diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 531e0a8..9331d02 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -2654,17 +2654,30 @@ int btrfs_find_item(struct btrfs_root *fs_root, struct btrfs_path *path,
 }
 
 /*
- * look for key in the tree.  path is filled in with nodes along the way
- * if key is found, we return zero and you can find the item in the leaf
- * level of the path (level 0)
+ * Search a btree for a specific key. Optionally update the btree for
+ * item insertion or removal.
  *
- * If the key isn't found, the path points to the slot where it should
- * be inserted, and 1 is returned.  If there are other errors during the
- * search a negative error number is returned.
+ *   @trans - Transaction handle (NULL for none, requires @cow == 0).
+ *   @root - The btree to be searched.
+ *   @key - key for search.
+ *   @p - Points to a btrfs_path to be populated with btree node and index
+ *      values encountered during traversal. (See also struct btrfs_path.)
+ *   @ins_len - byte length of item as follows:
+ *      >0: byte length of item for insertion.  Btree nodes/leaves are
+ *          split as required.
+ *      <0: byte length of item for deletion.  Btree nodes/leaves are
+ *          merged as required.
+ *       0: Do not modify the btree (search only).
+ *   @cow - 0 to nocow, or !0 to cow for btree update.
  *
- * if ins_len > 0, nodes and leaves will be split as we walk down the
- * tree.  if ins_len < 0, nodes will be merged as we walk down the tree (if
- * possible)
+ * Return values:
+ *   0: @key was found and @p was updated to indicate the leaf and item
+ *       slot where the item may be accessed.
+ *   1: @key was not found and @p was updated to indicate the leaf and
+ *       item slot where the item may be inserted.
+ *  <0: an error occurred.
+ *
+ * Note, insertion/removal updates will re-balance the btree as needed.
  */
 int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 		      const struct btrfs_key *key, struct btrfs_path *p,
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 2154b5a..e980caa 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -318,20 +318,36 @@ struct btrfs_node {
 } __attribute__ ((__packed__));
 
 /*
- * btrfs_paths remember the path taken from the root down to the leaf.
- * level 0 is always the leaf, and nodes[1...BTRFS_MAX_LEVEL] will point
- * to any other levels that are present.
- *
- * The slots array records the index of the item or block pointer
- * used while walking the tree.
+ * Used with btrfs_search_slot() to record a btree search path traversed from a
+ * root down to a leaf.
  */
 enum { READA_NONE = 0, READA_BACK, READA_FORWARD };
 struct btrfs_path {
+	/*
+	 * The nodes[] and slots[] arrays are indexed according to btree
+	 * levels. Index 0 corresponds to the lowest (leaf) level. Increasing
+	 * indexes correspond to interior btree nodes at subsequently higher
+	 * levels.
+	 *
+	 * nodes[0] always points to a leaf. nodes[1] points to a btree node
+	 * which contains a block pointer referencing that leaf. For higher
+	 * indexes, node[n] points to a btree node which contains a block
+	 * pointer referencing node[n-1].
+	 */
 	struct extent_buffer *nodes[BTRFS_MAX_LEVEL];
+
+	/*
+	 * The slots[0] value identifies an item index in the leaf at nodes[0].
+	 * Each slots[n] value identifies a block pointer index in the
+	 * corresponding tree node at nodes[n].
+	 */
 	int slots[BTRFS_MAX_LEVEL];
+
 	/* if there is real range locking, this locks field will change */
 	u8 locks[BTRFS_MAX_LEVEL];
+
 	u8 reada;
+
 	/* keep some upper locks as we walk down */
 	u8 lowest_level;
 
diff --git a/fs/btrfs/extent_map.h b/fs/btrfs/extent_map.h
index 64365bb..34ea85c3 100644
--- a/fs/btrfs/extent_map.h
+++ b/fs/btrfs/extent_map.h
@@ -5,35 +5,57 @@
 #include <linux/rbtree.h>
 #include <linux/refcount.h>
 
-#define EXTENT_MAP_LAST_BYTE ((u64)-4)
-#define EXTENT_MAP_HOLE ((u64)-3)
-#define EXTENT_MAP_INLINE ((u64)-2)
-#define EXTENT_MAP_DELALLOC ((u64)-1)
-
-/* bits for the flags field */
-#define EXTENT_FLAG_PINNED 0 /* this entry not yet on disk, don't free it */
-#define EXTENT_FLAG_COMPRESSED 1
-#define EXTENT_FLAG_VACANCY 2 /* no file extent item found */
-#define EXTENT_FLAG_PREALLOC 3 /* pre-allocated extent */
-#define EXTENT_FLAG_LOGGING 4 /* Logging this extent */
-#define EXTENT_FLAG_FILLING 5 /* Filling in a preallocated extent */
-#define EXTENT_FLAG_FS_MAPPING 6 /* filesystem extent mapping type */
+/*
+ * Reserve some special-case values for the extent_map .start, .block_start,
+ *: and .orig_start fields.
+ */
+#define EXTENT_MAP_LAST_BYTE	((u64)-4)	/* Lowest reserved value */
+#define EXTENT_MAP_HOLE		((u64)-3)	/* Unwritten extent */
+#define EXTENT_MAP_INLINE	((u64)-2)	/* Inlined file data */
+#define EXTENT_MAP_DELALLOC	((u64)-1)	/* Delayed block allocation */
 
+/*
+ * Bit flags for the extent_map .flags field.
+ */
+#define EXTENT_FLAG_PINNED	0 /* entry not yet on disk, don't free it */
+#define EXTENT_FLAG_COMPRESSED	1
+#define EXTENT_FLAG_VACANCY	2 /* no file extent item found */
+#define EXTENT_FLAG_PREALLOC	3 /* pre-allocated extent */
+#define EXTENT_FLAG_LOGGING	4 /* Logging this extent */
+#define EXTENT_FLAG_FILLING	5 /* Filling in a preallocated extent */
+#define EXTENT_FLAG_FS_MAPPING	6 /* filesystem extent mapping type */
+
+/*
+ * In-memory representation of a file extent (regular/prealloc/inline).
+ */
 struct extent_map {
 	struct rb_node rb_node;
 
 	/* all of these are in bytes */
-	u64 start;
-	u64 len;
+	u64 start;	/* logical byte offset in the file */
+	u64 len;	/* byte length of extent in the file */
 	u64 mod_start;
 	u64 mod_len;
 	u64 orig_start;
 	u64 orig_block_len;
+
+	/* max. bytes needed to hold the (uncompressed) extent in memory. */
 	u64 ram_bytes;
+
+	/*
+	 * For regular/prealloc, block_start is a logical address denoting the
+	 * start of the extent data, and block_len is the on-disk byte length
+	 * of the extent (which may be comressed). block_start may be
+	 * EXTENT_MAP_HOLE if the extent is unwritten.  For inline, block_start
+	 * is EXTENT_MAP_INLINE and block_len is (u64)-1.  See also
+	 * btrfs_extent_item_to_extent_map() and btrfs_get_extent().
+	 */
 	u64 block_start;
 	u64 block_len;
-	u64 generation;
-	unsigned long flags;
+
+	u64 generation;		/* transaction id */
+	unsigned long flags;	/* EXTENT_FLAG_* bit flags as above */
+
 	union {
 		struct block_device *bdev;
 
@@ -44,7 +66,7 @@ struct extent_map {
 		struct map_lookup *map_lookup;
 	};
 	refcount_t refs;
-	unsigned int compress_type;
+	unsigned int compress_type;	/* BTRFS_COMPRESS_* */
 	struct list_head list;
 };
 
diff --git a/fs/btrfs/file-item.c b/fs/btrfs/file-item.c
index fdcb410..5d63172 100644
--- a/fs/btrfs/file-item.c
+++ b/fs/btrfs/file-item.c
@@ -929,6 +929,9 @@ int btrfs_csum_file_blocks(struct btrfs_trans_handle *trans,
 	goto out;
 }
 
+/*
+ * Populate an extent_map from the btrfs_file_extent_item contents.
+ */
 void btrfs_extent_item_to_extent_map(struct btrfs_inode *inode,
 				     const struct btrfs_path *path,
 				     struct btrfs_file_extent_item *fi,
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 72c7b38..b6d5d94 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -6916,17 +6916,28 @@ static noinline int uncompress_inline(struct btrfs_path *path,
 }
 
 /*
- * a bit scary, this does extent mapping from logical file offset to the disk.
- * the ugly parts come from merging extents from the disk with the in-ram
+ * btrfs_get_extent - find or create an extent_map for the (@start, @len) range.
+ *
+ *  @inode - inode containing the desired extent.
+ *  @page - page for inlined extent (NULL if none)
+ *  @page_offset - byte offset within @page
+ *  @start - logical byte offset in the file.
+ *  @len - byte length of the range.
+ *  @create - for inlined extents: 0 to update @page with extent data from
+ *            the item; 1 to bypass the update.
+ *
+ * A bit scary, this does extent mapping from logical file offset to the disk.
+ * The ugly parts come from merging extents from the disk with the in-ram
  * representation.  This gets more complex because of the data=ordered code,
  * where the in-ram extents might be locked pending data=ordered completion.
  *
  * This also copies inline extents directly into the page.
+ *
+ * Returns the extent_map, or error code.
  */
 struct extent_map *btrfs_get_extent(struct btrfs_inode *inode,
-		struct page *page,
-	    size_t pg_offset, u64 start, u64 len,
-		int create)
+				    struct page *page, size_t pg_offset,
+				    u64 start, u64 len, int create)
 {
 	struct btrfs_fs_info *fs_info = btrfs_sb(inode->vfs_inode.i_sb);
 	int ret;
diff --git a/include/uapi/linux/btrfs_tree.h b/include/uapi/linux/btrfs_tree.h
index 6d6e5da..693636a 100644
--- a/include/uapi/linux/btrfs_tree.h
+++ b/include/uapi/linux/btrfs_tree.h
@@ -730,6 +730,7 @@ struct btrfs_balance_item {
 	__le64 unused[4];
 } __attribute__ ((__packed__));
 
+/* Values for .type field in btrfs_file_extent_item */
 #define BTRFS_FILE_EXTENT_INLINE 0
 #define BTRFS_FILE_EXTENT_REG 1
 #define BTRFS_FILE_EXTENT_PREALLOC 2
