NFS: Only change the cookie verifier if the directory page cache is empty

The cached NFSv3/v4 readdir cookies are associated with a verifier,
which is checked by the server on subsequent calls to readdir, and is
only expected to change when the cookies (and hence also the page cache
contents) are considered invalid.
We therefore do have to store the verifier, but only when the page cache
is empty.

Fixes: b593c09f ("NFS: Improve handling of directory verifiers")
Signed-off-by: NTrond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/dir.c

@@ -929,7 +929,12 @@ static int find_and_lock_cache_page(struct nfs_readdir_descriptor *desc)
 			}
 			return res;
 		}
-		memcpy(nfsi->cookieverf, verf, sizeof(nfsi->cookieverf));
+		/*
+		 * Set the cookie verifier if the page cache was empty
+		 */
+		if (desc->page_index == 0)
+			memcpy(nfsi->cookieverf, verf,
+			       sizeof(nfsi->cookieverf));
 	}
 	res = nfs_readdir_search_array(desc);
 	if (res == 0) {