ceph: fix race during filling readdir cache

Readdir cache uses page cache to save dentry pointers. When adding
dentry pointers to middle of a page, we need to make sure the page
already exists. Otherwise the beginning part of the page will be
invalid pointers.
Signed-off-by: Yan, Zheng <zyan@redhat.com>

fs/ceph/inode.c

@@ -1349,15 +1349,20 @@ static int fill_readdir_cache(struct inode *dir, struct dentry *dn,
 
 	if (!ctl->page || pgoff != page_index(ctl->page)) {
 		ceph_readdir_cache_release(ctl);
-		ctl->page  = grab_cache_page(&dir->i_data, pgoff);
+		if (idx == 0)
+			ctl->page = grab_cache_page(&dir->i_data, pgoff);
+		else
+			ctl->page = find_lock_page(&dir->i_data, pgoff);
 		if (!ctl->page) {
 			ctl->index = -1;
-			return -ENOMEM;
+			return idx == 0 ? -ENOMEM : 0;
 		}
 		/* reading/filling the cache are serialized by
 		 * i_mutex, no need to use page lock */
 		unlock_page(ctl->page);
 		ctl->dentries = kmap(ctl->page);
+		if (idx == 0)
+			memset(ctl->dentries, 0, PAGE_CACHE_SIZE);
 	}
 
 	if (req->r_dir_release_cnt == atomic64_read(&ci->i_release_count) &&