Strane remount behaviour with ext3-2.4-0.9.4

Strane remount behaviour with ext3-2.4-0.9.4

[Sean Hunter]

[-- Attachment #1: Type: text/plain, Size: 1402 bytes --]

Following the announcement on lkml, I have started using ext3 on one of my
servers.  Since the server in question is a farily security-sensitive box, my
/usr partition is mounted read only except when I remount rw to install
packages.

I converted this partition to run ext3 with the mount options
"nodev,ro,data=writeback,defaults" figuring that when I need to install new
packages etc, that I could just mount rw as before and that metadata-only
journalling would be ok for this partition as it really sees very little write
activity.

When I try to remount it r/w I get a log message saying:
Jul 27 09:54:29 henry kernel: EXT3-fs: cannot change data mode on remount

...even if I give the full mount option list (including data=writeback) with
the remount instruction.

I can, however, remount it as ext2 read-write, but when I try to remount as
ext3 (even read only) I get the same problem.

Wierdly, "mount" lists it as being still an ext3 partition even though it has
been remounted as ext2.  I can't umount /usr because kjournald is currently
listed as using the partition.

The box in question is more-or-less RedHat 7.1, with ext3-2.4-0.9.4, kernel
2.4.7 and with the following relevant package versions:

mount-2.11g-4
util-linux-2.11f-3
e2fsprogs-1.22-2

...all from rawhide rpms.

I'm going to try one of the partitions mounted "data=ordered" to see if I see
the same sort of thing.

Sean


[-- Attachment #2: Type: application/pgp-signature, Size: 232 bytes --]

Re: Strane remount behaviour with ext3-2.4-0.9.4

[Andreas Dilger]

Sean writes:
> servers.  Since the server in question is a farily security-sensitive box, my
> /usr partition is mounted read only except when I remount rw to install
> packages.

If it is a security-sensitive box, you need to at least use data=ordered or
data=journal.  Using data=writeback allows the possibility that after a crash
one user might be able to read data from deleted files of another user (note
that reiserfs currently only runs the equivalent of data=writeback).

> When I try to remount it r/w I get a log message saying:
> Jul 27 09:54:29 henry kernel: EXT3-fs: cannot change data mode on remount
> 
> ...even if I give the full mount option list (including data=writeback) with
> the remount instruction.

You _could_ leave out the data=writeback from /etc/fstab (default is ordered),
and you will be able to remount OK.  Also, Andrew made a patch which allowed
you to specify the data= mode on remount, as long it is the same. 

> I can, however, remount it as ext2 read-write, but when I try to remount as
> ext3 (even read only) I get the same problem.

You can't change filesystem types via remount (ext2 and ext3 are different
filesystem drivers).  In the future, you might be able to use the ext3
driver to mount a filesystem in totally unjournaled (ext2) mode.

Cheers, Andreas
-- 
Andreas Dilger
http://sourceforge.net/projects/ext2resize/
http://www-mddsp.enel.ucalgary.ca/People/adilger/

Re: Strane remount behaviour with ext3-2.4-0.9.4

[Stephen C. Tweedie]

Hi,

On Fri, Jul 27, 2001 at 11:15:39AM -0600, Andreas Dilger wrote:

> > servers.  Since the server in question is a farily security-sensitive box, my
> > /usr partition is mounted read only except when I remount rw to install
> > packages.
> 
> If it is a security-sensitive box, you need to at least use data=ordered or
> data=journal.  Using data=writeback allows the possibility that after a crash
> one user might be able to read data from deleted files of another user (note
> that reiserfs currently only runs the equivalent of data=writeback).

Agreed, and for most workloads data=ordered will have very little
performance difference from data=writeback.

> You _could_ leave out the data=writeback from /etc/fstab (default is ordered),
> and you will be able to remount OK.  Also, Andrew made a patch which allowed
> you to specify the data= mode on remount, as long it is the same. 

Indeed, but I'll need to make sure that the remount with no data= mode
also works (that should be legal.)

Cheers,
 Stephen