Here is an example:
- Lets say /mnt has a mount that is shared.
+ Let's 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'
+ 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.
#ls /tmp
a b c
- Now lets say we mount a device at /tmp/a
- #mount /dev/sd0 /tmp/a
+ Now let's say we mount a device at /tmp/a
+ # mount /dev/sd0 /tmp/a
#ls /tmp/a
t1 t2 t2
Here is an example:
- Lets say /mnt has a mount which is shared.
- #mount --make-shared /mnt
+ Let's say /mnt has a mount which is shared.
+ # mount --make-shared /mnt
- Lets bind mount /mnt to /tmp
- #mount --bind /mnt /tmp
+ Let's bind mount /mnt to /tmp
+ # mount --bind /mnt /tmp
the new mount at /tmp becomes a shared mount and it is a replica of
the mount at /mnt.
- Now lets make the mount at /tmp; a slave of /mnt
- #mount --make-slave /tmp
- [or smount /tmp slave]
+ Now let's make the mount at /tmp; a slave of /mnt
+ # mount --make-slave /tmp
- lets mount /dev/sd0 on /mnt/a
- #mount /dev/sd0 /mnt/a
+ let's mount /dev/sd0 on /mnt/a
+ # mount /dev/sd0 /mnt/a
#ls /mnt/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
+ However let's see what happens if we mount something on the mount at /tmp
- #mount /dev/sd1 /tmp/b
+ # mount /dev/sd1 /tmp/b
#ls /tmp/b
s1 s2 s3
2d) A unbindable mount is a unbindable private mount
- lets say we have a mount at /mnt and we make is unbindable
+ let's say we have a mount at /mnt and we make is unbindable
- #mount --make-unbindable /mnt
- [ smount /mnt unbindable ]
+ # mount --make-unbindable /mnt
- Lets try to bind mount this mount somewhere else.
+ 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
3) smount command
- Currently the mount command is not aware of shared subtree features.
- Work is in progress to add the support in mount ( util-linux package ).
- Till then use the following program.
-
- ------------------------------------------------------------------------
- //
- //this code was developed my Miklos Szeredi <miklos@szeredi.hu>
- //and modified by Ram Pai <linuxram@us.ibm.com>
- // sample usage:
- // smount /tmp shared
- //
- #include <stdio.h>
- #include <stdlib.h>
- #include <unistd.h>
- #include <string.h>
- #include <sys/mount.h>
- #include <sys/fsuid.h>
-
- #ifndef MS_REC
- #define MS_REC 0x4000 /* 16384: Recursive loopback */
- #endif
-
- #ifndef MS_SHARED
- #define MS_SHARED 1<<20 /* Shared */
- #endif
-
- #ifndef MS_PRIVATE
- #define MS_PRIVATE 1<<18 /* Private */
- #endif
-
- #ifndef MS_SLAVE
- #define MS_SLAVE 1<<19 /* Slave */
- #endif
-
- #ifndef MS_UNBINDABLE
- #define MS_UNBINDABLE 1<<17 /* Unbindable */
- #endif
-
- int main(int argc, char *argv[])
- {
- int type;
- if(argc != 3) {
- fprintf(stderr, "usage: %s dir "
- "<rshared|rslave|rprivate|runbindable|shared|slave"
- "|private|unbindable>\n" , argv[0]);
- return 1;
- }
-
- fprintf(stdout, "%s %s %s\n", argv[0], argv[1], argv[2]);
-
- if (strcmp(argv[2],"rshared")==0)
- type=(MS_SHARED|MS_REC);
- else if (strcmp(argv[2],"rslave")==0)
- type=(MS_SLAVE|MS_REC);
- else if (strcmp(argv[2],"rprivate")==0)
- type=(MS_PRIVATE|MS_REC);
- else if (strcmp(argv[2],"runbindable")==0)
- type=(MS_UNBINDABLE|MS_REC);
- else if (strcmp(argv[2],"shared")==0)
- type=MS_SHARED;
- else if (strcmp(argv[2],"slave")==0)
- type=MS_SLAVE;
- else if (strcmp(argv[2],"private")==0)
- type=MS_PRIVATE;
- else if (strcmp(argv[2],"unbindable")==0)
- type=MS_UNBINDABLE;
- else {
- fprintf(stderr, "invalid operation: %s\n", argv[2]);
- return 1;
- }
- setfsuid(getuid());
-
- if(mount("", argv[1], "dontcare", type, "") == -1) {
- perror("mount");
- return 1;
- }
- return 0;
- }
- -----------------------------------------------------------------------
-
- Copy the above code snippet into smount.c
- gcc -o smount smount.c
-
-
- (i) To mark all the mounts under /mnt as shared execute the following
- command:
-
- smount /mnt rshared
- the corresponding syntax planned for mount command is
- mount --make-rshared /mnt
-
- just to mark a mount /mnt as shared, execute the following
- command:
- smount /mnt shared
- the corresponding syntax planned for mount command is
- mount --make-shared /mnt
-
- (ii) To mark all the shared mounts under /mnt as slave execute the
- following
-
- command:
- smount /mnt rslave
- the corresponding syntax planned for mount command is
- mount --make-rslave /mnt
-
- just to mark a mount /mnt as slave, execute the following
- command:
- smount /mnt slave
- the corresponding syntax planned for mount command is
- mount --make-slave /mnt
-
- (iii) To mark all the mounts under /mnt as private execute the
- following command:
-
- smount /mnt rprivate
- the corresponding syntax planned for mount command is
- mount --make-rprivate /mnt
-
- just to mark a mount /mnt as private, execute the following
- command:
- smount /mnt private
- the corresponding syntax planned for mount command is
- mount --make-private /mnt
-
- NOTE: by default all the mounts are created as private. But if
- you want to change some shared/slave/unbindable mount as
- private at a later point in time, this command can help.
-
- (iv) To mark all the mounts under /mnt as unbindable execute the
- following
-
- command:
- smount /mnt runbindable
- the corresponding syntax planned for mount command is
- mount --make-runbindable /mnt
-
- just to mark a mount /mnt as unbindable, execute the following
- command:
- smount /mnt unbindable
- the corresponding syntax planned for mount command is
- mount --make-unbindable /mnt
+ Modern mount(8) command is aware of shared subtree features,
+ so use it instead of the 'smount' command. [source code removed]
4) Use cases
then the subtree under the unbindable mount is pruned in the new
location.
- eg: lets say we have the following mount tree.
+ eg: let's say we have the following mount tree.
A
/ \
/ \ / \
D E F G
- Lets say all the mount except the mount C in the tree are
+ 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
'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
+ Example: Let's 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
+ let's 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
+ 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
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
+ 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
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'
+ its root dentry is 1. Let's call this mount 'A'
And then we have a second mount at /tmp1 with root
- dentry 2. Lets call this mount 'B'
+ dentry 2. Let's call this mount 'B'
Next we have a third mount at /mnt with root dentry
- mnt. Lets call this mount 'C'
+ mnt. Let's call this mount 'C'
'B' is the slave of 'A' and 'C' is a slave of 'B'
A -> B -> C
Q3 Why is unbindable mount needed?
- Lets say we want to replicate the mount tree at multiple
+ Let's say we want to replicate the mount tree at multiple
locations within the same subtree.
if one rbind mounts a tree within the same subtree 'n' times
mounts. Here is a example.
step 1:
- lets say the root tree has just two directories with
+ let's say the root tree has just two directories with
one vfsmount.
root
/ \
Unclonable mounts come in handy here.
step 1:
- lets say the root tree has just two directories with
+ let's say the root tree has just two directories with
one vfsmount.
root
/ \
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