Shared Subtrees - The Linux Kernel documentation

shared_sub_tree.pdf

findmnt -o SOURCE,TARGET,PROPAGATION

Overview

一个进程想拥有自己的mount namespace，但是仍然想访问最近刚挂载的挂载点。那么Shared Sub Tree就提供了这个功能。共享子树提供给了shared mount、slave mount、private mount、unbindable mount四种模式。

Features

• Shared mount

一个shared mount可以复制出多个不同的挂载点，这些挂载点内容都是相同的。例如下面这个例子:

// /mnt需要是一个挂载点
# mount --make-shared /mnt
# mount --bind /mnt /tmp
# ls /mnt
a b c
# ls /tmp
a b c

// /tmp目录下新创建的挂载点都会在/mnt目录下看到，反过来也是一样
Now let's say we mount a device at /tmp/a
# mount /dev/sd0  /tmp/a

#ls /tmp/a
t1 t2 t3

#ls /mnt/a
t1 t2 t3

• Slave mount

除了不传递mount/unmount事件外，其他的和Shared mount一样，也就是说在其中一个挂载点下又挂载了新的挂载点是不会传递到bind的另外一个挂载点下的。

Let's say /mnt has a mount which is shared.
# mount --make-shared /mnt

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.

// 让/tmp变成slave，起对应的master则是/mnt
Now let's make the mount at /tmp; a slave of /mnt
# mount --make-slave /tmp

// 在master的mnt下挂载，产生的mount事件是会传递到/tmp目录下的
// 但是反之不是
let's 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 let's see what happens if we mount something on the mount at /tmp
// 在slave中进行mount是不会传递到master的
# 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

• Private mount

默认都是私有mount，不会进行传递，这个适用于mount namespace，当创建了一个新的mount namespace，会clone parent namespace下所有的mount，但是因为所有的mount默认都是private的，因此parent namespace中的挂载点内有新的挂载点，那么克隆的namespace是看不到。

• Unbindable mount

let's say we have a mount at /mnt and we make it unbindable

# mount --make-unbindable /mnt

 Let's try to bind mount this mount somewhere else.
 # mount --bind /mnt /tmp
 mount: wrong fs type, bad option, bad superblock on /mnt,
        or too many mounted file systems

Binding a unbindable mount is a invalid operation.

Setting mount states