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/ocfs2_fs.h

@@ -416,6 +416,7 @@ static struct ocfs2_system_inode_info ocfs2_system_inodes[NUM_SYSTEM_INODES] = {
 #define OCFS2_DIR_REC_LEN(name_len)	(((name_len) + OCFS2_DIR_MEMBER_LEN + \
                                           OCFS2_DIR_ROUND) & \
 					 ~OCFS2_DIR_ROUND)
+#define OCFS2_DIR_MIN_REC_LEN	OCFS2_DIR_REC_LEN(1)
 
 #define OCFS2_LINK_MAX		32000
 
@@ -842,7 +843,9 @@ struct ocfs2_dx_root_block {
 	__le16		dr_reserved1;
 	__le64		dr_dir_blkno;		/* Pointer to parent inode */
 	__le64		dr_reserved2;
-	__le64		dr_reserved3[16];
+	__le64		dr_free_blk;		/* Pointer to head of free
+						 * unindexed block list. */
+	__le64		dr_reserved3[15];
 	union {
 		struct ocfs2_extent_list dr_list; /* Keep this aligned to 128
 						   * bits for maximum space