Linux-2.6.12-rc2

Initial git repository build. I'm not bothering with the full history,
even though we have it. We can create a separate "historical" git
archive of that later if we want to, and in the meantime it's about
3.2GB when imported into git - space that would just make the early
git days unnecessarily complicated, when we don't have a lot of good
infrastructure for it.

Let it rip!

fs/fs-writeback.c

@@ -0,0 +1,695 @@
+/*
+ * 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	akpm@zip.com.au
+ *		Split out of fs/inode.c
+ *		Additions for address_space-based writeback
+ */
+
+#include <linux/kernel.h>
+#include <linux/spinlock.h>
+#include <linux/sched.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include <linux/writeback.h>
+#include <linux/blkdev.h>
+#include <linux/backing-dev.h>
+#include <linux/buffer_head.h>
+
+extern struct super_block *blockdev_superblock;
+
+/**
+ *	__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.
+ *
+ * This function *must* be atomic for the I_DIRTY_PAGES case -
+ * set_page_dirty() is called under spinlock in several places.
+ *
+ * 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;
+
+	/*
+	 * 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);
+	}
+
+	/*
+	 * 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)) {
+		struct dentry *dentry = NULL;
+		const char *name = "?";
+
+		if (!list_empty(&inode->i_dentry)) {
+			dentry = list_entry(inode->i_dentry.next,
+					    struct dentry, d_alias);
+			if (dentry && dentry->d_name.name)
+				name = (const char *) dentry->d_name.name;
+		}
+
+		if (inode->i_ino || strcmp(inode->i_sb->s_id, "bdev"))
+			printk(KERN_DEBUG
+			       "%s(%d): dirtied inode %lu (%s) on %s\n",
+			       current->comm, current->pid, inode->i_ino,
+			       name, inode->i_sb->s_id);
+	}
+
+	spin_lock(&inode_lock);
+	if ((inode->i_state & flags) != flags) {
+		const int was_dirty = inode->i_state & I_DIRTY;
+
+		inode->i_state |= flags;
+
+		/*
+		 * If the inode is locked, 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_LOCK)
+			goto out;
+
+		/*
+		 * Only add valid (hashed) inodes to the superblock's
+		 * dirty list.  Add blockdev inodes as well.
+		 */
+		if (!S_ISBLK(inode->i_mode)) {
+			if (hlist_unhashed(&inode->i_hash))
+				goto out;
+		}
+		if (inode->i_state & (I_FREEING|I_CLEAR))
+			goto out;
+
+		/*
+		 * If the inode was already on s_dirty or s_io, don't
+		 * reposition it (that would break s_dirty time-ordering).
+		 */
+		if (!was_dirty) {
+			inode->dirtied_when = jiffies;
+			list_move(&inode->i_list, &sb->s_dirty);
+		}
+	}
+out:
+	spin_unlock(&inode_lock);
+}
+
+EXPORT_SYMBOL(__mark_inode_dirty);
+
+static int write_inode(struct inode *inode, int sync)
+{
+	if (inode->i_sb->s_op->write_inode && !is_bad_inode(inode))
+		return inode->i_sb->s_op->write_inode(inode, sync);
+	return 0;
+}
+
+/*
+ * Write a single inode's dirty pages and inode data out to disk.
+ * 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.
+ *
+ * Called under inode_lock.
+ */
+static int
+__sync_single_inode(struct inode *inode, struct writeback_control *wbc)
+{
+	unsigned dirty;
+	struct address_space *mapping = inode->i_mapping;
+	struct super_block *sb = inode->i_sb;
+	int wait = wbc->sync_mode == WB_SYNC_ALL;
+	int ret;
+
+	BUG_ON(inode->i_state & I_LOCK);
+
+	/* Set I_LOCK, reset I_DIRTY */
+	dirty = inode->i_state & I_DIRTY;
+	inode->i_state |= I_LOCK;
+	inode->i_state &= ~I_DIRTY;
+
+	spin_unlock(&inode_lock);
+
+	ret = do_writepages(mapping, wbc);
+
+	/* 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, wait);
+		if (ret == 0)
+			ret = err;
+	}
+
+	if (wait) {
+		int err = filemap_fdatawait(mapping);
+		if (ret == 0)
+			ret = err;
+	}
+
+	spin_lock(&inode_lock);
+	inode->i_state &= ~I_LOCK;
+	if (!(inode->i_state & I_FREEING)) {
+		if (!(inode->i_state & I_DIRTY) &&
+		    mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
+			/*
+			 * We didn't write back all the pages.  nfs_writepages()
+			 * sometimes bales out without doing anything. Redirty
+			 * the inode.  It is still on sb->s_io.
+			 */
+			if (wbc->for_kupdate) {
+				/*
+				 * For the kupdate function we leave the inode
+				 * at the head of sb_dirty so it will get more
+				 * writeout as soon as the queue becomes
+				 * uncongested.
+				 */
+				inode->i_state |= I_DIRTY_PAGES;
+				list_move_tail(&inode->i_list, &sb->s_dirty);
+			} else {
+				/*
+				 * Otherwise fully redirty the inode so that
+				 * other inodes on this superblock will get some
+				 * writeout.  Otherwise heavy writing to one
+				 * file would indefinitely suspend writeout of
+				 * all the other files.
+				 */
+				inode->i_state |= I_DIRTY_PAGES;
+				inode->dirtied_when = jiffies;
+				list_move(&inode->i_list, &sb->s_dirty);
+			}
+		} else if (inode->i_state & I_DIRTY) {
+			/*
+			 * Someone redirtied the inode while were writing back
+			 * the pages.
+			 */
+			list_move(&inode->i_list, &sb->s_dirty);
+		} else if (atomic_read(&inode->i_count)) {
+			/*
+			 * The inode is clean, inuse
+			 */
+			list_move(&inode->i_list, &inode_in_use);
+		} else {
+			/*
+			 * The inode is clean, unused
+			 */
+			list_move(&inode->i_list, &inode_unused);
+			inodes_stat.nr_unused++;
+		}
+	}
+	wake_up_inode(inode);
+	return ret;
+}
+
+/*
+ * Write out an inode's dirty pages.  Called under inode_lock.
+ */
+static int
+__writeback_single_inode(struct inode *inode,
+			struct writeback_control *wbc)
+{
+	wait_queue_head_t *wqh;
+
+	if ((wbc->sync_mode != WB_SYNC_ALL) && (inode->i_state & I_LOCK)) {
+		list_move(&inode->i_list, &inode->i_sb->s_dirty);
+		return 0;
+	}
+
+	/*
+	 * It's a data-integrity sync.  We must wait.
+	 */
+	if (inode->i_state & I_LOCK) {
+		DEFINE_WAIT_BIT(wq, &inode->i_state, __I_LOCK);
+
+		wqh = bit_waitqueue(&inode->i_state, __I_LOCK);
+		do {
+			__iget(inode);
+			spin_unlock(&inode_lock);
+			__wait_on_bit(wqh, &wq, inode_wait,
+							TASK_UNINTERRUPTIBLE);
+			iput(inode);
+			spin_lock(&inode_lock);
+		} while (inode->i_state & I_LOCK);
+	}
+	return __sync_single_inode(inode, wbc);
+}
+
+/*
+ * 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 we're a pdlfush thread, then implement pdflush collision avoidance
+ * against the entire list.
+ *
+ * WB_SYNC_HOLD is a hack for sys_sync(): reattach the inode to sb->s_dirty so
+ * that it can be located for waiting on in __writeback_single_inode().
+ *
+ * Called under inode_lock.
+ *
+ * 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.
+ *
+ * FIXME: this linear search could get expensive with many fileystems.  But
+ * how to fix?  We need to go from an address_space to all inodes which share
+ * a queue with that address_space.  (Easy: have a global "dirty superblocks"
+ * list).
+ *
+ * The inodes to be written are parked on sb->s_io.  They are moved back onto
+ * sb->s_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 __wait_on_inode.
+ */
+static void
+sync_sb_inodes(struct super_block *sb, struct writeback_control *wbc)
+{
+	const unsigned long start = jiffies;	/* livelock avoidance */
+
+	if (!wbc->for_kupdate || list_empty(&sb->s_io))
+		list_splice_init(&sb->s_dirty, &sb->s_io);
+
+	while (!list_empty(&sb->s_io)) {
+		struct inode *inode = list_entry(sb->s_io.prev,
+						struct inode, i_list);
+		struct address_space *mapping = inode->i_mapping;
+		struct backing_dev_info *bdi = mapping->backing_dev_info;
+		long pages_skipped;
+
+		if (!bdi_cap_writeback_dirty(bdi)) {
+			list_move(&inode->i_list, &sb->s_dirty);
+			if (sb == blockdev_superblock) {
+				/*
+				 * Dirty memory-backed blockdev: the ramdisk
+				 * driver does this.  Skip just this inode
+				 */
+				continue;
+			}
+			/*
+			 * Dirty memory-backed inode against a filesystem other
+			 * than the kernel-internal bdev filesystem.  Skip the
+			 * entire superblock.
+			 */
+			break;
+		}
+
+		if (wbc->nonblocking && bdi_write_congested(bdi)) {
+			wbc->encountered_congestion = 1;
+			if (sb != blockdev_superblock)
+				break;		/* Skip a congested fs */
+			list_move(&inode->i_list, &sb->s_dirty);
+			continue;		/* Skip a congested blockdev */
+		}
+
+		if (wbc->bdi && bdi != wbc->bdi) {
+			if (sb != blockdev_superblock)
+				break;		/* fs has the wrong queue */
+			list_move(&inode->i_list, &sb->s_dirty);
+			continue;		/* blockdev has wrong queue */
+		}
+
+		/* Was this inode dirtied after sync_sb_inodes was called? */
+		if (time_after(inode->dirtied_when, start))
+			break;
+
+		/* Was this inode dirtied too recently? */
+		if (wbc->older_than_this && time_after(inode->dirtied_when,
+						*wbc->older_than_this))
+			break;
+
+		/* Is another pdflush already flushing this queue? */
+		if (current_is_pdflush() && !writeback_acquire(bdi))
+			break;
+
+		BUG_ON(inode->i_state & I_FREEING);
+		__iget(inode);
+		pages_skipped = wbc->pages_skipped;
+		__writeback_single_inode(inode, wbc);
+		if (wbc->sync_mode == WB_SYNC_HOLD) {
+			inode->dirtied_when = jiffies;
+			list_move(&inode->i_list, &sb->s_dirty);
+		}
+		if (current_is_pdflush())
+			writeback_release(bdi);
+		if (wbc->pages_skipped != pages_skipped) {
+			/*
+			 * writeback is not making progress due to locked
+			 * buffers.  Skip this inode for now.
+			 */
+			list_move(&inode->i_list, &sb->s_dirty);
+		}
+		spin_unlock(&inode_lock);
+		cond_resched();
+		iput(inode);
+		spin_lock(&inode_lock);
+		if (wbc->nr_to_write <= 0)
+			break;
+	}
+	return;		/* Leave any unwritten inodes on s_io */
+}
+
+/*
+ * Start writeback of dirty pagecache data against all unlocked inodes.
+ *
+ * Note:
+ * We don't need to grab a reference to superblock here. If it has non-empty
+ * ->s_dirty it's hadn't been killed yet and kill_super() won't proceed
+ * past sync_inodes_sb() until both the ->s_dirty and ->s_io lists are
+ * empty. Since __sync_single_inode() regains inode_lock before it finally moves
+ * inode from superblock lists we are OK.
+ *
+ * If `older_than_this' is non-zero then only flush inodes which have a
+ * flushtime older than *older_than_this.
+ *
+ * If `bdi' is non-zero then we will scan the first inode against each
+ * superblock until we find the matching ones.  One group will be the dirty
+ * inodes against a filesystem.  Then when we hit the dummy blockdev superblock,
+ * sync_sb_inodes will seekout the blockdev which matches `bdi'.  Maybe not
+ * super-efficient but we're about to do a ton of I/O...
+ */
+void
+writeback_inodes(struct writeback_control *wbc)
+{
+	struct super_block *sb;
+
+	might_sleep();
+	spin_lock(&sb_lock);
+restart:
+	sb = sb_entry(super_blocks.prev);
+	for (; sb != sb_entry(&super_blocks); sb = sb_entry(sb->s_list.prev)) {
+		if (!list_empty(&sb->s_dirty) || !list_empty(&sb->s_io)) {
+			/* we're making our own get_super here */
+			sb->s_count++;
+			spin_unlock(&sb_lock);
+			/*
+			 * If we can't get the readlock, there's no sense in
+			 * waiting around, most of the time the FS is going to
+			 * be unmounted by the time it is released.
+			 */
+			if (down_read_trylock(&sb->s_umount)) {
+				if (sb->s_root) {
+					spin_lock(&inode_lock);
+					sync_sb_inodes(sb, wbc);
+					spin_unlock(&inode_lock);
+				}
+				up_read(&sb->s_umount);
+			}
+			spin_lock(&sb_lock);
+			if (__put_super_and_need_restart(sb))
+				goto restart;
+		}
+		if (wbc->nr_to_write <= 0)
+			break;
+	}
+	spin_unlock(&sb_lock);
+}
+
+/*
+ * writeback and wait upon the filesystem's dirty inodes.  The caller will
+ * do this in two passes - one to write, and one to wait.  WB_SYNC_HOLD is
+ * used to park the written inodes on sb->s_dirty for the wait pass.
+ *
+ * A finite limit is set on the number of pages which will be written.
+ * To prevent infinite livelock of sys_sync().
+ *
+ * We add in the number of potentially dirty inodes, because each inode write
+ * can dirty pagecache in the underlying blockdev.
+ */
+void sync_inodes_sb(struct super_block *sb, int wait)
+{
+	struct writeback_control wbc = {
+		.sync_mode	= wait ? WB_SYNC_ALL : WB_SYNC_HOLD,
+	};
+	unsigned long nr_dirty = read_page_state(nr_dirty);
+	unsigned long nr_unstable = read_page_state(nr_unstable);
+
+	wbc.nr_to_write = nr_dirty + nr_unstable +
+			(inodes_stat.nr_inodes - inodes_stat.nr_unused) +
+			nr_dirty + nr_unstable;
+	wbc.nr_to_write += wbc.nr_to_write / 2;		/* Bit more for luck */
+	spin_lock(&inode_lock);
+	sync_sb_inodes(sb, &wbc);
+	spin_unlock(&inode_lock);
+}
+
+/*
+ * Rather lame livelock avoidance.
+ */
+static void set_sb_syncing(int val)
+{
+	struct super_block *sb;
+	spin_lock(&sb_lock);
+	sb = sb_entry(super_blocks.prev);
+	for (; sb != sb_entry(&super_blocks); sb = sb_entry(sb->s_list.prev)) {
+		sb->s_syncing = val;
+	}
+	spin_unlock(&sb_lock);
+}
+
+/*
+ * Find a superblock with inodes that need to be synced
+ */
+static struct super_block *get_super_to_sync(void)
+{
+	struct super_block *sb;
+restart:
+	spin_lock(&sb_lock);
+	sb = sb_entry(super_blocks.prev);
+	for (; sb != sb_entry(&super_blocks); sb = sb_entry(sb->s_list.prev)) {
+		if (sb->s_syncing)
+			continue;
+		sb->s_syncing = 1;
+		sb->s_count++;
+		spin_unlock(&sb_lock);
+		down_read(&sb->s_umount);
+		if (!sb->s_root) {
+			drop_super(sb);
+			goto restart;
+		}
+		return sb;
+	}
+	spin_unlock(&sb_lock);
+	return NULL;
+}
+
+/**
+ * sync_inodes
+ *
+ * sync_inodes() goes through each super block's dirty inode list, writes the
+ * inodes out, waits on the writeout and puts the inodes back on the normal
+ * list.
+ *
+ * This is for sys_sync().  fsync_dev() uses the same algorithm.  The subtle
+ * part of the sync functions is that the blockdev "superblock" is processed
+ * last.  This is because the write_inode() function of a typical fs will
+ * perform no I/O, but will mark buffers in the blockdev mapping as dirty.
+ * What we want to do is to perform all that dirtying first, and then write
+ * back all those inode blocks via the blockdev mapping in one sweep.  So the
+ * additional (somewhat redundant) sync_blockdev() calls here are to make
+ * sure that really happens.  Because if we call sync_inodes_sb(wait=1) with
+ * outstanding dirty inodes, the writeback goes block-at-a-time within the
+ * filesystem's write_inode().  This is extremely slow.
+ */
+void sync_inodes(int wait)
+{
+	struct super_block *sb;
+
+	set_sb_syncing(0);
+	while ((sb = get_super_to_sync()) != NULL) {
+		sync_inodes_sb(sb, 0);
+		sync_blockdev(sb->s_bdev);
+		drop_super(sb);
+	}
+	if (wait) {
+		set_sb_syncing(0);
+		while ((sb = get_super_to_sync()) != NULL) {
+			sync_inodes_sb(sb, 1);
+			sync_blockdev(sb->s_bdev);
+			drop_super(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.
+ */
+ 
+int write_inode_now(struct inode *inode, int sync)
+{
+	int ret;
+	struct writeback_control wbc = {
+		.nr_to_write = LONG_MAX,
+		.sync_mode = WB_SYNC_ALL,
+	};
+
+	if (!mapping_cap_writeback_dirty(inode->i_mapping))
+		return 0;
+
+	might_sleep();
+	spin_lock(&inode_lock);
+	ret = __writeback_single_inode(inode, &wbc);
+	spin_unlock(&inode_lock);
+	if (sync)
+		wait_on_inode(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_lock);
+	ret = __writeback_single_inode(inode, wbc);
+	spin_unlock(&inode_lock);
+	return ret;
+}
+EXPORT_SYMBOL(sync_inode);
+
+/**
+ * generic_osync_inode - flush all dirty data for a given inode to disk
+ * @inode: inode to write
+ * @what:  what to write and wait upon
+ *
+ * This can be called by file_write functions for files which have the
+ * O_SYNC flag set, to flush dirty writes to disk.
+ *
+ * @what is a bitmask, specifying which part of the inode's data should be
+ * written and waited upon:
+ *
+ *    OSYNC_DATA:     i_mapping's dirty data
+ *    OSYNC_METADATA: the buffers at i_mapping->private_list
+ *    OSYNC_INODE:    the inode itself
+ */
+
+int generic_osync_inode(struct inode *inode, struct address_space *mapping, int what)
+{
+	int err = 0;
+	int need_write_inode_now = 0;
+	int err2;
+
+	current->flags |= PF_SYNCWRITE;
+	if (what & OSYNC_DATA)
+		err = filemap_fdatawrite(mapping);
+	if (what & (OSYNC_METADATA|OSYNC_DATA)) {
+		err2 = sync_mapping_buffers(mapping);
+		if (!err)
+			err = err2;
+	}
+	if (what & OSYNC_DATA) {
+		err2 = filemap_fdatawait(mapping);
+		if (!err)
+			err = err2;
+	}
+	current->flags &= ~PF_SYNCWRITE;
+
+	spin_lock(&inode_lock);
+	if ((inode->i_state & I_DIRTY) &&
+	    ((what & OSYNC_INODE) || (inode->i_state & I_DIRTY_DATASYNC)))
+		need_write_inode_now = 1;
+	spin_unlock(&inode_lock);
+
+	if (need_write_inode_now) {
+		err2 = write_inode_now(inode, 1);
+		if (!err)
+			err = err2;
+	}
+	else
+		wait_on_inode(inode);
+
+	return err;
+}
+
+EXPORT_SYMBOL(generic_osync_inode);
+
+/**
+ * writeback_acquire: attempt to get exclusive writeback access to a device
+ * @bdi: the device's backing_dev_info structure
+ *
+ * It is a waste of resources to have more than one pdflush thread blocked on
+ * a single request queue.  Exclusion at the request_queue level is obtained
+ * via a flag in the request_queue's backing_dev_info.state.
+ *
+ * Non-request_queue-backed address_spaces will share default_backing_dev_info,
+ * unless they implement their own.  Which is somewhat inefficient, as this
+ * may prevent concurrent writeback against multiple devices.
+ */
+int writeback_acquire(struct backing_dev_info *bdi)
+{
+	return !test_and_set_bit(BDI_pdflush, &bdi->state);
+}
+
+/**
+ * writeback_in_progress: determine whether there is writeback in progress
+ *                        against a backing device.
+ * @bdi: the device's backing_dev_info structure.
+ */
+int writeback_in_progress(struct backing_dev_info *bdi)
+{
+	return test_bit(BDI_pdflush, &bdi->state);
+}
+
+/**
+ * writeback_release: relinquish exclusive writeback access against a device.
+ * @bdi: the device's backing_dev_info structure
+ */
+void writeback_release(struct backing_dev_info *bdi)
+{
+	BUG_ON(!writeback_in_progress(bdi));
+	clear_bit(BDI_pdflush, &bdi->state);
+}