Btrfs: use signed integer instead of unsigned long integer for log transid

The log trans id is initialized to be 0 every time we create a log tree,
and the log tree need be re-created after a new transaction is started,
it means the log trans id is unlikely to be a huge number, so we can use
signed integer instead of unsigned long integer to save a bit space.
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>

fs/btrfs/btrfs_inode.h

@@ -109,14 +109,17 @@ struct btrfs_inode {
 	u64 last_trans;
 
 	/*
-	 * log transid when this inode was last modified
+	 * transid that last logged this inode
 	 */
-	u64 last_sub_trans;
+	u64 logged_trans;
 
 	/*
-	 * transid that last logged this inode
+	 * log transid when this inode was last modified
 	 */
-	u64 logged_trans;
+	int last_sub_trans;
+
+	/* a local copy of root's last_log_commit */
+	int last_log_commit;
 
 	/* total number of bytes pending delalloc, used by stat to calc the
 	 * real block usage of the file
@@ -155,9 +158,6 @@ struct btrfs_inode {
 	/* flags field from the on disk inode */
 	u32 flags;
 
-	/* a local copy of root's last_log_commit */
-	unsigned long last_log_commit;
-
 	/*
 	 * Counters to keep track of the number of extent item's we may use due
 	 * to delalloc and such.  outstanding_extents is the number of extent

fs/btrfs/ctree.h

@@ -1721,8 +1721,8 @@ struct btrfs_root {
 	atomic_t log_writers;
 	atomic_t log_commit[2];
 	atomic_t log_batch;
-	unsigned long log_transid;
-	unsigned long last_log_commit;
+	int log_transid;
+	int last_log_commit;
 	pid_t log_start_pid;
 	bool log_multiple_pids;
 

fs/btrfs/tree-log.c

@@ -2362,7 +2362,7 @@ static int update_log_root(struct btrfs_trans_handle *trans,
 }
 
 static int wait_log_commit(struct btrfs_trans_handle *trans,
-			   struct btrfs_root *root, unsigned long transid)
+			   struct btrfs_root *root, int transid)
 {
 	DEFINE_WAIT(wait);
 	int index = transid % 2;
@@ -2434,7 +2434,7 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
 	int ret;
 	struct btrfs_root *log = root->log_root;
 	struct btrfs_root *log_root_tree = root->fs_info->log_root_tree;
-	unsigned long log_transid = 0;
+	int log_transid = 0;
 	struct blk_plug plug;
 
 	mutex_lock(&root->log_mutex);