Contributors: 9
Author Tokens Token Proportion Commits Commit Proportion
Darrick J. Wong 2398 91.91% 45 66.18%
Christoph Hellwig 135 5.17% 3 4.41%
David Chinner 46 1.76% 14 20.59%
Russell Cattelan 15 0.57% 1 1.47%
Brian Foster 6 0.23% 1 1.47%
Michal Marek 4 0.15% 1 1.47%
Eric Sandeen 2 0.08% 1 1.47%
Alex Elder 2 0.08% 1 1.47%
Nathan Scott 1 0.04% 1 1.47%
Total 2609 68 


// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * Copyright (C) 2018-2023 Oracle.  All Rights Reserved.
 * Author: Darrick J. Wong <djwong@kernel.org>
 */
#include "xfs.h"
#include "xfs_fs.h"
#include "xfs_shared.h"
#include "xfs_format.h"
#include "xfs_trans_resv.h"
#include "xfs_mount.h"
#include "xfs_defer.h"
#include "xfs_btree.h"
#include "xfs_btree_staging.h"
#include "xfs_inode.h"
#include "xfs_bit.h"
#include "xfs_log_format.h"
#include "xfs_trans.h"
#include "xfs_sb.h"
#include "xfs_alloc.h"
#include "xfs_ialloc.h"
#include "xfs_rmap.h"
#include "xfs_rmap_btree.h"
#include "xfs_refcount.h"
#include "xfs_refcount_btree.h"
#include "xfs_error.h"
#include "xfs_ag.h"
#include "scrub/xfs_scrub.h"
#include "scrub/scrub.h"
#include "scrub/common.h"
#include "scrub/btree.h"
#include "scrub/trace.h"
#include "scrub/repair.h"
#include "scrub/bitmap.h"
#include "scrub/agb_bitmap.h"
#include "scrub/xfile.h"
#include "scrub/xfarray.h"
#include "scrub/newbt.h"
#include "scrub/reap.h"

/*
 * Rebuilding the Reference Count Btree
 * ====================================
 *
 * This algorithm is "borrowed" from xfs_repair.  Imagine the rmap
 * entries as rectangles representing extents of physical blocks, and
 * that the rectangles can be laid down to allow them to overlap each
 * other; then we know that we must emit a refcnt btree entry wherever
 * the amount of overlap changes, i.e. the emission stimulus is
 * level-triggered:
 *
 *                 -    ---
 *       --      ----- ----   ---        ------
 * --   ----     ----------- ----     ---------
 * -------------------------------- -----------
 * ^ ^  ^^ ^^    ^ ^^ ^^^  ^^^^  ^ ^^ ^  ^     ^
 * 2 1  23 21    3 43 234  2123  1 01 2  3     0
 *
 * For our purposes, a rmap is a tuple (startblock, len, fileoff, owner).
 *
 * Note that in the actual refcnt btree we don't store the refcount < 2
 * cases because the bnobt tells us which blocks are free; single-use
 * blocks aren't recorded in the bnobt or the refcntbt.  If the rmapbt
 * supports storing multiple entries covering a given block we could
 * theoretically dispense with the refcntbt and simply count rmaps, but
 * that's inefficient in the (hot) write path, so we'll take the cost of
 * the extra tree to save time.  Also there's no guarantee that rmap
 * will be enabled.
 *
 * Given an array of rmaps sorted by physical block number, a starting
 * physical block (sp), a bag to hold rmaps that cover sp, and the next
 * physical block where the level changes (np), we can reconstruct the
 * refcount btree as follows:
 *
 * While there are still unprocessed rmaps in the array,
 *  - Set sp to the physical block (pblk) of the next unprocessed rmap.
 *  - Add to the bag all rmaps in the array where startblock == sp.
 *  - Set np to the physical block where the bag size will change.  This
 *    is the minimum of (the pblk of the next unprocessed rmap) and
 *    (startblock + len of each rmap in the bag).
 *  - Record the bag size as old_bag_size.
 *
 *  - While the bag isn't empty,
 *     - Remove from the bag all rmaps where startblock + len == np.
 *     - Add to the bag all rmaps in the array where startblock == np.
 *     - If the bag size isn't old_bag_size, store the refcount entry
 *       (sp, np - sp, bag_size) in the refcnt btree.
 *     - If the bag is empty, break out of the inner loop.
 *     - Set old_bag_size to the bag size
 *     - Set sp = np.
 *     - Set np to the physical block where the bag size will change.
 *       This is the minimum of (the pblk of the next unprocessed rmap)
 *       and (startblock + len of each rmap in the bag).
 *
 * Like all the other repairers, we make a list of all the refcount
 * records we need, then reinitialize the refcount btree root and
 * insert all the records.
 */

/* The only parts of the rmap that we care about for computing refcounts. */
struct xrep_refc_rmap {
	xfs_agblock_t		startblock;
	xfs_extlen_t		blockcount;
} __packed;

struct xrep_refc {
	/* refcount extents */
	struct xfarray		*refcount_records;

	/* new refcountbt information */
	struct xrep_newbt	new_btree;

	/* old refcountbt blocks */
	struct xagb_bitmap	old_refcountbt_blocks;

	struct xfs_scrub	*sc;

	/* get_records()'s position in the refcount record array. */
	xfarray_idx_t		array_cur;

	/* # of refcountbt blocks */
	xfs_extlen_t		btblocks;
};

/* Check for any obvious conflicts with this shared/CoW staging extent. */
STATIC int
xrep_refc_check_ext(
	struct xfs_scrub		*sc,
	const struct xfs_refcount_irec	*rec)
{
	enum xbtree_recpacking		outcome;
	int				error;

	if (xfs_refcount_check_irec(sc->sa.pag, rec) != NULL)
		return -EFSCORRUPTED;

	/* Make sure this isn't free space. */
	error = xfs_alloc_has_records(sc->sa.bno_cur, rec->rc_startblock,
			rec->rc_blockcount, &outcome);
	if (error)
		return error;
	if (outcome != XBTREE_RECPACKING_EMPTY)
		return -EFSCORRUPTED;

	/* Must not be an inode chunk. */
	error = xfs_ialloc_has_inodes_at_extent(sc->sa.ino_cur,
			rec->rc_startblock, rec->rc_blockcount, &outcome);
	if (error)
		return error;
	if (outcome != XBTREE_RECPACKING_EMPTY)
		return -EFSCORRUPTED;

	return 0;
}

/* Record a reference count extent. */
STATIC int
xrep_refc_stash(
	struct xrep_refc		*rr,
	enum xfs_refc_domain		domain,
	xfs_agblock_t			agbno,
	xfs_extlen_t			len,
	uint64_t			refcount)
{
	struct xfs_refcount_irec	irec = {
		.rc_startblock		= agbno,
		.rc_blockcount		= len,
		.rc_domain		= domain,
	};
	struct xfs_scrub		*sc = rr->sc;
	int				error = 0;

	if (xchk_should_terminate(sc, &error))
		return error;

	irec.rc_refcount = min_t(uint64_t, MAXREFCOUNT, refcount);

	error = xrep_refc_check_ext(rr->sc, &irec);
	if (error)
		return error;

	trace_xrep_refc_found(sc->sa.pag, &irec);

	return xfarray_append(rr->refcount_records, &irec);
}

/* Record a CoW staging extent. */
STATIC int
xrep_refc_stash_cow(
	struct xrep_refc		*rr,
	xfs_agblock_t			agbno,
	xfs_extlen_t			len)
{
	return xrep_refc_stash(rr, XFS_REFC_DOMAIN_COW, agbno, len, 1);
}

/* Decide if an rmap could describe a shared extent. */
static inline bool
xrep_refc_rmap_shareable(
	struct xfs_mount		*mp,
	const struct xfs_rmap_irec	*rmap)
{
	/* AG metadata are never sharable */
	if (XFS_RMAP_NON_INODE_OWNER(rmap->rm_owner))
		return false;

	/* Metadata in files are never shareable */
	if (xfs_internal_inum(mp, rmap->rm_owner))
		return false;

	/* Metadata and unwritten file blocks are not shareable. */
	if (rmap->rm_flags & (XFS_RMAP_ATTR_FORK | XFS_RMAP_BMBT_BLOCK |
			      XFS_RMAP_UNWRITTEN))
		return false;

	return true;
}

/*
 * Walk along the reverse mapping records until we find one that could describe
 * a shared extent.
 */
STATIC int
xrep_refc_walk_rmaps(
	struct xrep_refc	*rr,
	struct xrep_refc_rmap	*rrm,
	bool			*have_rec)
{
	struct xfs_rmap_irec	rmap;
	struct xfs_btree_cur	*cur = rr->sc->sa.rmap_cur;
	struct xfs_mount	*mp = cur->bc_mp;
	int			have_gt;
	int			error = 0;

	*have_rec = false;

	/*
	 * Loop through the remaining rmaps.  Remember CoW staging
	 * extents and the refcountbt blocks from the old tree for later
	 * disposal.  We can only share written data fork extents, so
	 * keep looping until we find an rmap for one.
	 */
	do {
		if (xchk_should_terminate(rr->sc, &error))
			return error;

		error = xfs_btree_increment(cur, 0, &have_gt);
		if (error)
			return error;
		if (!have_gt)
			return 0;

		error = xfs_rmap_get_rec(cur, &rmap, &have_gt);
		if (error)
			return error;
		if (XFS_IS_CORRUPT(mp, !have_gt))
			return -EFSCORRUPTED;

		if (rmap.rm_owner == XFS_RMAP_OWN_COW) {
			error = xrep_refc_stash_cow(rr, rmap.rm_startblock,
					rmap.rm_blockcount);
			if (error)
				return error;
		} else if (rmap.rm_owner == XFS_RMAP_OWN_REFC) {
			/* refcountbt block, dump it when we're done. */
			rr->btblocks += rmap.rm_blockcount;
			error = xagb_bitmap_set(&rr->old_refcountbt_blocks,
					rmap.rm_startblock, rmap.rm_blockcount);
			if (error)
				return error;
		}
	} while (!xrep_refc_rmap_shareable(mp, &rmap));

	rrm->startblock = rmap.rm_startblock;
	rrm->blockcount = rmap.rm_blockcount;
	*have_rec = true;
	return 0;
}

static inline uint32_t
xrep_refc_encode_startblock(
	const struct xfs_refcount_irec	*irec)
{
	uint32_t			start;

	start = irec->rc_startblock & ~XFS_REFC_COWFLAG;
	if (irec->rc_domain == XFS_REFC_DOMAIN_COW)
		start |= XFS_REFC_COWFLAG;

	return start;
}

/* Sort in the same order as the ondisk records. */
static int
xrep_refc_extent_cmp(
	const void			*a,
	const void			*b)
{
	const struct xfs_refcount_irec	*ap = a;
	const struct xfs_refcount_irec	*bp = b;
	uint32_t			sa, sb;

	sa = xrep_refc_encode_startblock(ap);
	sb = xrep_refc_encode_startblock(bp);

	if (sa > sb)
		return 1;
	if (sa < sb)
		return -1;
	return 0;
}

/*
 * Sort the refcount extents by startblock or else the btree records will be in
 * the wrong order.  Make sure the records do not overlap in physical space.
 */
STATIC int
xrep_refc_sort_records(
	struct xrep_refc		*rr)
{
	struct xfs_refcount_irec	irec;
	xfarray_idx_t			cur;
	enum xfs_refc_domain		dom = XFS_REFC_DOMAIN_SHARED;
	xfs_agblock_t			next_agbno = 0;
	int				error;

	error = xfarray_sort(rr->refcount_records, xrep_refc_extent_cmp,
			XFARRAY_SORT_KILLABLE);
	if (error)
		return error;

	foreach_xfarray_idx(rr->refcount_records, cur) {
		if (xchk_should_terminate(rr->sc, &error))
			return error;

		error = xfarray_load(rr->refcount_records, cur, &irec);
		if (error)
			return error;

		if (dom == XFS_REFC_DOMAIN_SHARED &&
		    irec.rc_domain == XFS_REFC_DOMAIN_COW) {
			dom = irec.rc_domain;
			next_agbno = 0;
		}

		if (dom != irec.rc_domain)
			return -EFSCORRUPTED;
		if (irec.rc_startblock < next_agbno)
			return -EFSCORRUPTED;

		next_agbno = irec.rc_startblock + irec.rc_blockcount;
	}

	return error;
}

#define RRM_NEXT(r)	((r).startblock + (r).blockcount)
/*
 * Find the next block where the refcount changes, given the next rmap we
 * looked at and the ones we're already tracking.
 */
static inline int
xrep_refc_next_edge(
	struct xfarray		*rmap_bag,
	struct xrep_refc_rmap	*next_rrm,
	bool			next_valid,
	xfs_agblock_t		*nbnop)
{
	struct xrep_refc_rmap	rrm;
	xfarray_idx_t		array_cur = XFARRAY_CURSOR_INIT;
	xfs_agblock_t		nbno = NULLAGBLOCK;
	int			error;

	if (next_valid)
		nbno = next_rrm->startblock;

	while ((error = xfarray_iter(rmap_bag, &array_cur, &rrm)) == 1)
		nbno = min_t(xfs_agblock_t, nbno, RRM_NEXT(rrm));

	if (error)
		return error;

	/*
	 * We should have found /something/ because either next_rrm is the next
	 * interesting rmap to look at after emitting this refcount extent, or
	 * there are other rmaps in rmap_bag contributing to the current
	 * sharing count.  But if something is seriously wrong, bail out.
	 */
	if (nbno == NULLAGBLOCK)
		return -EFSCORRUPTED;

	*nbnop = nbno;
	return 0;
}

/*
 * Walk forward through the rmap btree to collect all rmaps starting at
 * @bno in @rmap_bag.  These represent the file(s) that share ownership of
 * the current block.  Upon return, the rmap cursor points to the last record
 * satisfying the startblock constraint.
 */
static int
xrep_refc_push_rmaps_at(
	struct xrep_refc	*rr,
	struct xfarray		*rmap_bag,
	xfs_agblock_t		bno,
	struct xrep_refc_rmap	*rrm,
	bool			*have,
	uint64_t		*stack_sz)
{
	struct xfs_scrub	*sc = rr->sc;
	int			have_gt;
	int			error;

	while (*have && rrm->startblock == bno) {
		error = xfarray_store_anywhere(rmap_bag, rrm);
		if (error)
			return error;
		(*stack_sz)++;
		error = xrep_refc_walk_rmaps(rr, rrm, have);
		if (error)
			return error;
	}

	error = xfs_btree_decrement(sc->sa.rmap_cur, 0, &have_gt);
	if (error)
		return error;
	if (XFS_IS_CORRUPT(sc->mp, !have_gt))
		return -EFSCORRUPTED;

	return 0;
}

/* Iterate all the rmap records to generate reference count data. */
STATIC int
xrep_refc_find_refcounts(
	struct xrep_refc	*rr)
{
	struct xrep_refc_rmap	rrm;
	struct xfs_scrub	*sc = rr->sc;
	struct xfarray		*rmap_bag;
	char			*descr;
	uint64_t		old_stack_sz;
	uint64_t		stack_sz = 0;
	xfs_agblock_t		sbno;
	xfs_agblock_t		cbno;
	xfs_agblock_t		nbno;
	bool			have;
	int			error;

	xrep_ag_btcur_init(sc, &sc->sa);

	/*
	 * Set up a sparse array to store all the rmap records that we're
	 * tracking to generate a reference count record.  If this exceeds
	 * MAXREFCOUNT, we clamp rc_refcount.
	 */
	descr = xchk_xfile_ag_descr(sc, "rmap record bag");
	error = xfarray_create(descr, 0, sizeof(struct xrep_refc_rmap),
			&rmap_bag);
	kfree(descr);
	if (error)
		goto out_cur;

	/* Start the rmapbt cursor to the left of all records. */
	error = xfs_btree_goto_left_edge(sc->sa.rmap_cur);
	if (error)
		goto out_bag;

	/* Process reverse mappings into refcount data. */
	while (xfs_btree_has_more_records(sc->sa.rmap_cur)) {
		/* Push all rmaps with pblk == sbno onto the stack */
		error = xrep_refc_walk_rmaps(rr, &rrm, &have);
		if (error)
			goto out_bag;
		if (!have)
			break;
		sbno = cbno = rrm.startblock;
		error = xrep_refc_push_rmaps_at(rr, rmap_bag, sbno,
					&rrm, &have, &stack_sz);
		if (error)
			goto out_bag;

		/* Set nbno to the bno of the next refcount change */
		error = xrep_refc_next_edge(rmap_bag, &rrm, have, &nbno);
		if (error)
			goto out_bag;

		ASSERT(nbno > sbno);
		old_stack_sz = stack_sz;

		/* While stack isn't empty... */
		while (stack_sz) {
			xfarray_idx_t	array_cur = XFARRAY_CURSOR_INIT;

			/* Pop all rmaps that end at nbno */
			while ((error = xfarray_iter(rmap_bag, &array_cur,
								&rrm)) == 1) {
				if (RRM_NEXT(rrm) != nbno)
					continue;
				error = xfarray_unset(rmap_bag, array_cur - 1);
				if (error)
					goto out_bag;
				stack_sz--;
			}
			if (error)
				goto out_bag;

			/* Push array items that start at nbno */
			error = xrep_refc_walk_rmaps(rr, &rrm, &have);
			if (error)
				goto out_bag;
			if (have) {
				error = xrep_refc_push_rmaps_at(rr, rmap_bag,
						nbno, &rrm, &have, &stack_sz);
				if (error)
					goto out_bag;
			}

			/* Emit refcount if necessary */
			ASSERT(nbno > cbno);
			if (stack_sz != old_stack_sz) {
				if (old_stack_sz > 1) {
					error = xrep_refc_stash(rr,
							XFS_REFC_DOMAIN_SHARED,
							cbno, nbno - cbno,
							old_stack_sz);
					if (error)
						goto out_bag;
				}
				cbno = nbno;
			}

			/* Stack empty, go find the next rmap */
			if (stack_sz == 0)
				break;
			old_stack_sz = stack_sz;
			sbno = nbno;

			/* Set nbno to the bno of the next refcount change */
			error = xrep_refc_next_edge(rmap_bag, &rrm, have,
					&nbno);
			if (error)
				goto out_bag;

			ASSERT(nbno > sbno);
		}
	}

	ASSERT(stack_sz == 0);
out_bag:
	xfarray_destroy(rmap_bag);
out_cur:
	xchk_ag_btcur_free(&sc->sa);
	return error;
}
#undef RRM_NEXT

/* Retrieve refcountbt data for bulk load. */
STATIC int
xrep_refc_get_records(
	struct xfs_btree_cur		*cur,
	unsigned int			idx,
	struct xfs_btree_block		*block,
	unsigned int			nr_wanted,
	void				*priv)
{
	struct xfs_refcount_irec	*irec = &cur->bc_rec.rc;
	struct xrep_refc		*rr = priv;
	union xfs_btree_rec		*block_rec;
	unsigned int			loaded;
	int				error;

	for (loaded = 0; loaded < nr_wanted; loaded++, idx++) {
		error = xfarray_load(rr->refcount_records, rr->array_cur++,
				irec);
		if (error)
			return error;

		block_rec = xfs_btree_rec_addr(cur, idx, block);
		cur->bc_ops->init_rec_from_cur(cur, block_rec);
	}

	return loaded;
}

/* Feed one of the new btree blocks to the bulk loader. */
STATIC int
xrep_refc_claim_block(
	struct xfs_btree_cur	*cur,
	union xfs_btree_ptr	*ptr,
	void			*priv)
{
	struct xrep_refc        *rr = priv;

	return xrep_newbt_claim_block(cur, &rr->new_btree, ptr);
}

/* Update the AGF counters. */
STATIC int
xrep_refc_reset_counters(
	struct xrep_refc	*rr)
{
	struct xfs_scrub	*sc = rr->sc;
	struct xfs_perag	*pag = sc->sa.pag;

	/*
	 * After we commit the new btree to disk, it is possible that the
	 * process to reap the old btree blocks will race with the AIL trying
	 * to checkpoint the old btree blocks into the filesystem.  If the new
	 * tree is shorter than the old one, the refcountbt write verifier will
	 * fail and the AIL will shut down the filesystem.
	 *
	 * To avoid this, save the old incore btree height values as the alt
	 * height values before re-initializing the perag info from the updated
	 * AGF to capture all the new values.
	 */
	pag->pagf_repair_refcount_level = pag->pagf_refcount_level;

	/* Reinitialize with the values we just logged. */
	return xrep_reinit_pagf(sc);
}

/*
 * Use the collected refcount information to stage a new refcount btree.  If
 * this is successful we'll return with the new btree root information logged
 * to the repair transaction but not yet committed.
 */
STATIC int
xrep_refc_build_new_tree(
	struct xrep_refc	*rr)
{
	struct xfs_scrub	*sc = rr->sc;
	struct xfs_btree_cur	*refc_cur;
	struct xfs_perag	*pag = sc->sa.pag;
	xfs_fsblock_t		fsbno;
	int			error;

	error = xrep_refc_sort_records(rr);
	if (error)
		return error;

	/*
	 * Prepare to construct the new btree by reserving disk space for the
	 * new btree and setting up all the accounting information we'll need
	 * to root the new btree while it's under construction and before we
	 * attach it to the AG header.
	 */
	fsbno = XFS_AGB_TO_FSB(sc->mp, pag->pag_agno, xfs_refc_block(sc->mp));
	xrep_newbt_init_ag(&rr->new_btree, sc, &XFS_RMAP_OINFO_REFC, fsbno,
			XFS_AG_RESV_METADATA);
	rr->new_btree.bload.get_records = xrep_refc_get_records;
	rr->new_btree.bload.claim_block = xrep_refc_claim_block;

	/* Compute how many blocks we'll need. */
	refc_cur = xfs_refcountbt_stage_cursor(sc->mp, &rr->new_btree.afake,
			pag);
	error = xfs_btree_bload_compute_geometry(refc_cur,
			&rr->new_btree.bload,
			xfarray_length(rr->refcount_records));
	if (error)
		goto err_cur;

	/* Last chance to abort before we start committing fixes. */
	if (xchk_should_terminate(sc, &error))
		goto err_cur;

	/* Reserve the space we'll need for the new btree. */
	error = xrep_newbt_alloc_blocks(&rr->new_btree,
			rr->new_btree.bload.nr_blocks);
	if (error)
		goto err_cur;

	/*
	 * Due to btree slack factors, it's possible for a new btree to be one
	 * level taller than the old btree.  Update the incore btree height so
	 * that we don't trip the verifiers when writing the new btree blocks
	 * to disk.
	 */
	pag->pagf_repair_refcount_level = rr->new_btree.bload.btree_height;

	/* Add all observed refcount records. */
	rr->array_cur = XFARRAY_CURSOR_INIT;
	error = xfs_btree_bload(refc_cur, &rr->new_btree.bload, rr);
	if (error)
		goto err_level;

	/*
	 * Install the new btree in the AG header.  After this point the old
	 * btree is no longer accessible and the new tree is live.
	 */
	xfs_refcountbt_commit_staged_btree(refc_cur, sc->tp, sc->sa.agf_bp);
	xfs_btree_del_cursor(refc_cur, 0);

	/* Reset the AGF counters now that we've changed the btree shape. */
	error = xrep_refc_reset_counters(rr);
	if (error)
		goto err_newbt;

	/* Dispose of any unused blocks and the accounting information. */
	error = xrep_newbt_commit(&rr->new_btree);
	if (error)
		return error;

	return xrep_roll_ag_trans(sc);

err_level:
	pag->pagf_repair_refcount_level = 0;
err_cur:
	xfs_btree_del_cursor(refc_cur, error);
err_newbt:
	xrep_newbt_cancel(&rr->new_btree);
	return error;
}

/*
 * Now that we've logged the roots of the new btrees, invalidate all of the
 * old blocks and free them.
 */
STATIC int
xrep_refc_remove_old_tree(
	struct xrep_refc	*rr)
{
	struct xfs_scrub	*sc = rr->sc;
	struct xfs_perag	*pag = sc->sa.pag;
	int			error;

	/* Free the old refcountbt blocks if they're not in use. */
	error = xrep_reap_agblocks(sc, &rr->old_refcountbt_blocks,
			&XFS_RMAP_OINFO_REFC, XFS_AG_RESV_METADATA);
	if (error)
		return error;

	/*
	 * Now that we've zapped all the old refcountbt blocks we can turn off
	 * the alternate height mechanism and reset the per-AG space
	 * reservations.
	 */
	pag->pagf_repair_refcount_level = 0;
	sc->flags |= XREP_RESET_PERAG_RESV;
	return 0;
}

/* Rebuild the refcount btree. */
int
xrep_refcountbt(
	struct xfs_scrub	*sc)
{
	struct xrep_refc	*rr;
	struct xfs_mount	*mp = sc->mp;
	char			*descr;
	int			error;

	/* We require the rmapbt to rebuild anything. */
	if (!xfs_has_rmapbt(mp))
		return -EOPNOTSUPP;

	rr = kzalloc(sizeof(struct xrep_refc), XCHK_GFP_FLAGS);
	if (!rr)
		return -ENOMEM;
	rr->sc = sc;

	/* Set up enough storage to handle one refcount record per block. */
	descr = xchk_xfile_ag_descr(sc, "reference count records");
	error = xfarray_create(descr, mp->m_sb.sb_agblocks,
			sizeof(struct xfs_refcount_irec),
			&rr->refcount_records);
	kfree(descr);
	if (error)
		goto out_rr;

	/* Collect all reference counts. */
	xagb_bitmap_init(&rr->old_refcountbt_blocks);
	error = xrep_refc_find_refcounts(rr);
	if (error)
		goto out_bitmap;

	/* Rebuild the refcount information. */
	error = xrep_refc_build_new_tree(rr);
	if (error)
		goto out_bitmap;

	/* Kill the old tree. */
	error = xrep_refc_remove_old_tree(rr);
	if (error)
		goto out_bitmap;

out_bitmap:
	xagb_bitmap_destroy(&rr->old_refcountbt_blocks);
	xfarray_destroy(rr->refcount_records);
out_rr:
	kfree(rr);
	return error;
}