Linux内核延迟写机制学习

Linux内核延迟写机制

Linux内核延迟写的特点，是指在Linux通过write的场景下写入数据之后，会将数据直接标记为dirty，然后通过延迟读写的方式最后将数据回写到磁盘上。在本文的Linux-3.10的源码基础上简单学习一下相关机制。

Linux-bdi机制

Linux内核当前都是通过bdi_writeback函数来进行数据回写，该初始化方式如下。

static int __init default_bdi_init(void)
{int err;bdi_wq = alloc_workqueue("writeback", WQ_MEM_RECLAIM | WQ_FREEZABLE |WQ_UNBOUND | WQ_SYSFS, 0);   // 通过workqueue机制来进行的函数执行if (!bdi_wq)return -ENOMEM;err = bdi_init(&default_backing_dev_info);   // 初始化bdi相关信息if (!err)bdi_register(&default_backing_dev_info, NULL, "default");err = bdi_init(&noop_backing_dev_info);return err;
}
subsys_initcall(default_bdi_init);int bdi_init(struct backing_dev_info *bdi)
{int i, err;bdi->dev = NULL;bdi->min_ratio = 0;bdi->max_ratio = 100;bdi->max_prop_frac = FPROP_FRAC_BASE;spin_lock_init(&bdi->wb_lock);INIT_LIST_HEAD(&bdi->bdi_list);INIT_LIST_HEAD(&bdi->work_list);bdi_wb_init(&bdi->wb, bdi);   // 注册回调执行的函数类型for (i = 0; i < NR_BDI_STAT_ITEMS; i++) {err = percpu_counter_init(&bdi->bdi_stat[i], 0);if (err)goto err;}bdi->dirty_exceeded = 0;bdi->bw_time_stamp = jiffies;bdi->written_stamp = 0;bdi->balanced_dirty_ratelimit = INIT_BW;bdi->dirty_ratelimit = INIT_BW;bdi->write_bandwidth = INIT_BW;bdi->avg_write_bandwidth = INIT_BW;err = fprop_local_init_percpu(&bdi->completions);if (err) {err:while (i--)percpu_counter_destroy(&bdi->bdi_stat[i]);}return err;
}
EXPORT_SYMBOL(bdi_init);static void bdi_wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi)
{memset(wb, 0, sizeof(*wb));wb->bdi = bdi;wb->last_old_flush = jiffies;INIT_LIST_HEAD(&wb->b_dirty);INIT_LIST_HEAD(&wb->b_io);INIT_LIST_HEAD(&wb->b_more_io);spin_lock_init(&wb->list_lock);INIT_DELAYED_WORK(&wb->dwork, bdi_writeback_workfn);  // 注册bdi_writeback_workfn为延迟写入的回调处理函数
}

其中subsys_initcall函数会在start_kernel --> rest_init中通过kernel_init最终调用。

fs_initcall、early_initcall、__init宏_LPSTC123的博客-CSDN博客_early_initcall

主动调用sync触发回写操作

SYSCALL_DEFINE0(sync)
{int nowait = 0, wait = 1;wakeup_flusher_threads(0, WB_REASON_SYNC);iterate_supers(sync_inodes_one_sb, NULL);iterate_supers(sync_fs_one_sb, &nowait);iterate_supers(sync_fs_one_sb, &wait);iterate_bdevs(fdatawrite_one_bdev, NULL);iterate_bdevs(fdatawait_one_bdev, NULL);if (unlikely(laptop_mode))laptop_sync_completion();return 0;
}

主动唤醒了wakeup_flusher_threads线程来进行数据回写的操作。

/** Start writeback of `nr_pages' pages.  If `nr_pages' is zero, write back* the whole world.*/
void wakeup_flusher_threads(long nr_pages, enum wb_reason reason)
{struct backing_dev_info *bdi;if (!nr_pages) {nr_pages = global_page_state(NR_FILE_DIRTY) +global_page_state(NR_UNSTABLE_NFS);}rcu_read_lock();list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) {if (!bdi_has_dirty_io(bdi))continue;__bdi_start_writeback(bdi, nr_pages, false, reason); // 开启回写的page的任务}rcu_read_unlock();
}

如果需要回写的数据为dirty状态，则主动调用__bdi_start_writeback来回写。

static void
__bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages,bool range_cyclic, enum wb_reason reason)
{struct wb_writeback_work *work;/** This is WB_SYNC_NONE writeback, so if allocation fails just* wakeup the thread for old dirty data writeback*/work = kzalloc(sizeof(*work), GFP_ATOMIC);if (!work) {trace_writeback_nowork(bdi);mod_delayed_work(bdi_wq, &bdi->wb.dwork, 0);return;}work->sync_mode   = WB_SYNC_NONE;work->nr_pages   = nr_pages;work->range_cyclic = range_cyclic;work->reason   = reason;bdi_queue_work(bdi, work);  // 将回写的work放入到任务队列中进行处理
}static void bdi_queue_work(struct backing_dev_info *bdi,struct wb_writeback_work *work)
{trace_writeback_queue(bdi, work);spin_lock_bh(&bdi->wb_lock);list_add_tail(&work->list, &bdi->work_list); spin_unlock_bh(&bdi->wb_lock);mod_delayed_work(bdi_wq, &bdi->wb.dwork, 0);  // 放入队列中
}

通过mod_delayed_work函数放入到队列中进行延迟处理。


/*** mod_delayed_work - modify delay of or queue a delayed work* @wq: workqueue to use* @dwork: work to queue* @delay: number of jiffies to wait before queueing** mod_delayed_work_on() on local CPU.*/
static inline bool mod_delayed_work(struct workqueue_struct *wq,struct delayed_work *dwork,unsigned long delay)
{return mod_delayed_work_on(WORK_CPU_UNBOUND, wq, dwork, delay);
}bool mod_delayed_work_on(int cpu, struct workqueue_struct *wq,struct delayed_work *dwork, unsigned long delay)
{unsigned long flags;int ret;do {ret = try_to_grab_pending(&dwork->work, true, &flags);} while (unlikely(ret == -EAGAIN));if (likely(ret >= 0)) {__queue_delayed_work(cpu, wq, dwork, delay);  // 放入队列中进行处理local_irq_restore(flags);}/* -ENOENT from try_to_grab_pending() becomes %true */return ret;
}
EXPORT_SYMBOL_GPL(mod_delayed_work_on);

此时就会调用在work_queue中注册的回调函数。

bdi_writeback_workfn回写函数

/** Handle writeback of dirty data for the device backed by this bdi. Also* reschedules periodically and does kupdated style flushing.*/
void bdi_writeback_workfn(struct work_struct *work)
{struct bdi_writeback *wb = container_of(to_delayed_work(work),struct bdi_writeback, dwork);struct backing_dev_info *bdi = wb->bdi;long pages_written;set_worker_desc("flush-%s", dev_name(bdi->dev));current->flags |= PF_SWAPWRITE;if (likely(!current_is_workqueue_rescuer() ||list_empty(&bdi->bdi_list))) {/** The normal path.  Keep writing back @bdi until its* work_list is empty.  Note that this path is also taken* if @bdi is shutting down even when we're running off the* rescuer as work_list needs to be drained.*/do {pages_written = wb_do_writeback(wb, 0);   // 处理回调pagetrace_writeback_pages_written(pages_written);} while (!list_empty(&bdi->work_list));} else {/** bdi_wq can't get enough workers and we're running off* the emergency worker.  Don't hog it.  Hopefully, 1024 is* enough for efficient IO.*/pages_written = writeback_inodes_wb(&bdi->wb, 1024,WB_REASON_FORKER_THREAD);trace_writeback_pages_written(pages_written);}if (!list_empty(&bdi->work_list) ||(wb_has_dirty_io(wb) && dirty_writeback_interval))queue_delayed_work(bdi_wq, &wb->dwork,msecs_to_jiffies(dirty_writeback_interval * 10)); // 如果未处理完则放入队列继续处理current->flags &= ~PF_SWAPWRITE;
}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);while ((work = get_next_work_item(bdi)) != NULL) {  // 获取队列/** 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);wrote += wb_writeback(wb, work);  // 处理work/** Notify the caller of completion if this is a synchronous* work item, otherwise just free it.*/if (work->done)complete(work->done);elsekfree(work);}/** Check for periodic writeback, kupdated() style*/wrote += wb_check_old_data_flush(wb);wrote += wb_check_background_flush(wb);clear_bit(BDI_writeback_running, &wb->bdi->state);return wrote;
}static long wb_writeback(struct bdi_writeback *wb,struct wb_writeback_work *work)
{unsigned long wb_start = jiffies;long nr_pages = work->nr_pages;unsigned long oldest_jif;struct inode *inode;long progress;oldest_jif = jiffies;work->older_than_this = &oldest_jif;spin_lock(&wb->list_lock);for (;;) {/** Stop writeback when nr_pages has been consumed*/if (work->nr_pages <= 0)break;/** Background writeout and kupdate-style writeback may* run forever. Stop them if there is other work to do* so that e.g. sync can proceed. They'll be restarted* after the other works are all done.*/if ((work->for_background || work->for_kupdate) &&!list_empty(&wb->bdi->work_list))break;/** For background writeout, stop when we are below the* background dirty threshold*/if (work->for_background && !over_bground_thresh(wb->bdi))break;/** Kupdate and background works are special and we want to* include all inodes that need writing. Livelock avoidance is* handled by these works yielding to any other work so we are* safe.*/if (work->for_kupdate) {oldest_jif = jiffies -msecs_to_jiffies(dirty_expire_interval * 10);} else if (work->for_background)oldest_jif = jiffies;trace_writeback_start(wb->bdi, work);if (list_empty(&wb->b_io))queue_io(wb, work);if (work->sb)progress = writeback_sb_inodes(work->sb, wb, work);  // 写入elseprogress = __writeback_inodes_wb(wb, work);trace_writeback_written(wb->bdi, work);wb_update_bandwidth(wb, wb_start);/** Did we write something? Try for more** Dirty inodes are moved to b_io for writeback in batches.* The completion of the current batch does not necessarily* mean the overall work is done. So we keep looping as long* as made some progress on cleaning pages or inodes.*/if (progress)continue;/** No more inodes for IO, bail*/if (list_empty(&wb->b_more_io))break;/** Nothing written. Wait for some inode to* become available for writeback. Otherwise* we'll just busyloop.*/if (!list_empty(&wb->b_more_io))  {trace_writeback_wait(wb->bdi, work);inode = wb_inode(wb->b_more_io.prev);spin_lock(&inode->i_lock);spin_unlock(&wb->list_lock);/* This function drops i_lock... */inode_sleep_on_writeback(inode);spin_lock(&wb->list_lock);}}spin_unlock(&wb->list_lock);return nr_pages - work->nr_pages;
}

在进入wb_writeback函数之后就是一个死循环进行处理。接着就调用writeback_sb_inodes进一步写入。

/** Write a portion of b_io inodes which belong to @sb.** Return the number of pages and/or inodes written.*/
static long writeback_sb_inodes(struct super_block *sb,struct bdi_writeback *wb,struct wb_writeback_work *work)
{struct writeback_control wbc = {.sync_mode        = work->sync_mode,.tagged_writepages    = work->tagged_writepages,.for_kupdate      = work->for_kupdate,.for_background     = work->for_background,.range_cyclic        = work->range_cyclic,.range_start       = 0,.range_end     = LLONG_MAX,};unsigned long start_time = jiffies;long write_chunk;long wrote = 0;  /* count both pages and inodes */while (!list_empty(&wb->b_io)) {   // 检查队列是否为空struct inode *inode = wb_inode(wb->b_io.prev);if (inode->i_sb != sb) {if (work->sb) {/** We only want to write back data for this* superblock, move all inodes not belonging* to it back onto the dirty list.*/redirty_tail(inode, wb);continue;}/** The inode belongs to a different superblock.* Bounce back to the caller to unpin this and* pin the next superblock.*/break;}/** Don't bother with new inodes or inodes being freed, first* kind does not need periodic writeout yet, and for the latter* kind writeout is handled by the freer.*/spin_lock(&inode->i_lock);if (inode->i_state & (I_NEW | I_FREEING | I_WILL_FREE)) {spin_unlock(&inode->i_lock);redirty_tail(inode, wb);continue;}if ((inode->i_state & I_SYNC) && wbc.sync_mode != WB_SYNC_ALL) {/** If this inode is locked for writeback and we are not* doing writeback-for-data-integrity, move it to* b_more_io so that writeback can proceed with the* other inodes on s_io.** We'll have another go at writing back this inode* when we completed a full scan of b_io.*/spin_unlock(&inode->i_lock);requeue_io(inode, wb);trace_writeback_sb_inodes_requeue(inode);continue;}spin_unlock(&wb->list_lock);/** We already requeued the inode if it had I_SYNC set and we* are doing WB_SYNC_NONE writeback. So this catches only the* WB_SYNC_ALL case.*/if (inode->i_state & I_SYNC) {/* Wait for I_SYNC. This function drops i_lock... */inode_sleep_on_writeback(inode);/* Inode may be gone, start again */spin_lock(&wb->list_lock);continue;}inode->i_state |= I_SYNC;   // 设置同步状态spin_unlock(&inode->i_lock);write_chunk = writeback_chunk_size(wb->bdi, work);wbc.nr_to_write = write_chunk;wbc.pages_skipped = 0;/** We use I_SYNC to pin the inode in memory. While it is set* evict_inode() will wait so the inode cannot be freed.*/__writeback_single_inode(inode, &wbc);   // 调用single_inode函数进行处理work->nr_pages -= write_chunk - wbc.nr_to_write;wrote += write_chunk - wbc.nr_to_write;spin_lock(&wb->list_lock);spin_lock(&inode->i_lock);if (!(inode->i_state & I_DIRTY))wrote++;requeue_inode(inode, wb, &wbc);inode_sync_complete(inode);spin_unlock(&inode->i_lock);cond_resched_lock(&wb->list_lock);/** bail out to wb_writeback() often enough to check* background threshold and other termination conditions.*/if (wrote) {if (time_is_before_jiffies(start_time + HZ / 10UL))break;if (work->nr_pages <= 0)break;}}return wrote;
}static int
__writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
{struct address_space *mapping = inode->i_mapping;long nr_to_write = wbc->nr_to_write;unsigned dirty;int ret;WARN_ON(!(inode->i_state & I_SYNC));trace_writeback_single_inode_start(inode, wbc, nr_to_write);ret = do_writepages(mapping, wbc);/** Make sure to wait on the data before writing out the metadata.* This is important for filesystems that modify metadata on data* I/O completion.*/if (wbc->sync_mode == WB_SYNC_ALL) {int err = filemap_fdatawait(mapping);if (ret == 0)ret = err;}/** Some filesystems may redirty the inode during the writeback* due to delalloc, clear dirty metadata flags right before* write_inode()*/spin_lock(&inode->i_lock);/* Clear I_DIRTY_PAGES if we've written out all dirty pages */if (!mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))inode->i_state &= ~I_DIRTY_PAGES;dirty = inode->i_state & I_DIRTY;inode->i_state &= ~(I_DIRTY_SYNC | I_DIRTY_DATASYNC);spin_unlock(&inode->i_lock);/* Don't write the inode if only I_DIRTY_PAGES was set */if (dirty & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) {int err = write_inode(inode, wbc);if (ret == 0)ret = err;}trace_writeback_single_inode(inode, wbc, nr_to_write);return ret;
}int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
{int ret;if (wbc->nr_to_write <= 0)return 0;if (mapping->a_ops->writepages)ret = mapping->a_ops->writepages(mapping, wbc); // 调用文件系统注册的回调的函数elseret = generic_writepages(mapping, wbc);return ret;
}

至此，就将回写的操作分配到了底层的文件系统来进行处理。如果是ext4的文件系统则调用的为ext4_writepage函数进行下一步数据的写入操作。

总结

本文主要是通过资料跟着源码学习了一下数据回写的机制，主要是通过bdi机制，通过workqueue来进行数据的写入，并且在数据写入的时候其实就写到了page层就返回，然后再延迟写入机制将page中的数据落入到底层的存储介质中。触发的时机也引用到下文中的内容，即可以通过系统调用sync主动调用，也可以等待系统配置的定时来执行，在内存分配数据不足的时候也会触发数据回写的流程。由于本人才疏学浅，如有错误请批评指正。

page cache回写的几种触发方式_程序猿Ricky的日常干货的博客-CSDN博客_pagecache 如何回写

https://zhuanlan.zhihu.com/p/532262364

writeback bdi脏页回写原理linux内核源码解析_dongzhiyan_hjp的博客-CSDN博客_脏页回写

vm内核参数之内存脏页dirty_writeback_centisecs和dirty_expire_centisecs_Blue summer的博客-CSDN博客_dirty_writeback_centisecs

https://www.leviathan.vip/2019/06/01/Linux%E5%86%85%E6%A0%B8%E6%BA%90%E7%A0%81%E5%88%86%E6%9E%90-Page-Cache%E5%8E%9F%E7%90%86%E5%88%86%E6%9E%90/#%E5%90%8E%E5%8F%B0%E7%BA%BF%E7%A8%8B%E5%91%A8%E6%9C%9F%E5%9B%9E%E5%86%99

【linux内核源码】io操作之page cache - 简书

浅谈Linux dirty data配置 - 腾讯云开发者社区-腾讯云
Linux缓存回写——基于linux-4.15_SweeNeil的博客-CSDN博客
https://ywang-wnlo.github.io/posts/646202b9.html