前面2篇介紹了ext4磁碟上的佈局，在使用過程中，核心需要頻繁的訪問某些資料結構，所以會把磁碟上面資料讀出裝在記憶體中相應的資料結構。

超級塊

ext4在記憶體中的超級塊結構定義如下：

/*
 * fourth extended-fs super-block data in memory
 */
struct ext4_sb_info {
	unsigned long s_desc_size;	/* Size of a group descriptor in bytes */
	unsigned long s_inodes_per_block;/* Number of inodes per block */
	unsigned long s_blocks_per_group;/* Number of blocks in a group */
	unsigned long s_clusters_per_group; /* Number of clusters in a group */
	unsigned long s_inodes_per_group;/* Number of inodes in a group */
	unsigned long s_itb_per_group;	/* Number of inode table blocks per group */
	unsigned long s_gdb_count;	/* Number of group descriptor blocks */
	unsigned long s_desc_per_block;	/* Number of group descriptors per block */
	ext4_group_t s_groups_count;	/* Number of groups in the fs */
	ext4_group_t s_blockfile_groups;/* Groups acceptable for non-extent files */
	unsigned long s_overhead;  /* # of fs overhead clusters */
	unsigned int s_cluster_ratio;	/* Number of blocks per cluster */
	unsigned int s_cluster_bits;	/* log2 of s_cluster_ratio */
	loff_t s_bitmap_maxbytes;	/* max bytes for bitmap files */
	struct buffer_head * s_sbh;	/* Buffer containing the super block */
	struct ext4_super_block *s_es;	/* Pointer to the super block in the buffer */
	struct buffer_head **s_group_desc;
	unsigned int s_mount_opt;
	unsigned int s_mount_opt2;
	unsigned int s_mount_flags;
	unsigned int s_def_mount_opt;
	ext4_fsblk_t s_sb_block;
	atomic64_t s_resv_clusters;
	kuid_t s_resuid;
	kgid_t s_resgid;
	unsigned short s_mount_state;
	unsigned short s_pad;
	int s_addr_per_block_bits;
	int s_desc_per_block_bits;
	int s_inode_size;
	int s_first_ino;
	unsigned int s_inode_readahead_blks;
	unsigned int s_inode_goal;
	spinlock_t s_next_gen_lock;
	u32 s_next_generation;
	u32 s_hash_seed[4];
	int s_def_hash_version;
	int s_hash_unsigned;	/* 3 if hash should be signed, 0 if not */
	struct percpu_counter s_freeclusters_counter;
	struct percpu_counter s_freeinodes_counter;
	struct percpu_counter s_dirs_counter;
	struct percpu_counter s_dirtyclusters_counter;
	struct blockgroup_lock *s_blockgroup_lock;
	struct proc_dir_entry *s_proc;
	struct kobject s_kobj;
	struct completion s_kobj_unregister;
	struct super_block *s_sb;

	/* Journaling */
	struct journal_s *s_journal;
	struct list_head s_orphan;
	struct mutex s_orphan_lock;
	unsigned long s_resize_flags;		/* Flags indicating if there
						   is a resizer */
	unsigned long s_commit_interval;
	u32 s_max_batch_time;
	u32 s_min_batch_time;
	struct block_device *journal_bdev;
#ifdef CONFIG_QUOTA
	char *s_qf_names[EXT4_MAXQUOTAS];	/* Names of quota files with journalled quota */
	int s_jquota_fmt;			/* Format of quota to use */
#endif
	unsigned int s_want_extra_isize; /* New inodes should reserve # bytes */
	struct rb_root system_blks;

#ifdef EXTENTS_STATS
	/* ext4 extents stats */
	unsigned long s_ext_min;
	unsigned long s_ext_max;
	unsigned long s_depth_max;
	spinlock_t s_ext_stats_lock;
	unsigned long s_ext_blocks;
	unsigned long s_ext_extents;
#endif

	/* for buddy allocator */
	struct ext4_group_info ***s_group_info;
	struct inode *s_buddy_cache;
	spinlock_t s_md_lock;
	unsigned short *s_mb_offsets;
	unsigned int *s_mb_maxs;
	unsigned int s_group_info_size;

	/* tunables */
	unsigned long s_stripe;
	unsigned int s_mb_stream_request;
	unsigned int s_mb_max_to_scan;
	unsigned int s_mb_min_to_scan;
	unsigned int s_mb_stats;
	unsigned int s_mb_order2_reqs;
	unsigned int s_mb_group_prealloc;
	unsigned int s_max_dir_size_kb;
	/* where last allocation was done - for stream allocation */
	unsigned long s_mb_last_group;
	unsigned long s_mb_last_start;

	/* stats for buddy allocator */
	atomic_t s_bal_reqs;	/* number of reqs with len > 1 */
	atomic_t s_bal_success;	/* we found long enough chunks */
	atomic_t s_bal_allocated;	/* in blocks */
	atomic_t s_bal_ex_scanned;	/* total extents scanned */
	atomic_t s_bal_goals;	/* goal hits */
	atomic_t s_bal_breaks;	/* too long searches */
	atomic_t s_bal_2orders;	/* 2^order hits */
	spinlock_t s_bal_lock;
	unsigned long s_mb_buddies_generated;
	unsigned long long s_mb_generation_time;
	atomic_t s_mb_lost_chunks;
	atomic_t s_mb_preallocated;
	atomic_t s_mb_discarded;
	atomic_t s_lock_busy;

	/* locality groups */
	struct ext4_locality_group __percpu *s_locality_groups;

	/* for write statistics */
	unsigned long s_sectors_written_start;
	u64 s_kbytes_written;

	/* the size of zero-out chunk */
	unsigned int s_extent_max_zeroout_kb;

	unsigned int s_log_groups_per_flex;
	struct flex_groups *s_flex_groups;
	ext4_group_t s_flex_groups_allocated;

	/* workqueue for reserved extent conversions (buffered io) */
	struct workqueue_struct *rsv_conversion_wq;

	/* timer for periodic error stats printing */
	struct timer_list s_err_report;

	/* Lazy inode table initialization info */
	struct ext4_li_request *s_li_request;
	/* Wait multiplier for lazy initialization thread */
	unsigned int s_li_wait_mult;

	/* Kernel thread for multiple mount protection */
	struct task_struct *s_mmp_tsk;

	/* record the last minlen when FITRIM is called. */
	atomic_t s_last_trim_minblks;

	/* Reference to checksum algorithm driver via cryptoapi */
	struct crypto_shash *s_chksum_driver;

	/* Precomputed FS UUID checksum for seeding other checksums */
	__u32 s_csum_seed;

	/* Reclaim extents from extent status tree */
	struct shrinker s_es_shrinker;
	struct list_head s_es_list;	/* List of inodes with reclaimable extents */
	long s_es_nr_inode;
	struct ext4_es_stats s_es_stats;
	struct mb_cache *s_mb_cache;
	spinlock_t s_es_lock ____cacheline_aligned_in_smp;

	/* Ratelimit ext4 messages. */
	struct ratelimit_state s_err_ratelimit_state;
	struct ratelimit_state s_warning_ratelimit_state;
	struct ratelimit_state s_msg_ratelimit_state;

#ifdef CONFIG_EXT4_FS_ENCRYPTION
	/* Encryption */
	uint32_t s_file_encryption_mode;
	uint32_t s_dir_encryption_mode;
#endif
};

記憶體中的超級塊結構與磁碟中的超級塊結構大體一致，在驅動初始化時，核心會把磁碟上面的ext4檔案系統資料讀出裝入到記憶體中的磁碟資料結構體中，由於核心頻繁使用這些結構資料，所以這些資料是常駐記憶體的。

結構體成員s_sbh指向磁碟超級塊緩衝區頭部；

結構體成員s_group_desc指向磁碟組描述符緩衝區頭部；

結構體成員s_es指向磁碟超級塊結構的記憶體首地址；

ext4_sb_info的建立是在ext4_fill_super函式中完成的，程式碼如下：

struct ext4_sb_info {
	struct buffer_head * s_sbh;	/* Buffer containing the super block */
	struct ext4_super_block *s_es;	/* Pointer to the super block in the buffer */
	struct buffer_head **s_group_desc;
};

static int ext4_fill_super(struct super_block *sb, void *data, int silent)
{
	struct ext4_sb_info *sbi;記憶體超級塊
	struct buffer_head *bh;磁碟超級塊邏輯資料
	struct ext4_super_block *es = NULL;磁碟超級塊

	bh = sb_bread_unmovable(sb, logical_sb_block)讀出磁碟超級塊資料到緩衝區
	es = (struct ext4_super_block *) (bh->b_data + offset);資料裝載到磁碟超級塊記憶體資料結構

	sbi->s_sbh = bh;VFS與記憶體和磁碟超級塊的聯絡
	sbi->s_es = es;
	sb->s_fs_info = sbi;
	sbi->s_sb = sb;

	sbi->s_group_desc = ext4_kvmalloc(db_count *
					  sizeof(struct buffer_head *),
					  GFP_KERNEL);
	sbi->s_group_desc[i] = sb_bread_unmovable(sb, block);讀出組描述符資料

inode

ext4記憶體中inode資料結構如下：

/*
 * fourth extended file system inode data in memory
 */
struct ext4_inode_info {
	__le32	i_data[15];	/* unconverted */
	__u32	i_dtime;
	ext4_fsblk_t	i_file_acl;

	/*
	 * i_block_group is the number of the block group which contains
	 * this file's inode.  Constant across the lifetime of the inode,
	 * it is ued for making block allocation decisions - we try to
	 * place a file's data blocks near its inode block, and new inodes
	 * near to their parent directory's inode.
	 */
	ext4_group_t	i_block_group;
	ext4_lblk_t	i_dir_start_lookup;
#if (BITS_PER_LONG < 64)
	unsigned long	i_state_flags;		/* Dynamic state flags */
#endif
	unsigned long	i_flags;

	/*
	 * Extended attributes can be read independently of the main file
	 * data. Taking i_mutex even when reading would cause contention
	 * between readers of EAs and writers of regular file data, so
	 * instead we synchronize on xattr_sem when reading or changing
	 * EAs.
	 */
	struct rw_semaphore xattr_sem;

	struct list_head i_orphan;	/* unlinked but open inodes */

	/*
	 * i_disksize keeps track of what the inode size is ON DISK, not
	 * in memory.  During truncate, i_size is set to the new size by
	 * the VFS prior to calling ext4_truncate(), but the filesystem won't
	 * set i_disksize to 0 until the truncate is actually under way.
	 *
	 * The intent is that i_disksize always represents the blocks which
	 * are used by this file.  This allows recovery to restart truncate
	 * on orphans if we crash during truncate.  We actually write i_disksize
	 * into the on-disk inode when writing inodes out, instead of i_size.
	 *
	 * The only time when i_disksize and i_size may be different is when
	 * a truncate is in progress.  The only things which change i_disksize
	 * are ext4_get_block (growth) and ext4_truncate (shrinkth).
	 */
	loff_t	i_disksize;

	/*
	 * i_data_sem is for serialising ext4_truncate() against
	 * ext4_getblock().  In the 2.4 ext2 design, great chunks of inode's
	 * data tree are chopped off during truncate. We can't do that in
	 * ext4 because whenever we perform intermediate commits during
	 * truncate, the inode and all the metadata blocks *must* be in a
	 * consistent state which allows truncation of the orphans to restart
	 * during recovery.  Hence we must fix the get_block-vs-truncate race
	 * by other means, so we have i_data_sem.
	 */
	struct rw_semaphore i_data_sem;
	/*
	 * i_mmap_sem is for serializing page faults with truncate / punch hole
	 * operations. We have to make sure that new page cannot be faulted in
	 * a section of the inode that is being punched. We cannot easily use
	 * i_data_sem for this since we need protection for the whole punch
	 * operation and i_data_sem ranks below transaction start so we have
	 * to occasionally drop it.
	 */
	struct rw_semaphore i_mmap_sem;
	struct inode vfs_inode;
	struct jbd2_inode *jinode;

	spinlock_t i_raw_lock;	/* protects updates to the raw inode */

	/*
	 * File creation time. Its function is same as that of
	 * struct timespec i_{a,c,m}time in the generic inode.
	 */
	struct timespec i_crtime;

	/* mballoc */
	struct list_head i_prealloc_list;
	spinlock_t i_prealloc_lock;

	/* extents status tree */
	struct ext4_es_tree i_es_tree;
	rwlock_t i_es_lock;
	struct list_head i_es_list;
	unsigned int i_es_all_nr;	/* protected by i_es_lock */
	unsigned int i_es_shk_nr;	/* protected by i_es_lock */
	ext4_lblk_t i_es_shrink_lblk;	/* Offset where we start searching for
					   extents to shrink. Protected by
					   i_es_lock  */

	/* ialloc */
	ext4_group_t	i_last_alloc_group;

	/* allocation reservation info for delalloc */
	/* In case of bigalloc, these refer to clusters rather than blocks */
	unsigned int i_reserved_data_blocks;
	unsigned int i_reserved_meta_blocks;
	unsigned int i_allocated_meta_blocks;
	ext4_lblk_t i_da_metadata_calc_last_lblock;
	int i_da_metadata_calc_len;

	/* on-disk additional length */
	__u16 i_extra_isize;
	char i_crypt_policy_flags;

	/* Indicate the inline data space. */
	u16 i_inline_off;
	u16 i_inline_size;

#ifdef CONFIG_QUOTA
	/* quota space reservation, managed internally by quota code */
	qsize_t i_reserved_quota;
#endif

	/* Lock protecting lists below */
	spinlock_t i_completed_io_lock;
	/*
	 * Completed IOs that need unwritten extents handling and have
	 * transaction reserved
	 */
	struct list_head i_rsv_conversion_list;
	/*
	 * Completed IOs that need unwritten extents handling and don't have
	 * transaction reserved
	 */
	atomic_t i_ioend_count;	/* Number of outstanding io_end structs */
	atomic_t i_unwritten; /* Nr. of inflight conversions pending */
	struct work_struct i_rsv_conversion_work;

	spinlock_t i_block_reservation_lock;

	/*
	 * Transactions that contain inode's metadata needed to complete
	 * fsync and fdatasync, respectively.
	 */
	tid_t i_sync_tid;
	tid_t i_datasync_tid;

#ifdef CONFIG_QUOTA
	struct dquot *i_dquot[MAXQUOTAS];
#endif

	/* Precomputed uuid+inum+igen checksum for seeding inode checksums */
	__u32 i_csum_seed;

#ifdef CONFIG_EXT4_FS_ENCRYPTION
	/* Encryption params */
	struct ext4_encryption_key i_encryption_key;
#endif
};

結構體成員與磁碟上類似，具體怎麼用還不知。

磁碟上的bmap和imap也會讀取到記憶體中，用什麼資料結構儲存的呢？什麼時候讀的？

struct inode vfs_inode;這個vfs_inode是虛擬檔案系統的inode結構，下一篇介紹。