[PATCH] Complete description of shared subtrees.

Signed-off-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 1060 insertions, 0 deletions

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+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 togather 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 togather 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)