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.

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

v4, Thanks for Al Viro's commets for the logic of fs_pin.
1. add a completion for pin_kill waiting the reference is decreased to zero.
2. add a work_struct for pin_kill decreases the reference indirectly.
3. free svc_export/svc_expkey in pin_kill, not svc_export_put/svc_expkey_put.
4. svc_export_put/svc_expkey_put go though pin_kill logic.

v5,
let killing fs_pin under a reference of vfsmnt.

Kinglong Mee (6):
fs_pin: Initialize value for fs_pin explicitly
fs_pin: Export functions for specific filesystem
fs_pin: Kill fs_pin under a reference of vfsmnt
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                  |  4 ++
 fs/namei.c                   | 26 ++++++++++++
 fs/namespace.c               | 10 ++++-
 fs/nfsd/export.c             | 96 +++++++++++++++++++++++++++++++++++---------
 fs/nfsd/export.h             | 18 ++++++++-
 include/linux/fs_pin.h       |  6 +++
 include/linux/path.h         |  4 ++
 include/linux/sunrpc/cache.h | 11 +++++
 8 files changed, 154 insertions(+), 21 deletions(-)

2.4.3

Without initialized, done in fs_pin at stack space may contains strange value.

v3, v4, v5,
Adds macro for header file

---
 include/linux/fs_pin.h | 6 ++++++
 1 file changed, 6 insertions(+)

index 3886b3b..0dde7b7 100644
@@ -1,3 +1,6 @@
+#ifndef _LINUX_FS_PIN_H
+#define _LINUX_FS_PIN_H
+
 #include <linux/wait.h>
 
 struct fs_pin {

@@ -16,9 +19,12 @@ static inline void init_fs_pin(struct fs_pin *p, void (*kill)(struct fs_pin *))
 	INIT_HLIST_NODE(&p->s_list);
 	INIT_HLIST_NODE(&p->m_list);
 	p->kill = kill;
+	p->done = 0;
 }
 
 void pin_remove(struct fs_pin *);
 void pin_insert_group(struct fs_pin *, struct vfsmount *, struct hlist_head *);
 void pin_insert(struct fs_pin *, struct vfsmount *);
 void pin_kill(struct fs_pin *);
+
+#endif

2.4.3

Sorry for the bad title of v6, it should be v5.

Please ignore the threads of v6.

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