[PATCH RFC 0/4] proc: support multiple separate proc instances per pidnamespace

[Djalal Harouni]

Hi,

This RFC can be applied on top of Linus' tree 89970a04d7

This RFC implements support for multiple separate proc instances inside
the same pid namespace. This allows to solve lot of problems that
today's use case face.

Historically procfs was tied to pid namespaces, and mount options were
propagated to all other procfs instances in the same pid namespace. This
solved several use cases in that time. However today we face new
problems, there are mutliple container implementations there, some of
them want to hide pid entries, others want to hide non-pid entries,
others want to have sysctlfs, others want to share pid namespace with
private procfs mounts. All these with current implementation won't work
since all options will be propagated to all procfs mounts.

This series allow to have new instances of procfs per pid namespace where
each instance can have its own mount option inside the same pid namespace.
This was also suggested by Andy Lutomirski.


Now:
$ sudo mount -t proc -o unshare,hidepid=2 none /test

The option 'unshare' will allow to mount a new instance of procfs inside
the same pid namespace.

Before:
$ stat /proc/slabinfo

  File: â/proc/slabinfoâ
  Size: 0         	Blocks: 0          IO Block: 1024   regular empty file
Device: 4h/4d	Inode: 4026532046  Links: 1

$ stat /test3/slabinfo

  File: â/test3/slabinfoâ
  Size: 0         	Blocks: 0          IO Block: 1024   regular empty file
Device: 4h/4d	Inode: 4026532046  Links: 1


After:
$ stat /proc/slabinfo

  File: â/proc/slabinfoâ
  Size: 0         	Blocks: 0          IO Block: 1024   regular empty file
Device: 4h/4d	Inode: 4026532046  Links: 1

$ stat /test3/slabinfo

  File: â/test3/slabinfoâ
  Size: 0         	Blocks: 0          IO Block: 1024   regular empty file
Device: 31h/49d	Inode: 4026532046  Links: 1


Any better name for the option 'unshare' ? suggestions ?

I was going to use 'version=2' but then this may sound more like a
proc2 fs which currently impossible to implement since it will share
locks with the old proc.


Al, Eric any comments please ?

[Patch RFC 4/4] proc: support flushing dcache entries of a task on multiple procfs mounts
Is maybe not needed, and I have to test it further.

Thanks!