[PATCH 00/10 v7] NFSD: Pin to vfsmount for nfsd exports cache

Kinglong Mee
2015-07-11T12:46:23Z

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.

v6,
1. revert the change of v5
2. new helper legitimize_mntget() for nfsd exports/expkey cache
get vfsmount from fs_pin
3. cleanup some codes of sunrpc's cache
4. switch using list_head instead of single list for cache_head
in cache_detail
5. new functions validate/invalidate for processing of reference
increase/decrease change (nfsd exports/expkey using grab the
reference of mnt)
6. delete cache_head directly from cache_detail in pin_kill

v7,
implement self reference increase and decrease for nfsd exports/expkey

When reference of cahce_head increase(>1), grab a reference of mnt once.
and reference decrease to 1 (==1), drop the reference of mnt.

So after that,
When ref > 1, user cannot umount the filesystem with -EBUSY.
when ref ==1, means cache only reference by nfsd cache,
no other reference. So user can try umount,
1. before set MNT_UMOUNT (protected by mount_lock), nfsd cache is
referenced (ref > 1, legitimize_mntget), umount will fail with -EBUSY.
2. after set MNT_UMOUNT, nfsd cache is referenced (ref == 2),
legitimize_mntget will fail, and set cache to CACHE_NEGATIVE,
and the reference will be dropped, re-back to 1.
So, pin_kill can delete the cache and umount success.
3. when umountting, no reference to nfsd cache,
pin_kill can delete the cache and umount success.

Kinglong Mee (10):
fs_pin: Initialize value for fs_pin explicitly
fs_pin: Export functions for specific filesystem
path: New helpers path_get_pin/path_put_unpin for path pin
fs: New helper legitimize_mntget() for getting a legitimize mnt
sunrpc: Store cache_detail in seq_file's private directly
sunrpc/nfsd: Remove redundant code by exports seq_operations functions
sunrpc: Switch to using list_head instead single list
sunrpc: New helper cache_delete_entry for deleting cache_head directly
sunrpc: Support get_ref/put_ref for reference change in cache_head
nfsd: Allows user un-mounting filesystem where nfsd exports base on

fs/fs_pin.c | 4 +
fs/namei.c | 26 +++++
fs/namespace.c | 19 ++++
fs/nfsd/export.c | 259 ++++++++++++++++++++++++++++---------------
fs/nfsd/export.h | 22 +++-
include/linux/fs_pin.h | 6 +
include/linux/mount.h | 1 +
include/linux/path.h | 4 +
include/linux/sunrpc/cache.h | 25 ++++-
net/sunrpc/cache.c | 149 ++++++++++++++++---------
10 files changed, 368 insertions(+), 147 deletions(-)

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2.4.3