root/fs/xfs/libxfs/xfs_rmap_btree.c

DEFINITIONS

1. xfs_rmapbt_dup_cursor
2. xfs_rmapbt_set_root
3. xfs_rmapbt_alloc_block
4. xfs_rmapbt_free_block
5. xfs_rmapbt_get_minrecs
6. xfs_rmapbt_get_maxrecs
7. xfs_rmapbt_init_key_from_rec
8. xfs_rmapbt_init_high_key_from_rec
9. xfs_rmapbt_init_rec_from_cur
10. xfs_rmapbt_init_ptr_from_cur
11. xfs_rmapbt_key_diff
12. xfs_rmapbt_diff_two_keys
13. xfs_rmapbt_verify
14. xfs_rmapbt_read_verify
15. xfs_rmapbt_write_verify
16. xfs_rmapbt_keys_inorder
17. xfs_rmapbt_recs_inorder
18. xfs_rmapbt_init_cursor
19. xfs_rmapbt_maxrecs
20. xfs_rmapbt_compute_maxlevels
21. xfs_rmapbt_calc_size
22. xfs_rmapbt_max_size
23. xfs_rmapbt_calc_reserves

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Copyright (c) 2014 Red Hat, Inc.
   4  * All Rights Reserved.
   5  */
   6 #include "xfs.h"
   7 #include "xfs_fs.h"
   8 #include "xfs_shared.h"
   9 #include "xfs_format.h"
  10 #include "xfs_log_format.h"
  11 #include "xfs_trans_resv.h"
  12 #include "xfs_sb.h"
  13 #include "xfs_mount.h"
  14 #include "xfs_trans.h"
  15 #include "xfs_alloc.h"
  16 #include "xfs_btree.h"
  17 #include "xfs_rmap.h"
  18 #include "xfs_rmap_btree.h"
  19 #include "xfs_trace.h"
  20 #include "xfs_error.h"
  21 #include "xfs_extent_busy.h"
  22 #include "xfs_ag_resv.h"
  23 
  24 /*
  25  * Reverse map btree.
  26  *
  27  * This is a per-ag tree used to track the owner(s) of a given extent. With
  28  * reflink it is possible for there to be multiple owners, which is a departure
  29  * from classic XFS. Owner records for data extents are inserted when the
  30  * extent is mapped and removed when an extent is unmapped.  Owner records for
  31  * all other block types (i.e. metadata) are inserted when an extent is
  32  * allocated and removed when an extent is freed. There can only be one owner
  33  * of a metadata extent, usually an inode or some other metadata structure like
  34  * an AG btree.
  35  *
  36  * The rmap btree is part of the free space management, so blocks for the tree
  37  * are sourced from the agfl. Hence we need transaction reservation support for
  38  * this tree so that the freelist is always large enough. This also impacts on
  39  * the minimum space we need to leave free in the AG.
  40  *
  41  * The tree is ordered by [ag block, owner, offset]. This is a large key size,
  42  * but it is the only way to enforce unique keys when a block can be owned by
  43  * multiple files at any offset. There's no need to order/search by extent
  44  * size for online updating/management of the tree. It is intended that most
  45  * reverse lookups will be to find the owner(s) of a particular block, or to
  46  * try to recover tree and file data from corrupt primary metadata.
  47  */
  48 
  49 static struct xfs_btree_cur *
  50 xfs_rmapbt_dup_cursor(
  51         struct xfs_btree_cur    *cur)
  52 {
  53         return xfs_rmapbt_init_cursor(cur->bc_mp, cur->bc_tp,
  54                         cur->bc_private.a.agbp, cur->bc_private.a.agno);
  55 }
  56 
  57 STATIC void
  58 xfs_rmapbt_set_root(
  59         struct xfs_btree_cur    *cur,
  60         union xfs_btree_ptr     *ptr,
  61         int                     inc)
  62 {
  63         struct xfs_buf          *agbp = cur->bc_private.a.agbp;
  64         struct xfs_agf          *agf = XFS_BUF_TO_AGF(agbp);
  65         xfs_agnumber_t          seqno = be32_to_cpu(agf->agf_seqno);
  66         int                     btnum = cur->bc_btnum;
  67         struct xfs_perag        *pag = xfs_perag_get(cur->bc_mp, seqno);
  68 
  69         ASSERT(ptr->s != 0);
  70 
  71         agf->agf_roots[btnum] = ptr->s;
  72         be32_add_cpu(&agf->agf_levels[btnum], inc);
  73         pag->pagf_levels[btnum] += inc;
  74         xfs_perag_put(pag);
  75 
  76         xfs_alloc_log_agf(cur->bc_tp, agbp, XFS_AGF_ROOTS | XFS_AGF_LEVELS);
  77 }
  78 
  79 STATIC int
  80 xfs_rmapbt_alloc_block(
  81         struct xfs_btree_cur    *cur,
  82         union xfs_btree_ptr     *start,
  83         union xfs_btree_ptr     *new,
  84         int                     *stat)
  85 {
  86         struct xfs_buf          *agbp = cur->bc_private.a.agbp;
  87         struct xfs_agf          *agf = XFS_BUF_TO_AGF(agbp);
  88         int                     error;
  89         xfs_agblock_t           bno;
  90 
  91         /* Allocate the new block from the freelist. If we can't, give up.  */
  92         error = xfs_alloc_get_freelist(cur->bc_tp, cur->bc_private.a.agbp,
  93                                        &bno, 1);
  94         if (error)
  95                 return error;
  96 
  97         trace_xfs_rmapbt_alloc_block(cur->bc_mp, cur->bc_private.a.agno,
  98                         bno, 1);
  99         if (bno == NULLAGBLOCK) {
 100                 *stat = 0;
 101                 return 0;
 102         }
 103 
 104         xfs_extent_busy_reuse(cur->bc_mp, cur->bc_private.a.agno, bno, 1,
 105                         false);
 106 
 107         xfs_trans_agbtree_delta(cur->bc_tp, 1);
 108         new->s = cpu_to_be32(bno);
 109         be32_add_cpu(&agf->agf_rmap_blocks, 1);
 110         xfs_alloc_log_agf(cur->bc_tp, agbp, XFS_AGF_RMAP_BLOCKS);
 111 
 112         xfs_ag_resv_rmapbt_alloc(cur->bc_mp, cur->bc_private.a.agno);
 113 
 114         *stat = 1;
 115         return 0;
 116 }
 117 
 118 STATIC int
 119 xfs_rmapbt_free_block(
 120         struct xfs_btree_cur    *cur,
 121         struct xfs_buf          *bp)
 122 {
 123         struct xfs_buf          *agbp = cur->bc_private.a.agbp;
 124         struct xfs_agf          *agf = XFS_BUF_TO_AGF(agbp);
 125         xfs_agblock_t           bno;
 126         int                     error;
 127 
 128         bno = xfs_daddr_to_agbno(cur->bc_mp, XFS_BUF_ADDR(bp));
 129         trace_xfs_rmapbt_free_block(cur->bc_mp, cur->bc_private.a.agno,
 130                         bno, 1);
 131         be32_add_cpu(&agf->agf_rmap_blocks, -1);
 132         xfs_alloc_log_agf(cur->bc_tp, agbp, XFS_AGF_RMAP_BLOCKS);
 133         error = xfs_alloc_put_freelist(cur->bc_tp, agbp, NULL, bno, 1);
 134         if (error)
 135                 return error;
 136 
 137         xfs_extent_busy_insert(cur->bc_tp, be32_to_cpu(agf->agf_seqno), bno, 1,
 138                               XFS_EXTENT_BUSY_SKIP_DISCARD);
 139         xfs_trans_agbtree_delta(cur->bc_tp, -1);
 140 
 141         xfs_ag_resv_rmapbt_free(cur->bc_mp, cur->bc_private.a.agno);
 142 
 143         return 0;
 144 }
 145 
 146 STATIC int
 147 xfs_rmapbt_get_minrecs(
 148         struct xfs_btree_cur    *cur,
 149         int                     level)
 150 {
 151         return cur->bc_mp->m_rmap_mnr[level != 0];
 152 }
 153 
 154 STATIC int
 155 xfs_rmapbt_get_maxrecs(
 156         struct xfs_btree_cur    *cur,
 157         int                     level)
 158 {
 159         return cur->bc_mp->m_rmap_mxr[level != 0];
 160 }
 161 
 162 STATIC void
 163 xfs_rmapbt_init_key_from_rec(
 164         union xfs_btree_key     *key,
 165         union xfs_btree_rec     *rec)
 166 {
 167         key->rmap.rm_startblock = rec->rmap.rm_startblock;
 168         key->rmap.rm_owner = rec->rmap.rm_owner;
 169         key->rmap.rm_offset = rec->rmap.rm_offset;
 170 }
 171 
 172 /*
 173  * The high key for a reverse mapping record can be computed by shifting
 174  * the startblock and offset to the highest value that would still map
 175  * to that record.  In practice this means that we add blockcount-1 to
 176  * the startblock for all records, and if the record is for a data/attr
 177  * fork mapping, we add blockcount-1 to the offset too.
 178  */
 179 STATIC void
 180 xfs_rmapbt_init_high_key_from_rec(
 181         union xfs_btree_key     *key,
 182         union xfs_btree_rec     *rec)
 183 {
 184         uint64_t                off;
 185         int                     adj;
 186 
 187         adj = be32_to_cpu(rec->rmap.rm_blockcount) - 1;
 188 
 189         key->rmap.rm_startblock = rec->rmap.rm_startblock;
 190         be32_add_cpu(&key->rmap.rm_startblock, adj);
 191         key->rmap.rm_owner = rec->rmap.rm_owner;
 192         key->rmap.rm_offset = rec->rmap.rm_offset;
 193         if (XFS_RMAP_NON_INODE_OWNER(be64_to_cpu(rec->rmap.rm_owner)) ||
 194             XFS_RMAP_IS_BMBT_BLOCK(be64_to_cpu(rec->rmap.rm_offset)))
 195                 return;
 196         off = be64_to_cpu(key->rmap.rm_offset);
 197         off = (XFS_RMAP_OFF(off) + adj) | (off & ~XFS_RMAP_OFF_MASK);
 198         key->rmap.rm_offset = cpu_to_be64(off);
 199 }
 200 
 201 STATIC void
 202 xfs_rmapbt_init_rec_from_cur(
 203         struct xfs_btree_cur    *cur,
 204         union xfs_btree_rec     *rec)
 205 {
 206         rec->rmap.rm_startblock = cpu_to_be32(cur->bc_rec.r.rm_startblock);
 207         rec->rmap.rm_blockcount = cpu_to_be32(cur->bc_rec.r.rm_blockcount);
 208         rec->rmap.rm_owner = cpu_to_be64(cur->bc_rec.r.rm_owner);
 209         rec->rmap.rm_offset = cpu_to_be64(
 210                         xfs_rmap_irec_offset_pack(&cur->bc_rec.r));
 211 }
 212 
 213 STATIC void
 214 xfs_rmapbt_init_ptr_from_cur(
 215         struct xfs_btree_cur    *cur,
 216         union xfs_btree_ptr     *ptr)
 217 {
 218         struct xfs_agf          *agf = XFS_BUF_TO_AGF(cur->bc_private.a.agbp);
 219 
 220         ASSERT(cur->bc_private.a.agno == be32_to_cpu(agf->agf_seqno));
 221 
 222         ptr->s = agf->agf_roots[cur->bc_btnum];
 223 }
 224 
 225 STATIC int64_t
 226 xfs_rmapbt_key_diff(
 227         struct xfs_btree_cur    *cur,
 228         union xfs_btree_key     *key)
 229 {
 230         struct xfs_rmap_irec    *rec = &cur->bc_rec.r;
 231         struct xfs_rmap_key     *kp = &key->rmap;
 232         __u64                   x, y;
 233         int64_t                 d;
 234 
 235         d = (int64_t)be32_to_cpu(kp->rm_startblock) - rec->rm_startblock;
 236         if (d)
 237                 return d;
 238 
 239         x = be64_to_cpu(kp->rm_owner);
 240         y = rec->rm_owner;
 241         if (x > y)
 242                 return 1;
 243         else if (y > x)
 244                 return -1;
 245 
 246         x = XFS_RMAP_OFF(be64_to_cpu(kp->rm_offset));
 247         y = rec->rm_offset;
 248         if (x > y)
 249                 return 1;
 250         else if (y > x)
 251                 return -1;
 252         return 0;
 253 }
 254 
 255 STATIC int64_t
 256 xfs_rmapbt_diff_two_keys(
 257         struct xfs_btree_cur    *cur,
 258         union xfs_btree_key     *k1,
 259         union xfs_btree_key     *k2)
 260 {
 261         struct xfs_rmap_key     *kp1 = &k1->rmap;
 262         struct xfs_rmap_key     *kp2 = &k2->rmap;
 263         int64_t                 d;
 264         __u64                   x, y;
 265 
 266         d = (int64_t)be32_to_cpu(kp1->rm_startblock) -
 267                        be32_to_cpu(kp2->rm_startblock);
 268         if (d)
 269                 return d;
 270 
 271         x = be64_to_cpu(kp1->rm_owner);
 272         y = be64_to_cpu(kp2->rm_owner);
 273         if (x > y)
 274                 return 1;
 275         else if (y > x)
 276                 return -1;
 277 
 278         x = XFS_RMAP_OFF(be64_to_cpu(kp1->rm_offset));
 279         y = XFS_RMAP_OFF(be64_to_cpu(kp2->rm_offset));
 280         if (x > y)
 281                 return 1;
 282         else if (y > x)
 283                 return -1;
 284         return 0;
 285 }
 286 
 287 static xfs_failaddr_t
 288 xfs_rmapbt_verify(
 289         struct xfs_buf          *bp)
 290 {
 291         struct xfs_mount        *mp = bp->b_mount;
 292         struct xfs_btree_block  *block = XFS_BUF_TO_BLOCK(bp);
 293         struct xfs_perag        *pag = bp->b_pag;
 294         xfs_failaddr_t          fa;
 295         unsigned int            level;
 296 
 297         /*
 298          * magic number and level verification
 299          *
 300          * During growfs operations, we can't verify the exact level or owner as
 301          * the perag is not fully initialised and hence not attached to the
 302          * buffer.  In this case, check against the maximum tree depth.
 303          *
 304          * Similarly, during log recovery we will have a perag structure
 305          * attached, but the agf information will not yet have been initialised
 306          * from the on disk AGF. Again, we can only check against maximum limits
 307          * in this case.
 308          */
 309         if (!xfs_verify_magic(bp, block->bb_magic))
 310                 return __this_address;
 311 
 312         if (!xfs_sb_version_hasrmapbt(&mp->m_sb))
 313                 return __this_address;
 314         fa = xfs_btree_sblock_v5hdr_verify(bp);
 315         if (fa)
 316                 return fa;
 317 
 318         level = be16_to_cpu(block->bb_level);
 319         if (pag && pag->pagf_init) {
 320                 if (level >= pag->pagf_levels[XFS_BTNUM_RMAPi])
 321                         return __this_address;
 322         } else if (level >= mp->m_rmap_maxlevels)
 323                 return __this_address;
 324 
 325         return xfs_btree_sblock_verify(bp, mp->m_rmap_mxr[level != 0]);
 326 }
 327 
 328 static void
 329 xfs_rmapbt_read_verify(
 330         struct xfs_buf  *bp)
 331 {
 332         xfs_failaddr_t  fa;
 333 
 334         if (!xfs_btree_sblock_verify_crc(bp))
 335                 xfs_verifier_error(bp, -EFSBADCRC, __this_address);
 336         else {
 337                 fa = xfs_rmapbt_verify(bp);
 338                 if (fa)
 339                         xfs_verifier_error(bp, -EFSCORRUPTED, fa);
 340         }
 341 
 342         if (bp->b_error)
 343                 trace_xfs_btree_corrupt(bp, _RET_IP_);
 344 }
 345 
 346 static void
 347 xfs_rmapbt_write_verify(
 348         struct xfs_buf  *bp)
 349 {
 350         xfs_failaddr_t  fa;
 351 
 352         fa = xfs_rmapbt_verify(bp);
 353         if (fa) {
 354                 trace_xfs_btree_corrupt(bp, _RET_IP_);
 355                 xfs_verifier_error(bp, -EFSCORRUPTED, fa);
 356                 return;
 357         }
 358         xfs_btree_sblock_calc_crc(bp);
 359 
 360 }
 361 
 362 const struct xfs_buf_ops xfs_rmapbt_buf_ops = {
 363         .name                   = "xfs_rmapbt",
 364         .magic                  = { 0, cpu_to_be32(XFS_RMAP_CRC_MAGIC) },
 365         .verify_read            = xfs_rmapbt_read_verify,
 366         .verify_write           = xfs_rmapbt_write_verify,
 367         .verify_struct          = xfs_rmapbt_verify,
 368 };
 369 
 370 STATIC int
 371 xfs_rmapbt_keys_inorder(
 372         struct xfs_btree_cur    *cur,
 373         union xfs_btree_key     *k1,
 374         union xfs_btree_key     *k2)
 375 {
 376         uint32_t                x;
 377         uint32_t                y;
 378         uint64_t                a;
 379         uint64_t                b;
 380 
 381         x = be32_to_cpu(k1->rmap.rm_startblock);
 382         y = be32_to_cpu(k2->rmap.rm_startblock);
 383         if (x < y)
 384                 return 1;
 385         else if (x > y)
 386                 return 0;
 387         a = be64_to_cpu(k1->rmap.rm_owner);
 388         b = be64_to_cpu(k2->rmap.rm_owner);
 389         if (a < b)
 390                 return 1;
 391         else if (a > b)
 392                 return 0;
 393         a = XFS_RMAP_OFF(be64_to_cpu(k1->rmap.rm_offset));
 394         b = XFS_RMAP_OFF(be64_to_cpu(k2->rmap.rm_offset));
 395         if (a <= b)
 396                 return 1;
 397         return 0;
 398 }
 399 
 400 STATIC int
 401 xfs_rmapbt_recs_inorder(
 402         struct xfs_btree_cur    *cur,
 403         union xfs_btree_rec     *r1,
 404         union xfs_btree_rec     *r2)
 405 {
 406         uint32_t                x;
 407         uint32_t                y;
 408         uint64_t                a;
 409         uint64_t                b;
 410 
 411         x = be32_to_cpu(r1->rmap.rm_startblock);
 412         y = be32_to_cpu(r2->rmap.rm_startblock);
 413         if (x < y)
 414                 return 1;
 415         else if (x > y)
 416                 return 0;
 417         a = be64_to_cpu(r1->rmap.rm_owner);
 418         b = be64_to_cpu(r2->rmap.rm_owner);
 419         if (a < b)
 420                 return 1;
 421         else if (a > b)
 422                 return 0;
 423         a = XFS_RMAP_OFF(be64_to_cpu(r1->rmap.rm_offset));
 424         b = XFS_RMAP_OFF(be64_to_cpu(r2->rmap.rm_offset));
 425         if (a <= b)
 426                 return 1;
 427         return 0;
 428 }
 429 
 430 static const struct xfs_btree_ops xfs_rmapbt_ops = {
 431         .rec_len                = sizeof(struct xfs_rmap_rec),
 432         .key_len                = 2 * sizeof(struct xfs_rmap_key),
 433 
 434         .dup_cursor             = xfs_rmapbt_dup_cursor,
 435         .set_root               = xfs_rmapbt_set_root,
 436         .alloc_block            = xfs_rmapbt_alloc_block,
 437         .free_block             = xfs_rmapbt_free_block,
 438         .get_minrecs            = xfs_rmapbt_get_minrecs,
 439         .get_maxrecs            = xfs_rmapbt_get_maxrecs,
 440         .init_key_from_rec      = xfs_rmapbt_init_key_from_rec,
 441         .init_high_key_from_rec = xfs_rmapbt_init_high_key_from_rec,
 442         .init_rec_from_cur      = xfs_rmapbt_init_rec_from_cur,
 443         .init_ptr_from_cur      = xfs_rmapbt_init_ptr_from_cur,
 444         .key_diff               = xfs_rmapbt_key_diff,
 445         .buf_ops                = &xfs_rmapbt_buf_ops,
 446         .diff_two_keys          = xfs_rmapbt_diff_two_keys,
 447         .keys_inorder           = xfs_rmapbt_keys_inorder,
 448         .recs_inorder           = xfs_rmapbt_recs_inorder,
 449 };
 450 
 451 /*
 452  * Allocate a new allocation btree cursor.
 453  */
 454 struct xfs_btree_cur *
 455 xfs_rmapbt_init_cursor(
 456         struct xfs_mount        *mp,
 457         struct xfs_trans        *tp,
 458         struct xfs_buf          *agbp,
 459         xfs_agnumber_t          agno)
 460 {
 461         struct xfs_agf          *agf = XFS_BUF_TO_AGF(agbp);
 462         struct xfs_btree_cur    *cur;
 463 
 464         cur = kmem_zone_zalloc(xfs_btree_cur_zone, KM_NOFS);
 465         cur->bc_tp = tp;
 466         cur->bc_mp = mp;
 467         /* Overlapping btree; 2 keys per pointer. */
 468         cur->bc_btnum = XFS_BTNUM_RMAP;
 469         cur->bc_flags = XFS_BTREE_CRC_BLOCKS | XFS_BTREE_OVERLAPPING;
 470         cur->bc_blocklog = mp->m_sb.sb_blocklog;
 471         cur->bc_ops = &xfs_rmapbt_ops;
 472         cur->bc_nlevels = be32_to_cpu(agf->agf_levels[XFS_BTNUM_RMAP]);
 473         cur->bc_statoff = XFS_STATS_CALC_INDEX(xs_rmap_2);
 474 
 475         cur->bc_private.a.agbp = agbp;
 476         cur->bc_private.a.agno = agno;
 477 
 478         return cur;
 479 }
 480 
 481 /*
 482  * Calculate number of records in an rmap btree block.
 483  */
 484 int
 485 xfs_rmapbt_maxrecs(
 486         int                     blocklen,
 487         int                     leaf)
 488 {
 489         blocklen -= XFS_RMAP_BLOCK_LEN;
 490 
 491         if (leaf)
 492                 return blocklen / sizeof(struct xfs_rmap_rec);
 493         return blocklen /
 494                 (2 * sizeof(struct xfs_rmap_key) + sizeof(xfs_rmap_ptr_t));
 495 }
 496 
 497 /* Compute the maximum height of an rmap btree. */
 498 void
 499 xfs_rmapbt_compute_maxlevels(
 500         struct xfs_mount                *mp)
 501 {
 502         /*
 503          * On a non-reflink filesystem, the maximum number of rmap
 504          * records is the number of blocks in the AG, hence the max
 505          * rmapbt height is log_$maxrecs($agblocks).  However, with
 506          * reflink each AG block can have up to 2^32 (per the refcount
 507          * record format) owners, which means that theoretically we
 508          * could face up to 2^64 rmap records.
 509          *
 510          * That effectively means that the max rmapbt height must be
 511          * XFS_BTREE_MAXLEVELS.  "Fortunately" we'll run out of AG
 512          * blocks to feed the rmapbt long before the rmapbt reaches
 513          * maximum height.  The reflink code uses ag_resv_critical to
 514          * disallow reflinking when less than 10% of the per-AG metadata
 515          * block reservation since the fallback is a regular file copy.
 516          */
 517         if (xfs_sb_version_hasreflink(&mp->m_sb))
 518                 mp->m_rmap_maxlevels = XFS_BTREE_MAXLEVELS;
 519         else
 520                 mp->m_rmap_maxlevels = xfs_btree_compute_maxlevels(
 521                                 mp->m_rmap_mnr, mp->m_sb.sb_agblocks);
 522 }
 523 
 524 /* Calculate the refcount btree size for some records. */
 525 xfs_extlen_t
 526 xfs_rmapbt_calc_size(
 527         struct xfs_mount        *mp,
 528         unsigned long long      len)
 529 {
 530         return xfs_btree_calc_size(mp->m_rmap_mnr, len);
 531 }
 532 
 533 /*
 534  * Calculate the maximum refcount btree size.
 535  */
 536 xfs_extlen_t
 537 xfs_rmapbt_max_size(
 538         struct xfs_mount        *mp,
 539         xfs_agblock_t           agblocks)
 540 {
 541         /* Bail out if we're uninitialized, which can happen in mkfs. */
 542         if (mp->m_rmap_mxr[0] == 0)
 543                 return 0;
 544 
 545         return xfs_rmapbt_calc_size(mp, agblocks);
 546 }
 547 
 548 /*
 549  * Figure out how many blocks to reserve and how many are used by this btree.
 550  */
 551 int
 552 xfs_rmapbt_calc_reserves(
 553         struct xfs_mount        *mp,
 554         struct xfs_trans        *tp,
 555         xfs_agnumber_t          agno,
 556         xfs_extlen_t            *ask,
 557         xfs_extlen_t            *used)
 558 {
 559         struct xfs_buf          *agbp;
 560         struct xfs_agf          *agf;
 561         xfs_agblock_t           agblocks;
 562         xfs_extlen_t            tree_len;
 563         int                     error;
 564 
 565         if (!xfs_sb_version_hasrmapbt(&mp->m_sb))
 566                 return 0;
 567 
 568         error = xfs_alloc_read_agf(mp, tp, agno, 0, &agbp);
 569         if (error)
 570                 return error;
 571 
 572         agf = XFS_BUF_TO_AGF(agbp);
 573         agblocks = be32_to_cpu(agf->agf_length);
 574         tree_len = be32_to_cpu(agf->agf_rmap_blocks);
 575         xfs_trans_brelse(tp, agbp);
 576 
 577         /*
 578          * The log is permanently allocated, so the space it occupies will
 579          * never be available for the kinds of things that would require btree
 580          * expansion.  We therefore can pretend the space isn't there.
 581          */
 582         if (mp->m_sb.sb_logstart &&
 583             XFS_FSB_TO_AGNO(mp, mp->m_sb.sb_logstart) == agno)
 584                 agblocks -= mp->m_sb.sb_logblocks;
 585 
 586         /* Reserve 1% of the AG or enough for 1 block per record. */
 587         *ask += max(agblocks / 100, xfs_rmapbt_max_size(mp, agblocks));
 588         *used += tree_len;
 589 
 590         return error;
 591 }