proc: Dentry flushing without proc_mnt

Cleanly handling proc mount options require the internal mount of
proc to be removed (so mount options are not ignored), and quite
possibly multiple proc superblocks per pid namespace (so a
second mount of proc does not silently get the mount options of the
first mount of proc.  In either case being able to flush proc
dentries on process exit needs to be made to work without going
through proc_mnt.  After serveral discussions this is the set
of changes that work and no one objects to.

---

I have addressed all of the review comments as I understand them,
and fixed the small oversight the kernel test robot was able to
find. (I had failed to initialize the new field pid->inodes).

I did not hear any concerns from the 10,000 foot level last time
so I am assuming this set of changes (baring bugs) is good to go.

Unless some new issues appear my plan is to put this in my tree
and get this into linux-next.  Which will give Alexey something
to build his changes on.

I tested this set of changes by running:
 (while ls -1 -f /proc > /dev/null ; do :; done ) &
And monitoring the amount of free memory.

With the flushing disabled I saw the used memory in the system grow by
20M before the shrinker would bring it back down to where it started.
With the patch applied I saw the memory usage stay essentially fixed.

So flushing definitely keeps things working better.

Eric W. Biederman (6):
      proc: Rename in proc_inode rename sysctl_inodes sibling_inodes
      proc: Generalize proc_sys_prune_dcache into proc_prune_siblings_dcache
      proc: In proc_prune_siblings_dcache cache an aquired super block
      proc: Use d_invalidate in proc_prune_siblings_dcache
      proc: Clear the pieces of proc_inode that proc_evict_inode cares about
      proc: Use a list of inodes to flush from proc

 fs/proc/base.c          | 111 ++++++++++++++++--------------------------------
 fs/proc/inode.c         |  73 ++++++++++++++++++++++++++++---
 fs/proc/internal.h      |   4 +-
 fs/proc/proc_sysctl.c   |  45 +++-----------------
 include/linux/pid.h     |   1 +
 include/linux/proc_fs.h |   4 +-
 kernel/exit.c           |   4 +-
 kernel/pid.c            |   1 +
 8 files changed, 120 insertions(+), 123 deletions(-)

Link: https://lore.kernel.org/lkml/871rqk2brn.fsf_-_@x220.int.ebiederm.org/
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Merge branch 'proc-dentry-flushing-without-proc-mnt-v2' into HEAD