text: /*
   * fs/fs-writeback.c
   *
   * Copyright (C) 2002, Linus Torvalds.
   *
   * Contains all the functions related to writing back and waiting
   * upon dirty inodes against superblocks, and writing back dirty
   * pages against inodes.  ie: data writeback.  Writeout of the
   * inode itself is not handled here.
   *
   * 10Apr2002	Andrew Morton
   *		Split out of fs/inode.c
   *		Additions for address_space-based writeback
   */
  
  #include <linux/kernel.h>
  #include <linux/module.h>
  #include <linux/spinlock.h>
  #include <linux/slab.h>
  #include <linux/sched.h>
  #include <linux/fs.h>
  #include <linux/mm.h>
  #include <linux/kthread.h>
  #include <linux/freezer.h>
  #include <linux/writeback.h>
  #include <linux/blkdev.h>
  #include <linux/backing-dev.h>
  #include <linux/buffer_head.h>
  #include <linux/tracepoint.h>
  #include "internal.h"
  
  /*
   * Passed into wb_writeback(), essentially a subset of writeback_control
   */
  struct wb_writeback_work {
  	long nr_pages;
  	struct super_block *sb;
  	enum writeback_sync_modes sync_mode;
  	unsigned int for_kupdate:1;
  	unsigned int range_cyclic:1;
  	unsigned int for_background:1;
  
  	struct list_head list;		/* pending work list */
  	struct completion *done;	/* set if the caller waits */
  };
  
  /*
   * Include the creation of the trace points after defining the
   * wb_writeback_work structure so that the definition remains local to this
   * file.
   */
  #define CREATE_TRACE_POINTS
  #include <trace/events/writeback.h>
  
  /*
   * We don't actually have pdflush, but this one is exported though /proc...
   */
  int nr_pdflush_threads;
  
  /**
   * writeback_in_progress - determine whether there is writeback in progress
   * @bdi: the device's backing_dev_info structure.
   *
   * Determine whether there is writeback waiting to be handled against a
   * backing device.
   */
  int writeback_in_progress(struct backing_dev_info *bdi)
  {
  	return test_bit(BDI_writeback_running, &bdi->state);
  }
  
  static inline struct backing_dev_info *inode_to_bdi(struct inode *inode)
  {
  	struct super_block *sb = inode->i_sb;
  
  	if (strcmp(sb->s_type->name, "bdev") == 0)
  		return inode->i_mapping->backing_dev_info;
  
  	return sb->s_bdi;
  }
  
  static inline struct inode *wb_inode(struct list_head *head)
  {
  	return list_entry(head, struct inode, i_wb_list);
  }
  
  /* Wakeup flusher thread or forker thread to fork it. Requires bdi->wb_lock. */
  static void bdi_wakeup_flusher(struct backing_dev_info *bdi)
  {
  	if (bdi->wb.task) {
  		wake_up_process(bdi->wb.task);
  	} else {
  		/*
  		 * The bdi thread isn't there, wake up the forker thread which
  		 * will create and run it.
  		 */
  		wake_up_process(default_backing_dev_info.wb.task);
  	}
  }
  
  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);
  	if (!bdi->wb.task)
  		trace_writeback_nothread(bdi, work);
  	bdi_wakeup_flusher(bdi);
  	spin_unlock_bh(&bdi->wb_lock);
  }
  
  static void
  __bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages,
  		      bool range_cyclic)
  {
  	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) {
  		if (bdi->wb.task) {
  			trace_writeback_nowork(bdi);
  			wake_up_process(bdi->wb.task);
  		}
  		return;
  	}
  
  	work->sync_mode	= WB_SYNC_NONE;
  	work->nr_pages	= nr_pages;
  	work->range_cyclic = range_cyclic;
  
  	bdi_queue_work(bdi, work);
  }
  
  /**
   * bdi_start_writeback - start writeback
   * @bdi: the backing device to write from
   * @nr_pages: the number of pages to write
   *
   * Description:
   *   This does WB_SYNC_NONE opportunistic writeback. The IO is only
   *   started when this function returns, we make no guarantees on
   *   completion. Caller need not hold sb s_umount semaphore.
   *
   */
  void bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages)
  {
  	__bdi_start_writeback(bdi, nr_pages, true);
  }
  
  /**
   * bdi_start_background_writeback - start background writeback
   * @bdi: the backing device to write from
   *
   * Description:
   *   This makes sure WB_SYNC_NONE background writeback happens. When
   *   this function returns, it is only guaranteed that for given BDI
   *   some IO is happening if we are over background dirty threshold.
   *   Caller need not hold sb s_umount semaphore.
   */
  void bdi_start_background_writeback(struct backing_dev_info *bdi)
  {
  	/*
  	 * We just wake up the flusher thread. It will perform background
  	 * writeback as soon as there is no other work to do.
  	 */
  	trace_writeback_wake_background(bdi);
  	spin_lock_bh(&bdi->wb_lock);
  	bdi_wakeup_flusher(bdi);
  	spin_unlock_bh(&bdi->wb_lock);
  }
  
  /*
   * Remove the inode from the writeback list it is on.
   */
  void inode_wb_list_del(struct inode *inode)
  {
  	spin_lock(&inode_wb_list_lock);
  	list_del_init(&inode->i_wb_list);
  	spin_unlock(&inode_wb_list_lock);
  }
  
  
  /*
   * Redirty an inode: set its when-it-was dirtied timestamp and move it to the
   * furthest end of its superblock's dirty-inode list.
   *
   * Before stamping the inode's ->dirtied_when, we check to see whether it is
   * already the most-recently-dirtied inode on the b_dirty list.  If that is
   * the case then the inode must have been redirtied while it was being written
   * out and we don't reset its dirtied_when.
   */
  static void redirty_tail(struct inode *inode)
  {
  	struct bdi_writeback *wb = &inode_to_bdi(inode)->wb;
  
  	assert_spin_locked(&inode_wb_list_lock);
  	if (!list_empty(&wb->b_dirty)) {
  		struct inode *tail;
  
  		tail = wb_inode(wb->b_dirty.next);
  		if (time_before(inode->dirtied_when, tail->dirtied_when))
  			inode->dirtied_when = jiffies;
  	}
  	list_move(&inode->i_wb_list, &wb->b_dirty);
  }
  
  /*
   * requeue inode for re-scanning after bdi->b_io list is exhausted.
   */
  static void requeue_io(struct inode *inode)
  {
  	struct bdi_writeback *wb = &inode_to_bdi(inode)->wb;
  
  	assert_spin_locked(&inode_wb_list_lock);
  	list_move(&inode->i_wb_list, &wb->b_more_io);
  }
  
  static void inode_sync_complete(struct inode *inode)
  {
  	/*
  	 * Prevent speculative execution through
  	 * spin_unlock(&inode_wb_list_lock);
  	 */
  
  	smp_mb();
  	wake_up_bit(&inode->i_state, __I_SYNC);
  }
  
  static bool inode_dirtied_after(struct inode *inode, unsigned long t)
  {
  	bool ret = time_after(inode->dirtied_when, t);
  #ifndef CONFIG_64BIT
  	/*
  	 * For inodes being constantly redirtied, dirtied_when can get stuck.
  	 * It _appears_ to be in the future, but is actually in distant past.
  	 * This test is necessary to prevent such wrapped-around relative times
  	 * from permanently stopping the whole bdi writeback.
  	 */
  	ret = ret && time_before_eq(inode->dirtied_when, jiffies);
  #endif
  	return ret;
  }
  
  /*
   * Move expired dirty inodes from @delaying_queue to @dispatch_queue.
   */
  static void move_expired_inodes(struct list_head *delaying_queue,
  			       struct list_head *dispatch_queue,
  				unsigned long *older_than_this)
  {
  	LIST_HEAD(tmp);
  	struct list_head *pos, *node;
  	struct super_block *sb = NULL;
  	struct inode *inode;
  	int do_sb_sort = 0;
  
  	while (!list_empty(delaying_queue)) {
  		inode = wb_inode(delaying_queue->prev);
  		if (older_than_this &&
  		    inode_dirtied_after(inode, *older_than_this))
  			break;
  		if (sb && sb != inode->i_sb)
  			do_sb_sort = 1;
  		sb = inode->i_sb;
  		list_move(&inode->i_wb_list, &tmp);
  	}
  
  	/* just one sb in list, splice to dispatch_queue and we're done */
  	if (!do_sb_sort) {
  		list_splice(&tmp, dispatch_queue);
  		return;
  	}
  
  	/* Move inodes from one superblock together */
  	while (!list_empty(&tmp)) {
  		sb = wb_inode(tmp.prev)->i_sb;
  		list_for_each_prev_safe(pos, node, &tmp) {
  			inode = wb_inode(pos);
  			if (inode->i_sb == sb)
  				list_move(&inode->i_wb_list, dispatch_queue);
  		}
  	}
  }
  
  /*
   * Queue all expired dirty inodes for io, eldest first.
   * Before
   *         newly dirtied     b_dirty    b_io    b_more_io
   *         =============>    gf         edc     BA
   * After
   *         newly dirtied     b_dirty    b_io    b_more_io
   *         =============>    g          fBAedc
   *                                           |
   *                                           +--> dequeue for IO
   */
  static void queue_io(struct bdi_writeback *wb, unsigned long *older_than_this)
  {
  	assert_spin_locked(&inode_wb_list_lock);
  	list_splice_init(&wb->b_more_io, &wb->b_io);
  	move_expired_inodes(&wb->b_dirty, &wb->b_io, older_than_this);
  }
  
  static int write_inode(struct inode *inode, struct writeback_control *wbc)
  {
  	if (inode->i_sb->s_op->write_inode && !is_bad_inode(inode))
  		return inode->i_sb->s_op->write_inode(inode, wbc);
  	return 0;
  }
  
  /*
   * Wait for writeback on an inode to complete.
   */
  static void inode_wait_for_writeback(struct inode *inode)
  {
  	DEFINE_WAIT_BIT(wq, &inode->i_state, __I_SYNC);
  	wait_queue_head_t *wqh;
  
  	wqh = bit_waitqueue(&inode->i_state, __I_SYNC);
  	while (inode->i_state & I_SYNC) {
  		spin_unlock(&inode->i_lock);
  		spin_unlock(&inode_wb_list_lock);
  		__wait_on_bit(wqh, &wq, inode_wait, TASK_UNINTERRUPTIBLE);
  		spin_lock(&inode_wb_list_lock);
  		spin_lock(&inode->i_lock);
  	}
  }
  
  /*
   * Write out an inode's dirty pages.  Called under inode_wb_list_lock and
   * inode->i_lock.  Either the caller has an active reference on the inode or
   * the inode has I_WILL_FREE set.
   *
   * If `wait' is set, wait on the writeout.
   *
   * The whole writeout design is quite complex and fragile.  We want to avoid
   * starvation of particular inodes when others are being redirtied, prevent
   * livelocks, etc.
   */
  static int
  writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
  {
  	struct address_space *mapping = inode->i_mapping;
  	unsigned dirty;
  	int ret;
  
  	assert_spin_locked(&inode_wb_list_lock);
  	assert_spin_locked(&inode->i_lock);
  
  	if (!atomic_read(&inode->i_count))
  		WARN_ON(!(inode->i_state & (I_WILL_FREE|I_FREEING)));
  	else
  		WARN_ON(inode->i_state & I_WILL_FREE);
  
  	if (inode->i_state & I_SYNC) {
  		/*
  		 * 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.
  		 */
  		if (wbc->sync_mode != WB_SYNC_ALL) {
  			requeue_io(inode);
  			return 0;
  		}
  
  		/*
  		 * It's a data-integrity sync.  We must wait.
  		 */
  		inode_wait_for_writeback(inode);
  	}
  
  	BUG_ON(inode->i_state & I_SYNC);
  
  	/* Set I_SYNC, reset I_DIRTY_PAGES */
  	inode->i_state |= I_SYNC;
  	inode->i_state &= ~I_DIRTY_PAGES;
  	spin_unlock(&inode->i_lock);
  	spin_unlock(&inode_wb_list_lock);
  
  	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);
  	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;
  	}
  
  	spin_lock(&inode_wb_list_lock);
  	spin_lock(&inode->i_lock);
  	inode->i_state &= ~I_SYNC;
  	if (!(inode->i_state & I_FREEING)) {
  		if (mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
  			/*
  			 * We didn't write back all the pages.  nfs_writepages()
  			 * sometimes bales out without doing anything.
  			 */
  			inode->i_state |= I_DIRTY_PAGES;
  			if (wbc->nr_to_write <= 0) {
  				/*
  				 * slice used up: queue for next turn
  				 */
  				requeue_io(inode);
  			} else {
  				/*
  				 * Writeback blocked by something other than
  				 * congestion. Delay the inode for some time to
  				 * avoid spinning on the CPU (100% iowait)
  				 * retrying writeback of the dirty page/inode
  				 * that cannot be performed immediately.
  				 */
  				redirty_tail(inode);
  			}
  		} else if (inode->i_state & I_DIRTY) {
  			/*
  			 * Filesystems can dirty the inode during writeback
  			 * operations, such as delayed allocation during
  			 * submission or metadata updates after data IO
  			 * completion.
  			 */
  			redirty_tail(inode);
  		} else {
  			/*
  			 * The inode is clean.  At this point we either have
  			 * a reference to the inode or it's on it's way out.
  			 * No need to add it back to the LRU.
  			 */
  			list_del_init(&inode->i_wb_list);
  		}
  	}
  	inode_sync_complete(inode);
  	return ret;
  }
  
  /*
   * For background writeback the caller does not have the sb pinned
   * before calling writeback. So make sure that we do pin it, so it doesn't
   * go away while we are writing inodes from it.
   */
  static bool pin_sb_for_writeback(struct super_block *sb)
  {
  	spin_lock(&sb_lock);
  	if (list_empty(&sb->s_instances)) {
  		spin_unlock(&sb_lock);
  		return false;
  	}
  
  	sb->s_count++;
  	spin_unlock(&sb_lock);
  
  	if (down_read_trylock(&sb->s_umount)) {
  		if (sb->s_root)
  			return true;
  		up_read(&sb->s_umount);
  	}
  
  	put_super(sb);
  	return false;
  }
  
  /*
   * Write a portion of b_io inodes which belong to @sb.
   *
   * If @only_this_sb is true, then find and write all such
   * inodes. Otherwise write only ones which go sequentially
   * in reverse order.
   *
   * Return 1, if the caller writeback routine should be
   * interrupted. Otherwise return 0.
   */
  static int writeback_sb_inodes(struct super_block *sb, struct bdi_writeback *wb,
  		struct writeback_control *wbc, bool only_this_sb)
  {
  	while (!list_empty(&wb->b_io)) {
  		long pages_skipped;
  		struct inode *inode = wb_inode(wb->b_io.prev);
  
  		if (inode->i_sb != sb) {
  			if (only_this_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);
  				continue;
  			}
  
  			/*
  			 * The inode belongs to a different superblock.
  			 * Bounce back to the caller to unpin this and
  			 * pin the next superblock.
  			 */
  			return 0;
  		}
  
  		/*
  		 * Don't bother with new inodes or inodes beeing freed, first
  		 * kind does not need peridic 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);
  			requeue_io(inode);
  			continue;
  		}
  
  		/*
  		 * Was this inode dirtied after sync_sb_inodes was called?
  		 * This keeps sync from extra jobs and livelock.
  		 */
  		if (inode_dirtied_after(inode, wbc->wb_start)) {
  			spin_unlock(&inode->i_lock);
  			return 1;
  		}
  
  		__iget(inode);
  
  		pages_skipped = wbc->pages_skipped;
  		writeback_single_inode(inode, wbc);
  		if (wbc->pages_skipped != pages_skipped) {
  			/*
  			 * writeback is not making progress due to locked
  			 * buffers.  Skip this inode for now.
  			 */
  			redirty_tail(inode);
  		}
  		spin_unlock(&inode->i_lock);
  		spin_unlock(&inode_wb_list_lock);
  		iput(inode);
  		cond_resched();
  		spin_lock(&inode_wb_list_lock);
  		if (wbc->nr_to_write <= 0) {
  			wbc->more_io = 1;
  			return 1;
  		}
  		if (!list_empty(&wb->b_more_io))
  			wbc->more_io = 1;
  	}
  	/* b_io is empty */
  	return 1;
  }
  
  void writeback_inodes_wb(struct bdi_writeback *wb,
  		struct writeback_control *wbc)
  {
  	int ret = 0;
  
  	if (!wbc->wb_start)
  		wbc->wb_start = jiffies; /* livelock avoidance */
  	spin_lock(&inode_wb_list_lock);
  	if (!wbc->for_kupdate || list_empty(&wb->b_io))
  		queue_io(wb, wbc->older_than_this);
  
  	while (!list_empty(&wb->b_io)) {
  		struct inode *inode = wb_inode(wb->b_io.prev);
  		struct super_block *sb = inode->i_sb;
  
  		if (!pin_sb_for_writeback(sb)) {
  			requeue_io(inode);
  			continue;
  		}
  		ret = writeback_sb_inodes(sb, wb, wbc, false);
  		drop_super(sb);
  
  		if (ret)
  			break;
  	}
  	spin_unlock(&inode_wb_list_lock);
  	/* Leave any unwritten inodes on b_io */
  }
  
  static void __writeback_inodes_sb(struct super_block *sb,
  		struct bdi_writeback *wb, struct writeback_control *wbc)
  {
  	WARN_ON(!rwsem_is_locked(&sb->s_umount));
  
  	spin_lock(&inode_wb_list_lock);
  	if (!wbc->for_kupdate || list_empty(&wb->b_io))
  		queue_io(wb, wbc->older_than_this);
  	writeback_sb_inodes(sb, wb, wbc, true);
  	spin_unlock(&inode_wb_list_lock);
  }
  
  /*
   * The maximum number of pages to writeout in a single bdi flush/kupdate
   * operation.  We do this so we don't hold I_SYNC against an inode for
   * enormous amounts of time, which would block a userspace task which has
   * been forced to throttle against that inode.  Also, the code reevaluates
   * the dirty each time it has written this many pages.
   */
  #define MAX_WRITEBACK_PAGES     1024
  
  static inline bool over_bground_thresh(void)
  {
  	unsigned long background_thresh, dirty_thresh;
  
  	global_dirty_limits(&background_thresh, &dirty_thresh);
  
  	return (global_page_state(NR_FILE_DIRTY) +
  		global_page_state(NR_UNSTABLE_NFS) > background_thresh);
  }
  
  /*
   * Explicit flushing or periodic writeback of "old" data.
   *
   * Define "old": the first time one of an inode's pages is dirtied, we mark the
   * dirtying-time in the inode's address_space.  So this periodic writeback code
   * just walks the superblock inode list, writing back any inodes which are
   * older than a specific point in time.
   *
   * Try to run once per dirty_writeback_interval.  But if a writeback event
   * takes longer than a dirty_writeback_interval interval, then leave a
   * one-second gap.
   *
   * older_than_this takes precedence over nr_to_write.  So we'll only write back
   * all dirty pages if they are all attached to "old" mappings.
   */
  static long wb_writeback(struct bdi_writeback *wb,
  			 struct wb_writeback_work *work)
  {
  	struct writeback_control wbc = {
  		.sync_mode		= work->sync_mode,
  		.older_than_this	= NULL,
  		.for_kupdate		= work->for_kupdate,
  		.for_background		= work->for_background,
  		.range_cyclic		= work->range_cyclic,
  	};
  	unsigned long oldest_jif;
  	long wrote = 0;
  	long write_chunk;
  	struct inode *inode;
  
  	if (wbc.for_kupdate) {
  		wbc.older_than_this = &oldest_jif;
  		oldest_jif = jiffies -
  				msecs_to_jiffies(dirty_expire_interval * 10);
  	}
  	if (!wbc.range_cyclic) {
  		wbc.range_start = 0;
  		wbc.range_end = LLONG_MAX;
  	}
  
  	/*
  	 * WB_SYNC_ALL mode does livelock avoidance by syncing dirty
  	 * inodes/pages in one big loop. Setting wbc.nr_to_write=LONG_MAX
  	 * here avoids calling into writeback_inodes_wb() more than once.
  	 *
  	 * The intended call sequence for WB_SYNC_ALL writeback is:
  	 *
  	 *      wb_writeback()
  	 *          __writeback_inodes_sb()     <== called only once
  	 *              write_cache_pages()     <== called once for each inode
  	 *                   (quickly) tag currently dirty pages
  	 *                   (maybe slowly) sync all tagged pages
  	 */
  	if (wbc.sync_mode == WB_SYNC_NONE)
  		write_chunk = MAX_WRITEBACK_PAGES;
  	else
  		write_chunk = LONG_MAX;
  
  	wbc.wb_start = jiffies; /* livelock avoidance */
  	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())
  			break;
  
  		wbc.more_io = 0;
  		wbc.nr_to_write = write_chunk;
  		wbc.pages_skipped = 0;
  
  		trace_wbc_writeback_start(&wbc, wb->bdi);
  		if (work->sb)
  			__writeback_inodes_sb(work->sb, wb, &wbc);
  		else
  			writeback_inodes_wb(wb, &wbc);
  		trace_wbc_writeback_written(&wbc, wb->bdi);
  
  		work->nr_pages -= write_chunk - wbc.nr_to_write;
  		wrote += write_chunk - wbc.nr_to_write;
  
  		/*
  		 * If we consumed everything, see if we have more
  		 */
  		if (wbc.nr_to_write <= 0)
  			continue;
  		/*
  		 * Didn't write everything and we don't have more IO, bail
  		 */
  		if (!wbc.more_io)
  			break;
  		/*
  		 * Did we write something? Try for more
  		 */
  		if (wbc.nr_to_write < write_chunk)
  			continue;
  		/*
  		 * Nothing written. Wait for some inode to
  		 * become available for writeback. Otherwise
  		 * we'll just busyloop.
  		 */
  		spin_lock(&inode_wb_list_lock);
  		if (!list_empty(&wb->b_more_io))  {
  			inode = wb_inode(wb->b_more_io.prev);
  			trace_wbc_writeback_wait(&wbc, wb->bdi);
  			spin_lock(&inode->i_lock);
  			inode_wait_for_writeback(inode);
  			spin_unlock(&inode->i_lock);
  		}
  		spin_unlock(&inode_wb_list_lock);
  	}
  
  	return wrote;
  }
  
  /*
   * Return the next wb_writeback_work struct that hasn't been processed yet.
   */
  static struct wb_writeback_work *
  get_next_work_item(struct backing_dev_info *bdi)
  {
  	struct wb_writeback_work *work = NULL;
  
  	spin_lock_bh(&bdi->wb_lock);
  	if (!list_empty(&bdi->work_list)) {
  		work = list_entry(bdi->work_list.next,
  				  struct wb_writeback_work, list);
  		list_del_init(&work->list);
  	}
  	spin_unlock_bh(&bdi->wb_lock);
  	return work;
  }
  
  /*
   * Add in the number of potentially dirty inodes, because each inode
   * write can dirty pagecache in the underlying blockdev.
   */
  static unsigned long get_nr_dirty_pages(void)
  {
  	return global_page_state(NR_FILE_DIRTY) +
  		global_page_state(NR_UNSTABLE_NFS) +
  		get_nr_dirty_inodes();
  }
  
  static long wb_check_background_flush(struct bdi_writeback *wb)
  {
  	if (over_bground_thresh()) {
  
  		struct wb_writeback_work work = {
  			.nr_pages	= LONG_MAX,
  			.sync_mode	= WB_SYNC_NONE,
  			.for_background	= 1,
  			.range_cyclic	= 1,
  		};
  
  		return wb_writeback(wb, &work);
  	}
  
  	return 0;
  }
  
  static long wb_check_old_data_flush(struct bdi_writeback *wb)
  {
  	unsigned long expired;
  	long nr_pages;
  
  	/*
  	 * When set to zero, disable periodic writeback
  	 */
  	if (!dirty_writeback_interval)
  		return 0;
  
  	expired = wb->last_old_flush +
  			msecs_to_jiffies(dirty_writeback_interval * 10);
  	if (time_before(jiffies, expired))
  		return 0;
  
  	wb->last_old_flush = jiffies;
  	nr_pages = get_nr_dirty_pages();
  
  	if (nr_pages) {
  		struct wb_writeback_work work = {
  			.nr_pages	= nr_pages,
  			.sync_mode	= WB_SYNC_NONE,
  			.for_kupdate	= 1,
  			.range_cyclic	= 1,
  		};
  
  		return wb_writeback(wb, &work);
  	}
  
  	return 0;
  }
  
  /*
   * Retrieve work items and do the writeback they describe
   */
  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) {
  		/*
  		 * 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);
  
  		/*
  		 * Notify the caller of completion if this is a synchronous
  		 * work item, otherwise just free it.
  		 */
  		if (work->done)
  			complete(work->done);
  		else
  			kfree(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;
  }
  
  /*
   * Handle writeback of dirty data for the device backed by this bdi. Also
   * wakes up periodically and does kupdated style flushing.
   */
  int bdi_writeback_thread(void *data)
  {
  	struct bdi_writeback *wb = data;
  	struct backing_dev_info *bdi = wb->bdi;
  	long pages_written;
  
  	current->flags |= PF_SWAPWRITE;
  	set_freezable();
  	wb->last_active = jiffies;
  
  	/*
  	 * Our parent may run at a different priority, just set us to normal
  	 */
  	set_user_nice(current, 0);
  
  	trace_writeback_thread_start(bdi);
  
  	while (!kthread_should_stop()) {
  		/*
  		 * Remove own delayed wake-up timer, since we are already awake
  		 * and we'll take care of the preriodic write-back.
  		 */
  		del_timer(&wb->wakeup_timer);
  
  		pages_written = wb_do_writeback(wb, 0);
  
  		trace_writeback_pages_written(pages_written);
  
  		if (pages_written)
  			wb->last_active = jiffies;
  
  		set_current_state(TASK_INTERRUPTIBLE);
  		if (!list_empty(&bdi->work_list) || kthread_should_stop()) {
  			__set_current_state(TASK_RUNNING);
  			continue;
  		}
  
  		if (wb_has_dirty_io(wb) && dirty_writeback_interval)
  			schedule_timeout(msecs_to_jiffies(dirty_writeback_interval * 10));
  		else {
  			/*
  			 * We have nothing to do, so can go sleep without any
  			 * timeout and save power. When a work is queued or
  			 * something is made dirty - we will be woken up.
  			 */
  			schedule();
  		}
  
  		try_to_freeze();
  	}
  
  	/* Flush any work that raced with us exiting */
  	if (!list_empty(&bdi->work_list))
  		wb_do_writeback(wb, 1);
  
  	trace_writeback_thread_stop(bdi);
  	return 0;
  }
  
  
  /*
   * Start writeback of `nr_pages' pages.  If `nr_pages' is zero, write back
   * the whole world.
   */
  void wakeup_flusher_threads(long nr_pages)
  {
  	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);
  	}
  	rcu_read_unlock();
  }
  
  static noinline void block_dump___mark_inode_dirty(struct inode *inode)
  {
  	if (inode->i_ino || strcmp(inode->i_sb->s_id, "bdev")) {
  		struct dentry *dentry;
  		const char *name = "?";
  
  		dentry = d_find_alias(inode);
  		if (dentry) {
  			spin_lock(&dentry->d_lock);
  			name = (const char *) dentry->d_name.name;
  		}
  		printk(KERN_DEBUG
  		       "%s(%d): dirtied inode %lu (%s) on %s\n",
  		       current->comm, task_pid_nr(current), inode->i_ino,
  		       name, inode->i_sb->s_id);
  		if (dentry) {
  			spin_unlock(&dentry->d_lock);
  			dput(dentry);
  		}
  	}
  }
  
  /**
   *	__mark_inode_dirty -	internal function
   *	@inode: inode to mark
   *	@flags: what kind of dirty (i.e. I_DIRTY_SYNC)
   *	Mark an inode as dirty. Callers should use mark_inode_dirty or
   *  	mark_inode_dirty_sync.
   *
   * Put the inode on the super block's dirty list.
   *
   * CAREFUL! We mark it dirty unconditionally, but move it onto the
   * dirty list only if it is hashed or if it refers to a blockdev.
   * If it was not hashed, it will never be added to the dirty list
   * even if it is later hashed, as it will have been marked dirty already.
   *
   * In short, make sure you hash any inodes _before_ you start marking
   * them dirty.
   *
   * Note that for blockdevs, inode->dirtied_when represents the dirtying time of
   * the block-special inode (/dev/hda1) itself.  And the ->dirtied_when field of
   * the kernel-internal blockdev inode represents the dirtying time of the
   * blockdev's pages.  This is why for I_DIRTY_PAGES we always use
   * page->mapping->host, so the page-dirtying time is recorded in the internal
   * blockdev inode.
   */
  void __mark_inode_dirty(struct inode *inode, int flags)
  {
  	struct super_block *sb = inode->i_sb;
  	struct backing_dev_info *bdi = NULL;
  
  	/*
  	 * Don't do this for I_DIRTY_PAGES - that doesn't actually
  	 * dirty the inode itself
  	 */
  	if (flags & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) {
  		if (sb->s_op->dirty_inode)
  			sb->s_op->dirty_inode(inode, flags);
  	}
  
  	/*
  	 * make sure that changes are seen by all cpus before we test i_state
  	 * -- mikulas
  	 */
  	smp_mb();
  
  	/* avoid the locking if we can */
  	if ((inode->i_state & flags) == flags)
  		return;
  
  	if (unlikely(block_dump))
  		block_dump___mark_inode_dirty(inode);
  
  	spin_lock(&inode->i_lock);
  	if ((inode->i_state & flags) != flags) {
  		const int was_dirty = inode->i_state & I_DIRTY;
  
  		inode->i_state |= flags;
  
  		/*
  		 * If the inode is being synced, just update its dirty state.
  		 * The unlocker will place the inode on the appropriate
  		 * superblock list, based upon its state.
  		 */
  		if (inode->i_state & I_SYNC)
  			goto out_unlock_inode;
  
  		/*
  		 * Only add valid (hashed) inodes to the superblock's
  		 * dirty list.  Add blockdev inodes as well.
  		 */
  		if (!S_ISBLK(inode->i_mode)) {
  			if (inode_unhashed(inode))
  				goto out_unlock_inode;
  		}
  		if (inode->i_state & I_FREEING)
  			goto out_unlock_inode;
  
  		/*
  		 * If the inode was already on b_dirty/b_io/b_more_io, don't
  		 * reposition it (that would break b_dirty time-ordering).
  		 */
  		if (!was_dirty) {
  			bool wakeup_bdi = false;
  			bdi = inode_to_bdi(inode);
  
  			if (bdi_cap_writeback_dirty(bdi)) {
  				WARN(!test_bit(BDI_registered, &bdi->state),
  				     "bdi-%s not registered\n", bdi->name);
  
  				/*
  				 * If this is the first dirty inode for this
  				 * bdi, we have to wake-up the corresponding
  				 * bdi thread to make sure background
  				 * write-back happens later.
  				 */
  				if (!wb_has_dirty_io(&bdi->wb))
  					wakeup_bdi = true;
  			}
  
  			spin_unlock(&inode->i_lock);
  			spin_lock(&inode_wb_list_lock);
  			inode->dirtied_when = jiffies;
  			list_move(&inode->i_wb_list, &bdi->wb.b_dirty);
  			spin_unlock(&inode_wb_list_lock);
  
  			if (wakeup_bdi)
  				bdi_wakeup_thread_delayed(bdi);
  			return;
  		}
  	}
  out_unlock_inode:
  	spin_unlock(&inode->i_lock);
  
  }
  EXPORT_SYMBOL(__mark_inode_dirty);
  
  /*
   * Write out a superblock's list of dirty inodes.  A wait will be performed
   * upon no inodes, all inodes or the final one, depending upon sync_mode.
   *
   * If older_than_this is non-NULL, then only write out inodes which
   * had their first dirtying at a time earlier than *older_than_this.
   *
   * If `bdi' is non-zero then we're being asked to writeback a specific queue.
   * This function assumes that the blockdev superblock's inodes are backed by
   * a variety of queues, so all inodes are searched.  For other superblocks,
   * assume that all inodes are backed by the same queue.
   *
   * The inodes to be written are parked on bdi->b_io.  They are moved back onto
   * bdi->b_dirty as they are selected for writing.  This way, none can be missed
   * on the writer throttling path, and we get decent balancing between many
   * throttled threads: we don't want them all piling up on inode_sync_wait.
   */
  static void wait_sb_inodes(struct super_block *sb)
  {
  	struct inode *inode, *old_inode = NULL;
  
  	/*
  	 * We need to be protected against the filesystem going from
  	 * r/o to r/w or vice versa.
  	 */
  	WARN_ON(!rwsem_is_locked(&sb->s_umount));
  
  	spin_lock(&inode_sb_list_lock);
  
  	/*
  	 * Data integrity sync. Must wait for all pages under writeback,
  	 * because there may have been pages dirtied before our sync
  	 * call, but which had writeout started before we write it out.
  	 * In which case, the inode may not be on the dirty list, but
  	 * we still have to wait for that writeout.
  	 */
  	list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
  		struct address_space *mapping = inode->i_mapping;
  
  		spin_lock(&inode->i_lock);
  		if ((inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) ||
  		    (mapping->nrpages == 0)) {
  			spin_unlock(&inode->i_lock);
  			continue;
  		}
  		__iget(inode);
  		spin_unlock(&inode->i_lock);
  		spin_unlock(&inode_sb_list_lock);
  
  		/*
  		 * We hold a reference to 'inode' so it couldn't have been
  		 * removed from s_inodes list while we dropped the
  		 * inode_sb_list_lock.  We cannot iput the inode now as we can
  		 * be holding the last reference and we cannot iput it under
  		 * inode_sb_list_lock. So we keep the reference and iput it
  		 * later.
  		 */
  		iput(old_inode);
  		old_inode = inode;
  
  		filemap_fdatawait(mapping);
  
  		cond_resched();
  
  		spin_lock(&inode_sb_list_lock);
  	}
  	spin_unlock(&inode_sb_list_lock);
  	iput(old_inode);
  }
  
  /**
   * writeback_inodes_sb_nr -	writeback dirty inodes from given super_block
   * @sb: the superblock
   * @nr: the number of pages to write
   *
   * Start writeback on some inodes on this super_block. No guarantees are made
   * on how many (if any) will be written, and this function does not wait
   * for IO completion of submitted IO.
   */
  void writeback_inodes_sb_nr(struct super_block *sb, unsigned long nr)
  {
  	DECLARE_COMPLETION_ONSTACK(done);
  	struct wb_writeback_work work = {
  		.sb		= sb,
  		.sync_mode	= WB_SYNC_NONE,
  		.done		= &done,
  		.nr_pages	= nr,
  	};
  
  	WARN_ON(!rwsem_is_locked(&sb->s_umount));
  	bdi_queue_work(sb->s_bdi, &work);
  	wait_for_completion(&done);
  }
  EXPORT_SYMBOL(writeback_inodes_sb_nr);
  
  /**
   * writeback_inodes_sb	-	writeback dirty inodes from given super_block
   * @sb: the superblock
   *
   * Start writeback on some inodes on this super_block. No guarantees are made
   * on how many (if any) will be written, and this function does not wait
   * for IO completion of submitted IO.
   */
  void writeback_inodes_sb(struct super_block *sb)
  {
  	return writeback_inodes_sb_nr(sb, get_nr_dirty_pages());
  }
  EXPORT_SYMBOL(writeback_inodes_sb);
  
  /**
   * writeback_inodes_sb_if_idle	-	start writeback if none underway
   * @sb: the superblock
   *
   * Invoke writeback_inodes_sb if no writeback is currently underway.
   * Returns 1 if writeback was started, 0 if not.
   */
  int writeback_inodes_sb_if_idle(struct super_block *sb)
  {
  	if (!writeback_in_progress(sb->s_bdi)) {
  		down_read(&sb->s_umount);
  		writeback_inodes_sb(sb);
  		up_read(&sb->s_umount);
  		return 1;
  	} else
  		return 0;
  }
  EXPORT_SYMBOL(writeback_inodes_sb_if_idle);
  
  /**
   * writeback_inodes_sb_if_idle	-	start writeback if none underway
   * @sb: the superblock
   * @nr: the number of pages to write
   *
   * Invoke writeback_inodes_sb if no writeback is currently underway.
   * Returns 1 if writeback was started, 0 if not.
   */
  int writeback_inodes_sb_nr_if_idle(struct super_block *sb,
  				   unsigned long nr)
  {
  	if (!writeback_in_progress(sb->s_bdi)) {
  		down_read(&sb->s_umount);
  		writeback_inodes_sb_nr(sb, nr);
  		up_read(&sb->s_umount);
  		return 1;
  	} else
  		return 0;
  }
  EXPORT_SYMBOL(writeback_inodes_sb_nr_if_idle);
  
  /**
   * sync_inodes_sb	-	sync sb inode pages
   * @sb: the superblock
   *
   * This function writes and waits on any dirty inode belonging to this
   * super_block.
   */
  void sync_inodes_sb(struct super_block *sb)
  {
  	DECLARE_COMPLETION_ONSTACK(done);
  	struct wb_writeback_work work = {
  		.sb		= sb,
  		.sync_mode	= WB_SYNC_ALL,
  		.nr_pages	= LONG_MAX,
  		.range_cyclic	= 0,
  		.done		= &done,
  	};
  
  	WARN_ON(!rwsem_is_locked(&sb->s_umount));
  
  	bdi_queue_work(sb->s_bdi, &work);
  	wait_for_completion(&done);
  
  	wait_sb_inodes(sb);
  }
  EXPORT_SYMBOL(sync_inodes_sb);
  
  /**
   * write_inode_now	-	write an inode to disk
   * @inode: inode to write to disk
   * @sync: whether the write should be synchronous or not
   *
   * This function commits an inode to disk immediately if it is dirty. This is
   * primarily needed by knfsd.
   *
   * The caller must either have a ref on the inode or must have set I_WILL_FREE.
   */
  int write_inode_now(struct inode *inode, int sync)
  {
  	int ret;
  	struct writeback_control wbc = {
  		.nr_to_write = LONG_MAX,
  		.sync_mode = sync ? WB_SYNC_ALL : WB_SYNC_NONE,
  		.range_start = 0,
  		.range_end = LLONG_MAX,
  	};
  
  	if (!mapping_cap_writeback_dirty(inode->i_mapping))
  		wbc.nr_to_write = 0;
  
  	might_sleep();
  	spin_lock(&inode_wb_list_lock);
  	spin_lock(&inode->i_lock);
  	ret = writeback_single_inode(inode, &wbc);
  	spin_unlock(&inode->i_lock);
  	spin_unlock(&inode_wb_list_lock);
  	if (sync)
  		inode_sync_wait(inode);
  	return ret;
  }
  EXPORT_SYMBOL(write_inode_now);
  
  /**
   * sync_inode - write an inode and its pages to disk.
   * @inode: the inode to sync
   * @wbc: controls the writeback mode
   *
   * sync_inode() will write an inode and its pages to disk.  It will also
   * correctly update the inode on its superblock's dirty inode lists and will
   * update inode->i_state.
   *
   * The caller must have a ref on the inode.
   */
  int sync_inode(struct inode *inode, struct writeback_control *wbc)
  {
  	int ret;
  
  	spin_lock(&inode_wb_list_lock);
  	spin_lock(&inode->i_lock);
  	ret = writeback_single_inode(inode, wbc);
  	spin_unlock(&inode->i_lock);
  	spin_unlock(&inode_wb_list_lock);
  	return ret;
  }
  EXPORT_SYMBOL(sync_inode);
  
  /**
   * sync_inode_metadata - write an inode to disk
   * @inode: the inode to sync
   * @wait: wait for I/O to complete.
   *
   * Write an inode to disk and adjust its dirty state after completion.
   *
   * Note: only writes the actual inode, no associated data or other metadata.
   */
  int sync_inode_metadata(struct inode *inode, int wait)
  {
  	struct writeback_control wbc = {
  		.sync_mode = wait ? WB_SYNC_ALL : WB_SYNC_NONE,
  		.nr_to_write = 0, /* metadata-only */
  	};
  
  	return sync_inode(inode, &wbc);
  }
  EXPORT_SYMBOL(sync_inode_metadata);
  