Automatic NFSv4 state revocation on filesystem unmount

From: Chuck Lever <chuck.lever@oracle.com>

When an NFS server exports a filesystem and clients hold NFSv4 state
(opens, locks, delegations), unmounting the underlying filesystem
fails with EBUSY. The /proc/fs/nfsd/unlock_fs interface exists for
administrators to manually revoke state before retrying the unmount,
but this approach has significant operational drawbacks.

Manual intervention breaks automation workflows. Containerized NFS
servers, orchestration systems, and unattended maintenance scripts
cannot reliably unmount exported filesystems without implementing
custom logic to detect the failure and invoke unlock_fs. System
administrators managing many exports face tedious, error-prone
procedures when decommissioning storage.

This series enables the NFS server to detect filesystem unmount
events and automatically revoke associated state. The mechanism
uses the kernel's existing fs_pin infrastructure, which provides
callbacks during mount lifecycle transitions. When a filesystem
is unmounted, all NFSv4 opens, locks, and delegations referencing
it are revoked, async COPY operations are cancelled with
NFS4ERR_ADMIN_REVOKED sent to clients, NLM locks are released,
and cached file handles are closed.

With automatic revocation, unmount operations complete without
administrator intervention once the brief state cleanup finishes.
Clients receive immediate notification of state loss through
standard NFSv4 error codes, allowing applications to handle the
situation appropriately rather than encountering silent failures.

Based on the nfsd-testing branch of:

  https://git.kernel.org/pub/scm/linux/kernel/git/cel/linux...

Changes since v1:
- Explain why drop_client() is being renamed
- Finish implementing revocation on umount
- Rename pin_insert_group
- Clarified log output and code comments
- Hold nfsd_mutex while closing nfsd_files

Chuck Lever (6):
  nfsd: cancel async COPY operations when admin revokes filesystem state
  fs: export pin_insert and pin_remove for modular filesystems
  fs: add pin_insert_sb() for superblock-only pins
  fs: invoke group_pin_kill() during mount teardown
  nfsd: revoke NFSv4 state when filesystem is unmounted
  nfsd: close cached files on filesystem unmount

 fs/fs_pin.c            |  50 ++++++++
 fs/namespace.c         |   2 +
 fs/nfsd/Makefile       |   2 +-
 fs/nfsd/filecache.c    |  44 +++++++
 fs/nfsd/filecache.h    |   1 +
 fs/nfsd/netns.h        |   4 +
 fs/nfsd/nfs4proc.c     | 124 +++++++++++++++++--
 fs/nfsd/nfs4state.c    |  46 +++++--
 fs/nfsd/nfsctl.c       |  11 +-
 fs/nfsd/pin.c          | 274 +++++++++++++++++++++++++++++++++++++++++
 fs/nfsd/state.h        |   9 ++
 fs/nfsd/xdr4.h         |   1 +
 include/linux/fs_pin.h |   1 +
 13 files changed, 548 insertions(+), 21 deletions(-)
 create mode 100644 fs/nfsd/pin.c

-- 
2.52.0