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  2.6.25-rc4
Documentation
 sharedsubtree.txt
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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 <string.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)
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