From: J. Bruce Fields Date: Thu, 7 Feb 2008 08:13:35 +0000 (-0800) Subject: Documentation: move sharedsubtrees.txt to filesystems/ X-Git-Url: http://git.lede-project.org./?a=commitdiff_plain;h=d3cf91d0e201962a6367191e5926f5b0920b0339;p=openwrt%2Fstaging%2Fblogic.git Documentation: move sharedsubtrees.txt to filesystems/ This documentation is also vfs-related. Signed-off-by: J. Bruce Fields Acked-by: Randy Dunlap Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/Documentation/00-INDEX b/Documentation/00-INDEX index bb5e21034209..4d4dde447fe7 100644 --- a/Documentation/00-INDEX +++ b/Documentation/00-INDEX @@ -358,8 +358,6 @@ sgi-visws.txt - short blurb on the SGI Visual Workstations. sh/ - directory with info on porting Linux to a new architecture. -sharedsubtree.txt - - a description of shared subtrees for namespaces. smart-config.txt - description of the Smart Config makefile feature. sony-laptop.txt diff --git a/Documentation/filesystems/00-INDEX b/Documentation/filesystems/00-INDEX index 632fe3f376eb..e68021c08fbd 100644 --- a/Documentation/filesystems/00-INDEX +++ b/Documentation/filesystems/00-INDEX @@ -82,6 +82,8 @@ relay.txt - info on relay, for efficient streaming from kernel to user space. romfs.txt - description of the ROMFS filesystem. +sharedsubtree.txt + - a description of shared subtrees for namespaces. smbfs.txt - info on using filesystems with the SMB protocol (Win 3.11 and NT). spufs.txt diff --git a/Documentation/filesystems/sharedsubtree.txt b/Documentation/filesystems/sharedsubtree.txt new file mode 100644 index 000000000000..736540045dc7 --- /dev/null +++ b/Documentation/filesystems/sharedsubtree.txt @@ -0,0 +1,1061 @@ +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 + //and modified by Ram Pai + // sample usage: + // smount /tmp shared + // + #include + #include + #include + #include + #include + #include + + #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 " + "\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) diff --git a/Documentation/sharedsubtree.txt b/Documentation/sharedsubtree.txt deleted file mode 100644 index 736540045dc7..000000000000 --- a/Documentation/sharedsubtree.txt +++ /dev/null @@ -1,1061 +0,0 @@ -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 - //and modified by Ram Pai - // sample usage: - // smount /tmp shared - // - #include - #include - #include - #include - #include - #include - - #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 " - "\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)