Re: [PATCH 0/4 v2] NFSD: Pin to vfsmount for some nfsd exports cache

["J. Bruce Fields" <bfields@fieldses.org>]

On Sun, May 24, 2015 at 11:01:56PM +0800, Kinglong Mee wrote:
> 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.

Thanks for the update, apologies for the delayed response.

> 
> # 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).)
> 
> I don't think that's user expect, they want umount /nfs/test/.
> 
> It's caused by exports cache of nfsd holds the reference of
> the path (here is /nfs/test/), so, it can't be umounted.
> 
> v1 --> v2,
> 1. Adds an option named "allow_umount" for exports allowing user
>    un-mounting the filesystem where nfsd exports base on.

I don't think allow_umount is a useful option.  I'd rather just make the
code behave like allow_umount was on all the time.

The fact is nobody could ever *depend* on umount to fail on an exported
filesystem anyway.

I do think we might want a stronger "allow_umount" option that actually
revokes locks and such as necessary.  I just don't see the need for this
in between case.

--b.

> 2. New helpers path_get_pin/path_put_unpin for path pin.
> 3. Update exports according to the "allow_umount" option.
> 
> 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                 | 52 ++++++++++++++++++++++++++++++++--------
>  fs/nfsd/export.h                 | 11 ++++++++-
>  include/linux/fs_pin.h           |  6 +++++
>  include/linux/path.h             |  4 ++++
>  include/linux/sunrpc/cache.h     | 11 +++++++++
>  include/uapi/linux/nfsd/export.h |  3 ++-
>  8 files changed, 104 insertions(+), 12 deletions(-)
> 
> -- 
> 2.4.1