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doc/filesystems: remove smount program
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mount(8) handles shared subtrees just fine, so remove the smount program
from Documentation/filesystems/sharedsubtree.txt.

Fix annoying "Lets" -> "Let's".
Insert space between '#' prompt and "mount" command.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Miklos Szeredi <miklos@szeredi.hu>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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Randy Dunlap authored and Linus Torvalds committed Sep 24, 2009
1 parent 57f1f08 commit 0288b95
Showing 1 changed file with 34 additions and 175 deletions.
209 changes: 34 additions & 175 deletions Documentation/filesystems/sharedsubtree.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -41,14 +41,14 @@ replicas continue to be exactly same.

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.

Expand All @@ -58,8 +58,8 @@ replicas continue to be exactly same.
#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
Expand All @@ -80,21 +80,20 @@ replicas continue to be exactly same.

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
Expand All @@ -104,9 +103,9 @@ replicas continue to be exactly same.

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
Expand All @@ -124,12 +123,11 @@ replicas continue to be exactly same.

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
Expand All @@ -139,147 +137,8 @@ replicas continue to be exactly same.

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
Expand Down Expand Up @@ -558,15 +417,15 @@ replicas continue to be exactly same.
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
/ \
B C
/ \ / \
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
Expand Down Expand Up @@ -683,13 +542,13 @@ replicas continue to be exactly same.
'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
Expand All @@ -710,7 +569,7 @@ replicas continue to be exactly same.
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
Expand Down Expand Up @@ -759,11 +618,11 @@ replicas continue to be exactly same.
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
Expand Down Expand Up @@ -794,7 +653,7 @@ replicas continue to be exactly same.

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
Expand All @@ -803,7 +662,7 @@ replicas continue to be exactly same.
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
/ \
Expand Down Expand Up @@ -875,7 +734,7 @@ replicas continue to be exactly same.
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
/ \
Expand Down

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