libxfs/xfs_rmap_btree.c

   1 /*
   2  * Copyright (c) 2014 Red Hat, Inc.
   3  * All Rights Reserved.
   4  *
   5  * This program is free software; you can redistribute it and/or
   6  * modify it under the terms of the GNU General Public License as
   7  * published by the Free Software Foundation.
   8  *
   9  * This program is distributed in the hope that it would be useful,
  10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12  * GNU General Public License for more details.
  13  *
  14  * You should have received a copy of the GNU General Public License
  15  * along with this program; if not, write the Free Software Foundation,
  16  * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
  17  */
  18 #include "libxfs_priv.h"
  19 #include "xfs_fs.h"
  20 #include "xfs_shared.h"
  21 #include "xfs_format.h"
  22 #include "xfs_log_format.h"
  23 #include "xfs_trans_resv.h"
  24 #include "xfs_bit.h"
  25 #include "xfs_sb.h"
  26 #include "xfs_mount.h"
  27 #include "xfs_defer.h"
  28 #include "xfs_inode.h"
  29 #include "xfs_trans.h"
  30 #include "xfs_alloc.h"
  31 #include "xfs_btree.h"
  32 #include "xfs_rmap.h"
  33 #include "xfs_rmap_btree.h"
  34 #include "xfs_trace.h"
  35 #include "xfs_cksum.h"
  36 #include "xfs_ag_resv.h"
  37
  38 /*
  39  * Reverse map btree.
  40  *
  41  * This is a per-ag tree used to track the owner(s) of a given extent. With
  42  * reflink it is possible for there to be multiple owners, which is a departure
  43  * from classic XFS. Owner records for data extents are inserted when the
  44  * extent is mapped and removed when an extent is unmapped.  Owner records for
  45  * all other block types (i.e. metadata) are inserted when an extent is
  46  * allocated and removed when an extent is freed. There can only be one owner
  47  * of a metadata extent, usually an inode or some other metadata structure like
  48  * an AG btree.
  49  *
  50  * The rmap btree is part of the free space management, so blocks for the tree
  51  * are sourced from the agfl. Hence we need transaction reservation support for
  52  * this tree so that the freelist is always large enough. This also impacts on
  53  * the minimum space we need to leave free in the AG.
  54  *
  55  * The tree is ordered by [ag block, owner, offset]. This is a large key size,
  56  * but it is the only way to enforce unique keys when a block can be owned by
  57  * multiple files at any offset. There's no need to order/search by extent
  58  * size for online updating/management of the tree. It is intended that most
  59  * reverse lookups will be to find the owner(s) of a particular block, or to
  60  * try to recover tree and file data from corrupt primary metadata.
  61  */
  62
  63 static struct xfs_btree_cur *
  64 xfs_rmapbt_dup_cursor(
  65         struct xfs_btree_cur    *cur)
  66 {
  67         return xfs_rmapbt_init_cursor(cur->bc_mp, cur->bc_tp,
  68                         cur->bc_private.a.agbp, cur->bc_private.a.agno);
  69 }
  70
  71 STATIC void
  72 xfs_rmapbt_set_root(
  73         struct xfs_btree_cur    *cur,
  74         union xfs_btree_ptr     *ptr,
  75         int                     inc)
  76 {
  77         struct xfs_buf          *agbp = cur->bc_private.a.agbp;
  78         struct xfs_agf          *agf = XFS_BUF_TO_AGF(agbp);
  79         xfs_agnumber_t          seqno = be32_to_cpu(agf->agf_seqno);
  80         int                     btnum = cur->bc_btnum;
  81         struct xfs_perag        *pag = xfs_perag_get(cur->bc_mp, seqno);
  82
  83         ASSERT(ptr->s != 0);
  84
  85         agf->agf_roots[btnum] = ptr->s;
  86         be32_add_cpu(&agf->agf_levels[btnum], inc);
  87         pag->pagf_levels[btnum] += inc;
  88         xfs_perag_put(pag);
  89
  90         xfs_alloc_log_agf(cur->bc_tp, agbp, XFS_AGF_ROOTS | XFS_AGF_LEVELS);
  91 }
  92
  93 STATIC int
  94 xfs_rmapbt_alloc_block(
  95         struct xfs_btree_cur    *cur,
  96         union xfs_btree_ptr     *start,
  97         union xfs_btree_ptr     *new,
  98         int                     *stat)
  99 {
 100         struct xfs_buf          *agbp = cur->bc_private.a.agbp;
 101         struct xfs_agf          *agf = XFS_BUF_TO_AGF(agbp);
 102         int                     error;
 103         xfs_agblock_t           bno;
 104
 105         XFS_BTREE_TRACE_CURSOR(cur, XBT_ENTRY);
 106
 107         /* Allocate the new block from the freelist. If we can't, give up.  */
 108         error = xfs_alloc_get_freelist(cur->bc_tp, cur->bc_private.a.agbp,
 109                                        &bno, 1);
 110         if (error) {
 111                 XFS_BTREE_TRACE_CURSOR(cur, XBT_ERROR);
 112                 return error;
 113         }
 114
 115         trace_xfs_rmapbt_alloc_block(cur->bc_mp, cur->bc_private.a.agno,
 116                         bno, 1);
 117         if (bno == NULLAGBLOCK) {
 118                 XFS_BTREE_TRACE_CURSOR(cur, XBT_EXIT);
 119                 *stat = 0;
 120                 return 0;
 121         }
 122
 123         xfs_extent_busy_reuse(cur->bc_mp, cur->bc_private.a.agno, bno, 1,
 124                         false);
 125
 126         xfs_trans_agbtree_delta(cur->bc_tp, 1);
 127         new->s = cpu_to_be32(bno);
 128         be32_add_cpu(&agf->agf_rmap_blocks, 1);
 129         xfs_alloc_log_agf(cur->bc_tp, agbp, XFS_AGF_RMAP_BLOCKS);
 130
 131         XFS_BTREE_TRACE_CURSOR(cur, XBT_EXIT);
 132         *stat = 1;
 133         return 0;
 134 }
 135
 136 STATIC int
 137 xfs_rmapbt_free_block(
 138         struct xfs_btree_cur    *cur,
 139         struct xfs_buf          *bp)
 140 {
 141         struct xfs_buf          *agbp = cur->bc_private.a.agbp;
 142         struct xfs_agf          *agf = XFS_BUF_TO_AGF(agbp);
 143         xfs_agblock_t           bno;
 144         int                     error;
 145
 146         bno = xfs_daddr_to_agbno(cur->bc_mp, XFS_BUF_ADDR(bp));
 147         trace_xfs_rmapbt_free_block(cur->bc_mp, cur->bc_private.a.agno,
 148                         bno, 1);
 149         be32_add_cpu(&agf->agf_rmap_blocks, -1);
 150         xfs_alloc_log_agf(cur->bc_tp, agbp, XFS_AGF_RMAP_BLOCKS);
 151         error = xfs_alloc_put_freelist(cur->bc_tp, agbp, NULL, bno, 1);
 152         if (error)
 153                 return error;
 154
 155         xfs_extent_busy_insert(cur->bc_tp, be32_to_cpu(agf->agf_seqno), bno, 1,
 156                               XFS_EXTENT_BUSY_SKIP_DISCARD);
 157         xfs_trans_agbtree_delta(cur->bc_tp, -1);
 158
 159         return 0;
 160 }
 161
 162 STATIC int
 163 xfs_rmapbt_get_minrecs(
 164         struct xfs_btree_cur    *cur,
 165         int                     level)
 166 {
 167         return cur->bc_mp->m_rmap_mnr[level != 0];
 168 }
 169
 170 STATIC int
 171 xfs_rmapbt_get_maxrecs(
 172         struct xfs_btree_cur    *cur,
 173         int                     level)
 174 {
 175         return cur->bc_mp->m_rmap_mxr[level != 0];
 176 }
 177
 178 STATIC void
 179 xfs_rmapbt_init_key_from_rec(
 180         union xfs_btree_key     *key,
 181         union xfs_btree_rec     *rec)
 182 {
 183         key->rmap.rm_startblock = rec->rmap.rm_startblock;
 184         key->rmap.rm_owner = rec->rmap.rm_owner;
 185         key->rmap.rm_offset = rec->rmap.rm_offset;
 186 }
 187
 188 /*
 189  * The high key for a reverse mapping record can be computed by shifting
 190  * the startblock and offset to the highest value that would still map
 191  * to that record.  In practice this means that we add blockcount-1 to
 192  * the startblock for all records, and if the record is for a data/attr
 193  * fork mapping, we add blockcount-1 to the offset too.
 194  */
 195 STATIC void
 196 xfs_rmapbt_init_high_key_from_rec(
 197         union xfs_btree_key     *key,
 198         union xfs_btree_rec     *rec)
 199 {
 200         uint64_t                off;
 201         int                     adj;
 202
 203         adj = be32_to_cpu(rec->rmap.rm_blockcount) - 1;
 204
 205         key->rmap.rm_startblock = rec->rmap.rm_startblock;
 206         be32_add_cpu(&key->rmap.rm_startblock, adj);
 207         key->rmap.rm_owner = rec->rmap.rm_owner;
 208         key->rmap.rm_offset = rec->rmap.rm_offset;
 209         if (XFS_RMAP_NON_INODE_OWNER(be64_to_cpu(rec->rmap.rm_owner)) ||
 210             XFS_RMAP_IS_BMBT_BLOCK(be64_to_cpu(rec->rmap.rm_offset)))
 211                 return;
 212         off = be64_to_cpu(key->rmap.rm_offset);
 213         off = (XFS_RMAP_OFF(off) + adj) | (off & ~XFS_RMAP_OFF_MASK);
 214         key->rmap.rm_offset = cpu_to_be64(off);
 215 }
 216
 217 STATIC void
 218 xfs_rmapbt_init_rec_from_cur(
 219         struct xfs_btree_cur    *cur,
 220         union xfs_btree_rec     *rec)
 221 {
 222         rec->rmap.rm_startblock = cpu_to_be32(cur->bc_rec.r.rm_startblock);
 223         rec->rmap.rm_blockcount = cpu_to_be32(cur->bc_rec.r.rm_blockcount);
 224         rec->rmap.rm_owner = cpu_to_be64(cur->bc_rec.r.rm_owner);
 225         rec->rmap.rm_offset = cpu_to_be64(
 226                         xfs_rmap_irec_offset_pack(&cur->bc_rec.r));
 227 }
 228
 229 STATIC void
 230 xfs_rmapbt_init_ptr_from_cur(
 231         struct xfs_btree_cur    *cur,
 232         union xfs_btree_ptr     *ptr)
 233 {
 234         struct xfs_agf          *agf = XFS_BUF_TO_AGF(cur->bc_private.a.agbp);
 235
 236         ASSERT(cur->bc_private.a.agno == be32_to_cpu(agf->agf_seqno));
 237         ASSERT(agf->agf_roots[cur->bc_btnum] != 0);
 238
 239         ptr->s = agf->agf_roots[cur->bc_btnum];
 240 }
 241
 242 STATIC int64_t
 243 xfs_rmapbt_key_diff(
 244         struct xfs_btree_cur    *cur,
 245         union xfs_btree_key     *key)
 246 {
 247         struct xfs_rmap_irec    *rec = &cur->bc_rec.r;
 248         struct xfs_rmap_key     *kp = &key->rmap;
 249         __u64                   x, y;
 250         int64_t                 d;
 251
 252         d = (int64_t)be32_to_cpu(kp->rm_startblock) - rec->rm_startblock;
 253         if (d)
 254                 return d;
 255
 256         x = be64_to_cpu(kp->rm_owner);
 257         y = rec->rm_owner;
 258         if (x > y)
 259                 return 1;
 260         else if (y > x)
 261                 return -1;
 262
 263         x = XFS_RMAP_OFF(be64_to_cpu(kp->rm_offset));
 264         y = rec->rm_offset;
 265         if (x > y)
 266                 return 1;
 267         else if (y > x)
 268                 return -1;
 269         return 0;
 270 }
 271
 272 STATIC int64_t
 273 xfs_rmapbt_diff_two_keys(
 274         struct xfs_btree_cur    *cur,
 275         union xfs_btree_key     *k1,
 276         union xfs_btree_key     *k2)
 277 {
 278         struct xfs_rmap_key     *kp1 = &k1->rmap;
 279         struct xfs_rmap_key     *kp2 = &k2->rmap;
 280         int64_t                 d;
 281         __u64                   x, y;
 282
 283         d = (int64_t)be32_to_cpu(kp1->rm_startblock) -
 284                        be32_to_cpu(kp2->rm_startblock);
 285         if (d)
 286                 return d;
 287
 288         x = be64_to_cpu(kp1->rm_owner);
 289         y = be64_to_cpu(kp2->rm_owner);
 290         if (x > y)
 291                 return 1;
 292         else if (y > x)
 293                 return -1;
 294
 295         x = XFS_RMAP_OFF(be64_to_cpu(kp1->rm_offset));
 296         y = XFS_RMAP_OFF(be64_to_cpu(kp2->rm_offset));
 297         if (x > y)
 298                 return 1;
 299         else if (y > x)
 300                 return -1;
 301         return 0;
 302 }
 303
 304 static xfs_failaddr_t
 305 xfs_rmapbt_verify(
 306         struct xfs_buf          *bp)
 307 {
 308         struct xfs_mount        *mp = bp->b_target->bt_mount;
 309         struct xfs_btree_block  *block = XFS_BUF_TO_BLOCK(bp);
 310         struct xfs_perag        *pag = bp->b_pag;
 311         xfs_failaddr_t          fa;
 312         unsigned int            level;
 313
 314         /*
 315          * magic number and level verification
 316          *
 317          * During growfs operations, we can't verify the exact level or owner as
 318          * the perag is not fully initialised and hence not attached to the
 319          * buffer.  In this case, check against the maximum tree depth.
 320          *
 321          * Similarly, during log recovery we will have a perag structure
 322          * attached, but the agf information will not yet have been initialised
 323          * from the on disk AGF. Again, we can only check against maximum limits
 324          * in this case.
 325          */
 326         if (block->bb_magic != cpu_to_be32(XFS_RMAP_CRC_MAGIC))
 327                 return __this_address;
 328
 329         if (!xfs_sb_version_hasrmapbt(&mp->m_sb))
 330                 return __this_address;
 331         fa = xfs_btree_sblock_v5hdr_verify(bp);
 332         if (fa)
 333                 return fa;
 334
 335         level = be16_to_cpu(block->bb_level);
 336         if (pag && pag->pagf_init) {
 337                 if (level >= pag->pagf_levels[XFS_BTNUM_RMAPi])
 338                         return __this_address;
 339         } else if (level >= mp->m_rmap_maxlevels)
 340                 return __this_address;
 341
 342         return xfs_btree_sblock_verify(bp, mp->m_rmap_mxr[level != 0]);
 343 }
 344
 345 static void
 346 xfs_rmapbt_read_verify(
 347         struct xfs_buf  *bp)
 348 {
 349         xfs_failaddr_t  fa;
 350
 351         if (!xfs_btree_sblock_verify_crc(bp))
 352                 xfs_verifier_error(bp, -EFSBADCRC, __this_address);
 353         else {
 354                 fa = xfs_rmapbt_verify(bp);
 355                 if (fa)
 356                         xfs_verifier_error(bp, -EFSCORRUPTED, fa);
 357         }
 358
 359         if (bp->b_error)
 360                 trace_xfs_btree_corrupt(bp, _RET_IP_);
 361 }
 362
 363 static void
 364 xfs_rmapbt_write_verify(
 365         struct xfs_buf  *bp)
 366 {
 367         xfs_failaddr_t  fa;
 368
 369         fa = xfs_rmapbt_verify(bp);
 370         if (fa) {
 371                 trace_xfs_btree_corrupt(bp, _RET_IP_);
 372                 xfs_verifier_error(bp, -EFSCORRUPTED, fa);
 373                 return;
 374         }
 375         xfs_btree_sblock_calc_crc(bp);
 376
 377 }
 378
 379 const struct xfs_buf_ops xfs_rmapbt_buf_ops = {
 380         .name                   = "xfs_rmapbt",
 381         .verify_read            = xfs_rmapbt_read_verify,
 382         .verify_write           = xfs_rmapbt_write_verify,
 383         .verify_struct          = xfs_rmapbt_verify,
 384 };
 385
 386 STATIC int
 387 xfs_rmapbt_keys_inorder(
 388         struct xfs_btree_cur    *cur,
 389         union xfs_btree_key     *k1,
 390         union xfs_btree_key     *k2)
 391 {
 392         uint32_t                x;
 393         uint32_t                y;
 394         uint64_t                a;
 395         uint64_t                b;
 396
 397         x = be32_to_cpu(k1->rmap.rm_startblock);
 398         y = be32_to_cpu(k2->rmap.rm_startblock);
 399         if (x < y)
 400                 return 1;
 401         else if (x > y)
 402                 return 0;
 403         a = be64_to_cpu(k1->rmap.rm_owner);
 404         b = be64_to_cpu(k2->rmap.rm_owner);
 405         if (a < b)
 406                 return 1;
 407         else if (a > b)
 408                 return 0;
 409         a = XFS_RMAP_OFF(be64_to_cpu(k1->rmap.rm_offset));
 410         b = XFS_RMAP_OFF(be64_to_cpu(k2->rmap.rm_offset));
 411         if (a <= b)
 412                 return 1;
 413         return 0;
 414 }
 415
 416 STATIC int
 417 xfs_rmapbt_recs_inorder(
 418         struct xfs_btree_cur    *cur,
 419         union xfs_btree_rec     *r1,
 420         union xfs_btree_rec     *r2)
 421 {
 422         uint32_t                x;
 423         uint32_t                y;
 424         uint64_t                a;
 425         uint64_t                b;
 426
 427         x = be32_to_cpu(r1->rmap.rm_startblock);
 428         y = be32_to_cpu(r2->rmap.rm_startblock);
 429         if (x < y)
 430                 return 1;
 431         else if (x > y)
 432                 return 0;
 433         a = be64_to_cpu(r1->rmap.rm_owner);
 434         b = be64_to_cpu(r2->rmap.rm_owner);
 435         if (a < b)
 436                 return 1;
 437         else if (a > b)
 438                 return 0;
 439         a = XFS_RMAP_OFF(be64_to_cpu(r1->rmap.rm_offset));
 440         b = XFS_RMAP_OFF(be64_to_cpu(r2->rmap.rm_offset));
 441         if (a <= b)
 442                 return 1;
 443         return 0;
 444 }
 445
 446 static const struct xfs_btree_ops xfs_rmapbt_ops = {
 447         .rec_len                = sizeof(struct xfs_rmap_rec),
 448         .key_len                = 2 * sizeof(struct xfs_rmap_key),
 449
 450         .dup_cursor             = xfs_rmapbt_dup_cursor,
 451         .set_root               = xfs_rmapbt_set_root,
 452         .alloc_block            = xfs_rmapbt_alloc_block,
 453         .free_block             = xfs_rmapbt_free_block,
 454         .get_minrecs            = xfs_rmapbt_get_minrecs,
 455         .get_maxrecs            = xfs_rmapbt_get_maxrecs,
 456         .init_key_from_rec      = xfs_rmapbt_init_key_from_rec,
 457         .init_high_key_from_rec = xfs_rmapbt_init_high_key_from_rec,
 458         .init_rec_from_cur      = xfs_rmapbt_init_rec_from_cur,
 459         .init_ptr_from_cur      = xfs_rmapbt_init_ptr_from_cur,
 460         .key_diff               = xfs_rmapbt_key_diff,
 461         .buf_ops                = &xfs_rmapbt_buf_ops,
 462         .diff_two_keys          = xfs_rmapbt_diff_two_keys,
 463         .keys_inorder           = xfs_rmapbt_keys_inorder,
 464         .recs_inorder           = xfs_rmapbt_recs_inorder,
 465 };
 466
 467 /*
 468  * Allocate a new allocation btree cursor.
 469  */
 470 struct xfs_btree_cur *
 471 xfs_rmapbt_init_cursor(
 472         struct xfs_mount        *mp,
 473         struct xfs_trans        *tp,
 474         struct xfs_buf          *agbp,
 475         xfs_agnumber_t          agno)
 476 {
 477         struct xfs_agf          *agf = XFS_BUF_TO_AGF(agbp);
 478         struct xfs_btree_cur    *cur;
 479
 480         cur = kmem_zone_zalloc(xfs_btree_cur_zone, KM_NOFS);
 481         cur->bc_tp = tp;
 482         cur->bc_mp = mp;
 483         /* Overlapping btree; 2 keys per pointer. */
 484         cur->bc_btnum = XFS_BTNUM_RMAP;
 485         cur->bc_flags = XFS_BTREE_CRC_BLOCKS | XFS_BTREE_OVERLAPPING;
 486         cur->bc_blocklog = mp->m_sb.sb_blocklog;
 487         cur->bc_ops = &xfs_rmapbt_ops;
 488         cur->bc_nlevels = be32_to_cpu(agf->agf_levels[XFS_BTNUM_RMAP]);
 489         cur->bc_statoff = XFS_STATS_CALC_INDEX(xs_rmap_2);
 490
 491         cur->bc_private.a.agbp = agbp;
 492         cur->bc_private.a.agno = agno;
 493
 494         return cur;
 495 }
 496
 497 /*
 498  * Calculate number of records in an rmap btree block.
 499  */
 500 int
 501 xfs_rmapbt_maxrecs(
 502         struct xfs_mount        *mp,
 503         int                     blocklen,
 504         int                     leaf)
 505 {
 506         blocklen -= XFS_RMAP_BLOCK_LEN;
 507
 508         if (leaf)
 509                 return blocklen / sizeof(struct xfs_rmap_rec);
 510         return blocklen /
 511                 (2 * sizeof(struct xfs_rmap_key) + sizeof(xfs_rmap_ptr_t));
 512 }
 513
 514 /* Compute the maximum height of an rmap btree. */
 515 void
 516 xfs_rmapbt_compute_maxlevels(
 517         struct xfs_mount                *mp)
 518 {
 519         /*
 520          * On a non-reflink filesystem, the maximum number of rmap
 521          * records is the number of blocks in the AG, hence the max
 522          * rmapbt height is log_$maxrecs($agblocks).  However, with
 523          * reflink each AG block can have up to 2^32 (per the refcount
 524          * record format) owners, which means that theoretically we
 525          * could face up to 2^64 rmap records.
 526          *
 527          * That effectively means that the max rmapbt height must be
 528          * XFS_BTREE_MAXLEVELS.  "Fortunately" we'll run out of AG
 529          * blocks to feed the rmapbt long before the rmapbt reaches
 530          * maximum height.  The reflink code uses ag_resv_critical to
 531          * disallow reflinking when less than 10% of the per-AG metadata
 532          * block reservation since the fallback is a regular file copy.
 533          */
 534         if (xfs_sb_version_hasreflink(&mp->m_sb))
 535                 mp->m_rmap_maxlevels = XFS_BTREE_MAXLEVELS;
 536         else
 537                 mp->m_rmap_maxlevels = xfs_btree_compute_maxlevels(mp,
 538                                 mp->m_rmap_mnr, mp->m_sb.sb_agblocks);
 539 }
 540
 541 /* Calculate the refcount btree size for some records. */
 542 xfs_extlen_t
 543 xfs_rmapbt_calc_size(
 544         struct xfs_mount        *mp,
 545         unsigned long long      len)
 546 {
 547         return xfs_btree_calc_size(mp, mp->m_rmap_mnr, len);
 548 }
 549
 550 /*
 551  * Calculate the maximum refcount btree size.
 552  */
 553 xfs_extlen_t
 554 xfs_rmapbt_max_size(
 555         struct xfs_mount        *mp,
 556         xfs_agblock_t           agblocks)
 557 {
 558         /* Bail out if we're uninitialized, which can happen in mkfs. */
 559         if (mp->m_rmap_mxr[0] == 0)
 560                 return 0;
 561
 562         return xfs_rmapbt_calc_size(mp, agblocks);
 563 }
 564
 565 /*
 566  * Figure out how many blocks to reserve and how many are used by this btree.
 567  */
 568 int
 569 xfs_rmapbt_calc_reserves(
 570         struct xfs_mount        *mp,
 571         xfs_agnumber_t          agno,
 572         xfs_extlen_t            *ask,
 573         xfs_extlen_t            *used)
 574 {
 575         struct xfs_buf          *agbp;
 576         struct xfs_agf          *agf;
 577         xfs_agblock_t           agblocks;
 578         xfs_extlen_t            tree_len;
 579         int                     error;
 580
 581         if (!xfs_sb_version_hasrmapbt(&mp->m_sb))
 582                 return 0;
 583
 584         error = xfs_alloc_read_agf(mp, NULL, agno, 0, &agbp);
 585         if (error)
 586                 return error;
 587
 588         agf = XFS_BUF_TO_AGF(agbp);
 589         agblocks = be32_to_cpu(agf->agf_length);
 590         tree_len = be32_to_cpu(agf->agf_rmap_blocks);
 591         xfs_buf_relse(agbp);
 592
 593         /* Reserve 1% of the AG or enough for 1 block per record. */
 594         *ask += max(agblocks / 100, xfs_rmapbt_max_size(mp, agblocks));
 595         *used += tree_len;
 596
 597         return error;
 598 }