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 static 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 static 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 static 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         return sprintf(buf, "%u\n", p->discard_alignment);
 259 }
 260
 261 ssize_t part_stat_show(struct device *dev,
 262                        struct device_attribute *attr, char *buf)
 263 {
 264         struct hd_struct *p = dev_to_part(dev);
 265         int cpu;
 266
 267         cpu = part_stat_lock();
 268         part_round_stats(cpu, p);
 269         part_stat_unlock();
 270         return sprintf(buf,
 271                 "%8lu %8lu %8llu %8u "
 272                 "%8lu %8lu %8llu %8u "
 273                 "%8u %8u %8u"
 274                 "\n",
 275                 part_stat_read(p, ios[READ]),
 276                 part_stat_read(p, merges[READ]),
 277                 (unsigned long long)part_stat_read(p, sectors[READ]),
 278                 jiffies_to_msecs(part_stat_read(p, ticks[READ])),
 279                 part_stat_read(p, ios[WRITE]),
 280                 part_stat_read(p, merges[WRITE]),
 281                 (unsigned long long)part_stat_read(p, sectors[WRITE]),
 282                 jiffies_to_msecs(part_stat_read(p, ticks[WRITE])),
 283                 part_in_flight(p),
 284                 jiffies_to_msecs(part_stat_read(p, io_ticks)),
 285                 jiffies_to_msecs(part_stat_read(p, time_in_queue)));
 286 }
 287
 288 ssize_t part_inflight_show(struct device *dev,
 289                         struct device_attribute *attr, char *buf)
 290 {
 291         struct hd_struct *p = dev_to_part(dev);
 292
 293         return sprintf(buf, "%8u %8u\n", atomic_read(&p->in_flight[0]),
 294                 atomic_read(&p->in_flight[1]));
 295 }
 296
 297 #ifdef CONFIG_FAIL_MAKE_REQUEST
 298 ssize_t part_fail_show(struct device *dev,
 299                        struct device_attribute *attr, char *buf)
 300 {
 301         struct hd_struct *p = dev_to_part(dev);
 302
 303         return sprintf(buf, "%d\n", p->make_it_fail);
 304 }
 305
 306 ssize_t part_fail_store(struct device *dev,
 307                         struct device_attribute *attr,
 308                         const char *buf, size_t count)
 309 {
 310         struct hd_struct *p = dev_to_part(dev);
 311         int i;
 312
 313         if (count > 0 && sscanf(buf, "%d", &i) > 0)
 314                 p->make_it_fail = (i == 0) ? 0 : 1;
 315
 316         return count;
 317 }
 318 #endif
 319
 320 static DEVICE_ATTR(partition, S_IRUGO, part_partition_show, NULL);
 321 static DEVICE_ATTR(start, S_IRUGO, part_start_show, NULL);
 322 static DEVICE_ATTR(size, S_IRUGO, part_size_show, NULL);
 323 static DEVICE_ATTR(ro, S_IRUGO, part_ro_show, NULL);
 324 static DEVICE_ATTR(alignment_offset, S_IRUGO, part_alignment_offset_show, NULL);
 325 static DEVICE_ATTR(discard_alignment, S_IRUGO, part_discard_alignment_show,
 326                    NULL);
 327 static DEVICE_ATTR(stat, S_IRUGO, part_stat_show, NULL);
 328 static DEVICE_ATTR(inflight, S_IRUGO, part_inflight_show, NULL);
 329 #ifdef CONFIG_FAIL_MAKE_REQUEST
 330 static struct device_attribute dev_attr_fail =
 331         __ATTR(make-it-fail, S_IRUGO|S_IWUSR, part_fail_show, part_fail_store);
 332 #endif
 333
 334 static struct attribute *part_attrs[] = {
 335         &dev_attr_partition.attr,
 336         &dev_attr_start.attr,
 337         &dev_attr_size.attr,
 338         &dev_attr_ro.attr,
 339         &dev_attr_alignment_offset.attr,
 340         &dev_attr_discard_alignment.attr,
 341         &dev_attr_stat.attr,
 342         &dev_attr_inflight.attr,
 343 #ifdef CONFIG_FAIL_MAKE_REQUEST
 344         &dev_attr_fail.attr,
 345 #endif
 346         NULL
 347 };
 348
 349 static struct attribute_group part_attr_group = {
 350         .attrs = part_attrs,
 351 };
 352
 353 static const struct attribute_group *part_attr_groups[] = {
 354         &part_attr_group,
 355 #ifdef CONFIG_BLK_DEV_IO_TRACE
 356         &blk_trace_attr_group,
 357 #endif
 358         NULL
 359 };
 360
 361 static void part_release(struct device *dev)
 362 {
 363         struct hd_struct *p = dev_to_part(dev);
 364         free_part_stats(p);
 365         free_part_info(p);
 366         kfree(p);
 367 }
 368
 369 struct device_type part_type = {
 370         .name           = "partition",
 371         .groups         = part_attr_groups,
 372         .release        = part_release,
 373 };
 374
 375 static void delete_partition_rcu_cb(struct rcu_head *head)
 376 {
 377         struct hd_struct *part = container_of(head, struct hd_struct, rcu_head);
 378
 379         part->start_sect = 0;
 380         part->nr_sects = 0;
 381         part_stat_set_all(part, 0);
 382         put_device(part_to_dev(part));
 383 }
 384
 385 void __delete_partition(struct hd_struct *part)
 386 {
 387         call_rcu(&part->rcu_head, delete_partition_rcu_cb);
 388 }
 389
 390 void delete_partition(struct gendisk *disk, int partno)
 391 {
 392         struct disk_part_tbl *ptbl = disk->part_tbl;
 393         struct hd_struct *part;
 394
 395         if (partno >= ptbl->len)
 396                 return;
 397
 398         part = ptbl->part[partno];
 399         if (!part)
 400                 return;
 401
 402         blk_free_devt(part_devt(part));
 403         rcu_assign_pointer(ptbl->part[partno], NULL);
 404         rcu_assign_pointer(ptbl->last_lookup, NULL);
 405         kobject_put(part->holder_dir);
 406         device_del(part_to_dev(part));
 407
 408         hd_struct_put(part);
 409 }
 410
 411 static ssize_t whole_disk_show(struct device *dev,
 412                                struct device_attribute *attr, char *buf)
 413 {
 414         return 0;
 415 }
 416 static DEVICE_ATTR(whole_disk, S_IRUSR | S_IRGRP | S_IROTH,
 417                    whole_disk_show, NULL);
 418
 419 struct hd_struct *add_partition(struct gendisk *disk, int partno,
 420                                 sector_t start, sector_t len, int flags,
 421                                 struct partition_meta_info *info)
 422 {
 423         struct hd_struct *p;
 424         dev_t devt = MKDEV(0, 0);
 425         struct device *ddev = disk_to_dev(disk);
 426         struct device *pdev;
 427         struct disk_part_tbl *ptbl;
 428         const char *dname;
 429         int err;
 430
 431         err = disk_expand_part_tbl(disk, partno);
 432         if (err)
 433                 return ERR_PTR(err);
 434         ptbl = disk->part_tbl;
 435
 436         if (ptbl->part[partno])
 437                 return ERR_PTR(-EBUSY);
 438
 439         p = kzalloc(sizeof(*p), GFP_KERNEL);
 440         if (!p)
 441                 return ERR_PTR(-EBUSY);
 442
 443         if (!init_part_stats(p)) {
 444                 err = -ENOMEM;
 445                 goto out_free;
 446         }
 447         pdev = part_to_dev(p);
 448
 449         p->start_sect = start;
 450         p->alignment_offset =
 451                 queue_limit_alignment_offset(&disk->queue->limits, start);
 452         p->discard_alignment =
 453                 queue_limit_discard_alignment(&disk->queue->limits, start);
 454         p->nr_sects = len;
 455         p->partno = partno;
 456         p->policy = get_disk_ro(disk);
 457
 458         if (info) {
 459                 struct partition_meta_info *pinfo = alloc_part_info(disk);
 460                 if (!pinfo)
 461                         goto out_free_stats;
 462                 memcpy(pinfo, info, sizeof(*info));
 463                 p->info = pinfo;
 464         }
 465
 466         dname = dev_name(ddev);
 467         if (isdigit(dname[strlen(dname) - 1]))
 468                 dev_set_name(pdev, "%sp%d", dname, partno);
 469         else
 470                 dev_set_name(pdev, "%s%d", dname, partno);
 471
 472         device_initialize(pdev);
 473         pdev->class = &block_class;
 474         pdev->type = &part_type;
 475         pdev->parent = ddev;
 476
 477         err = blk_alloc_devt(p, &devt);
 478         if (err)
 479                 goto out_free_info;
 480         pdev->devt = devt;
 481
 482         /* delay uevent until 'holders' subdir is created */
 483         dev_set_uevent_suppress(pdev, 1);
 484         err = device_add(pdev);
 485         if (err)
 486                 goto out_put;
 487
 488         err = -ENOMEM;
 489         p->holder_dir = kobject_create_and_add("holders", &pdev->kobj);
 490         if (!p->holder_dir)
 491                 goto out_del;
 492
 493         dev_set_uevent_suppress(pdev, 0);
 494         if (flags & ADDPART_FLAG_WHOLEDISK) {
 495                 err = device_create_file(pdev, &dev_attr_whole_disk);
 496                 if (err)
 497                         goto out_del;
 498         }
 499
 500         /* everything is up and running, commence */
 501         rcu_assign_pointer(ptbl->part[partno], p);
 502
 503         /* suppress uevent if the disk suppresses it */
 504         if (!dev_get_uevent_suppress(ddev))
 505                 kobject_uevent(&pdev->kobj, KOBJ_ADD);
 506
 507         hd_ref_init(p);
 508         return p;
 509
 510 out_free_info:
 511         free_part_info(p);
 512 out_free_stats:
 513         free_part_stats(p);
 514 out_free:
 515         kfree(p);
 516         return ERR_PTR(err);
 517 out_del:
 518         kobject_put(p->holder_dir);
 519         device_del(pdev);
 520 out_put:
 521         put_device(pdev);
 522         blk_free_devt(devt);
 523         return ERR_PTR(err);
 524 }
 525
 526 static bool disk_unlock_native_capacity(struct gendisk *disk)
 527 {
 528         const struct block_device_operations *bdops = disk->fops;
 529
 530         if (bdops->unlock_native_capacity &&
 531             !(disk->flags & GENHD_FL_NATIVE_CAPACITY)) {
 532                 printk(KERN_CONT "enabling native capacity\n");
 533                 bdops->unlock_native_capacity(disk);
 534                 disk->flags |= GENHD_FL_NATIVE_CAPACITY;
 535                 return true;
 536         } else {
 537                 printk(KERN_CONT "truncated\n");
 538                 return false;
 539         }
 540 }
 541
 542 int rescan_partitions(struct gendisk *disk, struct block_device *bdev)
 543 {
 544         struct parsed_partitions *state = NULL;
 545         struct disk_part_iter piter;
 546         struct hd_struct *part;
 547         int p, highest, res;
 548 rescan:
 549         if (state && !IS_ERR(state)) {
 550                 kfree(state);
 551                 state = NULL;
 552         }
 553
 554         if (bdev->bd_part_count)
 555                 return -EBUSY;
 556         res = invalidate_partition(disk, 0);
 557         if (res)
 558                 return res;
 559
 560         disk_part_iter_init(&piter, disk, DISK_PITER_INCL_EMPTY);
 561         while ((part = disk_part_iter_next(&piter)))
 562                 delete_partition(disk, part->partno);
 563         disk_part_iter_exit(&piter);
 564
 565         if (disk->fops->revalidate_disk)
 566                 disk->fops->revalidate_disk(disk);
 567         check_disk_size_change(disk, bdev);
 568         bdev->bd_invalidated = 0;
 569         if (!get_capacity(disk) || !(state = check_partition(disk, bdev)))
 570                 return 0;
 571         if (IS_ERR(state)) {
 572                 /*
 573                  * I/O error reading the partition table.  If any
 574                  * partition code tried to read beyond EOD, retry
 575                  * after unlocking native capacity.
 576                  */
 577                 if (PTR_ERR(state) == -ENOSPC) {
 578                         printk(KERN_WARNING "%s: partition table beyond EOD, ",
 579                                disk->disk_name);
 580                         if (disk_unlock_native_capacity(disk))
 581                                 goto rescan;
 582                 }
 583                 return -EIO;
 584         }
 585         /*
 586          * If any partition code tried to read beyond EOD, try
 587          * unlocking native capacity even if partition table is
 588          * successfully read as we could be missing some partitions.
 589          */
 590         if (state->access_beyond_eod) {
 591                 printk(KERN_WARNING
 592                        "%s: partition table partially beyond EOD, ",
 593                        disk->disk_name);
 594                 if (disk_unlock_native_capacity(disk))
 595                         goto rescan;
 596         }
 597
 598         /* tell userspace that the media / partition table may have changed */
 599         kobject_uevent(&disk_to_dev(disk)->kobj, KOBJ_CHANGE);
 600
 601         /* Detect the highest partition number and preallocate
 602          * disk->part_tbl.  This is an optimization and not strictly
 603          * necessary.
 604          */
 605         for (p = 1, highest = 0; p < state->limit; p++)
 606                 if (state->parts[p].size)
 607                         highest = p;
 608
 609         disk_expand_part_tbl(disk, highest);
 610
 611         /* add partitions */
 612         for (p = 1; p < state->limit; p++) {
 613                 sector_t size, from;
 614                 struct partition_meta_info *info = NULL;
 615
 616                 size = state->parts[p].size;
 617                 if (!size)
 618                         continue;
 619
 620                 from = state->parts[p].from;
 621                 if (from >= get_capacity(disk)) {
 622                         printk(KERN_WARNING
 623                                "%s: p%d start %llu is beyond EOD, ",
 624                                disk->disk_name, p, (unsigned long long) from);
 625                         if (disk_unlock_native_capacity(disk))
 626                                 goto rescan;
 627                         continue;
 628                 }
 629
 630                 if (from + size > get_capacity(disk)) {
 631                         printk(KERN_WARNING
 632                                "%s: p%d size %llu extends beyond EOD, ",
 633                                disk->disk_name, p, (unsigned long long) size);
 634
 635                         if (disk_unlock_native_capacity(disk)) {
 636                                 /* free state and restart */
 637                                 goto rescan;
 638                         } else {
 639                                 /*
 640                                  * we can not ignore partitions of broken tables
 641                                  * created by for example camera firmware, but
 642                                  * we limit them to the end of the disk to avoid
 643                                  * creating invalid block devices
 644                                  */
 645                                 size = get_capacity(disk) - from;
 646                         }
 647                 }
 648
 649                 if (state->parts[p].has_info)
 650                         info = &state->parts[p].info;
 651                 part = add_partition(disk, p, from, size,
 652                                      state->parts[p].flags,
 653                                      &state->parts[p].info);
 654                 if (IS_ERR(part)) {
 655                         printk(KERN_ERR " %s: p%d could not be added: %ld\n",
 656                                disk->disk_name, p, -PTR_ERR(part));
 657                         continue;
 658                 }
 659 #ifdef CONFIG_BLK_DEV_MD
 660                 if (state->parts[p].flags & ADDPART_FLAG_RAID)
 661                         md_autodetect_dev(part_to_dev(part)->devt);
 662 #endif
 663         }
 664         kfree(state);
 665         return 0;
 666 }
 667
 668 unsigned char *read_dev_sector(struct block_device *bdev, sector_t n, Sector *p)
 669 {
 670         struct address_space *mapping = bdev->bd_inode->i_mapping;
 671         struct page *page;
 672
 673         page = read_mapping_page(mapping, (pgoff_t)(n >> (PAGE_CACHE_SHIFT-9)),
 674                                  NULL);
 675         if (!IS_ERR(page)) {
 676                 if (PageError(page))
 677                         goto fail;
 678                 p->v = page;
 679                 return (unsigned char *)page_address(page) +  ((n & ((1 << (PAGE_CACHE_SHIFT - 9)) - 1)) << 9);
 680 fail:
 681                 page_cache_release(page);
 682         }
 683         p->v = NULL;
 684         return NULL;
 685 }
 686
 687 EXPORT_SYMBOL(read_dev_sector);