[PATCH 1/2] btrfs: add extra comments on extent_map members

[Qu Wenruo <wqu@suse.com>]

The extent_map structure is very critical to btrfs, as it is involved
for both read and write paths.

Unfortunately the structure is not properly explained, making it pretty
hard to understand nor to do further improvement.

This patch would add extra comments explaining the major numbers base on
my code reading.
Hopefully we can find more members to cleanup in the future.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/extent_map.h | 62 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 61 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/extent_map.h b/fs/btrfs/extent_map.h
index c5a098c99cc6..30322defcd03 100644
--- a/fs/btrfs/extent_map.h
+++ b/fs/btrfs/extent_map.h
@@ -37,21 +37,81 @@ enum {
 };
 
 /*
+ * This extent_map structure is an in-memory representation of file extents,
+ * it would represent all file extents (including holes, no matter if we have
+ * hole file extents).
+ *
  * 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 of the file extent. matching key.offset of
+	 * (INO EXTENT_DATA FILEPOS) key.
+	 */
 	u64 start;
+
+	/*
+	 * Length of the file extent.
+	 * For non-inlined file extents it's btrfs_file_extent_item::num_bytes.
+	 * For inlined file extents it's sectorsize. (as there is no reliable
+	 * btrfs_file_extent::num_bytes).
+	 */
 	u64 len;
+
+	/*
+	 * The modified range start/length, these are in-memory-only
+	 * members for fsync/logtree optimization.
+	 */
 	u64 mod_start;
 	u64 mod_len;
+
+	/*
+	 * The file offset of the original file extent before splitting.
+	 *
+	 * This is an in-memory-only member, mathcing
+	 * em::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.
+	 *
+	 * For non-compressed extents, this matches orig_block_len.
+	 */
 	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 hole extents 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 em::len.
+	 * Otherwise -1 (aka doesn't make much sense).
+	 */
 	u64 block_len;
 
 	/*
-- 
2.44.0