NFSD: Trace boot verifier resets

According to commit bbf2f098 ("nfsd: Reset the boot verifier on
all write I/O errors"), the Linux NFS server forces all clients to
resend pending unstable writes if any server-side write or commit
operation encounters an error (say, ENOSPC). This is a rare and
quite exceptional event that could require administrative recovery
action, so it should be made trace-able. Example trace event:

nfsd-938   [002]  7174.945558: nfsd_writeverf_reset: boot_time=        61cc920d xid=0xdcd62036 error=-28 new verifier=0x08aecc6142515904
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/trace.h

@@ -574,6 +574,34 @@ DEFINE_EVENT(nfsd_net_class, nfsd_##name, \
 DEFINE_NET_EVENT(grace_start);
 DEFINE_NET_EVENT(grace_complete);
 
+TRACE_EVENT(nfsd_writeverf_reset,
+	TP_PROTO(
+		const struct nfsd_net *nn,
+		const struct svc_rqst *rqstp,
+		int error
+	),
+	TP_ARGS(nn, rqstp, error),
+	TP_STRUCT__entry(
+		__field(unsigned long long, boot_time)
+		__field(u32, xid)
+		__field(int, error)
+		__array(unsigned char, verifier, NFS4_VERIFIER_SIZE)
+	),
+	TP_fast_assign(
+		__entry->boot_time = nn->boot_time;
+		__entry->xid = be32_to_cpu(rqstp->rq_xid);
+		__entry->error = error;
+
+		/* avoid seqlock inside TP_fast_assign */
+		memcpy(__entry->verifier, nn->writeverf,
+		       NFS4_VERIFIER_SIZE);
+	),
+	TP_printk("boot_time=%16llx xid=0x%08x error=%d new verifier=0x%s",
+		__entry->boot_time, __entry->xid, __entry->error,
+		__print_hex_str(__entry->verifier, NFS4_VERIFIER_SIZE)
+	)
+);
+
 TRACE_EVENT(nfsd_clid_cred_mismatch,
 	TP_PROTO(
 		const struct nfs4_client *clp,

fs/nfsd/vfs.c

@@ -553,14 +553,17 @@ __be32 nfsd4_clone_file_range(struct svc_rqst *rqstp,
 		if (!status)
 			status = commit_inode_metadata(file_inode(src));
 		if (status < 0) {
+			struct nfsd_net *nn = net_generic(nf_dst->nf_net,
+							  nfsd_net_id);
+
 			trace_nfsd_clone_file_range_err(rqstp,
 					&nfsd4_get_cstate(rqstp)->save_fh,
 					src_pos,
 					&nfsd4_get_cstate(rqstp)->current_fh,
 					dst_pos,
 					count, status);
-			nfsd_reset_write_verifier(net_generic(nf_dst->nf_net,
-						  nfsd_net_id));
+			nfsd_reset_write_verifier(nn);
+			trace_nfsd_writeverf_reset(nn, rqstp, status);
 			ret = nfserrno(status);
 		}
 	}
@@ -1017,6 +1020,7 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
 	host_err = vfs_iter_write(file, &iter, &pos, flags);
 	if (host_err < 0) {
 		nfsd_reset_write_verifier(nn);
+		trace_nfsd_writeverf_reset(nn, rqstp, host_err);
 		goto out_nfserr;
 	}
 	*cnt = host_err;
@@ -1028,8 +1032,10 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
 
 	if (stable && use_wgather) {
 		host_err = wait_for_concurrent_writes(file);
-		if (host_err < 0)
+		if (host_err < 0) {
 			nfsd_reset_write_verifier(nn);
+			trace_nfsd_writeverf_reset(nn, rqstp, host_err);
+		}
 	}
 
 out_nfserr:
@@ -1151,6 +1157,7 @@ nfsd_commit(struct svc_rqst *rqstp, struct svc_fh *fhp,
 			break;
 		default:
 			nfsd_reset_write_verifier(nn);
+			trace_nfsd_writeverf_reset(nn, rqstp, err2);
 		}
 		err = nfserrno(err2);
 	} else