btrfs: Cleanup and simplify find_newest_super_backup

Backup roots are always written in a circular manner. By definition we
can only ever have 1 backup root whose generation equals to that of the
superblock. Hence, the 'if' in the for loop will trigger at most once.
This is sufficient to return the newest backup root.

Furthermore the newest_gen parameter is always set to the generation of
the superblock. This value can be obtained from the fs_info.

This patch removes the unnecessary code dealing with the wraparound
case and makes 'newest_gen' a local variable.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -1790,36 +1790,29 @@ static int transaction_kthread(void *arg)
 }
 
 /*
- * this will find the highest generation in the array of
- * root backups.  The index of the highest array is returned,
- * or -1 if we can't find anything.
+ * This will find the highest generation in the array of root backups.  The
+ * index of the highest array is returned, or -EINVAL if we can't find
+ * anything.
  *
  * We check to make sure the array is valid by comparing the
  * generation of the latest  root in the array with the generation
  * in the super block.  If they don't match we pitch it.
  */
-static int find_newest_super_backup(struct btrfs_fs_info *info, u64 newest_gen)
+static int find_newest_super_backup(struct btrfs_fs_info *info)
 {
+	const u64 newest_gen = btrfs_super_generation(info->super_copy);
 	u64 cur;
-	int newest_index = -1;
 	struct btrfs_root_backup *root_backup;
 	int i;
 
 	for (i = 0; i < BTRFS_NUM_BACKUP_ROOTS; i++) {
 		root_backup = info->super_copy->super_roots + i;
 		cur = btrfs_backup_tree_root_gen(root_backup);
 		if (cur == newest_gen)
-			newest_index = i;
+			return i;
 	}
 
-	/* check to see if we actually wrapped around */
-	if (newest_index == BTRFS_NUM_BACKUP_ROOTS - 1) {
-		root_backup = info->super_copy->super_roots;
-		cur = btrfs_backup_tree_root_gen(root_backup);
-		if (cur == newest_gen)
-			newest_index = 0;
-	}
-	return newest_index;
+	return -EINVAL;
 }
 
 
@@ -1831,11 +1824,11 @@ static int find_newest_super_backup(struct btrfs_fs_info *info, u64 newest_gen)
 static void find_oldest_super_backup(struct btrfs_fs_info *info,
 				     u64 newest_gen)
 {
-	int newest_index = -1;
+	int newest_index;
 
-	newest_index = find_newest_super_backup(info, newest_gen);
+	newest_index = find_newest_super_backup(info);
 	/* if there was garbage in there, just move along */
-	if (newest_index == -1) {
+	if (newest_index == -EINVAL) {
 		info->backup_root_index = 0;
 	} else {
 		info->backup_root_index = (newest_index + 1) % BTRFS_NUM_BACKUP_ROOTS;
@@ -1952,9 +1945,7 @@ static noinline int next_root_backup(struct btrfs_fs_info *info,
 	int newest = *backup_index;
 
 	if (*num_backups_tried == 0) {
-		u64 gen = btrfs_super_generation(super);
-
-		newest = find_newest_super_backup(info, gen);
+		newest = find_newest_super_backup(info);
 		if (newest == -1)
 			return -1;