nfsd: export NFSv4 callback op stats via netlink

Add a proc4cb-ops nested attribute to the server-stats netlink dump,
reusing the existing server-proc-entry (op/count) layout. The dump
gains a callback section that emits one entry per callback opcode
(OP_CB_GETATTR..OP_CB_OFFLOAD) from the per-netns callback counters,
paged across messages like the other per-operation sections.

This lets nfsstat report NFSv4 backchannel operation counts over
netlink, including CB_GETATTR which corresponds to the procfs
wdeleg_getattr line.

Assisted-by: LLM
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260717-exportd-netlink-v7-6-b7ce17b83b60@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>

Documentation/netlink/specs/nfsd.yaml

@@ -410,6 +410,11 @@ attribute-sets:
         type: nest
         nested-attributes: server-proc-entry
         multi-attr: true
+      -
+        name: proc4cb-ops
+        type: nest
+        nested-attributes: server-proc-entry
+        multi-attr: true
 
 operations:
   list:
@@ -621,6 +626,7 @@ operations:
             - proc3-ops
             - proc4-ops
             - proc4ops-ops
+            - proc4cb-ops
 
 mcast-groups:
   list:

fs/nfsd/nfsctl.c

@@ -2451,6 +2451,29 @@ static int nfsd_nl_server_stats_nfs4ops(struct sk_buff *skb,
 
 	return 0;
 }
+
+/*
+ * Emit NFSv4 callback (backchannel) per-operation counts, resuming at *idx,
+ * which counts from OP_CB_GETATTR. Same return convention as
+ * nfsd_nl_server_stats_proc().
+ */
+static int nfsd_nl_server_stats_cbops(struct sk_buff *skb,
+				      struct nfsd_net *nn, int *idx)
+{
+	int op;
+
+	for (op = OP_CB_GETATTR + *idx; op <= OP_CB_OFFLOAD; op++, (*idx)++) {
+		u64 cnt = percpu_counter_sum_positive(&nn->cb_counter[op]);
+
+		if (!cnt)
+			continue;
+		if (nfsd_nl_put_proc_entry(skb, NFSD_A_SERVER_STATS_PROC4CB_OPS,
+					   op, cnt))
+			return -EMSGSIZE;
+	}
+
+	return 0;
+}
 #endif
 
 /* Sections of the server-stats dump, emitted in order across messages. */
@@ -2459,6 +2482,7 @@ enum {
 	NFSD_SERVER_STATS_PROC2,
 	NFSD_SERVER_STATS_PROC3,
 	NFSD_SERVER_STATS_PROC4,
+	NFSD_SERVER_STATS_PROC4CB,
 	NFSD_SERVER_STATS_PROC4OPS,
 	NFSD_SERVER_STATS_DONE,
 };
@@ -2527,6 +2551,9 @@ int nfsd_nl_server_stats_get_dumpit(struct sk_buff *skb,
 					NFSD_A_SERVER_STATS_PROC4_OPS, &idx);
 			break;
 #ifdef CONFIG_NFSD_V4
+		case NFSD_SERVER_STATS_PROC4CB:
+			ret = nfsd_nl_server_stats_cbops(skb, nn, &idx);
+			break;
 		case NFSD_SERVER_STATS_PROC4OPS:
 			ret = nfsd_nl_server_stats_nfs4ops(skb, nn, &idx);
 			break;

include/uapi/linux/nfsd_netlink.h

@@ -254,6 +254,7 @@ enum {
 	NFSD_A_SERVER_STATS_PROC3_OPS,
 	NFSD_A_SERVER_STATS_PROC4_OPS,
 	NFSD_A_SERVER_STATS_PROC4OPS_OPS,
+	NFSD_A_SERVER_STATS_PROC4CB_OPS,
 
 	__NFSD_A_SERVER_STATS_MAX,
 	NFSD_A_SERVER_STATS_MAX = (__NFSD_A_SERVER_STATS_MAX - 1)