feel free.  :-)


root@kvm-xfstests:~# mkfs.ext4 /dev/vdc
mke2fs 1.47.0 (5-Feb-2023)
Discarding device blocks: done                            
Creating filesystem with 1310720 4k blocks and 327680 inodes
Filesystem UUID: fe434060-6731-4b40-a94a-3a8517df0660
Superblock backups stored on blocks: 
        32768, 98304, 163840, 229376, 294912, 819200, 884736

Allocating group tables: done                            
Writing inode tables: done                            
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done 

root@kvm-xfstests:~# md5sum /dev/vdc
fd38f9f8476ad63a744d179846ee7e18  /dev/vdc
root@kvm-xfstests:~# mount -o ro /dev/vdc /mnt
[  472.893614] EXT4-fs (vdc): orphan cleanup on readonly fs
[  472.894022] EXT4-fs (vdc): mounted filesystem fe434060-6731-4b40-a94a-3a8517df0660 ro with ordered data mode. Quota mode: none.
root@kvm-xfstests:~# umount /mnt
[  475.698053] EXT4-fs (vdc): unmounting filesystem fe434060-6731-4b40-a94a-3a8517df0660.
root@kvm-xfstests:~# md5sum /dev/vdc
fd38f9f8476ad63a744d179846ee7e18  /dev/vdc

Hmm.... OK, let's try it with LUKS, even though that *really*
shouldn't make a difference.  The cryptsetup lukeFormat and mkfs.ext4
steps are skipped here.  Also, note that I had to manually edit the
.config file to enable CONFIG_DM_CRYPT, since I dm_crypt is used by
xfstests, so my install-kconfig script doesn't enable CONFIG_DM_CRYPT.


root@kvm-xfstests:~# uname -a
Linux kvm-xfstests 6.4.0-rc6-xfstests-lockdep #200 SMP PREEMPT_DYNAMIC Fri Jun 23 00:33:39 EDT 2023 x86_64 GNU/Linux

root@kvm-xfstests:~# md5sum /dev/vdc
28b75cc094e1e2a62ac25a730fc1dfee  /dev/vdc
root@kvm-xfstests:~# cryptsetup luksOpen /dev/vdc test
Enter passphrase for /dev/vdc: 
root@kvm-xfstests:~# mount -o ro /dev/mapper/test /mnt
[  812.073771] EXT4-fs (dm-0): orphan cleanup on readonly fs
[  812.074306] EXT4-fs (dm-0): mounted filesystem ac3f76f1-da0a-426e-85b2-08526afb2224 ro with ordered data mode. Quota mode: none.
root@kvm-xfstests:~# umount /mnt
[  814.383016] EXT4-fs (dm-0): unmounting filesystem ac3f76f1-da0a-426e-85b2-08526afb2224.
root@kvm-xfstests:~# cryptsetup luksClose /dev/mapper/test
[  830.001992] dm-0: detected capacity change from 10452992 to 0
root@kvm-xfstests:~# md5sum /dev/vdc
28b75cc094e1e2a62ac25a730fc1dfee  /dev/vdc