ocfs2: Introduce dir free space list

The only operation which doesn't get faster with directory indexing is
insert, which still has to walk the entire unindexed directory portion to
find a free block. This patch provides an improvement in directory insert
performance by maintaining a singly linked list of directory leaf blocks
which have space for additional dirents.

Signed-off-by: Mark Fasheh <mfasheh@suse.com>
Acked-by: Joel Becker <joel.becker@oracle.com>

fs/ocfs2/dir.h

@@ -39,11 +39,19 @@ struct ocfs2_dir_lookup_result {
 
 	struct buffer_head		*dl_dx_root_bh;	/* Root of indexed
 							 * tree */
+
 	struct buffer_head		*dl_dx_leaf_bh;	/* Indexed leaf block */
 	struct ocfs2_dx_entry		*dl_dx_entry;	/* Target dx_entry in
 							 * indexed leaf */
 	struct ocfs2_dx_hinfo		dl_hinfo;	/* Name hash results */
+
+	struct buffer_head		*dl_prev_leaf_bh;/* Previous entry in
+							  * dir free space
+							  * list. NULL if
+							  * previous entry is
+							  * dx root block. */
 };
+
 void ocfs2_free_dir_lookup_result(struct ocfs2_dir_lookup_result *res);
 
 int ocfs2_find_entry(const char *name, int namelen,