f2fs: fix incorrect calculation with total/free inode num

Theoretically, our total inodes number is the same as total node number, but
there are three node ids are reserved in f2fs, they are 0, 1 (node nid), and 2
(meta nid), and they should never be used by user, so our total/free inode
number calculated in ->statfs is wrong.

This patch indroduces F2FS_RESERVED_NODE_NUM and then fixes this issue by
recalculating total/free inode number with the macro.
Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.c

@@ -1957,7 +1957,7 @@ static int init_node_manager(struct f2fs_sb_info *sbi)
 	nm_i->max_nid = NAT_ENTRY_PER_BLOCK * nat_blocks;
 
 	/* not used nids: 0, node, meta, (and root counted as valid node) */
-	nm_i->available_nids = nm_i->max_nid - 3;
+	nm_i->available_nids = nm_i->max_nid - F2FS_RESERVED_NODE_NUM;
 	nm_i->fcnt = 0;
 	nm_i->nat_cnt = 0;
 	nm_i->ram_thresh = DEF_RAM_THRESHOLD;

fs/f2fs/super.c

@@ -508,8 +508,8 @@ static int f2fs_statfs(struct dentry *dentry, struct kstatfs *buf)
 	buf->f_bfree = buf->f_blocks - valid_user_blocks(sbi) - ovp_count;
 	buf->f_bavail = user_block_count - valid_user_blocks(sbi);
 
-	buf->f_files = sbi->total_node_count;
-	buf->f_ffree = sbi->total_node_count - valid_inode_count(sbi);
+	buf->f_files = sbi->total_node_count - F2FS_RESERVED_NODE_NUM;
+	buf->f_ffree = buf->f_files - valid_inode_count(sbi);
 
 	buf->f_namelen = F2FS_NAME_LEN;
 	buf->f_fsid.val[0] = (u32)id;

include/linux/f2fs_fs.h

@@ -24,6 +24,9 @@
 #define NULL_ADDR		((block_t)0)	/* used as block_t addresses */
 #define NEW_ADDR		((block_t)-1)	/* used as block_t addresses */
 
+/* 0, 1(node nid), 2(meta nid) are reserved node id */
+#define F2FS_RESERVED_NODE_NUM		3
+
 #define F2FS_ROOT_INO(sbi)	(sbi->root_ino_num)
 #define F2FS_NODE_INO(sbi)	(sbi->node_ino_num)
 #define F2FS_META_INO(sbi)	(sbi->meta_ino_num)