Mixed back reference (FORWARD ROLLING FORMAT CHANGE)

This commit introduces a new kind of back reference for btrfs metadata.
Once a filesystem has been mounted with this commit, IT WILL NO LONGER
BE MOUNTABLE BY OLDER KERNELS.

The new back ref provides information about pointer's key, level and in which
tree the pointer lives. This information allow us to find the pointer by
searching the tree. The shortcoming of the new back ref is that it only works
for pointers in tree blocks referenced by their owner trees.

This is mostly a problem for snapshots, where resolving one of these fuzzy back
references would be O(number_of_snapshots) and quite slow.  The solution used
here is to use the fuzzy back references in the common case where a given tree
block is only referenced by one root, and use the full back references when
multiple roots have a reference

1 files changed, 52 insertions, 20 deletions

diff --git a/btrfs-image.c b/btrfs-image.c
index 62b3dd8..f2bbcc8 100644
--- a/btrfs-image.c
+++ b/btrfs-image.c
@@ -440,6 +440,42 @@ static int add_metadata(u64 start, u64 size, struct metadump_struct *md)
 	return 0;
 }
 
+#ifdef BTRFS_COMPAT_EXTENT_TREE_V0
+static int is_tree_block(struct btrfs_root *extent_root,
+			 struct btrfs_path *path, u64 bytenr)
+{
+	struct extent_buffer *leaf;
+	struct btrfs_key key;
+	u64 ref_objectid;
+	int ret;
+
+	leaf = path->nodes[0];
+	while (1) {
+		struct btrfs_extent_ref_v0 *ref_item;
+		path->slots[0]++;
+		if (path->slots[0] >= btrfs_header_nritems(leaf)) {
+			ret = btrfs_next_leaf(extent_root, path);
+			BUG_ON(ret < 0);
+			if (ret > 0)
+				break;
+			leaf = path->nodes[0];
+		}
+		btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
+		if (key.objectid != bytenr)
+			break;
+		if (key.type != BTRFS_EXTENT_REF_V0_KEY)
+			continue;
+		ref_item = btrfs_item_ptr(leaf, path->slots[0],
+					  struct btrfs_extent_ref_v0);
+		ref_objectid = btrfs_ref_objectid_v0(leaf, ref_item);
+		if (ref_objectid < BTRFS_FIRST_FREE_OBJECTID)
+			return 1;
+		break;
+	}
+	return 0;
+}
+#endif
+
 static int create_metadump(const char *input, FILE *out, int num_threads,
 			   int compress_level)
 {
@@ -447,12 +483,11 @@ static int create_metadump(const char *input, FILE *out, int num_threads,
 	struct btrfs_root *extent_root;
 	struct btrfs_path *path;
 	struct extent_buffer *leaf;
-	struct btrfs_extent_ref *ref_item;
+	struct btrfs_extent_item *ei;
 	struct btrfs_key key;
 	struct metadump_struct metadump;
 	u64 bytenr;
 	u64 num_bytes;
-	u64 ref_objectid;
 	int ret;
 
 	root = open_ctree(input, 0, 0);
@@ -495,29 +530,26 @@ static int create_metadump(const char *input, FILE *out, int num_threads,
 
 		bytenr = key.objectid;
 		num_bytes = key.offset;
-		while (1) {
-			path->slots[0]++;
-			if (path->slots[0] >= btrfs_header_nritems(leaf)) {
-				ret = btrfs_next_leaf(extent_root, path);
-				BUG_ON(ret < 0);
-				if (ret > 0)
-					break;
-				leaf = path->nodes[0];
+
+		if (btrfs_item_size_nr(leaf, path->slots[0]) > sizeof(*ei)) {
+			ei = btrfs_item_ptr(leaf, path->slots[0],
+					    struct btrfs_extent_item);
+			if (btrfs_extent_flags(leaf, ei) &
+			    BTRFS_EXTENT_FLAG_TREE_BLOCK) {
+				ret = add_metadata(bytenr, num_bytes,
+						   &metadump);
+				BUG_ON(ret);
 			}
-			btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
-			if (key.objectid != bytenr)
-				break;
-			if (key.type != BTRFS_EXTENT_REF_KEY)
-				continue;
-			ref_item = btrfs_item_ptr(leaf, path->slots[0],
-						  struct btrfs_extent_ref);
-			ref_objectid = btrfs_ref_objectid(leaf, ref_item);
-			if (ref_objectid < BTRFS_FIRST_FREE_OBJECTID) {
+		} else {
+#ifdef BTRFS_COMPAT_EXTENT_TREE_V0
+			if (is_tree_block(extent_root, path, bytenr)) {
 				ret = add_metadata(bytenr, num_bytes,
 						   &metadump);
 				BUG_ON(ret);
-				break;
 			}
+#else
+			BUG_ON(1);
+#endif
 		}
 		bytenr += num_bytes;
 	}