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[linux-2.6] / Documentation / sharedsubtree.txt

Shared Subtrees
---------------

Contents:
        1) Overview
        2) Features
        3) smount command
        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:

        Lets say /mnt has a mount that is shared.
        mount --make-shared /mnt

        note: mount command does not yet support the --make-shared flag.
        I have included a small C program which does the same by executing
        'smount /mnt shared'

        #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 lets say we mount a device at /tmp/a
        #mount /dev/sd0  /tmp/a

        #ls /tmp/a
        t1 t2 t2

        #ls /mnt/a
        t1 t2 t2

        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:

        Lets say /mnt has a mount which is shared.
        #mount --make-shared /mnt

        Lets 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 lets make the mount at /tmp; a slave of /mnt
        #mount --make-slave /tmp
        [or smount /tmp slave]

        lets 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 lets 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

        lets say we have a mount at /mnt and we make is unbindable

        #mount --make-unbindable /mnt
         [ smount /mnt  unbindable ]

         Lets 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) smount command

        Currently the mount command is not aware of shared subtree features.
        Work is in progress to add the support in mount ( util-linux package ).
        Till then use the following program.

        ------------------------------------------------------------------------
        //
        //this code was developed my Miklos Szeredi <miklos@szeredi.hu>
        //and modified by Ram Pai <linuxram@us.ibm.com>
        // sample usage:
        //              smount /tmp shared
        //
        #include <stdio.h>
        #include <stdlib.h>
        #include <unistd.h>
        #include <sys/mount.h>
        #include <sys/fsuid.h>

        #ifndef MS_REC
        #define MS_REC          0x4000  /* 16384: Recursive loopback */
        #endif

        #ifndef MS_SHARED
        #define MS_SHARED               1<<20   /* Shared */
        #endif

        #ifndef MS_PRIVATE
        #define MS_PRIVATE              1<<18   /* Private */
        #endif

        #ifndef MS_SLAVE
        #define MS_SLAVE                1<<19   /* Slave */
        #endif

        #ifndef MS_UNBINDABLE
        #define MS_UNBINDABLE           1<<17   /* Unbindable */
        #endif

        int main(int argc, char *argv[])
        {
                int type;
                if(argc != 3) {
                        fprintf(stderr, "usage: %s dir "
                        "<rshared|rslave|rprivate|runbindable|shared|slave"
                        "|private|unbindable>\n" , argv[0]);
                        return 1;
                }

                fprintf(stdout, "%s %s %s\n", argv[0], argv[1], argv[2]);

                if (strcmp(argv[2],"rshared")==0)
                        type=(MS_SHARED|MS_REC);
                else if (strcmp(argv[2],"rslave")==0)
                        type=(MS_SLAVE|MS_REC);
                else if (strcmp(argv[2],"rprivate")==0)
                        type=(MS_PRIVATE|MS_REC);
                else if (strcmp(argv[2],"runbindable")==0)
                        type=(MS_UNBINDABLE|MS_REC);
                else if (strcmp(argv[2],"shared")==0)
                        type=MS_SHARED;
                else if (strcmp(argv[2],"slave")==0)
                        type=MS_SLAVE;
                else if (strcmp(argv[2],"private")==0)
                        type=MS_PRIVATE;
                else if (strcmp(argv[2],"unbindable")==0)
                        type=MS_UNBINDABLE;
                else {
                        fprintf(stderr, "invalid operation: %s\n", argv[2]);
                        return 1;
                }
                setfsuid(getuid());

                if(mount("", argv[1], "dontcare", type, "") == -1) {
                        perror("mount");
                        return 1;
                }
                return 0;
        }
        -----------------------------------------------------------------------

        Copy the above code snippet into smount.c
        gcc -o smount smount.c


        (i) To mark all the mounts under /mnt as shared execute the following
        command:

                smount /mnt rshared
                the corresponding syntax planned for mount command is
                mount --make-rshared /mnt

            just to mark a mount /mnt as shared, execute the following
            command:
                smount /mnt shared
                the corresponding syntax planned for mount command is
                mount --make-shared /mnt

        (ii) To mark all the shared mounts under /mnt as slave execute the
        following

             command:
                smount /mnt rslave
                the corresponding syntax planned for mount command is
                mount --make-rslave /mnt

            just to mark a mount /mnt as slave, execute the following
            command:
                smount /mnt slave
                the corresponding syntax planned for mount command is
                mount --make-slave /mnt

        (iii) To mark all the mounts under /mnt as private execute the
        following command:

                smount /mnt rprivate
                the corresponding syntax planned for mount command is
                mount --make-rprivate /mnt

            just to mark a mount /mnt as private, execute the following
            command:
                smount /mnt private
                the corresponding syntax planned for mount command is
                mount --make-private /mnt

              NOTE: by default all the mounts are created as private. But if
              you want to change some shared/slave/unbindable  mount as
              private at a later point in time, this command can help.

        (iv) To mark all the mounts under /mnt as unbindable execute the
        following

             command:
                smount /mnt runbindable
                the corresponding syntax planned for mount command is
                mount --make-runbindable /mnt

            just to mark a mount /mnt as unbindable, execute the following
            command:
                smount /mnt unbindable
                the corresponding syntax planned for mount command is
                mount --make-unbindable /mnt


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, than 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 --bin /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: lets say we have the following mount tree.

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

             Lets 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: Lets say 'B1', 'B2', 'B3' are shared mounts that propagate to
        each other.

        lets say 'A1', 'A2', 'A3' are first mounted at dentry 'b' on mount
        'B1', 'B2' and 'B3' respectively.

        lets 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.

        Lets 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. Lets call this mount 'A'
                And then we have a second mount at /tmp1 with root
                dentry 2. Lets call this mount 'B'
                Next we have a third mount at /mnt with root dentry
                mnt. Lets 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?

                Lets 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:
                   lets 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 wont' 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:
                   lets 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.

        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 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)