[PATCH 13/21] 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 both orderings end with the
post-change mapping.

Assisted-by: Claude:claude-fable-5
Signed-off-by: Benjamin Coddington <[email protected]>
---
 fs/nfs/callback_proc.c |  4 ++++
 fs/nfs/pnfs.h          |  1 +
 fs/nfs/pnfs_dev.c      | 25 +++++++++++++++++++++++++
 3 files changed, 30 insertions(+)

diff --git a/fs/nfs/callback_proc.c b/fs/nfs/callback_proc.c
index 64c994790d3f..1721c6dd2250 100644
--- a/fs/nfs/callback_proc.c
+++ b/fs/nfs/callback_proc.c
@@ -394,7 +394,11 @@ __be32 nfs4_callback_devicenotify(void *argp, void *resp,
 		 * Unhash the cached device first so re-resolution cannot
 		 * re-pin the stale node, then re-point any references
 		 * pinned under live layouts (RFC 8881 Section 12.2.10).
+		 * The epoch bump lets an in-flight GETDEVICEINFO detect
+		 * that its reply may predate the change.
 		 */
+		if (dev->cbd_notify_type == NOTIFY_DEVICEID4_CHANGE)
+			nfs4_deviceid_bump_change_epoch();
 		nfs4_delete_deviceid(ld, cps->clp, &dev->cbd_dev_id);
 		if (dev->cbd_notify_type == NOTIFY_DEVICEID4_CHANGE)
 			pnfs_layout_reresolve_deviceid_byclid(cps->clp, ld,
diff --git a/fs/nfs/pnfs.h b/fs/nfs/pnfs.h
index 9ec0ffebe3c4..f3c55c86c256 100644
--- a/fs/nfs/pnfs.h
+++ b/fs/nfs/pnfs.h
@@ -399,6 +399,7 @@ nfs4_find_get_deviceid(struct nfs_server *server,
 		const struct nfs4_deviceid *id, const struct cred *cred,
 		gfp_t gfp_mask);
 void nfs4_delete_deviceid(const struct pnfs_layoutdriver_type *, const struct nfs_client *, const struct nfs4_deviceid *);
+void nfs4_deviceid_bump_change_epoch(void);
 void nfs4_init_deviceid_node(struct nfs4_deviceid_node *, struct nfs_server *,
 			     const struct nfs4_deviceid *);
 bool nfs4_put_deviceid_node(struct nfs4_deviceid_node *);
diff --git a/fs/nfs/pnfs_dev.c b/fs/nfs/pnfs_dev.c
index 274abdd6d5f3..0c39ae8325b3 100644
--- a/fs/nfs/pnfs_dev.c
+++ b/fs/nfs/pnfs_dev.c
@@ -181,6 +181,22 @@ __nfs4_find_get_deviceid(struct nfs_server *server,
 	return d;
 }
 
+/*
+ * Bumped when a CB_NOTIFY_DEVICEID CHANGE is processed.  A GETDEVICEINFO
+ * reply fetched across a change may carry the pre-change mapping; the
+ * epoch lets nfs4_find_get_deviceid() detect that and refetch.  The
+ * counter is ordered by nfs4_deviceid_lock: the notification handler
+ * bumps it before unhashing the stale entry under the lock, so an
+ * insert serialized after the unhash observes the new epoch.
+ */
+static atomic_t nfs4_deviceid_change_epoch = ATOMIC_INIT(0);
+
+void
+nfs4_deviceid_bump_change_epoch(void)
+{
+	atomic_inc(&nfs4_deviceid_change_epoch);
+}
+
 struct nfs4_deviceid_node *
 nfs4_find_get_deviceid(struct nfs_server *server,
 		const struct nfs4_deviceid *id, const struct cred *cred,
@@ -188,11 +204,14 @@ nfs4_find_get_deviceid(struct nfs_server *server,
 {
 	long hash = nfs4_deviceid_hash(id);
 	struct nfs4_deviceid_node *d, *new;
+	int epoch;
 
+retry:
 	d = __nfs4_find_get_deviceid(server, id, hash);
 	if (d)
 		goto found;
 
+	epoch = atomic_read(&nfs4_deviceid_change_epoch);
 	new = nfs4_get_device_info(server, id, cred, gfp_mask);
 	if (!new) {
 		trace_nfs4_find_deviceid(server, id, -ENOENT);
@@ -200,6 +219,12 @@ nfs4_find_get_deviceid(struct nfs_server *server,
 	}
 
 	spin_lock(&nfs4_deviceid_lock);
+	if (atomic_read(&nfs4_deviceid_change_epoch) != epoch) {
+		/* a mapping changed while we fetched; ours may be stale */
+		spin_unlock(&nfs4_deviceid_lock);
+		server->pnfs_curr_ld->free_deviceid_node(new);
+		goto retry;
+	}
 	d = __nfs4_find_get_deviceid(server, id, hash);
 	if (d) {
 		spin_unlock(&nfs4_deviceid_lock);
-- 
2.53.0