pNFS: Discard a GETDEVICEINFO reply that raced a CHANGE notification

RFC 8881 Section 18.40.4: a GETDEVICEINFO reply in flight while the
server changes the device mapping may carry the pre-change mapping; if
it is inserted into the cache after the CHANGE notification unhashed
the stale entry, the client re-caches stale data.

Track a change epoch, bumped when a CHANGE notification is processed
before the stale entry is unhashed.  nfs4_find_get_deviceid() snapshots
the epoch before issuing GETDEVICEINFO and, serialized against the
unhash by nfs4_deviceid_lock at insert time, discards the reply and
refetches if the epoch moved.  A stale insert that instead precedes the
unhash is removed by the unhash itself, so the cache does not retain the
pre-change entry either way; a reference already handed to a caller in
that ordering is dropped by the re-resolve walk instead.

The refetch is bounded.  The epoch is bumped once per CHANGE entry --
that is, at a rate the server chooses -- so an unbounded retry would let
a server drive GETDEVICEINFO traffic without limit, and each discarded
node can carry a DS client teardown and reconnect with it.  After
NFS4_DEVICEID_FETCH_RETRIES attempts the reply is accepted.  That is
safe because discarding is an optimisation rather than a correctness
requirement: it avoids caching a mapping already known to be superseded,
but before this patch the client cached whatever the reply carried, so
the bounded case is no worse than the previous behaviour and a mapping
that really is stale is corrected by the notification that follows.

The epoch lives on the nfs_client, so a CHANGE delivered on one server's
callback channel does not force an unrelated server's in-flight lookup
to discard its reply and refetch.  Mounts that share an nfs_client do
share the counter; the deviceid cache is keyed per client ID, so that is
the granularity the race is defined at.

Assisted-by: Claude:claude-fable-5
Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
Signed-off-by: Anna Schumaker <anna.schumaker@hammerspace.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/nfs_fs_sb.h b/include/linux/nfs_fs_sb.h
index 3e6bae7e5221..482daf1fa4b0 100644
--- a/include/linux/nfs_fs_sb.h
+++ b/include/linux/nfs_fs_sb.h
@@ -74,6 +74,8 @@ struct nfs_client {
 	u64			cl_clientid;	/* constant */
 	nfs4_verifier		cl_confirm;	/* Clientid verifier */
 	unsigned long		cl_state;
+	/* bumped on each CB_NOTIFY_DEVICEID CHANGE for this client */
+	atomic_t		cl_deviceid_change_epoch;
 
 	spinlock_t		cl_lock;