[PATCH 3/3] btrfs: factor out reading of bg from find_frist_block_group

[Johannes Thumshirn <johannes.thumshirn@wdc.com>]

When find_first_block_group() finds a block group item in the extent-tree,
it does a lookup of the object in the extent mapping tree and does further
checks on the item.

Factor out this step from find_first_block_group() so we can further
simplify the code.

As a bonus we even get a slight decrease in size:
$ ./scripts/bloat-o-meter btrfs_old.ko btrfs.ko
add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-2503 (-2503)
Function                                     old     new   delta
btrfs_read_block_groups.cold                 462     337    -125
btrfs_read_block_groups                     4787    2409   -2378
Total: Before=2369371, After=2366868, chg -0.11%

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
---
 fs/btrfs/block-group.c | 95 ++++++++++++++++++++++--------------------
 1 file changed, 49 insertions(+), 46 deletions(-)

diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
index c4462e4c8413..3d9e0ee1d1be 100644
--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -1522,6 +1522,52 @@ void btrfs_mark_bg_unused(struct btrfs_block_group *bg)
 	spin_unlock(&fs_info->unused_bgs_lock);
 }
 
+static int read_bg_from_eb(struct btrfs_fs_info *fs_info, struct btrfs_key *key,
+			   struct extent_buffer *leaf, int slot)
+{
+	struct extent_map_tree *em_tree;
+	struct extent_map *em;
+	struct btrfs_block_group_item bg;
+	u64 flags;
+	int ret = 0;
+
+	em_tree = &fs_info->mapping_tree;
+	read_lock(&em_tree->lock);
+	em = lookup_extent_mapping(em_tree, key->objectid, key->offset);
+	read_unlock(&em_tree->lock);
+	if (!em) {
+		btrfs_err(fs_info,
+			  "logical %llu len %llu found bg but no related chunk",
+			  key->objectid, key->offset);
+		return -ENOENT;
+	}
+
+	if (em->start != key->objectid || em->len != key->offset) {
+		btrfs_err(fs_info,
+			  "block group %llu len %llu mismatch with chunk %llu len %llu",
+			  key->objectid, key->offset, em->start, em->len);
+		ret = -EUCLEAN;
+		goto out_free_em;
+	}
+
+	read_extent_buffer(leaf, &bg, btrfs_item_ptr_offset(leaf, slot),
+			   sizeof(bg));
+	flags = btrfs_stack_block_group_flags(&bg) &
+		BTRFS_BLOCK_GROUP_TYPE_MASK;
+
+	if (flags != (em->map_lookup->type & BTRFS_BLOCK_GROUP_TYPE_MASK)) {
+		btrfs_err(fs_info,
+			  "block group %llu len %llu type flags 0x%llx mismatch with chunk type flags 0x%llx",
+			  key->objectid, key->offset, flags,
+			  (BTRFS_BLOCK_GROUP_TYPE_MASK & em->map_lookup->type));
+		ret = -EUCLEAN;
+	}
+
+out_free_em:
+	free_extent_map(em);
+	return ret;
+}
+
 static int find_first_block_group(struct btrfs_fs_info *fs_info,
 				  struct btrfs_path *path,
 				  struct btrfs_key *key)
@@ -1530,8 +1576,6 @@ static int find_first_block_group(struct btrfs_fs_info *fs_info,
 	int ret;
 	struct btrfs_key found_key;
 	struct extent_buffer *leaf;
-	struct btrfs_block_group_item bg;
-	u64 flags;
 	int slot;
 
 	ret = btrfs_search_slot(NULL, root, key, path, 0, 0);
@@ -1552,50 +1596,9 @@ static int find_first_block_group(struct btrfs_fs_info *fs_info,
 		btrfs_item_key_to_cpu(leaf, &found_key, slot);
 
 		if (found_key.objectid >= key->objectid &&
-		    found_key.type == BTRFS_BLOCK_GROUP_ITEM_KEY) {
-			struct extent_map_tree *em_tree;
-			struct extent_map *em;
-
-			em_tree = &fs_info->mapping_tree;
-			read_lock(&em_tree->lock);
-			em = lookup_extent_mapping(em_tree, found_key.objectid,
-						   found_key.offset);
-			read_unlock(&em_tree->lock);
-			if (!em) {
-				btrfs_err(fs_info,
-			"logical %llu len %llu found bg but no related chunk",
-					  found_key.objectid, found_key.offset);
-				ret = -ENOENT;
-			} else if (em->start != found_key.objectid ||
-				   em->len != found_key.offset) {
-				btrfs_err(fs_info,
-		"block group %llu len %llu mismatch with chunk %llu len %llu",
-					  found_key.objectid, found_key.offset,
-					  em->start, em->len);
-				ret = -EUCLEAN;
-			} else {
-				read_extent_buffer(leaf, &bg,
-					btrfs_item_ptr_offset(leaf, slot),
-					sizeof(bg));
-				flags = btrfs_stack_block_group_flags(&bg) &
-					BTRFS_BLOCK_GROUP_TYPE_MASK;
-
-				if (flags != (em->map_lookup->type &
-					      BTRFS_BLOCK_GROUP_TYPE_MASK)) {
-					btrfs_err(fs_info,
-"block group %llu len %llu type flags 0x%llx mismatch with chunk type flags 0x%llx",
-						found_key.objectid,
-						found_key.offset, flags,
-						(BTRFS_BLOCK_GROUP_TYPE_MASK &
-						 em->map_lookup->type));
-					ret = -EUCLEAN;
-				} else {
-					ret = 0;
-				}
-			}
-			free_extent_map(em);
-			return ret;
-		}
+		    found_key.type == BTRFS_BLOCK_GROUP_ITEM_KEY)
+			return read_bg_from_eb(fs_info, &found_key, leaf, slot);
+
 		path->slots[0]++;
 	}
 
-- 
2.24.1