writeback机制源码分析


writeback相关数据结构

与writeback相关的数据结构主要有:

1,backing_dev_info,该数据结构描述了backing_dev的所有信息,通常块设备的request queue中会包含backing_dev对象。

2,bdi_writeback,该数据结构封装了writeback的内核线程以及需要操作的inode队列。

3,wb_writeback_work,该数据结构封装了writeback的工作任务。

各数据结构之间的关系如下图所示:

下面对各个数据结构做简要介绍。

bdi information

bdi对象在块设备添加的时候需要注册到系统的bdi队列中。对于ext3而言,在mount的时候需要将底层块设备的bdi对象联系到ext3 root_inode中。bdi对象数据结构定义如下:

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对象 */
spinlock_t wb_lock; /* protects work_list */

/* 任务链表 */
struct list_head work_list;

struct device *dev;
/* 在laptop模式下应用的定时器 */
struct timer_list laptop_mode_wb_timer;

#ifdef CONFIG_DEBUG_FS
struct dentry *debug_dir;
struct dentry *debug_stats;
#endif
};

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