fs/fat/fatent.c

   1 /*
   2  * Copyright (C) 2004, OGAWA Hirofumi
   3  * Released under GPL v2.
   4  */
   5
   6 #include <linux/module.h>
   7 #include <linux/fs.h>
   8 #include <linux/msdos_fs.h>
   9
  10 struct fatent_operations {
  11         void (*ent_blocknr)(struct super_block *, int, int *, sector_t *);
  12         void (*ent_set_ptr)(struct fat_entry *, int);
  13         int (*ent_bread)(struct super_block *, struct fat_entry *,
  14                          int, sector_t);
  15         int (*ent_get)(struct fat_entry *);
  16         void (*ent_put)(struct fat_entry *, int);
  17         int (*ent_next)(struct fat_entry *);
  18 };
  19
  20 static DEFINE_SPINLOCK(fat12_entry_lock);
  21
  22 static void fat12_ent_blocknr(struct super_block *sb, int entry,
  23                               int *offset, sector_t *blocknr)
  24 {
  25         struct msdos_sb_info *sbi = MSDOS_SB(sb);
  26         int bytes = entry + (entry >> 1);
  27         WARN_ON(entry < FAT_START_ENT || sbi->max_cluster <= entry);
  28         *offset = bytes & (sb->s_blocksize - 1);
  29         *blocknr = sbi->fat_start + (bytes >> sb->s_blocksize_bits);
  30 }
  31
  32 static void fat_ent_blocknr(struct super_block *sb, int entry,
  33                             int *offset, sector_t *blocknr)
  34 {
  35         struct msdos_sb_info *sbi = MSDOS_SB(sb);
  36         int bytes = (entry << sbi->fatent_shift);
  37         WARN_ON(entry < FAT_START_ENT || sbi->max_cluster <= entry);
  38         *offset = bytes & (sb->s_blocksize - 1);
  39         *blocknr = sbi->fat_start + (bytes >> sb->s_blocksize_bits);
  40 }
  41
  42 static void fat12_ent_set_ptr(struct fat_entry *fatent, int offset)
  43 {
  44         struct buffer_head **bhs = fatent->bhs;
  45         if (fatent->nr_bhs == 1) {
  46                 WARN_ON(offset >= (bhs[0]->b_size - 1));
  47                 fatent->u.ent12_p[0] = bhs[0]->b_data + offset;
  48                 fatent->u.ent12_p[1] = bhs[0]->b_data + (offset + 1);
  49         } else {
  50                 WARN_ON(offset != (bhs[0]->b_size - 1));
  51                 fatent->u.ent12_p[0] = bhs[0]->b_data + offset;
  52                 fatent->u.ent12_p[1] = bhs[1]->b_data;
  53         }
  54 }
  55
  56 static void fat16_ent_set_ptr(struct fat_entry *fatent, int offset)
  57 {
  58         WARN_ON(offset & (2 - 1));
  59         fatent->u.ent16_p = (__le16 *)(fatent->bhs[0]->b_data + offset);
  60 }
  61
  62 static void fat32_ent_set_ptr(struct fat_entry *fatent, int offset)
  63 {
  64         WARN_ON(offset & (4 - 1));
  65         fatent->u.ent32_p = (__le32 *)(fatent->bhs[0]->b_data + offset);
  66 }
  67
  68 static int fat12_ent_bread(struct super_block *sb, struct fat_entry *fatent,
  69                            int offset, sector_t blocknr)
  70 {
  71         struct buffer_head **bhs = fatent->bhs;
  72
  73         WARN_ON(blocknr < MSDOS_SB(sb)->fat_start);
  74         bhs[0] = sb_bread(sb, blocknr);
  75         if (!bhs[0])
  76                 goto err;
  77
  78         if ((offset + 1) < sb->s_blocksize)
  79                 fatent->nr_bhs = 1;
  80         else {
  81                 /* This entry is block boundary, it needs the next block */
  82                 blocknr++;
  83                 bhs[1] = sb_bread(sb, blocknr);
  84                 if (!bhs[1])
  85                         goto err_brelse;
  86                 fatent->nr_bhs = 2;
  87         }
  88         fat12_ent_set_ptr(fatent, offset);
  89         return 0;
  90
  91 err_brelse:
  92         brelse(bhs[0]);
  93 err:
  94         printk(KERN_ERR "FAT: FAT read failed (blocknr %llu)\n",
  95                (unsigned long long)blocknr);
  96         return -EIO;
  97 }
  98
  99 static int fat_ent_bread(struct super_block *sb, struct fat_entry *fatent,
 100                          int offset, sector_t blocknr)
 101 {
 102         struct fatent_operations *ops = MSDOS_SB(sb)->fatent_ops;
 103
 104         WARN_ON(blocknr < MSDOS_SB(sb)->fat_start);
 105         fatent->bhs[0] = sb_bread(sb, blocknr);
 106         if (!fatent->bhs[0]) {
 107                 printk(KERN_ERR "FAT: FAT read failed (blocknr %llu)\n",
 108                        (unsigned long long)blocknr);
 109                 return -EIO;
 110         }
 111         fatent->nr_bhs = 1;
 112         ops->ent_set_ptr(fatent, offset);
 113         return 0;
 114 }
 115
 116 static int fat12_ent_get(struct fat_entry *fatent)
 117 {
 118         u8 **ent12_p = fatent->u.ent12_p;
 119         int next;
 120
 121         spin_lock(&fat12_entry_lock);
 122         if (fatent->entry & 1)
 123                 next = (*ent12_p[0] >> 4) | (*ent12_p[1] << 4);
 124         else
 125                 next = (*ent12_p[1] << 8) | *ent12_p[0];
 126         spin_unlock(&fat12_entry_lock);
 127
 128         next &= 0x0fff;
 129         if (next >= BAD_FAT12)
 130                 next = FAT_ENT_EOF;
 131         return next;
 132 }
 133
 134 static int fat16_ent_get(struct fat_entry *fatent)
 135 {
 136         int next = le16_to_cpu(*fatent->u.ent16_p);
 137         WARN_ON((unsigned long)fatent->u.ent16_p & (2 - 1));
 138         if (next >= BAD_FAT16)
 139                 next = FAT_ENT_EOF;
 140         return next;
 141 }
 142
 143 static int fat32_ent_get(struct fat_entry *fatent)
 144 {
 145         int next = le32_to_cpu(*fatent->u.ent32_p) & 0x0fffffff;
 146         WARN_ON((unsigned long)fatent->u.ent32_p & (4 - 1));
 147         if (next >= BAD_FAT32)
 148                 next = FAT_ENT_EOF;
 149         return next;
 150 }
 151
 152 static void fat12_ent_put(struct fat_entry *fatent, int new)
 153 {
 154         u8 **ent12_p = fatent->u.ent12_p;
 155
 156         if (new == FAT_ENT_EOF)
 157                 new = EOF_FAT12;
 158
 159         spin_lock(&fat12_entry_lock);
 160         if (fatent->entry & 1) {
 161                 *ent12_p[0] = (new << 4) | (*ent12_p[0] & 0x0f);
 162                 *ent12_p[1] = new >> 4;
 163         } else {
 164                 *ent12_p[0] = new & 0xff;
 165                 *ent12_p[1] = (*ent12_p[1] & 0xf0) | (new >> 8);
 166         }
 167         spin_unlock(&fat12_entry_lock);
 168
 169         mark_buffer_dirty(fatent->bhs[0]);
 170         if (fatent->nr_bhs == 2)
 171                 mark_buffer_dirty(fatent->bhs[1]);
 172 }
 173
 174 static void fat16_ent_put(struct fat_entry *fatent, int new)
 175 {
 176         if (new == FAT_ENT_EOF)
 177                 new = EOF_FAT16;
 178
 179         *fatent->u.ent16_p = cpu_to_le16(new);
 180         mark_buffer_dirty(fatent->bhs[0]);
 181 }
 182
 183 static void fat32_ent_put(struct fat_entry *fatent, int new)
 184 {
 185         if (new == FAT_ENT_EOF)
 186                 new = EOF_FAT32;
 187
 188         WARN_ON(new & 0xf0000000);
 189         new |= le32_to_cpu(*fatent->u.ent32_p) & ~0x0fffffff;
 190         *fatent->u.ent32_p = cpu_to_le32(new);
 191         mark_buffer_dirty(fatent->bhs[0]);
 192 }
 193
 194 static int fat12_ent_next(struct fat_entry *fatent)
 195 {
 196         u8 **ent12_p = fatent->u.ent12_p;
 197         struct buffer_head **bhs = fatent->bhs;
 198         u8 *nextp = ent12_p[1] + 1 + (fatent->entry & 1);
 199
 200         fatent->entry++;
 201         if (fatent->nr_bhs == 1) {
 202                 WARN_ON(ent12_p[0] > (u8 *)(bhs[0]->b_data + (bhs[0]->b_size - 2)));
 203                 WARN_ON(ent12_p[1] > (u8 *)(bhs[0]->b_data + (bhs[0]->b_size - 1)));
 204                 if (nextp < (u8 *)(bhs[0]->b_data + (bhs[0]->b_size - 1))) {
 205                         ent12_p[0] = nextp - 1;
 206                         ent12_p[1] = nextp;
 207                         return 1;
 208                 }
 209         } else {
 210                 WARN_ON(ent12_p[0] != (u8 *)(bhs[0]->b_data + (bhs[0]->b_size - 1)));
 211                 WARN_ON(ent12_p[1] != (u8 *)bhs[1]->b_data);
 212                 ent12_p[0] = nextp - 1;
 213                 ent12_p[1] = nextp;
 214                 brelse(bhs[0]);
 215                 bhs[0] = bhs[1];
 216                 fatent->nr_bhs = 1;
 217                 return 1;
 218         }
 219         ent12_p[0] = NULL;
 220         ent12_p[1] = NULL;
 221         return 0;
 222 }
 223
 224 static int fat16_ent_next(struct fat_entry *fatent)
 225 {
 226         const struct buffer_head *bh = fatent->bhs[0];
 227         fatent->entry++;
 228         if (fatent->u.ent16_p < (__le16 *)(bh->b_data + (bh->b_size - 2))) {
 229                 fatent->u.ent16_p++;
 230                 return 1;
 231         }
 232         fatent->u.ent16_p = NULL;
 233         return 0;
 234 }
 235
 236 static int fat32_ent_next(struct fat_entry *fatent)
 237 {
 238         const struct buffer_head *bh = fatent->bhs[0];
 239         fatent->entry++;
 240         if (fatent->u.ent32_p < (__le32 *)(bh->b_data + (bh->b_size - 4))) {
 241                 fatent->u.ent32_p++;
 242                 return 1;
 243         }
 244         fatent->u.ent32_p = NULL;
 245         return 0;
 246 }
 247
 248 static struct fatent_operations fat12_ops = {
 249         .ent_blocknr    = fat12_ent_blocknr,
 250         .ent_set_ptr    = fat12_ent_set_ptr,
 251         .ent_bread      = fat12_ent_bread,
 252         .ent_get        = fat12_ent_get,
 253         .ent_put        = fat12_ent_put,
 254         .ent_next       = fat12_ent_next,
 255 };
 256
 257 static struct fatent_operations fat16_ops = {
 258         .ent_blocknr    = fat_ent_blocknr,
 259         .ent_set_ptr    = fat16_ent_set_ptr,
 260         .ent_bread      = fat_ent_bread,
 261         .ent_get        = fat16_ent_get,
 262         .ent_put        = fat16_ent_put,
 263         .ent_next       = fat16_ent_next,
 264 };
 265
 266 static struct fatent_operations fat32_ops = {
 267         .ent_blocknr    = fat_ent_blocknr,
 268         .ent_set_ptr    = fat32_ent_set_ptr,
 269         .ent_bread      = fat_ent_bread,
 270         .ent_get        = fat32_ent_get,
 271         .ent_put        = fat32_ent_put,
 272         .ent_next       = fat32_ent_next,
 273 };
 274
 275 static inline void lock_fat(struct msdos_sb_info *sbi)
 276 {
 277         mutex_lock(&sbi->fat_lock);
 278 }
 279
 280 static inline void unlock_fat(struct msdos_sb_info *sbi)
 281 {
 282         mutex_unlock(&sbi->fat_lock);
 283 }
 284
 285 void fat_ent_access_init(struct super_block *sb)
 286 {
 287         struct msdos_sb_info *sbi = MSDOS_SB(sb);
 288
 289         mutex_init(&sbi->fat_lock);
 290
 291         switch (sbi->fat_bits) {
 292         case 32:
 293                 sbi->fatent_shift = 2;
 294                 sbi->fatent_ops = &fat32_ops;
 295                 break;
 296         case 16:
 297                 sbi->fatent_shift = 1;
 298                 sbi->fatent_ops = &fat16_ops;
 299                 break;
 300         case 12:
 301                 sbi->fatent_shift = -1;
 302                 sbi->fatent_ops = &fat12_ops;
 303                 break;
 304         }
 305 }
 306
 307 static inline int fat_ent_update_ptr(struct super_block *sb,
 308                                      struct fat_entry *fatent,
 309                                      int offset, sector_t blocknr)
 310 {
 311         struct msdos_sb_info *sbi = MSDOS_SB(sb);
 312         struct fatent_operations *ops = sbi->fatent_ops;
 313         struct buffer_head **bhs = fatent->bhs;
 314
 315         /* Is this fatent's blocks including this entry? */
 316         if (!fatent->nr_bhs || bhs[0]->b_blocknr != blocknr)
 317                 return 0;
 318         /* Does this entry need the next block? */
 319         if (sbi->fat_bits == 12 && (offset + 1) >= sb->s_blocksize) {
 320                 if (fatent->nr_bhs != 2 || bhs[1]->b_blocknr != (blocknr + 1))
 321                         return 0;
 322         }
 323         ops->ent_set_ptr(fatent, offset);
 324         return 1;
 325 }
 326
 327 int fat_ent_read(struct inode *inode, struct fat_entry *fatent, int entry)
 328 {
 329         struct super_block *sb = inode->i_sb;
 330         struct msdos_sb_info *sbi = MSDOS_SB(inode->i_sb);
 331         struct fatent_operations *ops = sbi->fatent_ops;
 332         int err, offset;
 333         sector_t blocknr;
 334
 335         if (entry < FAT_START_ENT || sbi->max_cluster <= entry) {
 336                 fatent_brelse(fatent);
 337                 fat_fs_panic(sb, "invalid access to FAT (entry 0x%08x)", entry);
 338                 return -EIO;
 339         }
 340
 341         fatent_set_entry(fatent, entry);
 342         ops->ent_blocknr(sb, entry, &offset, &blocknr);
 343
 344         if (!fat_ent_update_ptr(sb, fatent, offset, blocknr)) {
 345                 fatent_brelse(fatent);
 346                 err = ops->ent_bread(sb, fatent, offset, blocknr);
 347                 if (err)
 348                         return err;
 349         }
 350         return ops->ent_get(fatent);
 351 }
 352
 353 /* FIXME: We can write the blocks as more big chunk. */
 354 static int fat_mirror_bhs(struct super_block *sb, struct buffer_head **bhs,
 355                           int nr_bhs)
 356 {
 357         struct msdos_sb_info *sbi = MSDOS_SB(sb);
 358         struct buffer_head *c_bh;
 359         int err, n, copy;
 360
 361         err = 0;
 362         for (copy = 1; copy < sbi->fats; copy++) {
 363                 sector_t backup_fat = sbi->fat_length * copy;
 364
 365                 for (n = 0; n < nr_bhs; n++) {
 366                         c_bh = sb_getblk(sb, backup_fat + bhs[n]->b_blocknr);
 367                         if (!c_bh) {
 368                                 err = -ENOMEM;
 369                                 goto error;
 370                         }
 371                         memcpy(c_bh->b_data, bhs[n]->b_data, sb->s_blocksize);
 372                         set_buffer_uptodate(c_bh);
 373                         mark_buffer_dirty(c_bh);
 374                         if (sb->s_flags & MS_SYNCHRONOUS)
 375                                 err = sync_dirty_buffer(c_bh);
 376                         brelse(c_bh);
 377                         if (err)
 378                                 goto error;
 379                 }
 380         }
 381 error:
 382         return err;
 383 }
 384
 385 int fat_ent_write(struct inode *inode, struct fat_entry *fatent,
 386                   int new, int wait)
 387 {
 388         struct super_block *sb = inode->i_sb;
 389         struct fatent_operations *ops = MSDOS_SB(sb)->fatent_ops;
 390         int err;
 391
 392         ops->ent_put(fatent, new);
 393         if (wait) {
 394                 err = fat_sync_bhs(fatent->bhs, fatent->nr_bhs);
 395                 if (err)
 396                         return err;
 397         }
 398         return fat_mirror_bhs(sb, fatent->bhs, fatent->nr_bhs);
 399 }
 400
 401 static inline int fat_ent_next(struct msdos_sb_info *sbi,
 402                                struct fat_entry *fatent)
 403 {
 404         if (sbi->fatent_ops->ent_next(fatent)) {
 405                 if (fatent->entry < sbi->max_cluster)
 406                         return 1;
 407         }
 408         return 0;
 409 }
 410
 411 static inline int fat_ent_read_block(struct super_block *sb,
 412                                      struct fat_entry *fatent)
 413 {
 414         struct fatent_operations *ops = MSDOS_SB(sb)->fatent_ops;
 415         sector_t blocknr;
 416         int offset;
 417
 418         fatent_brelse(fatent);
 419         ops->ent_blocknr(sb, fatent->entry, &offset, &blocknr);
 420         return ops->ent_bread(sb, fatent, offset, blocknr);
 421 }
 422
 423 static void fat_collect_bhs(struct buffer_head **bhs, int *nr_bhs,
 424                             struct fat_entry *fatent)
 425 {
 426         int n, i;
 427
 428         for (n = 0; n < fatent->nr_bhs; n++) {
 429                 for (i = 0; i < *nr_bhs; i++) {
 430                         if (fatent->bhs[n] == bhs[i])
 431                                 break;
 432                 }
 433                 if (i == *nr_bhs) {
 434                         get_bh(fatent->bhs[n]);
 435                         bhs[i] = fatent->bhs[n];
 436                         (*nr_bhs)++;
 437                 }
 438         }
 439 }
 440
 441 int fat_alloc_clusters(struct inode *inode, int *cluster, int nr_cluster)
 442 {
 443         struct super_block *sb = inode->i_sb;
 444         struct msdos_sb_info *sbi = MSDOS_SB(sb);
 445         struct fatent_operations *ops = sbi->fatent_ops;
 446         struct fat_entry fatent, prev_ent;
 447         struct buffer_head *bhs[MAX_BUF_PER_PAGE];
 448         int i, count, err, nr_bhs, idx_clus;
 449
 450         BUG_ON(nr_cluster > (MAX_BUF_PER_PAGE / 2));    /* fixed limit */
 451
 452         lock_fat(sbi);
 453         if (sbi->free_clusters != -1 && sbi->free_clus_valid &&
 454             sbi->free_clusters < nr_cluster) {
 455                 unlock_fat(sbi);
 456                 return -ENOSPC;
 457         }
 458
 459         err = nr_bhs = idx_clus = 0;
 460         count = FAT_START_ENT;
 461         fatent_init(&prev_ent);
 462         fatent_init(&fatent);
 463         fatent_set_entry(&fatent, sbi->prev_free + 1);
 464         while (count < sbi->max_cluster) {
 465                 if (fatent.entry >= sbi->max_cluster)
 466                         fatent.entry = FAT_START_ENT;
 467                 fatent_set_entry(&fatent, fatent.entry);
 468                 err = fat_ent_read_block(sb, &fatent);
 469                 if (err)
 470                         goto out;
 471
 472                 /* Find the free entries in a block */
 473                 do {
 474                         if (ops->ent_get(&fatent) == FAT_ENT_FREE) {
 475                                 int entry = fatent.entry;
 476
 477                                 /* make the cluster chain */
 478                                 ops->ent_put(&fatent, FAT_ENT_EOF);
 479                                 if (prev_ent.nr_bhs)
 480                                         ops->ent_put(&prev_ent, entry);
 481
 482                                 fat_collect_bhs(bhs, &nr_bhs, &fatent);
 483
 484                                 sbi->prev_free = entry;
 485                                 if (sbi->free_clusters != -1)
 486                                         sbi->free_clusters--;
 487                                 sb->s_dirt = 1;
 488
 489                                 cluster[idx_clus] = entry;
 490                                 idx_clus++;
 491                                 if (idx_clus == nr_cluster)
 492                                         goto out;
 493
 494                                 /*
 495                                  * fat_collect_bhs() gets ref-count of bhs,
 496                                  * so we can still use the prev_ent.
 497                                  */
 498                                 prev_ent = fatent;
 499                         }
 500                         count++;
 501                         if (count == sbi->max_cluster)
 502                                 break;
 503                 } while (fat_ent_next(sbi, &fatent));
 504         }
 505
 506         /* Couldn't allocate the free entries */
 507         sbi->free_clusters = 0;
 508         sbi->free_clus_valid = 1;
 509         sb->s_dirt = 1;
 510         err = -ENOSPC;
 511
 512 out:
 513         unlock_fat(sbi);
 514         fatent_brelse(&fatent);
 515         if (!err) {
 516                 if (inode_needs_sync(inode))
 517                         err = fat_sync_bhs(bhs, nr_bhs);
 518                 if (!err)
 519                         err = fat_mirror_bhs(sb, bhs, nr_bhs);
 520         }
 521         for (i = 0; i < nr_bhs; i++)
 522                 brelse(bhs[i]);
 523
 524         if (err && idx_clus)
 525                 fat_free_clusters(inode, cluster[0]);
 526
 527         return err;
 528 }
 529
 530 int fat_free_clusters(struct inode *inode, int cluster)
 531 {
 532         struct super_block *sb = inode->i_sb;
 533         struct msdos_sb_info *sbi = MSDOS_SB(sb);
 534         struct fatent_operations *ops = sbi->fatent_ops;
 535         struct fat_entry fatent;
 536         struct buffer_head *bhs[MAX_BUF_PER_PAGE];
 537         int i, err, nr_bhs;
 538
 539         nr_bhs = 0;
 540         fatent_init(&fatent);
 541         lock_fat(sbi);
 542         do {
 543                 cluster = fat_ent_read(inode, &fatent, cluster);
 544                 if (cluster < 0) {
 545                         err = cluster;
 546                         goto error;
 547                 } else if (cluster == FAT_ENT_FREE) {
 548                         fat_fs_panic(sb, "%s: deleting FAT entry beyond EOF",
 549                                      __FUNCTION__);
 550                         err = -EIO;
 551                         goto error;
 552                 }
 553
 554                 ops->ent_put(&fatent, FAT_ENT_FREE);
 555                 if (sbi->free_clusters != -1) {
 556                         sbi->free_clusters++;
 557                         sb->s_dirt = 1;
 558                 }
 559
 560                 if (nr_bhs + fatent.nr_bhs > MAX_BUF_PER_PAGE) {
 561                         if (sb->s_flags & MS_SYNCHRONOUS) {
 562                                 err = fat_sync_bhs(bhs, nr_bhs);
 563                                 if (err)
 564                                         goto error;
 565                         }
 566                         err = fat_mirror_bhs(sb, bhs, nr_bhs);
 567                         if (err)
 568                                 goto error;
 569                         for (i = 0; i < nr_bhs; i++)
 570                                 brelse(bhs[i]);
 571                         nr_bhs = 0;
 572                 }
 573                 fat_collect_bhs(bhs, &nr_bhs, &fatent);
 574         } while (cluster != FAT_ENT_EOF);
 575
 576         if (sb->s_flags & MS_SYNCHRONOUS) {
 577                 err = fat_sync_bhs(bhs, nr_bhs);
 578                 if (err)
 579                         goto error;
 580         }
 581         err = fat_mirror_bhs(sb, bhs, nr_bhs);
 582 error:
 583         fatent_brelse(&fatent);
 584         for (i = 0; i < nr_bhs; i++)
 585                 brelse(bhs[i]);
 586         unlock_fat(sbi);
 587
 588         fat_clusters_flush(sb);
 589
 590         return err;
 591 }
 592
 593 EXPORT_SYMBOL_GPL(fat_free_clusters);
 594
 595 /* 128kb is the whole sectors for FAT12 and FAT16 */
 596 #define FAT_READA_SIZE          (128 * 1024)
 597
 598 static void fat_ent_reada(struct super_block *sb, struct fat_entry *fatent,
 599                           unsigned long reada_blocks)
 600 {
 601         struct fatent_operations *ops = MSDOS_SB(sb)->fatent_ops;
 602         sector_t blocknr;
 603         int i, offset;
 604
 605         ops->ent_blocknr(sb, fatent->entry, &offset, &blocknr);
 606
 607         for (i = 0; i < reada_blocks; i++)
 608                 sb_breadahead(sb, blocknr + i);
 609 }
 610
 611 int fat_count_free_clusters(struct super_block *sb)
 612 {
 613         struct msdos_sb_info *sbi = MSDOS_SB(sb);
 614         struct fatent_operations *ops = sbi->fatent_ops;
 615         struct fat_entry fatent;
 616         unsigned long reada_blocks, reada_mask, cur_block;
 617         int err = 0, free;
 618
 619         lock_fat(sbi);
 620         if (sbi->free_clusters != -1 && sbi->free_clus_valid)
 621                 goto out;
 622
 623         reada_blocks = FAT_READA_SIZE >> sb->s_blocksize_bits;
 624         reada_mask = reada_blocks - 1;
 625         cur_block = 0;
 626
 627         free = 0;
 628         fatent_init(&fatent);
 629         fatent_set_entry(&fatent, FAT_START_ENT);
 630         while (fatent.entry < sbi->max_cluster) {
 631                 /* readahead of fat blocks */
 632                 if ((cur_block & reada_mask) == 0) {
 633                         unsigned long rest = sbi->fat_length - cur_block;
 634                         fat_ent_reada(sb, &fatent, min(reada_blocks, rest));
 635                 }
 636                 cur_block++;
 637
 638                 err = fat_ent_read_block(sb, &fatent);
 639                 if (err)
 640                         goto out;
 641
 642                 do {
 643                         if (ops->ent_get(&fatent) == FAT_ENT_FREE)
 644                                 free++;
 645                 } while (fat_ent_next(sbi, &fatent));
 646         }
 647         sbi->free_clusters = free;
 648         sbi->free_clus_valid = 1;
 649         sb->s_dirt = 1;
 650         fatent_brelse(&fatent);
 651 out:
 652         unlock_fat(sbi);
 653         return err;
 654 }