[systemd-devel] Shared root fs by default

[Tvrtko Ursulin]

On Friday 05 April 2013 17:55:46 Lennart Poettering wrote:
> On Tue, 02.04.13 10:27, Tvrtko Ursulin (tvrtko.ursulin at onelan.co.uk) wrote:
> > You need to try it since you don't seem to believe me. :)
> > 
> > + M1=testmp1
> > + M2=testmp2
> > + SM=submount
> > + mkdir -p testmp1
> > + mkdir -p testmp2
> > + mount none -t tmpfs testmp1
> > + mkdir -p testmp1/submount
> > + mount none -t tmpfs testmp1/submount
> > + strace -f -e trace=mount,umount mount testmp1 --rbind testmp2
> > mount("/root/3/testmp1", "testmp2", 0x7f70e796a67c,
> > MS_MGC_VAL|MS_BIND|MS_REC, NULL) = 0
> > + strace -f -e trace=mount,umount umount -l testmp2
> > umount("/root/3/testmp2", MNT_DETACH)   = 0
> > + mountpoint testmp1/submount
> > testmp1/submount is not a mountpoint
> 
> Hmm, does this always happen this way, or is the MS_REC flag "sticky"
> and causes the MNT_DETACH to be recursive?
>
> That looks a bit like a kernel misfeature, no?

To me it looks like the kernel is working as designed, but perhaps I am not 
getting what exactly are you asking. You can read all the details in about 
shared mounts and event propagation in 
Documentation/filesystems/sharedsubtree.txt.

Use case described there is that if you clone (bind) a shared tree you need to 
make it a slave to shut down the propagation in the backward direction (it's 
bi-directional for shared trees by default). 

> > So now just imagine "testmp1/submount" was "/dev/pts".
> 
> Hmm, so in nspawn we don't bind mount /dev/pts anymore, we instantiate a
> new instance...

/dev/pts was just an example, point was I would lose everything once my script 
started tearing down the chroot.

Regards,

Tvrtko