Re: mount behavior question.

[Ram Pai <linuxram@us.ibm.com>]

On Thu, 2005-07-28 at 04:56, Miklos Szeredi wrote:
> > Here is a scenario with shared subtree. Sorry it is complex.
> > 
> > 
> > mount --bind /mnt /mnt
> > mount --make-shared /mnt
> > mkdir -p /mnt/p
> > mount --bind /usr  /mnt/1
> > mount --bind /mnt  /mnt/2
> > 
> > At this stage the mount at /mnt/2 and /mnt belong to the same pnode
> > which means mounts under them propogate to each other.
> > 
> > mount --bind /var /mnt/1
> > 
> > the contents of /var will be visible under /mnt/1 and not under /mnt/2
> > But if mount --bind /var /mnt/2 is executed, the contents of /var is
> > visible under /mnt/1 as well as /mnt/2 . Isn't this freaky?
> 
> I don't understand.
> 
> 'mount --bind /var /mnt/1' should propagate to /mnt/2/1, not /mnt/2. 

yes it should propogate to /mnt/2/1 , thats what I meant when I said
under /mnt/2, but yes I was not clear. Hope I have a clearer
explanation below.

>  No?
> 
> 'mount --bind /var/ /mnt/2' should propagate to /mnt.  What am I
> missing?


step 1: mount --bind /mnt /mnt
            a new mount 'A' is created at /mnt

step 2: mount --make-shared /mnt
           mounts under 'A' are made shared. But in this case
       there are no other mounts. So only 'A' will be made shared.
          
         
step 3: mkdir -p /mnt/1 /mnt/2
		nothing special here
		
step 4: mount --bind /usr  /mnt/1
                a new mount 'B' is created  at /mnt/1 which is
		 'shared;.
                		

step 5: mount --bind /mnt  /mnt/2

		a new mount 'C' is created at /mnt/2
		and propogation is set between 'A' and 'C'.
		note: 'C' is made shared.



lets say, at this point I try 
		mount --bind /var /mnt/1

	this is going to mount 'D' on top of mount 'B'.  However
	there is no other mount to which 'B' propogates to. So that is 
	it. the contents of /var is only visible at /mnt/1 and it
	 propogates no where else.

but lets say, we tried mount --bind /var /mnt/2/1
	/mnt/2/1 belongs to mount 'C'. And mounts under 'C' propogates to 'A'
too. So in this case a new mount 'E' is created at mnt/1/2
i.e on top of 'C' at dentry '2'  and due to propogation a new mount
'F' is created at /mnt/1 i.e on top of mount 'A' at dentry '1'
 But note: /mnt/1 already has a mount 'B' on top of it.   The new mount
'F' as per the 'most-current mount rule' obscures 'B' even though the
mount is on top of 'A'. As a result the contents of /var are now
visible both at /mnt/2/1 and /mnt/1


Ok the net effect is, mount at /mnt/1 is visible only under /mnt/1
but mount at /mnt/2/1 is visible at mount /mnt/2/1 and /mnt/1
This makes it confusing. If the 'top-most mount rule' is applied
'F' though mounted on 'A', will not be visible because it will get 
obscured by 'B' and the confusion is avoided.

So the point I am driving at is, is there any special reason 
for having 'most-recent mount visible rule' instead of 'top-most mount
visible rule'?
RP


> Miklos