nfsd: reset write verifier on deferred writeback errors

commit 2090b05 upstream.

nfsd_vfs_write() and nfsd_commit() both call filemap_check_wb_err() to
detect deferred writeback errors, but neither rotates the server's write
verifier (nn->writeverf) when this check fails. Every other
durable-storage-failure path in these functions calls
commit_reset_write_verifier() before returning an error.

The missing rotation means clients holding UNSTABLE write data under the
current verifier will COMMIT, receive the unchanged verifier back, and
conclude their data is durable — silently dropping data that failed
writeback. This violates the UNSTABLE+COMMIT durability contract
(RFC 1813 §3.3.7, RFC 8881 §18.32).

Add commit_reset_write_verifier() calls at both filemap_check_wb_err()
error sites, matching the pattern used by adjacent error paths in the
same functions. The helper already filters -EAGAIN and -ESTALE
internally, so the calls are unconditionally safe.

Reported-by: Chris Mason <clm@meta.com>
Fixes: 555dbf1 ("nfsd: Replace use of rwsem with errseq_t")
Cc: stable@vger.kernel.org
Assisted-by: kres:claude-opus-4-6
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
[ cel: 6.6.y predates the commit_reset_write_verifier() helper (v6.7);
  open-coded nfsd_reset_write_verifier() and the reset tracepoint at
  both sites, matching the other reset paths in these functions ]
Signed-off-by: Chuck Lever <cel@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: jtlayton

fs/nfsd/vfs.c

@@ -1185,8 +1185,11 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
 	nfsd_stats_io_write_add(nn, exp, *cnt);
 	fsnotify_modify(file);
 	host_err = filemap_check_wb_err(file->f_mapping, since);
-	if (host_err < 0)
+	if (host_err < 0) {
+		nfsd_reset_write_verifier(nn);
+		trace_nfsd_writeverf_reset(nn, rqstp, host_err);
 		goto out_nfserr;
+	}
 
 	if (stable && use_wgather) {
 		host_err = wait_for_concurrent_writes(file);
@@ -1330,6 +1333,10 @@ nfsd_commit(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
 			nfsd_copy_write_verifier(verf, nn);
 			err2 = filemap_check_wb_err(nf->nf_file->f_mapping,
 						    since);
+			if (err2 < 0) {
+				nfsd_reset_write_verifier(nn);
+				trace_nfsd_writeverf_reset(nn, rqstp, err2);
+			}
 			err = nfserrno(err2);
 			break;
 		case -EINVAL: