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docs: mark Mount Points 2.0 as legacy and redirect to Mount V2

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Signed-off-by: Andrei Vagin <avagin@google.com>
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Documentation/under-the-hood/mount-points20.md
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# Mount Points 2.0
# Mount Points 2.0 (Legacy)
## The Problem
A mount namespace is a tree of mount points. In addition, mounts have group-based dependencies: a mount can be a slave in one group and a member of another. Currently, these groups cannot be set directly; they must be inherited from a source mount. Recreating these properties alongside other mount attributes requires a specific sequence of operations.
> **Note**: This document describes an early design iteration for mount restoration. The current and much more advanced implementation is documented in [Mount V2](mount-v2.md).
Additional challenges include:
- **Over-mounts**: Multiple mounts may occupy the same location, or processes may hold file descriptors to over-mounted files.
- **User Namespaces**: Bind mounts cannot be created between different namespaces directly; each filesystem must be mounted from its respective user namespace.
## The Solution
Given the complexity of recreating a mount tree using standard commands, we proposed [a new flag](https://patchwork.kernel.org/patch/9703885/) for the `mount()` system call that allows adding a mount to an existing group.
With this capability, the restoration algorithm is simplified:
1. Create a temporary "root yard" mount.
1. Create all necessary namespaces (within their respective user namespaces).
1. Add the root yards from all namespaces into a single shared group so that a mount created in one namespace propagates to others.
1. Create all mounts in separate directories within the root yards.
1. Restore open files (at this stage, no mounts are over-mounted).
1. Assemble the final mount trees by moving mounts to their correct locations.
1. Perform `pivot_root()` in all namespaces.
### Example Configuration
| mnt_id | parent | shared | master |
| :--- | :--- | :--- | :--- |
| 1 | 0 | | |
| 2 | 1 | 1 | |
| 3 | 2 | 2 | |
| 4 | 2 | 3 | |
| 5 | 1 | | |
| 6 | 0 | | |
| 7 | 6 | 1 | |
| 8 | 7 | 2 | |
| 9 | 7 | 4 | 3 |
| 10 | 6 | | |
The original tree:
![File:mntns-2.0-tree.svg](File:mntns-2.0-tree.svg)
First, each mount and shared group is restored separately within its respective namespace:
![File:mntns-2.0-tree-2.svg](File:mntns-2.0-tree-2.svg)
Finally, the mounts are moved into their proper positions and their group relationships are established:
![File:mntns-2.0-tree-3.svg](File:mntns-2.0-tree-3.svg)
## Restoring UNIX Sockets
UNIX sockets can be bound to files. However, a socket's address and its underlying file are not intrinsically linked within the kernel. For example, if a socket file is moved, tools like `ss` still show the original address. Addresses may also use relative paths (e.g., `../socket_name`).
While `socket_diag` provides the device and inode of a socket file, it does not provide the path or mount point. To address this, we introduced the `SIOCUNIXFILE` ioctl, which returns a file descriptor for the socket file.
To restore a UNIX socket:
1. Create a temporary directory and mount `tmpfs` onto it.
1. Restore the sockets.
1. Create a socket address directory where the final component is a symlink to the correct directory on the required mount point.
1. `chroot()` into the temporary directory.
1. Bind the socket to the specified address.
1. If restoring a server socket, retrieve the file descriptor for its file and use it to restore client sockets by calling `connect()` on `/proc/self/fd/[SK_FILE_FD]`.
1. Unmount the temporary `tmpfs` and remove the directory.
## Source Code
- [github.com/avagin/criu/tree/mntns-2.0](https://github.com/avagin/criu/tree/mntns-2.0)
- [[PATCH] fs: add an ioctl to get an owning userns for a superblock](https://lkml.org/lkml/2017/5/9/634)
For detailed information on the modern mount restoration algorithm, including the use of detached mounts and `move_mount`, please refer to:
* [Mount V2 Overview](mount-v2.md)
* [Mount V2 Detailed Algorithm](mounts-v2-virtuozzo.md)
* [Mount Points (General)](mount-points.md)