writeback相關數據結構
與writeback相關的數據結構主要有：
1，backing_dev_info，該數據結構描述了backing_dev的所有信息，通常塊設備的request queue中會包含backing_dev對象。
2，bdi_writeback，該數據結構封裝了writeback的內核線程以及需要操作的inode隊列。
3，wb_writeback_work，該數據結構封裝了writeback的工作任務。
各數據結構之間的關系如下圖所示：
下面對各個數據結構做簡要介紹。
bdi information
bdi對象在塊設備添加的時候需要注冊到系統的bdi隊列中。對於ext3而言，在mount的時候需要將底層塊設備的bdi對象聯系到ext3 root_inode中。bdi對象數據結構定義如下：
struct backing_dev_info {
struct list_head bdi_list;
unsigned long ra_pages; /* max readahead in PAGE_CACHE_SIZE units */
unsigned long state; /* Always use atomic bitops on this */
unsigned int capabilities; /* Device capabilities */
congested_fn *congested_fn; /* Function pointer if device is md/dm */
void *congested_data; /* Pointer to aux data for congested func */
char *name;
struct percpu_counter bdi_stat[NR_BDI_STAT_ITEMS];
unsigned long bw_time_stamp; /* last time write bw is updated */
unsigned long dirtied_stamp;
unsigned long written_stamp; /* pages written at bw_time_stamp */
unsigned long write_bandwidth; /* the estimated write bandwidth */
unsigned long avg_write_bandwidth; /* further smoothed write bw */
/*
* The base dirty throttle rate, re-calculated on every 200ms.
* All the bdi tasks' dirty rate will be curbed under it.
* @dirty_ratelimit tracks the estimated @balanced_dirty_ratelimit
* in small steps and is much more smooth/stable than the latter.
*/
unsigned long dirty_ratelimit;
unsigned long balanced_dirty_ratelimit;
struct prop_local_percpu completions;
int dirty_exceeded;
unsigned int min_ratio;
unsigned int max_ratio, max_prop_frac;
struct bdi_writeback wb; /* default writeback info for this bdi，writeback對象 */
spinlock_t wb_lock; /* protects work_list */
/* 任務鏈表 */
struct list_head work_list;
struct device *dev;
/* 在laptop模式下應用的定時器 */
struct timer_list laptop_mode_wb_timer;
#ifdef CONFIG_DEBUG_FS
struct dentry *debug_dir;
struct dentry *debug_stats;
#endif
};
