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diff --git a/Documentation/filesystems/sharedsubtree.rst b/Documentation/filesystems/sharedsubtree.rst index 1cf56489ed48..8b7dc9159083 100644 --- a/Documentation/filesystems/sharedsubtree.rst +++ b/Documentation/filesystems/sharedsubtree.rst @@ -31,965 +31,960 @@ and versioned filesystem. ----------- Shared subtree provides four different flavors of mounts; struct vfsmount to be -precise +precise: - a. shared mount - b. slave mount - c. private mount - d. unbindable mount +a) A **shared mount** can be replicated to as many mountpoints and all the + replicas continue to be exactly same. -2a) A shared mount can be replicated to as many mountpoints and all the -replicas continue to be exactly same. + Here is an example: - Here is an example: + Let's say /mnt has a mount that is shared:: - Let's say /mnt has a mount that is shared:: + # mount --make-shared /mnt - 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. - 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 - # 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. - 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 /mnt - a b c + #ls /tmp + a b c - #ls /tmp - a b c + Now let's say we mount a device at /tmp/a:: - Now let's say we mount a device at /tmp/a:: + # mount /dev/sd0 /tmp/a - # mount /dev/sd0 /tmp/a + # ls /tmp/a + t1 t2 t3 - #ls /tmp/a - t1 t2 t3 + # ls /mnt/a + t1 t2 t3 - #ls /mnt/a - t1 t2 t3 + Note that the mount has propagated to the mount at /mnt as well. - 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. - And the same is true even when /dev/sd0 is mounted on /mnt/a. The - contents will be visible under /tmp/a too. +b) A **slave mount** is like a shared mount except that mount and umount events + only propagate towards it. -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. - All slave mounts have a master mount which is a shared. + Here is an example: - Here is an example: + Let's say /mnt has a mount which is shared:: - Let's say /mnt has a mount which is shared. - # mount --make-shared /mnt + # mount --make-shared /mnt - Let's bind mount /mnt to /tmp - # mount --bind /mnt /tmp + Let's bind mount /mnt to /tmp:: - the new mount at /tmp becomes a shared mount and it is a replica of - the mount at /mnt. + # mount --bind /mnt /tmp - Now let's make the mount at /tmp; a slave of /mnt - # mount --make-slave /tmp + the new mount at /tmp becomes a shared mount and it is a replica of + the mount at /mnt. - let's mount /dev/sd0 on /mnt/a - # mount /dev/sd0 /mnt/a + Now let's make the mount at /tmp; a slave of /mnt:: - #ls /mnt/a - t1 t2 t3 + # mount --make-slave /tmp - #ls /tmp/a - t1 t2 t3 + let's mount /dev/sd0 on /mnt/a:: - Note the mount event has propagated to the mount at /tmp + # mount /dev/sd0 /mnt/a - However let's see what happens if we mount something on the mount at /tmp + # ls /mnt/a + t1 t2 t3 - # mount /dev/sd1 /tmp/b + # ls /tmp/a + t1 t2 t3 - #ls /tmp/b - s1 s2 s3 + Note the mount event has propagated to the mount at /tmp - #ls /mnt/b + However let's see what happens if we mount something on the mount at + /tmp:: - Note how the mount event has not propagated to the mount at - /mnt + # mount /dev/sd1 /tmp/b + # ls /tmp/b + s1 s2 s3 -2c) A private mount does not forward or receive propagation. + # ls /mnt/b - This is the mount we are familiar with. Its the default type. + Note how the mount event has not propagated to the mount at + /mnt -2d) A unbindable mount is a unbindable private mount +c) A **private mount** does not forward or receive propagation. - let's say we have a mount at /mnt and we make it unbindable:: + This is the mount we are familiar with. Its the default type. - # mount --make-unbindable /mnt - Let's try to bind mount this mount somewhere else:: +d) An **unbindable mount** is, as the name suggests, an unbindable private + mount. - # mount --bind /mnt /tmp - mount: wrong fs type, bad option, bad superblock on /mnt, - or too many mounted file systems + let's say we have a mount at /mnt and we make it unbindable:: - Binding a unbindable mount is a invalid operation. + # 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:: +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 + 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. +A) A process wants to clone its own namespace, but still wants to + access the CD that got mounted recently. - Solution: + Solution: - The system administrator can make the mount at /cdrom shared:: + The system administrator can make the mount at /cdrom shared:: - mount --bind /cdrom /cdrom - mount --make-shared /cdrom + 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. + 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. + 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. +B) A process wants its mounts invisible to any other process, but + still be able to see the other system mounts. - Solution: + Solution: - To begin with, the administrator can mark the entire mount tree - as shareable:: + To begin with, the administrator can mark the entire mount tree + as shareable:: - mount --make-rshared / + mount --make-rshared / - A new process can clone off a new namespace. And mark some part - of its namespace as slave:: + A new process can clone off a new namespace. And mark some part + of its namespace as slave:: - mount --make-rslave /myprivatetree + 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. + 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: +Apart from the above semantics this feature provides the +building blocks to solve the following problems: - C) Per-user namespace +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. + 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 +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. + 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:: + An example is:: - mount --make-shared / - mount --rbind / /view/v1 - mount --rbind / /view/v2 - mount --rbind / /view/v3 - mount --rbind / /view/v4 + 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 + 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. + 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. +The section below explains the detailed semantics of +bind, rbind, move, mount, umount and clone-namespace operations. -5a) Mount states +.. Note:: + the word 'vfsmount' and the noun 'mount' have been used + to mean the same thing, throughout this document. - A given mount can be in one of the following states +a) Mount 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 '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. - A 'peer group' is defined as a group of vfsmounts that propagate - events to each other. + A given mount can be in one of the following states: - (1) Shared mounts + (1) Shared mounts - A 'shared mount' is defined as a vfsmount that belongs to a - 'peer group'. + A **shared mount** is defined as a vfsmount that belongs to a + peer group. - For example:: + For example:: - mount --make-shared /mnt - mount --bind /mnt /tmp + 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. + 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 + (2) Slave mounts - A 'slave mount' is defined as a vfsmount that receives - propagation events and does not forward propagation events. + 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:: + 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 + mount --make-slave mount - A shared mount that is made as a slave is no more shared unless - modified to become shared. + A shared mount that is made as a slave is no more shared unless + modified to become shared. - (3) Shared and Slave + (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. + 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. + 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:: + Only a slave vfsmount can be made as 'shared and slave' by + either executing the following command:: - mount --make-shared mount + mount --make-shared mount - or by moving the slave vfsmount under a shared vfsmount. + or by moving the slave vfsmount under a shared vfsmount. - (4) Private mount + (4) Private mount - A 'private mount' is defined as vfsmount that does not - receive or forward any propagation events. + A **private mount** is defined as vfsmount that does not + receive or forward any propagation events. - (5) Unbindable mount + (5) Unbindable mount - A 'unbindable mount' is defined as vfsmount that does not - receive or forward any propagation events and cannot - be bind mounted. + A **unbindable mount** is defined as vfsmount that does not + receive or forward any propagation events and cannot + be bind mounted. - State diagram: + State diagram: - The state diagram below explains the state transition of a mount, - in response to various commands:: + 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 | - ------------------------------------------------------------------------ + ----------------------------------------------------------------------- + | |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. + * 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. + ** 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. + 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 +b) Bind semantics - Consider the following command:: + Consider the following command:: - mount --bind A/a B/b + 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. + 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:: + 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 | - ************************************************************************** + -------------------------------------------------------------------------- + | 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: + 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'. + 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'. + 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 + 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. + +c) 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' + + + +d) 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. + 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:: + 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 | - *************************************************************************** + --------------------------------------------------------------------------- + | 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. + .. Note:: moving a mount residing under a shared mount is invalid. - Details follow: + 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'. + 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'. + 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'. + 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. + 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'. + 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. + 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'. + 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. + 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 +e) Mount semantics - Consider the following command:: + Consider the following command:: - mount device B/b + mount device B/b - 'B' is the destination mount and 'b' is the dentry in the destination - mount. + '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. + The above operation is the same as bind operation with the exception + that the source mount is always a private mount. -5f) Unmount semantics +f) Unmount semantics - Consider the following command:: + Consider the following command:: - umount A + umount A - where 'A' is a mount mounted on mount 'B' at dentry 'b'. + 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. + 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. + 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 '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. + 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. + 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'. + '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. + 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. + 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 +g) Clone Namespace - A cloned namespace contains all the mounts as that of the parent - 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. + 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 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 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 a private mount, then 'B' is a private mount too. - If 'A' is unbindable mount, then 'B' is a unbindable 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? +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 + 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? + 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? +B. What is the result of the following command sequence? - :: + :: - mount --make-rshared / - mkdir -p /v/1 - mount --rbind / /v/1 + mount --make-rshared / + mkdir -p /v/1 + mount --rbind / /v/1 - what should be the content of /v/1/v/1 be? + what should be the content of /v/1/v/1 be? - C. What is the result of the following command sequence? +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 + 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' + 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 + '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 + at this point if we execute the following command:: - mount --bind /bin /tmp/test + mount --bind /bin /tmp/test - The mount is attempted on 'A' + The mount is attempted on 'A' - will the mount propagate to 'B' and 'C' ? + will the mount propagate to 'B' and 'C' ? - what would be the contents of - /mnt/1/test be? + 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. +1. Why is bind mount needed? How is it different from symbolic links? - Q2. Why can't the shared subtree be implemented using exportfs? + 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. - 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? +2. Why can't the shared subtree be implemented using exportfs? - Q3 Why is unbindable mount needed? + 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? - Let's say we want to replicate the mount tree at multiple - locations within the same subtree. +3. Why is unbindable mount needed? - 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. + Let's say we want to replicate the mount tree at multiple + locations within the same subtree. - step 1: - let's say the root tree has just two directories with - one vfsmount:: + 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. - root - / \ - tmp usr + step 1: + let's say the root tree has just two directories with + one vfsmount:: - And we want to replicate the tree at multiple - mountpoints under /root/tmp + root + / \ + tmp usr - step 2: - :: + And we want to replicate the tree at multiple + mountpoints under /root/tmp + step 2: + :: - mount --make-shared /root - mkdir -p /tmp/m1 + mount --make-shared /root - mount --rbind /root /tmp/m1 + mkdir -p /tmp/m1 - the new tree now looks like this:: + mount --rbind /root /tmp/m1 - root - / \ - tmp usr - / - m1 - / \ - tmp usr - / - m1 + the new tree now looks like this:: - it has two vfsmounts + root + / \ + tmp usr + / + m1 + / \ + tmp usr + / + m1 - step 3: - :: + it has two vfsmounts - mkdir -p /tmp/m2 - mount --rbind /root /tmp/m2 + step 3: + :: - the new tree now looks like this:: + mkdir -p /tmp/m2 + mount --rbind /root /tmp/m2 - root - / \ - tmp usr - / \ - m1 m2 - / \ / \ - tmp usr tmp usr - / \ / - m1 m2 m1 - / \ / \ - tmp usr tmp usr - / / \ - m1 m1 m2 - / \ - tmp usr - / \ - m1 m2 + the new tree now looks like this:: - it has 6 vfsmounts + root + / \ + tmp usr + / \ + m1 m2 + / \ / \ + tmp usr tmp usr + / \ / + m1 m2 m1 + / \ / \ + tmp usr tmp usr + / / \ + m1 m1 m2 + / \ + tmp usr + / \ + m1 m2 - step 4: - :: - mkdir -p /tmp/m3 - mount --rbind /root /tmp/m3 + it has 6 vfsmounts - I won't draw the tree..but it has 24 vfsmounts + step 4: + :: + mkdir -p /tmp/m3 + mount --rbind /root /tmp/m3 - 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. + I won't draw the tree..but it has 24 vfsmounts - 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:: + 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. - root - / \ - tmp usr + 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. - How do we set up the same tree at multiple locations under - /root/tmp + step 1: + let's say the root tree has just two directories with + one vfsmount:: - step 2: - :: + root + / \ + tmp usr + How do we set up the same tree at multiple locations under + /root/tmp - mount --bind /root/tmp /root/tmp + step 2: + :: - mount --make-rshared /root - mount --make-unbindable /root/tmp - mkdir -p /tmp/m1 + mount --bind /root/tmp /root/tmp - mount --rbind /root /tmp/m1 + mount --make-rshared /root + mount --make-unbindable /root/tmp - the new tree now looks like this:: + mkdir -p /tmp/m1 - root - / \ - tmp usr - / - m1 - / \ - tmp usr + mount --rbind /root /tmp/m1 - step 3: - :: + the new tree now looks like this:: - mkdir -p /tmp/m2 - mount --rbind /root /tmp/m2 + root + / \ + tmp usr + / + m1 + / \ + tmp usr - the new tree now looks like this:: + step 3: + :: - root - / \ - tmp usr - / \ - m1 m2 - / \ / \ - tmp usr tmp usr + mkdir -p /tmp/m2 + mount --rbind /root /tmp/m2 - step 4: - :: + the new tree now looks like this:: - mkdir -p /tmp/m3 - mount --rbind /root /tmp/m3 + root + / \ + tmp usr + / \ + m1 m2 + / \ / \ + tmp usr tmp usr - the new tree now looks like this:: + step 4: + :: - root - / \ - tmp usr - / \ \ - m1 m2 m3 - / \ / \ / \ - tmp usr tmp usr tmp usr + 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 +A) Datastructure + + Several new fields are introduced to struct vfsmount: + + ->mnt_share + Links together all the mount to/from which this vfsmount + send/receives propagation events. - 4 new fields are introduced to struct vfsmount: + ->mnt_slave_list + Links all the mounts to which this vfsmount propagates + to. - * ->mnt_share - * ->mnt_slave_list - * ->mnt_slave - * ->mnt_master + ->mnt_slave + Links together all the slaves that its master vfsmount + propagates to. - ->mnt_share - links together all the mount to/from which this vfsmount - send/receives propagation events. + ->mnt_master + Points to the master vfsmount from which this vfsmount + receives propagation. - ->mnt_slave_list - links all the mounts to which this vfsmount propagates - to. + ->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. - ->mnt_slave - links together all the slaves that its master vfsmount - propagates to. + All the shared vfsmounts in a peer group form a cyclic list through + ->mnt_share. - ->mnt_master - points to the master vfsmount from which this vfsmount - receives propagation. + 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_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. + ->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 the shared vfsmounts in a peer group form a cyclic list through - ->mnt_share. + 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. - All vfsmounts with the same ->mnt_master form on a cyclic list anchored - in ->mnt_master->mnt_slave_list and going through ->mnt_slave. + A example propagation tree looks as shown in the figure below. - ->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. + .. note:: + Though it looks like a forest, if we consider all the shared + mounts as a conceptual entity called 'pnode', it becomes a tree. - 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 - 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' - 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_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' - 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's ->mnt_share links with ->mnt_share of K + 'E', 'K', 'F', 'G' have their ->mnt_master point to struct vfsmount of 'A' - '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' - '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' - 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' - 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 - 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' - 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'. - 'H' and 'I' have their ->mnt_master pointing to struct vfsmount of 'D'. + NOTE: The propagation tree is orthogonal to the mount tree. - NOTE: The propagation tree is orthogonal to the mount tree. +B) Locking: -8B Locking: + ->mnt_share, ->mnt_slave, ->mnt_slave_list, ->mnt_master are protected + by namespace_sem (exclusive for modifications, shared for reading). - ->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. - 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. +C) Algorithm: -8C Algorithm: + The crux of the implementation resides in rbind/move operation. - 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()) - The overall algorithm breaks the operation into 3 phases: (look at - attach_recursive_mnt() and propagate_mnt()) + 1. Prepare phase. - 1. prepare phase. - 2. commit phases. - 3. abort phases. + For each mount in the source tree: - Prepare phase: + 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. - for each mount in the source tree: + 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 - 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. + Also there should be 'm' new mount trees, where 'm' is + the number of mounts to which the destination mount + propagates to. - 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 + If any memory allocations fail, go to the abort phase. - Also there should be 'm' new mount trees, where 'm' is - the number of mounts to which the destination mount - propagates to. + 2. Commit phase. - if any memory allocations fail, go to the abort phase. + Attach each of the mount trees to their corresponding + destination mounts. - Commit phase - attach each of the mount trees to their corresponding - destination mounts. + 3. Abort phase. - Abort phase - delete all the newly created trees. + Delete all the newly created trees. - .. Note:: - all the propagation related functionality resides in the file pnode.c + .. Note:: + all the propagation related functionality resides in the file pnode.c ------------------------------------------------------------------------ |