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aops.c

/**
 * 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/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;

                  ofs = 0;
                  if (file_ofs < init_size)
                        ofs = init_size - file_ofs;
                  local_irq_save(flags);
                  zero_user_page(page, bh_offset(bh) + ofs,
                               bh->b_size - ofs, 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(page, i * blocksize, blocksize, KM_USER0);
            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(page, 0, PAGE_CACHE_SIZE, KM_USER0);
            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(page, 0, PAGE_CACHE_SIZE, KM_USER0);
            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(page, bh_offset(bh), blocksize,
                              KM_USER0);
                  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 (unlikely(test_set_buffer_locked(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_page(page, ofs, PAGE_CACHE_SIZE - ofs,
                              KM_USER0);
            }
            /* 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. */
      .sync_page  = block_sync_page,      /* Currently, just unplugs the
                                       disk request queue. */
#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. */
};

/**
 * 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. */
      .sync_page  = block_sync_page,      /* Currently, just unplugs the
                                       disk request queue. */
#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. */
};

#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 */

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