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compress.c

/**
 * compress.c - NTFS kernel compressed attributes handling.
 *          Part of the Linux-NTFS project.
 *
 * Copyright (c) 2001-2004 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/fs.h>
#include <linux/buffer_head.h>
#include <linux/blkdev.h>
#include <linux/vmalloc.h>

#include "attrib.h"
#include "inode.h"
#include "debug.h"
#include "ntfs.h"

/**
 * ntfs_compression_constants - enum of constants used in the compression code
 */
typedef enum {
      /* Token types and access mask. */
      NTFS_SYMBOL_TOKEN =     0,
      NTFS_PHRASE_TOKEN =     1,
      NTFS_TOKEN_MASK         =     1,

      /* Compression sub-block constants. */
      NTFS_SB_SIZE_MASK =     0x0fff,
      NTFS_SB_SIZE            =     0x1000,
      NTFS_SB_IS_COMPRESSED   =     0x8000,

      /*
       * The maximum compression block size is by definition 16 * the cluster
       * size, with the maximum supported cluster size being 4kiB. Thus the
       * maximum compression buffer size is 64kiB, so we use this when
       * initializing the compression buffer.
       */
      NTFS_MAX_CB_SIZE  = 64 * 1024,
} ntfs_compression_constants;

/**
 * ntfs_compression_buffer - one buffer for the decompression engine
 */
static u8 *ntfs_compression_buffer = NULL;

/**
 * ntfs_cb_lock - spinlock which protects ntfs_compression_buffer
 */
static DEFINE_SPINLOCK(ntfs_cb_lock);

/**
 * allocate_compression_buffers - allocate the decompression buffers
 *
 * Caller has to hold the ntfs_lock mutex.
 *
 * Return 0 on success or -ENOMEM if the allocations failed.
 */
int allocate_compression_buffers(void)
{
      BUG_ON(ntfs_compression_buffer);

      ntfs_compression_buffer = vmalloc(NTFS_MAX_CB_SIZE);
      if (!ntfs_compression_buffer)
            return -ENOMEM;
      return 0;
}

/**
 * free_compression_buffers - free the decompression buffers
 *
 * Caller has to hold the ntfs_lock mutex.
 */
void free_compression_buffers(void)
{
      BUG_ON(!ntfs_compression_buffer);
      vfree(ntfs_compression_buffer);
      ntfs_compression_buffer = NULL;
}

/**
 * zero_partial_compressed_page - zero out of bounds compressed page region
 */
static void zero_partial_compressed_page(struct page *page,
            const s64 initialized_size)
{
      u8 *kp = page_address(page);
      unsigned int kp_ofs;

      ntfs_debug("Zeroing page region outside initialized size.");
      if (((s64)page->index << PAGE_CACHE_SHIFT) >= initialized_size) {
            /*
             * FIXME: Using clear_page() will become wrong when we get
             * PAGE_CACHE_SIZE != PAGE_SIZE but for now there is no problem.
             */
            clear_page(kp);
            return;
      }
      kp_ofs = initialized_size & ~PAGE_CACHE_MASK;
      memset(kp + kp_ofs, 0, PAGE_CACHE_SIZE - kp_ofs);
      return;
}

/**
 * handle_bounds_compressed_page - test for&handle out of bounds compressed page
 */
static inline void handle_bounds_compressed_page(struct page *page,
            const loff_t i_size, const s64 initialized_size)
{
      if ((page->index >= (initialized_size >> PAGE_CACHE_SHIFT)) &&
                  (initialized_size < i_size))
            zero_partial_compressed_page(page, initialized_size);
      return;
}

/**
 * ntfs_decompress - decompress a compression block into an array of pages
 * @dest_pages:         destination array of pages
 * @dest_index:         current index into @dest_pages (IN/OUT)
 * @dest_ofs:           current offset within @dest_pages[@dest_index] (IN/OUT)
 * @dest_max_index:     maximum index into @dest_pages (IN)
 * @dest_max_ofs: maximum offset within @dest_pages[@dest_max_index] (IN)
 * @xpage:        the target page (-1 if none) (IN)
 * @xpage_done:         set to 1 if xpage was completed successfully (IN/OUT)
 * @cb_start:           compression block to decompress (IN)
 * @cb_size:            size of compression block @cb_start in bytes (IN)
 * @i_size:       file size when we started the read (IN)
 * @initialized_size:   initialized file size when we started the read (IN)
 *
 * The caller must have disabled preemption. ntfs_decompress() reenables it when
 * the critical section is finished.
 *
 * This decompresses the compression block @cb_start into the array of
 * destination pages @dest_pages starting at index @dest_index into @dest_pages
 * and at offset @dest_pos into the page @dest_pages[@dest_index].
 *
 * When the page @dest_pages[@xpage] is completed, @xpage_done is set to 1.
 * If xpage is -1 or @xpage has not been completed, @xpage_done is not modified.
 *
 * @cb_start is a pointer to the compression block which needs decompressing
 * and @cb_size is the size of @cb_start in bytes (8-64kiB).
 *
 * Return 0 if success or -EOVERFLOW on error in the compressed stream.
 * @xpage_done indicates whether the target page (@dest_pages[@xpage]) was
 * completed during the decompression of the compression block (@cb_start).
 *
 * Warning: This function *REQUIRES* PAGE_CACHE_SIZE >= 4096 or it will blow up
 * unpredicatbly! You have been warned!
 *
 * Note to hackers: This function may not sleep until it has finished accessing
 * the compression block @cb_start as it is a per-CPU buffer.
 */
static int ntfs_decompress(struct page *dest_pages[], int *dest_index,
            int *dest_ofs, const int dest_max_index, const int dest_max_ofs,
            const int xpage, char *xpage_done, u8 *const cb_start,
            const u32 cb_size, const loff_t i_size,
            const s64 initialized_size)
{
      /*
       * Pointers into the compressed data, i.e. the compression block (cb),
       * and the therein contained sub-blocks (sb).
       */
      u8 *cb_end = cb_start + cb_size; /* End of cb. */
      u8 *cb = cb_start;      /* Current position in cb. */
      u8 *cb_sb_start = cb;   /* Beginning of the current sb in the cb. */
      u8 *cb_sb_end;          /* End of current sb / beginning of next sb. */

      /* Variables for uncompressed data / destination. */
      struct page *dp;  /* Current destination page being worked on. */
      u8 *dp_addr;            /* Current pointer into dp. */
      u8 *dp_sb_start;  /* Start of current sub-block in dp. */
      u8 *dp_sb_end;          /* End of current sb in dp (dp_sb_start +
                           NTFS_SB_SIZE). */
      u16 do_sb_start;  /* @dest_ofs when starting this sub-block. */
      u16 do_sb_end;          /* @dest_ofs of end of this sb (do_sb_start +
                           NTFS_SB_SIZE). */

      /* Variables for tag and token parsing. */
      u8 tag;                 /* Current tag. */
      int token;        /* Loop counter for the eight tokens in tag. */

      /* Need this because we can't sleep, so need two stages. */
      int completed_pages[dest_max_index - *dest_index + 1];
      int nr_completed_pages = 0;

      /* Default error code. */
      int err = -EOVERFLOW;

      ntfs_debug("Entering, cb_size = 0x%x.", cb_size);
do_next_sb:
      ntfs_debug("Beginning sub-block at offset = 0x%zx in the cb.",
                  cb - cb_start);
      /*
       * Have we reached the end of the compression block or the end of the
       * decompressed data?  The latter can happen for example if the current
       * position in the compression block is one byte before its end so the
       * first two checks do not detect it.
       */
      if (cb == cb_end || !le16_to_cpup((le16*)cb) ||
                  (*dest_index == dest_max_index &&
                  *dest_ofs == dest_max_ofs)) {
            int i;

            ntfs_debug("Completed. Returning success (0).");
            err = 0;
return_error:
            /* We can sleep from now on, so we drop lock. */
            spin_unlock(&ntfs_cb_lock);
            /* Second stage: finalize completed pages. */
            if (nr_completed_pages > 0) {
                  for (i = 0; i < nr_completed_pages; i++) {
                        int di = completed_pages[i];

                        dp = dest_pages[di];
                        /*
                         * If we are outside the initialized size, zero
                         * the out of bounds page range.
                         */
                        handle_bounds_compressed_page(dp, i_size,
                                    initialized_size);
                        flush_dcache_page(dp);
                        kunmap(dp);
                        SetPageUptodate(dp);
                        unlock_page(dp);
                        if (di == xpage)
                              *xpage_done = 1;
                        else
                              page_cache_release(dp);
                        dest_pages[di] = NULL;
                  }
            }
            return err;
      }

      /* Setup offsets for the current sub-block destination. */
      do_sb_start = *dest_ofs;
      do_sb_end = do_sb_start + NTFS_SB_SIZE;

      /* Check that we are still within allowed boundaries. */
      if (*dest_index == dest_max_index && do_sb_end > dest_max_ofs)
            goto return_overflow;

      /* Does the minimum size of a compressed sb overflow valid range? */
      if (cb + 6 > cb_end)
            goto return_overflow;

      /* Setup the current sub-block source pointers and validate range. */
      cb_sb_start = cb;
      cb_sb_end = cb_sb_start + (le16_to_cpup((le16*)cb) & NTFS_SB_SIZE_MASK)
                  + 3;
      if (cb_sb_end > cb_end)
            goto return_overflow;

      /* Get the current destination page. */
      dp = dest_pages[*dest_index];
      if (!dp) {
            /* No page present. Skip decompression of this sub-block. */
            cb = cb_sb_end;

            /* Advance destination position to next sub-block. */
            *dest_ofs = (*dest_ofs + NTFS_SB_SIZE) & ~PAGE_CACHE_MASK;
            if (!*dest_ofs && (++*dest_index > dest_max_index))
                  goto return_overflow;
            goto do_next_sb;
      }

      /* We have a valid destination page. Setup the destination pointers. */
      dp_addr = (u8*)page_address(dp) + do_sb_start;

      /* Now, we are ready to process the current sub-block (sb). */
      if (!(le16_to_cpup((le16*)cb) & NTFS_SB_IS_COMPRESSED)) {
            ntfs_debug("Found uncompressed sub-block.");
            /* This sb is not compressed, just copy it into destination. */

            /* Advance source position to first data byte. */
            cb += 2;

            /* An uncompressed sb must be full size. */
            if (cb_sb_end - cb != NTFS_SB_SIZE)
                  goto return_overflow;

            /* Copy the block and advance the source position. */
            memcpy(dp_addr, cb, NTFS_SB_SIZE);
            cb += NTFS_SB_SIZE;

            /* Advance destination position to next sub-block. */
            *dest_ofs += NTFS_SB_SIZE;
            if (!(*dest_ofs &= ~PAGE_CACHE_MASK)) {
finalize_page:
                  /*
                   * First stage: add current page index to array of
                   * completed pages.
                   */
                  completed_pages[nr_completed_pages++] = *dest_index;
                  if (++*dest_index > dest_max_index)
                        goto return_overflow;
            }
            goto do_next_sb;
      }
      ntfs_debug("Found compressed sub-block.");
      /* This sb is compressed, decompress it into destination. */

      /* Setup destination pointers. */
      dp_sb_start = dp_addr;
      dp_sb_end = dp_sb_start + NTFS_SB_SIZE;

      /* Forward to the first tag in the sub-block. */
      cb += 2;
do_next_tag:
      if (cb == cb_sb_end) {
            /* Check if the decompressed sub-block was not full-length. */
            if (dp_addr < dp_sb_end) {
                  int nr_bytes = do_sb_end - *dest_ofs;

                  ntfs_debug("Filling incomplete sub-block with "
                              "zeroes.");
                  /* Zero remainder and update destination position. */
                  memset(dp_addr, 0, nr_bytes);
                  *dest_ofs += nr_bytes;
            }
            /* We have finished the current sub-block. */
            if (!(*dest_ofs &= ~PAGE_CACHE_MASK))
                  goto finalize_page;
            goto do_next_sb;
      }

      /* Check we are still in range. */
      if (cb > cb_sb_end || dp_addr > dp_sb_end)
            goto return_overflow;

      /* Get the next tag and advance to first token. */
      tag = *cb++;

      /* Parse the eight tokens described by the tag. */
      for (token = 0; token < 8; token++, tag >>= 1) {
            u16 lg, pt, length, max_non_overlap;
            register u16 i;
            u8 *dp_back_addr;

            /* Check if we are done / still in range. */
            if (cb >= cb_sb_end || dp_addr > dp_sb_end)
                  break;

            /* Determine token type and parse appropriately.*/
            if ((tag & NTFS_TOKEN_MASK) == NTFS_SYMBOL_TOKEN) {
                  /*
                   * We have a symbol token, copy the symbol across, and
                   * advance the source and destination positions.
                   */
                  *dp_addr++ = *cb++;
                  ++*dest_ofs;

                  /* Continue with the next token. */
                  continue;
            }

            /*
             * We have a phrase token. Make sure it is not the first tag in
             * the sb as this is illegal and would confuse the code below.
             */
            if (dp_addr == dp_sb_start)
                  goto return_overflow;

            /*
             * Determine the number of bytes to go back (p) and the number
             * of bytes to copy (l). We use an optimized algorithm in which
             * we first calculate log2(current destination position in sb),
             * which allows determination of l and p in O(1) rather than
             * O(n). We just need an arch-optimized log2() function now.
             */
            lg = 0;
            for (i = *dest_ofs - do_sb_start - 1; i >= 0x10; i >>= 1)
                  lg++;

            /* Get the phrase token into i. */
            pt = le16_to_cpup((le16*)cb);

            /*
             * Calculate starting position of the byte sequence in
             * the destination using the fact that p = (pt >> (12 - lg)) + 1
             * and make sure we don't go too far back.
             */
            dp_back_addr = dp_addr - (pt >> (12 - lg)) - 1;
            if (dp_back_addr < dp_sb_start)
                  goto return_overflow;

            /* Now calculate the length of the byte sequence. */
            length = (pt & (0xfff >> lg)) + 3;

            /* Advance destination position and verify it is in range. */
            *dest_ofs += length;
            if (*dest_ofs > do_sb_end)
                  goto return_overflow;

            /* The number of non-overlapping bytes. */
            max_non_overlap = dp_addr - dp_back_addr;

            if (length <= max_non_overlap) {
                  /* The byte sequence doesn't overlap, just copy it. */
                  memcpy(dp_addr, dp_back_addr, length);

                  /* Advance destination pointer. */
                  dp_addr += length;
            } else {
                  /*
                   * The byte sequence does overlap, copy non-overlapping
                   * part and then do a slow byte by byte copy for the
                   * overlapping part. Also, advance the destination
                   * pointer.
                   */
                  memcpy(dp_addr, dp_back_addr, max_non_overlap);
                  dp_addr += max_non_overlap;
                  dp_back_addr += max_non_overlap;
                  length -= max_non_overlap;
                  while (length--)
                        *dp_addr++ = *dp_back_addr++;
            }

            /* Advance source position and continue with the next token. */
            cb += 2;
      }

      /* No tokens left in the current tag. Continue with the next tag. */
      goto do_next_tag;

return_overflow:
      ntfs_error(NULL, "Failed. Returning -EOVERFLOW.");
      goto return_error;
}

/**
 * ntfs_read_compressed_block - read a compressed block into the page cache
 * @page:   locked page in the compression block(s) we need to read
 *
 * When we are called the page has already been verified to be locked and the
 * attribute is known to be non-resident, not encrypted, but compressed.
 *
 * 1. Determine which compression block(s) @page is in.
 * 2. Get hold of all pages corresponding to this/these compression block(s).
 * 3. Read the (first) compression block.
 * 4. Decompress it into the corresponding pages.
 * 5. Throw the compressed data away and proceed to 3. for the next compression
 *    block or return success if no more compression blocks left.
 *
 * Warning: We have to be careful what we do about existing pages. They might
 * have been written to so that we would lose data if we were to just overwrite
 * them with the out-of-date uncompressed data.
 *
 * FIXME: For PAGE_CACHE_SIZE > cb_size we are not doing the Right Thing(TM) at
 * the end of the file I think. We need to detect this case and zero the out
 * of bounds remainder of the page in question and mark it as handled. At the
 * moment we would just return -EIO on such a page. This bug will only become
 * apparent if pages are above 8kiB and the NTFS volume only uses 512 byte
 * clusters so is probably not going to be seen by anyone. Still this should
 * be fixed. (AIA)
 *
 * FIXME: Again for PAGE_CACHE_SIZE > cb_size we are screwing up both in
 * handling sparse and compressed cbs. (AIA)
 *
 * FIXME: At the moment we don't do any zeroing out in the case that
 * initialized_size is less than data_size. This should be safe because of the
 * nature of the compression algorithm used. Just in case we check and output
 * an error message in read inode if the two sizes are not equal for a
 * compressed file. (AIA)
 */
int ntfs_read_compressed_block(struct page *page)
{
      loff_t i_size;
      s64 initialized_size;
      struct address_space *mapping = page->mapping;
      ntfs_inode *ni = NTFS_I(mapping->host);
      ntfs_volume *vol = ni->vol;
      struct super_block *sb = vol->sb;
      runlist_element *rl;
      unsigned long flags, block_size = sb->s_blocksize;
      unsigned char block_size_bits = sb->s_blocksize_bits;
      u8 *cb, *cb_pos, *cb_end;
      struct buffer_head **bhs;
      unsigned long offset, index = page->index;
      u32 cb_size = ni->itype.compressed.block_size;
      u64 cb_size_mask = cb_size - 1UL;
      VCN vcn;
      LCN lcn;
      /* The first wanted vcn (minimum alignment is PAGE_CACHE_SIZE). */
      VCN start_vcn = (((s64)index << PAGE_CACHE_SHIFT) & ~cb_size_mask) >>
                  vol->cluster_size_bits;
      /*
       * The first vcn after the last wanted vcn (minumum alignment is again
       * PAGE_CACHE_SIZE.
       */
      VCN end_vcn = ((((s64)(index + 1UL) << PAGE_CACHE_SHIFT) + cb_size - 1)
                  & ~cb_size_mask) >> vol->cluster_size_bits;
      /* Number of compression blocks (cbs) in the wanted vcn range. */
      unsigned int nr_cbs = (end_vcn - start_vcn) << vol->cluster_size_bits
                  >> ni->itype.compressed.block_size_bits;
      /*
       * Number of pages required to store the uncompressed data from all
       * compression blocks (cbs) overlapping @page. Due to alignment
       * guarantees of start_vcn and end_vcn, no need to round up here.
       */
      unsigned int nr_pages = (end_vcn - start_vcn) <<
                  vol->cluster_size_bits >> PAGE_CACHE_SHIFT;
      unsigned int xpage, max_page, cur_page, cur_ofs, i;
      unsigned int cb_clusters, cb_max_ofs;
      int block, max_block, cb_max_page, bhs_size, nr_bhs, err = 0;
      struct page **pages;
      unsigned char xpage_done = 0;

      ntfs_debug("Entering, page->index = 0x%lx, cb_size = 0x%x, nr_pages = "
                  "%i.", index, cb_size, nr_pages);
      /*
       * Bad things happen if we get here for anything that is not an
       * unnamed $DATA attribute.
       */
      BUG_ON(ni->type != AT_DATA);
      BUG_ON(ni->name_len);

      pages = kmalloc(nr_pages * sizeof(struct page *), GFP_NOFS);

      /* Allocate memory to store the buffer heads we need. */
      bhs_size = cb_size / block_size * sizeof(struct buffer_head *);
      bhs = kmalloc(bhs_size, GFP_NOFS);

      if (unlikely(!pages || !bhs)) {
            kfree(bhs);
            kfree(pages);
            unlock_page(page);
            ntfs_error(vol->sb, "Failed to allocate internal buffers.");
            return -ENOMEM;
      }

      /*
       * We have already been given one page, this is the one we must do.
       * Once again, the alignment guarantees keep it simple.
       */
      offset = start_vcn << vol->cluster_size_bits >> PAGE_CACHE_SHIFT;
      xpage = index - offset;
      pages[xpage] = page;
      /*
       * The remaining pages need to be allocated and inserted into the page
       * cache, alignment guarantees keep all the below much simpler. (-8
       */
      read_lock_irqsave(&ni->size_lock, flags);
      i_size = i_size_read(VFS_I(ni));
      initialized_size = ni->initialized_size;
      read_unlock_irqrestore(&ni->size_lock, flags);
      max_page = ((i_size + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT) -
                  offset;
      /* Is the page fully outside i_size? (truncate in progress) */
      if (xpage >= max_page) {
            kfree(bhs);
            kfree(pages);
            zero_user_page(page, 0, PAGE_CACHE_SIZE, KM_USER0);
            ntfs_debug("Compressed read outside i_size - truncated?");
            SetPageUptodate(page);
            unlock_page(page);
            return 0;
      }
      if (nr_pages < max_page)
            max_page = nr_pages;
      for (i = 0; i < max_page; i++, offset++) {
            if (i != xpage)
                  pages[i] = grab_cache_page_nowait(mapping, offset);
            page = pages[i];
            if (page) {
                  /*
                   * We only (re)read the page if it isn't already read
                   * in and/or dirty or we would be losing data or at
                   * least wasting our time.
                   */
                  if (!PageDirty(page) && (!PageUptodate(page) ||
                              PageError(page))) {
                        ClearPageError(page);
                        kmap(page);
                        continue;
                  }
                  unlock_page(page);
                  page_cache_release(page);
                  pages[i] = NULL;
            }
      }

      /*
       * We have the runlist, and all the destination pages we need to fill.
       * Now read the first compression block.
       */
      cur_page = 0;
      cur_ofs = 0;
      cb_clusters = ni->itype.compressed.block_clusters;
do_next_cb:
      nr_cbs--;
      nr_bhs = 0;

      /* Read all cb buffer heads one cluster at a time. */
      rl = NULL;
      for (vcn = start_vcn, start_vcn += cb_clusters; vcn < start_vcn;
                  vcn++) {
            bool is_retry = false;

            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;
            ntfs_debug("Reading vcn = 0x%llx, lcn = 0x%llx.",
                        (unsigned long long)vcn,
                        (unsigned long long)lcn);
            if (lcn < 0) {
                  /*
                   * When we reach the first sparse cluster we have
                   * finished with the cb.
                   */
                  if (lcn == LCN_HOLE)
                        break;
                  if (is_retry || lcn != LCN_RL_NOT_MAPPED)
                        goto rl_err;
                  is_retry = true;
                  /*
                   * Attempt to map runlist, dropping lock for the
                   * duration.
                   */
                  up_read(&ni->runlist.lock);
                  if (!ntfs_map_runlist(ni, vcn))
                        goto lock_retry_remap;
                  goto map_rl_err;
            }
            block = lcn << vol->cluster_size_bits >> block_size_bits;
            /* Read the lcn from device in chunks of block_size bytes. */
            max_block = block + (vol->cluster_size >> block_size_bits);
            do {
                  ntfs_debug("block = 0x%x.", block);
                  if (unlikely(!(bhs[nr_bhs] = sb_getblk(sb, block))))
                        goto getblk_err;
                  nr_bhs++;
            } while (++block < max_block);
      }

      /* Release the lock if we took it. */
      if (rl)
            up_read(&ni->runlist.lock);

      /* Setup and initiate io on all buffer heads. */
      for (i = 0; i < nr_bhs; i++) {
            struct buffer_head *tbh = bhs[i];

            if (unlikely(test_set_buffer_locked(tbh)))
                  continue;
            if (unlikely(buffer_uptodate(tbh))) {
                  unlock_buffer(tbh);
                  continue;
            }
            get_bh(tbh);
            tbh->b_end_io = end_buffer_read_sync;
            submit_bh(READ, tbh);
      }

      /* Wait for io completion on all buffer heads. */
      for (i = 0; i < nr_bhs; i++) {
            struct buffer_head *tbh = bhs[i];

            if (buffer_uptodate(tbh))
                  continue;
            wait_on_buffer(tbh);
            /*
             * We need an optimization barrier here, otherwise we start
             * hitting the below fixup code when accessing a loopback
             * mounted ntfs partition. This indicates either there is a
             * race condition in the loop driver or, more likely, gcc
             * overoptimises the code without the barrier and it doesn't
             * do the Right Thing(TM).
             */
            barrier();
            if (unlikely(!buffer_uptodate(tbh))) {
                  ntfs_warning(vol->sb, "Buffer is unlocked but not "
                              "uptodate! Unplugging the disk queue "
                              "and rescheduling.");
                  get_bh(tbh);
                  blk_run_address_space(mapping);
                  schedule();
                  put_bh(tbh);
                  if (unlikely(!buffer_uptodate(tbh)))
                        goto read_err;
                  ntfs_warning(vol->sb, "Buffer is now uptodate. Good.");
            }
      }

      /*
       * Get the compression buffer. We must not sleep any more
       * until we are finished with it.
       */
      spin_lock(&ntfs_cb_lock);
      cb = ntfs_compression_buffer;

      BUG_ON(!cb);

      cb_pos = cb;
      cb_end = cb + cb_size;

      /* Copy the buffer heads into the contiguous buffer. */
      for (i = 0; i < nr_bhs; i++) {
            memcpy(cb_pos, bhs[i]->b_data, block_size);
            cb_pos += block_size;
      }

      /* Just a precaution. */
      if (cb_pos + 2 <= cb + cb_size)
            *(u16*)cb_pos = 0;

      /* Reset cb_pos back to the beginning. */
      cb_pos = cb;

      /* We now have both source (if present) and destination. */
      ntfs_debug("Successfully read the compression block.");

      /* The last page and maximum offset within it for the current cb. */
      cb_max_page = (cur_page << PAGE_CACHE_SHIFT) + cur_ofs + cb_size;
      cb_max_ofs = cb_max_page & ~PAGE_CACHE_MASK;
      cb_max_page >>= PAGE_CACHE_SHIFT;

      /* Catch end of file inside a compression block. */
      if (cb_max_page > max_page)
            cb_max_page = max_page;

      if (vcn == start_vcn - cb_clusters) {
            /* Sparse cb, zero out page range overlapping the cb. */
            ntfs_debug("Found sparse compression block.");
            /* We can sleep from now on, so we drop lock. */
            spin_unlock(&ntfs_cb_lock);
            if (cb_max_ofs)
                  cb_max_page--;
            for (; cur_page < cb_max_page; cur_page++) {
                  page = pages[cur_page];
                  if (page) {
                        /*
                         * FIXME: Using clear_page() will become wrong
                         * when we get PAGE_CACHE_SIZE != PAGE_SIZE but
                         * for now there is no problem.
                         */
                        if (likely(!cur_ofs))
                              clear_page(page_address(page));
                        else
                              memset(page_address(page) + cur_ofs, 0,
                                          PAGE_CACHE_SIZE -
                                          cur_ofs);
                        flush_dcache_page(page);
                        kunmap(page);
                        SetPageUptodate(page);
                        unlock_page(page);
                        if (cur_page == xpage)
                              xpage_done = 1;
                        else
                              page_cache_release(page);
                        pages[cur_page] = NULL;
                  }
                  cb_pos += PAGE_CACHE_SIZE - cur_ofs;
                  cur_ofs = 0;
                  if (cb_pos >= cb_end)
                        break;
            }
            /* If we have a partial final page, deal with it now. */
            if (cb_max_ofs && cb_pos < cb_end) {
                  page = pages[cur_page];
                  if (page)
                        memset(page_address(page) + cur_ofs, 0,
                                    cb_max_ofs - cur_ofs);
                  /*
                   * No need to update cb_pos at this stage:
                   *    cb_pos += cb_max_ofs - cur_ofs;
                   */
                  cur_ofs = cb_max_ofs;
            }
      } else if (vcn == start_vcn) {
            /* We can't sleep so we need two stages. */
            unsigned int cur2_page = cur_page;
            unsigned int cur_ofs2 = cur_ofs;
            u8 *cb_pos2 = cb_pos;

            ntfs_debug("Found uncompressed compression block.");
            /* Uncompressed cb, copy it to the destination pages. */
            /*
             * TODO: As a big optimization, we could detect this case
             * before we read all the pages and use block_read_full_page()
             * on all full pages instead (we still have to treat partial
             * pages especially but at least we are getting rid of the
             * synchronous io for the majority of pages.
             * Or if we choose not to do the read-ahead/-behind stuff, we
             * could just return block_read_full_page(pages[xpage]) as long
             * as PAGE_CACHE_SIZE <= cb_size.
             */
            if (cb_max_ofs)
                  cb_max_page--;
            /* First stage: copy data into destination pages. */
            for (; cur_page < cb_max_page; cur_page++) {
                  page = pages[cur_page];
                  if (page)
                        memcpy(page_address(page) + cur_ofs, cb_pos,
                                    PAGE_CACHE_SIZE - cur_ofs);
                  cb_pos += PAGE_CACHE_SIZE - cur_ofs;
                  cur_ofs = 0;
                  if (cb_pos >= cb_end)
                        break;
            }
            /* If we have a partial final page, deal with it now. */
            if (cb_max_ofs && cb_pos < cb_end) {
                  page = pages[cur_page];
                  if (page)
                        memcpy(page_address(page) + cur_ofs, cb_pos,
                                    cb_max_ofs - cur_ofs);
                  cb_pos += cb_max_ofs - cur_ofs;
                  cur_ofs = cb_max_ofs;
            }
            /* We can sleep from now on, so drop lock. */
            spin_unlock(&ntfs_cb_lock);
            /* Second stage: finalize pages. */
            for (; cur2_page < cb_max_page; cur2_page++) {
                  page = pages[cur2_page];
                  if (page) {
                        /*
                         * If we are outside the initialized size, zero
                         * the out of bounds page range.
                         */
                        handle_bounds_compressed_page(page, i_size,
                                    initialized_size);
                        flush_dcache_page(page);
                        kunmap(page);
                        SetPageUptodate(page);
                        unlock_page(page);
                        if (cur2_page == xpage)
                              xpage_done = 1;
                        else
                              page_cache_release(page);
                        pages[cur2_page] = NULL;
                  }
                  cb_pos2 += PAGE_CACHE_SIZE - cur_ofs2;
                  cur_ofs2 = 0;
                  if (cb_pos2 >= cb_end)
                        break;
            }
      } else {
            /* Compressed cb, decompress it into the destination page(s). */
            unsigned int prev_cur_page = cur_page;

            ntfs_debug("Found compressed compression block.");
            err = ntfs_decompress(pages, &cur_page, &cur_ofs,
                        cb_max_page, cb_max_ofs, xpage, &xpage_done,
                        cb_pos,     cb_size - (cb_pos - cb), i_size,
                        initialized_size);
            /*
             * We can sleep from now on, lock already dropped by
             * ntfs_decompress().
             */
            if (err) {
                  ntfs_error(vol->sb, "ntfs_decompress() failed in inode "
                              "0x%lx with error code %i. Skipping "
                              "this compression block.",
                              ni->mft_no, -err);
                  /* Release the unfinished pages. */
                  for (; prev_cur_page < cur_page; prev_cur_page++) {
                        page = pages[prev_cur_page];
                        if (page) {
                              flush_dcache_page(page);
                              kunmap(page);
                              unlock_page(page);
                              if (prev_cur_page != xpage)
                                    page_cache_release(page);
                              pages[prev_cur_page] = NULL;
                        }
                  }
            }
      }

      /* Release the buffer heads. */
      for (i = 0; i < nr_bhs; i++)
            brelse(bhs[i]);

      /* Do we have more work to do? */
      if (nr_cbs)
            goto do_next_cb;

      /* We no longer need the list of buffer heads. */
      kfree(bhs);

      /* Clean up if we have any pages left. Should never happen. */
      for (cur_page = 0; cur_page < max_page; cur_page++) {
            page = pages[cur_page];
            if (page) {
                  ntfs_error(vol->sb, "Still have pages left! "
                              "Terminating them with extreme "
                              "prejudice.  Inode 0x%lx, page index "
                              "0x%lx.", ni->mft_no, page->index);
                  flush_dcache_page(page);
                  kunmap(page);
                  unlock_page(page);
                  if (cur_page != xpage)
                        page_cache_release(page);
                  pages[cur_page] = NULL;
            }
      }

      /* We no longer need the list of pages. */
      kfree(pages);

      /* If we have completed the requested page, we return success. */
      if (likely(xpage_done))
            return 0;

      ntfs_debug("Failed. Returning error code %s.", err == -EOVERFLOW ?
                  "EOVERFLOW" : (!err ? "EIO" : "unkown error"));
      return err < 0 ? err : -EIO;

read_err:
      ntfs_error(vol->sb, "IO error while reading compressed data.");
      /* Release the buffer heads. */
      for (i = 0; i < nr_bhs; i++)
            brelse(bhs[i]);
      goto err_out;

map_rl_err:
      ntfs_error(vol->sb, "ntfs_map_runlist() failed. Cannot read "
                  "compression block.");
      goto err_out;

rl_err:
      up_read(&ni->runlist.lock);
      ntfs_error(vol->sb, "ntfs_rl_vcn_to_lcn() failed. Cannot read "
                  "compression block.");
      goto err_out;

getblk_err:
      up_read(&ni->runlist.lock);
      ntfs_error(vol->sb, "getblk() failed. Cannot read compression block.");

err_out:
      kfree(bhs);
      for (i = cur_page; i < max_page; i++) {
            page = pages[i];
            if (page) {
                  flush_dcache_page(page);
                  kunmap(page);
                  unlock_page(page);
                  if (i != xpage)
                        page_cache_release(page);
            }
      }
      kfree(pages);
      return -EIO;
}

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