HAMMER 60I/Many: Mirroring

[dragonfly.git] / sys / vfs / hammer / hammer_io.c

/*
 * Copyright (c) 2007-2008 The DragonFly Project.  All rights reserved.
 * 
 * This code is derived from software contributed to The DragonFly Project
 * by Matthew Dillon <dillon@backplane.com>
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name of The DragonFly Project nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific, prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 * 
 * $DragonFly: src/sys/vfs/hammer/hammer_io.c,v 1.48 2008/06/29 07:50:40 dillon Exp $
 */
/*
 * IO Primitives and buffer cache management
 *
 * All major data-tracking structures in HAMMER contain a struct hammer_io
 * which is used to manage their backing store.  We use filesystem buffers
 * for backing store and we leave them passively associated with their
 * HAMMER structures.
 *
 * If the kernel tries to destroy a passively associated buf which we cannot
 * yet let go we set B_LOCKED in the buffer and then actively released it
 * later when we can.
 */

#include "hammer.h"
#include <sys/fcntl.h>
#include <sys/nlookup.h>
#include <sys/buf.h>
#include <sys/buf2.h>

static void hammer_io_modify(hammer_io_t io, int count);
static void hammer_io_deallocate(struct buf *bp);
static int hammer_io_direct_uncache_callback(hammer_inode_t ip, void *data);

/*
 * Initialize a new, already-zero'd hammer_io structure, or reinitialize
 * an existing hammer_io structure which may have switched to another type.
 */
void
hammer_io_init(hammer_io_t io, hammer_mount_t hmp, enum hammer_io_type type)
{
        io->hmp = hmp;
        io->type = type;
}

/*
 * Helper routine to disassociate a buffer cache buffer from an I/O
 * structure.  The buffer is unlocked and marked appropriate for reclamation.
 *
 * The io may have 0 or 1 references depending on who called us.  The
 * caller is responsible for dealing with the refs.
 *
 * This call can only be made when no action is required on the buffer.
 *
 * The caller must own the buffer and the IO must indicate that the
 * structure no longer owns it (io.released != 0).
 */
static void
hammer_io_disassociate(hammer_io_structure_t iou)
{
        struct buf *bp = iou->io.bp;

        KKASSERT(iou->io.released);
        KKASSERT(iou->io.modified == 0);
        KKASSERT(LIST_FIRST(&bp->b_dep) == (void *)iou);
        buf_dep_init(bp);
        iou->io.bp = NULL;

        /*
         * If the buffer was locked someone wanted to get rid of it.
         */
        if (bp->b_flags & B_LOCKED) {
                --hammer_count_io_locked;
                bp->b_flags &= ~B_LOCKED;
        }
        if (iou->io.reclaim) {
                bp->b_flags |= B_NOCACHE|B_RELBUF;
                iou->io.reclaim = 0;
        }

        switch(iou->io.type) {
        case HAMMER_STRUCTURE_VOLUME:
                iou->volume.ondisk = NULL;
                break;
        case HAMMER_STRUCTURE_DATA_BUFFER:
        case HAMMER_STRUCTURE_META_BUFFER:
        case HAMMER_STRUCTURE_UNDO_BUFFER:
                iou->buffer.ondisk = NULL;
                break;
        }
}

/*
 * Wait for any physical IO to complete
 */
static void
hammer_io_wait(hammer_io_t io)
{
        if (io->running) {
                crit_enter();
                tsleep_interlock(io);
                io->waiting = 1;
                for (;;) {
                        tsleep(io, 0, "hmrflw", 0);
                        if (io->running == 0)
                                break;
                        tsleep_interlock(io);
                        io->waiting = 1;
                        if (io->running == 0)
                                break;
                }
                crit_exit();
        }
}

/*
 * Wait for all hammer_io-initated write I/O's to complete.  This is not
 * supposed to count direct I/O's but some can leak through (for
 * non-full-sized direct I/Os).
 */
void
hammer_io_wait_all(hammer_mount_t hmp, const char *ident)
{
        crit_enter();
        while (hmp->io_running_space)
                tsleep(&hmp->io_running_space, 0, ident, 0);
        crit_exit();
}

#define HAMMER_MAXRA    4

/*
 * Load bp for a HAMMER structure.  The io must be exclusively locked by
 * the caller.
 *
 * This routine is mostly used on meta-data and small-data blocks.  Generally
 * speaking HAMMER assumes some locality of reference and will cluster 
 * a 64K read.
 *
 * Note that clustering occurs at the device layer, not the logical layer.
 * If the buffers do not apply to the current operation they may apply to
 * some other.
 */
int
hammer_io_read(struct vnode *devvp, struct hammer_io *io, hammer_off_t limit)
{
        struct buf *bp;
        int   error;

        if ((bp = io->bp) == NULL) {
                hammer_count_io_running_read += io->bytes;
#if 1
                error = cluster_read(devvp, limit, io->offset, io->bytes,
                                     HAMMER_CLUSTER_SIZE,
                                     HAMMER_CLUSTER_BUFS, &io->bp);
#else
                error = bread(devvp, io->offset, io->bytes, &io->bp);
#endif
                hammer_count_io_running_read -= io->bytes;
                if (error == 0) {
                        bp = io->bp;
                        bp->b_ops = &hammer_bioops;
                        KKASSERT(LIST_FIRST(&bp->b_dep) == NULL);
                        LIST_INSERT_HEAD(&bp->b_dep, &io->worklist, node);
                        BUF_KERNPROC(bp);
                }
                KKASSERT(io->modified == 0);
                KKASSERT(io->running == 0);
                KKASSERT(io->waiting == 0);
                io->released = 0;       /* we hold an active lock on bp */
        } else {
                error = 0;
        }
        return(error);
}

/*
 * Similar to hammer_io_read() but returns a zero'd out buffer instead.
 * Must be called with the IO exclusively locked.
 *
 * vfs_bio_clrbuf() is kinda nasty, enforce serialization against background
 * I/O by forcing the buffer to not be in a released state before calling
 * it.
 *
 * This function will also mark the IO as modified but it will not
 * increment the modify_refs count.
 */
int
hammer_io_new(struct vnode *devvp, struct hammer_io *io)
{
        struct buf *bp;

        if ((bp = io->bp) == NULL) {
                io->bp = getblk(devvp, io->offset, io->bytes, 0, 0);
                bp = io->bp;
                bp->b_ops = &hammer_bioops;
                KKASSERT(LIST_FIRST(&bp->b_dep) == NULL);
                LIST_INSERT_HEAD(&bp->b_dep, &io->worklist, node);
                io->released = 0;
                KKASSERT(io->running == 0);
                io->waiting = 0;
                BUF_KERNPROC(bp);
        } else {
                if (io->released) {
                        regetblk(bp);
                        BUF_KERNPROC(bp);
                        io->released = 0;
                }
        }
        hammer_io_modify(io, 0);
        vfs_bio_clrbuf(bp);
        return(0);
}

/*
 * Remove potential device level aliases against buffers managed by high level
 * vnodes.
 */
void
hammer_io_inval(hammer_volume_t volume, hammer_off_t zone2_offset)
{
        hammer_io_structure_t iou;
        hammer_off_t phys_offset;
        struct buf *bp;

        phys_offset = volume->ondisk->vol_buf_beg +
                      (zone2_offset & HAMMER_OFF_SHORT_MASK);
        crit_enter();
        if ((bp = findblk(volume->devvp, phys_offset)) != NULL) {
                bp = getblk(volume->devvp, phys_offset, bp->b_bufsize, 0, 0);
                if ((iou = (void *)LIST_FIRST(&bp->b_dep)) != NULL) {
                        hammer_io_clear_modify(&iou->io, 1);
                        bundirty(bp);
                        iou->io.reclaim = 1;
                        hammer_io_deallocate(bp);
                } else {
                        KKASSERT((bp->b_flags & B_LOCKED) == 0);
                        bundirty(bp);
                        bp->b_flags |= B_NOCACHE|B_RELBUF;
                }
                brelse(bp);
        }
        crit_exit();
}

/*
 * This routine is called on the last reference to a hammer structure.
 * The io is usually interlocked with io.loading and io.refs must be 1.
 *
 * This routine may return a non-NULL bp to the caller for dispoal.  Disposal
 * simply means the caller finishes decrementing the ref-count on the 
 * IO structure then brelse()'s the bp.  The bp may or may not still be
 * passively associated with the IO.
 * 
 * The only requirement here is that modified meta-data and volume-header
 * buffer may NOT be disassociated from the IO structure, and consequently
 * we also leave such buffers actively associated with the IO if they already
 * are (since the kernel can't do anything with them anyway).  Only the
 * flusher is allowed to write such buffers out.  Modified pure-data and
 * undo buffers are returned to the kernel but left passively associated
 * so we can track when the kernel writes the bp out.
 */
struct buf *
hammer_io_release(struct hammer_io *io, int flush)
{
        union hammer_io_structure *iou = (void *)io;
        struct buf *bp;

        if ((bp = io->bp) == NULL)
                return(NULL);

        /*
         * Try to flush a dirty IO to disk if asked to by the
         * caller or if the kernel tried to flush the buffer in the past.
         *
         * Kernel-initiated flushes are only allowed for pure-data buffers.
         * meta-data and volume buffers can only be flushed explicitly
         * by HAMMER.
         */
        if (io->modified) {
                if (flush) {
                        hammer_io_flush(io);
                } else if (bp->b_flags & B_LOCKED) {
                        switch(io->type) {
                        case HAMMER_STRUCTURE_DATA_BUFFER:
                        case HAMMER_STRUCTURE_UNDO_BUFFER:
                                hammer_io_flush(io);
                                break;
                        default:
                                break;
                        }
                } /* else no explicit request to flush the buffer */
        }

        /*
         * Wait for the IO to complete if asked to.
         */
        if (io->waitdep && io->running) {
                hammer_io_wait(io);
        }

        /*
         * Return control of the buffer to the kernel (with the provisio
         * that our bioops can override kernel decisions with regards to
         * the buffer).
         */
        if ((flush || io->reclaim) && io->modified == 0 && io->running == 0) {
                /*
                 * Always disassociate the bp if an explicit flush
                 * was requested and the IO completed with no error
                 * (so unmount can really clean up the structure).
                 */
                if (io->released) {
                        regetblk(bp);
                        BUF_KERNPROC(bp);
                } else {
                        io->released = 1;
                }
                hammer_io_disassociate((hammer_io_structure_t)io);
                /* return the bp */
        } else if (io->modified) {
                /*
                 * Only certain IO types can be released to the kernel if
                 * the buffer has been modified.
                 *
                 * volume and meta-data IO types may only be explicitly
                 * flushed by HAMMER.
                 */
                switch(io->type) {
                case HAMMER_STRUCTURE_DATA_BUFFER:
                case HAMMER_STRUCTURE_UNDO_BUFFER:
                        if (io->released == 0) {
                                io->released = 1;
                                bdwrite(bp);
                        }
                        break;
                default:
                        break;
                }
                bp = NULL;      /* bp left associated */
        } else if (io->released == 0) {
                /*
                 * Clean buffers can be generally released to the kernel.
                 * We leave the bp passively associated with the HAMMER
                 * structure and use bioops to disconnect it later on
                 * if the kernel wants to discard the buffer.
                 *
                 * We can steal the structure's ownership of the bp.
                 */
                io->released = 1;
                if (bp->b_flags & B_LOCKED) {
                        hammer_io_disassociate(iou);
                        /* return the bp */
                } else {
                        if (io->reclaim) {
                                hammer_io_disassociate(iou);
                                /* return the bp */
                        } else {
                                /* return the bp (bp passively associated) */
                        }
                }
        } else {
                /*
                 * A released buffer is passively associate with our
                 * hammer_io structure.  The kernel cannot destroy it
                 * without making a bioops call.  If the kernel (B_LOCKED)
                 * or we (reclaim) requested that the buffer be destroyed
                 * we destroy it, otherwise we do a quick get/release to
                 * reset its position in the kernel's LRU list.
                 *
                 * Leaving the buffer passively associated allows us to
                 * use the kernel's LRU buffer flushing mechanisms rather
                 * then rolling our own.
                 *
                 * XXX there are two ways of doing this.  We can re-acquire
                 * and passively release to reset the LRU, or not.
                 */
                if (io->running == 0) {
                        regetblk(bp);
                        if ((bp->b_flags & B_LOCKED) || io->reclaim) {
                                hammer_io_disassociate(iou);
                                /* return the bp */
                        } else {
                                /* return the bp (bp passively associated) */
                        }
                } else {
                        /*
                         * bp is left passively associated but we do not
                         * try to reacquire it.  Interactions with the io
                         * structure will occur on completion of the bp's
                         * I/O.
                         */
                        bp = NULL;
                }
        }
        return(bp);
}

/*
 * This routine is called with a locked IO when a flush is desired and
 * no other references to the structure exists other then ours.  This
 * routine is ONLY called when HAMMER believes it is safe to flush a
 * potentially modified buffer out.
 */
void
hammer_io_flush(struct hammer_io *io)
{
        struct buf *bp;

        /*
         * Degenerate case - nothing to flush if nothing is dirty.
         */
        if (io->modified == 0) {
                return;
        }

        KKASSERT(io->bp);
        KKASSERT(io->modify_refs <= 0);

        /*
         * Acquire ownership of the bp, particularly before we clear our
         * modified flag.
         *
         * We are going to bawrite() this bp.  Don't leave a window where
         * io->released is set, we actually own the bp rather then our
         * buffer.
         */
        bp = io->bp;
        if (io->released) {
                regetblk(bp);
                /* BUF_KERNPROC(io->bp); */
                /* io->released = 0; */
                KKASSERT(io->released);
                KKASSERT(io->bp == bp);
        }
        io->released = 1;

        /*
         * Acquire exclusive access to the bp and then clear the modified
         * state of the buffer prior to issuing I/O to interlock any
         * modifications made while the I/O is in progress.  This shouldn't
         * happen anyway but losing data would be worse.  The modified bit
         * will be rechecked after the IO completes.
         *
         * NOTE: This call also finalizes the buffer's content (inval == 0).
         *
         * This is only legal when lock.refs == 1 (otherwise we might clear
         * the modified bit while there are still users of the cluster
         * modifying the data).
         *
         * Do this before potentially blocking so any attempt to modify the
         * ondisk while we are blocked blocks waiting for us.
         */
        hammer_io_clear_modify(io, 0);

        /*
         * Transfer ownership to the kernel and initiate I/O.
         */
        io->running = 1;
        io->hmp->io_running_space += io->bytes;
        hammer_count_io_running_write += io->bytes;
        bawrite(bp);
}

/************************************************************************
 *                              BUFFER DIRTYING                         *
 ************************************************************************
 *
 * These routines deal with dependancies created when IO buffers get
 * modified.  The caller must call hammer_modify_*() on a referenced
 * HAMMER structure prior to modifying its on-disk data.
 *
 * Any intent to modify an IO buffer acquires the related bp and imposes
 * various write ordering dependancies.
 */

/*
 * Mark a HAMMER structure as undergoing modification.  Meta-data buffers
 * are locked until the flusher can deal with them, pure data buffers
 * can be written out.
 */
static
void
hammer_io_modify(hammer_io_t io, int count)
{
        struct hammer_mount *hmp = io->hmp;

        /*
         * io->modify_refs must be >= 0
         */
        while (io->modify_refs < 0) {
                io->waitmod = 1;
                tsleep(io, 0, "hmrmod", 0);
        }

        /*
         * Shortcut if nothing to do.
         */
        KKASSERT(io->lock.refs != 0 && io->bp != NULL);
        io->modify_refs += count;
        if (io->modified && io->released == 0)
                return;

        hammer_lock_ex(&io->lock);
        if (io->modified == 0) {
                KKASSERT(io->mod_list == NULL);
                switch(io->type) {
                case HAMMER_STRUCTURE_VOLUME:
                        io->mod_list = &hmp->volu_list;
                        hmp->locked_dirty_space += io->bytes;
                        hammer_count_dirtybufspace += io->bytes;
                        break;
                case HAMMER_STRUCTURE_META_BUFFER:
                        io->mod_list = &hmp->meta_list;
                        hmp->locked_dirty_space += io->bytes;
                        hammer_count_dirtybufspace += io->bytes;
                        break;
                case HAMMER_STRUCTURE_UNDO_BUFFER:
                        io->mod_list = &hmp->undo_list;
                        break;
                case HAMMER_STRUCTURE_DATA_BUFFER:
                        io->mod_list = &hmp->data_list;
                        break;
                }
                TAILQ_INSERT_TAIL(io->mod_list, io, mod_entry);
                io->modified = 1;
        }
        if (io->released) {
                regetblk(io->bp);
                BUF_KERNPROC(io->bp);
                io->released = 0;
                KKASSERT(io->modified != 0);
        }
        hammer_unlock(&io->lock);
}

static __inline
void
hammer_io_modify_done(hammer_io_t io)
{
        KKASSERT(io->modify_refs > 0);
        --io->modify_refs;
        if (io->modify_refs == 0 && io->waitmod) {
                io->waitmod = 0;
                wakeup(io);
        }
}

void
hammer_io_write_interlock(hammer_io_t io)
{
        while (io->modify_refs != 0) {
                io->waitmod = 1;
                tsleep(io, 0, "hmrmod", 0);
        }
        io->modify_refs = -1;
}

void
hammer_io_done_interlock(hammer_io_t io)
{
        KKASSERT(io->modify_refs == -1);
        io->modify_refs = 0;
        if (io->waitmod) {
                io->waitmod = 0;
                wakeup(io);
        }
}

/*
 * Caller intends to modify a volume's ondisk structure.
 *
 * This is only allowed if we are the flusher or we have a ref on the
 * sync_lock.
 */
void
hammer_modify_volume(hammer_transaction_t trans, hammer_volume_t volume,
                     void *base, int len)
{
        KKASSERT (trans == NULL || trans->sync_lock_refs > 0);

        hammer_io_modify(&volume->io, 1);
        if (len) {
                intptr_t rel_offset = (intptr_t)base - (intptr_t)volume->ondisk;
                KKASSERT((rel_offset & ~(intptr_t)HAMMER_BUFMASK) == 0);
                hammer_generate_undo(trans, &volume->io,
                         HAMMER_ENCODE_RAW_VOLUME(volume->vol_no, rel_offset),
                         base, len);
        }
}

/*
 * Caller intends to modify a buffer's ondisk structure.
 *
 * This is only allowed if we are the flusher or we have a ref on the
 * sync_lock.
 */
void
hammer_modify_buffer(hammer_transaction_t trans, hammer_buffer_t buffer,
                     void *base, int len)
{
        KKASSERT (trans == NULL || trans->sync_lock_refs > 0);

        hammer_io_modify(&buffer->io, 1);
        if (len) {
                intptr_t rel_offset = (intptr_t)base - (intptr_t)buffer->ondisk;
                KKASSERT((rel_offset & ~(intptr_t)HAMMER_BUFMASK) == 0);
                hammer_generate_undo(trans, &buffer->io,
                                     buffer->zone2_offset + rel_offset,
                                     base, len);
        }
}

void
hammer_modify_volume_done(hammer_volume_t volume)
{
        hammer_io_modify_done(&volume->io);
}

void
hammer_modify_buffer_done(hammer_buffer_t buffer)
{
        hammer_io_modify_done(&buffer->io);
}

/*
 * Mark an entity as not being dirty any more and finalize any
 * delayed adjustments to the buffer.
 *
 * Delayed adjustments are an important performance enhancement, allowing
 * us to avoid recalculating B-Tree node CRCs over and over again when
 * making bulk-modifications to the B-Tree.
 *
 * If inval is non-zero delayed adjustments are ignored.
 */
void
hammer_io_clear_modify(struct hammer_io *io, int inval)
{
        if (io->modified == 0)
                return;

        /*
         * Take us off the mod-list and clear the modified bit.
         */
        KKASSERT(io->mod_list != NULL);
        if (io->mod_list == &io->hmp->volu_list ||
            io->mod_list == &io->hmp->meta_list) {
                io->hmp->locked_dirty_space -= io->bytes;
                hammer_count_dirtybufspace -= io->bytes;
        }
        TAILQ_REMOVE(io->mod_list, io, mod_entry);
        io->mod_list = NULL;
        io->modified = 0;

        /*
         * If this bit is not set there are no delayed adjustments.
         */
        if (io->gencrc == 0)
                return;
        io->gencrc = 0;

        /*
         * Finalize requested CRCs.  The NEEDSCRC flag also holds a reference
         * on the node (& underlying buffer).  Release the node after clearing
         * the flag.
         */
        if (io->type == HAMMER_STRUCTURE_META_BUFFER) {
                hammer_buffer_t buffer = (void *)io;
                hammer_node_t node;

restart:
                TAILQ_FOREACH(node, &buffer->clist, entry) {
                        if ((node->flags & HAMMER_NODE_NEEDSCRC) == 0)
                                continue;
                        node->flags &= ~HAMMER_NODE_NEEDSCRC;
                        KKASSERT(node->ondisk);
                        if (inval == 0)
                                node->ondisk->crc = crc32(&node->ondisk->crc + 1, HAMMER_BTREE_CRCSIZE);
                        hammer_rel_node(node);
                        goto restart;
                }
        }

}

/*
 * Clear the IO's modify list.  Even though the IO is no longer modified
 * it may still be on the lose_list.  This routine is called just before
 * the governing hammer_buffer is destroyed.
 */
void
hammer_io_clear_modlist(struct hammer_io *io)
{
        KKASSERT(io->modified == 0);
        if (io->mod_list) {
                crit_enter();   /* biodone race against list */
                KKASSERT(io->mod_list == &io->hmp->lose_list);
                TAILQ_REMOVE(io->mod_list, io, mod_entry);
                io->mod_list = NULL;
                crit_exit();
        }
}

/************************************************************************
 *                              HAMMER_BIOOPS                           *
 ************************************************************************
 *
 */

/*
 * Pre-IO initiation kernel callback - cluster build only
 */
static void
hammer_io_start(struct buf *bp)
{
}

/*
 * Post-IO completion kernel callback - MAY BE CALLED FROM INTERRUPT!
 *
 * NOTE: HAMMER may modify a buffer after initiating I/O.  The modified bit
 * may also be set if we were marking a cluster header open.  Only remove
 * our dependancy if the modified bit is clear.
 */
static void
hammer_io_complete(struct buf *bp)
{
        union hammer_io_structure *iou = (void *)LIST_FIRST(&bp->b_dep);

        KKASSERT(iou->io.released == 1);

        /*
         * Deal with people waiting for I/O to drain
         */
        if (iou->io.running) {
                hammer_count_io_running_write -= iou->io.bytes;
                iou->io.hmp->io_running_space -= iou->io.bytes;
                if (iou->io.hmp->io_running_space == 0)
                        wakeup(&iou->io.hmp->io_running_space);
                KKASSERT(iou->io.hmp->io_running_space >= 0);
                iou->io.running = 0;
        }

        if (iou->io.waiting) {
                iou->io.waiting = 0;
                wakeup(iou);
        }

        /*
         * If B_LOCKED is set someone wanted to deallocate the bp at some
         * point, do it now if refs has become zero.
         */
        if ((bp->b_flags & B_LOCKED) && iou->io.lock.refs == 0) {
                KKASSERT(iou->io.modified == 0);
                --hammer_count_io_locked;
                bp->b_flags &= ~B_LOCKED;
                hammer_io_deallocate(bp);
                /* structure may be dead now */
        }
}

/*
 * Callback from kernel when it wishes to deallocate a passively
 * associated structure.  This mostly occurs with clean buffers
 * but it may be possible for a holding structure to be marked dirty
 * while its buffer is passively associated.  The caller owns the bp.
 *
 * If we cannot disassociate we set B_LOCKED to prevent the buffer
 * from getting reused.
 *
 * WARNING: Because this can be called directly by getnewbuf we cannot
 * recurse into the tree.  If a bp cannot be immediately disassociated
 * our only recourse is to set B_LOCKED.
 *
 * WARNING: This may be called from an interrupt via hammer_io_complete()
 */
static void
hammer_io_deallocate(struct buf *bp)
{
        hammer_io_structure_t iou = (void *)LIST_FIRST(&bp->b_dep);

        KKASSERT((bp->b_flags & B_LOCKED) == 0 && iou->io.running == 0);
        if (iou->io.lock.refs > 0 || iou->io.modified) {
                /*
                 * It is not legal to disassociate a modified buffer.  This
                 * case really shouldn't ever occur.
                 */
                bp->b_flags |= B_LOCKED;
                ++hammer_count_io_locked;
        } else {
                /*
                 * Disassociate the BP.  If the io has no refs left we
                 * have to add it to the loose list.
                 */
                hammer_io_disassociate(iou);
                if (iou->io.type != HAMMER_STRUCTURE_VOLUME) {
                        KKASSERT(iou->io.bp == NULL);
                        KKASSERT(iou->io.mod_list == NULL);
                        crit_enter();   /* biodone race against list */
                        iou->io.mod_list = &iou->io.hmp->lose_list;
                        TAILQ_INSERT_TAIL(iou->io.mod_list, &iou->io, mod_entry);
                        crit_exit();
                }
        }
}

static int
hammer_io_fsync(struct vnode *vp)
{
        return(0);
}

/*
 * NOTE: will not be called unless we tell the kernel about the
 * bioops.  Unused... we use the mount's VFS_SYNC instead.
 */
static int
hammer_io_sync(struct mount *mp)
{
        return(0);
}

static void
hammer_io_movedeps(struct buf *bp1, struct buf *bp2)
{
}

/*
 * I/O pre-check for reading and writing.  HAMMER only uses this for
 * B_CACHE buffers so checkread just shouldn't happen, but if it does
 * allow it.
 *
 * Writing is a different case.  We don't want the kernel to try to write
 * out a buffer that HAMMER may be modifying passively or which has a
 * dependancy.  In addition, kernel-demanded writes can only proceed for
 * certain types of buffers (i.e. UNDO and DATA types).  Other dirty
 * buffer types can only be explicitly written by the flusher.
 *
 * checkwrite will only be called for bdwrite()n buffers.  If we return
 * success the kernel is guaranteed to initiate the buffer write.
 */
static int
hammer_io_checkread(struct buf *bp)
{
        return(0);
}

static int
hammer_io_checkwrite(struct buf *bp)
{
        hammer_io_t io = (void *)LIST_FIRST(&bp->b_dep);

        /*
         * This shouldn't happen under normal operation.
         */
        if (io->type == HAMMER_STRUCTURE_VOLUME ||
            io->type == HAMMER_STRUCTURE_META_BUFFER) {
                if (!panicstr)
                        panic("hammer_io_checkwrite: illegal buffer");
                if ((bp->b_flags & B_LOCKED) == 0) {
                        bp->b_flags |= B_LOCKED;
                        ++hammer_count_io_locked;
                }
                return(1);
        }

        /*
         * We can only clear the modified bit if the IO is not currently
         * undergoing modification.  Otherwise we may miss changes.
         */
        if (io->modify_refs == 0 && io->modified)
                hammer_io_clear_modify(io, 0);

        /*
         * The kernel is going to start the IO, set io->running.
         */
        KKASSERT(io->running == 0);
        io->running = 1;
        io->hmp->io_running_space += io->bytes;
        hammer_count_io_running_write += io->bytes;
        return(0);
}

/*
 * Return non-zero if we wish to delay the kernel's attempt to flush
 * this buffer to disk.
 */
static int
hammer_io_countdeps(struct buf *bp, int n)
{
        return(0);
}

struct bio_ops hammer_bioops = {
        .io_start       = hammer_io_start,
        .io_complete    = hammer_io_complete,
        .io_deallocate  = hammer_io_deallocate,
        .io_fsync       = hammer_io_fsync,
        .io_sync        = hammer_io_sync,
        .io_movedeps    = hammer_io_movedeps,
        .io_countdeps   = hammer_io_countdeps,
        .io_checkread   = hammer_io_checkread,
        .io_checkwrite  = hammer_io_checkwrite,
};

/************************************************************************
 *                              DIRECT IO OPS                           *
 ************************************************************************
 *
 * These functions operate directly on the buffer cache buffer associated
 * with a front-end vnode rather then a back-end device vnode.
 */

/*
 * Read a buffer associated with a front-end vnode directly from the
 * disk media.  The bio may be issued asynchronously.
 *
 * A second-level bio already resolved to a zone-2 offset (typically by
 * the BMAP code, or by a previous hammer_io_direct_write()), is passed. 
 */
int
hammer_io_direct_read(hammer_mount_t hmp, struct bio *bio)
{
        hammer_off_t zone2_offset;
        hammer_volume_t volume;
        struct buf *bp;
        struct bio *nbio;
        int vol_no;
        int error;

        zone2_offset = bio->bio_offset;

        KKASSERT((zone2_offset & HAMMER_OFF_ZONE_MASK) ==
                 HAMMER_ZONE_RAW_BUFFER);

        vol_no = HAMMER_VOL_DECODE(zone2_offset);
        volume = hammer_get_volume(hmp, vol_no, &error);
        if (error == 0 && zone2_offset >= volume->maxbuf_off)
                error = EIO;

        /*
         * Third level bio - raw offset specific to the
         * correct volume.
         */
        if (error == 0) {
                zone2_offset &= HAMMER_OFF_SHORT_MASK;

                nbio = push_bio(bio);
                nbio->bio_offset = volume->ondisk->vol_buf_beg +
                                   zone2_offset;
                vn_strategy(volume->devvp, nbio);
        }
        hammer_rel_volume(volume, 0);

        if (error) {
                kprintf("hammer_direct_read: failed @ %016llx\n",
                        zone2_offset);
                bp = bio->bio_buf;
                bp->b_error = error;
                bp->b_flags |= B_ERROR;
                biodone(bio);
        }
        return(error);
}

/*
 * Write a buffer associated with a front-end vnode directly to the
 * disk media.  The bio may be issued asynchronously.
 */
int
hammer_io_direct_write(hammer_mount_t hmp, hammer_btree_leaf_elm_t leaf,
                       struct bio *bio)
{
        hammer_off_t buf_offset;
        hammer_off_t zone2_offset;
        hammer_volume_t volume;
        hammer_buffer_t buffer;
        struct buf *bp;
        struct bio *nbio;
        char *ptr;
        int vol_no;
        int error;

        buf_offset = leaf->data_offset;

        KKASSERT(buf_offset > HAMMER_ZONE_BTREE);
        KKASSERT(bio->bio_buf->b_cmd == BUF_CMD_WRITE);

        if ((buf_offset & HAMMER_BUFMASK) == 0 &&
            leaf->data_len >= HAMMER_BUFSIZE) {
                /*
                 * We are using the vnode's bio to write directly to the
                 * media, any hammer_buffer at the same zone-X offset will
                 * now have stale data.
                 */
                zone2_offset = hammer_blockmap_lookup(hmp, buf_offset, &error);
                vol_no = HAMMER_VOL_DECODE(zone2_offset);
                volume = hammer_get_volume(hmp, vol_no, &error);

                if (error == 0 && zone2_offset >= volume->maxbuf_off)
                        error = EIO;
                if (error == 0) {
                        bp = bio->bio_buf;
                        KKASSERT((bp->b_bufsize & HAMMER_BUFMASK) == 0);
                        hammer_del_buffers(hmp, buf_offset,
                                           zone2_offset, bp->b_bufsize);
                        /*
                         * Second level bio - cached zone2 offset.
                         */
                        nbio = push_bio(bio);
                        nbio->bio_offset = zone2_offset;

                        /*
                         * Third level bio - raw offset specific to the
                         * correct volume.
                         */
                        zone2_offset &= HAMMER_OFF_SHORT_MASK;
                        nbio = push_bio(nbio);
                        nbio->bio_offset = volume->ondisk->vol_buf_beg +
                                           zone2_offset;
                        vn_strategy(volume->devvp, nbio);
                }
                hammer_rel_volume(volume, 0);
        } else {
                /* must fit in a standard HAMMER buffer */
                KKASSERT(((buf_offset ^ (buf_offset + leaf->data_len - 1)) & ~HAMMER_BUFMASK64) == 0);
                buffer = NULL;
                ptr = hammer_bread(hmp, buf_offset, &error, &buffer);
                if (error == 0) {
                        bp = bio->bio_buf;
                        bp->b_flags |= B_AGE;
                        hammer_io_modify(&buffer->io, 1);
                        bcopy(bp->b_data, ptr, leaf->data_len);
                        hammer_io_modify_done(&buffer->io);
                        hammer_rel_buffer(buffer, 0);
                        bp->b_resid = 0;
                        biodone(bio);
                }
        }
        if (error) {
                kprintf("hammer_direct_write: failed @ %016llx\n",
                        leaf->data_offset);
                bp = bio->bio_buf;
                bp->b_resid = 0;
                bp->b_error = EIO;
                bp->b_flags |= B_ERROR;
                biodone(bio);
        }
        return(error);
}

/*
 * This is called to remove the second-level cached zone-2 offset from
 * frontend buffer cache buffers, now stale due to a data relocation.
 * These offsets are generated by cluster_read() via VOP_BMAP, or directly
 * by hammer_vop_strategy_read().
 *
 * This is rather nasty because here we have something like the reblocker
 * scanning the raw B-Tree with no held references on anything, really,
 * other then a shared lock on the B-Tree node, and we have to access the
 * frontend's buffer cache to check for and clean out the association.
 * Specifically, if the reblocker is moving data on the disk, these cached
 * offsets will become invalid.
 *
 * Only data record types associated with the large-data zone are subject
 * to direct-io and need to be checked.
 *
 */
void
hammer_io_direct_uncache(hammer_mount_t hmp, hammer_btree_leaf_elm_t leaf)
{
        struct hammer_inode_info iinfo;
        int zone;

        if (leaf->base.rec_type != HAMMER_RECTYPE_DATA)
                return;
        zone = HAMMER_ZONE_DECODE(leaf->data_offset);
        if (zone != HAMMER_ZONE_LARGE_DATA_INDEX)
                return;
        iinfo.obj_id = leaf->base.obj_id;
        iinfo.obj_asof = 0;     /* unused */
        iinfo.obj_localization = leaf->base.localization &
                                 HAMMER_LOCALIZE_PSEUDOFS_MASK;
        iinfo.u.leaf = leaf;
        hammer_scan_inode_snapshots(hmp, &iinfo,
                                    hammer_io_direct_uncache_callback,
                                    leaf);
}

static int
hammer_io_direct_uncache_callback(hammer_inode_t ip, void *data)
{
        hammer_inode_info_t iinfo = data;
        hammer_off_t data_offset;
        hammer_off_t file_offset;
        struct vnode *vp;
        struct buf *bp;
        int blksize;

        if (ip->vp == NULL)
                return(0);
        data_offset = iinfo->u.leaf->data_offset;
        file_offset = iinfo->u.leaf->base.key - iinfo->u.leaf->data_len;
        blksize = iinfo->u.leaf->data_len;
        KKASSERT((blksize & HAMMER_BUFMASK) == 0);

        hammer_ref(&ip->lock);
        if (hammer_get_vnode(ip, &vp) == 0) {
                if ((bp = findblk(ip->vp, file_offset)) != NULL &&
                    bp->b_bio2.bio_offset != NOOFFSET) {
                        bp = getblk(ip->vp, file_offset, blksize, 0, 0);
                        bp->b_bio2.bio_offset = NOOFFSET;
                        brelse(bp);
                }
                vput(vp);
        }
        hammer_rel_inode(ip, 0);
        return(0);
}