/etc/mtab

[Ian Kent]

On Thu, 2010-12-23 at 12:12 +0100, Lennart Poettering wrote:
> On Wed, 22.12.10 22:38, Bernie Innocenti (bernie at codewiz.org) wrote:
> 
> > What's preventing us from symlinking /etc/mtab to /proc/mounts and get
> > rid of confusing situations when mtab gets out of sync with the
> > kernel?
> 
> Some of the mount options that /bin/mount uses currently are seen by
> userspace only and cannot be attached to the mount point in-kernel. The
> effect of that is that if you symlink /etc/mtab to /proc/mounts these
> options get silently dropped after mount, and while they might have
> effect at mount time, afterwards (i.e. umount time) they will have zero
> effect. An example of such an option is "user" and "users".
> 
> It is definitely planned to get rid of /etc/mtab, but before that
> happens the kernel must either learn to store userspace mount options
> along kernel mount options for each mount point in some way (Miklos
> Szeredi has been working on this IIRC), or /bin/mount must learn to
> store these options somewhere in userspace (for example /dev/.mount or
> so), and then augment its output with these options. (Karel Zak has been
> thinking about adding this).

And lets not forget the need for a pseudo mount option that can be used
by the user space tools to ignore certain mounts, such as autofs mounts.

> 
> Sooner or laer we will definitely make /etc/mtab a symlink, which should
> take us one definite step nearer to supporting r/o root by default. 
> 
> In fact, in systemd we already warn during boot if /etc/mtab is not a
> symlink, and we do not longer clear that file on bootup, all to put a
> tiny bit of pressure on the folks involved to fix this properly once and
> for all, in the F15 timeframe...
> 
> > I tried running my system for a few months this way. The only
> > regressions I'm seeing are:
> > 
> > 1) the Gnome Disk Mounter panel applet seems to get confused. After a
> > loopback mount gets unmounted, there's still an icon to remount the
> > device. (the loopback device is correctly cleaned up at umount time).
> 
> Loop devices use userspace-only mount options too, afair.
> 
> > 2) on boot, the system tries to remount /sys even though it's already
> > mounted, thus spitting this harmless error:
> > 
> > Remounting root filesystem in read-write mode: [  OK  ]
> > mount: according to mtab, /dev/sda1 is already mounted on /
> > Mounting local filesystems:  mount: sysfs already mounted or /sys busy
> > mount: according to mtab, /sys is already mounted on /sys [FAILED]
> > Enabling /etc/fstab swaps: [  OK  ]
> 
> This problem is gone on F15/Rawhide anyway, since systemd mounts this dir.
> 
> Lennart
> 
> -- 
> Lennart Poettering - Red Hat, Inc.