// SPDX-License-Identifier: GPL-2.0-or-later

  /**
   * compress.c - NTFS kernel compressed attributes handling.
   *		Part of the Linux-NTFS project.
   *
   * Copyright (c) 2001-2004 Anton Altaparmakov
   * Copyright (c) 2002 Richard Russon

   */
  
  #include <linux/fs.h>
  #include <linux/buffer_head.h>
  #include <linux/blkdev.h>
  #include <linux/vmalloc.h>

  #include <linux/slab.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;

  
  /**
   * 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_SHIFT) >= initialized_size) {

  		clear_page(kp);
  		return;
  	}

  	kp_ofs = initialized_size & ~PAGE_MASK;
  	memset(kp + kp_ofs, 0, PAGE_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_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

   * @completed_pages:	scratch space to track completed 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_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 completed_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. */

  	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

  					put_page(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_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_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_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_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_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_SIZE). */

  	VCN start_vcn = (((s64)index << PAGE_SHIFT) & ~cb_size_mask) >>

  			vol->cluster_size_bits;
  	/*

  	 * The first vcn after the last wanted vcn (minimum alignment is again

  	 * PAGE_SIZE.

  	 */

  	VCN end_vcn = ((((s64)(index + 1UL) << PAGE_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_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;

  	int *completed_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_array(nr_pages, sizeof(struct page *), GFP_NOFS);

  	completed_pages = kmalloc_array(nr_pages + 1, sizeof(int), 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 || !completed_pages)) {

  		kfree(bhs);
  		kfree(pages);

  		kfree(completed_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_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_SIZE - 1) >> PAGE_SHIFT) -

  			offset;

  	/* Is the page fully outside i_size? (truncate in progress) */
  	if (xpage >= max_page) {
  		kfree(bhs);
  		kfree(pages);

  		kfree(completed_pages);

  		zero_user(page, 0, PAGE_SIZE);

  		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);

  			put_page(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 (!trylock_buffer(tbh))

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

  		submit_bh(REQ_OP_READ, 0, 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);

  			io_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_SHIFT) + cur_ofs + cb_size;
  	cb_max_ofs = cb_max_page & ~PAGE_MASK;
  	cb_max_page >>= PAGE_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) {

  				if (likely(!cur_ofs))
  					clear_page(page_address(page));
  				else
  					memset(page_address(page) + cur_ofs, 0,

  							PAGE_SIZE -

  							cur_ofs);
  				flush_dcache_page(page);
  				kunmap(page);
  				SetPageUptodate(page);
  				unlock_page(page);
  				if (cur_page == xpage)
  					xpage_done = 1;
  				else

  					put_page(page);

  				pages[cur_page] = NULL;
  			}

  			cb_pos += PAGE_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_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_SIZE - cur_ofs);
  			cb_pos += PAGE_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

  					put_page(page);

  				pages[cur2_page] = NULL;
  			}

  			cb_pos2 += PAGE_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, completed_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)

  						put_page(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)

  				put_page(page);

  			pages[cur_page] = NULL;
  		}
  	}
  
  	/* We no longer need the list of pages. */
  	kfree(pages);

  	kfree(completed_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" : "unknown 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)

  				put_page(page);

  		}
  	}
  	kfree(pages);

  	kfree(completed_pages);

  	return -EIO;
  }