1 files changed, 24 insertions, 0 deletions

diff --git a/fs/ext4/ext4_extents.h b/fs/ext4/ext4_extents.h
index 0f58b86e3a0..cb1b2c91996 100644
--- a/fs/ext4/ext4_extents.h
+++ b/fs/ext4/ext4_extents.h
@@ -63,9 +63,22 @@
  * ext4_inode has i_block array (60 bytes total).
  * The first 12 bytes store ext4_extent_header;
  * the remainder stores an array of ext4_extent.
+ * For non-inode extent blocks, ext4_extent_tail
+ * follows the array.
  */
 
 /*
+ * This is the extent tail on-disk structure.
+ * All other extent structures are 12 bytes long.  It turns out that
+ * block_size % 12 >= 4 for at least all powers of 2 greater than 512, which
+ * covers all valid ext4 block sizes.  Therefore, this tail structure can be
+ * crammed into the end of the block without having to rebalance the tree.
+ */
+struct ext4_extent_tail {
+        __le32  et_checksum;    /* crc32c(uuid+inum+extent_block) */
+};
+
+/*
  * This is the extent on-disk structure.
  * It's used at the bottom of the tree.
  */
@@ -101,6 +114,17 @@ struct ext4_extent_header {
 
 #define EXT4_EXT_MAGIC          cpu_to_le16(0xf30a)
 
+#define EXT4_EXTENT_TAIL_OFFSET(hdr) \
+        (sizeof(struct ext4_extent_header) + \
+         (sizeof(struct ext4_extent) * le16_to_cpu((hdr)->eh_max)))
+
+static inline struct ext4_extent_tail *
+find_ext4_extent_tail(struct ext4_extent_header *eh)
+{
+        return (struct ext4_extent_tail *)(((void *)eh) +
+                                           EXT4_EXTENT_TAIL_OFFSET(eh));
+}
+
 /*
  * Array of ext4_ext_path contains path to some extent.
  * Creation/lookup routines use it for traversal/splitting/etc.