Node Address Table是用来记录Main Area中node地址的结构。维护了一张nid-block_addr表。

nat的物理结构比较简单，就是文件映射。每个f2fs_nat_block存储了455个f2fs_nat_entry。如果nid==ino则该node是inode。

struct f2fs_nat_entry {
	__u8 version;		/* latest version of cached nat entry */
	__le32 ino;		/* inode number */
	__le32 block_addr;	/* block address */
} __packed;

struct f2fs_nat_block {
	struct f2fs_nat_entry entries[NAT_ENTRY_PER_BLOCK];
} __packed;

NAT在内存中的管理结构是struct f2fs_nm_info，在build_node_manager函数进行初始化。

struct f2fs_nm_info {
	block_t nat_blkaddr;		/* base disk address of NAT */
	nid_t max_nid;			/* maximum possible node ids */
	nid_t available_nids;		/* # of available node ids */
	nid_t next_scan_nid;		/* the next nid to be scanned */
	nid_t max_rf_node_blocks;	/* max # of nodes for recovery */
	unsigned int ram_thresh;	/* control the memory footprint */
	unsigned int ra_nid_pages;	/* # of nid pages to be readaheaded */
	unsigned int dirty_nats_ratio;	/* control dirty nats ratio threshold */

	/* NAT cache management */
	struct radix_tree_root nat_root;/* root of the nat entry cache */
	struct radix_tree_root nat_set_root;/* root of the nat set cache */
	struct f2fs_rwsem nat_tree_lock;	/* protect nat entry tree */
	struct list_head nat_entries;	/* cached nat entry list (clean) */
	spinlock_t nat_list_lock;	/* protect clean nat entry list */
	unsigned int nat_cnt[MAX_NAT_STATE]; /* the # of cached nat entries */
	unsigned int nat_blocks;	/* # of nat blocks */

	/* free node ids management */
	struct radix_tree_root free_nid_root;/* root of the free_nid cache */
	struct list_head free_nid_list;		/* list for free nids excluding preallocated nids */
	unsigned int nid_cnt[MAX_NID_STATE];	/* the number of free node id */
	spinlock_t nid_list_lock;	/* protect nid lists ops */
	struct mutex build_lock;	/* lock for build free nids */
	unsigned char **free_nid_bitmap;
	unsigned char *nat_block_bitmap;
	unsigned short *free_nid_count;	/* free nid count of NAT block */

	/* for checkpoint */
	char *nat_bitmap;		/* NAT bitmap pointer */

	unsigned int nat_bits_blocks;	/* # of nat bits blocks */
	unsigned char *nat_bits;	/* NAT bits blocks */
	unsigned char *full_nat_bits;	/* full NAT pages */
	unsigned char *empty_nat_bits;	/* empty NAT pages */
#ifdef CONFIG_F2FS_CHECK_FS
	char *nat_bitmap_mir;		/* NAT bitmap mirror */
#endif
	int bitmap_size;		/* bitmap size */
};