Btrfs: Optimize tree log block allocations

Since tree log blocks get freed every transaction, they never really
need to be written to disk.  This skips the step where we update
metadata to record they were allocated.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 14 insertions, 5 deletions

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 3181759da1cf..c479d71e2869 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -1872,8 +1872,15 @@ static int __btrfs_free_extent(struct btrfs_trans_handle *trans,
 		return 0;
 	}
 	/* if metadata always pin */
-	if (owner_objectid < BTRFS_FIRST_FREE_OBJECTID)
+	if (owner_objectid < BTRFS_FIRST_FREE_OBJECTID) {
+		if (root->root_key.objectid == BTRFS_TREE_LOG_OBJECTID) {
+			/* btrfs_free_reserved_extent */
+			set_extent_dirty(&root->fs_info->free_space_cache,
+				 bytenr, bytenr + num_bytes - 1, GFP_NOFS);
+			return 0;
+		}
 		pin = 1;
+	}
 
 	/* if data pin when any transaction has committed this */
 	if (ref_generation != trans->transid)
@@ -2361,11 +2368,13 @@ int btrfs_alloc_extent(struct btrfs_trans_handle *trans,
 				     min_alloc_size, empty_size, hint_byte,
 				     search_end, ins, data);
 	BUG_ON(ret);
-	ret = __btrfs_alloc_reserved_extent(trans, root, root_objectid,
-					    ref_generation, owner,
-					    owner_offset, ins);
-	BUG_ON(ret);
+	if (root_objectid != BTRFS_TREE_LOG_OBJECTID) {
+		ret = __btrfs_alloc_reserved_extent(trans, root, root_objectid,
+						    ref_generation, owner,
+						    owner_offset, ins);
+		BUG_ON(ret);
 
+	}
 	maybe_unlock_mutex(root);
 	return ret;
 }