docs: filesystems: convert sharedsubtree.txt to ReST
- Add a SPDX header;
- Adjust document and section titles;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add table markups;
- Add it to filesystems/index.rst
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/6692b8abc177130e9e53aace94117a2ad076cab5.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/filesystems/sharedsubtree.rst b/Documentation/filesystems/sharedsubtree.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+===============
+Shared Subtrees
+===============
+
+.. Contents:
+ 1) Overview
+ 2) Features
+ 3) Setting mount states
+ 4) Use-case
+ 5) Detailed semantics
+ 6) Quiz
+ 7) FAQ
+ 8) Implementation
+
+
+1) Overview
+-----------
+
+Consider the following situation:
+
+A process wants to clone its own namespace, but still wants to access the CD
+that got mounted recently. Shared subtree semantics provide the necessary
+mechanism to accomplish the above.
+
+It provides the necessary building blocks for features like per-user-namespace
+and versioned filesystem.
+
+2) Features
+-----------
+
+Shared subtree provides four different flavors of mounts; struct vfsmount to be
+precise
+
+ a. shared mount
+ b. slave mount
+ c. private mount
+ d. unbindable mount
+
+
+2a) A shared mount can be replicated to as many mountpoints and all the
+replicas continue to be exactly same.
+
+ Here is an example:
+
+ Let's say /mnt has a mount that is shared::
+
+ mount --make-shared /mnt
+
+ Note: mount(8) command now supports the --make-shared flag,
+ so the sample 'smount' program is no longer needed and has been
+ removed.
+
+ ::
+
+ # mount --bind /mnt /tmp
+
+ The above command replicates the mount at /mnt to the mountpoint /tmp
+ and the contents of both the mounts remain identical.
+
+ ::
+
+ #ls /mnt
+ a b c
+
+ #ls /tmp
+ a b c
+
+ Now let's say we mount a device at /tmp/a::
+
+ # mount /dev/sd0 /tmp/a
+
+ #ls /tmp/a
+ t1 t2 t3
+
+ #ls /mnt/a
+ t1 t2 t3
+
+ Note that the mount has propagated to the mount at /mnt as well.
+
+ And the same is true even when /dev/sd0 is mounted on /mnt/a. The
+ contents will be visible under /tmp/a too.
+
+
+2b) A slave mount is like a shared mount except that mount and umount events
+ only propagate towards it.
+
+ All slave mounts have a master mount which is a shared.
+
+ Here is an example:
+
+ Let's say /mnt has a mount which is shared.
+ # mount --make-shared /mnt
+
+ Let's bind mount /mnt to /tmp
+ # mount --bind /mnt /tmp
+
+ the new mount at /tmp becomes a shared mount and it is a replica of
+ the mount at /mnt.
+
+ Now let's make the mount at /tmp; a slave of /mnt
+ # mount --make-slave /tmp
+
+ let's mount /dev/sd0 on /mnt/a
+ # mount /dev/sd0 /mnt/a
+
+ #ls /mnt/a
+ t1 t2 t3
+
+ #ls /tmp/a
+ t1 t2 t3
+
+ Note the mount event has propagated to the mount at /tmp
+
+ However let's see what happens if we mount something on the mount at /tmp
+
+ # mount /dev/sd1 /tmp/b
+
+ #ls /tmp/b
+ s1 s2 s3
+
+ #ls /mnt/b
+
+ Note how the mount event has not propagated to the mount at
+ /mnt
+
+
+2c) A private mount does not forward or receive propagation.
+
+ This is the mount we are familiar with. Its the default type.
+
+
+2d) A unbindable mount is a unbindable private mount
+
+ let's say we have a mount at /mnt and we make it unbindable::
+
+ # mount --make-unbindable /mnt
+
+ Let's try to bind mount this mount somewhere else::
+
+ # mount --bind /mnt /tmp
+ mount: wrong fs type, bad option, bad superblock on /mnt,
+ or too many mounted file systems
+
+ Binding a unbindable mount is a invalid operation.
+
+
+3) Setting mount states
+
+ The mount command (util-linux package) can be used to set mount
+ states::
+
+ mount --make-shared mountpoint
+ mount --make-slave mountpoint
+ mount --make-private mountpoint
+ mount --make-unbindable mountpoint
+
+
+4) Use cases
+------------
+
+ A) A process wants to clone its own namespace, but still wants to
+ access the CD that got mounted recently.
+
+ Solution:
+
+ The system administrator can make the mount at /cdrom shared::
+
+ mount --bind /cdrom /cdrom
+ mount --make-shared /cdrom
+
+ Now any process that clones off a new namespace will have a
+ mount at /cdrom which is a replica of the same mount in the
+ parent namespace.
+
+ So when a CD is inserted and mounted at /cdrom that mount gets
+ propagated to the other mount at /cdrom in all the other clone
+ namespaces.
+
+ B) A process wants its mounts invisible to any other process, but
+ still be able to see the other system mounts.
+
+ Solution:
+
+ To begin with, the administrator can mark the entire mount tree
+ as shareable::
+
+ mount --make-rshared /
+
+ A new process can clone off a new namespace. And mark some part
+ of its namespace as slave::
+
+ mount --make-rslave /myprivatetree
+
+ Hence forth any mounts within the /myprivatetree done by the
+ process will not show up in any other namespace. However mounts
+ done in the parent namespace under /myprivatetree still shows
+ up in the process's namespace.
+
+
+ Apart from the above semantics this feature provides the
+ building blocks to solve the following problems:
+
+ C) Per-user namespace
+
+ The above semantics allows a way to share mounts across
+ namespaces. But namespaces are associated with processes. If
+ namespaces are made first class objects with user API to
+ associate/disassociate a namespace with userid, then each user
+ could have his/her own namespace and tailor it to his/her
+ requirements. This needs to be supported in PAM.
+
+ D) Versioned files
+
+ If the entire mount tree is visible at multiple locations, then
+ an underlying versioning file system can return different
+ versions of the file depending on the path used to access that
+ file.
+
+ An example is::
+
+ mount --make-shared /
+ mount --rbind / /view/v1
+ mount --rbind / /view/v2
+ mount --rbind / /view/v3
+ mount --rbind / /view/v4
+
+ and if /usr has a versioning filesystem mounted, then that
+ mount appears at /view/v1/usr, /view/v2/usr, /view/v3/usr and
+ /view/v4/usr too
+
+ A user can request v3 version of the file /usr/fs/namespace.c
+ by accessing /view/v3/usr/fs/namespace.c . The underlying
+ versioning filesystem can then decipher that v3 version of the
+ filesystem is being requested and return the corresponding
+ inode.
+
+5) Detailed semantics
+---------------------
+ The section below explains the detailed semantics of
+ bind, rbind, move, mount, umount and clone-namespace operations.
+
+ Note: the word 'vfsmount' and the noun 'mount' have been used
+ to mean the same thing, throughout this document.
+
+5a) Mount states
+
+ A given mount can be in one of the following states
+
+ 1) shared
+ 2) slave
+ 3) shared and slave
+ 4) private
+ 5) unbindable
+
+ A 'propagation event' is defined as event generated on a vfsmount
+ that leads to mount or unmount actions in other vfsmounts.
+
+ A 'peer group' is defined as a group of vfsmounts that propagate
+ events to each other.
+
+ (1) Shared mounts
+
+ A 'shared mount' is defined as a vfsmount that belongs to a
+ 'peer group'.
+
+ For example::
+
+ mount --make-shared /mnt
+ mount --bind /mnt /tmp
+
+ The mount at /mnt and that at /tmp are both shared and belong
+ to the same peer group. Anything mounted or unmounted under
+ /mnt or /tmp reflect in all the other mounts of its peer
+ group.
+
+
+ (2) Slave mounts
+
+ A 'slave mount' is defined as a vfsmount that receives
+ propagation events and does not forward propagation events.
+
+ A slave mount as the name implies has a master mount from which
+ mount/unmount events are received. Events do not propagate from
+ the slave mount to the master. Only a shared mount can be made
+ a slave by executing the following command::
+
+ mount --make-slave mount
+
+ A shared mount that is made as a slave is no more shared unless
+ modified to become shared.
+
+ (3) Shared and Slave
+
+ A vfsmount can be both shared as well as slave. This state
+ indicates that the mount is a slave of some vfsmount, and
+ has its own peer group too. This vfsmount receives propagation
+ events from its master vfsmount, and also forwards propagation
+ events to its 'peer group' and to its slave vfsmounts.
+
+ Strictly speaking, the vfsmount is shared having its own
+ peer group, and this peer-group is a slave of some other
+ peer group.
+
+ Only a slave vfsmount can be made as 'shared and slave' by
+ either executing the following command::
+
+ mount --make-shared mount
+
+ or by moving the slave vfsmount under a shared vfsmount.
+
+ (4) Private mount
+
+ A 'private mount' is defined as vfsmount that does not
+ receive or forward any propagation events.
+
+ (5) Unbindable mount
+
+ A 'unbindable mount' is defined as vfsmount that does not
+ receive or forward any propagation events and cannot
+ be bind mounted.
+
+
+ State diagram:
+
+ The state diagram below explains the state transition of a mount,
+ in response to various commands::
+
+ -----------------------------------------------------------------------
+ | |make-shared | make-slave | make-private |make-unbindab|
+ --------------|------------|--------------|--------------|-------------|
+ |shared |shared |*slave/private| private | unbindable |
+ | | | | | |
+ |-------------|------------|--------------|--------------|-------------|
+ |slave |shared | **slave | private | unbindable |
+ | |and slave | | | |
+ |-------------|------------|--------------|--------------|-------------|
+ |shared |shared | slave | private | unbindable |
+ |and slave |and slave | | | |
+ |-------------|------------|--------------|--------------|-------------|
+ |private |shared | **private | private | unbindable |
+ |-------------|------------|--------------|--------------|-------------|
+ |unbindable |shared |**unbindable | private | unbindable |
+ ------------------------------------------------------------------------
+
+ * if the shared mount is the only mount in its peer group, making it
+ slave, makes it private automatically. Note that there is no master to
+ which it can be slaved to.
+
+ ** slaving a non-shared mount has no effect on the mount.
+
+ Apart from the commands listed below, the 'move' operation also changes
+ the state of a mount depending on type of the destination mount. Its
+ explained in section 5d.
+
+5b) Bind semantics
+
+ Consider the following command::
+
+ mount --bind A/a B/b
+
+ where 'A' is the source mount, 'a' is the dentry in the mount 'A', 'B'
+ is the destination mount and 'b' is the dentry in the destination mount.
+
+ The outcome depends on the type of mount of 'A' and 'B'. The table
+ below contains quick reference::
+
+ --------------------------------------------------------------------------
+ | BIND MOUNT OPERATION |
+ |************************************************************************|
+ |source(A)->| shared | private | slave | unbindable |
+ | dest(B) | | | | |
+ | | | | | | |
+ | v | | | | |
+ |************************************************************************|
+ | shared | shared | shared | shared & slave | invalid |
+ | | | | | |
+ |non-shared| shared | private | slave | invalid |
+ **************************************************************************
+
+ Details:
+
+ 1. 'A' is a shared mount and 'B' is a shared mount. A new mount 'C'
+ which is clone of 'A', is created. Its root dentry is 'a' . 'C' is
+ mounted on mount 'B' at dentry 'b'. Also new mount 'C1', 'C2', 'C3' ...
+ are created and mounted at the dentry 'b' on all mounts where 'B'
+ propagates to. A new propagation tree containing 'C1',..,'Cn' is
+ created. This propagation tree is identical to the propagation tree of
+ 'B'. And finally the peer-group of 'C' is merged with the peer group
+ of 'A'.
+
+ 2. 'A' is a private mount and 'B' is a shared mount. A new mount 'C'
+ which is clone of 'A', is created. Its root dentry is 'a'. 'C' is
+ mounted on mount 'B' at dentry 'b'. Also new mount 'C1', 'C2', 'C3' ...
+ are created and mounted at the dentry 'b' on all mounts where 'B'
+ propagates to. A new propagation tree is set containing all new mounts
+ 'C', 'C1', .., 'Cn' with exactly the same configuration as the
+ propagation tree for 'B'.
+
+ 3. 'A' is a slave mount of mount 'Z' and 'B' is a shared mount. A new
+ mount 'C' which is clone of 'A', is created. Its root dentry is 'a' .
+ 'C' is mounted on mount 'B' at dentry 'b'. Also new mounts 'C1', 'C2',
+ 'C3' ... are created and mounted at the dentry 'b' on all mounts where
+ 'B' propagates to. A new propagation tree containing the new mounts
+ 'C','C1',.. 'Cn' is created. This propagation tree is identical to the
+ propagation tree for 'B'. And finally the mount 'C' and its peer group
+ is made the slave of mount 'Z'. In other words, mount 'C' is in the
+ state 'slave and shared'.
+
+ 4. 'A' is a unbindable mount and 'B' is a shared mount. This is a
+ invalid operation.
+
+ 5. 'A' is a private mount and 'B' is a non-shared(private or slave or
+ unbindable) mount. A new mount 'C' which is clone of 'A', is created.
+ Its root dentry is 'a'. 'C' is mounted on mount 'B' at dentry 'b'.
+
+ 6. 'A' is a shared mount and 'B' is a non-shared mount. A new mount 'C'
+ which is a clone of 'A' is created. Its root dentry is 'a'. 'C' is
+ mounted on mount 'B' at dentry 'b'. 'C' is made a member of the
+ peer-group of 'A'.
+
+ 7. 'A' is a slave mount of mount 'Z' and 'B' is a non-shared mount. A
+ new mount 'C' which is a clone of 'A' is created. Its root dentry is
+ 'a'. 'C' is mounted on mount 'B' at dentry 'b'. Also 'C' is set as a
+ slave mount of 'Z'. In other words 'A' and 'C' are both slave mounts of
+ 'Z'. All mount/unmount events on 'Z' propagates to 'A' and 'C'. But
+ mount/unmount on 'A' do not propagate anywhere else. Similarly
+ mount/unmount on 'C' do not propagate anywhere else.
+
+ 8. 'A' is a unbindable mount and 'B' is a non-shared mount. This is a
+ invalid operation. A unbindable mount cannot be bind mounted.
+
+5c) Rbind semantics
+
+ rbind is same as bind. Bind replicates the specified mount. Rbind
+ replicates all the mounts in the tree belonging to the specified mount.
+ Rbind mount is bind mount applied to all the mounts in the tree.
+
+ If the source tree that is rbind has some unbindable mounts,
+ then the subtree under the unbindable mount is pruned in the new
+ location.
+
+ eg:
+
+ let's say we have the following mount tree::
+
+ A
+ / \
+ B C
+ / \ / \
+ D E F G
+
+ Let's say all the mount except the mount C in the tree are
+ of a type other than unbindable.
+
+ If this tree is rbound to say Z
+
+ We will have the following tree at the new location::
+
+ Z
+ |
+ A'
+ /
+ B' Note how the tree under C is pruned
+ / \ in the new location.
+ D' E'
+
+
+
+5d) Move semantics
+
+ Consider the following command
+
+ mount --move A B/b
+
+ where 'A' is the source mount, 'B' is the destination mount and 'b' is
+ the dentry in the destination mount.
+
+ The outcome depends on the type of the mount of 'A' and 'B'. The table
+ below is a quick reference::
+
+ ---------------------------------------------------------------------------
+ | MOVE MOUNT OPERATION |
+ |**************************************************************************
+ | source(A)->| shared | private | slave | unbindable |
+ | dest(B) | | | | |
+ | | | | | | |
+ | v | | | | |
+ |**************************************************************************
+ | shared | shared | shared |shared and slave| invalid |
+ | | | | | |
+ |non-shared| shared | private | slave | unbindable |
+ ***************************************************************************
+
+ .. Note:: moving a mount residing under a shared mount is invalid.
+
+ Details follow:
+
+ 1. 'A' is a shared mount and 'B' is a shared mount. The mount 'A' is
+ mounted on mount 'B' at dentry 'b'. Also new mounts 'A1', 'A2'...'An'
+ are created and mounted at dentry 'b' on all mounts that receive
+ propagation from mount 'B'. A new propagation tree is created in the
+ exact same configuration as that of 'B'. This new propagation tree
+ contains all the new mounts 'A1', 'A2'... 'An'. And this new
+ propagation tree is appended to the already existing propagation tree
+ of 'A'.
+
+ 2. 'A' is a private mount and 'B' is a shared mount. The mount 'A' is
+ mounted on mount 'B' at dentry 'b'. Also new mount 'A1', 'A2'... 'An'
+ are created and mounted at dentry 'b' on all mounts that receive
+ propagation from mount 'B'. The mount 'A' becomes a shared mount and a
+ propagation tree is created which is identical to that of
+ 'B'. This new propagation tree contains all the new mounts 'A1',
+ 'A2'... 'An'.
+
+ 3. 'A' is a slave mount of mount 'Z' and 'B' is a shared mount. The
+ mount 'A' is mounted on mount 'B' at dentry 'b'. Also new mounts 'A1',
+ 'A2'... 'An' are created and mounted at dentry 'b' on all mounts that
+ receive propagation from mount 'B'. A new propagation tree is created
+ in the exact same configuration as that of 'B'. This new propagation
+ tree contains all the new mounts 'A1', 'A2'... 'An'. And this new
+ propagation tree is appended to the already existing propagation tree of
+ 'A'. Mount 'A' continues to be the slave mount of 'Z' but it also
+ becomes 'shared'.
+
+ 4. 'A' is a unbindable mount and 'B' is a shared mount. The operation
+ is invalid. Because mounting anything on the shared mount 'B' can
+ create new mounts that get mounted on the mounts that receive
+ propagation from 'B'. And since the mount 'A' is unbindable, cloning
+ it to mount at other mountpoints is not possible.
+
+ 5. 'A' is a private mount and 'B' is a non-shared(private or slave or
+ unbindable) mount. The mount 'A' is mounted on mount 'B' at dentry 'b'.
+
+ 6. 'A' is a shared mount and 'B' is a non-shared mount. The mount 'A'
+ is mounted on mount 'B' at dentry 'b'. Mount 'A' continues to be a
+ shared mount.
+
+ 7. 'A' is a slave mount of mount 'Z' and 'B' is a non-shared mount.
+ The mount 'A' is mounted on mount 'B' at dentry 'b'. Mount 'A'
+ continues to be a slave mount of mount 'Z'.
+
+ 8. 'A' is a unbindable mount and 'B' is a non-shared mount. The mount
+ 'A' is mounted on mount 'B' at dentry 'b'. Mount 'A' continues to be a
+ unbindable mount.
+
+5e) Mount semantics
+
+ Consider the following command::
+
+ mount device B/b
+
+ 'B' is the destination mount and 'b' is the dentry in the destination
+ mount.
+
+ The above operation is the same as bind operation with the exception
+ that the source mount is always a private mount.
+
+
+5f) Unmount semantics
+
+ Consider the following command::
+
+ umount A
+
+ where 'A' is a mount mounted on mount 'B' at dentry 'b'.
+
+ If mount 'B' is shared, then all most-recently-mounted mounts at dentry
+ 'b' on mounts that receive propagation from mount 'B' and does not have
+ sub-mounts within them are unmounted.
+
+ Example: Let's say 'B1', 'B2', 'B3' are shared mounts that propagate to
+ each other.
+
+ let's say 'A1', 'A2', 'A3' are first mounted at dentry 'b' on mount
+ 'B1', 'B2' and 'B3' respectively.
+
+ let's say 'C1', 'C2', 'C3' are next mounted at the same dentry 'b' on
+ mount 'B1', 'B2' and 'B3' respectively.
+
+ if 'C1' is unmounted, all the mounts that are most-recently-mounted on
+ 'B1' and on the mounts that 'B1' propagates-to are unmounted.
+
+ 'B1' propagates to 'B2' and 'B3'. And the most recently mounted mount
+ on 'B2' at dentry 'b' is 'C2', and that of mount 'B3' is 'C3'.
+
+ So all 'C1', 'C2' and 'C3' should be unmounted.
+
+ If any of 'C2' or 'C3' has some child mounts, then that mount is not
+ unmounted, but all other mounts are unmounted. However if 'C1' is told
+ to be unmounted and 'C1' has some sub-mounts, the umount operation is
+ failed entirely.
+
+5g) Clone Namespace
+
+ A cloned namespace contains all the mounts as that of the parent
+ namespace.
+
+ Let's say 'A' and 'B' are the corresponding mounts in the parent and the
+ child namespace.
+
+ If 'A' is shared, then 'B' is also shared and 'A' and 'B' propagate to
+ each other.
+
+ If 'A' is a slave mount of 'Z', then 'B' is also the slave mount of
+ 'Z'.
+
+ If 'A' is a private mount, then 'B' is a private mount too.
+
+ If 'A' is unbindable mount, then 'B' is a unbindable mount too.
+
+
+6) Quiz
+
+ A. What is the result of the following command sequence?
+
+ ::
+
+ mount --bind /mnt /mnt
+ mount --make-shared /mnt
+ mount --bind /mnt /tmp
+ mount --move /tmp /mnt/1
+
+ what should be the contents of /mnt /mnt/1 /mnt/1/1 should be?
+ Should they all be identical? or should /mnt and /mnt/1 be
+ identical only?
+
+
+ B. What is the result of the following command sequence?
+
+ ::
+
+ mount --make-rshared /
+ mkdir -p /v/1
+ mount --rbind / /v/1
+
+ what should be the content of /v/1/v/1 be?
+
+
+ C. What is the result of the following command sequence?
+
+ ::
+
+ mount --bind /mnt /mnt
+ mount --make-shared /mnt
+ mkdir -p /mnt/1/2/3 /mnt/1/test
+ mount --bind /mnt/1 /tmp
+ mount --make-slave /mnt
+ mount --make-shared /mnt
+ mount --bind /mnt/1/2 /tmp1
+ mount --make-slave /mnt
+
+ At this point we have the first mount at /tmp and
+ its root dentry is 1. Let's call this mount 'A'
+ And then we have a second mount at /tmp1 with root
+ dentry 2. Let's call this mount 'B'
+ Next we have a third mount at /mnt with root dentry
+ mnt. Let's call this mount 'C'
+
+ 'B' is the slave of 'A' and 'C' is a slave of 'B'
+ A -> B -> C
+
+ at this point if we execute the following command
+
+ mount --bind /bin /tmp/test
+
+ The mount is attempted on 'A'
+
+ will the mount propagate to 'B' and 'C' ?
+
+ what would be the contents of
+ /mnt/1/test be?
+
+7) FAQ
+
+ Q1. Why is bind mount needed? How is it different from symbolic links?
+ symbolic links can get stale if the destination mount gets
+ unmounted or moved. Bind mounts continue to exist even if the
+ other mount is unmounted or moved.
+
+ Q2. Why can't the shared subtree be implemented using exportfs?
+
+ exportfs is a heavyweight way of accomplishing part of what
+ shared subtree can do. I cannot imagine a way to implement the
+ semantics of slave mount using exportfs?
+
+ Q3 Why is unbindable mount needed?
+
+ Let's say we want to replicate the mount tree at multiple
+ locations within the same subtree.
+
+ if one rbind mounts a tree within the same subtree 'n' times
+ the number of mounts created is an exponential function of 'n'.
+ Having unbindable mount can help prune the unneeded bind
+ mounts. Here is an example.
+
+ step 1:
+ let's say the root tree has just two directories with
+ one vfsmount::
+
+ root
+ / \
+ tmp usr
+
+ And we want to replicate the tree at multiple
+ mountpoints under /root/tmp
+
+ step 2:
+ ::
+
+
+ mount --make-shared /root
+
+ mkdir -p /tmp/m1
+
+ mount --rbind /root /tmp/m1
+
+ the new tree now looks like this::
+
+ root
+ / \
+ tmp usr
+ /
+ m1
+ / \
+ tmp usr
+ /
+ m1
+
+ it has two vfsmounts
+
+ step 3:
+ ::
+
+ mkdir -p /tmp/m2
+ mount --rbind /root /tmp/m2
+
+ the new tree now looks like this::
+
+ root
+ / \
+ tmp usr
+ / \
+ m1 m2
+ / \ / \
+ tmp usr tmp usr
+ / \ /
+ m1 m2 m1
+ / \ / \
+ tmp usr tmp usr
+ / / \
+ m1 m1 m2
+ / \
+ tmp usr
+ / \
+ m1 m2
+
+ it has 6 vfsmounts
+
+ step 4:
+ ::
+ mkdir -p /tmp/m3
+ mount --rbind /root /tmp/m3
+
+ I won't draw the tree..but it has 24 vfsmounts
+
+
+ at step i the number of vfsmounts is V[i] = i*V[i-1].
+ This is an exponential function. And this tree has way more
+ mounts than what we really needed in the first place.
+
+ One could use a series of umount at each step to prune
+ out the unneeded mounts. But there is a better solution.
+ Unclonable mounts come in handy here.
+
+ step 1:
+ let's say the root tree has just two directories with
+ one vfsmount::
+
+ root
+ / \
+ tmp usr
+
+ How do we set up the same tree at multiple locations under
+ /root/tmp
+
+ step 2:
+ ::
+
+
+ mount --bind /root/tmp /root/tmp
+
+ mount --make-rshared /root
+ mount --make-unbindable /root/tmp
+
+ mkdir -p /tmp/m1
+
+ mount --rbind /root /tmp/m1
+
+ the new tree now looks like this::
+
+ root
+ / \
+ tmp usr
+ /
+ m1
+ / \
+ tmp usr
+
+ step 3:
+ ::
+
+ mkdir -p /tmp/m2
+ mount --rbind /root /tmp/m2
+
+ the new tree now looks like this::
+
+ root
+ / \
+ tmp usr
+ / \
+ m1 m2
+ / \ / \
+ tmp usr tmp usr
+
+ step 4:
+ ::
+
+ mkdir -p /tmp/m3
+ mount --rbind /root /tmp/m3
+
+ the new tree now looks like this::
+
+ root
+ / \
+ tmp usr
+ / \ \
+ m1 m2 m3
+ / \ / \ / \
+ tmp usr tmp usr tmp usr
+
+8) Implementation
+
+8A) Datastructure
+
+ 4 new fields are introduced to struct vfsmount:
+
+ * ->mnt_share
+ * ->mnt_slave_list
+ * ->mnt_slave
+ * ->mnt_master
+
+ ->mnt_share
+ links together all the mount to/from which this vfsmount
+ send/receives propagation events.
+
+ ->mnt_slave_list
+ links all the mounts to which this vfsmount propagates
+ to.
+
+ ->mnt_slave
+ links together all the slaves that its master vfsmount
+ propagates to.
+
+ ->mnt_master
+ points to the master vfsmount from which this vfsmount
+ receives propagation.
+
+ ->mnt_flags
+ takes two more flags to indicate the propagation status of
+ the vfsmount. MNT_SHARE indicates that the vfsmount is a shared
+ vfsmount. MNT_UNCLONABLE indicates that the vfsmount cannot be
+ replicated.
+
+ All the shared vfsmounts in a peer group form a cyclic list through
+ ->mnt_share.
+
+ All vfsmounts with the same ->mnt_master form on a cyclic list anchored
+ in ->mnt_master->mnt_slave_list and going through ->mnt_slave.
+
+ ->mnt_master can point to arbitrary (and possibly different) members
+ of master peer group. To find all immediate slaves of a peer group
+ you need to go through _all_ ->mnt_slave_list of its members.
+ Conceptually it's just a single set - distribution among the
+ individual lists does not affect propagation or the way propagation
+ tree is modified by operations.
+
+ All vfsmounts in a peer group have the same ->mnt_master. If it is
+ non-NULL, they form a contiguous (ordered) segment of slave list.
+
+ A example propagation tree looks as shown in the figure below.
+ [ NOTE: Though it looks like a forest, if we consider all the shared
+ mounts as a conceptual entity called 'pnode', it becomes a tree]::
+
+
+ A <--> B <--> C <---> D
+ /|\ /| |\
+ / F G J K H I
+ /
+ E<-->K
+ /|\
+ M L N
+
+ In the above figure A,B,C and D all are shared and propagate to each
+ other. 'A' has got 3 slave mounts 'E' 'F' and 'G' 'C' has got 2 slave
+ mounts 'J' and 'K' and 'D' has got two slave mounts 'H' and 'I'.
+ 'E' is also shared with 'K' and they propagate to each other. And
+ 'K' has 3 slaves 'M', 'L' and 'N'
+
+ A's ->mnt_share links with the ->mnt_share of 'B' 'C' and 'D'
+
+ A's ->mnt_slave_list links with ->mnt_slave of 'E', 'K', 'F' and 'G'
+
+ E's ->mnt_share links with ->mnt_share of K
+
+ 'E', 'K', 'F', 'G' have their ->mnt_master point to struct vfsmount of 'A'
+
+ 'M', 'L', 'N' have their ->mnt_master point to struct vfsmount of 'K'
+
+ K's ->mnt_slave_list links with ->mnt_slave of 'M', 'L' and 'N'
+
+ C's ->mnt_slave_list links with ->mnt_slave of 'J' and 'K'
+
+ J and K's ->mnt_master points to struct vfsmount of C
+
+ and finally D's ->mnt_slave_list links with ->mnt_slave of 'H' and 'I'
+
+ 'H' and 'I' have their ->mnt_master pointing to struct vfsmount of 'D'.
+
+
+ NOTE: The propagation tree is orthogonal to the mount tree.
+
+8B Locking:
+
+ ->mnt_share, ->mnt_slave, ->mnt_slave_list, ->mnt_master are protected
+ by namespace_sem (exclusive for modifications, shared for reading).
+
+ Normally we have ->mnt_flags modifications serialized by vfsmount_lock.
+ There are two exceptions: do_add_mount() and clone_mnt().
+ The former modifies a vfsmount that has not been visible in any shared
+ data structures yet.
+ The latter holds namespace_sem and the only references to vfsmount
+ are in lists that can't be traversed without namespace_sem.
+
+8C Algorithm:
+
+ The crux of the implementation resides in rbind/move operation.
+
+ The overall algorithm breaks the operation into 3 phases: (look at
+ attach_recursive_mnt() and propagate_mnt())
+
+ 1. prepare phase.
+ 2. commit phases.
+ 3. abort phases.
+
+ Prepare phase:
+
+ for each mount in the source tree:
+
+ a) Create the necessary number of mount trees to
+ be attached to each of the mounts that receive
+ propagation from the destination mount.
+ b) Do not attach any of the trees to its destination.
+ However note down its ->mnt_parent and ->mnt_mountpoint
+ c) Link all the new mounts to form a propagation tree that
+ is identical to the propagation tree of the destination
+ mount.
+
+ If this phase is successful, there should be 'n' new
+ propagation trees; where 'n' is the number of mounts in the
+ source tree. Go to the commit phase
+
+ Also there should be 'm' new mount trees, where 'm' is
+ the number of mounts to which the destination mount
+ propagates to.
+
+ if any memory allocations fail, go to the abort phase.
+
+ Commit phase
+ attach each of the mount trees to their corresponding
+ destination mounts.
+
+ Abort phase
+ delete all the newly created trees.
+
+ .. Note::
+ all the propagation related functionality resides in the file pnode.c
+
+
+------------------------------------------------------------------------
+
+version 0.1 (created the initial document, Ram Pai linuxram@us.ibm.com)
+
+version 0.2 (Incorporated comments from Al Viro)