[RFC PATCH 0/8] namespace-aware configfs

[Hannes Reinecke <hare@kernel.org>]

Hey all,

during discussions at LSF/MM I thought it would be quite helpful
to make configfs namespace aware, such that one could do:

unshare --mnt
mount -t configfs none /sys/kernel/config

and get a new instance of configfs. This will allow to have distinct
configurations for each namespace.
This is particularly helpful if you want to run eg nvmet in a container;
with this patchset each container can have its own configuration, and
one can simulate a multi-node nvmet cluster using containers.

How it works:
The configfs 'root' entries are stored in 'configfs_super_info' which
is stored in an xarray using the namespace ID as the index.
A configfs subsystem can be converted to be namespace aware by implementing
the new subsyste callbacks 'fill_subsystem()' and 'clear_subsystem', which
are responsible for populating the subsystem structure.
Upon registration via 'register_subsystem()' the subsystem is put in a
linked list within 'configfs_super_info'. So when mount() is called from
a different namespace this list is traversed, and new subsystems are created
in the new namespace using the callbacks.

Open Issues:
- I've added a new function 'mnt_clone_direct()' to clone the vfsmount
  entry (the original code just did a simple_pin_fs()). Not sure if
  that's correct. Christian?
- The current cloning mechanism is not really a hierarchy, but rather
  always using the default namespace to find registered subsystems.
  Meaning you can only call 'register_subsystem()' for the default
  namespace. But then one shouldn't call modprobe in a container, so
  that's okay I guess.
- The original content of the configfs remains visible even from
  within the container, and the new 'mount' will just overlay that.
  Ideally I would have the container start off with an empty
  /sys/kernel/config to avoid configuration issues. But again I've
  no idea how to do that (or if it's even possible).

Otherwise, as usual, commands and reviews are welcome.

Hannes Reinecke (8):
  fs/configfs: rework configfs_is_root()
  fs/configfs: dynamically allocate super_info
  fs/configfs: separate out configfs_{link,unlink}_root()
  fs/namespace: implement mnt_clone_direct()
  fs/configfs: add superblock as attribute to configfs_pin_fs()
  fs/configfs: add 'fill_subsystem' and 'clear_subsystem' callbacks
  fs/configfs: switch to get_tree_keyed()
  nvmet: make configfs setup namespace aware

 drivers/nvme/target/configfs.c  | 183 ++++++++++++++++++++++-----
 drivers/nvme/target/core.c      |  35 +++---
 drivers/nvme/target/discovery.c | 101 +++++++++++----
 drivers/nvme/target/nvmet.h     |  13 +-
 fs/configfs/configfs_internal.h |  21 +++-
 fs/configfs/dir.c               | 167 +++++++++++++++++++-----
 fs/configfs/mount.c             | 216 +++++++++++++++++++++++++++-----
 fs/namespace.c                  |  11 ++
 include/linux/configfs.h        |   7 ++
 include/linux/mount.h           |   1 +
 11 files changed, 615 insertions(+), 144 deletions(-)

-- 
2.51.0