libxfs: refactor manage_zones()

[thirdparty/xfsprogs-dev.git] / libxfs / xfs_refcount.c

// SPDX-License-Identifier: GPL-2.0+
/*
 * Copyright (C) 2016 Oracle.  All Rights Reserved.
 * Author: Darrick J. Wong <darrick.wong@oracle.com>
 */
#include "libxfs_priv.h"
#include "xfs_fs.h"
#include "xfs_shared.h"
#include "xfs_format.h"
#include "xfs_log_format.h"
#include "xfs_trans_resv.h"
#include "xfs_sb.h"
#include "xfs_mount.h"
#include "xfs_defer.h"
#include "xfs_btree.h"
#include "xfs_bmap.h"
#include "xfs_refcount_btree.h"
#include "xfs_alloc.h"
#include "xfs_errortag.h"
#include "xfs_trace.h"
#include "xfs_cksum.h"
#include "xfs_trans.h"
#include "xfs_bit.h"
#include "xfs_refcount.h"
#include "xfs_rmap.h"

/* Allowable refcount adjustment amounts. */
enum xfs_refc_adjust_op {
        XFS_REFCOUNT_ADJUST_INCREASE    = 1,
        XFS_REFCOUNT_ADJUST_DECREASE    = -1,
        XFS_REFCOUNT_ADJUST_COW_ALLOC   = 0,
        XFS_REFCOUNT_ADJUST_COW_FREE    = -1,
};

STATIC int __xfs_refcount_cow_alloc(struct xfs_btree_cur *rcur,
                xfs_agblock_t agbno, xfs_extlen_t aglen);
STATIC int __xfs_refcount_cow_free(struct xfs_btree_cur *rcur,
                xfs_agblock_t agbno, xfs_extlen_t aglen);

/*
 * Look up the first record less than or equal to [bno, len] in the btree
 * given by cur.
 */
int
xfs_refcount_lookup_le(
        struct xfs_btree_cur    *cur,
        xfs_agblock_t           bno,
        int                     *stat)
{
        trace_xfs_refcount_lookup(cur->bc_mp, cur->bc_private.a.agno, bno,
                        XFS_LOOKUP_LE);
        cur->bc_rec.rc.rc_startblock = bno;
        cur->bc_rec.rc.rc_blockcount = 0;
        return xfs_btree_lookup(cur, XFS_LOOKUP_LE, stat);
}

/*
 * Look up the first record greater than or equal to [bno, len] in the btree
 * given by cur.
 */
int
xfs_refcount_lookup_ge(
        struct xfs_btree_cur    *cur,
        xfs_agblock_t           bno,
        int                     *stat)
{
        trace_xfs_refcount_lookup(cur->bc_mp, cur->bc_private.a.agno, bno,
                        XFS_LOOKUP_GE);
        cur->bc_rec.rc.rc_startblock = bno;
        cur->bc_rec.rc.rc_blockcount = 0;
        return xfs_btree_lookup(cur, XFS_LOOKUP_GE, stat);
}

/*
 * Look up the first record equal to [bno, len] in the btree
 * given by cur.
 */
int
xfs_refcount_lookup_eq(
        struct xfs_btree_cur    *cur,
        xfs_agblock_t           bno,
        int                     *stat)
{
        trace_xfs_refcount_lookup(cur->bc_mp, cur->bc_private.a.agno, bno,
                        XFS_LOOKUP_LE);
        cur->bc_rec.rc.rc_startblock = bno;
        cur->bc_rec.rc.rc_blockcount = 0;
        return xfs_btree_lookup(cur, XFS_LOOKUP_EQ, stat);
}

/* Convert on-disk record to in-core format. */
void
xfs_refcount_btrec_to_irec(
        union xfs_btree_rec             *rec,
        struct xfs_refcount_irec        *irec)
{
        irec->rc_startblock = be32_to_cpu(rec->refc.rc_startblock);
        irec->rc_blockcount = be32_to_cpu(rec->refc.rc_blockcount);
        irec->rc_refcount = be32_to_cpu(rec->refc.rc_refcount);
}

/*
 * Get the data from the pointed-to record.
 */
int
xfs_refcount_get_rec(
        struct xfs_btree_cur            *cur,
        struct xfs_refcount_irec        *irec,
        int                             *stat)
{
        struct xfs_mount                *mp = cur->bc_mp;
        xfs_agnumber_t                  agno = cur->bc_private.a.agno;
        union xfs_btree_rec             *rec;
        int                             error;
        xfs_agblock_t                   realstart;

        error = xfs_btree_get_rec(cur, &rec, stat);
        if (error || !*stat)
                return error;

        xfs_refcount_btrec_to_irec(rec, irec);

        agno = cur->bc_private.a.agno;
        if (irec->rc_blockcount == 0 || irec->rc_blockcount > MAXREFCEXTLEN)
                goto out_bad_rec;

        /* handle special COW-staging state */
        realstart = irec->rc_startblock;
        if (realstart & XFS_REFC_COW_START) {
                if (irec->rc_refcount != 1)
                        goto out_bad_rec;
                realstart &= ~XFS_REFC_COW_START;
        } else if (irec->rc_refcount < 2) {
                goto out_bad_rec;
        }

        /* check for valid extent range, including overflow */
        if (!xfs_verify_agbno(mp, agno, realstart))
                goto out_bad_rec;
        if (realstart > realstart + irec->rc_blockcount)
                goto out_bad_rec;
        if (!xfs_verify_agbno(mp, agno, realstart + irec->rc_blockcount - 1))
                goto out_bad_rec;

        if (irec->rc_refcount == 0 || irec->rc_refcount > MAXREFCOUNT)
                goto out_bad_rec;

        trace_xfs_refcount_get(cur->bc_mp, cur->bc_private.a.agno, irec);
        return 0;

out_bad_rec:
        xfs_warn(mp,
                "Refcount BTree record corruption in AG %d detected!", agno);
        xfs_warn(mp,
                "Start block 0x%x, block count 0x%x, references 0x%x",
                irec->rc_startblock, irec->rc_blockcount, irec->rc_refcount);
        return -EFSCORRUPTED;
}

/*
 * Update the record referred to by cur to the value given
 * by [bno, len, refcount].
 * This either works (return 0) or gets an EFSCORRUPTED error.
 */
STATIC int
xfs_refcount_update(
        struct xfs_btree_cur            *cur,
        struct xfs_refcount_irec        *irec)
{
        union xfs_btree_rec     rec;
        int                     error;

        trace_xfs_refcount_update(cur->bc_mp, cur->bc_private.a.agno, irec);
        rec.refc.rc_startblock = cpu_to_be32(irec->rc_startblock);
        rec.refc.rc_blockcount = cpu_to_be32(irec->rc_blockcount);
        rec.refc.rc_refcount = cpu_to_be32(irec->rc_refcount);
        error = xfs_btree_update(cur, &rec);
        if (error)
                trace_xfs_refcount_update_error(cur->bc_mp,
                                cur->bc_private.a.agno, error, _RET_IP_);
        return error;
}

/*
 * Insert the record referred to by cur to the value given
 * by [bno, len, refcount].
 * This either works (return 0) or gets an EFSCORRUPTED error.
 */
int
xfs_refcount_insert(
        struct xfs_btree_cur            *cur,
        struct xfs_refcount_irec        *irec,
        int                             *i)
{
        int                             error;

        trace_xfs_refcount_insert(cur->bc_mp, cur->bc_private.a.agno, irec);
        cur->bc_rec.rc.rc_startblock = irec->rc_startblock;
        cur->bc_rec.rc.rc_blockcount = irec->rc_blockcount;
        cur->bc_rec.rc.rc_refcount = irec->rc_refcount;
        error = xfs_btree_insert(cur, i);
        if (error)
                goto out_error;
        XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, *i == 1, out_error);

out_error:
        if (error)
                trace_xfs_refcount_insert_error(cur->bc_mp,
                                cur->bc_private.a.agno, error, _RET_IP_);
        return error;
}

/*
 * Remove the record referred to by cur, then set the pointer to the spot
 * where the record could be re-inserted, in case we want to increment or
 * decrement the cursor.
 * This either works (return 0) or gets an EFSCORRUPTED error.
 */
STATIC int
xfs_refcount_delete(
        struct xfs_btree_cur    *cur,
        int                     *i)
{
        struct xfs_refcount_irec        irec;
        int                     found_rec;
        int                     error;

        error = xfs_refcount_get_rec(cur, &irec, &found_rec);
        if (error)
                goto out_error;
        XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, found_rec == 1, out_error);
        trace_xfs_refcount_delete(cur->bc_mp, cur->bc_private.a.agno, &irec);
        error = xfs_btree_delete(cur, i);
        XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, *i == 1, out_error);
        if (error)
                goto out_error;
        error = xfs_refcount_lookup_ge(cur, irec.rc_startblock, &found_rec);
out_error:
        if (error)
                trace_xfs_refcount_delete_error(cur->bc_mp,
                                cur->bc_private.a.agno, error, _RET_IP_);
        return error;
}

/*
 * Adjusting the Reference Count
 *
 * As stated elsewhere, the reference count btree (refcbt) stores
 * >1 reference counts for extents of physical blocks.  In this
 * operation, we're either raising or lowering the reference count of
 * some subrange stored in the tree:
 *
 *      <------ adjustment range ------>
 * ----+   +---+-----+ +--+--------+---------
 *  2  |   | 3 |  4  | |17|   55   |   10
 * ----+   +---+-----+ +--+--------+---------
 * X axis is physical blocks number;
 * reference counts are the numbers inside the rectangles
 *
 * The first thing we need to do is to ensure that there are no
 * refcount extents crossing either boundary of the range to be
 * adjusted.  For any extent that does cross a boundary, split it into
 * two extents so that we can increment the refcount of one of the
 * pieces later:
 *
 *      <------ adjustment range ------>
 * ----+   +---+-----+ +--+--------+----+----
 *  2  |   | 3 |  2  | |17|   55   | 10 | 10
 * ----+   +---+-----+ +--+--------+----+----
 *
 * For this next step, let's assume that all the physical blocks in
 * the adjustment range are mapped to a file and are therefore in use
 * at least once.  Therefore, we can infer that any gap in the
 * refcount tree within the adjustment range represents a physical
 * extent with refcount == 1:
 *
 *      <------ adjustment range ------>
 * ----+---+---+-----+-+--+--------+----+----
 *  2  |"1"| 3 |  2  |1|17|   55   | 10 | 10
 * ----+---+---+-----+-+--+--------+----+----
 *      ^
 *
 * For each extent that falls within the interval range, figure out
 * which extent is to the left or the right of that extent.  Now we
 * have a left, current, and right extent.  If the new reference count
 * of the center extent enables us to merge left, center, and right
 * into one record covering all three, do so.  If the center extent is
 * at the left end of the range, abuts the left extent, and its new
 * reference count matches the left extent's record, then merge them.
 * If the center extent is at the right end of the range, abuts the
 * right extent, and the reference counts match, merge those.  In the
 * example, we can left merge (assuming an increment operation):
 *
 *      <------ adjustment range ------>
 * --------+---+-----+-+--+--------+----+----
 *    2    | 3 |  2  |1|17|   55   | 10 | 10
 * --------+---+-----+-+--+--------+----+----
 *          ^
 *
 * For all other extents within the range, adjust the reference count
 * or delete it if the refcount falls below 2.  If we were
 * incrementing, the end result looks like this:
 *
 *      <------ adjustment range ------>
 * --------+---+-----+-+--+--------+----+----
 *    2    | 4 |  3  |2|18|   56   | 11 | 10
 * --------+---+-----+-+--+--------+----+----
 *
 * The result of a decrement operation looks as such:
 *
 *      <------ adjustment range ------>
 * ----+   +---+       +--+--------+----+----
 *  2  |   | 2 |       |16|   54   |  9 | 10
 * ----+   +---+       +--+--------+----+----
 *      DDDD    111111DD
 *
 * The blocks marked "D" are freed; the blocks marked "1" are only
 * referenced once and therefore the record is removed from the
 * refcount btree.
 */

/* Next block after this extent. */
static inline xfs_agblock_t
xfs_refc_next(
        struct xfs_refcount_irec        *rc)
{
        return rc->rc_startblock + rc->rc_blockcount;
}

/*
 * Split a refcount extent that crosses agbno.
 */
STATIC int
xfs_refcount_split_extent(
        struct xfs_btree_cur            *cur,
        xfs_agblock_t                   agbno,
        bool                            *shape_changed)
{
        struct xfs_refcount_irec        rcext, tmp;
        int                             found_rec;
        int                             error;

        *shape_changed = false;
        error = xfs_refcount_lookup_le(cur, agbno, &found_rec);
        if (error)
                goto out_error;
        if (!found_rec)
                return 0;

        error = xfs_refcount_get_rec(cur, &rcext, &found_rec);
        if (error)
                goto out_error;
        XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, found_rec == 1, out_error);
        if (rcext.rc_startblock == agbno || xfs_refc_next(&rcext) <= agbno)
                return 0;

        *shape_changed = true;
        trace_xfs_refcount_split_extent(cur->bc_mp, cur->bc_private.a.agno,
                        &rcext, agbno);

        /* Establish the right extent. */
        tmp = rcext;
        tmp.rc_startblock = agbno;
        tmp.rc_blockcount -= (agbno - rcext.rc_startblock);
        error = xfs_refcount_update(cur, &tmp);
        if (error)
                goto out_error;

        /* Insert the left extent. */
        tmp = rcext;
        tmp.rc_blockcount = agbno - rcext.rc_startblock;
        error = xfs_refcount_insert(cur, &tmp, &found_rec);
        if (error)
                goto out_error;
        XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, found_rec == 1, out_error);
        return error;

out_error:
        trace_xfs_refcount_split_extent_error(cur->bc_mp,
                        cur->bc_private.a.agno, error, _RET_IP_);
        return error;
}

/*
 * Merge the left, center, and right extents.
 */
STATIC int
xfs_refcount_merge_center_extents(
        struct xfs_btree_cur            *cur,
        struct xfs_refcount_irec        *left,
        struct xfs_refcount_irec        *center,
        struct xfs_refcount_irec        *right,
        unsigned long long              extlen,
        xfs_extlen_t                    *aglen)
{
        int                             error;
        int                             found_rec;

        trace_xfs_refcount_merge_center_extents(cur->bc_mp,
                        cur->bc_private.a.agno, left, center, right);

        /*
         * Make sure the center and right extents are not in the btree.
         * If the center extent was synthesized, the first delete call
         * removes the right extent and we skip the second deletion.
         * If center and right were in the btree, then the first delete
         * call removes the center and the second one removes the right
         * extent.
         */
        error = xfs_refcount_lookup_ge(cur, center->rc_startblock,
                        &found_rec);
        if (error)
                goto out_error;
        XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, found_rec == 1, out_error);

        error = xfs_refcount_delete(cur, &found_rec);
        if (error)
                goto out_error;
        XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, found_rec == 1, out_error);

        if (center->rc_refcount > 1) {
                error = xfs_refcount_delete(cur, &found_rec);
                if (error)
                        goto out_error;
                XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, found_rec == 1,
                                out_error);
        }

        /* Enlarge the left extent. */
        error = xfs_refcount_lookup_le(cur, left->rc_startblock,
                        &found_rec);
        if (error)
                goto out_error;
        XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, found_rec == 1, out_error);

        left->rc_blockcount = extlen;
        error = xfs_refcount_update(cur, left);
        if (error)
                goto out_error;

        *aglen = 0;
        return error;

out_error:
        trace_xfs_refcount_merge_center_extents_error(cur->bc_mp,
                        cur->bc_private.a.agno, error, _RET_IP_);
        return error;
}

/*
 * Merge with the left extent.
 */
STATIC int
xfs_refcount_merge_left_extent(
        struct xfs_btree_cur            *cur,
        struct xfs_refcount_irec        *left,
        struct xfs_refcount_irec        *cleft,
        xfs_agblock_t                   *agbno,
        xfs_extlen_t                    *aglen)
{
        int                             error;
        int                             found_rec;

        trace_xfs_refcount_merge_left_extent(cur->bc_mp,
                        cur->bc_private.a.agno, left, cleft);

        /* If the extent at agbno (cleft) wasn't synthesized, remove it. */
        if (cleft->rc_refcount > 1) {
                error = xfs_refcount_lookup_le(cur, cleft->rc_startblock,
                                &found_rec);
                if (error)
                        goto out_error;
                XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, found_rec == 1,
                                out_error);

                error = xfs_refcount_delete(cur, &found_rec);
                if (error)
                        goto out_error;
                XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, found_rec == 1,
                                out_error);
        }

        /* Enlarge the left extent. */
        error = xfs_refcount_lookup_le(cur, left->rc_startblock,
                        &found_rec);
        if (error)
                goto out_error;
        XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, found_rec == 1, out_error);

        left->rc_blockcount += cleft->rc_blockcount;
        error = xfs_refcount_update(cur, left);
        if (error)
                goto out_error;

        *agbno += cleft->rc_blockcount;
        *aglen -= cleft->rc_blockcount;
        return error;

out_error:
        trace_xfs_refcount_merge_left_extent_error(cur->bc_mp,
                        cur->bc_private.a.agno, error, _RET_IP_);
        return error;
}

/*
 * Merge with the right extent.
 */
STATIC int
xfs_refcount_merge_right_extent(
        struct xfs_btree_cur            *cur,
        struct xfs_refcount_irec        *right,
        struct xfs_refcount_irec        *cright,
        xfs_extlen_t                    *aglen)
{
        int                             error;
        int                             found_rec;

        trace_xfs_refcount_merge_right_extent(cur->bc_mp,
                        cur->bc_private.a.agno, cright, right);

        /*
         * If the extent ending at agbno+aglen (cright) wasn't synthesized,
         * remove it.
         */
        if (cright->rc_refcount > 1) {
                error = xfs_refcount_lookup_le(cur, cright->rc_startblock,
                        &found_rec);
                if (error)
                        goto out_error;
                XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, found_rec == 1,
                                out_error);

                error = xfs_refcount_delete(cur, &found_rec);
                if (error)
                        goto out_error;
                XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, found_rec == 1,
                                out_error);
        }

        /* Enlarge the right extent. */
        error = xfs_refcount_lookup_le(cur, right->rc_startblock,
                        &found_rec);
        if (error)
                goto out_error;
        XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, found_rec == 1, out_error);

        right->rc_startblock -= cright->rc_blockcount;
        right->rc_blockcount += cright->rc_blockcount;
        error = xfs_refcount_update(cur, right);
        if (error)
                goto out_error;

        *aglen -= cright->rc_blockcount;
        return error;

out_error:
        trace_xfs_refcount_merge_right_extent_error(cur->bc_mp,
                        cur->bc_private.a.agno, error, _RET_IP_);
        return error;
}

#define XFS_FIND_RCEXT_SHARED   1
#define XFS_FIND_RCEXT_COW      2
/*
 * Find the left extent and the one after it (cleft).  This function assumes
 * that we've already split any extent crossing agbno.
 */
STATIC int
xfs_refcount_find_left_extents(
        struct xfs_btree_cur            *cur,
        struct xfs_refcount_irec        *left,
        struct xfs_refcount_irec        *cleft,
        xfs_agblock_t                   agbno,
        xfs_extlen_t                    aglen,
        int                             flags)
{
        struct xfs_refcount_irec        tmp;
        int                             error;
        int                             found_rec;

        left->rc_startblock = cleft->rc_startblock = NULLAGBLOCK;
        error = xfs_refcount_lookup_le(cur, agbno - 1, &found_rec);
        if (error)
                goto out_error;
        if (!found_rec)
                return 0;

        error = xfs_refcount_get_rec(cur, &tmp, &found_rec);
        if (error)
                goto out_error;
        XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, found_rec == 1, out_error);

        if (xfs_refc_next(&tmp) != agbno)
                return 0;
        if ((flags & XFS_FIND_RCEXT_SHARED) && tmp.rc_refcount < 2)
                return 0;
        if ((flags & XFS_FIND_RCEXT_COW) && tmp.rc_refcount > 1)
                return 0;
        /* We have a left extent; retrieve (or invent) the next right one */
        *left = tmp;

        error = xfs_btree_increment(cur, 0, &found_rec);
        if (error)
                goto out_error;
        if (found_rec) {
                error = xfs_refcount_get_rec(cur, &tmp, &found_rec);
                if (error)
                        goto out_error;
                XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, found_rec == 1,
                                out_error);

                /* if tmp starts at the end of our range, just use that */
                if (tmp.rc_startblock == agbno)
                        *cleft = tmp;
                else {
                        /*
                         * There's a gap in the refcntbt at the start of the
                         * range we're interested in (refcount == 1) so
                         * synthesize the implied extent and pass it back.
                         * We assume here that the agbno/aglen range was
                         * passed in from a data fork extent mapping and
                         * therefore is allocated to exactly one owner.
                         */
                        cleft->rc_startblock = agbno;
                        cleft->rc_blockcount = min(aglen,
                                        tmp.rc_startblock - agbno);
                        cleft->rc_refcount = 1;
                }
        } else {
                /*
                 * No extents, so pretend that there's one covering the whole
                 * range.
                 */
                cleft->rc_startblock = agbno;
                cleft->rc_blockcount = aglen;
                cleft->rc_refcount = 1;
        }
        trace_xfs_refcount_find_left_extent(cur->bc_mp, cur->bc_private.a.agno,
                        left, cleft, agbno);
        return error;

out_error:
        trace_xfs_refcount_find_left_extent_error(cur->bc_mp,
                        cur->bc_private.a.agno, error, _RET_IP_);
        return error;
}

/*
 * Find the right extent and the one before it (cright).  This function
 * assumes that we've already split any extents crossing agbno + aglen.
 */
STATIC int
xfs_refcount_find_right_extents(
        struct xfs_btree_cur            *cur,
        struct xfs_refcount_irec        *right,
        struct xfs_refcount_irec        *cright,
        xfs_agblock_t                   agbno,
        xfs_extlen_t                    aglen,
        int                             flags)
{
        struct xfs_refcount_irec        tmp;
        int                             error;
        int                             found_rec;

        right->rc_startblock = cright->rc_startblock = NULLAGBLOCK;
        error = xfs_refcount_lookup_ge(cur, agbno + aglen, &found_rec);
        if (error)
                goto out_error;
        if (!found_rec)
                return 0;

        error = xfs_refcount_get_rec(cur, &tmp, &found_rec);
        if (error)
                goto out_error;
        XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, found_rec == 1, out_error);

        if (tmp.rc_startblock != agbno + aglen)
                return 0;
        if ((flags & XFS_FIND_RCEXT_SHARED) && tmp.rc_refcount < 2)
                return 0;
        if ((flags & XFS_FIND_RCEXT_COW) && tmp.rc_refcount > 1)
                return 0;
        /* We have a right extent; retrieve (or invent) the next left one */
        *right = tmp;

        error = xfs_btree_decrement(cur, 0, &found_rec);
        if (error)
                goto out_error;
        if (found_rec) {
                error = xfs_refcount_get_rec(cur, &tmp, &found_rec);
                if (error)
                        goto out_error;
                XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, found_rec == 1,
                                out_error);

                /* if tmp ends at the end of our range, just use that */
                if (xfs_refc_next(&tmp) == agbno + aglen)
                        *cright = tmp;
                else {
                        /*
                         * There's a gap in the refcntbt at the end of the
                         * range we're interested in (refcount == 1) so
                         * create the implied extent and pass it back.
                         * We assume here that the agbno/aglen range was
                         * passed in from a data fork extent mapping and
                         * therefore is allocated to exactly one owner.
                         */
                        cright->rc_startblock = max(agbno, xfs_refc_next(&tmp));
                        cright->rc_blockcount = right->rc_startblock -
                                        cright->rc_startblock;
                        cright->rc_refcount = 1;
                }
        } else {
                /*
                 * No extents, so pretend that there's one covering the whole
                 * range.
                 */
                cright->rc_startblock = agbno;
                cright->rc_blockcount = aglen;
                cright->rc_refcount = 1;
        }
        trace_xfs_refcount_find_right_extent(cur->bc_mp, cur->bc_private.a.agno,
                        cright, right, agbno + aglen);
        return error;

out_error:
        trace_xfs_refcount_find_right_extent_error(cur->bc_mp,
                        cur->bc_private.a.agno, error, _RET_IP_);
        return error;
}

/* Is this extent valid? */
static inline bool
xfs_refc_valid(
        struct xfs_refcount_irec        *rc)
{
        return rc->rc_startblock != NULLAGBLOCK;
}

/*
 * Try to merge with any extents on the boundaries of the adjustment range.
 */
STATIC int
xfs_refcount_merge_extents(
        struct xfs_btree_cur    *cur,
        xfs_agblock_t           *agbno,
        xfs_extlen_t            *aglen,
        enum xfs_refc_adjust_op adjust,
        int                     flags,
        bool                    *shape_changed)
{
        struct xfs_refcount_irec        left = {0}, cleft = {0};
        struct xfs_refcount_irec        cright = {0}, right = {0};
        int                             error;
        unsigned long long              ulen;
        bool                            cequal;

        *shape_changed = false;
        /*
         * Find the extent just below agbno [left], just above agbno [cleft],
         * just below (agbno + aglen) [cright], and just above (agbno + aglen)
         * [right].
         */
        error = xfs_refcount_find_left_extents(cur, &left, &cleft, *agbno,
                        *aglen, flags);
        if (error)
                return error;
        error = xfs_refcount_find_right_extents(cur, &right, &cright, *agbno,
                        *aglen, flags);
        if (error)
                return error;

        /* No left or right extent to merge; exit. */
        if (!xfs_refc_valid(&left) && !xfs_refc_valid(&right))
                return 0;

        cequal = (cleft.rc_startblock == cright.rc_startblock) &&
                 (cleft.rc_blockcount == cright.rc_blockcount);

        /* Try to merge left, cleft, and right.  cleft must == cright. */
        ulen = (unsigned long long)left.rc_blockcount + cleft.rc_blockcount +
                        right.rc_blockcount;
        if (xfs_refc_valid(&left) && xfs_refc_valid(&right) &&
            xfs_refc_valid(&cleft) && xfs_refc_valid(&cright) && cequal &&
            left.rc_refcount == cleft.rc_refcount + adjust &&
            right.rc_refcount == cleft.rc_refcount + adjust &&
            ulen < MAXREFCEXTLEN) {
                *shape_changed = true;
                return xfs_refcount_merge_center_extents(cur, &left, &cleft,
                                &right, ulen, aglen);
        }

        /* Try to merge left and cleft. */
        ulen = (unsigned long long)left.rc_blockcount + cleft.rc_blockcount;
        if (xfs_refc_valid(&left) && xfs_refc_valid(&cleft) &&
            left.rc_refcount == cleft.rc_refcount + adjust &&
            ulen < MAXREFCEXTLEN) {
                *shape_changed = true;
                error = xfs_refcount_merge_left_extent(cur, &left, &cleft,
                                agbno, aglen);
                if (error)
                        return error;

                /*
                 * If we just merged left + cleft and cleft == cright,
                 * we no longer have a cright to merge with right.  We're done.
                 */
                if (cequal)
                        return 0;
        }

        /* Try to merge cright and right. */
        ulen = (unsigned long long)right.rc_blockcount + cright.rc_blockcount;
        if (xfs_refc_valid(&right) && xfs_refc_valid(&cright) &&
            right.rc_refcount == cright.rc_refcount + adjust &&
            ulen < MAXREFCEXTLEN) {
                *shape_changed = true;
                return xfs_refcount_merge_right_extent(cur, &right, &cright,
                                aglen);
        }

        return error;
}

/*
 * XXX: This is a pretty hand-wavy estimate.  The penalty for guessing
 * true incorrectly is a shutdown FS; the penalty for guessing false
 * incorrectly is more transaction rolls than might be necessary.
 * Be conservative here.
 */
static bool
xfs_refcount_still_have_space(
        struct xfs_btree_cur            *cur)
{
        unsigned long                   overhead;

        overhead = cur->bc_private.a.priv.refc.shape_changes *
                        xfs_allocfree_log_count(cur->bc_mp, 1);
        overhead *= cur->bc_mp->m_sb.sb_blocksize;

        /*
         * Only allow 2 refcount extent updates per transaction if the
         * refcount continue update "error" has been injected.
         */
        if (cur->bc_private.a.priv.refc.nr_ops > 2 &&
            XFS_TEST_ERROR(false, cur->bc_mp,
                        XFS_ERRTAG_REFCOUNT_CONTINUE_UPDATE))
                return false;

        if (cur->bc_private.a.priv.refc.nr_ops == 0)
                return true;
        else if (overhead > cur->bc_tp->t_log_res)
                return false;
        return  cur->bc_tp->t_log_res - overhead >
                cur->bc_private.a.priv.refc.nr_ops * XFS_REFCOUNT_ITEM_OVERHEAD;
}

/*
 * Adjust the refcounts of middle extents.  At this point we should have
 * split extents that crossed the adjustment range; merged with adjacent
 * extents; and updated agbno/aglen to reflect the merges.  Therefore,
 * all we have to do is update the extents inside [agbno, agbno + aglen].
 */
STATIC int
xfs_refcount_adjust_extents(
        struct xfs_btree_cur    *cur,
        xfs_agblock_t           *agbno,
        xfs_extlen_t            *aglen,
        enum xfs_refc_adjust_op adj,
        struct xfs_owner_info   *oinfo)
{
        struct xfs_refcount_irec        ext, tmp;
        int                             error;
        int                             found_rec, found_tmp;
        xfs_fsblock_t                   fsbno;

        /* Merging did all the work already. */
        if (*aglen == 0)
                return 0;

        error = xfs_refcount_lookup_ge(cur, *agbno, &found_rec);
        if (error)
                goto out_error;

        while (*aglen > 0 && xfs_refcount_still_have_space(cur)) {
                error = xfs_refcount_get_rec(cur, &ext, &found_rec);
                if (error)
                        goto out_error;
                if (!found_rec) {
                        ext.rc_startblock = cur->bc_mp->m_sb.sb_agblocks;
                        ext.rc_blockcount = 0;
                        ext.rc_refcount = 0;
                }

                /*
                 * Deal with a hole in the refcount tree; if a file maps to
                 * these blocks and there's no refcountbt record, pretend that
                 * there is one with refcount == 1.
                 */
                if (ext.rc_startblock != *agbno) {
                        tmp.rc_startblock = *agbno;
                        tmp.rc_blockcount = min(*aglen,
                                        ext.rc_startblock - *agbno);
                        tmp.rc_refcount = 1 + adj;
                        trace_xfs_refcount_modify_extent(cur->bc_mp,
                                        cur->bc_private.a.agno, &tmp);

                        /*
                         * Either cover the hole (increment) or
                         * delete the range (decrement).
                         */
                        if (tmp.rc_refcount) {
                                error = xfs_refcount_insert(cur, &tmp,
                                                &found_tmp);
                                if (error)
                                        goto out_error;
                                XFS_WANT_CORRUPTED_GOTO(cur->bc_mp,
                                                found_tmp == 1, out_error);
                                cur->bc_private.a.priv.refc.nr_ops++;
                        } else {
                                fsbno = XFS_AGB_TO_FSB(cur->bc_mp,
                                                cur->bc_private.a.agno,
                                                tmp.rc_startblock);
                                xfs_bmap_add_free(cur->bc_tp, fsbno,
                                                  tmp.rc_blockcount, oinfo);
                        }

                        (*agbno) += tmp.rc_blockcount;
                        (*aglen) -= tmp.rc_blockcount;

                        error = xfs_refcount_lookup_ge(cur, *agbno,
                                        &found_rec);
                        if (error)
                                goto out_error;
                }

                /* Stop if there's nothing left to modify */
                if (*aglen == 0 || !xfs_refcount_still_have_space(cur))
                        break;

                /*
                 * Adjust the reference count and either update the tree
                 * (incr) or free the blocks (decr).
                 */
                if (ext.rc_refcount == MAXREFCOUNT)
                        goto skip;
                ext.rc_refcount += adj;
                trace_xfs_refcount_modify_extent(cur->bc_mp,
                                cur->bc_private.a.agno, &ext);
                if (ext.rc_refcount > 1) {
                        error = xfs_refcount_update(cur, &ext);
                        if (error)
                                goto out_error;
                        cur->bc_private.a.priv.refc.nr_ops++;
                } else if (ext.rc_refcount == 1) {
                        error = xfs_refcount_delete(cur, &found_rec);
                        if (error)
                                goto out_error;
                        XFS_WANT_CORRUPTED_GOTO(cur->bc_mp,
                                        found_rec == 1, out_error);
                        cur->bc_private.a.priv.refc.nr_ops++;
                        goto advloop;
                } else {
                        fsbno = XFS_AGB_TO_FSB(cur->bc_mp,
                                        cur->bc_private.a.agno,
                                        ext.rc_startblock);
                        xfs_bmap_add_free(cur->bc_tp, fsbno, ext.rc_blockcount,
                                          oinfo);
                }

skip:
                error = xfs_btree_increment(cur, 0, &found_rec);
                if (error)
                        goto out_error;

advloop:
                (*agbno) += ext.rc_blockcount;
                (*aglen) -= ext.rc_blockcount;
        }

        return error;
out_error:
        trace_xfs_refcount_modify_extent_error(cur->bc_mp,
                        cur->bc_private.a.agno, error, _RET_IP_);
        return error;
}

/* Adjust the reference count of a range of AG blocks. */
STATIC int
xfs_refcount_adjust(
        struct xfs_btree_cur    *cur,
        xfs_agblock_t           agbno,
        xfs_extlen_t            aglen,
        xfs_agblock_t           *new_agbno,
        xfs_extlen_t            *new_aglen,
        enum xfs_refc_adjust_op adj,
        struct xfs_owner_info   *oinfo)
{
        bool                    shape_changed;
        int                     shape_changes = 0;
        int                     error;

        *new_agbno = agbno;
        *new_aglen = aglen;
        if (adj == XFS_REFCOUNT_ADJUST_INCREASE)
                trace_xfs_refcount_increase(cur->bc_mp, cur->bc_private.a.agno,
                                agbno, aglen);
        else
                trace_xfs_refcount_decrease(cur->bc_mp, cur->bc_private.a.agno,
                                agbno, aglen);

        /*
         * Ensure that no rcextents cross the boundary of the adjustment range.
         */
        error = xfs_refcount_split_extent(cur, agbno, &shape_changed);
        if (error)
                goto out_error;
        if (shape_changed)
                shape_changes++;

        error = xfs_refcount_split_extent(cur, agbno + aglen, &shape_changed);
        if (error)
                goto out_error;
        if (shape_changed)
                shape_changes++;

        /*
         * Try to merge with the left or right extents of the range.
         */
        error = xfs_refcount_merge_extents(cur, new_agbno, new_aglen, adj,
                        XFS_FIND_RCEXT_SHARED, &shape_changed);
        if (error)
                goto out_error;
        if (shape_changed)
                shape_changes++;
        if (shape_changes)
                cur->bc_private.a.priv.refc.shape_changes++;

        /* Now that we've taken care of the ends, adjust the middle extents */
        error = xfs_refcount_adjust_extents(cur, new_agbno, new_aglen,
                        adj, oinfo);
        if (error)
                goto out_error;

        return 0;

out_error:
        trace_xfs_refcount_adjust_error(cur->bc_mp, cur->bc_private.a.agno,
                        error, _RET_IP_);
        return error;
}

/* Clean up after calling xfs_refcount_finish_one. */
void
xfs_refcount_finish_one_cleanup(
        struct xfs_trans        *tp,
        struct xfs_btree_cur    *rcur,
        int                     error)
{
        struct xfs_buf          *agbp;

        if (rcur == NULL)
                return;
        agbp = rcur->bc_private.a.agbp;
        xfs_btree_del_cursor(rcur, error);
        if (error)
                xfs_trans_brelse(tp, agbp);
}

/*
 * Process one of the deferred refcount operations.  We pass back the
 * btree cursor to maintain our lock on the btree between calls.
 * This saves time and eliminates a buffer deadlock between the
 * superblock and the AGF because we'll always grab them in the same
 * order.
 */
int
xfs_refcount_finish_one(
        struct xfs_trans                *tp,
        enum xfs_refcount_intent_type   type,
        xfs_fsblock_t                   startblock,
        xfs_extlen_t                    blockcount,
        xfs_fsblock_t                   *new_fsb,
        xfs_extlen_t                    *new_len,
        struct xfs_btree_cur            **pcur)
{
        struct xfs_mount                *mp = tp->t_mountp;
        struct xfs_btree_cur            *rcur;
        struct xfs_buf                  *agbp = NULL;
        int                             error = 0;
        xfs_agnumber_t                  agno;
        xfs_agblock_t                   bno;
        xfs_agblock_t                   new_agbno;
        unsigned long                   nr_ops = 0;
        int                             shape_changes = 0;

        agno = XFS_FSB_TO_AGNO(mp, startblock);
        ASSERT(agno != NULLAGNUMBER);
        bno = XFS_FSB_TO_AGBNO(mp, startblock);

        trace_xfs_refcount_deferred(mp, XFS_FSB_TO_AGNO(mp, startblock),
                        type, XFS_FSB_TO_AGBNO(mp, startblock),
                        blockcount);

        if (XFS_TEST_ERROR(false, mp,
                        XFS_ERRTAG_REFCOUNT_FINISH_ONE))
                return -EIO;

        /*
         * If we haven't gotten a cursor or the cursor AG doesn't match
         * the startblock, get one now.
         */
        rcur = *pcur;
        if (rcur != NULL && rcur->bc_private.a.agno != agno) {
                nr_ops = rcur->bc_private.a.priv.refc.nr_ops;
                shape_changes = rcur->bc_private.a.priv.refc.shape_changes;
                xfs_refcount_finish_one_cleanup(tp, rcur, 0);
                rcur = NULL;
                *pcur = NULL;
        }
        if (rcur == NULL) {
                error = xfs_alloc_read_agf(tp->t_mountp, tp, agno,
                                XFS_ALLOC_FLAG_FREEING, &agbp);
                if (error)
                        return error;
                if (!agbp)
                        return -EFSCORRUPTED;

                rcur = xfs_refcountbt_init_cursor(mp, tp, agbp, agno);
                if (!rcur) {
                        error = -ENOMEM;
                        goto out_cur;
                }
                rcur->bc_private.a.priv.refc.nr_ops = nr_ops;
                rcur->bc_private.a.priv.refc.shape_changes = shape_changes;
        }
        *pcur = rcur;

        switch (type) {
        case XFS_REFCOUNT_INCREASE:
                error = xfs_refcount_adjust(rcur, bno, blockcount, &new_agbno,
                        new_len, XFS_REFCOUNT_ADJUST_INCREASE, NULL);
                *new_fsb = XFS_AGB_TO_FSB(mp, agno, new_agbno);
                break;
        case XFS_REFCOUNT_DECREASE:
                error = xfs_refcount_adjust(rcur, bno, blockcount, &new_agbno,
                        new_len, XFS_REFCOUNT_ADJUST_DECREASE, NULL);
                *new_fsb = XFS_AGB_TO_FSB(mp, agno, new_agbno);
                break;
        case XFS_REFCOUNT_ALLOC_COW:
                *new_fsb = startblock + blockcount;
                *new_len = 0;
                error = __xfs_refcount_cow_alloc(rcur, bno, blockcount);
                break;
        case XFS_REFCOUNT_FREE_COW:
                *new_fsb = startblock + blockcount;
                *new_len = 0;
                error = __xfs_refcount_cow_free(rcur, bno, blockcount);
                break;
        default:
                ASSERT(0);
                error = -EFSCORRUPTED;
        }
        if (!error && *new_len > 0)
                trace_xfs_refcount_finish_one_leftover(mp, agno, type,
                                bno, blockcount, new_agbno, *new_len);
        return error;

out_cur:
        xfs_trans_brelse(tp, agbp);

        return error;
}

/*
 * Record a refcount intent for later processing.
 */
static int
__xfs_refcount_add(
        struct xfs_trans                *tp,
        enum xfs_refcount_intent_type   type,
        xfs_fsblock_t                   startblock,
        xfs_extlen_t                    blockcount)
{
        struct xfs_refcount_intent      *ri;

        trace_xfs_refcount_defer(tp->t_mountp,
                        XFS_FSB_TO_AGNO(tp->t_mountp, startblock),
                        type, XFS_FSB_TO_AGBNO(tp->t_mountp, startblock),
                        blockcount);

        ri = kmem_alloc(sizeof(struct xfs_refcount_intent),
                        KM_SLEEP | KM_NOFS);
        INIT_LIST_HEAD(&ri->ri_list);
        ri->ri_type = type;
        ri->ri_startblock = startblock;
        ri->ri_blockcount = blockcount;

        xfs_defer_add(tp, XFS_DEFER_OPS_TYPE_REFCOUNT, &ri->ri_list);
        return 0;
}

/*
 * Increase the reference count of the blocks backing a file's extent.
 */
int
xfs_refcount_increase_extent(
        struct xfs_trans                *tp,
        struct xfs_bmbt_irec            *PREV)
{
        if (!xfs_sb_version_hasreflink(&tp->t_mountp->m_sb))
                return 0;

        return __xfs_refcount_add(tp, XFS_REFCOUNT_INCREASE,
                        PREV->br_startblock, PREV->br_blockcount);
}

/*
 * Decrease the reference count of the blocks backing a file's extent.
 */
int
xfs_refcount_decrease_extent(
        struct xfs_trans                *tp,
        struct xfs_bmbt_irec            *PREV)
{
        if (!xfs_sb_version_hasreflink(&tp->t_mountp->m_sb))
                return 0;

        return __xfs_refcount_add(tp, XFS_REFCOUNT_DECREASE,
                        PREV->br_startblock, PREV->br_blockcount);
}

/*
 * Given an AG extent, find the lowest-numbered run of shared blocks
 * within that range and return the range in fbno/flen.  If
 * find_end_of_shared is set, return the longest contiguous extent of
 * shared blocks; if not, just return the first extent we find.  If no
 * shared blocks are found, fbno and flen will be set to NULLAGBLOCK
 * and 0, respectively.
 */
int
xfs_refcount_find_shared(
        struct xfs_btree_cur            *cur,
        xfs_agblock_t                   agbno,
        xfs_extlen_t                    aglen,
        xfs_agblock_t                   *fbno,
        xfs_extlen_t                    *flen,
        bool                            find_end_of_shared)
{
        struct xfs_refcount_irec        tmp;
        int                             i;
        int                             have;
        int                             error;

        trace_xfs_refcount_find_shared(cur->bc_mp, cur->bc_private.a.agno,
                        agbno, aglen);

        /* By default, skip the whole range */
        *fbno = NULLAGBLOCK;
        *flen = 0;

        /* Try to find a refcount extent that crosses the start */
        error = xfs_refcount_lookup_le(cur, agbno, &have);
        if (error)
                goto out_error;
        if (!have) {
                /* No left extent, look at the next one */
                error = xfs_btree_increment(cur, 0, &have);
                if (error)
                        goto out_error;
                if (!have)
                        goto done;
        }
        error = xfs_refcount_get_rec(cur, &tmp, &i);
        if (error)
                goto out_error;
        XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, i == 1, out_error);

        /* If the extent ends before the start, look at the next one */
        if (tmp.rc_startblock + tmp.rc_blockcount <= agbno) {
                error = xfs_btree_increment(cur, 0, &have);
                if (error)
                        goto out_error;
                if (!have)
                        goto done;
                error = xfs_refcount_get_rec(cur, &tmp, &i);
                if (error)
                        goto out_error;
                XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, i == 1, out_error);
        }

        /* If the extent starts after the range we want, bail out */
        if (tmp.rc_startblock >= agbno + aglen)
                goto done;

        /* We found the start of a shared extent! */
        if (tmp.rc_startblock < agbno) {
                tmp.rc_blockcount -= (agbno - tmp.rc_startblock);
                tmp.rc_startblock = agbno;
        }

        *fbno = tmp.rc_startblock;
        *flen = min(tmp.rc_blockcount, agbno + aglen - *fbno);
        if (!find_end_of_shared)
                goto done;

        /* Otherwise, find the end of this shared extent */
        while (*fbno + *flen < agbno + aglen) {
                error = xfs_btree_increment(cur, 0, &have);
                if (error)
                        goto out_error;
                if (!have)
                        break;
                error = xfs_refcount_get_rec(cur, &tmp, &i);
                if (error)
                        goto out_error;
                XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, i == 1, out_error);
                if (tmp.rc_startblock >= agbno + aglen ||
                    tmp.rc_startblock != *fbno + *flen)
                        break;
                *flen = min(*flen + tmp.rc_blockcount, agbno + aglen - *fbno);
        }

done:
        trace_xfs_refcount_find_shared_result(cur->bc_mp,
                        cur->bc_private.a.agno, *fbno, *flen);

out_error:
        if (error)
                trace_xfs_refcount_find_shared_error(cur->bc_mp,
                                cur->bc_private.a.agno, error, _RET_IP_);
        return error;
}

/*
 * Recovering CoW Blocks After a Crash
 *
 * Due to the way that the copy on write mechanism works, there's a window of
 * opportunity in which we can lose track of allocated blocks during a crash.
 * Because CoW uses delayed allocation in the in-core CoW fork, writeback
 * causes blocks to be allocated and stored in the CoW fork.  The blocks are
 * no longer in the free space btree but are not otherwise recorded anywhere
 * until the write completes and the blocks are mapped into the file.  A crash
 * in between allocation and remapping results in the replacement blocks being
 * lost.  This situation is exacerbated by the CoW extent size hint because
 * allocations can hang around for long time.
 *
 * However, there is a place where we can record these allocations before they
 * become mappings -- the reference count btree.  The btree does not record
 * extents with refcount == 1, so we can record allocations with a refcount of
 * 1.  Blocks being used for CoW writeout cannot be shared, so there should be
 * no conflict with shared block records.  These mappings should be created
 * when we allocate blocks to the CoW fork and deleted when they're removed
 * from the CoW fork.
 *
 * Minor nit: records for in-progress CoW allocations and records for shared
 * extents must never be merged, to preserve the property that (except for CoW
 * allocations) there are no refcount btree entries with refcount == 1.  The
 * only time this could potentially happen is when unsharing a block that's
 * adjacent to CoW allocations, so we must be careful to avoid this.
 *
 * At mount time we recover lost CoW allocations by searching the refcount
 * btree for these refcount == 1 mappings.  These represent CoW allocations
 * that were in progress at the time the filesystem went down, so we can free
 * them to get the space back.
 *
 * This mechanism is superior to creating EFIs for unmapped CoW extents for
 * several reasons -- first, EFIs pin the tail of the log and would have to be
 * periodically relogged to avoid filling up the log.  Second, CoW completions
 * will have to file an EFD and create new EFIs for whatever remains in the
 * CoW fork; this partially takes care of (1) but extent-size reservations
 * will have to periodically relog even if there's no writeout in progress.
 * This can happen if the CoW extent size hint is set, which you really want.
 * Third, EFIs cannot currently be automatically relogged into newer
 * transactions to advance the log tail.  Fourth, stuffing the log full of
 * EFIs places an upper bound on the number of CoW allocations that can be
 * held filesystem-wide at any given time.  Recording them in the refcount
 * btree doesn't require us to maintain any state in memory and doesn't pin
 * the log.
 */
/*
 * Adjust the refcounts of CoW allocations.  These allocations are "magic"
 * in that they're not referenced anywhere else in the filesystem, so we
 * stash them in the refcount btree with a refcount of 1 until either file
 * remapping (or CoW cancellation) happens.
 */
STATIC int
xfs_refcount_adjust_cow_extents(
        struct xfs_btree_cur    *cur,
        xfs_agblock_t           agbno,
        xfs_extlen_t            aglen,
        enum xfs_refc_adjust_op adj)
{
        struct xfs_refcount_irec        ext, tmp;
        int                             error;
        int                             found_rec, found_tmp;

        if (aglen == 0)
                return 0;

        /* Find any overlapping refcount records */
        error = xfs_refcount_lookup_ge(cur, agbno, &found_rec);
        if (error)
                goto out_error;
        error = xfs_refcount_get_rec(cur, &ext, &found_rec);
        if (error)
                goto out_error;
        if (!found_rec) {
                ext.rc_startblock = cur->bc_mp->m_sb.sb_agblocks +
                                XFS_REFC_COW_START;
                ext.rc_blockcount = 0;
                ext.rc_refcount = 0;
        }

        switch (adj) {
        case XFS_REFCOUNT_ADJUST_COW_ALLOC:
                /* Adding a CoW reservation, there should be nothing here. */
                XFS_WANT_CORRUPTED_GOTO(cur->bc_mp,
                                ext.rc_startblock >= agbno + aglen, out_error);

                tmp.rc_startblock = agbno;
                tmp.rc_blockcount = aglen;
                tmp.rc_refcount = 1;
                trace_xfs_refcount_modify_extent(cur->bc_mp,
                                cur->bc_private.a.agno, &tmp);

                error = xfs_refcount_insert(cur, &tmp,
                                &found_tmp);
                if (error)
                        goto out_error;
                XFS_WANT_CORRUPTED_GOTO(cur->bc_mp,
                                found_tmp == 1, out_error);
                break;
        case XFS_REFCOUNT_ADJUST_COW_FREE:
                /* Removing a CoW reservation, there should be one extent. */
                XFS_WANT_CORRUPTED_GOTO(cur->bc_mp,
                        ext.rc_startblock == agbno, out_error);
                XFS_WANT_CORRUPTED_GOTO(cur->bc_mp,
                        ext.rc_blockcount == aglen, out_error);
                XFS_WANT_CORRUPTED_GOTO(cur->bc_mp,
                        ext.rc_refcount == 1, out_error);

                ext.rc_refcount = 0;
                trace_xfs_refcount_modify_extent(cur->bc_mp,
                                cur->bc_private.a.agno, &ext);
                error = xfs_refcount_delete(cur, &found_rec);
                if (error)
                        goto out_error;
                XFS_WANT_CORRUPTED_GOTO(cur->bc_mp,
                                found_rec == 1, out_error);
                break;
        default:
                ASSERT(0);
        }

        return error;
out_error:
        trace_xfs_refcount_modify_extent_error(cur->bc_mp,
                        cur->bc_private.a.agno, error, _RET_IP_);
        return error;
}

/*
 * Add or remove refcount btree entries for CoW reservations.
 */
STATIC int
xfs_refcount_adjust_cow(
        struct xfs_btree_cur    *cur,
        xfs_agblock_t           agbno,
        xfs_extlen_t            aglen,
        enum xfs_refc_adjust_op adj)
{
        bool                    shape_changed;
        int                     error;

        agbno += XFS_REFC_COW_START;

        /*
         * Ensure that no rcextents cross the boundary of the adjustment range.
         */
        error = xfs_refcount_split_extent(cur, agbno, &shape_changed);
        if (error)
                goto out_error;

        error = xfs_refcount_split_extent(cur, agbno + aglen, &shape_changed);
        if (error)
                goto out_error;

        /*
         * Try to merge with the left or right extents of the range.
         */
        error = xfs_refcount_merge_extents(cur, &agbno, &aglen, adj,
                        XFS_FIND_RCEXT_COW, &shape_changed);
        if (error)
                goto out_error;

        /* Now that we've taken care of the ends, adjust the middle extents */
        error = xfs_refcount_adjust_cow_extents(cur, agbno, aglen, adj);
        if (error)
                goto out_error;

        return 0;

out_error:
        trace_xfs_refcount_adjust_cow_error(cur->bc_mp, cur->bc_private.a.agno,
                        error, _RET_IP_);
        return error;
}

/*
 * Record a CoW allocation in the refcount btree.
 */
STATIC int
__xfs_refcount_cow_alloc(
        struct xfs_btree_cur    *rcur,
        xfs_agblock_t           agbno,
        xfs_extlen_t            aglen)
{
        trace_xfs_refcount_cow_increase(rcur->bc_mp, rcur->bc_private.a.agno,
                        agbno, aglen);

        /* Add refcount btree reservation */
        return xfs_refcount_adjust_cow(rcur, agbno, aglen,
                        XFS_REFCOUNT_ADJUST_COW_ALLOC);
}

/*
 * Remove a CoW allocation from the refcount btree.
 */
STATIC int
__xfs_refcount_cow_free(
        struct xfs_btree_cur    *rcur,
        xfs_agblock_t           agbno,
        xfs_extlen_t            aglen)
{
        trace_xfs_refcount_cow_decrease(rcur->bc_mp, rcur->bc_private.a.agno,
                        agbno, aglen);

        /* Remove refcount btree reservation */
        return xfs_refcount_adjust_cow(rcur, agbno, aglen,
                        XFS_REFCOUNT_ADJUST_COW_FREE);
}

/* Record a CoW staging extent in the refcount btree. */
int
xfs_refcount_alloc_cow_extent(
        struct xfs_trans                *tp,
        xfs_fsblock_t                   fsb,
        xfs_extlen_t                    len)
{
        struct xfs_mount                *mp = tp->t_mountp;
        int                             error;

        if (!xfs_sb_version_hasreflink(&mp->m_sb))
                return 0;

        error = __xfs_refcount_add(tp, XFS_REFCOUNT_ALLOC_COW, fsb, len);
        if (error)
                return error;

        /* Add rmap entry */
        return xfs_rmap_alloc_extent(tp, XFS_FSB_TO_AGNO(mp, fsb),
                        XFS_FSB_TO_AGBNO(mp, fsb), len, XFS_RMAP_OWN_COW);
}

/* Forget a CoW staging event in the refcount btree. */
int
xfs_refcount_free_cow_extent(
        struct xfs_trans                *tp,
        xfs_fsblock_t                   fsb,
        xfs_extlen_t                    len)
{
        struct xfs_mount                *mp = tp->t_mountp;
        int                             error;

        if (!xfs_sb_version_hasreflink(&mp->m_sb))
                return 0;

        /* Remove rmap entry */
        error = xfs_rmap_free_extent(tp, XFS_FSB_TO_AGNO(mp, fsb),
                        XFS_FSB_TO_AGBNO(mp, fsb), len, XFS_RMAP_OWN_COW);
        if (error)
                return error;

        return __xfs_refcount_add(tp, XFS_REFCOUNT_FREE_COW, fsb, len);
}

struct xfs_refcount_recovery {
        struct list_head                rr_list;
        struct xfs_refcount_irec        rr_rrec;
};

/* Stuff an extent on the recovery list. */
STATIC int
xfs_refcount_recover_extent(
        struct xfs_btree_cur            *cur,
        union xfs_btree_rec             *rec,
        void                            *priv)
{
        struct list_head                *debris = priv;
        struct xfs_refcount_recovery    *rr;

        if (be32_to_cpu(rec->refc.rc_refcount) != 1)
                return -EFSCORRUPTED;

        rr = kmem_alloc(sizeof(struct xfs_refcount_recovery), KM_SLEEP);
        xfs_refcount_btrec_to_irec(rec, &rr->rr_rrec);
        list_add_tail(&rr->rr_list, debris);

        return 0;
}

/* Find and remove leftover CoW reservations. */
int
xfs_refcount_recover_cow_leftovers(
        struct xfs_mount                *mp,
        xfs_agnumber_t                  agno)
{
        struct xfs_trans                *tp;
        struct xfs_btree_cur            *cur;
        struct xfs_buf                  *agbp;
        struct xfs_refcount_recovery    *rr, *n;
        struct list_head                debris;
        union xfs_btree_irec            low;
        union xfs_btree_irec            high;
        xfs_fsblock_t                   fsb;
        xfs_agblock_t                   agbno;
        int                             error;

        if (mp->m_sb.sb_agblocks >= XFS_REFC_COW_START)
                return -EOPNOTSUPP;

        INIT_LIST_HEAD(&debris);

        /*
         * In this first part, we use an empty transaction to gather up
         * all the leftover CoW extents so that we can subsequently
         * delete them.  The empty transaction is used to avoid
         * a buffer lock deadlock if there happens to be a loop in the
         * refcountbt because we're allowed to re-grab a buffer that is
         * already attached to our transaction.  When we're done
         * recording the CoW debris we cancel the (empty) transaction
         * and everything goes away cleanly.
         */
        error = xfs_trans_alloc_empty(mp, &tp);
        if (error)
                return error;

        error = xfs_alloc_read_agf(mp, tp, agno, 0, &agbp);
        if (error)
                goto out_trans;
        if (!agbp) {
                error = -ENOMEM;
                goto out_trans;
        }
        cur = xfs_refcountbt_init_cursor(mp, tp, agbp, agno);

        /* Find all the leftover CoW staging extents. */
        memset(&low, 0, sizeof(low));
        memset(&high, 0, sizeof(high));
        low.rc.rc_startblock = XFS_REFC_COW_START;
        high.rc.rc_startblock = -1U;
        error = xfs_btree_query_range(cur, &low, &high,
                        xfs_refcount_recover_extent, &debris);
        xfs_btree_del_cursor(cur, error);
        xfs_trans_brelse(tp, agbp);
        xfs_trans_cancel(tp);
        if (error)
                goto out_free;

        /* Now iterate the list to free the leftovers */
        list_for_each_entry_safe(rr, n, &debris, rr_list) {
                /* Set up transaction. */
                error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, 0, 0, 0, &tp);
                if (error)
                        goto out_free;

                trace_xfs_refcount_recover_extent(mp, agno, &rr->rr_rrec);

                /* Free the orphan record */
                agbno = rr->rr_rrec.rc_startblock - XFS_REFC_COW_START;
                fsb = XFS_AGB_TO_FSB(mp, agno, agbno);
                error = xfs_refcount_free_cow_extent(tp, fsb,
                                rr->rr_rrec.rc_blockcount);
                if (error)
                        goto out_trans;

                /* Free the block. */
                xfs_bmap_add_free(tp, fsb, rr->rr_rrec.rc_blockcount, NULL);

                error = xfs_trans_commit(tp);
                if (error)
                        goto out_free;

                list_del(&rr->rr_list);
                kmem_free(rr);
        }

        return error;
out_trans:
        xfs_trans_cancel(tp);
out_free:
        /* Free the leftover list */
        list_for_each_entry_safe(rr, n, &debris, rr_list) {
                list_del(&rr->rr_list);
                kmem_free(rr);
        }
        return error;
}

/* Is there a record covering a given extent? */
int
xfs_refcount_has_record(
        struct xfs_btree_cur    *cur,
        xfs_agblock_t           bno,
        xfs_extlen_t            len,
        bool                    *exists)
{
        union xfs_btree_irec    low;
        union xfs_btree_irec    high;

        memset(&low, 0, sizeof(low));
        low.rc.rc_startblock = bno;
        memset(&high, 0xFF, sizeof(high));
        high.rc.rc_startblock = bno + len - 1;

        return xfs_btree_has_record(cur, &low, &high, exists);
}