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前面2篇介绍了ext4磁盘上的布局，在使用过程中，内核需要频繁的访问某些数据结构，所以会把磁盘上面数据读出装在内存中相应的数据结构。
超级块
ext4在内存中的超级块结构定义如下：
/** 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 thereis a resizer */unsigned long s_commit_interval;u32 s_max_batch_time;u32 s_min_batch_time;struct block_device *journal_bdev;
#ifdef CONFIG_QUOTAchar *s_qf_names[EXT4_MAXQUOTAS]; /* Names of quota files with journalled quota */int s_jquota_fmt; /* Format of quota to use */
#endifunsigned 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
};内存中的超级块结构与磁盘中的超级块结构大体一致，在驱动初始化时，内核会把磁盘上面的ext4文件系统数据读出装入到内存中的磁盘数据结构体中，由于内核频繁使用这些结构数据，所以这些数据是常驻内存的。
结构体成员s_sbh指向磁盘超级块缓冲区头部；
结构体成员s_group_desc指向磁盘组描述符缓冲区头部；
结构体成员s_es指向磁盘超级块结构的内存首地址；
ext4_sb_info的建立是在ext4_fill_super函数中完成的，代码如下：
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;内存超级块struct buffer_head *bh;磁盘超级块逻辑数据struct ext4_super_block *es = NULL;磁盘超级块bh = sb_bread_unmovable(sb, logical_sb_block)读出磁盘超级块数据到缓冲区es = (struct ext4_super_block *) (bh->b_data + offset);数据装载到磁盘超级块内存数据结构sbi->s_sbh = bh;VFS与内存和磁盘超级块的联系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);读出组描述符数据inode
ext4内存中inode数据结构如下：
/** 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 */
#endifunsigned 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 forextents to shrink. Protected byi_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_QUOTAstruct 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
};结构体成员与磁盘上类似，具体怎么用还不知。
磁盘上的bmap和imap也会读取到内存中，用什么数据结构存储的呢？什么时候读的？
struct inode vfs_inode;这个vfs_inode是虚拟文件系统的inode结构，下一篇介绍。
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