NTFS: Repeat a failed ntfs_truncate() in fs/ntfs/aops.c::ntfs_writepage()

[safe/jmp/linux-2.6] / fs / ntfs / aops.c

/**
 * aops.c - NTFS kernel address space operations and page cache 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/errno.h>
#include <linux/mm.h>
#include <linux/pagemap.h>
#include <linux/swap.h>
#include <linux/buffer_head.h>
#include <linux/writeback.h>

#include "aops.h"
#include "attrib.h"
#include "debug.h"
#include "inode.h"
#include "mft.h"
#include "runlist.h"
#include "types.h"
#include "ntfs.h"

/**
 * ntfs_end_buffer_async_read - async io completion for reading attributes
 * @bh:         buffer head on which io is completed
 * @uptodate:   whether @bh is now uptodate or not
 *
 * Asynchronous I/O completion handler for reading pages belonging to the
 * attribute address space of an inode.  The inodes can either be files or
 * directories or they can be fake inodes describing some attribute.
 *
 * If NInoMstProtected(), perform the post read mst fixups when all IO on the
 * page has been completed and mark the page uptodate or set the error bit on
 * the page.  To determine the size of the records that need fixing up, we
 * cheat a little bit by setting the index_block_size in ntfs_inode to the ntfs
 * record size, and index_block_size_bits, to the log(base 2) of the ntfs
 * record size.
 */
static void ntfs_end_buffer_async_read(struct buffer_head *bh, int uptodate)
{
        static DEFINE_SPINLOCK(page_uptodate_lock);
        unsigned long flags;
        struct buffer_head *tmp;
        struct page *page;
        ntfs_inode *ni;
        int page_uptodate = 1;

        page = bh->b_page;
        ni = NTFS_I(page->mapping->host);

        if (likely(uptodate)) {
                s64 file_ofs, initialized_size;

                set_buffer_uptodate(bh);

                file_ofs = ((s64)page->index << PAGE_CACHE_SHIFT) +
                                bh_offset(bh);
                read_lock_irqsave(&ni->size_lock, flags);
                initialized_size = ni->initialized_size;
                read_unlock_irqrestore(&ni->size_lock, flags);
                /* Check for the current buffer head overflowing. */
                if (file_ofs + bh->b_size > initialized_size) {
                        char *addr;
                        int ofs = 0;

                        if (file_ofs < initialized_size)
                                ofs = initialized_size - file_ofs;
                        addr = kmap_atomic(page, KM_BIO_SRC_IRQ);
                        memset(addr + bh_offset(bh) + ofs, 0, bh->b_size - ofs);
                        flush_dcache_page(page);
                        kunmap_atomic(addr, KM_BIO_SRC_IRQ);
                }
        } else {
                clear_buffer_uptodate(bh);
                ntfs_error(ni->vol->sb, "Buffer I/O error, logical block %llu.",
                                (unsigned long long)bh->b_blocknr);
                SetPageError(page);
        }
        spin_lock_irqsave(&page_uptodate_lock, flags);
        clear_buffer_async_read(bh);
        unlock_buffer(bh);
        tmp = bh;
        do {
                if (!buffer_uptodate(tmp))
                        page_uptodate = 0;
                if (buffer_async_read(tmp)) {
                        if (likely(buffer_locked(tmp)))
                                goto still_busy;
                        /* Async buffers must be locked. */
                        BUG();
                }
                tmp = tmp->b_this_page;
        } while (tmp != bh);
        spin_unlock_irqrestore(&page_uptodate_lock, flags);
        /*
         * If none of the buffers had errors then we can set the page uptodate,
         * but we first have to perform the post read mst fixups, if the
         * attribute is mst protected, i.e. if NInoMstProteced(ni) is true.
         * Note we ignore fixup errors as those are detected when
         * map_mft_record() is called which gives us per record granularity
         * rather than per page granularity.
         */
        if (!NInoMstProtected(ni)) {
                if (likely(page_uptodate && !PageError(page)))
                        SetPageUptodate(page);
        } else {
                char *addr;
                unsigned int i, recs;
                u32 rec_size;

                rec_size = ni->itype.index.block_size;
                recs = PAGE_CACHE_SIZE / rec_size;
                /* Should have been verified before we got here... */
                BUG_ON(!recs);
                addr = kmap_atomic(page, KM_BIO_SRC_IRQ);
                for (i = 0; i < recs; i++)
                        post_read_mst_fixup((NTFS_RECORD*)(addr +
                                        i * rec_size), rec_size);
                flush_dcache_page(page);
                kunmap_atomic(addr, KM_BIO_SRC_IRQ);
                if (likely(!PageError(page) && page_uptodate))
                        SetPageUptodate(page);
        }
        unlock_page(page);
        return;
still_busy:
        spin_unlock_irqrestore(&page_uptodate_lock, flags);
        return;
}

/**
 * ntfs_read_block - fill a @page of an address space with data
 * @page:       page cache page to fill with data
 *
 * Fill the page @page of the address space belonging to the @page->host inode.
 * We read each buffer asynchronously and when all buffers are read in, our io
 * completion handler ntfs_end_buffer_read_async(), if required, automatically
 * applies the mst fixups to the page before finally marking it uptodate and
 * unlocking it.
 *
 * We only enforce allocated_size limit because i_size is checked for in
 * generic_file_read().
 *
 * Return 0 on success and -errno on error.
 *
 * Contains an adapted version of fs/buffer.c::block_read_full_page().
 */
static int ntfs_read_block(struct page *page)
{
        VCN vcn;
        LCN lcn;
        ntfs_inode *ni;
        ntfs_volume *vol;
        runlist_element *rl;
        struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
        sector_t iblock, lblock, zblock;
        unsigned long flags;
        unsigned int blocksize, vcn_ofs;
        int i, nr;
        unsigned char blocksize_bits;

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

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

        blocksize_bits = VFS_I(ni)->i_blkbits;
        blocksize = 1 << blocksize_bits;

        if (!page_has_buffers(page))
                create_empty_buffers(page, blocksize, 0);
        bh = head = page_buffers(page);
        if (unlikely(!bh)) {
                unlock_page(page);
                return -ENOMEM;
        }

        iblock = (s64)page->index << (PAGE_CACHE_SHIFT - blocksize_bits);
        read_lock_irqsave(&ni->size_lock, flags);
        lblock = (ni->allocated_size + blocksize - 1) >> blocksize_bits;
        zblock = (ni->initialized_size + blocksize - 1) >> blocksize_bits;
        read_unlock_irqrestore(&ni->size_lock, flags);

        /* Loop through all the buffers in the page. */
        rl = NULL;
        nr = i = 0;
        do {
                u8 *kaddr;

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

                        /* Convert iblock into corresponding vcn and offset. */
                        vcn = (VCN)iblock << blocksize_bits >>
                                        vol->cluster_size_bits;
                        vcn_ofs = ((VCN)iblock << blocksize_bits) &
                                        vol->cluster_size_mask;
                        if (!rl) {
lock_retry_remap:
                                down_read(&ni->runlist.lock);
                                rl = ni->runlist.rl;
                        }
                        if (likely(rl != NULL)) {
                                /* Seek to element containing target vcn. */
                                while (rl->length && rl[1].vcn <= vcn)
                                        rl++;
                                lcn = ntfs_rl_vcn_to_lcn(rl, vcn);
                        } else
                                lcn = LCN_RL_NOT_MAPPED;
                        /* Successful remap. */
                        if (lcn >= 0) {
                                /* Setup buffer head to correct block. */
                                bh->b_blocknr = ((lcn << vol->cluster_size_bits)
                                                + vcn_ofs) >> blocksize_bits;
                                set_buffer_mapped(bh);
                                /* Only read initialized data blocks. */
                                if (iblock < zblock) {
                                        arr[nr++] = bh;
                                        continue;
                                }
                                /* Fully non-initialized data block, zero it. */
                                goto handle_zblock;
                        }
                        /* It is a hole, need to zero it. */
                        if (lcn == LCN_HOLE)
                                goto handle_hole;
                        /* If first try and runlist unmapped, map and retry. */
                        if (!is_retry && lcn == LCN_RL_NOT_MAPPED) {
                                int err;
                                is_retry = TRUE;
                                /*
                                 * Attempt to map runlist, dropping lock for
                                 * the duration.
                                 */
                                up_read(&ni->runlist.lock);
                                err = ntfs_map_runlist(ni, vcn);
                                if (likely(!err))
                                        goto lock_retry_remap;
                                rl = NULL;
                                lcn = err;
                        }
                        /* Hard error, zero out region. */
                        bh->b_blocknr = -1;
                        SetPageError(page);
                        ntfs_error(vol->sb, "Failed to read from inode 0x%lx, "
                                        "attribute type 0x%x, vcn 0x%llx, "
                                        "offset 0x%x because its location on "
                                        "disk could not be determined%s "
                                        "(error code %lli).", ni->mft_no,
                                        ni->type, (unsigned long long)vcn,
                                        vcn_ofs, is_retry ? " even after "
                                        "retrying" : "", (long long)lcn);
                }
                /*
                 * Either iblock was outside lblock limits or
                 * ntfs_rl_vcn_to_lcn() returned error.  Just zero that portion
                 * of the page and set the buffer uptodate.
                 */
handle_hole:
                bh->b_blocknr = -1UL;
                clear_buffer_mapped(bh);
handle_zblock:
                kaddr = kmap_atomic(page, KM_USER0);
                memset(kaddr + i * blocksize, 0, blocksize);
                flush_dcache_page(page);
                kunmap_atomic(kaddr, KM_USER0);
                set_buffer_uptodate(bh);
        } while (i++, iblock++, (bh = bh->b_this_page) != head);

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

        /* Check we have at least one buffer ready for i/o. */
        if (nr) {
                struct buffer_head *tbh;

                /* Lock the buffers. */
                for (i = 0; i < nr; i++) {
                        tbh = arr[i];
                        lock_buffer(tbh);
                        tbh->b_end_io = ntfs_end_buffer_async_read;
                        set_buffer_async_read(tbh);
                }
                /* Finally, start i/o on the buffers. */
                for (i = 0; i < nr; i++) {
                        tbh = arr[i];
                        if (likely(!buffer_uptodate(tbh)))
                                submit_bh(READ, tbh);
                        else
                                ntfs_end_buffer_async_read(tbh, 1);
                }
                return 0;
        }
        /* No i/o was scheduled on any of the buffers. */
        if (likely(!PageError(page)))
                SetPageUptodate(page);
        else /* Signal synchronous i/o error. */
                nr = -EIO;
        unlock_page(page);
        return nr;
}

/**
 * ntfs_readpage - fill a @page of a @file with data from the device
 * @file:       open file to which the page @page belongs or NULL
 * @page:       page cache page to fill with data
 *
 * For non-resident attributes, ntfs_readpage() fills the @page of the open
 * file @file by calling the ntfs version of the generic block_read_full_page()
 * function, ntfs_read_block(), which in turn creates and reads in the buffers
 * associated with the page asynchronously.
 *
 * For resident attributes, OTOH, ntfs_readpage() fills @page by copying the
 * data from the mft record (which at this stage is most likely in memory) and
 * fills the remainder with zeroes. Thus, in this case, I/O is synchronous, as
 * even if the mft record is not cached at this point in time, we need to wait
 * for it to be read in before we can do the copy.
 *
 * Return 0 on success and -errno on error.
 */
static int ntfs_readpage(struct file *file, struct page *page)
{
        loff_t i_size;
        ntfs_inode *ni, *base_ni;
        u8 *kaddr;
        ntfs_attr_search_ctx *ctx;
        MFT_RECORD *mrec;
        u32 attr_len;
        int err = 0;

        BUG_ON(!PageLocked(page));
        /*
         * This can potentially happen because we clear PageUptodate() during
         * ntfs_writepage() of MstProtected() attributes.
         */
        if (PageUptodate(page)) {
                unlock_page(page);
                return 0;
        }
        ni = NTFS_I(page->mapping->host);

        /* NInoNonResident() == NInoIndexAllocPresent() */
        if (NInoNonResident(ni)) {
                /*
                 * Only unnamed $DATA attributes can be compressed or
                 * encrypted.
                 */
                if (ni->type == AT_DATA && !ni->name_len) {
                        /* If file is encrypted, deny access, just like NT4. */
                        if (NInoEncrypted(ni)) {
                                err = -EACCES;
                                goto err_out;
                        }
                        /* Compressed data streams are handled in compress.c. */
                        if (NInoCompressed(ni))
                                return ntfs_read_compressed_block(page);
                }
                /* Normal data stream. */
                return ntfs_read_block(page);
        }
        /*
         * Attribute is resident, implying it is not compressed or encrypted.
         * This also means the attribute is smaller than an mft record and
         * hence smaller than a page, so can simply zero out any pages with
         * index above 0.  We can also do this if the file size is 0.
         */
        if (unlikely(page->index > 0 || !i_size_read(VFS_I(ni)))) {
                kaddr = kmap_atomic(page, KM_USER0);
                memset(kaddr, 0, PAGE_CACHE_SIZE);
                flush_dcache_page(page);
                kunmap_atomic(kaddr, KM_USER0);
                goto done;
        }
        if (!NInoAttr(ni))
                base_ni = ni;
        else
                base_ni = ni->ext.base_ntfs_ino;
        /* Map, pin, and lock the mft record. */
        mrec = map_mft_record(base_ni);
        if (IS_ERR(mrec)) {
                err = PTR_ERR(mrec);
                goto err_out;
        }
        ctx = ntfs_attr_get_search_ctx(base_ni, mrec);
        if (unlikely(!ctx)) {
                err = -ENOMEM;
                goto unm_err_out;
        }
        err = ntfs_attr_lookup(ni->type, ni->name, ni->name_len,
                        CASE_SENSITIVE, 0, NULL, 0, ctx);
        if (unlikely(err))
                goto put_unm_err_out;
        attr_len = le32_to_cpu(ctx->attr->data.resident.value_length);
        i_size = i_size_read(VFS_I(ni));
        if (unlikely(attr_len > i_size))
                attr_len = i_size;
        kaddr = kmap_atomic(page, KM_USER0);
        /* Copy the data to the page. */
        memcpy(kaddr, (u8*)ctx->attr +
                        le16_to_cpu(ctx->attr->data.resident.value_offset),
                        attr_len);
        /* Zero the remainder of the page. */
        memset(kaddr + attr_len, 0, PAGE_CACHE_SIZE - attr_len);
        flush_dcache_page(page);
        kunmap_atomic(kaddr, KM_USER0);
put_unm_err_out:
        ntfs_attr_put_search_ctx(ctx);
unm_err_out:
        unmap_mft_record(base_ni);
done:
        SetPageUptodate(page);
err_out:
        unlock_page(page);
        return err;
}

#ifdef NTFS_RW

/**
 * ntfs_write_block - write a @page to the backing store
 * @page:       page cache page to write out
 * @wbc:        writeback control structure
 *
 * This function is for writing pages belonging to non-resident, non-mst
 * protected attributes to their backing store.
 *
 * For a page with buffers, map and write the dirty buffers asynchronously
 * under page writeback. For a page without buffers, create buffers for the
 * page, then proceed as above.
 *
 * If a page doesn't have buffers the page dirty state is definitive. If a page
 * does have buffers, the page dirty state is just a hint, and the buffer dirty
 * state is definitive. (A hint which has rules: dirty buffers against a clean
 * page is illegal. Other combinations are legal and need to be handled. In
 * particular a dirty page containing clean buffers for example.)
 *
 * Return 0 on success and -errno on error.
 *
 * Based on ntfs_read_block() and __block_write_full_page().
 */
static int ntfs_write_block(struct page *page, struct writeback_control *wbc)
{
        VCN vcn;
        LCN lcn;
        s64 initialized_size;
        loff_t i_size;
        sector_t block, dblock, iblock;
        struct inode *vi;
        ntfs_inode *ni;
        ntfs_volume *vol;
        runlist_element *rl;
        struct buffer_head *bh, *head;
        unsigned long flags;
        unsigned int blocksize, vcn_ofs;
        int err;
        BOOL need_end_writeback;
        unsigned char blocksize_bits;

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

        ntfs_debug("Entering for inode 0x%lx, attribute type 0x%x, page index "
                        "0x%lx.", ni->mft_no, ni->type, page->index);

        BUG_ON(!NInoNonResident(ni));
        BUG_ON(NInoMstProtected(ni));

        blocksize_bits = vi->i_blkbits;
        blocksize = 1 << blocksize_bits;

        if (!page_has_buffers(page)) {
                BUG_ON(!PageUptodate(page));
                create_empty_buffers(page, blocksize,
                                (1 << BH_Uptodate) | (1 << BH_Dirty));
        }
        bh = head = page_buffers(page);
        if (unlikely(!bh)) {
                ntfs_warning(vol->sb, "Error allocating page buffers. "
                                "Redirtying page so we try again later.");
                /*
                 * Put the page back on mapping->dirty_pages, but leave its
                 * buffer's dirty state as-is.
                 */
                redirty_page_for_writepage(wbc, page);
                unlock_page(page);
                return 0;
        }

        /* NOTE: Different naming scheme to ntfs_read_block()! */

        /* The first block in the page. */
        block = (s64)page->index << (PAGE_CACHE_SHIFT - blocksize_bits);

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

        /* The first out of bounds block for the data size. */
        dblock = (i_size + blocksize - 1) >> blocksize_bits;

        /* The last (fully or partially) initialized block. */
        iblock = initialized_size >> blocksize_bits;

        /*
         * Be very careful.  We have no exclusion from __set_page_dirty_buffers
         * here, and the (potentially unmapped) buffers may become dirty at
         * any time.  If a buffer becomes dirty here after we've inspected it
         * then we just miss that fact, and the page stays dirty.
         *
         * Buffers outside i_size may be dirtied by __set_page_dirty_buffers;
         * handle that here by just cleaning them.
         */

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

                if (unlikely(block >= dblock)) {
                        /*
                         * Mapped buffers outside i_size will occur, because
                         * this page can be outside i_size when there is a
                         * truncate in progress. The contents of such buffers
                         * were zeroed by ntfs_writepage().
                         *
                         * FIXME: What about the small race window where
                         * ntfs_writepage() has not done any clearing because
                         * the page was within i_size but before we get here,
                         * vmtruncate() modifies i_size?
                         */
                        clear_buffer_dirty(bh);
                        set_buffer_uptodate(bh);
                        continue;
                }

                /* Clean buffers are not written out, so no need to map them. */
                if (!buffer_dirty(bh))
                        continue;

                /* Make sure we have enough initialized size. */
                if (unlikely((block >= iblock) &&
                                (initialized_size < i_size))) {
                        /*
                         * If this page is fully outside initialized size, zero
                         * out all pages between the current initialized size
                         * and the current page. Just use ntfs_readpage() to do
                         * the zeroing transparently.
                         */
                        if (block > iblock) {
                                // TODO:
                                // For each page do:
                                // - read_cache_page()
                                // Again for each page do:
                                // - wait_on_page_locked()
                                // - Check (PageUptodate(page) &&
                                //                      !PageError(page))
                                // Update initialized size in the attribute and
                                // in the inode.
                                // Again, for each page do:
                                //      __set_page_dirty_buffers();
                                // page_cache_release()
                                // We don't need to wait on the writes.
                                // Update iblock.
                        }
                        /*
                         * The current page straddles initialized size. Zero
                         * all non-uptodate buffers and set them uptodate (and
                         * dirty?). Note, there aren't any non-uptodate buffers
                         * if the page is uptodate.
                         * FIXME: For an uptodate page, the buffers may need to
                         * be written out because they were not initialized on
                         * disk before.
                         */
                        if (!PageUptodate(page)) {
                                // TODO:
                                // Zero any non-uptodate buffers up to i_size.
                                // Set them uptodate and dirty.
                        }
                        // TODO:
                        // Update initialized size in the attribute and in the
                        // inode (up to i_size).
                        // Update iblock.
                        // FIXME: This is inefficient. Try to batch the two
                        // size changes to happen in one go.
                        ntfs_error(vol->sb, "Writing beyond initialized size "
                                        "is not supported yet. Sorry.");
                        err = -EOPNOTSUPP;
                        break;
                        // Do NOT set_buffer_new() BUT DO clear buffer range
                        // outside write request range.
                        // set_buffer_uptodate() on complete buffers as well as
                        // set_buffer_dirty().
                }

                /* No need to map buffers that are already mapped. */
                if (buffer_mapped(bh))
                        continue;

                /* Unmapped, dirty buffer. Need to map it. */
                bh->b_bdev = vol->sb->s_bdev;

                /* Convert block into corresponding vcn and offset. */
                vcn = (VCN)block << blocksize_bits;
                vcn_ofs = vcn & vol->cluster_size_mask;
                vcn >>= vol->cluster_size_bits;
                if (!rl) {
lock_retry_remap:
                        down_read(&ni->runlist.lock);
                        rl = ni->runlist.rl;
                }
                if (likely(rl != NULL)) {
                        /* Seek to element containing target vcn. */
                        while (rl->length && rl[1].vcn <= vcn)
                                rl++;
                        lcn = ntfs_rl_vcn_to_lcn(rl, vcn);
                } else
                        lcn = LCN_RL_NOT_MAPPED;
                /* Successful remap. */
                if (lcn >= 0) {
                        /* Setup buffer head to point to correct block. */
                        bh->b_blocknr = ((lcn << vol->cluster_size_bits) +
                                        vcn_ofs) >> blocksize_bits;
                        set_buffer_mapped(bh);
                        continue;
                }
                /* It is a hole, need to instantiate it. */
                if (lcn == LCN_HOLE) {
                        // TODO: Instantiate the hole.
                        // clear_buffer_new(bh);
                        // unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);
                        ntfs_error(vol->sb, "Writing into sparse regions is "
                                        "not supported yet. Sorry.");
                        err = -EOPNOTSUPP;
                        break;
                }
                /* If first try and runlist unmapped, map and retry. */
                if (!is_retry && lcn == LCN_RL_NOT_MAPPED) {
                        is_retry = TRUE;
                        /*
                         * Attempt to map runlist, dropping lock for
                         * the duration.
                         */
                        up_read(&ni->runlist.lock);
                        err = ntfs_map_runlist(ni, vcn);
                        if (likely(!err))
                                goto lock_retry_remap;
                        rl = NULL;
                        lcn = err;
                }
                /* Failed to map the buffer, even after retrying. */
                bh->b_blocknr = -1;
                ntfs_error(vol->sb, "Failed to write to inode 0x%lx, "
                                "attribute type 0x%x, vcn 0x%llx, offset 0x%x "
                                "because its location on disk could not be "
                                "determined%s (error code %lli).", ni->mft_no,
                                ni->type, (unsigned long long)vcn,
                                vcn_ofs, is_retry ? " even after "
                                "retrying" : "", (long long)lcn);
                if (!err)
                        err = -EIO;
                break;
        } while (block++, (bh = bh->b_this_page) != head);

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

        /* For the error case, need to reset bh to the beginning. */
        bh = head;

        /* Just an optimization, so ->readpage() isn't called later. */
        if (unlikely(!PageUptodate(page))) {
                int uptodate = 1;
                do {
                        if (!buffer_uptodate(bh)) {
                                uptodate = 0;
                                bh = head;
                                break;
                        }
                } while ((bh = bh->b_this_page) != head);
                if (uptodate)
                        SetPageUptodate(page);
        }

        /* Setup all mapped, dirty buffers for async write i/o. */
        do {
                get_bh(bh);
                if (buffer_mapped(bh) && buffer_dirty(bh)) {
                        lock_buffer(bh);
                        if (test_clear_buffer_dirty(bh)) {
                                BUG_ON(!buffer_uptodate(bh));
                                mark_buffer_async_write(bh);
                        } else
                                unlock_buffer(bh);
                } else if (unlikely(err)) {
                        /*
                         * For the error case. The buffer may have been set
                         * dirty during attachment to a dirty page.
                         */
                        if (err != -ENOMEM)
                                clear_buffer_dirty(bh);
                }
        } while ((bh = bh->b_this_page) != head);

        if (unlikely(err)) {
                // TODO: Remove the -EOPNOTSUPP check later on...
                if (unlikely(err == -EOPNOTSUPP))
                        err = 0;
                else if (err == -ENOMEM) {
                        ntfs_warning(vol->sb, "Error allocating memory. "
                                        "Redirtying page so we try again "
                                        "later.");
                        /*
                         * Put the page back on mapping->dirty_pages, but
                         * leave its buffer's dirty state as-is.
                         */
                        redirty_page_for_writepage(wbc, page);
                        err = 0;
                } else
                        SetPageError(page);
        }

        BUG_ON(PageWriteback(page));
        set_page_writeback(page);       /* Keeps try_to_free_buffers() away. */
        unlock_page(page);

        /*
         * Submit the prepared buffers for i/o. Note the page is unlocked,
         * and the async write i/o completion handler can end_page_writeback()
         * at any time after the *first* submit_bh(). So the buffers can then
         * disappear...
         */
        need_end_writeback = TRUE;
        do {
                struct buffer_head *next = bh->b_this_page;
                if (buffer_async_write(bh)) {
                        submit_bh(WRITE, bh);
                        need_end_writeback = FALSE;
                }
                put_bh(bh);
                bh = next;
        } while (bh != head);

        /* If no i/o was started, need to end_page_writeback(). */
        if (unlikely(need_end_writeback))
                end_page_writeback(page);

        ntfs_debug("Done.");
        return err;
}

/**
 * ntfs_write_mst_block - write a @page to the backing store
 * @page:       page cache page to write out
 * @wbc:        writeback control structure
 *
 * This function is for writing pages belonging to non-resident, mst protected
 * attributes to their backing store.  The only supported attributes are index
 * allocation and $MFT/$DATA.  Both directory inodes and index inodes are
 * supported for the index allocation case.
 *
 * The page must remain locked for the duration of the write because we apply
 * the mst fixups, write, and then undo the fixups, so if we were to unlock the
 * page before undoing the fixups, any other user of the page will see the
 * page contents as corrupt.
 *
 * We clear the page uptodate flag for the duration of the function to ensure
 * exclusion for the $MFT/$DATA case against someone mapping an mft record we
 * are about to apply the mst fixups to.
 *
 * Return 0 on success and -errno on error.
 *
 * Based on ntfs_write_block(), ntfs_mft_writepage(), and
 * write_mft_record_nolock().
 */
static int ntfs_write_mst_block(struct page *page,
                struct writeback_control *wbc)
{
        sector_t block, dblock, rec_block;
        struct inode *vi = page->mapping->host;
        ntfs_inode *ni = NTFS_I(vi);
        ntfs_volume *vol = ni->vol;
        u8 *kaddr;
        unsigned char bh_size_bits = vi->i_blkbits;
        unsigned int bh_size = 1 << bh_size_bits;
        unsigned int rec_size = ni->itype.index.block_size;
        ntfs_inode *locked_nis[PAGE_CACHE_SIZE / rec_size];
        struct buffer_head *bh, *head, *tbh, *rec_start_bh;
        int max_bhs = PAGE_CACHE_SIZE / bh_size;
        struct buffer_head *bhs[max_bhs];
        runlist_element *rl;
        int i, nr_locked_nis, nr_recs, nr_bhs, bhs_per_rec, err, err2;
        unsigned rec_size_bits;
        BOOL sync, is_mft, page_is_dirty, rec_is_dirty;

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

        /* Were we called for sync purposes? */
        sync = (wbc->sync_mode == WB_SYNC_ALL);

        /* Make sure we have mapped buffers. */
        BUG_ON(!page_has_buffers(page));
        bh = head = page_buffers(page);
        BUG_ON(!bh);

        rec_size_bits = ni->itype.index.block_size_bits;
        BUG_ON(!(PAGE_CACHE_SIZE >> rec_size_bits));
        bhs_per_rec = rec_size >> bh_size_bits;
        BUG_ON(!bhs_per_rec);

        /* The first block in the page. */
        rec_block = block = (sector_t)page->index <<
                        (PAGE_CACHE_SHIFT - bh_size_bits);

        /* The first out of bounds block for the data size. */
        dblock = (i_size_read(vi) + bh_size - 1) >> bh_size_bits;

        rl = NULL;
        err = err2 = nr_bhs = nr_recs = nr_locked_nis = 0;
        page_is_dirty = rec_is_dirty = FALSE;
        rec_start_bh = NULL;
        do {
                BOOL is_retry = FALSE;

                if (likely(block < rec_block)) {
                        if (unlikely(block >= dblock)) {
                                clear_buffer_dirty(bh);
                                continue;
                        }
                        /*
                         * This block is not the first one in the record.  We
                         * ignore the buffer's dirty state because we could
                         * have raced with a parallel mark_ntfs_record_dirty().
                         */
                        if (!rec_is_dirty)
                                continue;
                        if (unlikely(err2)) {
                                if (err2 != -ENOMEM)
                                        clear_buffer_dirty(bh);
                                continue;
                        }
                } else /* if (block == rec_block) */ {
                        BUG_ON(block > rec_block);
                        /* This block is the first one in the record. */
                        rec_block += bhs_per_rec;
                        err2 = 0;
                        if (unlikely(block >= dblock)) {
                                clear_buffer_dirty(bh);
                                continue;
                        }
                        if (!buffer_dirty(bh)) {
                                /* Clean records are not written out. */
                                rec_is_dirty = FALSE;
                                continue;
                        }
                        rec_is_dirty = TRUE;
                        rec_start_bh = bh;
                }
                /* Need to map the buffer if it is not mapped already. */
                if (unlikely(!buffer_mapped(bh))) {
                        VCN vcn;
                        LCN lcn;
                        unsigned int vcn_ofs;

                        /* Obtain the vcn and offset of the current block. */
                        vcn = (VCN)block << bh_size_bits;
                        vcn_ofs = vcn & vol->cluster_size_mask;
                        vcn >>= vol->cluster_size_bits;
                        if (!rl) {
lock_retry_remap:
                                down_read(&ni->runlist.lock);
                                rl = ni->runlist.rl;
                        }
                        if (likely(rl != NULL)) {
                                /* Seek to element containing target vcn. */
                                while (rl->length && rl[1].vcn <= vcn)
                                        rl++;
                                lcn = ntfs_rl_vcn_to_lcn(rl, vcn);
                        } else
                                lcn = LCN_RL_NOT_MAPPED;
                        /* Successful remap. */
                        if (likely(lcn >= 0)) {
                                /* Setup buffer head to correct block. */
                                bh->b_blocknr = ((lcn <<
                                                vol->cluster_size_bits) +
                                                vcn_ofs) >> bh_size_bits;
                                set_buffer_mapped(bh);
                        } else {
                                /*
                                 * Remap failed.  Retry to map the runlist once
                                 * unless we are working on $MFT which always
                                 * has the whole of its runlist in memory.
                                 */
                                if (!is_mft && !is_retry &&
                                                lcn == LCN_RL_NOT_MAPPED) {
                                        is_retry = TRUE;
                                        /*
                                         * Attempt to map runlist, dropping
                                         * lock for the duration.
                                         */
                                        up_read(&ni->runlist.lock);
                                        err2 = ntfs_map_runlist(ni, vcn);
                                        if (likely(!err2))
                                                goto lock_retry_remap;
                                        if (err2 == -ENOMEM)
                                                page_is_dirty = TRUE;
                                        lcn = err2;
                                } else
                                        err2 = -EIO;
                                /* Hard error.  Abort writing this record. */
                                if (!err || err == -ENOMEM)
                                        err = err2;
                                bh->b_blocknr = -1;
                                ntfs_error(vol->sb, "Cannot write ntfs record "
                                                "0x%llx (inode 0x%lx, "
                                                "attribute type 0x%x) because "
                                                "its location on disk could "
                                                "not be determined (error "
                                                "code %lli).", (s64)block <<
                                                bh_size_bits >>
                                                vol->mft_record_size_bits,
                                                ni->mft_no, ni->type,
                                                (long long)lcn);
                                /*
                                 * If this is not the first buffer, remove the
                                 * buffers in this record from the list of
                                 * buffers to write and clear their dirty bit
                                 * if not error -ENOMEM.
                                 */
                                if (rec_start_bh != bh) {
                                        while (bhs[--nr_bhs] != rec_start_bh)
                                                ;
                                        if (err2 != -ENOMEM) {
                                                do {
                                                        clear_buffer_dirty(
                                                                rec_start_bh);
                                                } while ((rec_start_bh =
                                                                rec_start_bh->
                                                                b_this_page) !=
                                                                bh);
                                        }
                                }
                                continue;
                        }
                }
                BUG_ON(!buffer_uptodate(bh));
                BUG_ON(nr_bhs >= max_bhs);
                bhs[nr_bhs++] = bh;
        } while (block++, (bh = bh->b_this_page) != head);
        if (unlikely(rl))
                up_read(&ni->runlist.lock);
        /* If there were no dirty buffers, we are done. */
        if (!nr_bhs)
                goto done;
        /* Map the page so we can access its contents. */
        kaddr = kmap(page);
        /* Clear the page uptodate flag whilst the mst fixups are applied. */
        BUG_ON(!PageUptodate(page));
        ClearPageUptodate(page);
        for (i = 0; i < nr_bhs; i++) {
                unsigned int ofs;

                /* Skip buffers which are not at the beginning of records. */
                if (i % bhs_per_rec)
                        continue;
                tbh = bhs[i];
                ofs = bh_offset(tbh);
                if (is_mft) {
                        ntfs_inode *tni;
                        unsigned long mft_no;

                        /* Get the mft record number. */
                        mft_no = (((s64)page->index << PAGE_CACHE_SHIFT) + ofs)
                                        >> rec_size_bits;
                        /* Check whether to write this mft record. */
                        tni = NULL;
                        if (!ntfs_may_write_mft_record(vol, mft_no,
                                        (MFT_RECORD*)(kaddr + ofs), &tni)) {
                                /*
                                 * The record should not be written.  This
                                 * means we need to redirty the page before
                                 * returning.
                                 */
                                page_is_dirty = TRUE;
                                /*
                                 * Remove the buffers in this mft record from
                                 * the list of buffers to write.
                                 */
                                do {
                                        bhs[i] = NULL;
                                } while (++i % bhs_per_rec);
                                continue;
                        }
                        /*
                         * The record should be written.  If a locked ntfs
                         * inode was returned, add it to the array of locked
                         * ntfs inodes.
                         */
                        if (tni)
                                locked_nis[nr_locked_nis++] = tni;
                }
                /* Apply the mst protection fixups. */
                err2 = pre_write_mst_fixup((NTFS_RECORD*)(kaddr + ofs),
                                rec_size);
                if (unlikely(err2)) {
                        if (!err || err == -ENOMEM)
                                err = -EIO;
                        ntfs_error(vol->sb, "Failed to apply mst fixups "
                                        "(inode 0x%lx, attribute type 0x%x, "
                                        "page index 0x%lx, page offset 0x%x)!"
                                        "  Unmount and run chkdsk.", vi->i_ino,
                                        ni->type, page->index, ofs);
                        /*
                         * Mark all the buffers in this record clean as we do
                         * not want to write corrupt data to disk.
                         */
                        do {
                                clear_buffer_dirty(bhs[i]);
                                bhs[i] = NULL;
                        } while (++i % bhs_per_rec);
                        continue;
                }
                nr_recs++;
        }
        /* If no records are to be written out, we are done. */
        if (!nr_recs)
                goto unm_done;
        flush_dcache_page(page);
        /* Lock buffers and start synchronous write i/o on them. */
        for (i = 0; i < nr_bhs; i++) {
                tbh = bhs[i];
                if (!tbh)
                        continue;
                if (unlikely(test_set_buffer_locked(tbh)))
                        BUG();
                /* The buffer dirty state is now irrelevant, just clean it. */
                clear_buffer_dirty(tbh);
                BUG_ON(!buffer_uptodate(tbh));
                BUG_ON(!buffer_mapped(tbh));
                get_bh(tbh);
                tbh->b_end_io = end_buffer_write_sync;
                submit_bh(WRITE, tbh);
        }
        /* Synchronize the mft mirror now if not @sync. */
        if (is_mft && !sync)
                goto do_mirror;
do_wait:
        /* Wait on i/o completion of buffers. */
        for (i = 0; i < nr_bhs; i++) {
                tbh = bhs[i];
                if (!tbh)
                        continue;
                wait_on_buffer(tbh);
                if (unlikely(!buffer_uptodate(tbh))) {
                        ntfs_error(vol->sb, "I/O error while writing ntfs "
                                        "record buffer (inode 0x%lx, "
                                        "attribute type 0x%x, page index "
                                        "0x%lx, page offset 0x%lx)!  Unmount "
                                        "and run chkdsk.", vi->i_ino, ni->type,
                                        page->index, bh_offset(tbh));
                        if (!err || err == -ENOMEM)
                                err = -EIO;
                        /*
                         * Set the buffer uptodate so the page and buffer
                         * states do not become out of sync.
                         */
                        set_buffer_uptodate(tbh);
                }
        }
        /* If @sync, now synchronize the mft mirror. */
        if (is_mft && sync) {
do_mirror:
                for (i = 0; i < nr_bhs; i++) {
                        unsigned long mft_no;
                        unsigned int ofs;

                        /*
                         * Skip buffers which are not at the beginning of
                         * records.
                         */
                        if (i % bhs_per_rec)
                                continue;
                        tbh = bhs[i];
                        /* Skip removed buffers (and hence records). */
                        if (!tbh)
                                continue;
                        ofs = bh_offset(tbh);
                        /* Get the mft record number. */
                        mft_no = (((s64)page->index << PAGE_CACHE_SHIFT) + ofs)
                                        >> rec_size_bits;
                        if (mft_no < vol->mftmirr_size)
                                ntfs_sync_mft_mirror(vol, mft_no,
                                                (MFT_RECORD*)(kaddr + ofs),
                                                sync);
                }
                if (!sync)
                        goto do_wait;
        }
        /* Remove the mst protection fixups again. */
        for (i = 0; i < nr_bhs; i++) {
                if (!(i % bhs_per_rec)) {
                        tbh = bhs[i];
                        if (!tbh)
                                continue;
                        post_write_mst_fixup((NTFS_RECORD*)(kaddr +
                                        bh_offset(tbh)));
                }
        }
        flush_dcache_page(page);
unm_done:
        /* Unlock any locked inodes. */
        while (nr_locked_nis-- > 0) {
                ntfs_inode *tni, *base_tni;
                
                tni = locked_nis[nr_locked_nis];
                /* Get the base inode. */
                down(&tni->extent_lock);
                if (tni->nr_extents >= 0)
                        base_tni = tni;
                else {
                        base_tni = tni->ext.base_ntfs_ino;
                        BUG_ON(!base_tni);
                }
                up(&tni->extent_lock);
                ntfs_debug("Unlocking %s inode 0x%lx.",
                                tni == base_tni ? "base" : "extent",
                                tni->mft_no);
                up(&tni->mrec_lock);
                atomic_dec(&tni->count);
                iput(VFS_I(base_tni));
        }
        SetPageUptodate(page);
        kunmap(page);
done:
        if (unlikely(err && err != -ENOMEM)) {
                /*
                 * Set page error if there is only one ntfs record in the page.
                 * Otherwise we would loose per-record granularity.
                 */
                if (ni->itype.index.block_size == PAGE_CACHE_SIZE)
                        SetPageError(page);
                NVolSetErrors(vol);
        }
        if (page_is_dirty) {
                ntfs_debug("Page still contains one or more dirty ntfs "
                                "records.  Redirtying the page starting at "
                                "record 0x%lx.", page->index <<
                                (PAGE_CACHE_SHIFT - rec_size_bits));
                redirty_page_for_writepage(wbc, page);
                unlock_page(page);
        } else {
                /*
                 * Keep the VM happy.  This must be done otherwise the
                 * radix-tree tag PAGECACHE_TAG_DIRTY remains set even though
                 * the page is clean.
                 */
                BUG_ON(PageWriteback(page));
                set_page_writeback(page);
                unlock_page(page);
                end_page_writeback(page);
        }
        if (likely(!err))
                ntfs_debug("Done.");
        return err;
}

/**
 * ntfs_writepage - write a @page to the backing store
 * @page:       page cache page to write out
 * @wbc:        writeback control structure
 *
 * This is called from the VM when it wants to have a dirty ntfs page cache
 * page cleaned.  The VM has already locked the page and marked it clean.
 *
 * For non-resident attributes, ntfs_writepage() writes the @page by calling
 * the ntfs version of the generic block_write_full_page() function,
 * ntfs_write_block(), which in turn if necessary creates and writes the
 * buffers associated with the page asynchronously.
 *
 * For resident attributes, OTOH, ntfs_writepage() writes the @page by copying
 * the data to the mft record (which at this stage is most likely in memory).
 * The mft record is then marked dirty and written out asynchronously via the
 * vfs inode dirty code path for the inode the mft record belongs to or via the
 * vm page dirty code path for the page the mft record is in.
 *
 * Based on ntfs_readpage() and fs/buffer.c::block_write_full_page().
 *
 * Return 0 on success and -errno on error.
 */
static int ntfs_writepage(struct page *page, struct writeback_control *wbc)
{
        loff_t i_size;
        struct inode *vi = page->mapping->host;
        ntfs_inode *base_ni = NULL, *ni = NTFS_I(vi);
        char *kaddr;
        ntfs_attr_search_ctx *ctx = NULL;
        MFT_RECORD *m = NULL;
        u32 attr_len;
        int err;

        BUG_ON(!PageLocked(page));
        /*
         * If a previous ntfs_truncate() failed, repeat it and abort if it
         * fails again.
         */
        if (unlikely(NInoTruncateFailed(ni))) {
                down_write(&vi->i_alloc_sem);
                err = ntfs_truncate(vi);
                up_write(&vi->i_alloc_sem);
                if (err || NInoTruncateFailed(ni)) {
                        if (!err)
                                err = -EIO;
                        goto err_out;
                }
        }
        i_size = i_size_read(vi);
        /* Is the page fully outside i_size? (truncate in progress) */
        if (unlikely(page->index >= (i_size + PAGE_CACHE_SIZE - 1) >>
                        PAGE_CACHE_SHIFT)) {
                /*
                 * The page may have dirty, unmapped buffers.  Make them
                 * freeable here, so the page does not leak.
                 */
                block_invalidatepage(page, 0);
                unlock_page(page);
                ntfs_debug("Write outside i_size - truncated?");
                return 0;
        }
        /* NInoNonResident() == NInoIndexAllocPresent() */
        if (NInoNonResident(ni)) {
                /*
                 * Only unnamed $DATA attributes can be compressed, encrypted,
                 * and/or sparse.
                 */
                if (ni->type == AT_DATA && !ni->name_len) {
                        /* If file is encrypted, deny access, just like NT4. */
                        if (NInoEncrypted(ni)) {
                                unlock_page(page);
                                ntfs_debug("Denying write access to encrypted "
                                                "file.");
                                return -EACCES;
                        }
                        /* Compressed data streams are handled in compress.c. */
                        if (NInoCompressed(ni)) {
                                // TODO: Implement and replace this check with
                                // return ntfs_write_compressed_block(page);
                                unlock_page(page);
                                ntfs_error(vi->i_sb, "Writing to compressed "
                                                "files is not supported yet. "
                                                "Sorry.");
                                return -EOPNOTSUPP;
                        }
                        // TODO: Implement and remove this check.
                        if (NInoSparse(ni)) {
                                unlock_page(page);
                                ntfs_error(vi->i_sb, "Writing to sparse files "
                                                "is not supported yet. Sorry.");
                                return -EOPNOTSUPP;
                        }
                }
                /* We have to zero every time due to mmap-at-end-of-file. */
                if (page->index >= (i_size >> PAGE_CACHE_SHIFT)) {
                        /* The page straddles i_size. */
                        unsigned int ofs = i_size & ~PAGE_CACHE_MASK;
                        kaddr = kmap_atomic(page, KM_USER0);
                        memset(kaddr + ofs, 0, PAGE_CACHE_SIZE - ofs);
                        flush_dcache_page(page);
                        kunmap_atomic(kaddr, KM_USER0);
                }
                /* Handle mst protected attributes. */
                if (NInoMstProtected(ni))
                        return ntfs_write_mst_block(page, wbc);
                /* Normal data stream. */
                return ntfs_write_block(page, wbc);
        }
        /*
         * Attribute is resident, implying it is not compressed, encrypted,
         * sparse, or mst protected.  This also means the attribute is smaller
         * than an mft record and hence smaller than a page, so can simply
         * return error on any pages with index above 0.
         */
        BUG_ON(page_has_buffers(page));
        BUG_ON(!PageUptodate(page));
        if (unlikely(page->index > 0)) {
                ntfs_error(vi->i_sb, "BUG()! page->index (0x%lx) > 0.  "
                                "Aborting write.", page->index);
                BUG_ON(PageWriteback(page));
                set_page_writeback(page);
                unlock_page(page);
                end_page_writeback(page);
                return -EIO;
        }
        if (!NInoAttr(ni))
                base_ni = ni;
        else
                base_ni = ni->ext.base_ntfs_ino;
        /* Map, pin, and lock the mft record. */
        m = map_mft_record(base_ni);
        if (IS_ERR(m)) {
                err = PTR_ERR(m);
                m = NULL;
                ctx = NULL;
                goto err_out;
        }
        ctx = ntfs_attr_get_search_ctx(base_ni, m);
        if (unlikely(!ctx)) {
                err = -ENOMEM;
                goto err_out;
        }
        err = ntfs_attr_lookup(ni->type, ni->name, ni->name_len,
                        CASE_SENSITIVE, 0, NULL, 0, ctx);
        if (unlikely(err))
                goto err_out;
        /*
         * Keep the VM happy.  This must be done otherwise the radix-tree tag
         * PAGECACHE_TAG_DIRTY remains set even though the page is clean.
         */
        BUG_ON(PageWriteback(page));
        set_page_writeback(page);
        unlock_page(page);

        /*
         * Here, we don't need to zero the out of bounds area everytime because
         * the below memcpy() already takes care of the mmap-at-end-of-file
         * requirements. If the file is converted to a non-resident one, then
         * the code path use is switched to the non-resident one where the
         * zeroing happens on each ntfs_writepage() invocation.
         *
         * The above also applies nicely when i_size is decreased.
         *
         * When i_size is increased, the memory between the old and new i_size
         * _must_ be zeroed (or overwritten with new data). Otherwise we will
         * expose data to userspace/disk which should never have been exposed.
         *
         * FIXME: Ensure that i_size increases do the zeroing/overwriting and
         * if we cannot guarantee that, then enable the zeroing below.  If the
         * zeroing below is enabled, we MUST move the unlock_page() from above
         * to after the kunmap_atomic(), i.e. just before the
         * end_page_writeback().
         * UPDATE: ntfs_prepare/commit_write() do the zeroing on i_size
         * increases for resident attributes so those are ok.
         * TODO: ntfs_truncate(), others?
         */

        attr_len = le32_to_cpu(ctx->attr->data.resident.value_length);
        i_size = i_size_read(vi);
        kaddr = kmap_atomic(page, KM_USER0);
        if (unlikely(attr_len > i_size)) {
                /* Zero out of bounds area in the mft record. */
                memset((u8*)ctx->attr + le16_to_cpu(
                                ctx->attr->data.resident.value_offset) +
                                i_size, 0, attr_len - i_size);
                attr_len = i_size;
        }
        /* Copy the data from the page to the mft record. */
        memcpy((u8*)ctx->attr +
                        le16_to_cpu(ctx->attr->data.resident.value_offset),
                        kaddr, attr_len);
        flush_dcache_mft_record_page(ctx->ntfs_ino);
        /* Zero out of bounds area in the page cache page. */
        memset(kaddr + attr_len, 0, PAGE_CACHE_SIZE - attr_len);
        flush_dcache_page(page);
        kunmap_atomic(kaddr, KM_USER0);

        end_page_writeback(page);

        /* Mark the mft record dirty, so it gets written back. */
        mark_mft_record_dirty(ctx->ntfs_ino);
        ntfs_attr_put_search_ctx(ctx);
        unmap_mft_record(base_ni);
        return 0;
err_out:
        if (err == -ENOMEM) {
                ntfs_warning(vi->i_sb, "Error allocating memory. Redirtying "
                                "page so we try again later.");
                /*
                 * Put the page back on mapping->dirty_pages, but leave its
                 * buffers' dirty state as-is.
                 */
                redirty_page_for_writepage(wbc, page);
                err = 0;
        } else {
                ntfs_error(vi->i_sb, "Resident attribute write failed with "
                                "error %i.", err);
                SetPageError(page);
                NVolSetErrors(ni->vol);
                make_bad_inode(vi);
        }
        unlock_page(page);
        if (ctx)
                ntfs_attr_put_search_ctx(ctx);
        if (m)
                unmap_mft_record(base_ni);
        return err;
}

/**
 * ntfs_prepare_nonresident_write -
 *
 */
static int ntfs_prepare_nonresident_write(struct page *page,
                unsigned from, unsigned to)
{
        VCN vcn;
        LCN lcn;
        s64 initialized_size;
        loff_t i_size;
        sector_t block, ablock, iblock;
        struct inode *vi;
        ntfs_inode *ni;
        ntfs_volume *vol;
        runlist_element *rl;
        struct buffer_head *bh, *head, *wait[2], **wait_bh = wait;
        unsigned long flags;
        unsigned int vcn_ofs, block_start, block_end, blocksize;
        int err;
        BOOL is_retry;
        unsigned char blocksize_bits;

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

        ntfs_debug("Entering for inode 0x%lx, attribute type 0x%x, page index "
                        "0x%lx, from = %u, to = %u.", ni->mft_no, ni->type,
                        page->index, from, to);

        BUG_ON(!NInoNonResident(ni));

        blocksize_bits = vi->i_blkbits;
        blocksize = 1 << blocksize_bits;

        /*
         * create_empty_buffers() will create uptodate/dirty buffers if the
         * page is uptodate/dirty.
         */
        if (!page_has_buffers(page))
                create_empty_buffers(page, blocksize, 0);
        bh = head = page_buffers(page);
        if (unlikely(!bh))
                return -ENOMEM;

        /* The first block in the page. */
        block = (s64)page->index << (PAGE_CACHE_SHIFT - blocksize_bits);

        read_lock_irqsave(&ni->size_lock, flags);
        /*
         * The first out of bounds block for the allocated size. No need to
         * round up as allocated_size is in multiples of cluster size and the
         * minimum cluster size is 512 bytes, which is equal to the smallest
         * blocksize.
         */
        ablock = ni->allocated_size >> blocksize_bits;

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

        /* The last (fully or partially) initialized block. */
        iblock = initialized_size >> blocksize_bits;

        /* Loop through all the buffers in the page. */
        block_start = 0;
        rl = NULL;
        err = 0;
        do {
                block_end = block_start + blocksize;
                /*
                 * If buffer @bh is outside the write, just mark it uptodate
                 * if the page is uptodate and continue with the next buffer.
                 */
                if (block_end <= from || block_start >= to) {
                        if (PageUptodate(page)) {
                                if (!buffer_uptodate(bh))
                                        set_buffer_uptodate(bh);
                        }
                        continue;
                }
                /*
                 * @bh is at least partially being written to.
                 * Make sure it is not marked as new.
                 */
                //if (buffer_new(bh))
                //      clear_buffer_new(bh);

                if (block >= ablock) {
                        // TODO: block is above allocated_size, need to
                        // allocate it. Best done in one go to accommodate not
                        // only block but all above blocks up to and including:
                        // ((page->index << PAGE_CACHE_SHIFT) + to + blocksize
                        // - 1) >> blobksize_bits. Obviously will need to round
                        // up to next cluster boundary, too. This should be
                        // done with a helper function, so it can be reused.
                        ntfs_error(vol->sb, "Writing beyond allocated size "
                                        "is not supported yet. Sorry.");
                        err = -EOPNOTSUPP;
                        goto err_out;
                        // Need to update ablock.
                        // Need to set_buffer_new() on all block bhs that are
                        // newly allocated.
                }
                /*
                 * Now we have enough allocated size to fulfill the whole
                 * request, i.e. block < ablock is true.
                 */
                if (unlikely((block >= iblock) &&
                                (initialized_size < i_size))) {
                        /*
                         * If this page is fully outside initialized size, zero
                         * out all pages between the current initialized size
                         * and the current page. Just use ntfs_readpage() to do
                         * the zeroing transparently.
                         */
                        if (block > iblock) {
                                // TODO:
                                // For each page do:
                                // - read_cache_page()
                                // Again for each page do:
                                // - wait_on_page_locked()
                                // - Check (PageUptodate(page) &&
                                //                      !PageError(page))
                                // Update initialized size in the attribute and
                                // in the inode.
                                // Again, for each page do:
                                //      __set_page_dirty_buffers();
                                // page_cache_release()
                                // We don't need to wait on the writes.
                                // Update iblock.
                        }
                        /*
                         * The current page straddles initialized size. Zero
                         * all non-uptodate buffers and set them uptodate (and
                         * dirty?). Note, there aren't any non-uptodate buffers
                         * if the page is uptodate.
                         * FIXME: For an uptodate page, the buffers may need to
                         * be written out because they were not initialized on
                         * disk before.
                         */
                        if (!PageUptodate(page)) {
                                // TODO:
                                // Zero any non-uptodate buffers up to i_size.
                                // Set them uptodate and dirty.
                        }
                        // TODO:
                        // Update initialized size in the attribute and in the
                        // inode (up to i_size).
                        // Update iblock.
                        // FIXME: This is inefficient. Try to batch the two
                        // size changes to happen in one go.
                        ntfs_error(vol->sb, "Writing beyond initialized size "
                                        "is not supported yet. Sorry.");
                        err = -EOPNOTSUPP;
                        goto err_out;
                        // Do NOT set_buffer_new() BUT DO clear buffer range
                        // outside write request range.
                        // set_buffer_uptodate() on complete buffers as well as
                        // set_buffer_dirty().
                }

                /* Need to map unmapped buffers. */
                if (!buffer_mapped(bh)) {
                        /* Unmapped buffer. Need to map it. */
                        bh->b_bdev = vol->sb->s_bdev;

                        /* Convert block into corresponding vcn and offset. */
                        vcn = (VCN)block << blocksize_bits >>
                                        vol->cluster_size_bits;
                        vcn_ofs = ((VCN)block << blocksize_bits) &
                                        vol->cluster_size_mask;

                        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;
                        if (unlikely(lcn < 0)) {
                                /*
                                 * We extended the attribute allocation above.
                                 * If we hit an ENOENT here it means that the
                                 * allocation was insufficient which is a bug.
                                 */
                                BUG_ON(lcn == LCN_ENOENT);

                                /* It is a hole, need to instantiate it. */
                                if (lcn == LCN_HOLE) {
                                        // TODO: Instantiate the hole.
                                        // clear_buffer_new(bh);
                                        // unmap_underlying_metadata(bh->b_bdev,
                                        //              bh->b_blocknr);
                                        // For non-uptodate buffers, need to
                                        // zero out the region outside the
                                        // request in this bh or all bhs,
                                        // depending on what we implemented
                                        // above.
                                        // Need to flush_dcache_page().
                                        // Or could use set_buffer_new()
                                        // instead?
                                        ntfs_error(vol->sb, "Writing into "
                                                        "sparse regions is "
                                                        "not supported yet. "
                                                        "Sorry.");
                                        err = -EOPNOTSUPP;
                                        goto err_out;
                                } else if (!is_retry &&
                                                lcn == LCN_RL_NOT_MAPPED) {
                                        is_retry = TRUE;
                                        /*
                                         * Attempt to map runlist, dropping
                                         * lock for the duration.
                                         */
                                        up_read(&ni->runlist.lock);
                                        err = ntfs_map_runlist(ni, vcn);
                                        if (likely(!err))
                                                goto lock_retry_remap;
                                        rl = NULL;
                                        lcn = err;
                                }
                                /*
                                 * Failed to map the buffer, even after
                                 * retrying.
                                 */
                                bh->b_blocknr = -1;
                                ntfs_error(vol->sb, "Failed to write to inode "
                                                "0x%lx, attribute type 0x%x, "
                                                "vcn 0x%llx, offset 0x%x "
                                                "because its location on disk "
                                                "could not be determined%s "
                                                "(error code %lli).",
                                                ni->mft_no, ni->type,
                                                (unsigned long long)vcn,
                                                vcn_ofs, is_retry ? " even "
                                                "after retrying" : "",
                                                (long long)lcn);
                                if (!err)
                                        err = -EIO;
                                goto err_out;
                        }
                        /* We now have a successful remap, i.e. lcn >= 0. */

                        /* Setup buffer head to correct block. */
                        bh->b_blocknr = ((lcn << vol->cluster_size_bits)
                                        + vcn_ofs) >> blocksize_bits;
                        set_buffer_mapped(bh);

                        // FIXME: Something analogous to this is needed for
                        // each newly allocated block, i.e. BH_New.
                        // FIXME: Might need to take this out of the
                        // if (!buffer_mapped(bh)) {}, depending on how we
                        // implement things during the allocated_size and
                        // initialized_size extension code above.
                        if (buffer_new(bh)) {
                                clear_buffer_new(bh);
                                unmap_underlying_metadata(bh->b_bdev,
                                                bh->b_blocknr);
                                if (PageUptodate(page)) {
                                        set_buffer_uptodate(bh);
                                        continue;
                                }
                                /*
                                 * Page is _not_ uptodate, zero surrounding
                                 * region. NOTE: This is how we decide if to
                                 * zero or not!
                                 */
                                if (block_end > to || block_start < from) {
                                        void *kaddr;

                                        kaddr = kmap_atomic(page, KM_USER0);
                                        if (block_end > to)
                                                memset(kaddr + to, 0,
                                                                block_end - to);
                                        if (block_start < from)
                                                memset(kaddr + block_start, 0,
                                                                from -
                                                                block_start);
                                        flush_dcache_page(page);
                                        kunmap_atomic(kaddr, KM_USER0);
                                }
                                continue;
                        }
                }
                /* @bh is mapped, set it uptodate if the page is uptodate. */
                if (PageUptodate(page)) {
                        if (!buffer_uptodate(bh))
                                set_buffer_uptodate(bh);
                        continue;
                }
                /*
                 * The page is not uptodate. The buffer is mapped. If it is not
                 * uptodate, and it is only partially being written to, we need
                 * to read the buffer in before the write, i.e. right now.
                 */
                if (!buffer_uptodate(bh) &&
                                (block_start < from || block_end > to)) {
                        ll_rw_block(READ, 1, &bh);
                        *wait_bh++ = bh;
                }
        } while (block++, block_start = block_end,
                        (bh = bh->b_this_page) != head);

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

        /* If we issued read requests, let them complete. */
        while (wait_bh > wait) {
                wait_on_buffer(*--wait_bh);
                if (!buffer_uptodate(*wait_bh))
                        return -EIO;
        }

        ntfs_debug("Done.");
        return 0;
err_out:
        /*
         * Zero out any newly allocated blocks to avoid exposing stale data.
         * If BH_New is set, we know that the block was newly allocated in the
         * above loop.
         * FIXME: What about initialized_size increments? Have we done all the
         * required zeroing above? If not this error handling is broken, and
         * in particular the if (block_end <= from) check is completely bogus.
         */
        bh = head;
        block_start = 0;
        is_retry = FALSE;
        do {
                block_end = block_start + blocksize;
                if (block_end <= from)
                        continue;
                if (block_start >= to)
                        break;
                if (buffer_new(bh)) {
                        void *kaddr;

                        clear_buffer_new(bh);
                        kaddr = kmap_atomic(page, KM_USER0);
                        memset(kaddr + block_start, 0, bh->b_size);
                        kunmap_atomic(kaddr, KM_USER0);
                        set_buffer_uptodate(bh);
                        mark_buffer_dirty(bh);
                        is_retry = TRUE;
                }
        } while (block_start = block_end, (bh = bh->b_this_page) != head);
        if (is_retry)
                flush_dcache_page(page);
        if (rl)
                up_read(&ni->runlist.lock);
        return err;
}

/**
 * ntfs_prepare_write - prepare a page for receiving data
 *
 * This is called from generic_file_write() with i_sem held on the inode
 * (@page->mapping->host).  The @page is locked but not kmap()ped.  The source
 * data has not yet been copied into the @page.
 *
 * Need to extend the attribute/fill in holes if necessary, create blocks and
 * make partially overwritten blocks uptodate,
 *
 * i_size is not to be modified yet.
 *
 * Return 0 on success or -errno on error.
 *
 * Should be using block_prepare_write() [support for sparse files] or
 * cont_prepare_write() [no support for sparse files].  Cannot do that due to
 * ntfs specifics but can look at them for implementation guidance.
 *
 * Note: In the range, @from is inclusive and @to is exclusive, i.e. @from is
 * the first byte in the page that will be written to and @to is the first byte
 * after the last byte that will be written to.
 */
static int ntfs_prepare_write(struct file *file, struct page *page,
                unsigned from, unsigned to)
{
        s64 new_size;
        struct inode *vi = page->mapping->host;
        ntfs_inode *base_ni = NULL, *ni = NTFS_I(vi);
        ntfs_volume *vol = ni->vol;
        ntfs_attr_search_ctx *ctx = NULL;
        MFT_RECORD *m = NULL;
        ATTR_RECORD *a;
        u8 *kaddr;
        u32 attr_len;
        int err;

        ntfs_debug("Entering for inode 0x%lx, attribute type 0x%x, page index "
                        "0x%lx, from = %u, to = %u.", vi->i_ino, ni->type,
                        page->index, from, to);
        BUG_ON(!PageLocked(page));
        BUG_ON(from > PAGE_CACHE_SIZE);
        BUG_ON(to > PAGE_CACHE_SIZE);
        BUG_ON(from > to);
        BUG_ON(NInoMstProtected(ni));
        /*
         * If a previous ntfs_truncate() failed, repeat it and abort if it
         * fails again.
         */
        if (unlikely(NInoTruncateFailed(ni))) {
                down_write(&vi->i_alloc_sem);
                err = ntfs_truncate(vi);
                up_write(&vi->i_alloc_sem);
                if (err || NInoTruncateFailed(ni)) {
                        if (!err)
                                err = -EIO;
                        goto err_out;
                }
        }
        /* If the attribute is not resident, deal with it elsewhere. */
        if (NInoNonResident(ni)) {
                /*
                 * Only unnamed $DATA attributes can be compressed, encrypted,
                 * and/or sparse.
                 */
                if (ni->type == AT_DATA && !ni->name_len) {
                        /* If file is encrypted, deny access, just like NT4. */
                        if (NInoEncrypted(ni)) {
                                ntfs_debug("Denying write access to encrypted "
                                                "file.");
                                return -EACCES;
                        }
                        /* Compressed data streams are handled in compress.c. */
                        if (NInoCompressed(ni)) {
                                // TODO: Implement and replace this check with
                                // return ntfs_write_compressed_block(page);
                                ntfs_error(vi->i_sb, "Writing to compressed "
                                                "files is not supported yet. "
                                                "Sorry.");
                                return -EOPNOTSUPP;
                        }
                        // TODO: Implement and remove this check.
                        if (NInoSparse(ni)) {
                                ntfs_error(vi->i_sb, "Writing to sparse files "
                                                "is not supported yet. Sorry.");
                                return -EOPNOTSUPP;
                        }
                }
                /* Normal data stream. */
                return ntfs_prepare_nonresident_write(page, from, to);
        }
        /*
         * Attribute is resident, implying it is not compressed, encrypted, or
         * sparse.
         */
        BUG_ON(page_has_buffers(page));
        new_size = ((s64)page->index << PAGE_CACHE_SHIFT) + to;
        /* If we do not need to resize the attribute allocation we are done. */
        if (new_size <= i_size_read(vi))
                goto done;

        // FIXME: We abort for now as this code is not safe.
        ntfs_error(vi->i_sb, "Changing the file size is not supported yet.  "
                        "Sorry.");
        return -EOPNOTSUPP;

        /* Map, pin, and lock the (base) mft record. */
        if (!NInoAttr(ni))
                base_ni = ni;
        else
                base_ni = ni->ext.base_ntfs_ino;
        m = map_mft_record(base_ni);
        if (IS_ERR(m)) {
                err = PTR_ERR(m);
                m = NULL;
                ctx = NULL;
                goto err_out;
        }
        ctx = ntfs_attr_get_search_ctx(base_ni, m);
        if (unlikely(!ctx)) {
                err = -ENOMEM;
                goto err_out;
        }
        err = ntfs_attr_lookup(ni->type, ni->name, ni->name_len,
                        CASE_SENSITIVE, 0, NULL, 0, ctx);
        if (unlikely(err)) {
                if (err == -ENOENT)
                        err = -EIO;
                goto err_out;
        }
        m = ctx->mrec;
        a = ctx->attr;
        /* The total length of the attribute value. */
        attr_len = le32_to_cpu(a->data.resident.value_length);
        BUG_ON(i_size_read(vi) != attr_len);
        /* Check if new size is allowed in $AttrDef. */
        err = ntfs_attr_size_bounds_check(vol, ni->type, new_size);
        if (unlikely(err)) {
                if (err == -ERANGE) {
                        ntfs_error(vol->sb, "Write would cause the inode "
                                        "0x%lx to exceed the maximum size for "
                                        "its attribute type (0x%x).  Aborting "
                                        "write.", vi->i_ino,
                                        le32_to_cpu(ni->type));
                } else {
                        ntfs_error(vol->sb, "Inode 0x%lx has unknown "
                                        "attribute type 0x%x.  Aborting "
                                        "write.", vi->i_ino,
                                        le32_to_cpu(ni->type));
                        err = -EIO;
                }
                goto err_out2;
        }
        /*
         * Extend the attribute record to be able to store the new attribute
         * size.
         */
        if (new_size >= vol->mft_record_size || ntfs_attr_record_resize(m, a,
                        le16_to_cpu(a->data.resident.value_offset) +
                        new_size)) {
                /* Not enough space in the mft record. */
                ntfs_error(vol->sb, "Not enough space in the mft record for "
                                "the resized attribute value.  This is not "
                                "supported yet.  Aborting write.");
                err = -EOPNOTSUPP;
                goto err_out2;
        }
        /*
         * We have enough space in the mft record to fit the write.  This
         * implies the attribute is smaller than the mft record and hence the
         * attribute must be in a single page and hence page->index must be 0.
         */
        BUG_ON(page->index);
        /*
         * If the beginning of the write is past the old size, enlarge the
         * attribute value up to the beginning of the write and fill it with
         * zeroes.
         */
        if (from > attr_len) {
                memset((u8*)a + le16_to_cpu(a->data.resident.value_offset) +
                                attr_len, 0, from - attr_len);
                a->data.resident.value_length = cpu_to_le32(from);
                /* Zero the corresponding area in the page as well. */
                if (PageUptodate(page)) {
                        kaddr = kmap_atomic(page, KM_USER0);
                        memset(kaddr + attr_len, 0, from - attr_len);
                        kunmap_atomic(kaddr, KM_USER0);
                        flush_dcache_page(page);
                }
        }
        flush_dcache_mft_record_page(ctx->ntfs_ino);
        mark_mft_record_dirty(ctx->ntfs_ino);
        ntfs_attr_put_search_ctx(ctx);
        unmap_mft_record(base_ni);
        /*
         * Because resident attributes are handled by memcpy() to/from the
         * corresponding MFT record, and because this form of i/o is byte
         * aligned rather than block aligned, there is no need to bring the
         * page uptodate here as in the non-resident case where we need to
         * bring the buffers straddled by the write uptodate before
         * generic_file_write() does the copying from userspace.
         *
         * We thus defer the uptodate bringing of the page region outside the
         * region written to to ntfs_commit_write(), which makes the code
         * simpler and saves one atomic kmap which is good.
         */
done:
        ntfs_debug("Done.");
        return 0;
err_out:
        if (err == -ENOMEM)
                ntfs_warning(vi->i_sb, "Error allocating memory required to "
                                "prepare the write.");
        else {
                ntfs_error(vi->i_sb, "Resident attribute prepare write failed "
                                "with error %i.", err);
                NVolSetErrors(vol);
                make_bad_inode(vi);
        }
err_out2:
        if (ctx)
                ntfs_attr_put_search_ctx(ctx);
        if (m)
                unmap_mft_record(base_ni);
        return err;
}

/**
 * ntfs_commit_nonresident_write -
 *
 */
static int ntfs_commit_nonresident_write(struct page *page,
                unsigned from, unsigned to)
{
        s64 pos = ((s64)page->index << PAGE_CACHE_SHIFT) + to;
        struct inode *vi = page->mapping->host;
        struct buffer_head *bh, *head;
        unsigned int block_start, block_end, blocksize;
        BOOL partial;

        ntfs_debug("Entering for inode 0x%lx, attribute type 0x%x, page index "
                        "0x%lx, from = %u, to = %u.", vi->i_ino,
                        NTFS_I(vi)->type, page->index, from, to);
        blocksize = 1 << vi->i_blkbits;

        // FIXME: We need a whole slew of special cases in here for compressed
        // files for example...
        // For now, we know ntfs_prepare_write() would have failed so we can't
        // get here in any of the cases which we have to special case, so we
        // are just a ripped off, unrolled generic_commit_write().

        bh = head = page_buffers(page);
        block_start = 0;
        partial = FALSE;
        do {
                block_end = block_start + blocksize;
                if (block_end <= from || block_start >= to) {
                        if (!buffer_uptodate(bh))
                                partial = TRUE;
                } else {
                        set_buffer_uptodate(bh);
                        mark_buffer_dirty(bh);
                }
        } while (block_start = block_end, (bh = bh->b_this_page) != head);
        /*
         * If this is a partial write which happened to make all buffers
         * uptodate then we can optimize away a bogus ->readpage() for the next
         * read().  Here we 'discover' whether the page went uptodate as a
         * result of this (potentially partial) write.
         */
        if (!partial)
                SetPageUptodate(page);
        /*
         * Not convinced about this at all.  See disparity comment above.  For
         * now we know ntfs_prepare_write() would have failed in the write
         * exceeds i_size case, so this will never trigger which is fine.
         */
        if (pos > i_size_read(vi)) {
                ntfs_error(vi->i_sb, "Writing beyond the existing file size is "
                                "not supported yet.  Sorry.");
                return -EOPNOTSUPP;
                // vi->i_size = pos;
                // mark_inode_dirty(vi);
        }
        ntfs_debug("Done.");
        return 0;
}

/**
 * ntfs_commit_write - commit the received data
 *
 * This is called from generic_file_write() with i_sem held on the inode
 * (@page->mapping->host).  The @page is locked but not kmap()ped.  The source
 * data has already been copied into the @page.  ntfs_prepare_write() has been
 * called before the data copied and it returned success so we can take the
 * results of various BUG checks and some error handling for granted.
 *
 * Need to mark modified blocks dirty so they get written out later when
 * ntfs_writepage() is invoked by the VM.
 *
 * Return 0 on success or -errno on error.
 *
 * Should be using generic_commit_write().  This marks buffers uptodate and
 * dirty, sets the page uptodate if all buffers in the page are uptodate, and
 * updates i_size if the end of io is beyond i_size.  In that case, it also
 * marks the inode dirty.
 *
 * Cannot use generic_commit_write() due to ntfs specialities but can look at
 * it for implementation guidance.
 *
 * If things have gone as outlined in ntfs_prepare_write(), then we do not
 * need to do any page content modifications here at all, except in the write
 * to resident attribute case, where we need to do the uptodate bringing here
 * which we combine with the copying into the mft record which means we save
 * one atomic kmap.
 */
static int ntfs_commit_write(struct file *file, struct page *page,
                unsigned from, unsigned to)
{
        struct inode *vi = page->mapping->host;
        ntfs_inode *base_ni, *ni = NTFS_I(vi);
        char *kaddr, *kattr;
        ntfs_attr_search_ctx *ctx;
        MFT_RECORD *m;
        ATTR_RECORD *a;
        u32 attr_len;
        int err;

        ntfs_debug("Entering for inode 0x%lx, attribute type 0x%x, page index "
                        "0x%lx, from = %u, to = %u.", vi->i_ino, ni->type,
                        page->index, from, to);
        /* If the attribute is not resident, deal with it elsewhere. */
        if (NInoNonResident(ni)) {
                /* Only unnamed $DATA attributes can be compressed/encrypted. */
                if (ni->type == AT_DATA && !ni->name_len) {
                        /* Encrypted files need separate handling. */
                        if (NInoEncrypted(ni)) {
                                // We never get here at present!
                                BUG();
                        }
                        /* Compressed data streams are handled in compress.c. */
                        if (NInoCompressed(ni)) {
                                // TODO: Implement this!
                                // return ntfs_write_compressed_block(page);
                                // We never get here at present!
                                BUG();
                        }
                }
                /* Normal data stream. */
                return ntfs_commit_nonresident_write(page, from, to);
        }
        /*
         * Attribute is resident, implying it is not compressed, encrypted, or
         * sparse.
         */
        if (!NInoAttr(ni))
                base_ni = ni;
        else
                base_ni = ni->ext.base_ntfs_ino;
        /* Map, pin, and lock the mft record. */
        m = map_mft_record(base_ni);
        if (IS_ERR(m)) {
                err = PTR_ERR(m);
                m = NULL;
                ctx = NULL;
                goto err_out;
        }
        ctx = ntfs_attr_get_search_ctx(base_ni, m);
        if (unlikely(!ctx)) {
                err = -ENOMEM;
                goto err_out;
        }
        err = ntfs_attr_lookup(ni->type, ni->name, ni->name_len,
                        CASE_SENSITIVE, 0, NULL, 0, ctx);
        if (unlikely(err)) {
                if (err == -ENOENT)
                        err = -EIO;
                goto err_out;
        }
        a = ctx->attr;
        /* The total length of the attribute value. */
        attr_len = le32_to_cpu(a->data.resident.value_length);
        BUG_ON(from > attr_len);
        kattr = (u8*)a + le16_to_cpu(a->data.resident.value_offset);
        kaddr = kmap_atomic(page, KM_USER0);
        /* Copy the received data from the page to the mft record. */
        memcpy(kattr + from, kaddr + from, to - from);
        /* Update the attribute length if necessary. */
        if (to > attr_len) {
                attr_len = to;
                a->data.resident.value_length = cpu_to_le32(attr_len);
        }
        /*
         * If the page is not uptodate, bring the out of bounds area(s)
         * uptodate by copying data from the mft record to the page.
         */
        if (!PageUptodate(page)) {
                if (from > 0)
                        memcpy(kaddr, kattr, from);
                if (to < attr_len)
                        memcpy(kaddr + to, kattr + to, attr_len - to);
                /* Zero the region outside the end of the attribute value. */
                if (attr_len < PAGE_CACHE_SIZE)
                        memset(kaddr + attr_len, 0, PAGE_CACHE_SIZE - attr_len);
                /*
                 * The probability of not having done any of the above is
                 * extremely small, so we just flush unconditionally.
                 */
                flush_dcache_page(page);
                SetPageUptodate(page);
        }
        kunmap_atomic(kaddr, KM_USER0);
        /* Update i_size if necessary. */
        if (i_size_read(vi) < attr_len) {
                unsigned long flags;

                write_lock_irqsave(&ni->size_lock, flags);
                ni->allocated_size = ni->initialized_size = attr_len;
                i_size_write(vi, attr_len);
                write_unlock_irqrestore(&ni->size_lock, flags);
        }
        /* Mark the mft record dirty, so it gets written back. */
        flush_dcache_mft_record_page(ctx->ntfs_ino);
        mark_mft_record_dirty(ctx->ntfs_ino);
        ntfs_attr_put_search_ctx(ctx);
        unmap_mft_record(base_ni);
        ntfs_debug("Done.");
        return 0;
err_out:
        if (err == -ENOMEM) {
                ntfs_warning(vi->i_sb, "Error allocating memory required to "
                                "commit the write.");
                if (PageUptodate(page)) {
                        ntfs_warning(vi->i_sb, "Page is uptodate, setting "
                                        "dirty so the write will be retried "
                                        "later on by the VM.");
                        /*
                         * Put the page on mapping->dirty_pages, but leave its
                         * buffers' dirty state as-is.
                         */
                        __set_page_dirty_nobuffers(page);
                        err = 0;
                } else
                        ntfs_error(vi->i_sb, "Page is not uptodate.  Written "
                                        "data has been lost.");
        } else {
                ntfs_error(vi->i_sb, "Resident attribute commit write failed "
                                "with error %i.", err);
                NVolSetErrors(ni->vol);
                make_bad_inode(vi);
        }
        if (ctx)
                ntfs_attr_put_search_ctx(ctx);
        if (m)
                unmap_mft_record(base_ni);
        return err;
}

#endif  /* NTFS_RW */

/**
 * ntfs_aops - general address space operations for inodes and attributes
 */
struct address_space_operations ntfs_aops = {
        .readpage       = ntfs_readpage,        /* Fill page with data. */
        .sync_page      = block_sync_page,      /* Currently, just unplugs the
                                                   disk request queue. */
#ifdef NTFS_RW
        .writepage      = ntfs_writepage,       /* Write dirty page to disk. */
        .prepare_write  = ntfs_prepare_write,   /* Prepare page and buffers
                                                   ready to receive data. */
        .commit_write   = ntfs_commit_write,    /* Commit received data. */
#endif /* NTFS_RW */
};

/**
 * ntfs_mst_aops - general address space operations for mst protecteed inodes
 *                 and attributes
 */
struct address_space_operations ntfs_mst_aops = {
        .readpage       = ntfs_readpage,        /* Fill page with data. */
        .sync_page      = block_sync_page,      /* Currently, just unplugs the
                                                   disk request queue. */
#ifdef NTFS_RW
        .writepage      = ntfs_writepage,       /* Write dirty page to disk. */
        .set_page_dirty = __set_page_dirty_nobuffers,   /* Set the page dirty
                                                   without touching the buffers
                                                   belonging to the page. */
#endif /* NTFS_RW */
};

#ifdef NTFS_RW

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

        BUG_ON(!PageUptodate(page));
        end = ofs + ni->itype.index.block_size;
        bh_size = 1 << VFS_I(ni)->i_blkbits;
        spin_lock(&mapping->private_lock);
        if (unlikely(!page_has_buffers(page))) {
                spin_unlock(&mapping->private_lock);
                bh = head = alloc_page_buffers(page, bh_size, 1);
                spin_lock(&mapping->private_lock);
                if (likely(!page_has_buffers(page))) {
                        struct buffer_head *tail;

                        do {
                                set_buffer_uptodate(bh);
                                tail = bh;
                                bh = bh->b_this_page;
                        } while (bh);
                        tail->b_this_page = head;
                        attach_page_buffers(page, head);
                } else
                        buffers_to_free = bh;
        }
        bh = head = page_buffers(page);
        do {
                bh_ofs = bh_offset(bh);
                if (bh_ofs + bh_size <= ofs)
                        continue;
                if (unlikely(bh_ofs >= end))
                        break;
                set_buffer_dirty(bh);
        } while ((bh = bh->b_this_page) != head);
        spin_unlock(&mapping->private_lock);
        __set_page_dirty_nobuffers(page);
        if (unlikely(buffers_to_free)) {
                do {
                        bh = buffers_to_free->b_this_page;
                        free_buffer_head(buffers_to_free);
                        buffers_to_free = bh;
                } while (buffers_to_free);
        }
}

#endif /* NTFS_RW */