/*
   *  linux/fs/buffer.c
   *
   *  Copyright (C) 1991, 1992, 2002  Linus Torvalds
   */
  
  /*
   * Start bdflush() with kernel_thread not syscall - Paul Gortmaker, 12/95
   *
   * Removed a lot of unnecessary code and simplified things now that
   * the buffer cache isn't our primary cache - Andrew Tridgell 12/96
   *
   * Speed up hash, lru, and free list operations.  Use gfp() for allocating
   * hash table, use SLAB cache for buffer heads. SMP threading.  -DaveM
   *
   * Added 32k buffer block sizes - these are required older ARM systems. - RMK
   *
   * async buffer flushing, 1999 Andrea Arcangeli <andrea@suse.de>
   */

  #include <linux/kernel.h>

  #include <linux/sched/signal.h>

  #include <linux/syscalls.h>
  #include <linux/fs.h>

  #include <linux/iomap.h>

  #include <linux/mm.h>
  #include <linux/percpu.h>
  #include <linux/slab.h>

  #include <linux/capability.h>

  #include <linux/blkdev.h>
  #include <linux/file.h>
  #include <linux/quotaops.h>
  #include <linux/highmem.h>

  #include <linux/export.h>

  #include <linux/backing-dev.h>

  #include <linux/writeback.h>
  #include <linux/hash.h>
  #include <linux/suspend.h>
  #include <linux/buffer_head.h>

  #include <linux/task_io_accounting_ops.h>

  #include <linux/bio.h>
  #include <linux/notifier.h>
  #include <linux/cpu.h>
  #include <linux/bitops.h>
  #include <linux/mpage.h>

  #include <linux/bit_spinlock.h>

  #include <linux/pagevec.h>

  #include <trace/events/block.h>

  
  static int fsync_buffers_list(spinlock_t *lock, struct list_head *list);

  static int submit_bh_wbc(int op, int op_flags, struct buffer_head *bh,

  			 enum rw_hint hint, struct writeback_control *wbc);

  
  #define BH_ENTRY(list) list_entry((list), struct buffer_head, b_assoc_buffers)

  void init_buffer(struct buffer_head *bh, bh_end_io_t *handler, void *private)

  {
  	bh->b_end_io = handler;
  	bh->b_private = private;
  }

  EXPORT_SYMBOL(init_buffer);

  inline void touch_buffer(struct buffer_head *bh)
  {

  	trace_block_touch_buffer(bh);

  	mark_page_accessed(bh->b_page);
  }
  EXPORT_SYMBOL(touch_buffer);

  void __lock_buffer(struct buffer_head *bh)

  {

  	wait_on_bit_lock_io(&bh->b_state, BH_Lock, TASK_UNINTERRUPTIBLE);

  }
  EXPORT_SYMBOL(__lock_buffer);

  void unlock_buffer(struct buffer_head *bh)

  {

  	clear_bit_unlock(BH_Lock, &bh->b_state);

  	smp_mb__after_atomic();

  	wake_up_bit(&bh->b_state, BH_Lock);
  }

  EXPORT_SYMBOL(unlock_buffer);

  
  /*

   * Returns if the page has dirty or writeback buffers. If all the buffers
   * are unlocked and clean then the PageDirty information is stale. If
   * any of the pages are locked, it is assumed they are locked for IO.
   */
  void buffer_check_dirty_writeback(struct page *page,
  				     bool *dirty, bool *writeback)
  {
  	struct buffer_head *head, *bh;
  	*dirty = false;
  	*writeback = false;
  
  	BUG_ON(!PageLocked(page));
  
  	if (!page_has_buffers(page))
  		return;
  
  	if (PageWriteback(page))
  		*writeback = true;
  
  	head = page_buffers(page);
  	bh = head;
  	do {
  		if (buffer_locked(bh))
  			*writeback = true;
  
  		if (buffer_dirty(bh))
  			*dirty = true;
  
  		bh = bh->b_this_page;
  	} while (bh != head);
  }
  EXPORT_SYMBOL(buffer_check_dirty_writeback);
  
  /*

   * Block until a buffer comes unlocked.  This doesn't stop it
   * from becoming locked again - you have to lock it yourself
   * if you want to preserve its state.
   */
  void __wait_on_buffer(struct buffer_head * bh)
  {

  	wait_on_bit_io(&bh->b_state, BH_Lock, TASK_UNINTERRUPTIBLE);

  }

  EXPORT_SYMBOL(__wait_on_buffer);

  
  static void
  __clear_page_buffers(struct page *page)
  {
  	ClearPagePrivate(page);

  	set_page_private(page, 0);

  	put_page(page);

  }

  static void buffer_io_error(struct buffer_head *bh, char *msg)

  {

  	if (!test_bit(BH_Quiet, &bh->b_state))
  		printk_ratelimited(KERN_ERR

  			"Buffer I/O error on dev %pg, logical block %llu%s
  ",
  			bh->b_bdev, (unsigned long long)bh->b_blocknr, msg);

  }
  
  /*

   * End-of-IO handler helper function which does not touch the bh after
   * unlocking it.
   * Note: unlock_buffer() sort-of does touch the bh after unlocking it, but
   * a race there is benign: unlock_buffer() only use the bh's address for
   * hashing after unlocking the buffer, so it doesn't actually touch the bh
   * itself.

   */

  static void __end_buffer_read_notouch(struct buffer_head *bh, int uptodate)

  {
  	if (uptodate) {
  		set_buffer_uptodate(bh);
  	} else {

  		/* This happens, due to failed read-ahead attempts. */

  		clear_buffer_uptodate(bh);
  	}
  	unlock_buffer(bh);

  }
  
  /*
   * Default synchronous end-of-IO handler..  Just mark it up-to-date and
   * unlock the buffer. This is what ll_rw_block uses too.
   */
  void end_buffer_read_sync(struct buffer_head *bh, int uptodate)
  {
  	__end_buffer_read_notouch(bh, uptodate);

  	put_bh(bh);
  }

  EXPORT_SYMBOL(end_buffer_read_sync);

  
  void end_buffer_write_sync(struct buffer_head *bh, int uptodate)
  {

  	if (uptodate) {
  		set_buffer_uptodate(bh);
  	} else {

  		buffer_io_error(bh, ", lost sync page write");

  		mark_buffer_write_io_error(bh);

  		clear_buffer_uptodate(bh);
  	}
  	unlock_buffer(bh);
  	put_bh(bh);
  }

  EXPORT_SYMBOL(end_buffer_write_sync);

  
  /*

   * Various filesystems appear to want __find_get_block to be non-blocking.
   * But it's the page lock which protects the buffers.  To get around this,
   * we get exclusion from try_to_free_buffers with the blockdev mapping's
   * private_lock.
   *
   * Hack idea: for the blockdev mapping, i_bufferlist_lock contention
   * may be quite high.  This code could TryLock the page, and if that
   * succeeds, there is no need to take private_lock. (But if
   * private_lock is contended then so is mapping->tree_lock).
   */
  static struct buffer_head *

  __find_get_block_slow(struct block_device *bdev, sector_t block)

  {
  	struct inode *bd_inode = bdev->bd_inode;
  	struct address_space *bd_mapping = bd_inode->i_mapping;
  	struct buffer_head *ret = NULL;
  	pgoff_t index;
  	struct buffer_head *bh;
  	struct buffer_head *head;
  	struct page *page;
  	int all_mapped = 1;

  	index = block >> (PAGE_SHIFT - bd_inode->i_blkbits);

  	page = find_get_page_flags(bd_mapping, index, FGP_ACCESSED);

  	if (!page)
  		goto out;
  
  	spin_lock(&bd_mapping->private_lock);
  	if (!page_has_buffers(page))
  		goto out_unlock;
  	head = page_buffers(page);
  	bh = head;
  	do {

  		if (!buffer_mapped(bh))
  			all_mapped = 0;
  		else if (bh->b_blocknr == block) {

  			ret = bh;
  			get_bh(bh);
  			goto out_unlock;
  		}

  		bh = bh->b_this_page;
  	} while (bh != head);
  
  	/* we might be here because some of the buffers on this page are
  	 * not mapped.  This is due to various races between
  	 * file io on the block device and getblk.  It gets dealt with
  	 * elsewhere, don't buffer_error if we had some unmapped buffers
  	 */
  	if (all_mapped) {
  		printk("__find_get_block_slow() failed. "
  			"block=%llu, b_blocknr=%llu
  ",

  			(unsigned long long)block,
  			(unsigned long long)bh->b_blocknr);
  		printk("b_state=0x%08lx, b_size=%zu
  ",
  			bh->b_state, bh->b_size);

  		printk("device %pg blocksize: %d
  ", bdev,

  			1 << bd_inode->i_blkbits);

  	}
  out_unlock:
  	spin_unlock(&bd_mapping->private_lock);

  	put_page(page);

  out:
  	return ret;
  }

  /*

   * Kick the writeback threads then try to free up some ZONE_NORMAL memory.

   */
  static void free_more_memory(void)
  {

  	struct zoneref *z;

  	int nid;

  	wakeup_flusher_threads(1024, WB_REASON_FREE_MORE_MEM);

  	yield();

  	for_each_online_node(nid) {

  
  		z = first_zones_zonelist(node_zonelist(nid, GFP_NOFS),
  						gfp_zone(GFP_NOFS), NULL);
  		if (z->zone)

  			try_to_free_pages(node_zonelist(nid, GFP_NOFS), 0,

  						GFP_NOFS, NULL);

  	}
  }
  
  /*
   * I/O completion handler for block_read_full_page() - pages
   * which come unlocked at the end of I/O.
   */
  static void end_buffer_async_read(struct buffer_head *bh, int uptodate)
  {

  	unsigned long flags;

  	struct buffer_head *first;

  	struct buffer_head *tmp;
  	struct page *page;
  	int page_uptodate = 1;
  
  	BUG_ON(!buffer_async_read(bh));
  
  	page = bh->b_page;
  	if (uptodate) {
  		set_buffer_uptodate(bh);
  	} else {
  		clear_buffer_uptodate(bh);

  		buffer_io_error(bh, ", async page read");

  		SetPageError(page);
  	}
  
  	/*
  	 * Be _very_ careful from here on. Bad things can happen if
  	 * two buffer heads end IO at almost the same time and both
  	 * decide that the page is now completely done.
  	 */

  	first = page_buffers(page);
  	local_irq_save(flags);
  	bit_spin_lock(BH_Uptodate_Lock, &first->b_state);

  	clear_buffer_async_read(bh);
  	unlock_buffer(bh);
  	tmp = bh;
  	do {
  		if (!buffer_uptodate(tmp))
  			page_uptodate = 0;
  		if (buffer_async_read(tmp)) {
  			BUG_ON(!buffer_locked(tmp));
  			goto still_busy;
  		}
  		tmp = tmp->b_this_page;
  	} while (tmp != bh);

  	bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
  	local_irq_restore(flags);

  
  	/*
  	 * If none of the buffers had errors and they are all
  	 * uptodate then we can set the page uptodate.
  	 */
  	if (page_uptodate && !PageError(page))
  		SetPageUptodate(page);
  	unlock_page(page);
  	return;
  
  still_busy:

  	bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
  	local_irq_restore(flags);

  	return;
  }
  
  /*
   * Completion handler for block_write_full_page() - pages which are unlocked
   * during I/O, and which have PageWriteback cleared upon I/O completion.
   */

  void end_buffer_async_write(struct buffer_head *bh, int uptodate)

  {

  	unsigned long flags;

  	struct buffer_head *first;

  	struct buffer_head *tmp;
  	struct page *page;
  
  	BUG_ON(!buffer_async_write(bh));
  
  	page = bh->b_page;
  	if (uptodate) {
  		set_buffer_uptodate(bh);
  	} else {

  		buffer_io_error(bh, ", lost async page write");

  		mark_buffer_write_io_error(bh);

  		clear_buffer_uptodate(bh);
  		SetPageError(page);
  	}

  	first = page_buffers(page);
  	local_irq_save(flags);
  	bit_spin_lock(BH_Uptodate_Lock, &first->b_state);

  	clear_buffer_async_write(bh);
  	unlock_buffer(bh);
  	tmp = bh->b_this_page;
  	while (tmp != bh) {
  		if (buffer_async_write(tmp)) {
  			BUG_ON(!buffer_locked(tmp));
  			goto still_busy;
  		}
  		tmp = tmp->b_this_page;
  	}

  	bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
  	local_irq_restore(flags);

  	end_page_writeback(page);
  	return;
  
  still_busy:

  	bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
  	local_irq_restore(flags);

  	return;
  }

  EXPORT_SYMBOL(end_buffer_async_write);

  
  /*
   * If a page's buffers are under async readin (end_buffer_async_read
   * completion) then there is a possibility that another thread of
   * control could lock one of the buffers after it has completed
   * but while some of the other buffers have not completed.  This
   * locked buffer would confuse end_buffer_async_read() into not unlocking
   * the page.  So the absence of BH_Async_Read tells end_buffer_async_read()
   * that this buffer is not under async I/O.
   *
   * The page comes unlocked when it has no locked buffer_async buffers
   * left.
   *
   * PageLocked prevents anyone starting new async I/O reads any of
   * the buffers.
   *
   * PageWriteback is used to prevent simultaneous writeout of the same
   * page.
   *
   * PageLocked prevents anyone from starting writeback of a page which is
   * under read I/O (PageWriteback is only ever set against a locked page).
   */
  static void mark_buffer_async_read(struct buffer_head *bh)
  {
  	bh->b_end_io = end_buffer_async_read;
  	set_buffer_async_read(bh);
  }

  static void mark_buffer_async_write_endio(struct buffer_head *bh,
  					  bh_end_io_t *handler)

  {

  	bh->b_end_io = handler;

  	set_buffer_async_write(bh);
  }

  
  void mark_buffer_async_write(struct buffer_head *bh)
  {
  	mark_buffer_async_write_endio(bh, end_buffer_async_write);
  }

  EXPORT_SYMBOL(mark_buffer_async_write);
  
  
  /*
   * fs/buffer.c contains helper functions for buffer-backed address space's
   * fsync functions.  A common requirement for buffer-based filesystems is
   * that certain data from the backing blockdev needs to be written out for
   * a successful fsync().  For example, ext2 indirect blocks need to be
   * written back and waited upon before fsync() returns.
   *
   * The functions mark_buffer_inode_dirty(), fsync_inode_buffers(),
   * inode_has_buffers() and invalidate_inode_buffers() are provided for the
   * management of a list of dependent buffers at ->i_mapping->private_list.
   *
   * Locking is a little subtle: try_to_free_buffers() will remove buffers
   * from their controlling inode's queue when they are being freed.  But
   * try_to_free_buffers() will be operating against the *blockdev* mapping
   * at the time, not against the S_ISREG file which depends on those buffers.
   * So the locking for private_list is via the private_lock in the address_space
   * which backs the buffers.  Which is different from the address_space 
   * against which the buffers are listed.  So for a particular address_space,
   * mapping->private_lock does *not* protect mapping->private_list!  In fact,
   * mapping->private_list will always be protected by the backing blockdev's
   * ->private_lock.
   *
   * Which introduces a requirement: all buffers on an address_space's
   * ->private_list must be from the same address_space: the blockdev's.
   *
   * address_spaces which do not place buffers at ->private_list via these
   * utility functions are free to use private_lock and private_list for
   * whatever they want.  The only requirement is that list_empty(private_list)
   * be true at clear_inode() time.
   *
   * FIXME: clear_inode should not call invalidate_inode_buffers().  The
   * filesystems should do that.  invalidate_inode_buffers() should just go
   * BUG_ON(!list_empty).
   *
   * FIXME: mark_buffer_dirty_inode() is a data-plane operation.  It should
   * take an address_space, not an inode.  And it should be called
   * mark_buffer_dirty_fsync() to clearly define why those buffers are being
   * queued up.
   *
   * FIXME: mark_buffer_dirty_inode() doesn't need to add the buffer to the
   * list if it is already on a list.  Because if the buffer is on a list,
   * it *must* already be on the right one.  If not, the filesystem is being
   * silly.  This will save a ton of locking.  But first we have to ensure
   * that buffers are taken *off* the old inode's list when they are freed
   * (presumably in truncate).  That requires careful auditing of all
   * filesystems (do it inside bforget()).  It could also be done by bringing
   * b_inode back.
   */
  
  /*
   * The buffer's backing address_space's private_lock must be held
   */

  static void __remove_assoc_queue(struct buffer_head *bh)

  {
  	list_del_init(&bh->b_assoc_buffers);

  	WARN_ON(!bh->b_assoc_map);

  	bh->b_assoc_map = NULL;

  }
  
  int inode_has_buffers(struct inode *inode)
  {
  	return !list_empty(&inode->i_data.private_list);
  }
  
  /*
   * osync is designed to support O_SYNC io.  It waits synchronously for
   * all already-submitted IO to complete, but does not queue any new
   * writes to the disk.
   *
   * To do O_SYNC writes, just queue the buffer writes with ll_rw_block as
   * you dirty the buffers, and then use osync_inode_buffers to wait for
   * completion.  Any other dirty buffers which are not yet queued for
   * write will not be flushed to disk by the osync.
   */
  static int osync_buffers_list(spinlock_t *lock, struct list_head *list)
  {
  	struct buffer_head *bh;
  	struct list_head *p;
  	int err = 0;
  
  	spin_lock(lock);
  repeat:
  	list_for_each_prev(p, list) {
  		bh = BH_ENTRY(p);
  		if (buffer_locked(bh)) {
  			get_bh(bh);
  			spin_unlock(lock);
  			wait_on_buffer(bh);
  			if (!buffer_uptodate(bh))
  				err = -EIO;
  			brelse(bh);
  			spin_lock(lock);
  			goto repeat;
  		}
  	}
  	spin_unlock(lock);
  	return err;
  }

  static void do_thaw_one(struct super_block *sb, void *unused)

  {

  	while (sb->s_bdev && !thaw_bdev(sb->s_bdev, sb))

  		printk(KERN_WARNING "Emergency Thaw on %pg
  ", sb->s_bdev);

  }

  static void do_thaw_all(struct work_struct *work)
  {
  	iterate_supers(do_thaw_one, NULL);

  	kfree(work);

  	printk(KERN_WARNING "Emergency Thaw complete
  ");
  }
  
  /**
   * emergency_thaw_all -- forcibly thaw every frozen filesystem
   *
   * Used for emergency unfreeze of all filesystems via SysRq
   */
  void emergency_thaw_all(void)
  {

  	struct work_struct *work;
  
  	work = kmalloc(sizeof(*work), GFP_ATOMIC);
  	if (work) {
  		INIT_WORK(work, do_thaw_all);
  		schedule_work(work);
  	}

  }

  /**

   * sync_mapping_buffers - write out & wait upon a mapping's "associated" buffers

   * @mapping: the mapping which wants those buffers written

   *
   * Starts I/O against the buffers at mapping->private_list, and waits upon
   * that I/O.
   *

   * Basically, this is a convenience function for fsync().
   * @mapping is a file or directory which needs those buffers to be written for
   * a successful fsync().

   */
  int sync_mapping_buffers(struct address_space *mapping)
  {

  	struct address_space *buffer_mapping = mapping->private_data;

  
  	if (buffer_mapping == NULL || list_empty(&mapping->private_list))
  		return 0;
  
  	return fsync_buffers_list(&buffer_mapping->private_lock,
  					&mapping->private_list);
  }
  EXPORT_SYMBOL(sync_mapping_buffers);
  
  /*
   * Called when we've recently written block `bblock', and it is known that
   * `bblock' was for a buffer_boundary() buffer.  This means that the block at
   * `bblock + 1' is probably a dirty indirect block.  Hunt it down and, if it's
   * dirty, schedule it for IO.  So that indirects merge nicely with their data.
   */
  void write_boundary_block(struct block_device *bdev,
  			sector_t bblock, unsigned blocksize)
  {
  	struct buffer_head *bh = __find_get_block(bdev, bblock + 1, blocksize);
  	if (bh) {
  		if (buffer_dirty(bh))

  			ll_rw_block(REQ_OP_WRITE, 0, 1, &bh);

  		put_bh(bh);
  	}
  }
  
  void mark_buffer_dirty_inode(struct buffer_head *bh, struct inode *inode)
  {
  	struct address_space *mapping = inode->i_mapping;
  	struct address_space *buffer_mapping = bh->b_page->mapping;
  
  	mark_buffer_dirty(bh);

  	if (!mapping->private_data) {
  		mapping->private_data = buffer_mapping;

  	} else {

  		BUG_ON(mapping->private_data != buffer_mapping);

  	}

  	if (!bh->b_assoc_map) {

  		spin_lock(&buffer_mapping->private_lock);
  		list_move_tail(&bh->b_assoc_buffers,
  				&mapping->private_list);

  		bh->b_assoc_map = mapping;

  		spin_unlock(&buffer_mapping->private_lock);
  	}
  }
  EXPORT_SYMBOL(mark_buffer_dirty_inode);
  
  /*

   * Mark the page dirty, and set it dirty in the radix tree, and mark the inode
   * dirty.
   *
   * If warn is true, then emit a warning if the page is not uptodate and has
   * not been truncated.

   *

   * The caller must hold lock_page_memcg().

   */

  static void __set_page_dirty(struct page *page, struct address_space *mapping,

  			     int warn)

  {

  	unsigned long flags;
  
  	spin_lock_irqsave(&mapping->tree_lock, flags);

  	if (page->mapping) {	/* Race with truncate? */
  		WARN_ON_ONCE(warn && !PageUptodate(page));

  		account_page_dirtied(page, mapping);

  		radix_tree_tag_set(&mapping->page_tree,
  				page_index(page), PAGECACHE_TAG_DIRTY);
  	}

  	spin_unlock_irqrestore(&mapping->tree_lock, flags);

  }
  
  /*

   * Add a page to the dirty page list.
   *
   * It is a sad fact of life that this function is called from several places
   * deeply under spinlocking.  It may not sleep.
   *
   * If the page has buffers, the uptodate buffers are set dirty, to preserve
   * dirty-state coherency between the page and the buffers.  It the page does
   * not have buffers then when they are later attached they will all be set
   * dirty.
   *
   * The buffers are dirtied before the page is dirtied.  There's a small race
   * window in which a writepage caller may see the page cleanness but not the
   * buffer dirtiness.  That's fine.  If this code were to set the page dirty
   * before the buffers, a concurrent writepage caller could clear the page dirty
   * bit, see a bunch of clean buffers and we'd end up with dirty buffers/clean
   * page on the dirty page list.
   *
   * We use private_lock to lock against try_to_free_buffers while using the
   * page's buffer list.  Also use this to protect against clean buffers being
   * added to the page after it was set dirty.
   *
   * FIXME: may need to call ->reservepage here as well.  That's rather up to the
   * address_space though.
   */
  int __set_page_dirty_buffers(struct page *page)
  {

  	int newly_dirty;

  	struct address_space *mapping = page_mapping(page);

  
  	if (unlikely(!mapping))
  		return !TestSetPageDirty(page);

  
  	spin_lock(&mapping->private_lock);
  	if (page_has_buffers(page)) {
  		struct buffer_head *head = page_buffers(page);
  		struct buffer_head *bh = head;
  
  		do {
  			set_buffer_dirty(bh);
  			bh = bh->b_this_page;
  		} while (bh != head);
  	}

  	/*

  	 * Lock out page->mem_cgroup migration to keep PageDirty
  	 * synchronized with per-memcg dirty page counters.

  	 */

  	lock_page_memcg(page);

  	newly_dirty = !TestSetPageDirty(page);

  	spin_unlock(&mapping->private_lock);

  	if (newly_dirty)

  		__set_page_dirty(page, mapping, 1);

  	unlock_page_memcg(page);

  
  	if (newly_dirty)
  		__mark_inode_dirty(mapping->host, I_DIRTY_PAGES);

  	return newly_dirty;

  }
  EXPORT_SYMBOL(__set_page_dirty_buffers);
  
  /*
   * Write out and wait upon a list of buffers.
   *
   * We have conflicting pressures: we want to make sure that all
   * initially dirty buffers get waited on, but that any subsequently
   * dirtied buffers don't.  After all, we don't want fsync to last
   * forever if somebody is actively writing to the file.
   *
   * Do this in two main stages: first we copy dirty buffers to a
   * temporary inode list, queueing the writes as we go.  Then we clean
   * up, waiting for those writes to complete.
   * 
   * During this second stage, any subsequent updates to the file may end
   * up refiling the buffer on the original inode's dirty list again, so
   * there is a chance we will end up with a buffer queued for write but
   * not yet completed on that list.  So, as a final cleanup we go through
   * the osync code to catch these locked, dirty buffers without requeuing
   * any newly dirty buffers for write.
   */
  static int fsync_buffers_list(spinlock_t *lock, struct list_head *list)
  {
  	struct buffer_head *bh;
  	struct list_head tmp;

  	struct address_space *mapping;

  	int err = 0, err2;

  	struct blk_plug plug;

  
  	INIT_LIST_HEAD(&tmp);

  	blk_start_plug(&plug);

  
  	spin_lock(lock);
  	while (!list_empty(list)) {
  		bh = BH_ENTRY(list->next);

  		mapping = bh->b_assoc_map;

  		__remove_assoc_queue(bh);

  		/* Avoid race with mark_buffer_dirty_inode() which does
  		 * a lockless check and we rely on seeing the dirty bit */
  		smp_mb();

  		if (buffer_dirty(bh) || buffer_locked(bh)) {
  			list_add(&bh->b_assoc_buffers, &tmp);

  			bh->b_assoc_map = mapping;

  			if (buffer_dirty(bh)) {
  				get_bh(bh);
  				spin_unlock(lock);
  				/*
  				 * Ensure any pending I/O completes so that

  				 * write_dirty_buffer() actually writes the
  				 * current contents - it is a noop if I/O is
  				 * still in flight on potentially older
  				 * contents.

  				 */

  				write_dirty_buffer(bh, REQ_SYNC);

  
  				/*
  				 * Kick off IO for the previous mapping. Note
  				 * that we will not run the very last mapping,
  				 * wait_on_buffer() will do that for us
  				 * through sync_buffer().
  				 */

  				brelse(bh);
  				spin_lock(lock);
  			}
  		}
  	}

  	spin_unlock(lock);
  	blk_finish_plug(&plug);
  	spin_lock(lock);

  	while (!list_empty(&tmp)) {
  		bh = BH_ENTRY(tmp.prev);

  		get_bh(bh);

  		mapping = bh->b_assoc_map;
  		__remove_assoc_queue(bh);
  		/* Avoid race with mark_buffer_dirty_inode() which does
  		 * a lockless check and we rely on seeing the dirty bit */
  		smp_mb();
  		if (buffer_dirty(bh)) {
  			list_add(&bh->b_assoc_buffers,

  				 &mapping->private_list);

  			bh->b_assoc_map = mapping;
  		}

  		spin_unlock(lock);
  		wait_on_buffer(bh);
  		if (!buffer_uptodate(bh))
  			err = -EIO;
  		brelse(bh);
  		spin_lock(lock);
  	}
  	
  	spin_unlock(lock);
  	err2 = osync_buffers_list(lock, list);
  	if (err)
  		return err;
  	else
  		return err2;
  }
  
  /*
   * Invalidate any and all dirty buffers on a given inode.  We are
   * probably unmounting the fs, but that doesn't mean we have already
   * done a sync().  Just drop the buffers from the inode list.
   *
   * NOTE: we take the inode's blockdev's mapping's private_lock.  Which
   * assumes that all the buffers are against the blockdev.  Not true
   * for reiserfs.
   */
  void invalidate_inode_buffers(struct inode *inode)
  {
  	if (inode_has_buffers(inode)) {
  		struct address_space *mapping = &inode->i_data;
  		struct list_head *list = &mapping->private_list;

  		struct address_space *buffer_mapping = mapping->private_data;

  
  		spin_lock(&buffer_mapping->private_lock);
  		while (!list_empty(list))
  			__remove_assoc_queue(BH_ENTRY(list->next));
  		spin_unlock(&buffer_mapping->private_lock);
  	}
  }

  EXPORT_SYMBOL(invalidate_inode_buffers);

  
  /*
   * Remove any clean buffers from the inode's buffer list.  This is called
   * when we're trying to free the inode itself.  Those buffers can pin it.
   *
   * Returns true if all buffers were removed.
   */
  int remove_inode_buffers(struct inode *inode)
  {
  	int ret = 1;
  
  	if (inode_has_buffers(inode)) {
  		struct address_space *mapping = &inode->i_data;
  		struct list_head *list = &mapping->private_list;

  		struct address_space *buffer_mapping = mapping->private_data;

  
  		spin_lock(&buffer_mapping->private_lock);
  		while (!list_empty(list)) {
  			struct buffer_head *bh = BH_ENTRY(list->next);
  			if (buffer_dirty(bh)) {
  				ret = 0;
  				break;
  			}
  			__remove_assoc_queue(bh);
  		}
  		spin_unlock(&buffer_mapping->private_lock);
  	}
  	return ret;
  }
  
  /*
   * Create the appropriate buffers when given a page for data area and
   * the size of each buffer.. Use the bh->b_this_page linked list to
   * follow the buffers created.  Return NULL if unable to create more
   * buffers.
   *
   * The retry flag is used to differentiate async IO (paging, swapping)
   * which may not fail from ordinary buffer allocations.
   */
  struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
  		int retry)
  {
  	struct buffer_head *bh, *head;
  	long offset;
  
  try_again:
  	head = NULL;
  	offset = PAGE_SIZE;
  	while ((offset -= size) >= 0) {
  		bh = alloc_buffer_head(GFP_NOFS);
  		if (!bh)
  			goto no_grow;

  		bh->b_this_page = head;
  		bh->b_blocknr = -1;
  		head = bh;

  		bh->b_size = size;
  
  		/* Link the buffer to its page */
  		set_bh_page(bh, page, offset);

  	}
  	return head;
  /*
   * In case anything failed, we just free everything we got.
   */
  no_grow:
  	if (head) {
  		do {
  			bh = head;
  			head = head->b_this_page;
  			free_buffer_head(bh);
  		} while (head);
  	}
  
  	/*
  	 * Return failure for non-async IO requests.  Async IO requests
  	 * are not allowed to fail, so we have to wait until buffer heads
  	 * become available.  But we don't want tasks sleeping with 
  	 * partially complete buffers, so all were released above.
  	 */
  	if (!retry)
  		return NULL;
  
  	/* We're _really_ low on memory. Now we just
  	 * wait for old buffer heads to become free due to
  	 * finishing IO.  Since this is an async request and
  	 * the reserve list is empty, we're sure there are 
  	 * async buffer heads in use.
  	 */
  	free_more_memory();
  	goto try_again;
  }
  EXPORT_SYMBOL_GPL(alloc_page_buffers);
  
  static inline void
  link_dev_buffers(struct page *page, struct buffer_head *head)
  {
  	struct buffer_head *bh, *tail;
  
  	bh = head;
  	do {
  		tail = bh;
  		bh = bh->b_this_page;
  	} while (bh);
  	tail->b_this_page = head;
  	attach_page_buffers(page, head);
  }

  static sector_t blkdev_max_block(struct block_device *bdev, unsigned int size)
  {
  	sector_t retval = ~((sector_t)0);
  	loff_t sz = i_size_read(bdev->bd_inode);
  
  	if (sz) {
  		unsigned int sizebits = blksize_bits(size);
  		retval = (sz >> sizebits);
  	}
  	return retval;
  }

  /*
   * Initialise the state of a blockdev page's buffers.
   */ 

  static sector_t

  init_page_buffers(struct page *page, struct block_device *bdev,
  			sector_t block, int size)
  {
  	struct buffer_head *head = page_buffers(page);
  	struct buffer_head *bh = head;
  	int uptodate = PageUptodate(page);

  	sector_t end_block = blkdev_max_block(I_BDEV(bdev->bd_inode), size);

  
  	do {
  		if (!buffer_mapped(bh)) {
  			init_buffer(bh, NULL, NULL);
  			bh->b_bdev = bdev;
  			bh->b_blocknr = block;
  			if (uptodate)
  				set_buffer_uptodate(bh);

  			if (block < end_block)
  				set_buffer_mapped(bh);

  		}
  		block++;
  		bh = bh->b_this_page;
  	} while (bh != head);

  
  	/*
  	 * Caller needs to validate requested block against end of device.
  	 */
  	return end_block;

  }
  
  /*
   * Create the page-cache page that contains the requested block.
   *

   * This is used purely for blockdev mappings.

   */

  static int

  grow_dev_page(struct block_device *bdev, sector_t block,

  	      pgoff_t index, int size, int sizebits, gfp_t gfp)

  {
  	struct inode *inode = bdev->bd_inode;
  	struct page *page;
  	struct buffer_head *bh;

  	sector_t end_block;
  	int ret = 0;		/* Will call free_more_memory() */

  	gfp_t gfp_mask;

  	gfp_mask = mapping_gfp_constraint(inode->i_mapping, ~__GFP_FS) | gfp;

  	/*
  	 * XXX: __getblk_slow() can not really deal with failure and
  	 * will endlessly loop on improvised global reclaim.  Prefer
  	 * looping in the allocator rather than here, at least that
  	 * code knows what it's doing.
  	 */
  	gfp_mask |= __GFP_NOFAIL;
  
  	page = find_or_create_page(inode->i_mapping, index, gfp_mask);

  	if (!page)

  		return ret;

  	BUG_ON(!PageLocked(page));

  
  	if (page_has_buffers(page)) {
  		bh = page_buffers(page);
  		if (bh->b_size == size) {

  			end_block = init_page_buffers(page, bdev,

  						(sector_t)index << sizebits,
  						size);

  			goto done;

  		}
  		if (!try_to_free_buffers(page))
  			goto failed;
  	}
  
  	/*
  	 * Allocate some buffers for this page
  	 */
  	bh = alloc_page_buffers(page, size, 0);
  	if (!bh)
  		goto failed;
  
  	/*
  	 * Link the page to the buffers and initialise them.  Take the
  	 * lock to be atomic wrt __find_get_block(), which does not
  	 * run under the page lock.
  	 */
  	spin_lock(&inode->i_mapping->private_lock);
  	link_dev_buffers(page, bh);

  	end_block = init_page_buffers(page, bdev, (sector_t)index << sizebits,
  			size);

  	spin_unlock(&inode->i_mapping->private_lock);

  done:
  	ret = (block < end_block) ? 1 : -ENXIO;

  failed:

  	unlock_page(page);

  	put_page(page);

  	return ret;

  }
  
  /*
   * Create buffers for the specified block device block's page.  If
   * that page was dirty, the buffers are set dirty also.

   */

  static int

  grow_buffers(struct block_device *bdev, sector_t block, int size, gfp_t gfp)

  {

  	pgoff_t index;
  	int sizebits;
  
  	sizebits = -1;
  	do {
  		sizebits++;
  	} while ((size << sizebits) < PAGE_SIZE);
  
  	index = block >> sizebits;

  	/*
  	 * Check for a block which wants to lie outside our maximum possible
  	 * pagecache index.  (this comparison is done using sector_t types).
  	 */
  	if (unlikely(index != block >> sizebits)) {

  		printk(KERN_ERR "%s: requested out-of-range block %llu for "

  			"device %pg
  ",

  			__func__, (unsigned long long)block,

  			bdev);

  		return -EIO;
  	}

  	/* Create a page with the proper size buffers.. */

  	return grow_dev_page(bdev, block, index, size, sizebits, gfp);

  }

  static struct buffer_head *

  __getblk_slow(struct block_device *bdev, sector_t block,
  	     unsigned size, gfp_t gfp)

  {
  	/* Size must be multiple of hard sectorsize */

  	if (unlikely(size & (bdev_logical_block_size(bdev)-1) ||

  			(size < 512 || size > PAGE_SIZE))) {
  		printk(KERN_ERR "getblk(): invalid block size %d requested
  ",
  					size);

  		printk(KERN_ERR "logical block size: %d
  ",
  					bdev_logical_block_size(bdev));

  
  		dump_stack();
  		return NULL;
  	}

  	for (;;) {
  		struct buffer_head *bh;
  		int ret;

  
  		bh = __find_get_block(bdev, block, size);
  		if (bh)
  			return bh;

  		ret = grow_buffers(bdev, block, size, gfp);

  		if (ret < 0)
  			return NULL;
  		if (ret == 0)
  			free_more_memory();

  	}
  }
  
  /*
   * The relationship between dirty buffers and dirty pages:
   *
   * Whenever a page has any dirty buffers, the page's dirty bit is set, and
   * the page is tagged dirty in its radix tree.
   *
   * At all times, the dirtiness of the buffers represents the dirtiness of
   * subsections of the page.  If the page has buffers, the page dirty bit is
   * merely a hint about the true dirty state.
   *
   * When a page is set dirty in its entirety, all its buffers are marked dirty
   * (if the page has buffers).
   *
   * When a buffer is marked dirty, its page is dirtied, but the page's other
   * buffers are not.
   *
   * Also.  When blockdev buffers are explicitly read with bread(), they
   * individually become uptodate.  But their backing page remains not
   * uptodate - even if all of its buffers are uptodate.  A subsequent
   * block_read_full_page() against that page will discover all the uptodate
   * buffers, will set the page uptodate and will perform no I/O.
   */
  
  /**
   * mark_buffer_dirty - mark a buffer_head as needing writeout

   * @bh: the buffer_head to mark dirty

   *
   * mark_buffer_dirty() will set the dirty bit against the buffer, then set its
   * backing page dirty, then tag the page as dirty in its address_space's radix
   * tree and then attach the address_space's inode to its superblock's dirty
   * inode list.
   *
   * mark_buffer_dirty() is atomic.  It takes bh->b_page->mapping->private_lock,

   * mapping->tree_lock and mapping->host->i_lock.

   */

  void mark_buffer_dirty(struct buffer_head *bh)

  {

  	WARN_ON_ONCE(!buffer_uptodate(bh));

  	trace_block_dirty_buffer(bh);

  	/*
  	 * Very *carefully* optimize the it-is-already-dirty case.
  	 *
  	 * Don't let the final "is it dirty" escape to before we
  	 * perhaps modified the buffer.
  	 */
  	if (buffer_dirty(bh)) {
  		smp_mb();
  		if (buffer_dirty(bh))
  			return;
  	}

  	if (!test_set_buffer_dirty(bh)) {
  		struct page *page = bh->b_page;

  		struct address_space *mapping = NULL;

  		lock_page_memcg(page);

  		if (!TestSetPageDirty(page)) {

  			mapping = page_mapping(page);

  			if (mapping)

  				__set_page_dirty(page, mapping, 0);

  		}

  		unlock_page_memcg(page);

  		if (mapping)
  			__mark_inode_dirty(mapping->host, I_DIRTY_PAGES);

  	}

  }

  EXPORT_SYMBOL(mark_buffer_dirty);

  void mark_buffer_write_io_error(struct buffer_head *bh)
  {
  	set_buffer_write_io_error(bh);
  	/* FIXME: do we need to set this in both places? */
  	if (bh->b_page && bh->b_page->mapping)
  		mapping_set_error(bh->b_page->mapping, -EIO);
  	if (bh->b_assoc_map)
  		mapping_set_error(bh->b_assoc_map, -EIO);
  }
  EXPORT_SYMBOL(mark_buffer_write_io_error);

  /*
   * Decrement a buffer_head's reference count.  If all buffers against a page
   * have zero reference count, are clean and unlocked, and if the page is clean
   * and unlocked then try_to_free_buffers() may strip the buffers from the page
   * in preparation for freeing it (sometimes, rarely, buffers are removed from
   * a page but it ends up not being freed, and buffers may later be reattached).
   */
  void __brelse(struct buffer_head * buf)
  {
  	if (atomic_read(&buf->b_count)) {
  		put_bh(buf);
  		return;
  	}

  	WARN(1, KERN_ERR "VFS: brelse: Trying to free free buffer
  ");

  }

  EXPORT_SYMBOL(__brelse);

  
  /*
   * bforget() is like brelse(), except it discards any
   * potentially dirty data.
   */
  void __bforget(struct buffer_head *bh)
  {
  	clear_buffer_dirty(bh);

  	if (bh->b_assoc_map) {

  		struct address_space *buffer_mapping = bh->b_page->mapping;
  
  		spin_lock(&buffer_mapping->private_lock);
  		list_del_init(&bh->b_assoc_buffers);

  		bh->b_assoc_map = NULL;

  		spin_unlock(&buffer_mapping->private_lock);
  	}
  	__brelse(bh);
  }

  EXPORT_SYMBOL(__bforget);

  
  static struct buffer_head *__bread_slow(struct buffer_head *bh)
  {
  	lock_buffer(bh);
  	if (buffer_uptodate(bh)) {
  		unlock_buffer(bh);
  		return bh;
  	} else {
  		get_bh(bh);
  		bh->b_end_io = end_buffer_read_sync;

  		submit_bh(REQ_OP_READ, 0, bh);

  		wait_on_buffer(bh);
  		if (buffer_uptodate(bh))
  			return bh;
  	}
  	brelse(bh);
  	return NULL;
  }
  
  /*
   * Per-cpu buffer LRU implementation.  To reduce the cost of __find_get_block().
   * The bhs[] array is sorted - newest buffer is at bhs[0].  Buffers have their
   * refcount elevated by one when they're in an LRU.  A buffer can only appear
   * once in a particular CPU's LRU.  A single buffer can be present in multiple
   * CPU's LRUs at the same time.
   *
   * This is a transparent caching front-end to sb_bread(), sb_getblk() and
   * sb_find_get_block().
   *
   * The LRUs themselves only need locking against invalidate_bh_lrus.  We use
   * a local interrupt disable for that.
   */

  #define BH_LRU_SIZE	16

  
  struct bh_lru {
  	struct buffer_head *bhs[BH_LRU_SIZE];
  };
  
  static DEFINE_PER_CPU(struct bh_lru, bh_lrus) = {{ NULL }};
  
  #ifdef CONFIG_SMP
  #define bh_lru_lock()	local_irq_disable()
  #define bh_lru_unlock()	local_irq_enable()
  #else
  #define bh_lru_lock()	preempt_disable()
  #define bh_lru_unlock()	preempt_enable()
  #endif
  
  static inline void check_irqs_on(void)
  {
  #ifdef irqs_disabled
  	BUG_ON(irqs_disabled());
  #endif
  }
  
  /*

   * Install a buffer_head into this cpu's LRU.  If not already in the LRU, it is
   * inserted at the front, and the buffer_head at the back if any is evicted.
   * Or, if already in the LRU it is moved to the front.

   */
  static void bh_lru_install(struct buffer_head *bh)
  {

  	struct buffer_head *evictee = bh;
  	struct bh_lru *b;
  	int i;

  
  	check_irqs_on();
  	bh_lru_lock();

  	b = this_cpu_ptr(&bh_lrus);
  	for (i = 0; i < BH_LRU_SIZE; i++) {
  		swap(evictee, b->bhs[i]);
  		if (evictee == bh) {
  			bh_lru_unlock();
  			return;

  		}

  	}

  	get_bh(bh);
  	bh_lru_unlock();
  	brelse(evictee);

  }
  
  /*
   * Look up the bh in this cpu's LRU.  If it's there, move it to the head.
   */

  static struct buffer_head *

  lookup_bh_lru(struct block_device *bdev, sector_t block, unsigned size)

  {
  	struct buffer_head *ret = NULL;

  	unsigned int i;

  
  	check_irqs_on();
  	bh_lru_lock();

  	for (i = 0; i < BH_LRU_SIZE; i++) {

  		struct buffer_head *bh = __this_cpu_read(bh_lrus.bhs[i]);

  		if (bh && bh->b_blocknr == block && bh->b_bdev == bdev &&
  		    bh->b_size == size) {

  			if (i) {
  				while (i) {

  					__this_cpu_write(bh_lrus.bhs[i],
  						__this_cpu_read(bh_lrus.bhs[i - 1]));

  					i--;
  				}

  				__this_cpu_write(bh_lrus.bhs[0], bh);

  			}
  			get_bh(bh);
  			ret = bh;
  			break;
  		}
  	}
  	bh_lru_unlock();
  	return ret;
  }
  
  /*
   * Perform a pagecache lookup for the matching buffer.  If it's there, refresh
   * it in the LRU and mark it as accessed.  If it is not present then return
   * NULL
   */
  struct buffer_head *

  __find_get_block(struct block_device *bdev, sector_t block, unsigned size)

  {
  	struct buffer_head *bh = lookup_bh_lru(bdev, block, size);
  
  	if (bh == NULL) {

  		/* __find_get_block_slow will mark the page accessed */

  		bh = __find_get_block_slow(bdev, block);

  		if (bh)
  			bh_lru_install(bh);

  	} else

  		touch_buffer(bh);

  	return bh;
  }
  EXPORT_SYMBOL(__find_get_block);
  
  /*

   * __getblk_gfp() will locate (and, if necessary, create) the buffer_head

   * which corresponds to the passed block_device, block and size. The
   * returned buffer has its reference count incremented.
   *

   * __getblk_gfp() will lock up the machine if grow_dev_page's
   * try_to_free_buffers() attempt is failing.  FIXME, perhaps?

   */
  struct buffer_head *

  __getblk_gfp(struct block_device *bdev, sector_t block,
  	     unsigned size, gfp_t gfp)

  {
  	struct buffer_head *bh = __find_get_block(bdev, block, size);
  
  	might_sleep();
  	if (bh == NULL)

  		bh = __getblk_slow(bdev, block, size, gfp);

  	return bh;
  }

  EXPORT_SYMBOL(__getblk_gfp);

  
  /*
   * Do async read-ahead on a buffer..
   */

  void __breadahead(struct block_device *bdev, sector_t block, unsigned size)

  {
  	struct buffer_head *bh = __getblk(bdev, block, size);

  	if (likely(bh)) {

  		ll_rw_block(REQ_OP_READ, REQ_RAHEAD, 1, &bh);

  		brelse(bh);
  	}

  }
  EXPORT_SYMBOL(__breadahead);
  
  /**

   *  __bread_gfp() - reads a specified block and returns the bh

   *  @bdev: the block_device to read from

   *  @block: number of block
   *  @size: size (in bytes) to read

   *  @gfp: page allocation flag
   *

   *  Reads a specified block, and returns buffer head that contains it.

   *  The page cache can be allocated from non-movable area
   *  not to prevent page migration if you set gfp to zero.

   *  It returns NULL if the block was unreadable.
   */
  struct buffer_head *

  __bread_gfp(struct block_device *bdev, sector_t block,
  		   unsigned size, gfp_t gfp)

  {

  	struct buffer_head *bh = __getblk_gfp(bdev, block, size, gfp);

  	if (likely(bh) && !buffer_uptodate(bh))

  		bh = __bread_slow(bh);
  	return bh;
  }

  EXPORT_SYMBOL(__bread_gfp);

  
  /*
   * invalidate_bh_lrus() is called rarely - but not only at unmount.
   * This doesn't race because it runs in each cpu either in irq
   * or with preempt disabled.
   */
  static void invalidate_bh_lru(void *arg)
  {
  	struct bh_lru *b = &get_cpu_var(bh_lrus);
  	int i;
  
  	for (i = 0; i < BH_LRU_SIZE; i++) {
  		brelse(b->bhs[i]);
  		b->bhs[i] = NULL;
  	}
  	put_cpu_var(bh_lrus);
  }

  
  static bool has_bh_in_lru(int cpu, void *dummy)
  {
  	struct bh_lru *b = per_cpu_ptr(&bh_lrus, cpu);
  	int i;

  	for (i = 0; i < BH_LRU_SIZE; i++) {
  		if (b->bhs[i])
  			return 1;
  	}
  
  	return 0;
  }

  void invalidate_bh_lrus(void)

  {

  	on_each_cpu_cond(has_bh_in_lru, invalidate_bh_lru, NULL, 1, GFP_KERNEL);

  }

  EXPORT_SYMBOL_GPL(invalidate_bh_lrus);

  
  void set_bh_page(struct buffer_head *bh,
  		struct page *page, unsigned long offset)
  {
  	bh->b_page = page;

  	BUG_ON(offset >= PAGE_SIZE);

  	if (PageHighMem(page))
  		/*
  		 * This catches illegal uses and preserves the offset:
  		 */
  		bh->b_data = (char *)(0 + offset);
  	else
  		bh->b_data = page_address(page) + offset;
  }
  EXPORT_SYMBOL(set_bh_page);
  
  /*
   * Called when truncating a buffer on a page completely.
   */

  
  /* Bits that are cleared during an invalidate */
  #define BUFFER_FLAGS_DISCARD \
  	(1 << BH_Mapped | 1 << BH_New | 1 << BH_Req | \
  	 1 << BH_Delay | 1 << BH_Unwritten)

  static void discard_buffer(struct buffer_head * bh)

  {

  	unsigned long b_state, b_state_old;

  	lock_buffer(bh);
  	clear_buffer_dirty(bh);
  	bh->b_bdev = NULL;

  	b_state = bh->b_state;
  	for (;;) {
  		b_state_old = cmpxchg(&bh->b_state, b_state,
  				      (b_state & ~BUFFER_FLAGS_DISCARD));
  		if (b_state_old == b_state)
  			break;
  		b_state = b_state_old;
  	}

  	unlock_buffer(bh);
  }
  
  /**

   * block_invalidatepage - invalidate part or all of a buffer-backed page

   *
   * @page: the page which is affected

   * @offset: start of the range to invalidate
   * @length: length of the range to invalidate

   *
   * block_invalidatepage() is called when all or part of the page has become

   * invalidated by a truncate operation.

   *
   * block_invalidatepage() does not have to release all buffers, but it must
   * ensure that no dirty buffer is left outside @offset and that no I/O
   * is underway against any of the blocks which are outside the truncation
   * point.  Because the caller is about to free (and possibly reuse) those
   * blocks on-disk.
   */

  void block_invalidatepage(struct page *page, unsigned int offset,
  			  unsigned int length)

  {
  	struct buffer_head *head, *bh, *next;
  	unsigned int curr_off = 0;

  	unsigned int stop = length + offset;

  
  	BUG_ON(!PageLocked(page));
  	if (!page_has_buffers(page))
  		goto out;

  	/*
  	 * Check for overflow
  	 */

  	BUG_ON(stop > PAGE_SIZE || stop < length);

  	head = page_buffers(page);
  	bh = head;
  	do {
  		unsigned int next_off = curr_off + bh->b_size;
  		next = bh->b_this_page;
  
  		/*

  		 * Are we still fully in range ?
  		 */
  		if (next_off > stop)
  			goto out;
  
  		/*

  		 * is this block fully invalidated?
  		 */
  		if (offset <= curr_off)
  			discard_buffer(bh);
  		curr_off = next_off;
  		bh = next;
  	} while (bh != head);
  
  	/*
  	 * We release buffers only if the entire page is being invalidated.
  	 * The get_block cached value has been unconditionally invalidated,
  	 * so real IO is not possible anymore.
  	 */
  	if (offset == 0)

  		try_to_release_page(page, 0);

  out:

  	return;

  }
  EXPORT_SYMBOL(block_invalidatepage);

  /*
   * We attach and possibly dirty the buffers atomically wrt
   * __set_page_dirty_buffers() via private_lock.  try_to_free_buffers
   * is already excluded via the page lock.
   */
  void create_empty_buffers(struct page *page,
  			unsigned long blocksize, unsigned long b_state)
  {
  	struct buffer_head *bh, *head, *tail;
  
  	head = alloc_page_buffers(page, blocksize, 1);
  	bh = head;
  	do {
  		bh->b_state |= b_state;
  		tail = bh;
  		bh = bh->b_this_page;
  	} while (bh);
  	tail->b_this_page = head;
  
  	spin_lock(&page->mapping->private_lock);
  	if (PageUptodate(page) || PageDirty(page)) {
  		bh = head;
  		do {
  			if (PageDirty(page))
  				set_buffer_dirty(bh);
  			if (PageUptodate(page))
  				set_buffer_uptodate(bh);
  			bh = bh->b_this_page;
  		} while (bh != head);
  	}
  	attach_page_buffers(page, head);
  	spin_unlock(&page->mapping->private_lock);
  }
  EXPORT_SYMBOL(create_empty_buffers);

  /**
   * clean_bdev_aliases: clean a range of buffers in block device
   * @bdev: Block device to clean buffers in
   * @block: Start of a range of blocks to clean
   * @len: Number of blocks to clean

   *

   * We are taking a range of blocks for data and we don't want writeback of any
   * buffer-cache aliases starting from return from this function and until the
   * moment when something will explicitly mark the buffer dirty (hopefully that
   * will not happen until we will free that block ;-) We don't even need to mark
   * it not-uptodate - nobody can expect anything from a newly allocated buffer
   * anyway. We used to use unmap_buffer() for such invalidation, but that was
   * wrong. We definitely don't want to mark the alias unmapped, for example - it
   * would confuse anyone who might pick it with bread() afterwards...
   *
   * Also..  Note that bforget() doesn't lock the buffer.  So there can be
   * writeout I/O going on against recently-freed buffers.  We don't wait on that
   * I/O in bforget() - it's more efficient to wait on the I/O only if we really
   * need to.  That happens here.

   */

  void clean_bdev_aliases(struct block_device *bdev, sector_t block, sector_t len)

  {

  	struct inode *bd_inode = bdev->bd_inode;
  	struct address_space *bd_mapping = bd_inode->i_mapping;
  	struct pagevec pvec;
  	pgoff_t index = block >> (PAGE_SHIFT - bd_inode->i_blkbits);
  	pgoff_t end;

  	int i, count;

  	struct buffer_head *bh;
  	struct buffer_head *head;

  	end = (block + len - 1) >> (PAGE_SHIFT - bd_inode->i_blkbits);
  	pagevec_init(&pvec, 0);

  	while (pagevec_lookup_range(&pvec, bd_mapping, &index, end)) {

  		count = pagevec_count(&pvec);
  		for (i = 0; i < count; i++) {

  			struct page *page = pvec.pages[i];

  			if (!page_has_buffers(page))
  				continue;
  			/*
  			 * We use page lock instead of bd_mapping->private_lock
  			 * to pin buffers here since we can afford to sleep and
  			 * it scales better than a global spinlock lock.
  			 */
  			lock_page(page);
  			/* Recheck when the page is locked which pins bhs */
  			if (!page_has_buffers(page))
  				goto unlock_page;
  			head = page_buffers(page);
  			bh = head;
  			do {

  				if (!buffer_mapped(bh) || (bh->b_blocknr < block))

  					goto next;
  				if (bh->b_blocknr >= block + len)
  					break;
  				clear_buffer_dirty(bh);
  				wait_on_buffer(bh);
  				clear_buffer_req(bh);
  next:
  				bh = bh->b_this_page;
  			} while (bh != head);
  unlock_page:
  			unlock_page(page);
  		}
  		pagevec_release(&pvec);
  		cond_resched();

  		/* End of range already reached? */
  		if (index > end || !index)
  			break;

  	}
  }

  EXPORT_SYMBOL(clean_bdev_aliases);

  
  /*

   * Size is a power-of-two in the range 512..PAGE_SIZE,
   * and the case we care about most is PAGE_SIZE.
   *
   * So this *could* possibly be written with those
   * constraints in mind (relevant mostly if some
   * architecture has a slow bit-scan instruction)
   */
  static inline int block_size_bits(unsigned int blocksize)
  {
  	return ilog2(blocksize);
  }
  
  static struct buffer_head *create_page_buffers(struct page *page, struct inode *inode, unsigned int b_state)
  {
  	BUG_ON(!PageLocked(page));
  
  	if (!page_has_buffers(page))
  		create_empty_buffers(page, 1 << ACCESS_ONCE(inode->i_blkbits), b_state);
  	return page_buffers(page);
  }
  
  /*

   * NOTE! All mapped/uptodate combinations are valid:
   *
   *	Mapped	Uptodate	Meaning
   *
   *	No	No		"unknown" - must do get_block()
   *	No	Yes		"hole" - zero-filled
   *	Yes	No		"allocated" - allocated on disk, not read in
   *	Yes	Yes		"valid" - allocated and up-to-date in memory.
   *
   * "Dirty" is valid only with the last case (mapped+uptodate).
   */
  
  /*
   * While block_write_full_page is writing back the dirty buffers under
   * the page lock, whoever dirtied the buffers may decide to clean them
   * again at any time.  We handle that by only looking at the buffer
   * state inside lock_buffer().
   *
   * If block_write_full_page() is called for regular writeback
   * (wbc->sync_mode == WB_SYNC_NONE) then it will redirty a page which has a
   * locked buffer.   This only can happen if someone has written the buffer
   * directly, with submit_bh().  At the address_space level PageWriteback
   * prevents this contention from occurring.

   *
   * If block_write_full_page() is called with wbc->sync_mode ==

   * WB_SYNC_ALL, the writes are posted using REQ_SYNC; this

   * causes the writes to be flagged as synchronous writes.

   */

  int __block_write_full_page(struct inode *inode, struct page *page,

  			get_block_t *get_block, struct writeback_control *wbc,
  			bh_end_io_t *handler)

  {
  	int err;
  	sector_t block;
  	sector_t last_block;

  	struct buffer_head *bh, *head;

  	unsigned int blocksize, bbits;

  	int nr_underway = 0;

  	int write_flags = wbc_to_write_flags(wbc);

  	head = create_page_buffers(page, inode,

  					(1 << BH_Dirty)|(1 << BH_Uptodate));

  
  	/*
  	 * Be very careful.  We have no exclusion from __set_page_dirty_buffers
  	 * here, and the (potentially unmapped) buffers may become dirty at
  	 * any time.  If a buffer becomes dirty here after we've inspected it
  	 * then we just miss that fact, and the page stays dirty.
  	 *
  	 * Buffers outside i_size may be dirtied by __set_page_dirty_buffers;
  	 * handle that here by just cleaning them.
  	 */

  	bh = head;

  	blocksize = bh->b_size;
  	bbits = block_size_bits(blocksize);

  	block = (sector_t)page->index << (PAGE_SHIFT - bbits);

  	last_block = (i_size_read(inode) - 1) >> bbits;

  
  	/*
  	 * Get all the dirty buffers mapped to disk addresses and
  	 * handle any aliases from the underlying blockdev's mapping.
  	 */
  	do {
  		if (block > last_block) {
  			/*
  			 * mapped buffers outside i_size will occur, because
  			 * this page can be outside i_size when there is a
  			 * truncate in progress.
  			 */
  			/*
  			 * The buffer was zeroed by block_write_full_page()
  			 */
  			clear_buffer_dirty(bh);
  			set_buffer_uptodate(bh);

  		} else if ((!buffer_mapped(bh) || buffer_delay(bh)) &&
  			   buffer_dirty(bh)) {

  			WARN_ON(bh->b_size != blocksize);

  			err = get_block(inode, block, bh, 1);
  			if (err)
  				goto recover;

  			clear_buffer_delay(bh);

  			if (buffer_new(bh)) {
  				/* blockdev mappings never come here */
  				clear_buffer_new(bh);

  				clean_bdev_bh_alias(bh);

  			}
  		}
  		bh = bh->b_this_page;
  		block++;
  	} while (bh != head);
  
  	do {

  		if (!buffer_mapped(bh))
  			continue;
  		/*
  		 * If it's a fully non-blocking write attempt and we cannot
  		 * lock the buffer then redirty the page.  Note that this can

  		 * potentially cause a busy-wait loop from writeback threads
  		 * and kswapd activity, but those code paths have their own
  		 * higher-level throttling.

  		 */

  		if (wbc->sync_mode != WB_SYNC_NONE) {

  			lock_buffer(bh);

  		} else if (!trylock_buffer(bh)) {

  			redirty_page_for_writepage(wbc, page);
  			continue;
  		}
  		if (test_clear_buffer_dirty(bh)) {

  			mark_buffer_async_write_endio(bh, handler);

  		} else {
  			unlock_buffer(bh);
  		}
  	} while ((bh = bh->b_this_page) != head);
  
  	/*
  	 * The page and its buffers are protected by PageWriteback(), so we can
  	 * drop the bh refcounts early.
  	 */
  	BUG_ON(PageWriteback(page));
  	set_page_writeback(page);

  
  	do {
  		struct buffer_head *next = bh->b_this_page;
  		if (buffer_async_write(bh)) {

  			submit_bh_wbc(REQ_OP_WRITE, write_flags, bh,
  					inode->i_write_hint, wbc);

  			nr_underway++;
  		}

  		bh = next;
  	} while (bh != head);

  	unlock_page(page);

  
  	err = 0;
  done:
  	if (nr_underway == 0) {
  		/*
  		 * The page was marked dirty, but the buffers were
  		 * clean.  Someone wrote them back by hand with
  		 * ll_rw_block/submit_bh.  A rare case.
  		 */

  		end_page_writeback(page);

  		/*
  		 * The page and buffer_heads can be released at any time from
  		 * here on.
  		 */

  	}
  	return err;
  
  recover:
  	/*
  	 * ENOSPC, or some other error.  We may already have added some
  	 * blocks to the file, so we need to write these out to avoid
  	 * exposing stale data.
  	 * The page is currently locked and not marked for writeback
  	 */
  	bh = head;
  	/* Recovery: lock and submit the mapped buffers */
  	do {

  		if (buffer_mapped(bh) && buffer_dirty(bh) &&
  		    !buffer_delay(bh)) {

  			lock_buffer(bh);

  			mark_buffer_async_write_endio(bh, handler);

  		} else {
  			/*
  			 * The buffer may have been set dirty during
  			 * attachment to a dirty page.
  			 */
  			clear_buffer_dirty(bh);
  		}
  	} while ((bh = bh->b_this_page) != head);
  	SetPageError(page);
  	BUG_ON(PageWriteback(page));

  	mapping_set_error(page->mapping, err);

  	set_page_writeback(page);

  	do {
  		struct buffer_head *next = bh->b_this_page;
  		if (buffer_async_write(bh)) {
  			clear_buffer_dirty(bh);

  			submit_bh_wbc(REQ_OP_WRITE, write_flags, bh,
  					inode->i_write_hint, wbc);

  			nr_underway++;
  		}

  		bh = next;
  	} while (bh != head);

  	unlock_page(page);

  	goto done;
  }

  EXPORT_SYMBOL(__block_write_full_page);

  /*
   * If a page has any new buffers, zero them out here, and mark them uptodate
   * and dirty so they'll be written out (in order to prevent uninitialised
   * block data from leaking). And clear the new bit.
   */
  void page_zero_new_buffers(struct page *page, unsigned from, unsigned to)
  {
  	unsigned int block_start, block_end;
  	struct buffer_head *head, *bh;
  
  	BUG_ON(!PageLocked(page));
  	if (!page_has_buffers(page))
  		return;
  
  	bh = head = page_buffers(page);
  	block_start = 0;
  	do {
  		block_end = block_start + bh->b_size;
  
  		if (buffer_new(bh)) {
  			if (block_end > from && block_start < to) {
  				if (!PageUptodate(page)) {
  					unsigned start, size;
  
  					start = max(from, block_start);
  					size = min(to, block_end) - start;

  					zero_user(page, start, size);

  					set_buffer_uptodate(bh);
  				}
  
  				clear_buffer_new(bh);
  				mark_buffer_dirty(bh);
  			}
  		}
  
  		block_start = block_end;
  		bh = bh->b_this_page;
  	} while (bh != head);
  }
  EXPORT_SYMBOL(page_zero_new_buffers);

  static void
  iomap_to_bh(struct inode *inode, sector_t block, struct buffer_head *bh,
  		struct iomap *iomap)
  {
  	loff_t offset = block << inode->i_blkbits;
  
  	bh->b_bdev = iomap->bdev;
  
  	/*
  	 * Block points to offset in file we need to map, iomap contains
  	 * the offset at which the map starts. If the map ends before the
  	 * current block, then do not map the buffer and let the caller
  	 * handle it.
  	 */
  	BUG_ON(offset >= iomap->offset + iomap->length);
  
  	switch (iomap->type) {
  	case IOMAP_HOLE:
  		/*
  		 * If the buffer is not up to date or beyond the current EOF,
  		 * we need to mark it as new to ensure sub-block zeroing is
  		 * executed if necessary.
  		 */
  		if (!buffer_uptodate(bh) ||
  		    (offset >= i_size_read(inode)))
  			set_buffer_new(bh);
  		break;
  	case IOMAP_DELALLOC:
  		if (!buffer_uptodate(bh) ||
  		    (offset >= i_size_read(inode)))
  			set_buffer_new(bh);
  		set_buffer_uptodate(bh);
  		set_buffer_mapped(bh);
  		set_buffer_delay(bh);
  		break;
  	case IOMAP_UNWRITTEN:
  		/*
  		 * For unwritten regions, we always need to ensure that
  		 * sub-block writes cause the regions in the block we are not
  		 * writing to are zeroed. Set the buffer as new to ensure this.
  		 */
  		set_buffer_new(bh);
  		set_buffer_unwritten(bh);
  		/* FALLTHRU */
  	case IOMAP_MAPPED:
  		if (offset >= i_size_read(inode))
  			set_buffer_new(bh);
  		bh->b_blocknr = (iomap->blkno >> (inode->i_blkbits - 9)) +
  				((offset - iomap->offset) >> inode->i_blkbits);
  		set_buffer_mapped(bh);
  		break;
  	}
  }
  
  int __block_write_begin_int(struct page *page, loff_t pos, unsigned len,
  		get_block_t *get_block, struct iomap *iomap)

  {

  	unsigned from = pos & (PAGE_SIZE - 1);

  	unsigned to = from + len;

  	struct inode *inode = page->mapping->host;

  	unsigned block_start, block_end;
  	sector_t block;
  	int err = 0;
  	unsigned blocksize, bbits;
  	struct buffer_head *bh, *head, *wait[2], **wait_bh=wait;
  
  	BUG_ON(!PageLocked(page));

  	BUG_ON(from > PAGE_SIZE);
  	BUG_ON(to > PAGE_SIZE);

  	BUG_ON(from > to);

  	head = create_page_buffers(page, inode, 0);
  	blocksize = head->b_size;
  	bbits = block_size_bits(blocksize);

  	block = (sector_t)page->index << (PAGE_SHIFT - bbits);

  
  	for(bh = head, block_start = 0; bh != head || !block_start;
  	    block++, block_start=block_end, bh = bh->b_this_page) {
  		block_end = block_start + blocksize;
  		if (block_end <= from || block_start >= to) {
  			if (PageUptodate(page)) {
  				if (!buffer_uptodate(bh))
  					set_buffer_uptodate(bh);
  			}
  			continue;
  		}
  		if (buffer_new(bh))
  			clear_buffer_new(bh);
  		if (!buffer_mapped(bh)) {

  			WARN_ON(bh->b_size != blocksize);

  			if (get_block) {
  				err = get_block(inode, block, bh, 1);
  				if (err)
  					break;
  			} else {
  				iomap_to_bh(inode, block, bh, iomap);
  			}

  			if (buffer_new(bh)) {

  				clean_bdev_bh_alias(bh);

  				if (PageUptodate(page)) {

  					clear_buffer_new(bh);

  					set_buffer_uptodate(bh);

  					mark_buffer_dirty(bh);

  					continue;
  				}

  				if (block_end > to || block_start < from)
  					zero_user_segments(page,
  						to, block_end,
  						block_start, from);

  				continue;
  			}
  		}
  		if (PageUptodate(page)) {
  			if (!buffer_uptodate(bh))
  				set_buffer_uptodate(bh);
  			continue; 
  		}
  		if (!buffer_uptodate(bh) && !buffer_delay(bh) &&

  		    !buffer_unwritten(bh) &&

  		     (block_start < from || block_end > to)) {

  			ll_rw_block(REQ_OP_READ, 0, 1, &bh);

  			*wait_bh++=bh;
  		}
  	}
  	/*
  	 * If we issued read requests - let them complete.
  	 */
  	while(wait_bh > wait) {
  		wait_on_buffer(*--wait_bh);
  		if (!buffer_uptodate(*wait_bh))

  			err = -EIO;

  	}

  	if (unlikely(err))

  		page_zero_new_buffers(page, from, to);

  	return err;
  }

  
  int __block_write_begin(struct page *page, loff_t pos, unsigned len,
  		get_block_t *get_block)
  {
  	return __block_write_begin_int(page, pos, len, get_block, NULL);
  }

  EXPORT_SYMBOL(__block_write_begin);

  
  static int __block_commit_write(struct inode *inode, struct page *page,
  		unsigned from, unsigned to)
  {
  	unsigned block_start, block_end;
  	int partial = 0;
  	unsigned blocksize;
  	struct buffer_head *bh, *head;

  	bh = head = page_buffers(page);
  	blocksize = bh->b_size;

  	block_start = 0;
  	do {

  		block_end = block_start + blocksize;
  		if (block_end <= from || block_start >= to) {
  			if (!buffer_uptodate(bh))
  				partial = 1;
  		} else {
  			set_buffer_uptodate(bh);
  			mark_buffer_dirty(bh);
  		}

  		clear_buffer_new(bh);

  
  		block_start = block_end;
  		bh = bh->b_this_page;
  	} while (bh != head);

  
  	/*
  	 * If this is a partial write which happened to make all buffers
  	 * uptodate then we can optimize away a bogus readpage() for
  	 * the next read(). Here we 'discover' whether the page went
  	 * uptodate as a result of this (potentially partial) write.
  	 */
  	if (!partial)
  		SetPageUptodate(page);
  	return 0;
  }
  
  /*

   * block_write_begin takes care of the basic task of block allocation and
   * bringing partial write blocks uptodate first.
   *

   * The filesystem needs to handle block truncation upon failure.

   */

  int block_write_begin(struct address_space *mapping, loff_t pos, unsigned len,
  		unsigned flags, struct page **pagep, get_block_t *get_block)

  {

  	pgoff_t index = pos >> PAGE_SHIFT;

  	struct page *page;

  	int status;

  	page = grab_cache_page_write_begin(mapping, index, flags);
  	if (!page)
  		return -ENOMEM;

  	status = __block_write_begin(page, pos, len, get_block);

  	if (unlikely(status)) {

  		unlock_page(page);

  		put_page(page);

  		page = NULL;

  	}

  	*pagep = page;

  	return status;
  }
  EXPORT_SYMBOL(block_write_begin);
  
  int block_write_end(struct file *file, struct address_space *mapping,
  			loff_t pos, unsigned len, unsigned copied,
  			struct page *page, void *fsdata)
  {
  	struct inode *inode = mapping->host;
  	unsigned start;

  	start = pos & (PAGE_SIZE - 1);

  
  	if (unlikely(copied < len)) {
  		/*
  		 * The buffers that were written will now be uptodate, so we
  		 * don't have to worry about a readpage reading them and
  		 * overwriting a partial write. However if we have encountered
  		 * a short write and only partially written into a buffer, it
  		 * will not be marked uptodate, so a readpage might come in and
  		 * destroy our partial write.
  		 *
  		 * Do the simplest thing, and just treat any short write to a
  		 * non uptodate page as a zero-length write, and force the
  		 * caller to redo the whole thing.
  		 */
  		if (!PageUptodate(page))
  			copied = 0;
  
  		page_zero_new_buffers(page, start+copied, start+len);
  	}
  	flush_dcache_page(page);
  
  	/* This could be a short (even 0-length) commit */
  	__block_commit_write(inode, page, start, start+copied);
  
  	return copied;
  }
  EXPORT_SYMBOL(block_write_end);
  
  int generic_write_end(struct file *file, struct address_space *mapping,
  			loff_t pos, unsigned len, unsigned copied,
  			struct page *page, void *fsdata)
  {
  	struct inode *inode = mapping->host;

  	loff_t old_size = inode->i_size;

  	int i_size_changed = 0;

  
  	copied = block_write_end(file, mapping, pos, len, copied, page, fsdata);
  
  	/*
  	 * No need to use i_size_read() here, the i_size
  	 * cannot change under us because we hold i_mutex.
  	 *
  	 * But it's important to update i_size while still holding page lock:
  	 * page writeout could otherwise come in and zero beyond i_size.
  	 */
  	if (pos+copied > inode->i_size) {
  		i_size_write(inode, pos+copied);

  		i_size_changed = 1;

  	}
  
  	unlock_page(page);

  	put_page(page);

  	if (old_size < pos)
  		pagecache_isize_extended(inode, old_size, pos);

  	/*
  	 * Don't mark the inode dirty under page lock. First, it unnecessarily
  	 * makes the holding time of page lock longer. Second, it forces lock
  	 * ordering of page lock and transaction start for journaling
  	 * filesystems.
  	 */
  	if (i_size_changed)
  		mark_inode_dirty(inode);

  	return copied;
  }
  EXPORT_SYMBOL(generic_write_end);
  
  /*

   * block_is_partially_uptodate checks whether buffers within a page are
   * uptodate or not.
   *
   * Returns true if all buffers which correspond to a file portion
   * we want to read are uptodate.
   */

  int block_is_partially_uptodate(struct page *page, unsigned long from,
  					unsigned long count)

  {

  	unsigned block_start, block_end, blocksize;
  	unsigned to;
  	struct buffer_head *bh, *head;
  	int ret = 1;
  
  	if (!page_has_buffers(page))
  		return 0;

  	head = page_buffers(page);
  	blocksize = head->b_size;

  	to = min_t(unsigned, PAGE_SIZE - from, count);

  	to = from + to;

  	if (from < blocksize && to > PAGE_SIZE - blocksize)

  		return 0;

  	bh = head;
  	block_start = 0;
  	do {
  		block_end = block_start + blocksize;
  		if (block_end > from && block_start < to) {
  			if (!buffer_uptodate(bh)) {
  				ret = 0;
  				break;
  			}
  			if (block_end >= to)
  				break;
  		}
  		block_start = block_end;
  		bh = bh->b_this_page;
  	} while (bh != head);
  
  	return ret;
  }
  EXPORT_SYMBOL(block_is_partially_uptodate);
  
  /*

   * Generic "read page" function for block devices that have the normal
   * get_block functionality. This is most of the block device filesystems.
   * Reads the page asynchronously --- the unlock_buffer() and
   * set/clear_buffer_uptodate() functions propagate buffer state into the
   * page struct once IO has completed.
   */
  int block_read_full_page(struct page *page, get_block_t *get_block)
  {
  	struct inode *inode = page->mapping->host;
  	sector_t iblock, lblock;
  	struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];

  	unsigned int blocksize, bbits;

  	int nr, i;
  	int fully_mapped = 1;

  	head = create_page_buffers(page, inode, 0);
  	blocksize = head->b_size;
  	bbits = block_size_bits(blocksize);

  	iblock = (sector_t)page->index << (PAGE_SHIFT - bbits);

  	lblock = (i_size_read(inode)+blocksize-1) >> bbits;

  	bh = head;
  	nr = 0;
  	i = 0;
  
  	do {
  		if (buffer_uptodate(bh))
  			continue;
  
  		if (!buffer_mapped(bh)) {

  			int err = 0;

  			fully_mapped = 0;
  			if (iblock < lblock) {

  				WARN_ON(bh->b_size != blocksize);

  				err = get_block(inode, iblock, bh, 0);
  				if (err)

  					SetPageError(page);
  			}
  			if (!buffer_mapped(bh)) {

  				zero_user(page, i * blocksize, blocksize);

  				if (!err)
  					set_buffer_uptodate(bh);

  				continue;
  			}
  			/*
  			 * get_block() might have updated the buffer
  			 * synchronously
  			 */
  			if (buffer_uptodate(bh))
  				continue;
  		}
  		arr[nr++] = bh;
  	} while (i++, iblock++, (bh = bh->b_this_page) != head);
  
  	if (fully_mapped)
  		SetPageMappedToDisk(page);
  
  	if (!nr) {
  		/*
  		 * All buffers are uptodate - we can set the page uptodate
  		 * as well. But not if get_block() returned an error.
  		 */
  		if (!PageError(page))
  			SetPageUptodate(page);
  		unlock_page(page);
  		return 0;
  	}
  
  	/* Stage two: lock the buffers */
  	for (i = 0; i < nr; i++) {
  		bh = arr[i];
  		lock_buffer(bh);
  		mark_buffer_async_read(bh);
  	}
  
  	/*
  	 * Stage 3: start the IO.  Check for uptodateness
  	 * inside the buffer lock in case another process reading
  	 * the underlying blockdev brought it uptodate (the sct fix).
  	 */
  	for (i = 0; i < nr; i++) {
  		bh = arr[i];
  		if (buffer_uptodate(bh))
  			end_buffer_async_read(bh, 1);
  		else

  			submit_bh(REQ_OP_READ, 0, bh);

  	}
  	return 0;
  }

  EXPORT_SYMBOL(block_read_full_page);

  
  /* utility function for filesystems that need to do work on expanding

   * truncates.  Uses filesystem pagecache writes to allow the filesystem to

   * deal with the hole.  
   */

  int generic_cont_expand_simple(struct inode *inode, loff_t size)

  {
  	struct address_space *mapping = inode->i_mapping;
  	struct page *page;

  	void *fsdata;

  	int err;

  	err = inode_newsize_ok(inode, size);
  	if (err)

  		goto out;

  	err = pagecache_write_begin(NULL, mapping, size, 0,

  				    AOP_FLAG_CONT_EXPAND, &page, &fsdata);

  	if (err)

  		goto out;

  	err = pagecache_write_end(NULL, mapping, size, 0, 0, page, fsdata);
  	BUG_ON(err > 0);

  out:
  	return err;
  }

  EXPORT_SYMBOL(generic_cont_expand_simple);

  static int cont_expand_zero(struct file *file, struct address_space *mapping,
  			    loff_t pos, loff_t *bytes)

  {

  	struct inode *inode = mapping->host;

  	unsigned int blocksize = i_blocksize(inode);

  	struct page *page;
  	void *fsdata;
  	pgoff_t index, curidx;
  	loff_t curpos;
  	unsigned zerofrom, offset, len;
  	int err = 0;

  	index = pos >> PAGE_SHIFT;
  	offset = pos & ~PAGE_MASK;

  	while (index > (curidx = (curpos = *bytes)>>PAGE_SHIFT)) {
  		zerofrom = curpos & ~PAGE_MASK;

  		if (zerofrom & (blocksize-1)) {
  			*bytes |= (blocksize-1);
  			(*bytes)++;
  		}

  		len = PAGE_SIZE - zerofrom;

  		err = pagecache_write_begin(file, mapping, curpos, len, 0,
  					    &page, &fsdata);

  		if (err)
  			goto out;