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-rw-r--r--Documentation/filesystems/sharedsubtree.txt220
1 files changed, 42 insertions, 178 deletions
diff --git a/Documentation/filesystems/sharedsubtree.txt b/Documentation/filesystems/sharedsubtree.txt
index 736540045dc7..23a181074f94 100644
--- a/Documentation/filesystems/sharedsubtree.txt
+++ b/Documentation/filesystems/sharedsubtree.txt
@@ -4,7 +4,7 @@ Shared Subtrees
Contents:
1) Overview
2) Features
- 3) smount command
+ 3) Setting mount states
4) Use-case
5) Detailed semantics
6) Quiz
@@ -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.
@@ -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
@@ -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
@@ -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
@@ -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
@@ -137,149 +135,15 @@ replicas continue to be exactly same.
Binding a unbindable mount is a invalid operation.
-3) smount command
+3) Setting mount states
- 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.
+ The mount command (util-linux package) can be used to set mount
+ states:
- ------------------------------------------------------------------------
- //
- //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
+ mount --make-shared mountpoint
+ mount --make-slave mountpoint
+ mount --make-private mountpoint
+ mount --make-unbindable mountpoint
4) Use cases
@@ -350,7 +214,7 @@ replicas continue to be exactly same.
mount --rbind / /view/v3
mount --rbind / /view/v4
- and if /usr has a versioning filesystem mounted, than that
+ 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
@@ -390,7 +254,7 @@ replicas continue to be exactly same.
For example:
mount --make-shared /mnt
- mount --bin /mnt /tmp
+ 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
@@ -558,7 +422,7 @@ 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
/ \
@@ -566,7 +430,7 @@ replicas continue to be exactly same.
/ \ / \
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
@@ -683,13 +547,13 @@ 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
@@ -710,7 +574,7 @@ 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
@@ -759,11 +623,11 @@ 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
@@ -794,7 +658,7 @@ 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
@@ -803,7 +667,7 @@ 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
/ \
@@ -875,7 +739,7 @@ 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
/ \
generated by cgit v1.2.3 (git 2.39.0) at 2024-12-30 15:39:25 +0300