/**
   * aops.c - NTFS kernel address space operations and page cache handling.
   *	    Part of the Linux-NTFS project.
   *

   * Copyright (c) 2001-2007 Anton Altaparmakov

   * Copyright (c) 2002 Richard Russon
   *
   * This program/include file is free software; you can redistribute it and/or
   * modify it under the terms of the GNU General Public License as published
   * by the Free Software Foundation; either version 2 of the License, or
   * (at your option) any later version.
   *
   * This program/include file is distributed in the hope that it will be
   * useful, but WITHOUT ANY WARRANTY; without even the implied warranty
   * of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   * GNU General Public License for more details.
   *
   * You should have received a copy of the GNU General Public License
   * along with this program (in the main directory of the Linux-NTFS
   * distribution in the file COPYING); if not, write to the Free Software
   * Foundation,Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
   */
  
  #include <linux/errno.h>

  #include <linux/fs.h>

  #include <linux/gfp.h>

  #include <linux/mm.h>
  #include <linux/pagemap.h>
  #include <linux/swap.h>
  #include <linux/buffer_head.h>
  #include <linux/writeback.h>

  #include <linux/bit_spinlock.h>

  
  #include "aops.h"
  #include "attrib.h"
  #include "debug.h"
  #include "inode.h"
  #include "mft.h"
  #include "runlist.h"
  #include "types.h"
  #include "ntfs.h"
  
  /**
   * ntfs_end_buffer_async_read - async io completion for reading attributes
   * @bh:		buffer head on which io is completed
   * @uptodate:	whether @bh is now uptodate or not
   *
   * Asynchronous I/O completion handler for reading pages belonging to the
   * attribute address space of an inode.  The inodes can either be files or
   * directories or they can be fake inodes describing some attribute.
   *
   * If NInoMstProtected(), perform the post read mst fixups when all IO on the
   * page has been completed and mark the page uptodate or set the error bit on
   * the page.  To determine the size of the records that need fixing up, we
   * cheat a little bit by setting the index_block_size in ntfs_inode to the ntfs
   * record size, and index_block_size_bits, to the log(base 2) of the ntfs
   * record size.
   */
  static void ntfs_end_buffer_async_read(struct buffer_head *bh, int uptodate)
  {

  	unsigned long flags;

  	struct buffer_head *first, *tmp;

  	struct page *page;

  	struct inode *vi;

  	ntfs_inode *ni;
  	int page_uptodate = 1;
  
  	page = bh->b_page;

  	vi = page->mapping->host;
  	ni = NTFS_I(vi);

  
  	if (likely(uptodate)) {

  		loff_t i_size;
  		s64 file_ofs, init_size;

  
  		set_buffer_uptodate(bh);
  
  		file_ofs = ((s64)page->index << PAGE_CACHE_SHIFT) +
  				bh_offset(bh);

  		read_lock_irqsave(&ni->size_lock, flags);

  		init_size = ni->initialized_size;
  		i_size = i_size_read(vi);

  		read_unlock_irqrestore(&ni->size_lock, flags);

  		if (unlikely(init_size > i_size)) {
  			/* Race with shrinking truncate. */
  			init_size = i_size;
  		}

  		/* Check for the current buffer head overflowing. */

  		if (unlikely(file_ofs + bh->b_size > init_size)) {

  			int ofs;

  			void *kaddr;

  
  			ofs = 0;
  			if (file_ofs < init_size)
  				ofs = init_size - file_ofs;

  			local_irq_save(flags);

  			kaddr = kmap_atomic(page, KM_BIO_SRC_IRQ);
  			memset(kaddr + bh_offset(bh) + ofs, 0,
  					bh->b_size - ofs);
  			flush_dcache_page(page);
  			kunmap_atomic(kaddr, KM_BIO_SRC_IRQ);

  			local_irq_restore(flags);

  		}
  	} else {
  		clear_buffer_uptodate(bh);

  		SetPageError(page);

  		ntfs_error(ni->vol->sb, "Buffer I/O error, logical block "
  				"0x%llx.", (unsigned long long)bh->b_blocknr);

  	}

  	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)) {
  			if (likely(buffer_locked(tmp)))
  				goto still_busy;
  			/* Async buffers must be locked. */
  			BUG();
  		}
  		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 then we can set the page uptodate,
  	 * but we first have to perform the post read mst fixups, if the
  	 * attribute is mst protected, i.e. if NInoMstProteced(ni) is true.
  	 * Note we ignore fixup errors as those are detected when
  	 * map_mft_record() is called which gives us per record granularity
  	 * rather than per page granularity.
  	 */
  	if (!NInoMstProtected(ni)) {
  		if (likely(page_uptodate && !PageError(page)))
  			SetPageUptodate(page);
  	} else {

  		u8 *kaddr;

  		unsigned int i, recs;
  		u32 rec_size;
  
  		rec_size = ni->itype.index.block_size;
  		recs = PAGE_CACHE_SIZE / rec_size;
  		/* Should have been verified before we got here... */
  		BUG_ON(!recs);

  		local_irq_save(flags);

  		kaddr = kmap_atomic(page, KM_BIO_SRC_IRQ);

  		for (i = 0; i < recs; i++)

  			post_read_mst_fixup((NTFS_RECORD*)(kaddr +

  					i * rec_size), rec_size);

  		kunmap_atomic(kaddr, KM_BIO_SRC_IRQ);

  		local_irq_restore(flags);

  		flush_dcache_page(page);

  		if (likely(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;
  }
  
  /**
   * ntfs_read_block - fill a @page of an address space with data
   * @page:	page cache page to fill with data
   *
   * Fill the page @page of the address space belonging to the @page->host inode.
   * We read each buffer asynchronously and when all buffers are read in, our io
   * completion handler ntfs_end_buffer_read_async(), if required, automatically
   * applies the mst fixups to the page before finally marking it uptodate and
   * unlocking it.
   *
   * We only enforce allocated_size limit because i_size is checked for in
   * generic_file_read().
   *
   * Return 0 on success and -errno on error.
   *
   * Contains an adapted version of fs/buffer.c::block_read_full_page().
   */
  static int ntfs_read_block(struct page *page)
  {

  	loff_t i_size;

  	VCN vcn;
  	LCN lcn;

  	s64 init_size;
  	struct inode *vi;

  	ntfs_inode *ni;
  	ntfs_volume *vol;
  	runlist_element *rl;
  	struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
  	sector_t iblock, lblock, zblock;

  	unsigned long flags;

  	unsigned int blocksize, vcn_ofs;
  	int i, nr;
  	unsigned char blocksize_bits;

  	vi = page->mapping->host;
  	ni = NTFS_I(vi);

  	vol = ni->vol;
  
  	/* $MFT/$DATA must have its complete runlist in memory at all times. */
  	BUG_ON(!ni->runlist.rl && !ni->mft_no && !NInoAttr(ni));

  	blocksize = vol->sb->s_blocksize;
  	blocksize_bits = vol->sb->s_blocksize_bits;

  	if (!page_has_buffers(page)) {

  		create_empty_buffers(page, blocksize, 0);

  		if (unlikely(!page_has_buffers(page))) {
  			unlock_page(page);
  			return -ENOMEM;
  		}

  	}

  	bh = head = page_buffers(page);
  	BUG_ON(!bh);

  	/*
  	 * We may be racing with truncate.  To avoid some of the problems we
  	 * now take a snapshot of the various sizes and use those for the whole
  	 * of the function.  In case of an extending truncate it just means we
  	 * may leave some buffers unmapped which are now allocated.  This is
  	 * not a problem since these buffers will just get mapped when a write
  	 * occurs.  In case of a shrinking truncate, we will detect this later
  	 * on due to the runlist being incomplete and if the page is being
  	 * fully truncated, truncate will throw it away as soon as we unlock
  	 * it so no need to worry what we do with it.
  	 */

  	iblock = (s64)page->index << (PAGE_CACHE_SHIFT - blocksize_bits);

  	read_lock_irqsave(&ni->size_lock, flags);

  	lblock = (ni->allocated_size + blocksize - 1) >> blocksize_bits;

  	init_size = ni->initialized_size;
  	i_size = i_size_read(vi);

  	read_unlock_irqrestore(&ni->size_lock, flags);

  	if (unlikely(init_size > i_size)) {
  		/* Race with shrinking truncate. */
  		init_size = i_size;
  	}
  	zblock = (init_size + blocksize - 1) >> blocksize_bits;

  
  	/* Loop through all the buffers in the page. */
  	rl = NULL;
  	nr = i = 0;
  	do {

  		int err = 0;

  
  		if (unlikely(buffer_uptodate(bh)))
  			continue;
  		if (unlikely(buffer_mapped(bh))) {
  			arr[nr++] = bh;
  			continue;
  		}
  		bh->b_bdev = vol->sb->s_bdev;
  		/* Is the block within the allowed limits? */
  		if (iblock < lblock) {

  			bool is_retry = false;

  
  			/* Convert iblock into corresponding vcn and offset. */
  			vcn = (VCN)iblock << blocksize_bits >>
  					vol->cluster_size_bits;
  			vcn_ofs = ((VCN)iblock << blocksize_bits) &
  					vol->cluster_size_mask;
  			if (!rl) {
  lock_retry_remap:
  				down_read(&ni->runlist.lock);
  				rl = ni->runlist.rl;
  			}
  			if (likely(rl != NULL)) {
  				/* Seek to element containing target vcn. */
  				while (rl->length && rl[1].vcn <= vcn)
  					rl++;
  				lcn = ntfs_rl_vcn_to_lcn(rl, vcn);
  			} else
  				lcn = LCN_RL_NOT_MAPPED;
  			/* Successful remap. */
  			if (lcn >= 0) {
  				/* Setup buffer head to correct block. */
  				bh->b_blocknr = ((lcn << vol->cluster_size_bits)
  						+ vcn_ofs) >> blocksize_bits;
  				set_buffer_mapped(bh);
  				/* Only read initialized data blocks. */
  				if (iblock < zblock) {
  					arr[nr++] = bh;
  					continue;
  				}
  				/* Fully non-initialized data block, zero it. */
  				goto handle_zblock;
  			}
  			/* It is a hole, need to zero it. */
  			if (lcn == LCN_HOLE)
  				goto handle_hole;
  			/* If first try and runlist unmapped, map and retry. */
  			if (!is_retry && lcn == LCN_RL_NOT_MAPPED) {

  				is_retry = true;

  				/*
  				 * Attempt to map runlist, dropping lock for
  				 * the duration.
  				 */
  				up_read(&ni->runlist.lock);
  				err = ntfs_map_runlist(ni, vcn);
  				if (likely(!err))
  					goto lock_retry_remap;
  				rl = NULL;

  			} else if (!rl)
  				up_read(&ni->runlist.lock);

  			/*
  			 * If buffer is outside the runlist, treat it as a
  			 * hole.  This can happen due to concurrent truncate
  			 * for example.
  			 */
  			if (err == -ENOENT || lcn == LCN_ENOENT) {
  				err = 0;
  				goto handle_hole;
  			}

  			/* Hard error, zero out region. */

  			if (!err)
  				err = -EIO;

  			bh->b_blocknr = -1;
  			SetPageError(page);
  			ntfs_error(vol->sb, "Failed to read from inode 0x%lx, "
  					"attribute type 0x%x, vcn 0x%llx, "
  					"offset 0x%x because its location on "
  					"disk could not be determined%s "

  					"(error code %i).", ni->mft_no,

  					ni->type, (unsigned long long)vcn,
  					vcn_ofs, is_retry ? " even after "

  					"retrying" : "", err);

  		}
  		/*
  		 * Either iblock was outside lblock limits or
  		 * ntfs_rl_vcn_to_lcn() returned error.  Just zero that portion
  		 * of the page and set the buffer uptodate.
  		 */
  handle_hole:
  		bh->b_blocknr = -1UL;
  		clear_buffer_mapped(bh);
  handle_zblock:

  		zero_user(page, i * blocksize, blocksize);

  		if (likely(!err))
  			set_buffer_uptodate(bh);

  	} while (i++, iblock++, (bh = bh->b_this_page) != head);
  
  	/* Release the lock if we took it. */
  	if (rl)
  		up_read(&ni->runlist.lock);
  
  	/* Check we have at least one buffer ready for i/o. */
  	if (nr) {
  		struct buffer_head *tbh;
  
  		/* Lock the buffers. */
  		for (i = 0; i < nr; i++) {
  			tbh = arr[i];
  			lock_buffer(tbh);
  			tbh->b_end_io = ntfs_end_buffer_async_read;
  			set_buffer_async_read(tbh);
  		}
  		/* Finally, start i/o on the buffers. */
  		for (i = 0; i < nr; i++) {
  			tbh = arr[i];
  			if (likely(!buffer_uptodate(tbh)))
  				submit_bh(READ, tbh);
  			else
  				ntfs_end_buffer_async_read(tbh, 1);
  		}
  		return 0;
  	}
  	/* No i/o was scheduled on any of the buffers. */
  	if (likely(!PageError(page)))
  		SetPageUptodate(page);
  	else /* Signal synchronous i/o error. */
  		nr = -EIO;
  	unlock_page(page);
  	return nr;
  }
  
  /**
   * ntfs_readpage - fill a @page of a @file with data from the device
   * @file:	open file to which the page @page belongs or NULL
   * @page:	page cache page to fill with data
   *
   * For non-resident attributes, ntfs_readpage() fills the @page of the open
   * file @file by calling the ntfs version of the generic block_read_full_page()
   * function, ntfs_read_block(), which in turn creates and reads in the buffers
   * associated with the page asynchronously.
   *
   * For resident attributes, OTOH, ntfs_readpage() fills @page by copying the
   * data from the mft record (which at this stage is most likely in memory) and
   * fills the remainder with zeroes. Thus, in this case, I/O is synchronous, as
   * even if the mft record is not cached at this point in time, we need to wait
   * for it to be read in before we can do the copy.
   *
   * Return 0 on success and -errno on error.
   */
  static int ntfs_readpage(struct file *file, struct page *page)
  {

  	loff_t i_size;
  	struct inode *vi;

  	ntfs_inode *ni, *base_ni;

  	u8 *addr;

  	ntfs_attr_search_ctx *ctx;
  	MFT_RECORD *mrec;

  	unsigned long flags;

  	u32 attr_len;
  	int err = 0;

  retry_readpage:

  	BUG_ON(!PageLocked(page));

  	vi = page->mapping->host;
  	i_size = i_size_read(vi);
  	/* Is the page fully outside i_size? (truncate in progress) */
  	if (unlikely(page->index >= (i_size + PAGE_CACHE_SIZE - 1) >>
  			PAGE_CACHE_SHIFT)) {

  		zero_user(page, 0, PAGE_CACHE_SIZE);

  		ntfs_debug("Read outside i_size - truncated?");
  		goto done;
  	}

  	/*
  	 * This can potentially happen because we clear PageUptodate() during
  	 * ntfs_writepage() of MstProtected() attributes.
  	 */
  	if (PageUptodate(page)) {
  		unlock_page(page);
  		return 0;
  	}

  	ni = NTFS_I(vi);

  	/*
  	 * Only $DATA attributes can be encrypted and only unnamed $DATA
  	 * attributes can be compressed.  Index root can have the flags set but
  	 * this means to create compressed/encrypted files, not that the

  	 * attribute is compressed/encrypted.  Note we need to check for
  	 * AT_INDEX_ALLOCATION since this is the type of both directory and
  	 * index inodes.

  	 */

  	if (ni->type != AT_INDEX_ALLOCATION) {

  		/* If attribute is encrypted, deny access, just like NT4. */
  		if (NInoEncrypted(ni)) {
  			BUG_ON(ni->type != AT_DATA);
  			err = -EACCES;
  			goto err_out;
  		}
  		/* Compressed data streams are handled in compress.c. */
  		if (NInoNonResident(ni) && NInoCompressed(ni)) {
  			BUG_ON(ni->type != AT_DATA);
  			BUG_ON(ni->name_len);
  			return ntfs_read_compressed_block(page);
  		}
  	}

  	/* NInoNonResident() == NInoIndexAllocPresent() */
  	if (NInoNonResident(ni)) {

  		/* Normal, non-resident data stream. */

  		return ntfs_read_block(page);
  	}
  	/*
  	 * Attribute is resident, implying it is not compressed or encrypted.
  	 * This also means the attribute is smaller than an mft record and
  	 * hence smaller than a page, so can simply zero out any pages with

  	 * index above 0.  Note the attribute can actually be marked compressed
  	 * but if it is resident the actual data is not compressed so we are
  	 * ok to ignore the compressed flag here.

  	 */

  	if (unlikely(page->index > 0)) {

  		zero_user(page, 0, PAGE_CACHE_SIZE);

  		goto done;
  	}
  	if (!NInoAttr(ni))
  		base_ni = ni;
  	else
  		base_ni = ni->ext.base_ntfs_ino;
  	/* Map, pin, and lock the mft record. */
  	mrec = map_mft_record(base_ni);
  	if (IS_ERR(mrec)) {
  		err = PTR_ERR(mrec);
  		goto err_out;
  	}

  	/*
  	 * If a parallel write made the attribute non-resident, drop the mft
  	 * record and retry the readpage.
  	 */
  	if (unlikely(NInoNonResident(ni))) {
  		unmap_mft_record(base_ni);
  		goto retry_readpage;
  	}

  	ctx = ntfs_attr_get_search_ctx(base_ni, mrec);
  	if (unlikely(!ctx)) {
  		err = -ENOMEM;
  		goto unm_err_out;
  	}
  	err = ntfs_attr_lookup(ni->type, ni->name, ni->name_len,
  			CASE_SENSITIVE, 0, NULL, 0, ctx);
  	if (unlikely(err))
  		goto put_unm_err_out;
  	attr_len = le32_to_cpu(ctx->attr->data.resident.value_length);

  	read_lock_irqsave(&ni->size_lock, flags);
  	if (unlikely(attr_len > ni->initialized_size))
  		attr_len = ni->initialized_size;

  	i_size = i_size_read(vi);

  	read_unlock_irqrestore(&ni->size_lock, flags);

  	if (unlikely(attr_len > i_size)) {
  		/* Race with shrinking truncate. */
  		attr_len = i_size;
  	}

  	addr = kmap_atomic(page, KM_USER0);

  	/* Copy the data to the page. */

  	memcpy(addr, (u8*)ctx->attr +

  			le16_to_cpu(ctx->attr->data.resident.value_offset),
  			attr_len);
  	/* Zero the remainder of the page. */

  	memset(addr + attr_len, 0, PAGE_CACHE_SIZE - attr_len);

  	flush_dcache_page(page);

  	kunmap_atomic(addr, KM_USER0);

  put_unm_err_out:
  	ntfs_attr_put_search_ctx(ctx);
  unm_err_out:
  	unmap_mft_record(base_ni);
  done:
  	SetPageUptodate(page);
  err_out:
  	unlock_page(page);
  	return err;
  }
  
  #ifdef NTFS_RW
  
  /**
   * ntfs_write_block - write a @page to the backing store
   * @page:	page cache page to write out
   * @wbc:	writeback control structure
   *
   * This function is for writing pages belonging to non-resident, non-mst
   * protected attributes to their backing store.
   *
   * For a page with buffers, map and write the dirty buffers asynchronously
   * under page writeback. For a page without buffers, create buffers for the
   * page, then proceed as above.
   *
   * If a page doesn't have buffers the page dirty state is definitive. If a page
   * does have buffers, the page dirty state is just a hint, and the buffer dirty
   * state is definitive. (A hint which has rules: dirty buffers against a clean
   * page is illegal. Other combinations are legal and need to be handled. In
   * particular a dirty page containing clean buffers for example.)
   *
   * Return 0 on success and -errno on error.
   *
   * Based on ntfs_read_block() and __block_write_full_page().
   */
  static int ntfs_write_block(struct page *page, struct writeback_control *wbc)
  {
  	VCN vcn;
  	LCN lcn;

  	s64 initialized_size;
  	loff_t i_size;

  	sector_t block, dblock, iblock;
  	struct inode *vi;
  	ntfs_inode *ni;
  	ntfs_volume *vol;
  	runlist_element *rl;
  	struct buffer_head *bh, *head;

  	unsigned long flags;

  	unsigned int blocksize, vcn_ofs;
  	int err;

  	bool need_end_writeback;

  	unsigned char blocksize_bits;
  
  	vi = page->mapping->host;
  	ni = NTFS_I(vi);
  	vol = ni->vol;
  
  	ntfs_debug("Entering for inode 0x%lx, attribute type 0x%x, page index "
  			"0x%lx.", ni->mft_no, ni->type, page->index);
  
  	BUG_ON(!NInoNonResident(ni));
  	BUG_ON(NInoMstProtected(ni));

  	blocksize = vol->sb->s_blocksize;
  	blocksize_bits = vol->sb->s_blocksize_bits;

  	if (!page_has_buffers(page)) {
  		BUG_ON(!PageUptodate(page));
  		create_empty_buffers(page, blocksize,
  				(1 << BH_Uptodate) | (1 << BH_Dirty));

  		if (unlikely(!page_has_buffers(page))) {
  			ntfs_warning(vol->sb, "Error allocating page "
  					"buffers.  Redirtying page so we try "
  					"again later.");
  			/*
  			 * Put the page back on mapping->dirty_pages, but leave
  			 * its buffers' dirty state as-is.
  			 */
  			redirty_page_for_writepage(wbc, page);
  			unlock_page(page);
  			return 0;
  		}

  	}
  	bh = head = page_buffers(page);

  	BUG_ON(!bh);

  
  	/* NOTE: Different naming scheme to ntfs_read_block()! */
  
  	/* The first block in the page. */
  	block = (s64)page->index << (PAGE_CACHE_SHIFT - blocksize_bits);

  	read_lock_irqsave(&ni->size_lock, flags);
  	i_size = i_size_read(vi);
  	initialized_size = ni->initialized_size;
  	read_unlock_irqrestore(&ni->size_lock, flags);

  	/* The first out of bounds block for the data size. */

  	dblock = (i_size + blocksize - 1) >> blocksize_bits;

  
  	/* The last (fully or partially) initialized block. */

  	iblock = initialized_size >> blocksize_bits;

  
  	/*
  	 * 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.
  	 */
  
  	/*
  	 * Loop through all the buffers in the page, mapping all the dirty
  	 * buffers to disk addresses and handling any aliases from the
  	 * underlying block device's mapping.
  	 */
  	rl = NULL;
  	err = 0;
  	do {

  		bool is_retry = false;

  
  		if (unlikely(block >= dblock)) {
  			/*
  			 * Mapped buffers outside i_size will occur, because
  			 * this page can be outside i_size when there is a
  			 * truncate in progress. The contents of such buffers
  			 * were zeroed by ntfs_writepage().
  			 *
  			 * FIXME: What about the small race window where
  			 * ntfs_writepage() has not done any clearing because
  			 * the page was within i_size but before we get here,
  			 * vmtruncate() modifies i_size?
  			 */
  			clear_buffer_dirty(bh);
  			set_buffer_uptodate(bh);
  			continue;
  		}
  
  		/* Clean buffers are not written out, so no need to map them. */
  		if (!buffer_dirty(bh))
  			continue;
  
  		/* Make sure we have enough initialized size. */
  		if (unlikely((block >= iblock) &&

  				(initialized_size < i_size))) {

  			/*
  			 * If this page is fully outside initialized size, zero
  			 * out all pages between the current initialized size
  			 * and the current page. Just use ntfs_readpage() to do
  			 * the zeroing transparently.
  			 */
  			if (block > iblock) {
  				// TODO:
  				// For each page do:
  				// - read_cache_page()
  				// Again for each page do:
  				// - wait_on_page_locked()
  				// - Check (PageUptodate(page) &&
  				//			!PageError(page))
  				// Update initialized size in the attribute and
  				// in the inode.
  				// Again, for each page do:
  				//	__set_page_dirty_buffers();
  				// page_cache_release()
  				// We don't need to wait on the writes.
  				// Update iblock.
  			}
  			/*
  			 * The current page straddles initialized size. Zero
  			 * all non-uptodate buffers and set them uptodate (and
  			 * dirty?). Note, there aren't any non-uptodate buffers
  			 * if the page is uptodate.
  			 * FIXME: For an uptodate page, the buffers may need to
  			 * be written out because they were not initialized on
  			 * disk before.
  			 */
  			if (!PageUptodate(page)) {
  				// TODO:
  				// Zero any non-uptodate buffers up to i_size.
  				// Set them uptodate and dirty.
  			}
  			// TODO:
  			// Update initialized size in the attribute and in the
  			// inode (up to i_size).
  			// Update iblock.
  			// FIXME: This is inefficient. Try to batch the two
  			// size changes to happen in one go.
  			ntfs_error(vol->sb, "Writing beyond initialized size "
  					"is not supported yet. Sorry.");
  			err = -EOPNOTSUPP;
  			break;
  			// Do NOT set_buffer_new() BUT DO clear buffer range
  			// outside write request range.
  			// set_buffer_uptodate() on complete buffers as well as
  			// set_buffer_dirty().
  		}
  
  		/* No need to map buffers that are already mapped. */
  		if (buffer_mapped(bh))
  			continue;
  
  		/* Unmapped, dirty buffer. Need to map it. */
  		bh->b_bdev = vol->sb->s_bdev;
  
  		/* Convert block into corresponding vcn and offset. */
  		vcn = (VCN)block << blocksize_bits;
  		vcn_ofs = vcn & vol->cluster_size_mask;
  		vcn >>= vol->cluster_size_bits;
  		if (!rl) {
  lock_retry_remap:
  			down_read(&ni->runlist.lock);
  			rl = ni->runlist.rl;
  		}
  		if (likely(rl != NULL)) {
  			/* Seek to element containing target vcn. */
  			while (rl->length && rl[1].vcn <= vcn)
  				rl++;
  			lcn = ntfs_rl_vcn_to_lcn(rl, vcn);
  		} else
  			lcn = LCN_RL_NOT_MAPPED;
  		/* Successful remap. */
  		if (lcn >= 0) {
  			/* Setup buffer head to point to correct block. */
  			bh->b_blocknr = ((lcn << vol->cluster_size_bits) +
  					vcn_ofs) >> blocksize_bits;
  			set_buffer_mapped(bh);
  			continue;
  		}
  		/* It is a hole, need to instantiate it. */
  		if (lcn == LCN_HOLE) {

  			u8 *kaddr;
  			unsigned long *bpos, *bend;
  
  			/* Check if the buffer is zero. */
  			kaddr = kmap_atomic(page, KM_USER0);
  			bpos = (unsigned long *)(kaddr + bh_offset(bh));
  			bend = (unsigned long *)((u8*)bpos + blocksize);
  			do {
  				if (unlikely(*bpos))
  					break;
  			} while (likely(++bpos < bend));
  			kunmap_atomic(kaddr, KM_USER0);
  			if (bpos == bend) {
  				/*
  				 * Buffer is zero and sparse, no need to write
  				 * it.
  				 */
  				bh->b_blocknr = -1;
  				clear_buffer_dirty(bh);
  				continue;
  			}

  			// TODO: Instantiate the hole.
  			// clear_buffer_new(bh);
  			// unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);
  			ntfs_error(vol->sb, "Writing into sparse regions is "
  					"not supported yet. Sorry.");
  			err = -EOPNOTSUPP;
  			break;
  		}
  		/* If first try and runlist unmapped, map and retry. */
  		if (!is_retry && lcn == LCN_RL_NOT_MAPPED) {

  			is_retry = true;

  			/*
  			 * Attempt to map runlist, dropping lock for
  			 * the duration.
  			 */
  			up_read(&ni->runlist.lock);
  			err = ntfs_map_runlist(ni, vcn);
  			if (likely(!err))
  				goto lock_retry_remap;
  			rl = NULL;

  		} else if (!rl)
  			up_read(&ni->runlist.lock);

  		/*
  		 * If buffer is outside the runlist, truncate has cut it out
  		 * of the runlist.  Just clean and clear the buffer and set it
  		 * uptodate so it can get discarded by the VM.
  		 */
  		if (err == -ENOENT || lcn == LCN_ENOENT) {

  			bh->b_blocknr = -1;
  			clear_buffer_dirty(bh);

  			zero_user(page, bh_offset(bh), blocksize);

  			set_buffer_uptodate(bh);
  			err = 0;
  			continue;
  		}

  		/* Failed to map the buffer, even after retrying. */

  		if (!err)
  			err = -EIO;

  		bh->b_blocknr = -1;
  		ntfs_error(vol->sb, "Failed to write to inode 0x%lx, "
  				"attribute type 0x%x, vcn 0x%llx, offset 0x%x "
  				"because its location on disk could not be "

  				"determined%s (error code %i).", ni->mft_no,

  				ni->type, (unsigned long long)vcn,
  				vcn_ofs, is_retry ? " even after "

  				"retrying" : "", err);

  		break;
  	} while (block++, (bh = bh->b_this_page) != head);
  
  	/* Release the lock if we took it. */
  	if (rl)
  		up_read(&ni->runlist.lock);
  
  	/* For the error case, need to reset bh to the beginning. */
  	bh = head;

  	/* Just an optimization, so ->readpage() is not called later. */

  	if (unlikely(!PageUptodate(page))) {
  		int uptodate = 1;
  		do {
  			if (!buffer_uptodate(bh)) {
  				uptodate = 0;
  				bh = head;
  				break;
  			}
  		} while ((bh = bh->b_this_page) != head);
  		if (uptodate)
  			SetPageUptodate(page);
  	}
  
  	/* Setup all mapped, dirty buffers for async write i/o. */
  	do {

  		if (buffer_mapped(bh) && buffer_dirty(bh)) {
  			lock_buffer(bh);
  			if (test_clear_buffer_dirty(bh)) {
  				BUG_ON(!buffer_uptodate(bh));
  				mark_buffer_async_write(bh);
  			} else
  				unlock_buffer(bh);
  		} else if (unlikely(err)) {
  			/*
  			 * For the error case. The buffer may have been set
  			 * dirty during attachment to a dirty page.
  			 */
  			if (err != -ENOMEM)
  				clear_buffer_dirty(bh);
  		}
  	} while ((bh = bh->b_this_page) != head);
  
  	if (unlikely(err)) {
  		// TODO: Remove the -EOPNOTSUPP check later on...
  		if (unlikely(err == -EOPNOTSUPP))
  			err = 0;
  		else if (err == -ENOMEM) {
  			ntfs_warning(vol->sb, "Error allocating memory. "
  					"Redirtying page so we try again "
  					"later.");
  			/*
  			 * Put the page back on mapping->dirty_pages, but
  			 * leave its buffer's dirty state as-is.
  			 */
  			redirty_page_for_writepage(wbc, page);
  			err = 0;
  		} else
  			SetPageError(page);
  	}
  
  	BUG_ON(PageWriteback(page));
  	set_page_writeback(page);	/* Keeps try_to_free_buffers() away. */

  	/* Submit the prepared buffers for i/o. */

  	need_end_writeback = true;

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

  			need_end_writeback = false;

  		}

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

  	unlock_page(page);

  
  	/* If no i/o was started, need to end_page_writeback(). */
  	if (unlikely(need_end_writeback))
  		end_page_writeback(page);
  
  	ntfs_debug("Done.");
  	return err;
  }
  
  /**
   * ntfs_write_mst_block - write a @page to the backing store
   * @page:	page cache page to write out
   * @wbc:	writeback control structure
   *
   * This function is for writing pages belonging to non-resident, mst protected
   * attributes to their backing store.  The only supported attributes are index
   * allocation and $MFT/$DATA.  Both directory inodes and index inodes are
   * supported for the index allocation case.
   *
   * The page must remain locked for the duration of the write because we apply
   * the mst fixups, write, and then undo the fixups, so if we were to unlock the
   * page before undoing the fixups, any other user of the page will see the
   * page contents as corrupt.
   *
   * We clear the page uptodate flag for the duration of the function to ensure
   * exclusion for the $MFT/$DATA case against someone mapping an mft record we
   * are about to apply the mst fixups to.
   *
   * Return 0 on success and -errno on error.
   *
   * Based on ntfs_write_block(), ntfs_mft_writepage(), and
   * write_mft_record_nolock().
   */
  static int ntfs_write_mst_block(struct page *page,
  		struct writeback_control *wbc)
  {
  	sector_t block, dblock, rec_block;
  	struct inode *vi = page->mapping->host;
  	ntfs_inode *ni = NTFS_I(vi);
  	ntfs_volume *vol = ni->vol;
  	u8 *kaddr;

  	unsigned int rec_size = ni->itype.index.block_size;
  	ntfs_inode *locked_nis[PAGE_CACHE_SIZE / rec_size];
  	struct buffer_head *bh, *head, *tbh, *rec_start_bh;

  	struct buffer_head *bhs[MAX_BUF_PER_PAGE];

  	runlist_element *rl;

  	int i, nr_locked_nis, nr_recs, nr_bhs, max_bhs, bhs_per_rec, err, err2;
  	unsigned bh_size, rec_size_bits;

  	bool sync, is_mft, page_is_dirty, rec_is_dirty;

  	unsigned char bh_size_bits;

  
  	ntfs_debug("Entering for inode 0x%lx, attribute type 0x%x, page index "
  			"0x%lx.", vi->i_ino, ni->type, page->index);
  	BUG_ON(!NInoNonResident(ni));
  	BUG_ON(!NInoMstProtected(ni));
  	is_mft = (S_ISREG(vi->i_mode) && !vi->i_ino);
  	/*
  	 * NOTE: ntfs_write_mst_block() would be called for $MFTMirr if a page
  	 * in its page cache were to be marked dirty.  However this should
  	 * never happen with the current driver and considering we do not
  	 * handle this case here we do want to BUG(), at least for now.
  	 */
  	BUG_ON(!(is_mft || S_ISDIR(vi->i_mode) ||
  			(NInoAttr(ni) && ni->type == AT_INDEX_ALLOCATION)));

  	bh_size = vol->sb->s_blocksize;
  	bh_size_bits = vol->sb->s_blocksize_bits;

  	max_bhs = PAGE_CACHE_SIZE / bh_size;

  	BUG_ON(!max_bhs);

  	BUG_ON(max_bhs > MAX_BUF_PER_PAGE);

  
  	/* Were we called for sync purposes? */
  	sync = (wbc->sync_mode == WB_SYNC_ALL);
  
  	/* Make sure we have mapped buffers. */

  	bh = head = page_buffers(page);
  	BUG_ON(!bh);
  
  	rec_size_bits = ni->itype.index.block_size_bits;
  	BUG_ON(!(PAGE_CACHE_SIZE >> rec_size_bits));
  	bhs_per_rec = rec_size >> bh_size_bits;
  	BUG_ON(!bhs_per_rec);
  
  	/* The first block in the page. */
  	rec_block = block = (sector_t)page->index <<
  			(PAGE_CACHE_SHIFT - bh_size_bits);
  
  	/* The first out of bounds block for the data size. */

  	dblock = (i_size_read(vi) + bh_size - 1) >> bh_size_bits;

  
  	rl = NULL;
  	err = err2 = nr_bhs = nr_recs = nr_locked_nis = 0;

  	page_is_dirty = rec_is_dirty = false;

  	rec_start_bh = NULL;
  	do {

  		bool is_retry = false;

  
  		if (likely(block < rec_block)) {
  			if (unlikely(block >= dblock)) {
  				clear_buffer_dirty(bh);

  				set_buffer_uptodate(bh);

  				continue;
  			}
  			/*
  			 * This block is not the first one in the record.  We
  			 * ignore the buffer's dirty state because we could
  			 * have raced with a parallel mark_ntfs_record_dirty().
  			 */
  			if (!rec_is_dirty)
  				continue;
  			if (unlikely(err2)) {
  				if (err2 != -ENOMEM)
  					clear_buffer_dirty(bh);
  				continue;
  			}
  		} else /* if (block == rec_block) */ {
  			BUG_ON(block > rec_block);
  			/* This block is the first one in the record. */
  			rec_block += bhs_per_rec;
  			err2 = 0;
  			if (unlikely(block >= dblock)) {
  				clear_buffer_dirty(bh);
  				continue;
  			}
  			if (!buffer_dirty(bh)) {
  				/* Clean records are not written out. */

  				rec_is_dirty = false;

  				continue;
  			}

  			rec_is_dirty = true;

  			rec_start_bh = bh;
  		}
  		/* Need to map the buffer if it is not mapped already. */
  		if (unlikely(!buffer_mapped(bh))) {
  			VCN vcn;
  			LCN lcn;
  			unsigned int vcn_ofs;

  			bh->b_bdev = vol->sb->s_bdev;

  			/* Obtain the vcn and offset of the current block. */
  			vcn = (VCN)block << bh_size_bits;
  			vcn_ofs = vcn & vol->cluster_size_mask;
  			vcn >>= vol->cluster_size_bits;
  			if (!rl) {
  lock_retry_remap:
  				down_read(&ni->runlist.lock);
  				rl = ni->runlist.rl;
  			}
  			if (likely(rl != NULL)) {
  				/* Seek to element containing target vcn. */
  				while (rl->length && rl[1].vcn <= vcn)
  					rl++;
  				lcn = ntfs_rl_vcn_to_lcn(rl, vcn);
  			} else
  				lcn = LCN_RL_NOT_MAPPED;
  			/* Successful remap. */
  			if (likely(lcn >= 0)) {
  				/* Setup buffer head to correct block. */
  				bh->b_blocknr = ((lcn <<
  						vol->cluster_size_bits) +
  						vcn_ofs) >> bh_size_bits;
  				set_buffer_mapped(bh);
  			} else {
  				/*
  				 * Remap failed.  Retry to map the runlist once
  				 * unless we are working on $MFT which always
  				 * has the whole of its runlist in memory.
  				 */
  				if (!is_mft && !is_retry &&
  						lcn == LCN_RL_NOT_MAPPED) {

  					is_retry = true;

  					/*
  					 * Attempt to map runlist, dropping
  					 * lock for the duration.
  					 */
  					up_read(&ni->runlist.lock);
  					err2 = ntfs_map_runlist(ni, vcn);
  					if (likely(!err2))
  						goto lock_retry_remap;
  					if (err2 == -ENOMEM)

  						page_is_dirty = true;

  					lcn = err2;

  				} else {

  					err2 = -EIO;

  					if (!rl)
  						up_read(&ni->runlist.lock);
  				}

  				/* Hard error.  Abort writing this record. */
  				if (!err || err == -ENOMEM)
  					err = err2;
  				bh->b_blocknr = -1;
  				ntfs_error(vol->sb, "Cannot write ntfs record "
  						"0x%llx (inode 0x%lx, "
  						"attribute type 0x%x) because "
  						"its location on disk could "
  						"not be determined (error "

  						"code %lli).",
  						(long long)block <<

  						bh_size_bits >>
  						vol->mft_record_size_bits,
  						ni->mft_no, ni->type,
  						(long long)lcn);
  				/*
  				 * If this is not the first buffer, remove the
  				 * buffers in this record from the list of
  				 * buffers to write and clear their dirty bit
  				 * if not error -ENOMEM.
  				 */
  				if (rec_start_bh != bh) {
  					while (bhs[--nr_bhs] != rec_start_bh)
  						;
  					if (err2 != -ENOMEM) {
  						do {
  							clear_buffer_dirty(
  								rec_start_bh);
  						} while ((rec_start_bh =
  								rec_start_bh->
  								b_this_page) !=
  								bh);
  					}
  				}
  				continue;
  			}
  		}
  		BUG_ON(!buffer_uptodate(bh));
  		BUG_ON(nr_bhs >= max_bhs);
  		bhs[nr_bhs++] = bh;
  	} while (block++, (bh = bh->b_this_page) != head);
  	if (unlikely(rl))
  		up_read(&ni->runlist.lock);
  	/* If there were no dirty buffers, we are done. */
  	if (!nr_bhs)
  		goto done;
  	/* Map the page so we can access its contents. */
  	kaddr = kmap(page);
  	/* Clear the page uptodate flag whilst the mst fixups are applied. */
  	BUG_ON(!PageUptodate(page));
  	ClearPageUptodate(page);
  	for (i = 0; i < nr_bhs; i++) {
  		unsigned int ofs;
  
  		/* Skip buffers which are not at the beginning of records. */
  		if (i % bhs_per_rec)
  			continue;
  		tbh = bhs[i];
  		ofs = bh_offset(tbh);
  		if (is_mft) {
  			ntfs_inode *tni;
  			unsigned long mft_no;
  
  			/* Get the mft record number. */
  			mft_no = (((s64)page->index << PAGE_CACHE_SHIFT) + ofs)
  					>> rec_size_bits;
  			/* Check whether to write this mft record. */
  			tni = NULL;
  			if (!ntfs_may_write_mft_record(vol, mft_no,
  					(MFT_RECORD*)(kaddr + ofs), &tni)) {
  				/*
  				 * The record should not be written.  This
  				 * means we need to redirty the page before
  				 * returning.
  				 */

  				page_is_dirty = true;

  				/*
  				 * Remove the buffers in this mft record from
  				 * the list of buffers to write.
  				 */
  				do {
  					bhs[i] = NULL;
  				} while (++i % bhs_per_rec);
  				continue;
  			}
  			/*
  			 * The record should be written.  If a locked ntfs
  			 * inode was returned, add it to the array of locked
  			 * ntfs inodes.
  			 */
  			if (tni)
  				locked_nis[nr_locked_nis++] = tni;
  		}
  		/* Apply the mst protection fixups. */
  		err2 = pre_write_mst_fixup((NTFS_RECORD*)(kaddr + ofs),
  				rec_size);
  		if (unlikely(err2)) {
  			if (!err || err == -ENOMEM)
  				err = -EIO;
  			ntfs_error(vol->sb, "Failed to apply mst fixups "
  					"(inode 0x%lx, attribute type 0x%x, "
  					"page index 0x%lx, page offset 0x%x)!"
  					"  Unmount and run chkdsk.", vi->i_ino,
  					ni->type, page->index, ofs);
  			/*
  			 * Mark all the buffers in this record clean as we do
  			 * not want to write corrupt data to disk.
  			 */
  			do {
  				clear_buffer_dirty(bhs[i]);
  				bhs[i] = NULL;
  			} while (++i % bhs_per_rec);
  			continue;
  		}
  		nr_recs++;
  	}
  	/* If no records are to be written out, we are done. */
  	if (!nr_recs)
  		goto unm_done;
  	flush_dcache_page(page);
  	/* Lock buffers and start synchronous write i/o on them. */
  	for (i = 0; i < nr_bhs; i++) {
  		tbh = bhs[i];
  		if (!tbh)
  			continue;

  		if (!trylock_buffer(tbh))

  			BUG();
  		/* The buffer dirty state is now irrelevant, just clean it. */
  		clear_buffer_dirty(tbh);
  		BUG_ON(!buffer_uptodate(tbh));
  		BUG_ON(!buffer_mapped(tbh));
  		get_bh(tbh);
  		tbh->b_end_io = end_buffer_write_sync;
  		submit_bh(WRITE, tbh);
  	}
  	/* Synchronize the mft mirror now if not @sync. */
  	if (is_mft && !sync)
  		goto do_mirror;
  do_wait:
  	/* Wait on i/o completion of buffers. */
  	for (i = 0; i < nr_bhs; i++) {
  		tbh = bhs[i];
  		if (!tbh)
  			continue;
  		wait_on_buffer(tbh);
  		if (unlikely(!buffer_uptodate(tbh))) {
  			ntfs_error(vol->sb, "I/O error while writing ntfs "
  					"record buffer (inode 0x%lx, "
  					"attribute type 0x%x, page index "
  					"0x%lx, page offset 0x%lx)!  Unmount "
  					"and run chkdsk.", vi->i_ino, ni->type,
  					page->index, bh_offset(tbh));
  			if (!err || err == -ENOMEM)
  				err = -EIO;
  			/*
  			 * Set the buffer uptodate so the page and buffer
  			 * states do not become out of sync.
  			 */
  			set_buffer_uptodate(tbh);
  		}
  	}
  	/* If @sync, now synchronize the mft mirror. */
  	if (is_mft && sync) {
  do_mirror:
  		for (i = 0; i < nr_bhs; i++) {
  			unsigned long mft_no;
  			unsigned int ofs;
  
  			/*
  			 * Skip buffers which are not at the beginning of
  			 * records.
  			 */
  			if (i % bhs_per_rec)
  				continue;
  			tbh = bhs[i];
  			/* Skip removed buffers (and hence records). */
  			if (!tbh)
  				continue;
  			ofs = bh_offset(tbh);
  			/* Get the mft record number. */
  			mft_no = (((s64)page->index << PAGE_CACHE_SHIFT) + ofs)
  					>> rec_size_bits;
  			if (mft_no < vol->mftmirr_size)
  				ntfs_sync_mft_mirror(vol, mft_no,
  						(MFT_RECORD*)(kaddr + ofs),
  						sync);
  		}
  		if (!sync)
  			goto do_wait;
  	}
  	/* Remove the mst protection fixups again. */
  	for (i = 0; i < nr_bhs; i++) {
  		if (!(i % bhs_per_rec)) {
  			tbh = bhs[i];
  			if (!tbh)
  				continue;
  			post_write_mst_fixup((NTFS_RECORD*)(kaddr +
  					bh_offset(tbh)));
  		}
  	}
  	flush_dcache_page(page);
  unm_done:
  	/* Unlock any locked inodes. */
  	while (nr_locked_nis-- > 0) {
  		ntfs_inode *tni, *base_tni;
  		
  		tni = locked_nis[nr_locked_nis];
  		/* Get the base inode. */

  		mutex_lock(&tni->extent_lock);

  		if (tni->nr_extents >= 0)
  			base_tni = tni;
  		else {
  			base_tni = tni->ext.base_ntfs_ino;
  			BUG_ON(!base_tni);
  		}

  		mutex_unlock(&tni->extent_lock);

  		ntfs_debug("Unlocking %s inode 0x%lx.",
  				tni == base_tni ? "base" : "extent",
  				tni->mft_no);

  		mutex_unlock(&tni->mrec_lock);

  		atomic_dec(&tni->count);
  		iput(VFS_I(base_tni));
  	}
  	SetPageUptodate(page);
  	kunmap(page);
  done:
  	if (unlikely(err && err != -ENOMEM)) {
  		/*
  		 * Set page error if there is only one ntfs record in the page.
  		 * Otherwise we would loose per-record granularity.
  		 */
  		if (ni->itype.index.block_size == PAGE_CACHE_SIZE)
  			SetPageError(page);
  		NVolSetErrors(vol);
  	}
  	if (page_is_dirty) {
  		ntfs_debug("Page still contains one or more dirty ntfs "
  				"records.  Redirtying the page starting at "
  				"record 0x%lx.", page->index <<
  				(PAGE_CACHE_SHIFT - rec_size_bits));
  		redirty_page_for_writepage(wbc, page);
  		unlock_page(page);
  	} else {
  		/*
  		 * Keep the VM happy.  This must be done otherwise the
  		 * radix-tree tag PAGECACHE_TAG_DIRTY remains set even though
  		 * the page is clean.
  		 */
  		BUG_ON(PageWriteback(page));
  		set_page_writeback(page);
  		unlock_page(page);
  		end_page_writeback(page);
  	}
  	if (likely(!err))
  		ntfs_debug("Done.");
  	return err;
  }
  
  /**
   * ntfs_writepage - write a @page to the backing store
   * @page:	page cache page to write out
   * @wbc:	writeback control structure
   *
   * This is called from the VM when it wants to have a dirty ntfs page cache
   * page cleaned.  The VM has already locked the page and marked it clean.
   *
   * For non-resident attributes, ntfs_writepage() writes the @page by calling
   * the ntfs version of the generic block_write_full_page() function,
   * ntfs_write_block(), which in turn if necessary creates and writes the
   * buffers associated with the page asynchronously.
   *
   * For resident attributes, OTOH, ntfs_writepage() writes the @page by copying
   * the data to the mft record (which at this stage is most likely in memory).
   * The mft record is then marked dirty and written out asynchronously via the
   * vfs inode dirty code path for the inode the mft record belongs to or via the
   * vm page dirty code path for the page the mft record is in.
   *
   * Based on ntfs_readpage() and fs/buffer.c::block_write_full_page().
   *
   * Return 0 on success and -errno on error.
   */
  static int ntfs_writepage(struct page *page, struct writeback_control *wbc)
  {
  	loff_t i_size;

  	struct inode *vi = page->mapping->host;
  	ntfs_inode *base_ni = NULL, *ni = NTFS_I(vi);

  	char *addr;

  	ntfs_attr_search_ctx *ctx = NULL;
  	MFT_RECORD *m = NULL;

  	u32 attr_len;
  	int err;

  retry_writepage:

  	BUG_ON(!PageLocked(page));

  	i_size = i_size_read(vi);

  	/* Is the page fully outside i_size? (truncate in progress) */
  	if (unlikely(page->index >= (i_size + PAGE_CACHE_SIZE - 1) >>
  			PAGE_CACHE_SHIFT)) {
  		/*
  		 * The page may have dirty, unmapped buffers.  Make them
  		 * freeable here, so the page does not leak.
  		 */
  		block_invalidatepage(page, 0);
  		unlock_page(page);
  		ntfs_debug("Write outside i_size - truncated?");
  		return 0;
  	}

  	/*
  	 * Only $DATA attributes can be encrypted and only unnamed $DATA
  	 * attributes can be compressed.  Index root can have the flags set but
  	 * this means to create compressed/encrypted files, not that the

  	 * attribute is compressed/encrypted.  Note we need to check for
  	 * AT_INDEX_ALLOCATION since this is the type of both directory and
  	 * index inodes.

  	 */

  	if (ni->type != AT_INDEX_ALLOCATION) {

  		/* If file is encrypted, deny access, just like NT4. */
  		if (NInoEncrypted(ni)) {
  			unlock_page(page);
  			BUG_ON(ni->type != AT_DATA);

  			ntfs_debug("Denying write access to encrypted file.");

  			return -EACCES;
  		}
  		/* Compressed data streams are handled in compress.c. */
  		if (NInoNonResident(ni) && NInoCompressed(ni)) {
  			BUG_ON(ni->type != AT_DATA);
  			BUG_ON(ni->name_len);
  			// TODO: Implement and replace this with
  			// return ntfs_write_compressed_block(page);
  			unlock_page(page);
  			ntfs_error(vi->i_sb, "Writing to compressed files is "
  					"not supported yet.  Sorry.");
  			return -EOPNOTSUPP;
  		}
  		// TODO: Implement and remove this check.
  		if (NInoNonResident(ni) && NInoSparse(ni)) {
  			unlock_page(page);
  			ntfs_error(vi->i_sb, "Writing to sparse files is not "
  					"supported yet.  Sorry.");
  			return -EOPNOTSUPP;
  		}
  	}

  	/* NInoNonResident() == NInoIndexAllocPresent() */
  	if (NInoNonResident(ni)) {

  		/* We have to zero every time due to mmap-at-end-of-file. */
  		if (page->index >= (i_size >> PAGE_CACHE_SHIFT)) {
  			/* The page straddles i_size. */
  			unsigned int ofs = i_size & ~PAGE_CACHE_MASK;

  			zero_user_segment(page, ofs, PAGE_CACHE_SIZE);

  		}
  		/* Handle mst protected attributes. */
  		if (NInoMstProtected(ni))
  			return ntfs_write_mst_block(page, wbc);

  		/* Normal, non-resident data stream. */

  		return ntfs_write_block(page, wbc);
  	}
  	/*

  	 * Attribute is resident, implying it is not compressed, encrypted, or
  	 * mst protected.  This also means the attribute is smaller than an mft
  	 * record and hence smaller than a page, so can simply return error on
  	 * any pages with index above 0.  Note the attribute can actually be
  	 * marked compressed but if it is resident the actual data is not
  	 * compressed so we are ok to ignore the compressed flag here.

  	 */
  	BUG_ON(page_has_buffers(page));
  	BUG_ON(!PageUptodate(page));
  	if (unlikely(page->index > 0)) {
  		ntfs_error(vi->i_sb, "BUG()! page->index (0x%lx) > 0.  "
  				"Aborting write.", page->index);
  		BUG_ON(PageWriteback(page));
  		set_page_writeback(page);
  		unlock_page(page);
  		end_page_writeback(page);
  		return -EIO;
  	}
  	if (!NInoAttr(ni))
  		base_ni = ni;
  	else
  		base_ni = ni->ext.base_ntfs_ino;
  	/* Map, pin, and lock the mft record. */
  	m = map_mft_record(base_ni);
  	if (IS_ERR(m)) {
  		err = PTR_ERR(m);
  		m = NULL;
  		ctx = NULL;
  		goto err_out;
  	}

  	/*
  	 * If a parallel write made the attribute non-resident, drop the mft
  	 * record and retry the writepage.
  	 */
  	if (unlikely(NInoNonResident(ni))) {
  		unmap_mft_record(base_ni);
  		goto retry_writepage;
  	}

  	ctx = ntfs_attr_get_search_ctx(base_ni, m);
  	if (unlikely(!ctx)) {
  		err = -ENOMEM;
  		goto err_out;
  	}
  	err = ntfs_attr_lookup(ni->type, ni->name, ni->name_len,
  			CASE_SENSITIVE, 0, NULL, 0, ctx);
  	if (unlikely(err))
  		goto err_out;
  	/*
  	 * Keep the VM happy.  This must be done otherwise the radix-tree tag
  	 * PAGECACHE_TAG_DIRTY remains set even though the page is clean.
  	 */
  	BUG_ON(PageWriteback(page));
  	set_page_writeback(page);
  	unlock_page(page);

  	attr_len = le32_to_cpu(ctx->attr->data.resident.value_length);

  	i_size = i_size_read(vi);

  	if (unlikely(attr_len > i_size)) {

  		/* Race with shrinking truncate or a failed truncate. */

  		attr_len = i_size;

  		/*
  		 * If the truncate failed, fix it up now.  If a concurrent
  		 * truncate, we do its job, so it does not have to do anything.
  		 */
  		err = ntfs_resident_attr_value_resize(ctx->mrec, ctx->attr,
  				attr_len);
  		/* Shrinking cannot fail. */
  		BUG_ON(err);

  	}

  	addr = kmap_atomic(page, KM_USER0);

  	/* Copy the data from the page to the mft record. */
  	memcpy((u8*)ctx->attr +
  			le16_to_cpu(ctx->attr->data.resident.value_offset),

  			addr, attr_len);

  	/* Zero out of bounds area in the page cache page. */

  	memset(addr + attr_len, 0, PAGE_CACHE_SIZE - attr_len);
  	kunmap_atomic(addr, KM_USER0);

  	flush_dcache_page(page);

  	flush_dcache_mft_record_page(ctx->ntfs_ino);

  	/* We are done with the page. */

  	end_page_writeback(page);

  	/* Finally, mark the mft record dirty, so it gets written back. */

  	mark_mft_record_dirty(ctx->ntfs_ino);
  	ntfs_attr_put_search_ctx(ctx);
  	unmap_mft_record(base_ni);
  	return 0;
  err_out:
  	if (err == -ENOMEM) {
  		ntfs_warning(vi->i_sb, "Error allocating memory. Redirtying "
  				"page so we try again later.");
  		/*
  		 * Put the page back on mapping->dirty_pages, but leave its
  		 * buffers' dirty state as-is.
  		 */
  		redirty_page_for_writepage(wbc, page);
  		err = 0;
  	} else {
  		ntfs_error(vi->i_sb, "Resident attribute write failed with "

  				"error %i.", err);

  		SetPageError(page);

  		NVolSetErrors(ni->vol);

  	}
  	unlock_page(page);
  	if (ctx)
  		ntfs_attr_put_search_ctx(ctx);
  	if (m)
  		unmap_mft_record(base_ni);
  	return err;
  }

  #endif	/* NTFS_RW */
  
  /**
   * ntfs_aops - general address space operations for inodes and attributes
   */

  const struct address_space_operations ntfs_aops = {

  	.readpage	= ntfs_readpage,	/* Fill page with data. */

  #ifdef NTFS_RW
  	.writepage	= ntfs_writepage,	/* Write dirty page to disk. */

  #endif /* NTFS_RW */

  	.migratepage	= buffer_migrate_page,	/* Move a page cache page from
  						   one physical page to an
  						   other. */

  	.error_remove_page = generic_error_remove_page,

  };
  
  /**
   * ntfs_mst_aops - general address space operations for mst protecteed inodes
   *		   and attributes
   */

  const struct address_space_operations ntfs_mst_aops = {

  	.readpage	= ntfs_readpage,	/* Fill page with data. */

  #ifdef NTFS_RW
  	.writepage	= ntfs_writepage,	/* Write dirty page to disk. */
  	.set_page_dirty	= __set_page_dirty_nobuffers,	/* Set the page dirty
  						   without touching the buffers
  						   belonging to the page. */
  #endif /* NTFS_RW */

  	.migratepage	= buffer_migrate_page,	/* Move a page cache page from
  						   one physical page to an
  						   other. */

  	.error_remove_page = generic_error_remove_page,

  };
  
  #ifdef NTFS_RW
  
  /**
   * mark_ntfs_record_dirty - mark an ntfs record dirty
   * @page:	page containing the ntfs record to mark dirty
   * @ofs:	byte offset within @page at which the ntfs record begins
   *
   * Set the buffers and the page in which the ntfs record is located dirty.
   *
   * The latter also marks the vfs inode the ntfs record belongs to dirty
   * (I_DIRTY_PAGES only).
   *
   * If the page does not have buffers, we create them and set them uptodate.
   * The page may not be locked which is why we need to handle the buffers under
   * the mapping->private_lock.  Once the buffers are marked dirty we no longer
   * need the lock since try_to_free_buffers() does not free dirty buffers.
   */
  void mark_ntfs_record_dirty(struct page *page, const unsigned int ofs) {
  	struct address_space *mapping = page->mapping;
  	ntfs_inode *ni = NTFS_I(mapping->host);
  	struct buffer_head *bh, *head, *buffers_to_free = NULL;
  	unsigned int end, bh_size, bh_ofs;
  
  	BUG_ON(!PageUptodate(page));
  	end = ofs + ni->itype.index.block_size;

  	bh_size = VFS_I(ni)->i_sb->s_blocksize;

  	spin_lock(&mapping->private_lock);
  	if (unlikely(!page_has_buffers(page))) {
  		spin_unlock(&mapping->private_lock);
  		bh = head = alloc_page_buffers(page, bh_size, 1);
  		spin_lock(&mapping->private_lock);
  		if (likely(!page_has_buffers(page))) {
  			struct buffer_head *tail;
  
  			do {
  				set_buffer_uptodate(bh);
  				tail = bh;
  				bh = bh->b_this_page;
  			} while (bh);
  			tail->b_this_page = head;
  			attach_page_buffers(page, head);
  		} else
  			buffers_to_free = bh;
  	}
  	bh = head = page_buffers(page);

  	BUG_ON(!bh);

  	do {
  		bh_ofs = bh_offset(bh);
  		if (bh_ofs + bh_size <= ofs)
  			continue;
  		if (unlikely(bh_ofs >= end))
  			break;
  		set_buffer_dirty(bh);
  	} while ((bh = bh->b_this_page) != head);
  	spin_unlock(&mapping->private_lock);
  	__set_page_dirty_nobuffers(page);
  	if (unlikely(buffers_to_free)) {
  		do {
  			bh = buffers_to_free->b_this_page;
  			free_buffer_head(buffers_to_free);
  			buffers_to_free = bh;
  		} while (buffers_to_free);
  	}
  }
  
  #endif /* NTFS_RW */