fs/partitions/check.c

   1 /*
   2  *  fs/partitions/check.c
   3  *
   4  *  Code extracted from drivers/block/genhd.c
   5  *  Copyright (C) 1991-1998  Linus Torvalds
   6  *  Re-organised Feb 1998 Russell King
   7  *
   8  *  We now have independent partition support from the
   9  *  block drivers, which allows all the partition code to
  10  *  be grouped in one location, and it to be mostly self
  11  *  contained.
  12  *
  13  *  Added needed MAJORS for new pairs, {hdi,hdj}, {hdk,hdl}
  14  */
  15
  16 #include <linux/init.h>
  17 #include <linux/module.h>
  18 #include <linux/fs.h>
  19 #include <linux/slab.h>
  20 #include <linux/kmod.h>
  21 #include <linux/ctype.h>
  22 #include <linux/genhd.h>
  23 #include <linux/blktrace_api.h>
  24
  25 #include "check.h"
  26
  27 #include "acorn.h"
  28 #include "amiga.h"
  29 #include "atari.h"
  30 #include "ldm.h"
  31 #include "mac.h"
  32 #include "msdos.h"
  33 #include "osf.h"
  34 #include "sgi.h"
  35 #include "sun.h"
  36 #include "ibm.h"
  37 #include "ultrix.h"
  38 #include "efi.h"
  39 #include "karma.h"
  40 #include "sysv68.h"
  41
  42 #ifdef CONFIG_BLK_DEV_MD
  43 extern void md_autodetect_dev(dev_t dev);
  44 #endif
  45
  46 int warn_no_part = 1; /*This is ugly: should make genhd removable media aware*/
  47
  48 static int (*check_part[])(struct parsed_partitions *) = {
  49         /*
  50          * Probe partition formats with tables at disk address 0
  51          * that also have an ADFS boot block at 0xdc0.
  52          */
  53 #ifdef CONFIG_ACORN_PARTITION_ICS
  54         adfspart_check_ICS,
  55 #endif
  56 #ifdef CONFIG_ACORN_PARTITION_POWERTEC
  57         adfspart_check_POWERTEC,
  58 #endif
  59 #ifdef CONFIG_ACORN_PARTITION_EESOX
  60         adfspart_check_EESOX,
  61 #endif
  62
  63         /*
  64          * Now move on to formats that only have partition info at
  65          * disk address 0xdc0.  Since these may also have stale
  66          * PC/BIOS partition tables, they need to come before
  67          * the msdos entry.
  68          */
  69 #ifdef CONFIG_ACORN_PARTITION_CUMANA
  70         adfspart_check_CUMANA,
  71 #endif
  72 #ifdef CONFIG_ACORN_PARTITION_ADFS
  73         adfspart_check_ADFS,
  74 #endif
  75
  76 #ifdef CONFIG_EFI_PARTITION
  77         efi_partition,          /* this must come before msdos */
  78 #endif
  79 #ifdef CONFIG_SGI_PARTITION
  80         sgi_partition,
  81 #endif
  82 #ifdef CONFIG_LDM_PARTITION
  83         ldm_partition,          /* this must come before msdos */
  84 #endif
  85 #ifdef CONFIG_MSDOS_PARTITION
  86         msdos_partition,
  87 #endif
  88 #ifdef CONFIG_OSF_PARTITION
  89         osf_partition,
  90 #endif
  91 #ifdef CONFIG_SUN_PARTITION
  92         sun_partition,
  93 #endif
  94 #ifdef CONFIG_AMIGA_PARTITION
  95         amiga_partition,
  96 #endif
  97 #ifdef CONFIG_ATARI_PARTITION
  98         atari_partition,
  99 #endif
 100 #ifdef CONFIG_MAC_PARTITION
 101         mac_partition,
 102 #endif
 103 #ifdef CONFIG_ULTRIX_PARTITION
 104         ultrix_partition,
 105 #endif
 106 #ifdef CONFIG_IBM_PARTITION
 107         ibm_partition,
 108 #endif
 109 #ifdef CONFIG_KARMA_PARTITION
 110         karma_partition,
 111 #endif
 112 #ifdef CONFIG_SYSV68_PARTITION
 113         sysv68_partition,
 114 #endif
 115         NULL
 116 };
 117
 118 /*
 119  * disk_name() is used by partition check code and the genhd driver.
 120  * It formats the devicename of the indicated disk into
 121  * the supplied buffer (of size at least 32), and returns
 122  * a pointer to that same buffer (for convenience).
 123  */
 124
 125 char *disk_name(struct gendisk *hd, int partno, char *buf)
 126 {
 127         if (!partno)
 128                 snprintf(buf, BDEVNAME_SIZE, "%s", hd->disk_name);
 129         else if (isdigit(hd->disk_name[strlen(hd->disk_name)-1]))
 130                 snprintf(buf, BDEVNAME_SIZE, "%sp%d", hd->disk_name, partno);
 131         else
 132                 snprintf(buf, BDEVNAME_SIZE, "%s%d", hd->disk_name, partno);
 133
 134         return buf;
 135 }
 136
 137 const char *bdevname(struct block_device *bdev, char *buf)
 138 {
 139         return disk_name(bdev->bd_disk, bdev->bd_part->partno, buf);
 140 }
 141
 142 EXPORT_SYMBOL(bdevname);
 143
 144 /*
 145  * There's very little reason to use this, you should really
 146  * have a struct block_device just about everywhere and use
 147  * bdevname() instead.
 148  */
 149 const char *__bdevname(dev_t dev, char *buffer)
 150 {
 151         scnprintf(buffer, BDEVNAME_SIZE, "unknown-block(%u,%u)",
 152                                 MAJOR(dev), MINOR(dev));
 153         return buffer;
 154 }
 155
 156 EXPORT_SYMBOL(__bdevname);
 157
 158 static struct parsed_partitions *
 159 check_partition(struct gendisk *hd, struct block_device *bdev)
 160 {
 161         struct parsed_partitions *state;
 162         int i, res, err;
 163
 164         state = kzalloc(sizeof(struct parsed_partitions), GFP_KERNEL);
 165         if (!state)
 166                 return NULL;
 167         state->pp_buf = (char *)__get_free_page(GFP_KERNEL);
 168         if (!state->pp_buf) {
 169                 kfree(state);
 170                 return NULL;
 171         }
 172         state->pp_buf[0] = '\0';
 173
 174         state->bdev = bdev;
 175         disk_name(hd, 0, state->name);
 176         snprintf(state->pp_buf, PAGE_SIZE, " %s:", state->name);
 177         if (isdigit(state->name[strlen(state->name)-1]))
 178                 sprintf(state->name, "p");
 179
 180         state->limit = disk_max_parts(hd);
 181         i = res = err = 0;
 182         while (!res && check_part[i]) {
 183                 memset(&state->parts, 0, sizeof(state->parts));
 184                 res = check_part[i++](state);
 185                 if (res < 0) {
 186                         /* We have hit an I/O error which we don't report now.
 187                         * But record it, and let the others do their job.
 188                         */
 189                         err = res;
 190                         res = 0;
 191                 }
 192
 193         }
 194         if (res > 0) {
 195                 printk(KERN_INFO "%s", state->pp_buf);
 196
 197                 free_page((unsigned long)state->pp_buf);
 198                 return state;
 199         }
 200         if (state->access_beyond_eod)
 201                 err = -ENOSPC;
 202         if (err)
 203         /* The partition is unrecognized. So report I/O errors if there were any */
 204                 res = err;
 205         if (!res)
 206                 strlcat(state->pp_buf, " unknown partition table\n", PAGE_SIZE);
 207         else if (warn_no_part)
 208                 strlcat(state->pp_buf, " unable to read partition table\n", PAGE_SIZE);
 209
 210         printk(KERN_INFO "%s", state->pp_buf);
 211
 212         free_page((unsigned long)state->pp_buf);
 213         kfree(state);
 214         return ERR_PTR(res);
 215 }
 216
 217 static ssize_t part_partition_show(struct device *dev,
 218                                    struct device_attribute *attr, char *buf)
 219 {
 220         struct hd_struct *p = dev_to_part(dev);
 221
 222         return sprintf(buf, "%d\n", p->partno);
 223 }
 224
 225 static ssize_t part_start_show(struct device *dev,
 226                                struct device_attribute *attr, char *buf)
 227 {
 228         struct hd_struct *p = dev_to_part(dev);
 229
 230         return sprintf(buf, "%llu\n",(unsigned long long)p->start_sect);
 231 }
 232
 233 ssize_t part_size_show(struct device *dev,
 234                        struct device_attribute *attr, char *buf)
 235 {
 236         struct hd_struct *p = dev_to_part(dev);
 237         return sprintf(buf, "%llu\n",(unsigned long long)p->nr_sects);
 238 }
 239
 240 ssize_t part_ro_show(struct device *dev,
 241                        struct device_attribute *attr, char *buf)
 242 {
 243         struct hd_struct *p = dev_to_part(dev);
 244         return sprintf(buf, "%d\n", p->policy ? 1 : 0);
 245 }
 246
 247 ssize_t part_alignment_offset_show(struct device *dev,
 248                                    struct device_attribute *attr, char *buf)
 249 {
 250         struct hd_struct *p = dev_to_part(dev);
 251         return sprintf(buf, "%llu\n", (unsigned long long)p->alignment_offset);
 252 }
 253
 254 ssize_t part_discard_alignment_show(struct device *dev,
 255                                    struct device_attribute *attr, char *buf)
 256 {
 257         struct hd_struct *p = dev_to_part(dev);
 258         struct gendisk *disk = dev_to_disk(dev);
 259
 260         return sprintf(buf, "%u\n",
 261                         queue_limit_discard_alignment(&disk->queue->limits,
 262                                                         p->start_sect));
 263 }
 264
 265 ssize_t part_stat_show(struct device *dev,
 266                        struct device_attribute *attr, char *buf)
 267 {
 268         struct hd_struct *p = dev_to_part(dev);
 269         int cpu;
 270
 271         cpu = part_stat_lock();
 272         part_round_stats(cpu, p);
 273         part_stat_unlock();
 274         return sprintf(buf,
 275                 "%8lu %8lu %8llu %8u "
 276                 "%8lu %8lu %8llu %8u "
 277                 "%8u %8u %8u"
 278                 "\n",
 279                 part_stat_read(p, ios[READ]),
 280                 part_stat_read(p, merges[READ]),
 281                 (unsigned long long)part_stat_read(p, sectors[READ]),
 282                 jiffies_to_msecs(part_stat_read(p, ticks[READ])),
 283                 part_stat_read(p, ios[WRITE]),
 284                 part_stat_read(p, merges[WRITE]),
 285                 (unsigned long long)part_stat_read(p, sectors[WRITE]),
 286                 jiffies_to_msecs(part_stat_read(p, ticks[WRITE])),
 287                 part_in_flight(p),
 288                 jiffies_to_msecs(part_stat_read(p, io_ticks)),
 289                 jiffies_to_msecs(part_stat_read(p, time_in_queue)));
 290 }
 291
 292 ssize_t part_inflight_show(struct device *dev,
 293                         struct device_attribute *attr, char *buf)
 294 {
 295         struct hd_struct *p = dev_to_part(dev);
 296
 297         return sprintf(buf, "%8u %8u\n", atomic_read(&p->in_flight[0]),
 298                 atomic_read(&p->in_flight[1]));
 299 }
 300
 301 #ifdef CONFIG_FAIL_MAKE_REQUEST
 302 ssize_t part_fail_show(struct device *dev,
 303                        struct device_attribute *attr, char *buf)
 304 {
 305         struct hd_struct *p = dev_to_part(dev);
 306
 307         return sprintf(buf, "%d\n", p->make_it_fail);
 308 }
 309
 310 ssize_t part_fail_store(struct device *dev,
 311                         struct device_attribute *attr,
 312                         const char *buf, size_t count)
 313 {
 314         struct hd_struct *p = dev_to_part(dev);
 315         int i;
 316
 317         if (count > 0 && sscanf(buf, "%d", &i) > 0)
 318                 p->make_it_fail = (i == 0) ? 0 : 1;
 319
 320         return count;
 321 }
 322 #endif
 323
 324 static DEVICE_ATTR(partition, S_IRUGO, part_partition_show, NULL);
 325 static DEVICE_ATTR(start, S_IRUGO, part_start_show, NULL);
 326 static DEVICE_ATTR(size, S_IRUGO, part_size_show, NULL);
 327 static DEVICE_ATTR(ro, S_IRUGO, part_ro_show, NULL);
 328 static DEVICE_ATTR(alignment_offset, S_IRUGO, part_alignment_offset_show, NULL);
 329 static DEVICE_ATTR(discard_alignment, S_IRUGO, part_discard_alignment_show,
 330                    NULL);
 331 static DEVICE_ATTR(stat, S_IRUGO, part_stat_show, NULL);
 332 static DEVICE_ATTR(inflight, S_IRUGO, part_inflight_show, NULL);
 333 #ifdef CONFIG_FAIL_MAKE_REQUEST
 334 static struct device_attribute dev_attr_fail =
 335         __ATTR(make-it-fail, S_IRUGO|S_IWUSR, part_fail_show, part_fail_store);
 336 #endif
 337
 338 static struct attribute *part_attrs[] = {
 339         &dev_attr_partition.attr,
 340         &dev_attr_start.attr,
 341         &dev_attr_size.attr,
 342         &dev_attr_ro.attr,
 343         &dev_attr_alignment_offset.attr,
 344         &dev_attr_discard_alignment.attr,
 345         &dev_attr_stat.attr,
 346         &dev_attr_inflight.attr,
 347 #ifdef CONFIG_FAIL_MAKE_REQUEST
 348         &dev_attr_fail.attr,
 349 #endif
 350         NULL
 351 };
 352
 353 static struct attribute_group part_attr_group = {
 354         .attrs = part_attrs,
 355 };
 356
 357 static const struct attribute_group *part_attr_groups[] = {
 358         &part_attr_group,
 359 #ifdef CONFIG_BLK_DEV_IO_TRACE
 360         &blk_trace_attr_group,
 361 #endif
 362         NULL
 363 };
 364
 365 static void part_release(struct device *dev)
 366 {
 367         struct hd_struct *p = dev_to_part(dev);
 368         free_part_stats(p);
 369         free_part_info(p);
 370         kfree(p);
 371 }
 372
 373 struct device_type part_type = {
 374         .name           = "partition",
 375         .groups         = part_attr_groups,
 376         .release        = part_release,
 377 };
 378
 379 static void delete_partition_rcu_cb(struct rcu_head *head)
 380 {
 381         struct hd_struct *part = container_of(head, struct hd_struct, rcu_head);
 382
 383         part->start_sect = 0;
 384         part->nr_sects = 0;
 385         part_stat_set_all(part, 0);
 386         put_device(part_to_dev(part));
 387 }
 388
 389 void __delete_partition(struct hd_struct *part)
 390 {
 391         call_rcu(&part->rcu_head, delete_partition_rcu_cb);
 392 }
 393
 394 void delete_partition(struct gendisk *disk, int partno)
 395 {
 396         struct disk_part_tbl *ptbl = disk->part_tbl;
 397         struct hd_struct *part;
 398
 399         if (partno >= ptbl->len)
 400                 return;
 401
 402         part = ptbl->part[partno];
 403         if (!part)
 404                 return;
 405
 406         blk_free_devt(part_devt(part));
 407         rcu_assign_pointer(ptbl->part[partno], NULL);
 408         rcu_assign_pointer(ptbl->last_lookup, NULL);
 409         kobject_put(part->holder_dir);
 410         device_del(part_to_dev(part));
 411
 412         hd_struct_put(part);
 413 }
 414
 415 static ssize_t whole_disk_show(struct device *dev,
 416                                struct device_attribute *attr, char *buf)
 417 {
 418         return 0;
 419 }
 420 static DEVICE_ATTR(whole_disk, S_IRUSR | S_IRGRP | S_IROTH,
 421                    whole_disk_show, NULL);
 422
 423 struct hd_struct *add_partition(struct gendisk *disk, int partno,
 424                                 sector_t start, sector_t len, int flags,
 425                                 struct partition_meta_info *info)
 426 {
 427         struct hd_struct *p;
 428         dev_t devt = MKDEV(0, 0);
 429         struct device *ddev = disk_to_dev(disk);
 430         struct device *pdev;
 431         struct disk_part_tbl *ptbl;
 432         const char *dname;
 433         int err;
 434
 435         err = disk_expand_part_tbl(disk, partno);
 436         if (err)
 437                 return ERR_PTR(err);
 438         ptbl = disk->part_tbl;
 439
 440         if (ptbl->part[partno])
 441                 return ERR_PTR(-EBUSY);
 442
 443         p = kzalloc(sizeof(*p), GFP_KERNEL);
 444         if (!p)
 445                 return ERR_PTR(-EBUSY);
 446
 447         if (!init_part_stats(p)) {
 448                 err = -ENOMEM;
 449                 goto out_free;
 450         }
 451         pdev = part_to_dev(p);
 452
 453         p->start_sect = start;
 454         p->alignment_offset =
 455                 queue_limit_alignment_offset(&disk->queue->limits, start);
 456         p->nr_sects = len;
 457         p->partno = partno;
 458         p->policy = get_disk_ro(disk);
 459
 460         if (info) {
 461                 struct partition_meta_info *pinfo = alloc_part_info(disk);
 462                 if (!pinfo)
 463                         goto out_free_stats;
 464                 memcpy(pinfo, info, sizeof(*info));
 465                 p->info = pinfo;
 466         }
 467
 468         dname = dev_name(ddev);
 469         if (isdigit(dname[strlen(dname) - 1]))
 470                 dev_set_name(pdev, "%sp%d", dname, partno);
 471         else
 472                 dev_set_name(pdev, "%s%d", dname, partno);
 473
 474         device_initialize(pdev);
 475         pdev->class = &block_class;
 476         pdev->type = &part_type;
 477         pdev->parent = ddev;
 478
 479         err = blk_alloc_devt(p, &devt);
 480         if (err)
 481                 goto out_free_info;
 482         pdev->devt = devt;
 483
 484         /* delay uevent until 'holders' subdir is created */
 485         dev_set_uevent_suppress(pdev, 1);
 486         err = device_add(pdev);
 487         if (err)
 488                 goto out_put;
 489
 490         err = -ENOMEM;
 491         p->holder_dir = kobject_create_and_add("holders", &pdev->kobj);
 492         if (!p->holder_dir)
 493                 goto out_del;
 494
 495         dev_set_uevent_suppress(pdev, 0);
 496         if (flags & ADDPART_FLAG_WHOLEDISK) {
 497                 err = device_create_file(pdev, &dev_attr_whole_disk);
 498                 if (err)
 499                         goto out_del;
 500         }
 501
 502         /* everything is up and running, commence */
 503         rcu_assign_pointer(ptbl->part[partno], p);
 504
 505         /* suppress uevent if the disk suppresses it */
 506         if (!dev_get_uevent_suppress(ddev))
 507                 kobject_uevent(&pdev->kobj, KOBJ_ADD);
 508
 509         hd_ref_init(p);
 510         return p;
 511
 512 out_free_info:
 513         free_part_info(p);
 514 out_free_stats:
 515         free_part_stats(p);
 516 out_free:
 517         kfree(p);
 518         return ERR_PTR(err);
 519 out_del:
 520         kobject_put(p->holder_dir);
 521         device_del(pdev);
 522 out_put:
 523         put_device(pdev);
 524         blk_free_devt(devt);
 525         return ERR_PTR(err);
 526 }
 527
 528 static bool disk_unlock_native_capacity(struct gendisk *disk)
 529 {
 530         const struct block_device_operations *bdops = disk->fops;
 531
 532         if (bdops->unlock_native_capacity &&
 533             !(disk->flags & GENHD_FL_NATIVE_CAPACITY)) {
 534                 printk(KERN_CONT "enabling native capacity\n");
 535                 bdops->unlock_native_capacity(disk);
 536                 disk->flags |= GENHD_FL_NATIVE_CAPACITY;
 537                 return true;
 538         } else {
 539                 printk(KERN_CONT "truncated\n");
 540                 return false;
 541         }
 542 }
 543
 544 int rescan_partitions(struct gendisk *disk, struct block_device *bdev)
 545 {
 546         struct parsed_partitions *state = NULL;
 547         struct disk_part_iter piter;
 548         struct hd_struct *part;
 549         int p, highest, res;
 550 rescan:
 551         if (state && !IS_ERR(state)) {
 552                 kfree(state);
 553                 state = NULL;
 554         }
 555
 556         if (bdev->bd_part_count)
 557                 return -EBUSY;
 558         res = invalidate_partition(disk, 0);
 559         if (res)
 560                 return res;
 561
 562         disk_part_iter_init(&piter, disk, DISK_PITER_INCL_EMPTY);
 563         while ((part = disk_part_iter_next(&piter)))
 564                 delete_partition(disk, part->partno);
 565         disk_part_iter_exit(&piter);
 566
 567         if (disk->fops->revalidate_disk)
 568                 disk->fops->revalidate_disk(disk);
 569         check_disk_size_change(disk, bdev);
 570         bdev->bd_invalidated = 0;
 571         if (!get_capacity(disk) || !(state = check_partition(disk, bdev)))
 572                 return 0;
 573         if (IS_ERR(state)) {
 574                 /*
 575                  * I/O error reading the partition table.  If any
 576                  * partition code tried to read beyond EOD, retry
 577                  * after unlocking native capacity.
 578                  */
 579                 if (PTR_ERR(state) == -ENOSPC) {
 580                         printk(KERN_WARNING "%s: partition table beyond EOD, ",
 581                                disk->disk_name);
 582                         if (disk_unlock_native_capacity(disk))
 583                                 goto rescan;
 584                 }
 585                 return -EIO;
 586         }
 587         /*
 588          * If any partition code tried to read beyond EOD, try
 589          * unlocking native capacity even if partition table is
 590          * successfully read as we could be missing some partitions.
 591          */
 592         if (state->access_beyond_eod) {
 593                 printk(KERN_WARNING
 594                        "%s: partition table partially beyond EOD, ",
 595                        disk->disk_name);
 596                 if (disk_unlock_native_capacity(disk))
 597                         goto rescan;
 598         }
 599
 600         /* tell userspace that the media / partition table may have changed */
 601         kobject_uevent(&disk_to_dev(disk)->kobj, KOBJ_CHANGE);
 602
 603         /* Detect the highest partition number and preallocate
 604          * disk->part_tbl.  This is an optimization and not strictly
 605          * necessary.
 606          */
 607         for (p = 1, highest = 0; p < state->limit; p++)
 608                 if (state->parts[p].size)
 609                         highest = p;
 610
 611         disk_expand_part_tbl(disk, highest);
 612
 613         /* add partitions */
 614         for (p = 1; p < state->limit; p++) {
 615                 sector_t size, from;
 616                 struct partition_meta_info *info = NULL;
 617
 618                 size = state->parts[p].size;
 619                 if (!size)
 620                         continue;
 621
 622                 from = state->parts[p].from;
 623                 if (from >= get_capacity(disk)) {
 624                         printk(KERN_WARNING
 625                                "%s: p%d start %llu is beyond EOD, ",
 626                                disk->disk_name, p, (unsigned long long) from);
 627                         if (disk_unlock_native_capacity(disk))
 628                                 goto rescan;
 629                         continue;
 630                 }
 631
 632                 if (from + size > get_capacity(disk)) {
 633                         printk(KERN_WARNING
 634                                "%s: p%d size %llu extends beyond EOD, ",
 635                                disk->disk_name, p, (unsigned long long) size);
 636
 637                         if (disk_unlock_native_capacity(disk)) {
 638                                 /* free state and restart */
 639                                 goto rescan;
 640                         } else {
 641                                 /*
 642                                  * we can not ignore partitions of broken tables
 643                                  * created by for example camera firmware, but
 644                                  * we limit them to the end of the disk to avoid
 645                                  * creating invalid block devices
 646                                  */
 647                                 size = get_capacity(disk) - from;
 648                         }
 649                 }
 650
 651                 if (state->parts[p].has_info)
 652                         info = &state->parts[p].info;
 653                 part = add_partition(disk, p, from, size,
 654                                      state->parts[p].flags,
 655                                      &state->parts[p].info);
 656                 if (IS_ERR(part)) {
 657                         printk(KERN_ERR " %s: p%d could not be added: %ld\n",
 658                                disk->disk_name, p, -PTR_ERR(part));
 659                         continue;
 660                 }
 661 #ifdef CONFIG_BLK_DEV_MD
 662                 if (state->parts[p].flags & ADDPART_FLAG_RAID)
 663                         md_autodetect_dev(part_to_dev(part)->devt);
 664 #endif
 665         }
 666         kfree(state);
 667         return 0;
 668 }
 669
 670 unsigned char *read_dev_sector(struct block_device *bdev, sector_t n, Sector *p)
 671 {
 672         struct address_space *mapping = bdev->bd_inode->i_mapping;
 673         struct page *page;
 674
 675         page = read_mapping_page(mapping, (pgoff_t)(n >> (PAGE_CACHE_SHIFT-9)),
 676                                  NULL);
 677         if (!IS_ERR(page)) {
 678                 if (PageError(page))
 679                         goto fail;
 680                 p->v = page;
 681                 return (unsigned char *)page_address(page) +  ((n & ((1 << (PAGE_CACHE_SHIFT - 9)) - 1)) << 9);
 682 fail:
 683                 page_cache_release(page);
 684         }
 685         p->v = NULL;
 686         return NULL;
 687 }
 688
 689 EXPORT_SYMBOL(read_dev_sector);