Re: [PATCH v3 0/3] Automatic NFSv4 state revocation on filesystem unmount

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Feb 24, 2026 at 11:39:05AM -0500, Chuck Lever wrote:
> 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
> registers with a new SRCU notifier chain in VFS that fires during
> mount teardown, after processing stuck children but before
> fsnotify_vfsmount_delete(), while SB_ACTIVE is still set. 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.

So anyone can force that just with unshare -U -m date?  Creates
a new namespace, populated by clones of all mounts you see, runs
date(1) in that, then exits, with namespace dissolved.  At that
point all cloned mounts are released, each triggering your notifier
chain...