xfs: ensure buffer types are set correctly

[firefly-linux-kernel-4.4.55.git] / fs / xfs / xfs_buf_item.c

/*
 * Copyright (c) 2000-2005 Silicon Graphics, Inc.
 * All Rights Reserved.
 *
 * This program 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.
 *
 * This program is distributed in the hope that it would 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; if not, write the Free Software Foundation,
 * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
#include "xfs.h"
#include "xfs_fs.h"
#include "xfs_types.h"
#include "xfs_bit.h"
#include "xfs_log.h"
#include "xfs_trans.h"
#include "xfs_sb.h"
#include "xfs_ag.h"
#include "xfs_mount.h"
#include "xfs_buf_item.h"
#include "xfs_trans_priv.h"
#include "xfs_error.h"
#include "xfs_trace.h"


kmem_zone_t     *xfs_buf_item_zone;

static inline struct xfs_buf_log_item *BUF_ITEM(struct xfs_log_item *lip)
{
        return container_of(lip, struct xfs_buf_log_item, bli_item);
}

STATIC void     xfs_buf_do_callbacks(struct xfs_buf *bp);

/*
 * This returns the number of log iovecs needed to log the
 * given buf log item.
 *
 * It calculates this as 1 iovec for the buf log format structure
 * and 1 for each stretch of non-contiguous chunks to be logged.
 * Contiguous chunks are logged in a single iovec.
 *
 * If the XFS_BLI_STALE flag has been set, then log nothing.
 */
STATIC uint
xfs_buf_item_size_segment(
        struct xfs_buf_log_item *bip,
        struct xfs_buf_log_format *blfp)
{
        struct xfs_buf          *bp = bip->bli_buf;
        uint                    nvecs;
        int                     next_bit;
        int                     last_bit;

        last_bit = xfs_next_bit(blfp->blf_data_map, blfp->blf_map_size, 0);
        if (last_bit == -1)
                return 0;

        /*
         * initial count for a dirty buffer is 2 vectors - the format structure
         * and the first dirty region.
         */
        nvecs = 2;

        while (last_bit != -1) {
                /*
                 * This takes the bit number to start looking from and
                 * returns the next set bit from there.  It returns -1
                 * if there are no more bits set or the start bit is
                 * beyond the end of the bitmap.
                 */
                next_bit = xfs_next_bit(blfp->blf_data_map, blfp->blf_map_size,
                                        last_bit + 1);
                /*
                 * If we run out of bits, leave the loop,
                 * else if we find a new set of bits bump the number of vecs,
                 * else keep scanning the current set of bits.
                 */
                if (next_bit == -1) {
                        break;
                } else if (next_bit != last_bit + 1) {
                        last_bit = next_bit;
                        nvecs++;
                } else if (xfs_buf_offset(bp, next_bit * XFS_BLF_CHUNK) !=
                           (xfs_buf_offset(bp, last_bit * XFS_BLF_CHUNK) +
                            XFS_BLF_CHUNK)) {
                        last_bit = next_bit;
                        nvecs++;
                } else {
                        last_bit++;
                }
        }

        return nvecs;
}

/*
 * This returns the number of log iovecs needed to log the given buf log item.
 *
 * It calculates this as 1 iovec for the buf log format structure and 1 for each
 * stretch of non-contiguous chunks to be logged.  Contiguous chunks are logged
 * in a single iovec.
 *
 * Discontiguous buffers need a format structure per region that that is being
 * logged. This makes the changes in the buffer appear to log recovery as though
 * they came from separate buffers, just like would occur if multiple buffers
 * were used instead of a single discontiguous buffer. This enables
 * discontiguous buffers to be in-memory constructs, completely transparent to
 * what ends up on disk.
 *
 * If the XFS_BLI_STALE flag has been set, then log nothing but the buf log
 * format structures.
 */
STATIC uint
xfs_buf_item_size(
        struct xfs_log_item     *lip)
{
        struct xfs_buf_log_item *bip = BUF_ITEM(lip);
        uint                    nvecs;
        int                     i;

        ASSERT(atomic_read(&bip->bli_refcount) > 0);
        if (bip->bli_flags & XFS_BLI_STALE) {
                /*
                 * The buffer is stale, so all we need to log
                 * is the buf log format structure with the
                 * cancel flag in it.
                 */
                trace_xfs_buf_item_size_stale(bip);
                ASSERT(bip->__bli_format.blf_flags & XFS_BLF_CANCEL);
                return bip->bli_format_count;
        }

        ASSERT(bip->bli_flags & XFS_BLI_LOGGED);

        /*
         * the vector count is based on the number of buffer vectors we have
         * dirty bits in. This will only be greater than one when we have a
         * compound buffer with more than one segment dirty. Hence for compound
         * buffers we need to track which segment the dirty bits correspond to,
         * and when we move from one segment to the next increment the vector
         * count for the extra buf log format structure that will need to be
         * written.
         */
        nvecs = 0;
        for (i = 0; i < bip->bli_format_count; i++) {
                nvecs += xfs_buf_item_size_segment(bip, &bip->bli_formats[i]);
        }

        trace_xfs_buf_item_size(bip);
        return nvecs;
}

static struct xfs_log_iovec *
xfs_buf_item_format_segment(
        struct xfs_buf_log_item *bip,
        struct xfs_log_iovec    *vecp,
        uint                    offset,
        struct xfs_buf_log_format *blfp)
{
        struct xfs_buf  *bp = bip->bli_buf;
        uint            base_size;
        uint            nvecs;
        int             first_bit;
        int             last_bit;
        int             next_bit;
        uint            nbits;
        uint            buffer_offset;

        /* copy the flags across from the base format item */
        blfp->blf_flags = bip->__bli_format.blf_flags;

        /*
         * Base size is the actual size of the ondisk structure - it reflects
         * the actual size of the dirty bitmap rather than the size of the in
         * memory structure.
         */
        base_size = offsetof(struct xfs_buf_log_format, blf_data_map) +
                        (blfp->blf_map_size * sizeof(blfp->blf_data_map[0]));

        nvecs = 0;
        first_bit = xfs_next_bit(blfp->blf_data_map, blfp->blf_map_size, 0);
        if (!(bip->bli_flags & XFS_BLI_STALE) && first_bit == -1) {
                /*
                 * If the map is not be dirty in the transaction, mark
                 * the size as zero and do not advance the vector pointer.
                 */
                goto out;
        }

        vecp->i_addr = blfp;
        vecp->i_len = base_size;
        vecp->i_type = XLOG_REG_TYPE_BFORMAT;
        vecp++;
        nvecs = 1;

        if (bip->bli_flags & XFS_BLI_STALE) {
                /*
                 * The buffer is stale, so all we need to log
                 * is the buf log format structure with the
                 * cancel flag in it.
                 */
                trace_xfs_buf_item_format_stale(bip);
                ASSERT(blfp->blf_flags & XFS_BLF_CANCEL);
                goto out;
        }

        /*
         * Fill in an iovec for each set of contiguous chunks.
         */

        last_bit = first_bit;
        nbits = 1;
        for (;;) {
                /*
                 * This takes the bit number to start looking from and
                 * returns the next set bit from there.  It returns -1
                 * if there are no more bits set or the start bit is
                 * beyond the end of the bitmap.
                 */
                next_bit = xfs_next_bit(blfp->blf_data_map, blfp->blf_map_size,
                                        (uint)last_bit + 1);
                /*
                 * If we run out of bits fill in the last iovec and get
                 * out of the loop.
                 * Else if we start a new set of bits then fill in the
                 * iovec for the series we were looking at and start
                 * counting the bits in the new one.
                 * Else we're still in the same set of bits so just
                 * keep counting and scanning.
                 */
                if (next_bit == -1) {
                        buffer_offset = offset + first_bit * XFS_BLF_CHUNK;
                        vecp->i_addr = xfs_buf_offset(bp, buffer_offset);
                        vecp->i_len = nbits * XFS_BLF_CHUNK;
                        vecp->i_type = XLOG_REG_TYPE_BCHUNK;
                        nvecs++;
                        break;
                } else if (next_bit != last_bit + 1) {
                        buffer_offset = offset + first_bit * XFS_BLF_CHUNK;
                        vecp->i_addr = xfs_buf_offset(bp, buffer_offset);
                        vecp->i_len = nbits * XFS_BLF_CHUNK;
                        vecp->i_type = XLOG_REG_TYPE_BCHUNK;
                        nvecs++;
                        vecp++;
                        first_bit = next_bit;
                        last_bit = next_bit;
                        nbits = 1;
                } else if (xfs_buf_offset(bp, offset +
                                              (next_bit << XFS_BLF_SHIFT)) !=
                           (xfs_buf_offset(bp, offset +
                                               (last_bit << XFS_BLF_SHIFT)) +
                            XFS_BLF_CHUNK)) {
                        buffer_offset = offset + first_bit * XFS_BLF_CHUNK;
                        vecp->i_addr = xfs_buf_offset(bp, buffer_offset);
                        vecp->i_len = nbits * XFS_BLF_CHUNK;
                        vecp->i_type = XLOG_REG_TYPE_BCHUNK;
                        nvecs++;
                        vecp++;
                        first_bit = next_bit;
                        last_bit = next_bit;
                        nbits = 1;
                } else {
                        last_bit++;
                        nbits++;
                }
        }
out:
        blfp->blf_size = nvecs;
        return vecp;
}

/*
 * This is called to fill in the vector of log iovecs for the
 * given log buf item.  It fills the first entry with a buf log
 * format structure, and the rest point to contiguous chunks
 * within the buffer.
 */
STATIC void
xfs_buf_item_format(
        struct xfs_log_item     *lip,
        struct xfs_log_iovec    *vecp)
{
        struct xfs_buf_log_item *bip = BUF_ITEM(lip);
        struct xfs_buf          *bp = bip->bli_buf;
        uint                    offset = 0;
        int                     i;

        ASSERT(atomic_read(&bip->bli_refcount) > 0);
        ASSERT((bip->bli_flags & XFS_BLI_LOGGED) ||
               (bip->bli_flags & XFS_BLI_STALE));
        ASSERT((bip->bli_flags & XFS_BLI_STALE) ||
               (xfs_blft_from_flags(&bip->__bli_format) > XFS_BLFT_UNKNOWN_BUF
                && xfs_blft_from_flags(&bip->__bli_format) < XFS_BLFT_MAX_BUF));


        /*
         * If it is an inode buffer, transfer the in-memory state to the
         * format flags and clear the in-memory state. We do not transfer
         * this state if the inode buffer allocation has not yet been committed
         * to the log as setting the XFS_BLI_INODE_BUF flag will prevent
         * correct replay of the inode allocation.
         */
        if (bip->bli_flags & XFS_BLI_INODE_BUF) {
                if (!((bip->bli_flags & XFS_BLI_INODE_ALLOC_BUF) &&
                      xfs_log_item_in_current_chkpt(lip)))
                        bip->__bli_format.blf_flags |= XFS_BLF_INODE_BUF;
                bip->bli_flags &= ~XFS_BLI_INODE_BUF;
        }

        for (i = 0; i < bip->bli_format_count; i++) {
                vecp = xfs_buf_item_format_segment(bip, vecp, offset,
                                                &bip->bli_formats[i]);
                offset += bp->b_maps[i].bm_len;
        }

        /*
         * Check to make sure everything is consistent.
         */
        trace_xfs_buf_item_format(bip);
}

/*
 * This is called to pin the buffer associated with the buf log item in memory
 * so it cannot be written out.
 *
 * We also always take a reference to the buffer log item here so that the bli
 * is held while the item is pinned in memory. This means that we can
 * unconditionally drop the reference count a transaction holds when the
 * transaction is completed.
 */
STATIC void
xfs_buf_item_pin(
        struct xfs_log_item     *lip)
{
        struct xfs_buf_log_item *bip = BUF_ITEM(lip);

        ASSERT(atomic_read(&bip->bli_refcount) > 0);
        ASSERT((bip->bli_flags & XFS_BLI_LOGGED) ||
               (bip->bli_flags & XFS_BLI_STALE));

        trace_xfs_buf_item_pin(bip);

        atomic_inc(&bip->bli_refcount);
        atomic_inc(&bip->bli_buf->b_pin_count);
}

/*
 * This is called to unpin the buffer associated with the buf log
 * item which was previously pinned with a call to xfs_buf_item_pin().
 *
 * Also drop the reference to the buf item for the current transaction.
 * If the XFS_BLI_STALE flag is set and we are the last reference,
 * then free up the buf log item and unlock the buffer.
 *
 * If the remove flag is set we are called from uncommit in the
 * forced-shutdown path.  If that is true and the reference count on
 * the log item is going to drop to zero we need to free the item's
 * descriptor in the transaction.
 */
STATIC void
xfs_buf_item_unpin(
        struct xfs_log_item     *lip,
        int                     remove)
{
        struct xfs_buf_log_item *bip = BUF_ITEM(lip);
        xfs_buf_t       *bp = bip->bli_buf;
        struct xfs_ail  *ailp = lip->li_ailp;
        int             stale = bip->bli_flags & XFS_BLI_STALE;
        int             freed;

        ASSERT(bp->b_fspriv == bip);
        ASSERT(atomic_read(&bip->bli_refcount) > 0);

        trace_xfs_buf_item_unpin(bip);

        freed = atomic_dec_and_test(&bip->bli_refcount);

        if (atomic_dec_and_test(&bp->b_pin_count))
                wake_up_all(&bp->b_waiters);

        if (freed && stale) {
                ASSERT(bip->bli_flags & XFS_BLI_STALE);
                ASSERT(xfs_buf_islocked(bp));
                ASSERT(XFS_BUF_ISSTALE(bp));
                ASSERT(bip->__bli_format.blf_flags & XFS_BLF_CANCEL);

                trace_xfs_buf_item_unpin_stale(bip);

                if (remove) {
                        /*
                         * If we are in a transaction context, we have to
                         * remove the log item from the transaction as we are
                         * about to release our reference to the buffer.  If we
                         * don't, the unlock that occurs later in
                         * xfs_trans_uncommit() will try to reference the
                         * buffer which we no longer have a hold on.
                         */
                        if (lip->li_desc)
                                xfs_trans_del_item(lip);

                        /*
                         * Since the transaction no longer refers to the buffer,
                         * the buffer should no longer refer to the transaction.
                         */
                        bp->b_transp = NULL;
                }

                /*
                 * If we get called here because of an IO error, we may
                 * or may not have the item on the AIL. xfs_trans_ail_delete()
                 * will take care of that situation.
                 * xfs_trans_ail_delete() drops the AIL lock.
                 */
                if (bip->bli_flags & XFS_BLI_STALE_INODE) {
                        xfs_buf_do_callbacks(bp);
                        bp->b_fspriv = NULL;
                        bp->b_iodone = NULL;
                } else {
                        spin_lock(&ailp->xa_lock);
                        xfs_trans_ail_delete(ailp, lip, SHUTDOWN_LOG_IO_ERROR);
                        xfs_buf_item_relse(bp);
                        ASSERT(bp->b_fspriv == NULL);
                }
                xfs_buf_relse(bp);
        } else if (freed && remove) {
                /*
                 * There are currently two references to the buffer - the active
                 * LRU reference and the buf log item. What we are about to do
                 * here - simulate a failed IO completion - requires 3
                 * references.
                 *
                 * The LRU reference is removed by the xfs_buf_stale() call. The
                 * buf item reference is removed by the xfs_buf_iodone()
                 * callback that is run by xfs_buf_do_callbacks() during ioend
                 * processing (via the bp->b_iodone callback), and then finally
                 * the ioend processing will drop the IO reference if the buffer
                 * is marked XBF_ASYNC.
                 *
                 * Hence we need to take an additional reference here so that IO
                 * completion processing doesn't free the buffer prematurely.
                 */
                xfs_buf_lock(bp);
                xfs_buf_hold(bp);
                bp->b_flags |= XBF_ASYNC;
                xfs_buf_ioerror(bp, EIO);
                XFS_BUF_UNDONE(bp);
                xfs_buf_stale(bp);
                xfs_buf_ioend(bp, 0);
        }
}

STATIC uint
xfs_buf_item_push(
        struct xfs_log_item     *lip,
        struct list_head        *buffer_list)
{
        struct xfs_buf_log_item *bip = BUF_ITEM(lip);
        struct xfs_buf          *bp = bip->bli_buf;
        uint                    rval = XFS_ITEM_SUCCESS;

        if (xfs_buf_ispinned(bp))
                return XFS_ITEM_PINNED;
        if (!xfs_buf_trylock(bp)) {
                /*
                 * If we have just raced with a buffer being pinned and it has
                 * been marked stale, we could end up stalling until someone else
                 * issues a log force to unpin the stale buffer. Check for the
                 * race condition here so xfsaild recognizes the buffer is pinned
                 * and queues a log force to move it along.
                 */
                if (xfs_buf_ispinned(bp))
                        return XFS_ITEM_PINNED;
                return XFS_ITEM_LOCKED;
        }

        ASSERT(!(bip->bli_flags & XFS_BLI_STALE));

        trace_xfs_buf_item_push(bip);

        if (!xfs_buf_delwri_queue(bp, buffer_list))
                rval = XFS_ITEM_FLUSHING;
        xfs_buf_unlock(bp);
        return rval;
}

/*
 * Release the buffer associated with the buf log item.  If there is no dirty
 * logged data associated with the buffer recorded in the buf log item, then
 * free the buf log item and remove the reference to it in the buffer.
 *
 * This call ignores the recursion count.  It is only called when the buffer
 * should REALLY be unlocked, regardless of the recursion count.
 *
 * We unconditionally drop the transaction's reference to the log item. If the
 * item was logged, then another reference was taken when it was pinned, so we
 * can safely drop the transaction reference now.  This also allows us to avoid
 * potential races with the unpin code freeing the bli by not referencing the
 * bli after we've dropped the reference count.
 *
 * If the XFS_BLI_HOLD flag is set in the buf log item, then free the log item
 * if necessary but do not unlock the buffer.  This is for support of
 * xfs_trans_bhold(). Make sure the XFS_BLI_HOLD field is cleared if we don't
 * free the item.
 */
STATIC void
xfs_buf_item_unlock(
        struct xfs_log_item     *lip)
{
        struct xfs_buf_log_item *bip = BUF_ITEM(lip);
        struct xfs_buf          *bp = bip->bli_buf;
        int                     aborted, clean, i;
        uint                    hold;

        /* Clear the buffer's association with this transaction. */
        bp->b_transp = NULL;

        /*
         * If this is a transaction abort, don't return early.  Instead, allow
         * the brelse to happen.  Normally it would be done for stale
         * (cancelled) buffers at unpin time, but we'll never go through the
         * pin/unpin cycle if we abort inside commit.
         */
        aborted = (lip->li_flags & XFS_LI_ABORTED) != 0;

        /*
         * Before possibly freeing the buf item, determine if we should
         * release the buffer at the end of this routine.
         */
        hold = bip->bli_flags & XFS_BLI_HOLD;

        /* Clear the per transaction state. */
        bip->bli_flags &= ~(XFS_BLI_LOGGED | XFS_BLI_HOLD);

        /*
         * If the buf item is marked stale, then don't do anything.  We'll
         * unlock the buffer and free the buf item when the buffer is unpinned
         * for the last time.
         */
        if (bip->bli_flags & XFS_BLI_STALE) {
                trace_xfs_buf_item_unlock_stale(bip);
                ASSERT(bip->__bli_format.blf_flags & XFS_BLF_CANCEL);
                if (!aborted) {
                        atomic_dec(&bip->bli_refcount);
                        return;
                }
        }

        trace_xfs_buf_item_unlock(bip);

        /*
         * If the buf item isn't tracking any data, free it, otherwise drop the
         * reference we hold to it. If we are aborting the transaction, this may
         * be the only reference to the buf item, so we free it anyway
         * regardless of whether it is dirty or not. A dirty abort implies a
         * shutdown, anyway.
         */
        clean = 1;
        for (i = 0; i < bip->bli_format_count; i++) {
                if (!xfs_bitmap_empty(bip->bli_formats[i].blf_data_map,
                             bip->bli_formats[i].blf_map_size)) {
                        clean = 0;
                        break;
                }
        }
        if (clean)
                xfs_buf_item_relse(bp);
        else if (aborted) {
                if (atomic_dec_and_test(&bip->bli_refcount)) {
                        ASSERT(XFS_FORCED_SHUTDOWN(lip->li_mountp));
                        xfs_buf_item_relse(bp);
                }
        } else
                atomic_dec(&bip->bli_refcount);

        if (!hold)
                xfs_buf_relse(bp);
}

/*
 * This is called to find out where the oldest active copy of the
 * buf log item in the on disk log resides now that the last log
 * write of it completed at the given lsn.
 * We always re-log all the dirty data in a buffer, so usually the
 * latest copy in the on disk log is the only one that matters.  For
 * those cases we simply return the given lsn.
 *
 * The one exception to this is for buffers full of newly allocated
 * inodes.  These buffers are only relogged with the XFS_BLI_INODE_BUF
 * flag set, indicating that only the di_next_unlinked fields from the
 * inodes in the buffers will be replayed during recovery.  If the
 * original newly allocated inode images have not yet been flushed
 * when the buffer is so relogged, then we need to make sure that we
 * keep the old images in the 'active' portion of the log.  We do this
 * by returning the original lsn of that transaction here rather than
 * the current one.
 */
STATIC xfs_lsn_t
xfs_buf_item_committed(
        struct xfs_log_item     *lip,
        xfs_lsn_t               lsn)
{
        struct xfs_buf_log_item *bip = BUF_ITEM(lip);

        trace_xfs_buf_item_committed(bip);

        if ((bip->bli_flags & XFS_BLI_INODE_ALLOC_BUF) && lip->li_lsn != 0)
                return lip->li_lsn;
        return lsn;
}

STATIC void
xfs_buf_item_committing(
        struct xfs_log_item     *lip,
        xfs_lsn_t               commit_lsn)
{
}

/*
 * This is the ops vector shared by all buf log items.
 */
static const struct xfs_item_ops xfs_buf_item_ops = {
        .iop_size       = xfs_buf_item_size,
        .iop_format     = xfs_buf_item_format,
        .iop_pin        = xfs_buf_item_pin,
        .iop_unpin      = xfs_buf_item_unpin,
        .iop_unlock     = xfs_buf_item_unlock,
        .iop_committed  = xfs_buf_item_committed,
        .iop_push       = xfs_buf_item_push,
        .iop_committing = xfs_buf_item_committing
};

STATIC int
xfs_buf_item_get_format(
        struct xfs_buf_log_item *bip,
        int                     count)
{
        ASSERT(bip->bli_formats == NULL);
        bip->bli_format_count = count;

        if (count == 1) {
                bip->bli_formats = &bip->__bli_format;
                return 0;
        }

        bip->bli_formats = kmem_zalloc(count * sizeof(struct xfs_buf_log_format),
                                KM_SLEEP);
        if (!bip->bli_formats)
                return ENOMEM;
        return 0;
}

STATIC void
xfs_buf_item_free_format(
        struct xfs_buf_log_item *bip)
{
        if (bip->bli_formats != &bip->__bli_format) {
                kmem_free(bip->bli_formats);
                bip->bli_formats = NULL;
        }
}

/*
 * Allocate a new buf log item to go with the given buffer.
 * Set the buffer's b_fsprivate field to point to the new
 * buf log item.  If there are other item's attached to the
 * buffer (see xfs_buf_attach_iodone() below), then put the
 * buf log item at the front.
 */
void
xfs_buf_item_init(
        xfs_buf_t       *bp,
        xfs_mount_t     *mp)
{
        xfs_log_item_t          *lip = bp->b_fspriv;
        xfs_buf_log_item_t      *bip;
        int                     chunks;
        int                     map_size;
        int                     error;
        int                     i;

        /*
         * Check to see if there is already a buf log item for
         * this buffer.  If there is, it is guaranteed to be
         * the first.  If we do already have one, there is
         * nothing to do here so return.
         */
        ASSERT(bp->b_target->bt_mount == mp);
        if (lip != NULL && lip->li_type == XFS_LI_BUF)
                return;

        bip = kmem_zone_zalloc(xfs_buf_item_zone, KM_SLEEP);
        xfs_log_item_init(mp, &bip->bli_item, XFS_LI_BUF, &xfs_buf_item_ops);
        bip->bli_buf = bp;
        xfs_buf_hold(bp);

        /*
         * chunks is the number of XFS_BLF_CHUNK size pieces the buffer
         * can be divided into. Make sure not to truncate any pieces.
         * map_size is the size of the bitmap needed to describe the
         * chunks of the buffer.
         *
         * Discontiguous buffer support follows the layout of the underlying
         * buffer. This makes the implementation as simple as possible.
         */
        error = xfs_buf_item_get_format(bip, bp->b_map_count);
        ASSERT(error == 0);

        for (i = 0; i < bip->bli_format_count; i++) {
                chunks = DIV_ROUND_UP(BBTOB(bp->b_maps[i].bm_len),
                                      XFS_BLF_CHUNK);
                map_size = DIV_ROUND_UP(chunks, NBWORD);

                bip->bli_formats[i].blf_type = XFS_LI_BUF;
                bip->bli_formats[i].blf_blkno = bp->b_maps[i].bm_bn;
                bip->bli_formats[i].blf_len = bp->b_maps[i].bm_len;
                bip->bli_formats[i].blf_map_size = map_size;
        }

#ifdef XFS_TRANS_DEBUG
        /*
         * Allocate the arrays for tracking what needs to be logged
         * and what our callers request to be logged.  bli_orig
         * holds a copy of the original, clean buffer for comparison
         * against, and bli_logged keeps a 1 bit flag per byte in
         * the buffer to indicate which bytes the callers have asked
         * to have logged.
         */
        bip->bli_orig = kmem_alloc(BBTOB(bp->b_length), KM_SLEEP);
        memcpy(bip->bli_orig, bp->b_addr, BBTOB(bp->b_length));
        bip->bli_logged = kmem_zalloc(BBTOB(bp->b_length) / NBBY, KM_SLEEP);
#endif

        /*
         * Put the buf item into the list of items attached to the
         * buffer at the front.
         */
        if (bp->b_fspriv)
                bip->bli_item.li_bio_list = bp->b_fspriv;
        bp->b_fspriv = bip;
}


/*
 * Mark bytes first through last inclusive as dirty in the buf
 * item's bitmap.
 */
void
xfs_buf_item_log_segment(
        struct xfs_buf_log_item *bip,
        uint                    first,
        uint                    last,
        uint                    *map)
{
        uint            first_bit;
        uint            last_bit;
        uint            bits_to_set;
        uint            bits_set;
        uint            word_num;
        uint            *wordp;
        uint            bit;
        uint            end_bit;
        uint            mask;

        /*
         * Convert byte offsets to bit numbers.
         */
        first_bit = first >> XFS_BLF_SHIFT;
        last_bit = last >> XFS_BLF_SHIFT;

        /*
         * Calculate the total number of bits to be set.
         */
        bits_to_set = last_bit - first_bit + 1;

        /*
         * Get a pointer to the first word in the bitmap
         * to set a bit in.
         */
        word_num = first_bit >> BIT_TO_WORD_SHIFT;
        wordp = &map[word_num];

        /*
         * Calculate the starting bit in the first word.
         */
        bit = first_bit & (uint)(NBWORD - 1);

        /*
         * First set any bits in the first word of our range.
         * If it starts at bit 0 of the word, it will be
         * set below rather than here.  That is what the variable
         * bit tells us. The variable bits_set tracks the number
         * of bits that have been set so far.  End_bit is the number
         * of the last bit to be set in this word plus one.
         */
        if (bit) {
                end_bit = MIN(bit + bits_to_set, (uint)NBWORD);
                mask = ((1 << (end_bit - bit)) - 1) << bit;
                *wordp |= mask;
                wordp++;
                bits_set = end_bit - bit;
        } else {
                bits_set = 0;
        }

        /*
         * Now set bits a whole word at a time that are between
         * first_bit and last_bit.
         */
        while ((bits_to_set - bits_set) >= NBWORD) {
                *wordp |= 0xffffffff;
                bits_set += NBWORD;
                wordp++;
        }

        /*
         * Finally, set any bits left to be set in one last partial word.
         */
        end_bit = bits_to_set - bits_set;
        if (end_bit) {
                mask = (1 << end_bit) - 1;
                *wordp |= mask;
        }
}

/*
 * Mark bytes first through last inclusive as dirty in the buf
 * item's bitmap.
 */
void
xfs_buf_item_log(
        xfs_buf_log_item_t      *bip,
        uint                    first,
        uint                    last)
{
        int                     i;
        uint                    start;
        uint                    end;
        struct xfs_buf          *bp = bip->bli_buf;

        /*
         * Mark the item as having some dirty data for
         * quick reference in xfs_buf_item_dirty.
         */
        bip->bli_flags |= XFS_BLI_DIRTY;

        /*
         * walk each buffer segment and mark them dirty appropriately.
         */
        start = 0;
        for (i = 0; i < bip->bli_format_count; i++) {
                if (start > last)
                        break;
                end = start + BBTOB(bp->b_maps[i].bm_len);
                if (first > end) {
                        start += BBTOB(bp->b_maps[i].bm_len);
                        continue;
                }
                if (first < start)
                        first = start;
                if (end > last)
                        end = last;

                xfs_buf_item_log_segment(bip, first, end,
                                         &bip->bli_formats[i].blf_data_map[0]);

                start += bp->b_maps[i].bm_len;
        }
}


/*
 * Return 1 if the buffer has some data that has been logged (at any
 * point, not just the current transaction) and 0 if not.
 */
uint
xfs_buf_item_dirty(
        xfs_buf_log_item_t      *bip)
{
        return (bip->bli_flags & XFS_BLI_DIRTY);
}

STATIC void
xfs_buf_item_free(
        xfs_buf_log_item_t      *bip)
{
#ifdef XFS_TRANS_DEBUG
        kmem_free(bip->bli_orig);
        kmem_free(bip->bli_logged);
#endif /* XFS_TRANS_DEBUG */

        xfs_buf_item_free_format(bip);
        kmem_zone_free(xfs_buf_item_zone, bip);
}

/*
 * This is called when the buf log item is no longer needed.  It should
 * free the buf log item associated with the given buffer and clear
 * the buffer's pointer to the buf log item.  If there are no more
 * items in the list, clear the b_iodone field of the buffer (see
 * xfs_buf_attach_iodone() below).
 */
void
xfs_buf_item_relse(
        xfs_buf_t       *bp)
{
        xfs_buf_log_item_t      *bip;

        trace_xfs_buf_item_relse(bp, _RET_IP_);

        bip = bp->b_fspriv;
        bp->b_fspriv = bip->bli_item.li_bio_list;
        if (bp->b_fspriv == NULL)
                bp->b_iodone = NULL;

        xfs_buf_rele(bp);
        xfs_buf_item_free(bip);
}


/*
 * Add the given log item with its callback to the list of callbacks
 * to be called when the buffer's I/O completes.  If it is not set
 * already, set the buffer's b_iodone() routine to be
 * xfs_buf_iodone_callbacks() and link the log item into the list of
 * items rooted at b_fsprivate.  Items are always added as the second
 * entry in the list if there is a first, because the buf item code
 * assumes that the buf log item is first.
 */
void
xfs_buf_attach_iodone(
        xfs_buf_t       *bp,
        void            (*cb)(xfs_buf_t *, xfs_log_item_t *),
        xfs_log_item_t  *lip)
{
        xfs_log_item_t  *head_lip;

        ASSERT(xfs_buf_islocked(bp));

        lip->li_cb = cb;
        head_lip = bp->b_fspriv;
        if (head_lip) {
                lip->li_bio_list = head_lip->li_bio_list;
                head_lip->li_bio_list = lip;
        } else {
                bp->b_fspriv = lip;
        }

        ASSERT(bp->b_iodone == NULL ||
               bp->b_iodone == xfs_buf_iodone_callbacks);
        bp->b_iodone = xfs_buf_iodone_callbacks;
}

/*
 * We can have many callbacks on a buffer. Running the callbacks individually
 * can cause a lot of contention on the AIL lock, so we allow for a single
 * callback to be able to scan the remaining lip->li_bio_list for other items
 * of the same type and callback to be processed in the first call.
 *
 * As a result, the loop walking the callback list below will also modify the
 * list. it removes the first item from the list and then runs the callback.
 * The loop then restarts from the new head of the list. This allows the
 * callback to scan and modify the list attached to the buffer and we don't
 * have to care about maintaining a next item pointer.
 */
STATIC void
xfs_buf_do_callbacks(
        struct xfs_buf          *bp)
{
        struct xfs_log_item     *lip;

        while ((lip = bp->b_fspriv) != NULL) {
                bp->b_fspriv = lip->li_bio_list;
                ASSERT(lip->li_cb != NULL);
                /*
                 * Clear the next pointer so we don't have any
                 * confusion if the item is added to another buf.
                 * Don't touch the log item after calling its
                 * callback, because it could have freed itself.
                 */
                lip->li_bio_list = NULL;
                lip->li_cb(bp, lip);
        }
}

/*
 * This is the iodone() function for buffers which have had callbacks
 * attached to them by xfs_buf_attach_iodone().  It should remove each
 * log item from the buffer's list and call the callback of each in turn.
 * When done, the buffer's fsprivate field is set to NULL and the buffer
 * is unlocked with a call to iodone().
 */
void
xfs_buf_iodone_callbacks(
        struct xfs_buf          *bp)
{
        struct xfs_log_item     *lip = bp->b_fspriv;
        struct xfs_mount        *mp = lip->li_mountp;
        static ulong            lasttime;
        static xfs_buftarg_t    *lasttarg;

        if (likely(!xfs_buf_geterror(bp)))
                goto do_callbacks;

        /*
         * If we've already decided to shutdown the filesystem because of
         * I/O errors, there's no point in giving this a retry.
         */
        if (XFS_FORCED_SHUTDOWN(mp)) {
                xfs_buf_stale(bp);
                XFS_BUF_DONE(bp);
                trace_xfs_buf_item_iodone(bp, _RET_IP_);
                goto do_callbacks;
        }

        if (bp->b_target != lasttarg ||
            time_after(jiffies, (lasttime + 5*HZ))) {
                lasttime = jiffies;
                xfs_buf_ioerror_alert(bp, __func__);
        }
        lasttarg = bp->b_target;

        /*
         * If the write was asynchronous then no one will be looking for the
         * error.  Clear the error state and write the buffer out again.
         *
         * XXX: This helps against transient write errors, but we need to find
         * a way to shut the filesystem down if the writes keep failing.
         *
         * In practice we'll shut the filesystem down soon as non-transient
         * erorrs tend to affect the whole device and a failing log write
         * will make us give up.  But we really ought to do better here.
         */
        if (XFS_BUF_ISASYNC(bp)) {
                ASSERT(bp->b_iodone != NULL);

                trace_xfs_buf_item_iodone_async(bp, _RET_IP_);

                xfs_buf_ioerror(bp, 0); /* errno of 0 unsets the flag */

                if (!XFS_BUF_ISSTALE(bp)) {
                        bp->b_flags |= XBF_WRITE | XBF_ASYNC | XBF_DONE;
                        xfs_buf_iorequest(bp);
                } else {
                        xfs_buf_relse(bp);
                }

                return;
        }

        /*
         * If the write of the buffer was synchronous, we want to make
         * sure to return the error to the caller of xfs_bwrite().
         */
        xfs_buf_stale(bp);
        XFS_BUF_DONE(bp);

        trace_xfs_buf_error_relse(bp, _RET_IP_);

do_callbacks:
        xfs_buf_do_callbacks(bp);
        bp->b_fspriv = NULL;
        bp->b_iodone = NULL;
        xfs_buf_ioend(bp, 0);
}

/*
 * This is the iodone() function for buffers which have been
 * logged.  It is called when they are eventually flushed out.
 * It should remove the buf item from the AIL, and free the buf item.
 * It is called by xfs_buf_iodone_callbacks() above which will take
 * care of cleaning up the buffer itself.
 */
void
xfs_buf_iodone(
        struct xfs_buf          *bp,
        struct xfs_log_item     *lip)
{
        struct xfs_ail          *ailp = lip->li_ailp;

        ASSERT(BUF_ITEM(lip)->bli_buf == bp);

        xfs_buf_rele(bp);

        /*
         * If we are forcibly shutting down, this may well be
         * off the AIL already. That's because we simulate the
         * log-committed callbacks to unpin these buffers. Or we may never
         * have put this item on AIL because of the transaction was
         * aborted forcibly. xfs_trans_ail_delete() takes care of these.
         *
         * Either way, AIL is useless if we're forcing a shutdown.
         */
        spin_lock(&ailp->xa_lock);
        xfs_trans_ail_delete(ailp, lip, SHUTDOWN_CORRUPT_INCORE);
        xfs_buf_item_free(BUF_ITEM(lip));
}