I am confused about --make-{shared,slave,private,unbindable} flags of the mount command

Question

I am confused about --make-shared, --make-slave, --make-private and --make-unbindable flags of the mount command.

Can someone please explain in simple words , what each flag do ? and what is the difference between them ?

Answer 1

These flags to the mount command are advanced features which modify the way bind-mounts work. They also have the same effects on mount-namespaces, and this is sometimes used by systemd to allow a process to have its own private mounts.

The --make-unbindable flag prevents any later bind-mounting of either part or the whole of a mount:

mount --make-unbindable /master
#Both of these will fail
mount --rbind /master /slave
mount --rbind /master/subdir /slave

The other flags effect how changes to submounts within a mount are synchronized between future bind-mounts and namespaces. The use of --rbind instead of --bind is still needed to recursively share all existing submounts with the bind mount, except those made unbindable as above.

Like --bind, all these flags also have variants with an r prefix which are recursively applied to all contained mounts.

The --make-[r]shared flag (the initial state with systemd) enables synchronisation between a master mount and future copies. By example, for an existing mount at /master, both master-submount and slave-submount will be mounted under both /master and /slave:

mount --make-rshared /master
mount --rbind /master /slave
mount /dev/$DEVICE /master/master-submount
mount /dev/$DEVICE /slave/slave-submount

The --make-[r]private flag stops further synchronisation. This is irreversible for any existing shares. Continuing the above example below, unmounting /master/master-submount and /slave/slave-submount will not now unmount the copies at /slave/master-submount and /master/slave-submount:

mount --make-rprivate /slave
umount /master/master-submount
umount /slave/slave-submount

The --make-[r]slave flag can be used to break only updates to others, but still receive them. If rslave had been used instead of rprivate above, /master/slave-submount would still not be unmounted by /slave, but /slave/master-submount would also have been unmounted by /master.

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The full documentation for these flags is not in the man page, but is available at https://www.kernel.org/doc/Documentation/filesystems/sharedsubtree.txt.