Linux-2.6.12-rc2

[mirror_ubuntu-artful-kernel.git] / fs / ntfs / 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 semaphore.
 *
 * 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 semaphore.
 */
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(ntfs_inode *ni, struct page *page)
{
        u8 *kp = page_address(page);
        unsigned int kp_ofs;

        ntfs_debug("Zeroing page region outside initialized size.");
        if (((s64)page->index << PAGE_CACHE_SHIFT) >= ni->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 = ni->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(ntfs_inode *ni,
                struct page *page)
{
        if ((page->index >= (ni->initialized_size >> PAGE_CACHE_SHIFT)) &&
                        (ni->initialized_size < VFS_I(ni)->i_size))
                zero_partial_compressed_page(ni, page);
        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)
 *
 * 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)
{
        /*
         * 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) {
                        struct page *page = dest_pages[completed_pages[0]];
                        ntfs_inode *ni = NTFS_I(page->mapping->host);

                        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(ni, dp);
                                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)
{
        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 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);
                SetPageError(page);
                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
         */
        max_page = ((VFS_I(ni)->i_size + PAGE_CACHE_SIZE - 1) >>
                        PAGE_CACHE_SHIFT) - offset;
        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(ni, page);
                                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));
                /*
                 * 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) {
                                        if (prev_cur_page == xpage &&
                                                        !xpage_done)
                                                SetPageError(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);
                        if (cur_page == xpage && !xpage_done)
                                SetPageError(page);
                        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) {
                        if (i == xpage && !xpage_done)
                                SetPageError(page);
                        flush_dcache_page(page);
                        kunmap(page);
                        unlock_page(page);
                        if (i != xpage)
                                page_cache_release(page);
                }
        }
        kfree(pages);
        return -EIO;
}