NFSD: Pin to vfsmount for nfsd exports cache

If there are some mount points(not exported for nfs) under pseudo root, after client's operation of those entry under the root, anyone *can't* unmount those mount points until export cache expired.

# cat /etc/exports
/nfs/xfs        *(rw,insecure,no_subtree_check,no_root_squash)
/nfs/pnfs       *(rw,insecure,no_subtree_check,no_root_squash)
# ll /nfs/
total 0
drwxr-xr-x. 3 root root 84 Apr 21 22:27 pnfs
drwxr-xr-x. 3 root root 84 Apr 21 22:27 test
drwxr-xr-x. 2 root root  6 Apr 20 22:01 xfs
# mount /dev/sde /nfs/test
# df
Filesystem                      1K-blocks    Used Available Use% Mounted on
......
/dev/sdd                          1038336   32944   1005392   4% /nfs/pnfs
/dev/sdc                         10475520   32928  10442592   1% /nfs/xfs
/dev/sde                           999320    1284    929224   1% /nfs/test
# mount -t nfs 127.0.0.1:/nfs/ /mnt
# ll /mnt/*/
/mnt/pnfs/:
total 0
-rw-r--r--. 1 root root 0 Apr 21 22:23 attr
drwxr-xr-x. 2 root root 6 Apr 21 22:19 tmp

/mnt/xfs/:
total 0

# umount /nfs/test/
umount: /nfs/test/: target is busy
        (In some cases useful info about processes that
         use the device is found by lsof(8) or fuser(1).)

It's caused by exports cache of nfsd holds the reference of the path (here is /nfs/test/), so, it can't be umounted.

I don't think that's user expect, they want umount /nfs/test/. Bruce think user can also umount /nfs/pnfs/ and /nfs/xfs.

This patch site lets nfsd exports pinning to vfsmount, not using mntget, so user can umount any exports mountpoint now.

v1 --> v3,
1. New helpers path_get_pin/path_put_unpin for path pin.
2. Use kzalloc for allocating memory.

Kinglong Mee (5):
fs_pin: Fix uninitialized value in fs_pin
fs_pin: Export functions for specific filesystem
path: New helpers path_get_pin/path_put_unpin for path pin
sunrpc: New helper cache_force_expire for cache cleanup
nfsd: Allows user un-mounting filesystem where nfsd exports base on

 fs/fs_pin.c                  |  3 +++
 fs/namei.c                   | 26 ++++++++++++++++++++++++++
 fs/nfsd/export.c             | 38 ++++++++++++++++++++++++++++----------
 fs/nfsd/export.h             | 10 +++++++++-
 include/linux/fs_pin.h       |  6 ++++++
 include/linux/path.h         |  4 ++++
 include/linux/sunrpc/cache.h | 11 +++++++++++
 7 files changed, 87 insertions(+), 11 deletions(-)

2.4.2