: Enable multiple mounts of devpts

This patchset includes the patches to cleanup get_node() I had sent earlier, with the Sign-off added. No other changes were made to those patches since I last sent.

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Enable multiple mounts of devpts filesystem so each container can allocate ptys independently.

User interface:

Since supporting multiple mounts of devpts can break user-space, this feature is enabled only under a new mount option (-o newinstance).

If this option is never specified, this patchset should not change any behavior. i.e existing single-namespace semantics are preserved across any remounts of /dev/pts (as long as none of them use the new option). So old startup scripts should continue to work.

If the '-o newinstance' option is specified, then a new 'private' mount of devpts is created. Ptys in this private instance are independent of ptys created in other devpts instances. For this to be fully functional, /dev/ptmx must be a bind-mount of '/dev/pts/ptmx'

i.e
mount -t devpts -o newinstance lxcpts /dev/pts mount -o bind /dev/pts/ptmx /dev/ptmx

See [PATCH 11/11] for more details on usage.

Implementation notes:

1. To enable multiple mounts of /dev/pts, (most) devpts interfaces need to know which instance of devpts is being accessed. This patchset uses the 'struct inode' or 'struct tty_struct' of the device being accessed to identify the appropriate devpts instance. Hence the need for the /dev/pts/ptmx bind-mount.

2. See comments in get_sb_ref() in fs/super.c (could not find existing interfaces that accomplish it) Or is there a better way ?

3. Mount options must be parsed twice during mount (once to determine the mode of mount (single/multi-instance) and once to actually save the options. There does not seem to be an easy way to parse once and reuse (See 'safe_process_mount_opts()' in PATCH 11/11)

[PATCH 1/11]: Move tty lookup/reopen to tty_open [PATCH 2/11]: Add an instance parameter to devpts interfaces [PATCH 3/11]: Simplify devpts_get_tty()
[PATCH 4/11]: Simplify devpts_pty_new()
[PATCH 5/11]: Simplify devpts_pty_kill
[PATCH 6/11]: Remove devpts_root global
[PATCH 7/11]: Per-mount allocated_ptys
[PATCH 8/11]: Per-mount 'config' object
[PATCH 9/11]: Extract option parsing to new function [PATCH 10/11]: Ability to internally create ptmx [PATCH 11/11]: Enable multiple instances of devpts

Changelog [v3]:

- Port to 2008-08-28 ttydev tree
- Rename new mount options to 'ptmxmode' and 'newinstance'.
- [Alan Cox] Use tty driver data to identify devpts (this is used to cleanup get_node() in devpts_pty_kill()).
- [H. Peter Anvin] get_node() cleanup in devpts (which was enabled by the inode/tty parameter to devpts interfaces)
- Bugfix in multi-mount mode (see Patch 11/11).
- Executed pty tests in LTP (in both single-instance and multi-instance mode)
- Should be bisect-safe :-)

Changelog [v2]:

- New mount option '-o newmnt' added (patch 8/8)
- Support both single-mount and multi-mount semantics (patch 8/8)
- Automatically create ptmx node when devpts is mounted (patch 7/8)
- Extract option parsing code to new function (patch 6/8)
- Make 'config' params per-mount variables (patch 5/8)
- Slightly re-ordered existing patches in set (patches 1/8, 2/8)

TODO:
- Do we need a '-o ptmxuid' and '-o ptmxgid' options as well ?
- Add a config option to enable multiple-mounts of devpts.
- (Sometime in future) Remove even initial kernel mount of devpts
- Any other good test suites to test this (besides LTP, sshd).