Shared Subtrees
  ---------------
  
  Contents:
  	1) Overview
  	2) Features

  	3) Setting mount states

  	4) Use-case
  	5) Detailed semantics
  	6) Quiz
  	7) FAQ
  	8) Implementation
  
  
  1) Overview
  -----------
  
  Consider the following situation:
  
  A process wants to clone its own namespace, but still wants to access the CD
  that got mounted recently.  Shared subtree semantics provide the necessary
  mechanism to accomplish the above.
  
  It provides the necessary building blocks for features like per-user-namespace
  and versioned filesystem.
  
  2) Features
  -----------
  
  Shared subtree provides four different flavors of mounts; struct vfsmount to be
  precise
  
  	a. shared mount
  	b. slave mount
  	c. private mount
  	d. unbindable mount
  
  
  2a) A shared mount can be replicated to as many mountpoints and all the
  replicas continue to be exactly same.
  
  	Here is an example:

  	Let's say /mnt has a mount that is shared.

  	mount --make-shared /mnt

  	Note: mount(8) command now supports the --make-shared flag,
  	so the sample 'smount' program is no longer needed and has been
  	removed.

  	# mount --bind /mnt /tmp

  	The above command replicates the mount at /mnt to the mountpoint /tmp
  	and the contents of both the mounts remain identical.
  
  	#ls /mnt
  	a b c
  
  	#ls /tmp
  	a b c

  	Now let's say we mount a device at /tmp/a
  	# mount /dev/sd0  /tmp/a

  
  	#ls /tmp/a

  	t1 t2 t3

  
  	#ls /mnt/a

  	t1 t2 t3

  
  	Note that the mount has propagated to the mount at /mnt as well.
  
  	And the same is true even when /dev/sd0 is mounted on /mnt/a. The
  	contents will be visible under /tmp/a too.
  
  
  2b) A slave mount is like a shared mount except that mount and umount events
  	only propagate towards it.
  
  	All slave mounts have a master mount which is a shared.
  
  	Here is an example:

  	Let's say /mnt has a mount which is shared.
  	# mount --make-shared /mnt

  	Let's bind mount /mnt to /tmp
  	# mount --bind /mnt /tmp

  
  	the new mount at /tmp becomes a shared mount and it is a replica of
  	the mount at /mnt.

  	Now let's make the mount at /tmp; a slave of /mnt
  	# mount --make-slave /tmp

  	let's mount /dev/sd0 on /mnt/a
  	# mount /dev/sd0 /mnt/a

  
  	#ls /mnt/a
  	t1 t2 t3
  
  	#ls /tmp/a
  	t1 t2 t3
  
  	Note the mount event has propagated to the mount at /tmp

  	However let's see what happens if we mount something on the mount at /tmp

  	# mount /dev/sd1 /tmp/b

  
  	#ls /tmp/b
  	s1 s2 s3
  
  	#ls /mnt/b
  
  	Note how the mount event has not propagated to the mount at
  	/mnt
  
  
  2c) A private mount does not forward or receive propagation.
  
  	This is the mount we are familiar with. Its the default type.
  
  
  2d) A unbindable mount is a unbindable private mount

  	let's say we have a mount at /mnt and we make is unbindable

  	# mount --make-unbindable /mnt

  	 Let's try to bind mount this mount somewhere else.

  	 # mount --bind /mnt /tmp
  	 mount: wrong fs type, bad option, bad superblock on /mnt,
  	        or too many mounted file systems
  
  	Binding a unbindable mount is a invalid operation.

  3) Setting mount states

  	The mount command (util-linux package) can be used to set mount
  	states:
  
  	mount --make-shared mountpoint
  	mount --make-slave mountpoint
  	mount --make-private mountpoint
  	mount --make-unbindable mountpoint

  
  
  4) Use cases
  ------------
  
  	A) A process wants to clone its own namespace, but still wants to
  	   access the CD that got mounted recently.
  
  	   Solution:
  
  		The system administrator can make the mount at /cdrom shared
  		mount --bind /cdrom /cdrom
  		mount --make-shared /cdrom
  
  		Now any process that clones off a new namespace will have a
  		mount at /cdrom which is a replica of the same mount in the
  		parent namespace.
  
  		So when a CD is inserted and mounted at /cdrom that mount gets
  		propagated to the other mount at /cdrom in all the other clone
  		namespaces.
  
  	B) A process wants its mounts invisible to any other process, but
  	still be able to see the other system mounts.
  
  	   Solution:
  
  		To begin with, the administrator can mark the entire mount tree
  		as shareable.
  
  		mount --make-rshared /
  
  		A new process can clone off a new namespace. And mark some part
  		of its namespace as slave
  
  		mount --make-rslave /myprivatetree
  
  		Hence forth any mounts within the /myprivatetree done by the
  		process will not show up in any other namespace. However mounts
  		done in the parent namespace under /myprivatetree still shows
  		up in the process's namespace.
  
  
  	Apart from the above semantics this feature provides the
  	building blocks to solve the following problems:
  
  	C)  Per-user namespace
  
  		The above semantics allows a way to share mounts across
  		namespaces.  But namespaces are associated with processes. If
  		namespaces are made first class objects with user API to
  		associate/disassociate a namespace with userid, then each user
  		could have his/her own namespace and tailor it to his/her
  		requirements. Offcourse its needs support from PAM.
  
  	D)  Versioned files
  
  		If the entire mount tree is visible at multiple locations, then
  		a underlying versioning file system can return different
  		version of the file depending on the path used to access that
  		file.
  
  		An example is:
  
  		mount --make-shared /
  		mount --rbind / /view/v1
  		mount --rbind / /view/v2
  		mount --rbind / /view/v3
  		mount --rbind / /view/v4

  		and if /usr has a versioning filesystem mounted, then that

  		mount appears at /view/v1/usr, /view/v2/usr, /view/v3/usr and
  		/view/v4/usr too
  
  		A user can request v3 version of the file /usr/fs/namespace.c
  		by accessing /view/v3/usr/fs/namespace.c . The underlying
  		versioning filesystem can then decipher that v3 version of the
  		filesystem is being requested and return the corresponding
  		inode.
  
  5) Detailed semantics:
  -------------------
  	The section below explains the detailed semantics of
  	bind, rbind, move, mount, umount and clone-namespace operations.
  
  	Note: the word 'vfsmount' and the noun 'mount' have been used
  	to mean the same thing, throughout this document.
  
  5a) Mount states
  
  	A given mount can be in one of the following states
  	1) shared
  	2) slave
  	3) shared and slave
  	4) private
  	5) unbindable
  
  	A 'propagation event' is defined as event generated on a vfsmount
  	that leads to mount or unmount actions in other vfsmounts.
  
  	A 'peer group' is defined as a group of vfsmounts that propagate
  	events to each other.
  
  	(1) Shared mounts
  
  		A 'shared mount' is defined as a vfsmount that belongs to a
  		'peer group'.
  
  		For example:
  			mount --make-shared /mnt

  			mount --bind /mnt /tmp

  
  		The mount at /mnt and that at /tmp are both shared and belong
  		to the same peer group. Anything mounted or unmounted under
  		/mnt or /tmp reflect in all the other mounts of its peer
  		group.
  
  
  	(2) Slave mounts
  
  		A 'slave mount' is defined as a vfsmount that receives
  		propagation events and does not forward propagation events.
  
  		A slave mount as the name implies has a master mount from which
  		mount/unmount events are received. Events do not propagate from
  		the slave mount to the master.  Only a shared mount can be made
  		a slave by executing the following command
  
  			mount --make-slave mount
  
  		A shared mount that is made as a slave is no more shared unless
  		modified to become shared.
  
  	(3) Shared and Slave
  
  		A vfsmount can be both shared as well as slave.  This state
  		indicates that the mount is a slave of some vfsmount, and
  		has its own peer group too.  This vfsmount receives propagation
  		events from its master vfsmount, and also forwards propagation
  		events to its 'peer group' and to its slave vfsmounts.
  
  		Strictly speaking, the vfsmount is shared having its own
  		peer group, and this peer-group is a slave of some other
  		peer group.
  
  		Only a slave vfsmount can be made as 'shared and slave' by
  		either executing the following command
  			mount --make-shared mount
  		or by moving the slave vfsmount under a shared vfsmount.
  
  	(4) Private mount
  
  		A 'private mount' is defined as vfsmount that does not
  		receive or forward any propagation events.
  
  	(5) Unbindable mount
  
  		A 'unbindable mount' is defined as vfsmount that does not
  		receive or forward any propagation events and cannot
  		be bind mounted.
  
  
     	State diagram:
     	The state diagram below explains the state transition of a mount,
  	in response to various commands.
  	------------------------------------------------------------------------
  	|             |make-shared |  make-slave  | make-private |make-unbindab|
  	--------------|------------|--------------|--------------|-------------|
  	|shared	      |shared	   |*slave/private|   private	 | unbindable  |
  	|             |            |              |              |             |
  	|-------------|------------|--------------|--------------|-------------|
  	|slave	      |shared      |	**slave	  |    private   | unbindable  |
  	|             |and slave   |              |              |             |
  	|-------------|------------|--------------|--------------|-------------|
  	|shared	      |shared      |    slave	  |    private   | unbindable  |
  	|and slave    |and slave   |              |              |             |
  	|-------------|------------|--------------|--------------|-------------|
  	|private      |shared	   |  **private	  |    private   | unbindable  |
  	|-------------|------------|--------------|--------------|-------------|
  	|unbindable   |shared	   |**unbindable  |    private   | unbindable  |
  	------------------------------------------------------------------------
  
  	* if the shared mount is the only mount in its peer group, making it
  	slave, makes it private automatically. Note that there is no master to
  	which it can be slaved to.
  
  	** slaving a non-shared mount has no effect on the mount.
  
  	Apart from the commands listed below, the 'move' operation also changes
  	the state of a mount depending on type of the destination mount. Its
  	explained in section 5d.
  
  5b) Bind semantics
  
  	Consider the following command
  
  	mount --bind A/a  B/b
  
  	where 'A' is the source mount, 'a' is the dentry in the mount 'A', 'B'
  	is the destination mount and 'b' is the dentry in the destination mount.
  
  	The outcome depends on the type of mount of 'A' and 'B'. The table
  	below contains quick reference.
     ---------------------------------------------------------------------------
     |         BIND MOUNT OPERATION                                            |
     |**************************************************************************
     |source(A)->| shared       |       private  |       slave    | unbindable |
     | dest(B)  |               |                |                |            |
     |   |      |               |                |                |            |
     |   v      |               |                |                |            |
     |**************************************************************************
     |  shared  | shared        |     shared     | shared & slave |  invalid   |
     |          |               |                |                |            |
     |non-shared| shared        |      private   |      slave     |  invalid   |
     ***************************************************************************
  
       	Details:
  
  	1. 'A' is a shared mount and 'B' is a shared mount. A new mount 'C'
  	which is clone of 'A', is created. Its root dentry is 'a' . 'C' is
  	mounted on mount 'B' at dentry 'b'. Also new mount 'C1', 'C2', 'C3' ...
  	are created and mounted at the dentry 'b' on all mounts where 'B'
  	propagates to. A new propagation tree containing 'C1',..,'Cn' is
  	created. This propagation tree is identical to the propagation tree of
  	'B'.  And finally the peer-group of 'C' is merged with the peer group
  	of 'A'.
  
  	2. 'A' is a private mount and 'B' is a shared mount. A new mount 'C'
  	which is clone of 'A', is created. Its root dentry is 'a'. 'C' is
  	mounted on mount 'B' at dentry 'b'. Also new mount 'C1', 'C2', 'C3' ...
  	are created and mounted at the dentry 'b' on all mounts where 'B'
  	propagates to. A new propagation tree is set containing all new mounts
  	'C', 'C1', .., 'Cn' with exactly the same configuration as the
  	propagation tree for 'B'.
  
  	3. 'A' is a slave mount of mount 'Z' and 'B' is a shared mount. A new
  	mount 'C' which is clone of 'A', is created. Its root dentry is 'a' .
  	'C' is mounted on mount 'B' at dentry 'b'. Also new mounts 'C1', 'C2',
  	'C3' ... are created and mounted at the dentry 'b' on all mounts where
  	'B' propagates to. A new propagation tree containing the new mounts
  	'C','C1',..  'Cn' is created. This propagation tree is identical to the
  	propagation tree for 'B'. And finally the mount 'C' and its peer group
  	is made the slave of mount 'Z'.  In other words, mount 'C' is in the
  	state 'slave and shared'.
  
  	4. 'A' is a unbindable mount and 'B' is a shared mount. This is a
  	invalid operation.
  
  	5. 'A' is a private mount and 'B' is a non-shared(private or slave or
  	unbindable) mount. A new mount 'C' which is clone of 'A', is created.
  	Its root dentry is 'a'. 'C' is mounted on mount 'B' at dentry 'b'.
  
  	6. 'A' is a shared mount and 'B' is a non-shared mount. A new mount 'C'
  	which is a clone of 'A' is created. Its root dentry is 'a'. 'C' is
  	mounted on mount 'B' at dentry 'b'.  'C' is made a member of the
  	peer-group of 'A'.
  
  	7. 'A' is a slave mount of mount 'Z' and 'B' is a non-shared mount. A
  	new mount 'C' which is a clone of 'A' is created. Its root dentry is
  	'a'.  'C' is mounted on mount 'B' at dentry 'b'. Also 'C' is set as a
  	slave mount of 'Z'. In other words 'A' and 'C' are both slave mounts of
  	'Z'.  All mount/unmount events on 'Z' propagates to 'A' and 'C'. But
  	mount/unmount on 'A' do not propagate anywhere else. Similarly
  	mount/unmount on 'C' do not propagate anywhere else.
  
  	8. 'A' is a unbindable mount and 'B' is a non-shared mount. This is a
  	invalid operation. A unbindable mount cannot be bind mounted.
  
  5c) Rbind semantics
  
  	rbind is same as bind. Bind replicates the specified mount.  Rbind
  	replicates all the mounts in the tree belonging to the specified mount.
  	Rbind mount is bind mount applied to all the mounts in the tree.
  
  	If the source tree that is rbind has some unbindable mounts,
  	then the subtree under the unbindable mount is pruned in the new
  	location.

  	eg: let's say we have the following mount tree.

  
  		A
  	      /   \
  	      B   C
  	     / \ / \
  	     D E F G

  	     Let's say all the mount except the mount C in the tree are

  	     of a type other than unbindable.
  
  	     If this tree is rbound to say Z
  
  	     We will have the following tree at the new location.
  
  		Z
  		|
  		A'
  	       /
  	      B'		Note how the tree under C is pruned
  	     / \ 		in the new location.
  	    D' E'
  
  
  
  5d) Move semantics
  
  	Consider the following command
  
  	mount --move A  B/b
  
  	where 'A' is the source mount, 'B' is the destination mount and 'b' is
  	the dentry in the destination mount.
  
  	The outcome depends on the type of the mount of 'A' and 'B'. The table
  	below is a quick reference.
     ---------------------------------------------------------------------------
     |         		MOVE MOUNT OPERATION                                 |
     |**************************************************************************
     | source(A)->| shared      |       private  |       slave    | unbindable |
     | dest(B)  |               |                |                |            |
     |   |      |               |                |                |            |
     |   v      |               |                |                |            |
     |**************************************************************************
     |  shared  | shared        |     shared     |shared and slave|  invalid   |
     |          |               |                |                |            |
     |non-shared| shared        |      private   |    slave       | unbindable |
     ***************************************************************************
  	NOTE: moving a mount residing under a shared mount is invalid.
  
        Details follow:
  
  	1. 'A' is a shared mount and 'B' is a shared mount.  The mount 'A' is
  	mounted on mount 'B' at dentry 'b'.  Also new mounts 'A1', 'A2'...'An'
  	are created and mounted at dentry 'b' on all mounts that receive
  	propagation from mount 'B'. A new propagation tree is created in the
  	exact same configuration as that of 'B'. This new propagation tree
  	contains all the new mounts 'A1', 'A2'...  'An'.  And this new
  	propagation tree is appended to the already existing propagation tree
  	of 'A'.
  
  	2. 'A' is a private mount and 'B' is a shared mount. The mount 'A' is
  	mounted on mount 'B' at dentry 'b'. Also new mount 'A1', 'A2'... 'An'
  	are created and mounted at dentry 'b' on all mounts that receive
  	propagation from mount 'B'. The mount 'A' becomes a shared mount and a
  	propagation tree is created which is identical to that of
  	'B'. This new propagation tree contains all the new mounts 'A1',
  	'A2'...  'An'.
  
  	3. 'A' is a slave mount of mount 'Z' and 'B' is a shared mount.  The
  	mount 'A' is mounted on mount 'B' at dentry 'b'.  Also new mounts 'A1',
  	'A2'... 'An' are created and mounted at dentry 'b' on all mounts that
  	receive propagation from mount 'B'. A new propagation tree is created
  	in the exact same configuration as that of 'B'. This new propagation
  	tree contains all the new mounts 'A1', 'A2'...  'An'.  And this new
  	propagation tree is appended to the already existing propagation tree of
  	'A'.  Mount 'A' continues to be the slave mount of 'Z' but it also
  	becomes 'shared'.
  
  	4. 'A' is a unbindable mount and 'B' is a shared mount. The operation
  	is invalid. Because mounting anything on the shared mount 'B' can
  	create new mounts that get mounted on the mounts that receive
  	propagation from 'B'.  And since the mount 'A' is unbindable, cloning
  	it to mount at other mountpoints is not possible.
  
  	5. 'A' is a private mount and 'B' is a non-shared(private or slave or
  	unbindable) mount. The mount 'A' is mounted on mount 'B' at dentry 'b'.
  
  	6. 'A' is a shared mount and 'B' is a non-shared mount.  The mount 'A'
  	is mounted on mount 'B' at dentry 'b'.  Mount 'A' continues to be a
  	shared mount.
  
  	7. 'A' is a slave mount of mount 'Z' and 'B' is a non-shared mount.
  	The mount 'A' is mounted on mount 'B' at dentry 'b'.  Mount 'A'
  	continues to be a slave mount of mount 'Z'.
  
  	8. 'A' is a unbindable mount and 'B' is a non-shared mount. The mount
  	'A' is mounted on mount 'B' at dentry 'b'. Mount 'A' continues to be a
  	unbindable mount.
  
  5e) Mount semantics
  
  	Consider the following command
  
  	mount device  B/b
  
  	'B' is the destination mount and 'b' is the dentry in the destination
  	mount.
  
  	The above operation is the same as bind operation with the exception
  	that the source mount is always a private mount.
  
  
  5f) Unmount semantics
  
  	Consider the following command
  
  	umount A
  
  	where 'A' is a mount mounted on mount 'B' at dentry 'b'.
  
  	If mount 'B' is shared, then all most-recently-mounted mounts at dentry
  	'b' on mounts that receive propagation from mount 'B' and does not have
  	sub-mounts within them are unmounted.

  	Example: Let's say 'B1', 'B2', 'B3' are shared mounts that propagate to

  	each other.

  	let's say 'A1', 'A2', 'A3' are first mounted at dentry 'b' on mount

  	'B1', 'B2' and 'B3' respectively.

  	let's say 'C1', 'C2', 'C3' are next mounted at the same dentry 'b' on

  	mount 'B1', 'B2' and 'B3' respectively.
  
  	if 'C1' is unmounted, all the mounts that are most-recently-mounted on
  	'B1' and on the mounts that 'B1' propagates-to are unmounted.
  
  	'B1' propagates to 'B2' and 'B3'. And the most recently mounted mount
  	on 'B2' at dentry 'b' is 'C2', and that of mount 'B3' is 'C3'.
  
  	So all 'C1', 'C2' and 'C3' should be unmounted.
  
  	If any of 'C2' or 'C3' has some child mounts, then that mount is not
  	unmounted, but all other mounts are unmounted. However if 'C1' is told
  	to be unmounted and 'C1' has some sub-mounts, the umount operation is
  	failed entirely.
  
  5g) Clone Namespace
  
  	A cloned namespace contains all the mounts as that of the parent
  	namespace.

  	Let's say 'A' and 'B' are the corresponding mounts in the parent and the

  	child namespace.
  
  	If 'A' is shared, then 'B' is also shared and 'A' and 'B' propagate to
  	each other.
  
  	If 'A' is a slave mount of 'Z', then 'B' is also the slave mount of
  	'Z'.
  
  	If 'A' is a private mount, then 'B' is a private mount too.
  
  	If 'A' is unbindable mount, then 'B' is a unbindable mount too.
  
  
  6) Quiz
  
  	A. What is the result of the following command sequence?
  
  		mount --bind /mnt /mnt
  		mount --make-shared /mnt
  		mount --bind /mnt /tmp
  		mount --move /tmp /mnt/1
  
  		what should be the contents of /mnt /mnt/1 /mnt/1/1 should be?
  		Should they all be identical? or should /mnt and /mnt/1 be
  		identical only?
  
  
  	B. What is the result of the following command sequence?
  
  		mount --make-rshared /
  		mkdir -p /v/1
  		mount --rbind / /v/1
  
  		what should be the content of /v/1/v/1 be?
  
  
  	C. What is the result of the following command sequence?
  
  		mount --bind /mnt /mnt
  		mount --make-shared /mnt
  		mkdir -p /mnt/1/2/3 /mnt/1/test
  		mount --bind /mnt/1 /tmp
  		mount --make-slave /mnt
  		mount --make-shared /mnt
  		mount --bind /mnt/1/2 /tmp1
  		mount --make-slave /mnt
  
  		At this point we have the first mount at /tmp and

  		its root dentry is 1. Let's call this mount 'A'

  		And then we have a second mount at /tmp1 with root

  		dentry 2. Let's call this mount 'B'

  		Next we have a third mount at /mnt with root dentry

  		mnt. Let's call this mount 'C'

  
  		'B' is the slave of 'A' and 'C' is a slave of 'B'
  		A -> B -> C
  
  		at this point if we execute the following command
  
  		mount --bind /bin /tmp/test
  
  		The mount is attempted on 'A'
  
  		will the mount propagate to 'B' and 'C' ?
  
  		what would be the contents of
  		/mnt/1/test be?
  
  7) FAQ
  
  	Q1. Why is bind mount needed? How is it different from symbolic links?
  		symbolic links can get stale if the destination mount gets
  		unmounted or moved. Bind mounts continue to exist even if the
  		other mount is unmounted or moved.
  
  	Q2. Why can't the shared subtree be implemented using exportfs?
  
  		exportfs is a heavyweight way of accomplishing part of what
  		shared subtree can do. I cannot imagine a way to implement the
  		semantics of slave mount using exportfs?
  
  	Q3 Why is unbindable mount needed?

  		Let's say we want to replicate the mount tree at multiple

  		locations within the same subtree.
  
  		if one rbind mounts a tree within the same subtree 'n' times
  		the number of mounts created is an exponential function of 'n'.
  		Having unbindable mount can help prune the unneeded bind
  		mounts. Here is a example.
  
  		step 1:

  		   let's say the root tree has just two directories with

  		   one vfsmount.
  				    root
  				   /    \
  				  tmp    usr
  
  		    And we want to replicate the tree at multiple
  		    mountpoints under /root/tmp
  
  		step2:
  		      mount --make-shared /root
  
  		      mkdir -p /tmp/m1
  
  		      mount --rbind /root /tmp/m1
  
  		      the new tree now looks like this:
  
  				    root
  				   /    \
  				 tmp    usr
  				/
  			       m1
  			      /  \
  			     tmp  usr
  			     /
  			    m1
  
  			  it has two vfsmounts
  
  		step3:
  			    mkdir -p /tmp/m2
  			    mount --rbind /root /tmp/m2
  
  			the new tree now looks like this:
  
  				      root
  				     /    \
  				   tmp     usr
  				  /    \
  				m1       m2
  			       / \       /  \
  			     tmp  usr   tmp  usr
  			     / \          /
  			    m1  m2      m1
  				/ \     /  \
  			      tmp usr  tmp   usr
  			      /        / \
  			     m1       m1  m2
  			    /  \
  			  tmp   usr
  			  /  \
  			 m1   m2
  
  		       it has 6 vfsmounts
  
  		step 4:
  			  mkdir -p /tmp/m3
  			  mount --rbind /root /tmp/m3

  			  I won't draw the tree..but it has 24 vfsmounts

  
  
  		at step i the number of vfsmounts is V[i] = i*V[i-1].
  		This is an exponential function. And this tree has way more
  		mounts than what we really needed in the first place.
  
  		One could use a series of umount at each step to prune
  		out the unneeded mounts. But there is a better solution.
  		Unclonable mounts come in handy here.
  
  		step 1:

  		   let's say the root tree has just two directories with

  		   one vfsmount.
  				    root
  				   /    \
  				  tmp    usr
  
  		    How do we set up the same tree at multiple locations under
  		    /root/tmp
  
  		step2:
  		      mount --bind /root/tmp /root/tmp
  
  		      mount --make-rshared /root
  		      mount --make-unbindable /root/tmp
  
  		      mkdir -p /tmp/m1
  
  		      mount --rbind /root /tmp/m1
  
  		      the new tree now looks like this:
  
  				    root
  				   /    \
  				 tmp    usr
  				/
  			       m1
  			      /  \
  			     tmp  usr
  
  		step3:
  			    mkdir -p /tmp/m2
  			    mount --rbind /root /tmp/m2
  
  		      the new tree now looks like this:
  
  				    root
  				   /    \
  				 tmp    usr
  				/   \
  			       m1     m2
  			      /  \     / \
  			     tmp  usr tmp usr
  
  		step4:
  
  			    mkdir -p /tmp/m3
  			    mount --rbind /root /tmp/m3
  
  		      the new tree now looks like this:
  
  				    	  root
  				      /    	  \
  				     tmp    	   usr
  			         /    \    \
  			       m1     m2     m3
  			      /  \     / \    /  \
  			     tmp  usr tmp usr tmp usr
  
  8) Implementation
  
  8A) Datastructure
  
  	4 new fields are introduced to struct vfsmount
  	->mnt_share
  	->mnt_slave_list
  	->mnt_slave
  	->mnt_master

  	->mnt_share links together all the mount to/from which this vfsmount

  		send/receives propagation events.
  
  	->mnt_slave_list links all the mounts to which this vfsmount propagates
  		to.

  	->mnt_slave links together all the slaves that its master vfsmount

  		propagates to.
  
  	->mnt_master points to the master vfsmount from which this vfsmount
  		receives propagation.
  
  	->mnt_flags takes two more flags to indicate the propagation status of
  		the vfsmount.  MNT_SHARE indicates that the vfsmount is a shared
  		vfsmount.  MNT_UNCLONABLE indicates that the vfsmount cannot be
  		replicated.
  
  	All the shared vfsmounts in a peer group form a cyclic list through
  	->mnt_share.
  
  	All vfsmounts with the same ->mnt_master form on a cyclic list anchored
  	in ->mnt_master->mnt_slave_list and going through ->mnt_slave.
  
  	 ->mnt_master can point to arbitrary (and possibly different) members
  	 of master peer group.  To find all immediate slaves of a peer group
  	 you need to go through _all_ ->mnt_slave_list of its members.
  	 Conceptually it's just a single set - distribution among the
  	 individual lists does not affect propagation or the way propagation
  	 tree is modified by operations.

  	All vfsmounts in a peer group have the same ->mnt_master.  If it is
  	non-NULL, they form a contiguous (ordered) segment of slave list.

  	A example propagation tree looks as shown in the figure below.
  	[ NOTE: Though it looks like a forest, if we consider all the shared
  	mounts as a conceptual entity called 'pnode', it becomes a tree]
  
  
  		        A <--> B <--> C <---> D
  		       /|\	      /|      |\
  		      / F G	     J K      H I
  		     /
  		    E<-->K
  			/|\
  		       M L N
  
  	In the above figure  A,B,C and D all are shared and propagate to each
  	other.   'A' has got 3 slave mounts 'E' 'F' and 'G' 'C' has got 2 slave
  	mounts 'J' and 'K'  and  'D' has got two slave mounts 'H' and 'I'.
  	'E' is also shared with 'K' and they propagate to each other.  And
  	'K' has 3 slaves 'M', 'L' and 'N'
  
  	A's ->mnt_share links with the ->mnt_share of 'B' 'C' and 'D'
  
  	A's ->mnt_slave_list links with ->mnt_slave of 'E', 'K', 'F' and 'G'
  
  	E's ->mnt_share links with ->mnt_share of K
  	'E', 'K', 'F', 'G' have their ->mnt_master point to struct
  				vfsmount of 'A'
  	'M', 'L', 'N' have their ->mnt_master point to struct vfsmount of 'K'
  	K's ->mnt_slave_list links with ->mnt_slave of 'M', 'L' and 'N'
  
  	C's ->mnt_slave_list links with ->mnt_slave of 'J' and 'K'
  	J and K's ->mnt_master points to struct vfsmount of C
  	and finally D's ->mnt_slave_list links with ->mnt_slave of 'H' and 'I'
  	'H' and 'I' have their ->mnt_master pointing to struct vfsmount of 'D'.
  
  
  	NOTE: The propagation tree is orthogonal to the mount tree.

  8B Locking:
  
  	->mnt_share, ->mnt_slave, ->mnt_slave_list, ->mnt_master are protected
  	by namespace_sem (exclusive for modifications, shared for reading).
  
  	Normally we have ->mnt_flags modifications serialized by vfsmount_lock.
  	There are two exceptions: do_add_mount() and clone_mnt().
  	The former modifies a vfsmount that has not been visible in any shared
  	data structures yet.
  	The latter holds namespace_sem and the only references to vfsmount
  	are in lists that can't be traversed without namespace_sem.

  8C Algorithm:

  
  	The crux of the implementation resides in rbind/move operation.
  
  	The overall algorithm breaks the operation into 3 phases: (look at
  	attach_recursive_mnt() and propagate_mnt())
  
  	1. prepare phase.
  	2. commit phases.
  	3. abort phases.
  
  	Prepare phase:
  
  	for each mount in the source tree:
  		   a) Create the necessary number of mount trees to
  		   	be attached to each of the mounts that receive
  			propagation from the destination mount.
  		   b) Do not attach any of the trees to its destination.
  		      However note down its ->mnt_parent and ->mnt_mountpoint
  		   c) Link all the new mounts to form a propagation tree that
  		      is identical to the propagation tree of the destination
  		      mount.
  
  		   If this phase is successful, there should be 'n' new
  		   propagation trees; where 'n' is the number of mounts in the
  		   source tree.  Go to the commit phase
  
  		   Also there should be 'm' new mount trees, where 'm' is
  		   the number of mounts to which the destination mount
  		   propagates to.
  
  		   if any memory allocations fail, go to the abort phase.
  
  	Commit phase
  		attach each of the mount trees to their corresponding
  		destination mounts.
  
  	Abort phase
  		delete all the newly created trees.
  
  	NOTE: all the propagation related functionality resides in the file
  	pnode.c
  
  
  ------------------------------------------------------------------------
  
  version 0.1  (created the initial document, Ram Pai linuxram@us.ibm.com)
  version 0.2  (Incorporated comments from Al Viro)