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
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 };
在bdi数据结构中定义了一个writeback对象，该对象是对writeback内核线程的描述，并且封装了需要处理的inode队列。在bdi数据结构中有一条work_list，该work队列维护了writeback内核线程需要处理的任务。如果该队列上没有work可以处理，那么writeback内核线程将会睡眠等待。
writeback
writeback对象封装了内核线程task以及需要处理的inode队列。当page cache/buffer cache需要刷新radix tree上的inode时，可以将该inode挂载到writeback对象的b_dirty队列上，然后唤醒writeback线程。在处理过程中，inode会被移到b_io队列上进行处理。多条链表的方式可以降低多线程之间的资源共享。writeback数据结构具体定义如下：
struct bdi_writeback { struct backing_dev_info *bdi; /* our parent bdi */ unsigned int nr; unsigned long last_old_flush; /* last old data flush */ unsigned long last_active; /* last time bdi thread was active */ struct task_struct *task; /* writeback thread */ struct timer_list wakeup_timer; /* used for delayed bdi thread wakeup */ struct list_head b_dirty; /* dirty inodes */ struct list_head b_io; /* parked for writeback */ struct list_head b_more_io; /* parked for more writeback */ spinlock_t list_lock; /* protects the b_* lists */ };
writeback work
wb_writeback_work数据结构是对writeback任务的封装，不同的任务可以采用不同的刷新策略。writeback线程的处理对象就是writeback_work。如果writeback_work队列为空，那么内核线程就可以睡眠了。Writeback_work的数据结构定义如下：
struct wb_writeback_work { long nr_pages; struct super_block *sb; /* superblock对象 */ unsigned long *older_than_this; enum writeback_sync_modes sync_mode; unsigned int tagged_writepages:1; unsigned int for_kupdate:1; unsigned int range_cyclic:1; unsigned int for_background:1; enum wb_reason reason; /* why was writeback initiated? */ struct list_head list; /* pending work list，链入bdi-> work_list队列 */ struct completion *done; /* set if the caller waits，work完成时通知调用者 */ };
writeback主要函数分析
writeback机制的主要函数包括如下两个方面：
1，管理bdi对象并且fork相应的writeback内核线程处理cache数据的刷新工作。
2，writeback内核线程处理函数，实现dirty page的刷新操作
writeback线程管理
Linux中有一个内核守护线程，该线程用来管理系统bdi队列，并且负责为block device创建writeback thread。当bdi中有dirty page并且还没有为bdi分配内核线程的时候，bdi_forker_thread程序会为其分配线程资源；当一个writeback线程长时间处于空闲状态时，bdi_forker_thread程序会释放该线程资源。
writeback线程管理程序分析如下：
static int bdi_forker_thread(void *ptr) { struct bdi_writeback *me = ptr; current->flags |= PF_SWAPWRITE; set_freezable(); /* * Our parent may run at a different priority, just set us to normal */ set_user_nice(current, 0); for (;;) { struct task_struct *task = NULL; struct backing_dev_info *bdi; enum { NO_ACTION, /* Nothing to do */ FORK_THREAD, /* Fork bdi thread */ KILL_THREAD, /* Kill inactive bdi thread */ } action = NO_ACTION; /* * Temporary measure, we want to make sure we don't see * dirty data on the default backing_dev_info */ if (wb_has_dirty_io(me) || !list_empty(&me->bdi->work_list)) { del_timer(&me->wakeup_timer); wb_do_writeback(me, 0); } spin_lock_bh(&bdi_lock); /* * In the following loop we are going to check whether we have * some work to do without any synchronization with tasks * waking us up to do work for them. Set the task state here * so that we don't miss wakeups after verifying conditions. */ set_current_state(TASK_INTERRUPTIBLE); /* 遍历所有的bdi对象，检查这些bdi是否存在脏数据，如果有脏数据，那么需要为其fork线程，然后做writeback操作 */ list_for_each_entry(bdi, &bdi_list, bdi_list) { bool have_dirty_io; if (!bdi_cap_writeback_dirty(bdi) || bdi_cap_flush_forker(bdi)) continue; WARN(!test_bit(BDI_registered, &bdi->state), "bdi %p/%s is not registered!\n", bdi, bdi->name); /* 检查是否存在脏数据 */ have_dirty_io = !list_empty(&bdi->work_list) || wb_has_dirty_io(&bdi->wb); /* * If the bdi has work to do, but the thread does not * exist - create it. */ if (!bdi->wb.task && have_dirty_io) { /* * Set the pending bit - if someone will try to * unregister this bdi - it'll wait on this bit. */ /* 如果有脏数据，并且不存在线程，那么接下来做线程的FORK操作 */ set_bit(BDI_pending, &bdi->state); action = FORK_THREAD; break; } spin_lock(&bdi->wb_lock); /* * If there is no work to do and the bdi thread was * inactive long enough - kill it. The wb_lock is taken * to make sure no-one adds more work to this bdi and * wakes the bdi thread up. */ /* 如果一个bdi长时间没有脏数据，那么执行线程的KILL操作，结束掉该bdi对应的writeback线程 */ if (bdi->wb.task && !have_dirty_io && time_after(jiffies, bdi->wb.last_active + bdi_longest_inactive())) { task = bdi->wb.task; bdi->wb.task = NULL; spin_unlock(&bdi->wb_lock); set_bit(BDI_pending, &bdi->state); action = KILL_THREAD; break; } spin_unlock(&bdi->wb_lock); } spin_unlock_bh(&bdi_lock); /* Keep working if default bdi still has things to do */ if (!list_empty(&me->bdi->work_list)) __set_current_state(TASK_RUNNING); /* 执行线程的FORK和KILL操作 */ switch (action) { case FORK_THREAD: /* FORK一个bdi_writeback_thread线程，该线程的名字为flush-major:minor */ __set_current_state(TASK_RUNNING); task = kthread_create(bdi_writeback_thread, &bdi->wb, "flush-%s", dev_name(bdi->dev)); if (IS_ERR(task)) { /* * If thread creation fails, force writeout of * the bdi from the thread. Hopefully 1024 is * large enough for efficient IO. */ writeback_inodes_wb(&bdi->wb, 1024, WB_REASON_FORKER_THREAD); } else { /* * The spinlock makes sure we do not lose * wake-ups when racing with 'bdi_queue_work()'. * And as soon as the bdi thread is visible, we * can start it. */ spin_lock_bh(&bdi->wb_lock); bdi->wb.task = task; spin_unlock_bh(&bdi->wb_lock); wake_up_process(task); } bdi_clear_pending(bdi); break; case KILL_THREAD: /* KILL一个线程 */ __set_current_state(TASK_RUNNING); kthread_stop(task); bdi_clear_pending(bdi); break; case NO_ACTION: /* 如果没有可执行的动作，那么调度本线程睡眠一段时间 */ if (!wb_has_dirty_io(me) || !dirty_writeback_interval) /* * There are no dirty data. The only thing we * should now care about is checking for * inactive bdi threads and killing them. Thus, * let's sleep for longer time, save energy and * be friendly for battery-driven devices. */ schedule_timeout(bdi_longest_inactive()); else schedule_timeout(msecs_to_jiffies(dirty_writeback_interval * 10)); try_to_freeze(); break; } } return 0; }
writeback线程
writeback线程是bdi_forker_thread 创建的，该线程的任务就是处理等待的数据回刷任务。线程处理函数为bdi_writeback_thread，其会调用wb_do_writeback函数完成具体操作，该函数分析如下：
long wb_do_writeback(struct bdi_writeback *wb, int force_wait) { struct backing_dev_info *bdi = wb->bdi; struct wb_writeback_work *work; long wrote = 0; set_bit(BDI_writeback_running, &wb->bdi->state); /* 处理等待的work，所有等待work pengding在bdi->work_list上 */ while ((work = get_next_work_item(bdi)) != NULL) { /* * Override sync mode, in case we must wait for completion * because this thread is exiting now. */ if (force_wait) work->sync_mode = WB_SYNC_ALL; trace_writeback_exec(bdi, work); /* 调用wb_writeback函数处理相应的inode */ wrote += wb_writeback(wb, work); /* * Notify the caller of completion if this is a synchronous * work item, otherwise just free it. */ /* 通知上层软件，相应的work已经完成 */ if (work->done) complete(work->done); else kfree(work); } /* * Check for periodic writeback, kupdated() style */ /* 处理周期性的dirty page刷新作业，buffer cache就会走这条路径，在下面的函数中会创建work，并且调用wb_writeback函数进行处理 */ wrote += wb_check_old_data_flush(wb); wrote += wb_check_background_flush(wb); clear_bit(BDI_writeback_running, &wb->bdi->state); return wrote; }
小结
本文在linux-3.2的基础上对writeback代码进行了浏览。整体上来讲，writeback机制是比较简单的，其核心是通过一个常驻内核线程为bdi对象分配writeback线程，实现对cache中dirty page的数据回刷。
