nfsd4: don't try to encode conflicting owner if low on space

I ran into this corner case in testing: in theory clients can provide
state owners up to 1024 bytes long.  In the sessions case there might be
a risk of this pushing us over the DRC slot size.

The conflicting owner isn't really that important, so let's humor a
client that provides a small maxresponsize_cached by allowing ourselves
to return without the conflicting owner instead of outright failing the
operation.
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4proc.c

@@ -1432,7 +1432,8 @@ nfsd4_proc_compound(struct svc_rqst *rqstp,
 #define op_encode_change_info_maxsz	(5)
 #define nfs4_fattr_bitmap_maxsz		(4)
 
-#define op_encode_lockowner_maxsz	(1 + XDR_QUADLEN(IDMAP_NAMESZ))
+/* We'll fall back on returning no lockowner if run out of space: */
+#define op_encode_lockowner_maxsz	(0)
 #define op_encode_lock_denied_maxsz	(8 + op_encode_lockowner_maxsz)
 
 #define nfs4_owner_maxsz		(1 + XDR_QUADLEN(IDMAP_NAMESZ))

fs/nfsd/nfs4xdr.c

@@ -2878,9 +2878,20 @@ nfsd4_encode_lock_denied(struct xdr_stream *xdr, struct nfsd4_lock_denied *ld)
 	struct xdr_netobj *conf = &ld->ld_owner;
 	__be32 *p;
 
+again:
 	p = xdr_reserve_space(xdr, 32 + XDR_LEN(conf->len));
-	if (!p)
+	if (!p) {
+		/*
+		 * Don't fail to return the result just because we can't
+		 * return the conflicting open:
+		 */
+		if (conf->len) {
+			conf->len = 0;
+			conf->data = NULL;
+			goto again;
+		}
 		return nfserr_resource;
+	}
 	WRITE64(ld->ld_start);
 	WRITE64(ld->ld_length);
 	WRITE32(ld->ld_type);
@@ -2888,7 +2899,6 @@ nfsd4_encode_lock_denied(struct xdr_stream *xdr, struct nfsd4_lock_denied *ld)
 		WRITEMEM(&ld->ld_clientid, 8);
 		WRITE32(conf->len);
 		WRITEMEM(conf->data, conf->len);
-		kfree(conf->data);
 	}  else {  /* non - nfsv4 lock in conflict, no clientid nor owner */
 		WRITE64((u64)0); /* clientid */
 		WRITE32(0); /* length of owner name */
@@ -2905,7 +2915,7 @@ nfsd4_encode_lock(struct nfsd4_compoundres *resp, __be32 nfserr, struct nfsd4_lo
 		nfserr = nfsd4_encode_stateid(xdr, &lock->lk_resp_stateid);
 	else if (nfserr == nfserr_denied)
 		nfserr = nfsd4_encode_lock_denied(xdr, &lock->lk_denied);
-
+	kfree(lock->lk_denied.ld_owner.data);
 	return nfserr;
 }