Adding upstream version 4.19.249.upstream/4.19.249

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 939 insertions, 0 deletions

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+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
+
+		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)