@@ -35,19 +35,73 @@ enum {
};
/*
+ * This structure represents file extents and holes.
+ *
+ * Unlike on-disk file extent items, extent maps can be merged to
+ * save memory.
+ * This means members only match file extent items before any merging.
+ *
* Keep this structure as compact as possible, as we can have really large
* amounts of allocated extent maps at any time.
*/
struct extent_map {
struct rb_node rb_node;
- /* all of these are in bytes */
+ /* All of these are in bytes. */
+
+ /* File offset matching the offset of a BTRFS_EXTENT_ITEM_KEY key. */
u64 start;
+
+ /*
+ * Length of the file extent.
+ *
+ * For non-inlined file extents it's btrfs_file_extent_item::num_bytes.
+ * For inline extents it's sectorsize, since inline data starts at
+ * offsetof(struct btrfs_file_extent_item, disk_bytenr) thus
+ * btrfs_file_extent_item::num_bytes is not valid.
+ */
u64 len;
+
+ /*
+ * The file offset of the original file extent before splitting.
+ *
+ * This is an in-memory only member, matching
+ * extent_map::start - btrfs_file_extent_item::offset for
+ * regular/preallocated extents. EXTENT_MAP_HOLE otherwise.
+ */
u64 orig_start;
+
+ /*
+ * The full on-disk extent length, matching
+ * btrfs_file_extent_item::disk_num_bytes.
+ */
u64 orig_block_len;
+
+ /*
+ * The decompressed size of the whole on-disk extent, matching
+ * btrfs_file_extent_item::ram_bytes.
+ */
u64 ram_bytes;
+
+ /*
+ * The on-disk logical bytenr for the file extent.
+ *
+ * For compressed extents it matches btrfs_file_extent_item::disk_bytenr.
+ * For uncompressed extents it matches
+ * btrfs_file_extent_item::disk_bytenr + btrfs_file_extent_item::offset
+ *
+ * For holes it is EXTENT_MAP_HOLE and for inline extents it is
+ * EXTENT_MAP_INLINE.
+ */
u64 block_start;
+
+ /*
+ * The on-disk length for the file extent.
+ *
+ * For compressed extents it matches btrfs_file_extent_item::disk_num_bytes.
+ * For uncompressed extents it matches extent_map::len.
+ * For holes and inline extents it's -1 and shouldn't be used.
+ */
u64 block_len;
/*