Documentation/filesystems/sharedsubtree.rst

.. SPDX-License-Identifier: GPL-2.0

===============
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 it 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. This needs to be supported in PAM.

	D)  Versioned files

		If the entire mount tree is visible at multiple locations, then
		an underlying versioning file system can return different
		versions 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 an 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

		step 2:
		      ::


			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

		step 3:
		    ::

			    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

		step 2:
		      ::


			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

		step 3:
		      ::

			    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

		step 4:
		      ::

			    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)