Automatic NFSv4 state revocation on filesystem unmount

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_ip and
/proc/fs/nfsd/unlock_fs procfs interfaces handle this, but have
no netlink equivalents, and unlock_fs operates at whole-superblock
granularity.

This series adds three new NFSD netlink commands, each with its own
attribute set:

 - NFSD_CMD_UNLOCK_IP releases NLM locks held by a client IP
   address. Netlink equivalent of write_unlock_ip.

 - NFSD_CMD_UNLOCK_FILESYSTEM revokes all NFS state on a
   superblock. Netlink equivalent of write_unlock_fs.

 - NFSD_CMD_UNLOCK_EXPORT revokes NFSv4 state acquired through
   exports of a specific path, regardless of client.

UNLOCK_FILESYSTEM and UNLOCK_EXPORT serve different intents.
UNLOCK_FILESYSTEM means "unmounting /data, release everything
on this superblock." UNLOCK_EXPORT means "no clients remain for
/data/projectA, release only the state acquired through exports
of that path." Userspace (exportfs -u) sends UNLOCK_EXPORT after
removing the last client for a given path, enabling the underlying
filesystem to be unmounted.

The path-only design for UNLOCK_EXPORT avoids the auth_domain
naming complexity (use_ipaddr vs hostname-based domains) by not
requiring the caller to identify a specific client. Since this
mechanism is to be used to enable umount, this seemed like a
reasonable compromise.

---
Changes since v4:
- 1/9 has been queued in nfsd-testing
- Split single NFSD_CMD_UNLOCK into three separate commands
- UNLOCK_EXPORT takes path only, no client attribute to avoid
  auth_domain naming complexity with use_ipaddr

Changes since v3:
- All VFS changes replaced with new netlink "unlock" operation

Changes since v2:
- Replace fs_pin with an SRCU umount notifier chain in VFS
- Merge the pending COPY cancellation patch
- Replace xa_cmpxchg() with xa_insert()
- Use cancel_work_sync() instead of flush_workqueue()
- Remove rcu_barrier()
- Correct misleading claims in kdoc comments and commit messages

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 (7):
      NFSD: Extract revoke_one_stid() utility function
      NFSD: Add NFSD_CMD_UNLOCK_IP netlink command
      NFSD: Add NFSD_CMD_UNLOCK_FILESYSTEM netlink command
      NFSD: Replace idr_for_each_entry_ul in find_one_sb_stid()
      NFSD: Track svc_export in nfs4_stid
      NFSD: Add NFSD_CMD_UNLOCK_EXPORT netlink command
      NFSD: Close cached file handles when revoking export state

 Documentation/netlink/specs/nfsd.yaml |  61 +++++++++
 fs/nfsd/filecache.c                   |  46 +++++++
 fs/nfsd/filecache.h                   |   1 +
 fs/nfsd/netlink.c                     |  36 ++++++
 fs/nfsd/netlink.h                     |   3 +
 fs/nfsd/nfs4layouts.c                 |   2 +
 fs/nfsd/nfs4state.c                   | 226 +++++++++++++++++++++++-----------
 fs/nfsd/nfsctl.c                      | 126 ++++++++++++++++++-
 fs/nfsd/state.h                       |   6 +
 fs/nfsd/trace.h                       |  32 ++++-
 include/uapi/linux/nfsd_netlink.h     |  24 ++++
 11 files changed, 484 insertions(+), 79 deletions(-)
---
base-commit: 65058e9e9b20619f920397f529072e853dd43811
change-id: 20260318-umount-kills-nfsv4-state-138218f2f4e0

Best regards,
--  
Chuck Lever