fs/ext4/balloc.c

   1 /*
   2  *  linux/fs/ext4/balloc.c
   3  *
   4  * Copyright (C) 1992, 1993, 1994, 1995
   5  * Remy Card (card@masi.ibp.fr)
   6  * Laboratoire MASI - Institut Blaise Pascal
   7  * Universite Pierre et Marie Curie (Paris VI)
   8  *
   9  *  Enhanced block allocation by Stephen Tweedie (sct@redhat.com), 1993
  10  *  Big-endian to little-endian byte-swapping/bitmaps by
  11  *        David S. Miller (davem@caip.rutgers.edu), 1995
  12  */
  13
  14 #include <linux/time.h>
  15 #include <linux/capability.h>
  16 #include <linux/fs.h>
  17 #include <linux/jbd2.h>
  18 #include <linux/quotaops.h>
  19 #include <linux/buffer_head.h>
  20 #include "ext4.h"
  21 #include "ext4_jbd2.h"
  22 #include "mballoc.h"
  23
  24 #include <trace/events/ext4.h>
  25
  26 /*
  27  * balloc.c contains the blocks allocation and deallocation routines
  28  */
  29
  30 /*
  31  * Calculate the block group number and offset, given a block number
  32  */
  33 void ext4_get_group_no_and_offset(struct super_block *sb, ext4_fsblk_t blocknr,
  34                 ext4_group_t *blockgrpp, ext4_grpblk_t *offsetp)
  35 {
  36         struct ext4_super_block *es = EXT4_SB(sb)->s_es;
  37         ext4_grpblk_t offset;
  38
  39         blocknr = blocknr - le32_to_cpu(es->s_first_data_block);
  40         offset = do_div(blocknr, EXT4_BLOCKS_PER_GROUP(sb));
  41         if (offsetp)
  42                 *offsetp = offset;
  43         if (blockgrpp)
  44                 *blockgrpp = blocknr;
  45
  46 }
  47
  48 static int ext4_block_in_group(struct super_block *sb, ext4_fsblk_t block,
  49                         ext4_group_t block_group)
  50 {
  51         ext4_group_t actual_group;
  52         ext4_get_group_no_and_offset(sb, block, &actual_group, NULL);
  53         if (actual_group == block_group)
  54                 return 1;
  55         return 0;
  56 }
  57
  58 static int ext4_group_used_meta_blocks(struct super_block *sb,
  59                                        ext4_group_t block_group,
  60                                        struct ext4_group_desc *gdp)
  61 {
  62         ext4_fsblk_t tmp;
  63         struct ext4_sb_info *sbi = EXT4_SB(sb);
  64         /* block bitmap, inode bitmap, and inode table blocks */
  65         int used_blocks = sbi->s_itb_per_group + 2;
  66
  67         if (EXT4_HAS_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_FLEX_BG)) {
  68                 if (!ext4_block_in_group(sb, ext4_block_bitmap(sb, gdp),
  69                                         block_group))
  70                         used_blocks--;
  71
  72                 if (!ext4_block_in_group(sb, ext4_inode_bitmap(sb, gdp),
  73                                         block_group))
  74                         used_blocks--;
  75
  76                 tmp = ext4_inode_table(sb, gdp);
  77                 for (; tmp < ext4_inode_table(sb, gdp) +
  78                                 sbi->s_itb_per_group; tmp++) {
  79                         if (!ext4_block_in_group(sb, tmp, block_group))
  80                                 used_blocks -= 1;
  81                 }
  82         }
  83         return used_blocks;
  84 }
  85
  86 /* Initializes an uninitialized block bitmap if given, and returns the
  87  * number of blocks free in the group. */
  88 unsigned ext4_init_block_bitmap(struct super_block *sb, struct buffer_head *bh,
  89                  ext4_group_t block_group, struct ext4_group_desc *gdp)
  90 {
  91         int bit, bit_max;
  92         ext4_group_t ngroups = ext4_get_groups_count(sb);
  93         unsigned free_blocks, group_blocks;
  94         struct ext4_sb_info *sbi = EXT4_SB(sb);
  95
  96         if (bh) {
  97                 J_ASSERT_BH(bh, buffer_locked(bh));
  98
  99                 /* If checksum is bad mark all blocks used to prevent allocation
 100                  * essentially implementing a per-group read-only flag. */
 101                 if (!ext4_group_desc_csum_verify(sbi, block_group, gdp)) {
 102                         ext4_error(sb, "Checksum bad for group %u",
 103                                         block_group);
 104                         ext4_free_blks_set(sb, gdp, 0);
 105                         ext4_free_inodes_set(sb, gdp, 0);
 106                         ext4_itable_unused_set(sb, gdp, 0);
 107                         memset(bh->b_data, 0xff, sb->s_blocksize);
 108                         return 0;
 109                 }
 110                 memset(bh->b_data, 0, sb->s_blocksize);
 111         }
 112
 113         /* Check for superblock and gdt backups in this group */
 114         bit_max = ext4_bg_has_super(sb, block_group);
 115
 116         if (!EXT4_HAS_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_META_BG) ||
 117             block_group < le32_to_cpu(sbi->s_es->s_first_meta_bg) *
 118                           sbi->s_desc_per_block) {
 119                 if (bit_max) {
 120                         bit_max += ext4_bg_num_gdb(sb, block_group);
 121                         bit_max +=
 122                                 le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks);
 123                 }
 124         } else { /* For META_BG_BLOCK_GROUPS */
 125                 bit_max += ext4_bg_num_gdb(sb, block_group);
 126         }
 127
 128         if (block_group == ngroups - 1) {
 129                 /*
 130                  * Even though mke2fs always initialize first and last group
 131                  * if some other tool enabled the EXT4_BG_BLOCK_UNINIT we need
 132                  * to make sure we calculate the right free blocks
 133                  */
 134                 group_blocks = ext4_blocks_count(sbi->s_es) -
 135                         ext4_group_first_block_no(sb, ngroups - 1);
 136         } else {
 137                 group_blocks = EXT4_BLOCKS_PER_GROUP(sb);
 138         }
 139
 140         free_blocks = group_blocks - bit_max;
 141
 142         if (bh) {
 143                 ext4_fsblk_t start, tmp;
 144                 int flex_bg = 0;
 145
 146                 for (bit = 0; bit < bit_max; bit++)
 147                         ext4_set_bit(bit, bh->b_data);
 148
 149                 start = ext4_group_first_block_no(sb, block_group);
 150
 151                 if (EXT4_HAS_INCOMPAT_FEATURE(sb,
 152                                               EXT4_FEATURE_INCOMPAT_FLEX_BG))
 153                         flex_bg = 1;
 154
 155                 /* Set bits for block and inode bitmaps, and inode table */
 156                 tmp = ext4_block_bitmap(sb, gdp);
 157                 if (!flex_bg || ext4_block_in_group(sb, tmp, block_group))
 158                         ext4_set_bit(tmp - start, bh->b_data);
 159
 160                 tmp = ext4_inode_bitmap(sb, gdp);
 161                 if (!flex_bg || ext4_block_in_group(sb, tmp, block_group))
 162                         ext4_set_bit(tmp - start, bh->b_data);
 163
 164                 tmp = ext4_inode_table(sb, gdp);
 165                 for (; tmp < ext4_inode_table(sb, gdp) +
 166                                 sbi->s_itb_per_group; tmp++) {
 167                         if (!flex_bg ||
 168                                 ext4_block_in_group(sb, tmp, block_group))
 169                                 ext4_set_bit(tmp - start, bh->b_data);
 170                 }
 171                 /*
 172                  * Also if the number of blocks within the group is
 173                  * less than the blocksize * 8 ( which is the size
 174                  * of bitmap ), set rest of the block bitmap to 1
 175                  */
 176                 ext4_mark_bitmap_end(group_blocks, sb->s_blocksize * 8,
 177                                      bh->b_data);
 178         }
 179         return free_blocks - ext4_group_used_meta_blocks(sb, block_group, gdp);
 180 }
 181
 182
 183 /*
 184  * The free blocks are managed by bitmaps.  A file system contains several
 185  * blocks groups.  Each group contains 1 bitmap block for blocks, 1 bitmap
 186  * block for inodes, N blocks for the inode table and data blocks.
 187  *
 188  * The file system contains group descriptors which are located after the
 189  * super block.  Each descriptor contains the number of the bitmap block and
 190  * the free blocks count in the block.  The descriptors are loaded in memory
 191  * when a file system is mounted (see ext4_fill_super).
 192  */
 193
 194 /**
 195  * ext4_get_group_desc() -- load group descriptor from disk
 196  * @sb:                 super block
 197  * @block_group:        given block group
 198  * @bh:                 pointer to the buffer head to store the block
 199  *                      group descriptor
 200  */
 201 struct ext4_group_desc * ext4_get_group_desc(struct super_block *sb,
 202                                              ext4_group_t block_group,
 203                                              struct buffer_head **bh)
 204 {
 205         unsigned int group_desc;
 206         unsigned int offset;
 207         ext4_group_t ngroups = ext4_get_groups_count(sb);
 208         struct ext4_group_desc *desc;
 209         struct ext4_sb_info *sbi = EXT4_SB(sb);
 210
 211         if (block_group >= ngroups) {
 212                 ext4_error(sb, "block_group >= groups_count - block_group = %u,"
 213                            " groups_count = %u", block_group, ngroups);
 214
 215                 return NULL;
 216         }
 217
 218         group_desc = block_group >> EXT4_DESC_PER_BLOCK_BITS(sb);
 219         offset = block_group & (EXT4_DESC_PER_BLOCK(sb) - 1);
 220         if (!sbi->s_group_desc[group_desc]) {
 221                 ext4_error(sb, "Group descriptor not loaded - "
 222                            "block_group = %u, group_desc = %u, desc = %u",
 223                            block_group, group_desc, offset);
 224                 return NULL;
 225         }
 226
 227         desc = (struct ext4_group_desc *)(
 228                 (__u8 *)sbi->s_group_desc[group_desc]->b_data +
 229                 offset * EXT4_DESC_SIZE(sb));
 230         if (bh)
 231                 *bh = sbi->s_group_desc[group_desc];
 232         return desc;
 233 }
 234
 235 static int ext4_valid_block_bitmap(struct super_block *sb,
 236                                         struct ext4_group_desc *desc,
 237                                         unsigned int block_group,
 238                                         struct buffer_head *bh)
 239 {
 240         ext4_grpblk_t offset;
 241         ext4_grpblk_t next_zero_bit;
 242         ext4_fsblk_t bitmap_blk;
 243         ext4_fsblk_t group_first_block;
 244
 245         if (EXT4_HAS_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_FLEX_BG)) {
 246                 /* with FLEX_BG, the inode/block bitmaps and itable
 247                  * blocks may not be in the group at all
 248                  * so the bitmap validation will be skipped for those groups
 249                  * or it has to also read the block group where the bitmaps
 250                  * are located to verify they are set.
 251                  */
 252                 return 1;
 253         }
 254         group_first_block = ext4_group_first_block_no(sb, block_group);
 255
 256         /* check whether block bitmap block number is set */
 257         bitmap_blk = ext4_block_bitmap(sb, desc);
 258         offset = bitmap_blk - group_first_block;
 259         if (!ext4_test_bit(offset, bh->b_data))
 260                 /* bad block bitmap */
 261                 goto err_out;
 262
 263         /* check whether the inode bitmap block number is set */
 264         bitmap_blk = ext4_inode_bitmap(sb, desc);
 265         offset = bitmap_blk - group_first_block;
 266         if (!ext4_test_bit(offset, bh->b_data))
 267                 /* bad block bitmap */
 268                 goto err_out;
 269
 270         /* check whether the inode table block number is set */
 271         bitmap_blk = ext4_inode_table(sb, desc);
 272         offset = bitmap_blk - group_first_block;
 273         next_zero_bit = ext4_find_next_zero_bit(bh->b_data,
 274                                 offset + EXT4_SB(sb)->s_itb_per_group,
 275                                 offset);
 276         if (next_zero_bit >= offset + EXT4_SB(sb)->s_itb_per_group)
 277                 /* good bitmap for inode tables */
 278                 return 1;
 279
 280 err_out:
 281         ext4_error(sb, "Invalid block bitmap - block_group = %d, block = %llu",
 282                         block_group, bitmap_blk);
 283         return 0;
 284 }
 285 /**
 286  * ext4_read_block_bitmap()
 287  * @sb:                 super block
 288  * @block_group:        given block group
 289  *
 290  * Read the bitmap for a given block_group,and validate the
 291  * bits for block/inode/inode tables are set in the bitmaps
 292  *
 293  * Return buffer_head on success or NULL in case of failure.
 294  */
 295 struct buffer_head *
 296 ext4_read_block_bitmap(struct super_block *sb, ext4_group_t block_group)
 297 {
 298         struct ext4_group_desc *desc;
 299         struct buffer_head *bh = NULL;
 300         ext4_fsblk_t bitmap_blk;
 301
 302         desc = ext4_get_group_desc(sb, block_group, NULL);
 303         if (!desc)
 304                 return NULL;
 305         bitmap_blk = ext4_block_bitmap(sb, desc);
 306         bh = sb_getblk(sb, bitmap_blk);
 307         if (unlikely(!bh)) {
 308                 ext4_error(sb, "Cannot read block bitmap - "
 309                             "block_group = %u, block_bitmap = %llu",
 310                             block_group, bitmap_blk);
 311                 return NULL;
 312         }
 313
 314         if (bitmap_uptodate(bh))
 315                 return bh;
 316
 317         lock_buffer(bh);
 318         if (bitmap_uptodate(bh)) {
 319                 unlock_buffer(bh);
 320                 return bh;
 321         }
 322         ext4_lock_group(sb, block_group);
 323         if (desc->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) {
 324                 ext4_init_block_bitmap(sb, bh, block_group, desc);
 325                 set_bitmap_uptodate(bh);
 326                 set_buffer_uptodate(bh);
 327                 ext4_unlock_group(sb, block_group);
 328                 unlock_buffer(bh);
 329                 return bh;
 330         }
 331         ext4_unlock_group(sb, block_group);
 332         if (buffer_uptodate(bh)) {
 333                 /*
 334                  * if not uninit if bh is uptodate,
 335                  * bitmap is also uptodate
 336                  */
 337                 set_bitmap_uptodate(bh);
 338                 unlock_buffer(bh);
 339                 return bh;
 340         }
 341         /*
 342          * submit the buffer_head for read. We can
 343          * safely mark the bitmap as uptodate now.
 344          * We do it here so the bitmap uptodate bit
 345          * get set with buffer lock held.
 346          */
 347         trace_ext4_read_block_bitmap_load(sb, block_group);
 348         set_bitmap_uptodate(bh);
 349         if (bh_submit_read(bh) < 0) {
 350                 put_bh(bh);
 351                 ext4_error(sb, "Cannot read block bitmap - "
 352                             "block_group = %u, block_bitmap = %llu",
 353                             block_group, bitmap_blk);
 354                 return NULL;
 355         }
 356         ext4_valid_block_bitmap(sb, desc, block_group, bh);
 357         /*
 358          * file system mounted not to panic on error,
 359          * continue with corrupt bitmap
 360          */
 361         return bh;
 362 }
 363
 364 /**
 365  * ext4_has_free_blocks()
 366  * @sbi:        in-core super block structure.
 367  * @nblocks:    number of needed blocks
 368  *
 369  * Check if filesystem has nblocks free & available for allocation.
 370  * On success return 1, return 0 on failure.
 371  */
 372 static int ext4_has_free_blocks(struct ext4_sb_info *sbi, s64 nblocks)
 373 {
 374         s64 free_blocks, dirty_blocks, root_blocks;
 375         struct percpu_counter *fbc = &sbi->s_freeblocks_counter;
 376         struct percpu_counter *dbc = &sbi->s_dirtyblocks_counter;
 377
 378         free_blocks  = percpu_counter_read_positive(fbc);
 379         dirty_blocks = percpu_counter_read_positive(dbc);
 380         root_blocks = ext4_r_blocks_count(sbi->s_es);
 381
 382         if (free_blocks - (nblocks + root_blocks + dirty_blocks) <
 383                                                 EXT4_FREEBLOCKS_WATERMARK) {
 384                 free_blocks  = percpu_counter_sum_positive(fbc);
 385                 dirty_blocks = percpu_counter_sum_positive(dbc);
 386         }
 387         /* Check whether we have space after
 388          * accounting for current dirty blocks & root reserved blocks.
 389          */
 390         if (free_blocks >= ((root_blocks + nblocks) + dirty_blocks))
 391                 return 1;
 392
 393         /* Hm, nope.  Are (enough) root reserved blocks available? */
 394         if (sbi->s_resuid == current_fsuid() ||
 395             ((sbi->s_resgid != 0) && in_group_p(sbi->s_resgid)) ||
 396             capable(CAP_SYS_RESOURCE)) {
 397                 if (free_blocks >= (nblocks + dirty_blocks))
 398                         return 1;
 399         }
 400
 401         return 0;
 402 }
 403
 404 int ext4_claim_free_blocks(struct ext4_sb_info *sbi,
 405                                                 s64 nblocks)
 406 {
 407         if (ext4_has_free_blocks(sbi, nblocks)) {
 408                 percpu_counter_add(&sbi->s_dirtyblocks_counter, nblocks);
 409                 return 0;
 410         } else
 411                 return -ENOSPC;
 412 }
 413
 414 /**
 415  * ext4_should_retry_alloc()
 416  * @sb:                 super block
 417  * @retries             number of attemps has been made
 418  *
 419  * ext4_should_retry_alloc() is called when ENOSPC is returned, and if
 420  * it is profitable to retry the operation, this function will wait
 421  * for the current or committing transaction to complete, and then
 422  * return TRUE.
 423  *
 424  * if the total number of retries exceed three times, return FALSE.
 425  */
 426 int ext4_should_retry_alloc(struct super_block *sb, int *retries)
 427 {
 428         if (!ext4_has_free_blocks(EXT4_SB(sb), 1) ||
 429             (*retries)++ > 3 ||
 430             !EXT4_SB(sb)->s_journal)
 431                 return 0;
 432
 433         jbd_debug(1, "%s: retrying operation after ENOSPC\n", sb->s_id);
 434
 435         return jbd2_journal_force_commit_nested(EXT4_SB(sb)->s_journal);
 436 }
 437
 438 /*
 439  * ext4_new_meta_blocks() -- allocate block for meta data (indexing) blocks
 440  *
 441  * @handle:             handle to this transaction
 442  * @inode:              file inode
 443  * @goal:               given target block(filesystem wide)
 444  * @count:              pointer to total number of blocks needed
 445  * @errp:               error code
 446  *
 447  * Return 1st allocated block number on success, *count stores total account
 448  * error stores in errp pointer
 449  */
 450 ext4_fsblk_t ext4_new_meta_blocks(handle_t *handle, struct inode *inode,
 451                 ext4_fsblk_t goal, unsigned long *count, int *errp)
 452 {
 453         struct ext4_allocation_request ar;
 454         ext4_fsblk_t ret;
 455
 456         memset(&ar, 0, sizeof(ar));
 457         /* Fill with neighbour allocated blocks */
 458         ar.inode = inode;
 459         ar.goal = goal;
 460         ar.len = count ? *count : 1;
 461
 462         ret = ext4_mb_new_blocks(handle, &ar, errp);
 463         if (count)
 464                 *count = ar.len;
 465         /*
 466          * Account for the allocated meta blocks.  We will never
 467          * fail EDQUOT for metdata, but we do account for it.
 468          */
 469         if (!(*errp) &&
 470             ext4_test_inode_state(inode, EXT4_STATE_DELALLOC_RESERVED)) {
 471                 spin_lock(&EXT4_I(inode)->i_block_reservation_lock);
 472                 EXT4_I(inode)->i_allocated_meta_blocks += ar.len;
 473                 spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
 474                 dquot_alloc_block_nofail(inode, ar.len);
 475         }
 476         return ret;
 477 }
 478
 479 /**
 480  * ext4_count_free_blocks() -- count filesystem free blocks
 481  * @sb:         superblock
 482  *
 483  * Adds up the number of free blocks from each block group.
 484  */
 485 ext4_fsblk_t ext4_count_free_blocks(struct super_block *sb)
 486 {
 487         ext4_fsblk_t desc_count;
 488         struct ext4_group_desc *gdp;
 489         ext4_group_t i;
 490         ext4_group_t ngroups = ext4_get_groups_count(sb);
 491 #ifdef EXT4FS_DEBUG
 492         struct ext4_super_block *es;
 493         ext4_fsblk_t bitmap_count;
 494         unsigned int x;
 495         struct buffer_head *bitmap_bh = NULL;
 496
 497         es = EXT4_SB(sb)->s_es;
 498         desc_count = 0;
 499         bitmap_count = 0;
 500         gdp = NULL;
 501
 502         for (i = 0; i < ngroups; i++) {
 503                 gdp = ext4_get_group_desc(sb, i, NULL);
 504                 if (!gdp)
 505                         continue;
 506                 desc_count += ext4_free_blks_count(sb, gdp);
 507                 brelse(bitmap_bh);
 508                 bitmap_bh = ext4_read_block_bitmap(sb, i);
 509                 if (bitmap_bh == NULL)
 510                         continue;
 511
 512                 x = ext4_count_free(bitmap_bh, sb->s_blocksize);
 513                 printk(KERN_DEBUG "group %u: stored = %d, counted = %u\n",
 514                         i, ext4_free_blks_count(sb, gdp), x);
 515                 bitmap_count += x;
 516         }
 517         brelse(bitmap_bh);
 518         printk(KERN_DEBUG "ext4_count_free_blocks: stored = %llu"
 519                 ", computed = %llu, %llu\n", ext4_free_blocks_count(es),
 520                desc_count, bitmap_count);
 521         return bitmap_count;
 522 #else
 523         desc_count = 0;
 524         for (i = 0; i < ngroups; i++) {
 525                 gdp = ext4_get_group_desc(sb, i, NULL);
 526                 if (!gdp)
 527                         continue;
 528                 desc_count += ext4_free_blks_count(sb, gdp);
 529         }
 530
 531         return desc_count;
 532 #endif
 533 }
 534
 535 static inline int test_root(ext4_group_t a, int b)
 536 {
 537         int num = b;
 538
 539         while (a > num)
 540                 num *= b;
 541         return num == a;
 542 }
 543
 544 static int ext4_group_sparse(ext4_group_t group)
 545 {
 546         if (group <= 1)
 547                 return 1;
 548         if (!(group & 1))
 549                 return 0;
 550         return (test_root(group, 7) || test_root(group, 5) ||
 551                 test_root(group, 3));
 552 }
 553
 554 /**
 555  *      ext4_bg_has_super - number of blocks used by the superblock in group
 556  *      @sb: superblock for filesystem
 557  *      @group: group number to check
 558  *
 559  *      Return the number of blocks used by the superblock (primary or backup)
 560  *      in this group.  Currently this will be only 0 or 1.
 561  */
 562 int ext4_bg_has_super(struct super_block *sb, ext4_group_t group)
 563 {
 564         if (EXT4_HAS_RO_COMPAT_FEATURE(sb,
 565                                 EXT4_FEATURE_RO_COMPAT_SPARSE_SUPER) &&
 566                         !ext4_group_sparse(group))
 567                 return 0;
 568         return 1;
 569 }
 570
 571 static unsigned long ext4_bg_num_gdb_meta(struct super_block *sb,
 572                                         ext4_group_t group)
 573 {
 574         unsigned long metagroup = group / EXT4_DESC_PER_BLOCK(sb);
 575         ext4_group_t first = metagroup * EXT4_DESC_PER_BLOCK(sb);
 576         ext4_group_t last = first + EXT4_DESC_PER_BLOCK(sb) - 1;
 577
 578         if (group == first || group == first + 1 || group == last)
 579                 return 1;
 580         return 0;
 581 }
 582
 583 static unsigned long ext4_bg_num_gdb_nometa(struct super_block *sb,
 584                                         ext4_group_t group)
 585 {
 586         if (!ext4_bg_has_super(sb, group))
 587                 return 0;
 588
 589         if (EXT4_HAS_INCOMPAT_FEATURE(sb,EXT4_FEATURE_INCOMPAT_META_BG))
 590                 return le32_to_cpu(EXT4_SB(sb)->s_es->s_first_meta_bg);
 591         else
 592                 return EXT4_SB(sb)->s_gdb_count;
 593 }
 594
 595 /**
 596  *      ext4_bg_num_gdb - number of blocks used by the group table in group
 597  *      @sb: superblock for filesystem
 598  *      @group: group number to check
 599  *
 600  *      Return the number of blocks used by the group descriptor table
 601  *      (primary or backup) in this group.  In the future there may be a
 602  *      different number of descriptor blocks in each group.
 603  */
 604 unsigned long ext4_bg_num_gdb(struct super_block *sb, ext4_group_t group)
 605 {
 606         unsigned long first_meta_bg =
 607                         le32_to_cpu(EXT4_SB(sb)->s_es->s_first_meta_bg);
 608         unsigned long metagroup = group / EXT4_DESC_PER_BLOCK(sb);
 609
 610         if (!EXT4_HAS_INCOMPAT_FEATURE(sb,EXT4_FEATURE_INCOMPAT_META_BG) ||
 611                         metagroup < first_meta_bg)
 612                 return ext4_bg_num_gdb_nometa(sb, group);
 613
 614         return ext4_bg_num_gdb_meta(sb,group);
 615
 616 }
 617