block/partition-generic.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  *  Code extracted from drivers/block/genhd.c
   4  *  Copyright (C) 1991-1998  Linus Torvalds
   5  *  Re-organised Feb 1998 Russell King
   6  *
   7  *  We now have independent partition support from the
   8  *  block drivers, which allows all the partition code to
   9  *  be grouped in one location, and it to be mostly self
  10  *  contained.
  11  */
  12
  13 #include <linux/init.h>
  14 #include <linux/module.h>
  15 #include <linux/fs.h>
  16 #include <linux/slab.h>
  17 #include <linux/kmod.h>
  18 #include <linux/ctype.h>
  19 #include <linux/genhd.h>
  20 #include <linux/blktrace_api.h>
  21
  22 #include "partitions/check.h"
  23
  24 #ifdef CONFIG_BLK_DEV_MD
  25 extern void md_autodetect_dev(dev_t dev);
  26 #endif
  27
  28 /*
  29  * disk_name() is used by partition check code and the genhd driver.
  30  * It formats the devicename of the indicated disk into
  31  * the supplied buffer (of size at least 32), and returns
  32  * a pointer to that same buffer (for convenience).
  33  */
  34
  35 char *disk_name(struct gendisk *hd, int partno, char *buf)
  36 {
  37         if (!partno)
  38                 snprintf(buf, BDEVNAME_SIZE, "%s", hd->disk_name);
  39         else if (isdigit(hd->disk_name[strlen(hd->disk_name)-1]))
  40                 snprintf(buf, BDEVNAME_SIZE, "%sp%d", hd->disk_name, partno);
  41         else
  42                 snprintf(buf, BDEVNAME_SIZE, "%s%d", hd->disk_name, partno);
  43
  44         return buf;
  45 }
  46
  47 const char *bdevname(struct block_device *bdev, char *buf)
  48 {
  49         return disk_name(bdev->bd_disk, bdev->bd_part->partno, buf);
  50 }
  51
  52 EXPORT_SYMBOL(bdevname);
  53
  54 const char *bio_devname(struct bio *bio, char *buf)
  55 {
  56         return disk_name(bio->bi_disk, bio->bi_partno, buf);
  57 }
  58 EXPORT_SYMBOL(bio_devname);
  59
  60 /*
  61  * There's very little reason to use this, you should really
  62  * have a struct block_device just about everywhere and use
  63  * bdevname() instead.
  64  */
  65 const char *__bdevname(dev_t dev, char *buffer)
  66 {
  67         scnprintf(buffer, BDEVNAME_SIZE, "unknown-block(%u,%u)",
  68                                 MAJOR(dev), MINOR(dev));
  69         return buffer;
  70 }
  71
  72 EXPORT_SYMBOL(__bdevname);
  73
  74 static ssize_t part_partition_show(struct device *dev,
  75                                    struct device_attribute *attr, char *buf)
  76 {
  77         struct hd_struct *p = dev_to_part(dev);
  78
  79         return sprintf(buf, "%d\n", p->partno);
  80 }
  81
  82 static ssize_t part_start_show(struct device *dev,
  83                                struct device_attribute *attr, char *buf)
  84 {
  85         struct hd_struct *p = dev_to_part(dev);
  86
  87         return sprintf(buf, "%llu\n",(unsigned long long)p->start_sect);
  88 }
  89
  90 ssize_t part_size_show(struct device *dev,
  91                        struct device_attribute *attr, char *buf)
  92 {
  93         struct hd_struct *p = dev_to_part(dev);
  94         return sprintf(buf, "%llu\n",(unsigned long long)part_nr_sects_read(p));
  95 }
  96
  97 static ssize_t part_ro_show(struct device *dev,
  98                             struct device_attribute *attr, char *buf)
  99 {
 100         struct hd_struct *p = dev_to_part(dev);
 101         return sprintf(buf, "%d\n", p->policy ? 1 : 0);
 102 }
 103
 104 static ssize_t part_alignment_offset_show(struct device *dev,
 105                                           struct device_attribute *attr, char *buf)
 106 {
 107         struct hd_struct *p = dev_to_part(dev);
 108         return sprintf(buf, "%llu\n", (unsigned long long)p->alignment_offset);
 109 }
 110
 111 static ssize_t part_discard_alignment_show(struct device *dev,
 112                                            struct device_attribute *attr, char *buf)
 113 {
 114         struct hd_struct *p = dev_to_part(dev);
 115         return sprintf(buf, "%u\n", p->discard_alignment);
 116 }
 117
 118 ssize_t part_stat_show(struct device *dev,
 119                        struct device_attribute *attr, char *buf)
 120 {
 121         struct hd_struct *p = dev_to_part(dev);
 122         struct request_queue *q = part_to_disk(p)->queue;
 123         unsigned int inflight;
 124
 125         inflight = part_in_flight(q, p);
 126         return sprintf(buf,
 127                 "%8lu %8lu %8llu %8u "
 128                 "%8lu %8lu %8llu %8u "
 129                 "%8u %8u %8u "
 130                 "%8lu %8lu %8llu %8u"
 131                 "\n",
 132                 part_stat_read(p, ios[STAT_READ]),
 133                 part_stat_read(p, merges[STAT_READ]),
 134                 (unsigned long long)part_stat_read(p, sectors[STAT_READ]),
 135                 (unsigned int)part_stat_read_msecs(p, STAT_READ),
 136                 part_stat_read(p, ios[STAT_WRITE]),
 137                 part_stat_read(p, merges[STAT_WRITE]),
 138                 (unsigned long long)part_stat_read(p, sectors[STAT_WRITE]),
 139                 (unsigned int)part_stat_read_msecs(p, STAT_WRITE),
 140                 inflight,
 141                 jiffies_to_msecs(part_stat_read(p, io_ticks)),
 142                 jiffies_to_msecs(part_stat_read(p, time_in_queue)),
 143                 part_stat_read(p, ios[STAT_DISCARD]),
 144                 part_stat_read(p, merges[STAT_DISCARD]),
 145                 (unsigned long long)part_stat_read(p, sectors[STAT_DISCARD]),
 146                 (unsigned int)part_stat_read_msecs(p, STAT_DISCARD));
 147 }
 148
 149 ssize_t part_inflight_show(struct device *dev, struct device_attribute *attr,
 150                            char *buf)
 151 {
 152         struct hd_struct *p = dev_to_part(dev);
 153         struct request_queue *q = part_to_disk(p)->queue;
 154         unsigned int inflight[2];
 155
 156         part_in_flight_rw(q, p, inflight);
 157         return sprintf(buf, "%8u %8u\n", inflight[0], inflight[1]);
 158 }
 159
 160 #ifdef CONFIG_FAIL_MAKE_REQUEST
 161 ssize_t part_fail_show(struct device *dev,
 162                        struct device_attribute *attr, char *buf)
 163 {
 164         struct hd_struct *p = dev_to_part(dev);
 165
 166         return sprintf(buf, "%d\n", p->make_it_fail);
 167 }
 168
 169 ssize_t part_fail_store(struct device *dev,
 170                         struct device_attribute *attr,
 171                         const char *buf, size_t count)
 172 {
 173         struct hd_struct *p = dev_to_part(dev);
 174         int i;
 175
 176         if (count > 0 && sscanf(buf, "%d", &i) > 0)
 177                 p->make_it_fail = (i == 0) ? 0 : 1;
 178
 179         return count;
 180 }
 181 #endif
 182
 183 static DEVICE_ATTR(partition, 0444, part_partition_show, NULL);
 184 static DEVICE_ATTR(start, 0444, part_start_show, NULL);
 185 static DEVICE_ATTR(size, 0444, part_size_show, NULL);
 186 static DEVICE_ATTR(ro, 0444, part_ro_show, NULL);
 187 static DEVICE_ATTR(alignment_offset, 0444, part_alignment_offset_show, NULL);
 188 static DEVICE_ATTR(discard_alignment, 0444, part_discard_alignment_show, NULL);
 189 static DEVICE_ATTR(stat, 0444, part_stat_show, NULL);
 190 static DEVICE_ATTR(inflight, 0444, part_inflight_show, NULL);
 191 #ifdef CONFIG_FAIL_MAKE_REQUEST
 192 static struct device_attribute dev_attr_fail =
 193         __ATTR(make-it-fail, 0644, part_fail_show, part_fail_store);
 194 #endif
 195
 196 static struct attribute *part_attrs[] = {
 197         &dev_attr_partition.attr,
 198         &dev_attr_start.attr,
 199         &dev_attr_size.attr,
 200         &dev_attr_ro.attr,
 201         &dev_attr_alignment_offset.attr,
 202         &dev_attr_discard_alignment.attr,
 203         &dev_attr_stat.attr,
 204         &dev_attr_inflight.attr,
 205 #ifdef CONFIG_FAIL_MAKE_REQUEST
 206         &dev_attr_fail.attr,
 207 #endif
 208         NULL
 209 };
 210
 211 static struct attribute_group part_attr_group = {
 212         .attrs = part_attrs,
 213 };
 214
 215 static const struct attribute_group *part_attr_groups[] = {
 216         &part_attr_group,
 217 #ifdef CONFIG_BLK_DEV_IO_TRACE
 218         &blk_trace_attr_group,
 219 #endif
 220         NULL
 221 };
 222
 223 static void part_release(struct device *dev)
 224 {
 225         struct hd_struct *p = dev_to_part(dev);
 226         blk_free_devt(dev->devt);
 227         hd_free_part(p);
 228         kfree(p);
 229 }
 230
 231 static int part_uevent(struct device *dev, struct kobj_uevent_env *env)
 232 {
 233         struct hd_struct *part = dev_to_part(dev);
 234
 235         add_uevent_var(env, "PARTN=%u", part->partno);
 236         if (part->info && part->info->volname[0])
 237                 add_uevent_var(env, "PARTNAME=%s", part->info->volname);
 238         return 0;
 239 }
 240
 241 struct device_type part_type = {
 242         .name           = "partition",
 243         .groups         = part_attr_groups,
 244         .release        = part_release,
 245         .uevent         = part_uevent,
 246 };
 247
 248 static void delete_partition_work_fn(struct work_struct *work)
 249 {
 250         struct hd_struct *part = container_of(to_rcu_work(work), struct hd_struct,
 251                                         rcu_work);
 252
 253         part->start_sect = 0;
 254         part->nr_sects = 0;
 255         part_stat_set_all(part, 0);
 256         put_device(part_to_dev(part));
 257 }
 258
 259 void __delete_partition(struct percpu_ref *ref)
 260 {
 261         struct hd_struct *part = container_of(ref, struct hd_struct, ref);
 262         INIT_RCU_WORK(&part->rcu_work, delete_partition_work_fn);
 263         queue_rcu_work(system_wq, &part->rcu_work);
 264 }
 265
 266 /*
 267  * Must be called either with bd_mutex held, before a disk can be opened or
 268  * after all disk users are gone.
 269  */
 270 void delete_partition(struct gendisk *disk, int partno)
 271 {
 272         struct disk_part_tbl *ptbl =
 273                 rcu_dereference_protected(disk->part_tbl, 1);
 274         struct hd_struct *part;
 275
 276         if (partno >= ptbl->len)
 277                 return;
 278
 279         part = rcu_dereference_protected(ptbl->part[partno], 1);
 280         if (!part)
 281                 return;
 282
 283         rcu_assign_pointer(ptbl->part[partno], NULL);
 284         rcu_assign_pointer(ptbl->last_lookup, NULL);
 285         kobject_put(part->holder_dir);
 286         device_del(part_to_dev(part));
 287
 288         /*
 289          * Remove gendisk pointer from idr so that it cannot be looked up
 290          * while RCU period before freeing gendisk is running to prevent
 291          * use-after-free issues. Note that the device number stays
 292          * "in-use" until we really free the gendisk.
 293          */
 294         blk_invalidate_devt(part_devt(part));
 295         hd_struct_kill(part);
 296 }
 297
 298 static ssize_t whole_disk_show(struct device *dev,
 299                                struct device_attribute *attr, char *buf)
 300 {
 301         return 0;
 302 }
 303 static DEVICE_ATTR(whole_disk, 0444, whole_disk_show, NULL);
 304
 305 /*
 306  * Must be called either with bd_mutex held, before a disk can be opened or
 307  * after all disk users are gone.
 308  */
 309 struct hd_struct *add_partition(struct gendisk *disk, int partno,
 310                                 sector_t start, sector_t len, int flags,
 311                                 struct partition_meta_info *info)
 312 {
 313         struct hd_struct *p;
 314         dev_t devt = MKDEV(0, 0);
 315         struct device *ddev = disk_to_dev(disk);
 316         struct device *pdev;
 317         struct disk_part_tbl *ptbl;
 318         const char *dname;
 319         int err;
 320
 321         err = disk_expand_part_tbl(disk, partno);
 322         if (err)
 323                 return ERR_PTR(err);
 324         ptbl = rcu_dereference_protected(disk->part_tbl, 1);
 325
 326         if (ptbl->part[partno])
 327                 return ERR_PTR(-EBUSY);
 328
 329         p = kzalloc(sizeof(*p), GFP_KERNEL);
 330         if (!p)
 331                 return ERR_PTR(-EBUSY);
 332
 333         if (!init_part_stats(p)) {
 334                 err = -ENOMEM;
 335                 goto out_free;
 336         }
 337
 338         seqcount_init(&p->nr_sects_seq);
 339         pdev = part_to_dev(p);
 340
 341         p->start_sect = start;
 342         p->alignment_offset =
 343                 queue_limit_alignment_offset(&disk->queue->limits, start);
 344         p->discard_alignment =
 345                 queue_limit_discard_alignment(&disk->queue->limits, start);
 346         p->nr_sects = len;
 347         p->partno = partno;
 348         p->policy = get_disk_ro(disk);
 349
 350         if (info) {
 351                 struct partition_meta_info *pinfo = alloc_part_info(disk);
 352                 if (!pinfo) {
 353                         err = -ENOMEM;
 354                         goto out_free_stats;
 355                 }
 356                 memcpy(pinfo, info, sizeof(*info));
 357                 p->info = pinfo;
 358         }
 359
 360         dname = dev_name(ddev);
 361         if (isdigit(dname[strlen(dname) - 1]))
 362                 dev_set_name(pdev, "%sp%d", dname, partno);
 363         else
 364                 dev_set_name(pdev, "%s%d", dname, partno);
 365
 366         device_initialize(pdev);
 367         pdev->class = &block_class;
 368         pdev->type = &part_type;
 369         pdev->parent = ddev;
 370
 371         err = blk_alloc_devt(p, &devt);
 372         if (err)
 373                 goto out_free_info;
 374         pdev->devt = devt;
 375
 376         /* delay uevent until 'holders' subdir is created */
 377         dev_set_uevent_suppress(pdev, 1);
 378         err = device_add(pdev);
 379         if (err)
 380                 goto out_put;
 381
 382         err = -ENOMEM;
 383         p->holder_dir = kobject_create_and_add("holders", &pdev->kobj);
 384         if (!p->holder_dir)
 385                 goto out_del;
 386
 387         dev_set_uevent_suppress(pdev, 0);
 388         if (flags & ADDPART_FLAG_WHOLEDISK) {
 389                 err = device_create_file(pdev, &dev_attr_whole_disk);
 390                 if (err)
 391                         goto out_del;
 392         }
 393
 394         err = hd_ref_init(p);
 395         if (err) {
 396                 if (flags & ADDPART_FLAG_WHOLEDISK)
 397                         goto out_remove_file;
 398                 goto out_del;
 399         }
 400
 401         /* everything is up and running, commence */
 402         rcu_assign_pointer(ptbl->part[partno], p);
 403
 404         /* suppress uevent if the disk suppresses it */
 405         if (!dev_get_uevent_suppress(ddev))
 406                 kobject_uevent(&pdev->kobj, KOBJ_ADD);
 407         return p;
 408
 409 out_free_info:
 410         free_part_info(p);
 411 out_free_stats:
 412         free_part_stats(p);
 413 out_free:
 414         kfree(p);
 415         return ERR_PTR(err);
 416 out_remove_file:
 417         device_remove_file(pdev, &dev_attr_whole_disk);
 418 out_del:
 419         kobject_put(p->holder_dir);
 420         device_del(pdev);
 421 out_put:
 422         put_device(pdev);
 423         return ERR_PTR(err);
 424 }
 425
 426 static bool disk_unlock_native_capacity(struct gendisk *disk)
 427 {
 428         const struct block_device_operations *bdops = disk->fops;
 429
 430         if (bdops->unlock_native_capacity &&
 431             !(disk->flags & GENHD_FL_NATIVE_CAPACITY)) {
 432                 printk(KERN_CONT "enabling native capacity\n");
 433                 bdops->unlock_native_capacity(disk);
 434                 disk->flags |= GENHD_FL_NATIVE_CAPACITY;
 435                 return true;
 436         } else {
 437                 printk(KERN_CONT "truncated\n");
 438                 return false;
 439         }
 440 }
 441
 442 static int drop_partitions(struct gendisk *disk, struct block_device *bdev)
 443 {
 444         struct disk_part_iter piter;
 445         struct hd_struct *part;
 446         int res;
 447
 448         if (bdev->bd_part_count || bdev->bd_super)
 449                 return -EBUSY;
 450         res = invalidate_partition(disk, 0);
 451         if (res)
 452                 return res;
 453
 454         disk_part_iter_init(&piter, disk, DISK_PITER_INCL_EMPTY);
 455         while ((part = disk_part_iter_next(&piter)))
 456                 delete_partition(disk, part->partno);
 457         disk_part_iter_exit(&piter);
 458
 459         return 0;
 460 }
 461
 462 static bool part_zone_aligned(struct gendisk *disk,
 463                               struct block_device *bdev,
 464                               sector_t from, sector_t size)
 465 {
 466         unsigned int zone_sectors = bdev_zone_sectors(bdev);
 467
 468         /*
 469          * If this function is called, then the disk is a zoned block device
 470          * (host-aware or host-managed). This can be detected even if the
 471          * zoned block device support is disabled (CONFIG_BLK_DEV_ZONED not
 472          * set). In this case, however, only host-aware devices will be seen
 473          * as a block device is not created for host-managed devices. Without
 474          * zoned block device support, host-aware drives can still be used as
 475          * regular block devices (no zone operation) and their zone size will
 476          * be reported as 0. Allow this case.
 477          */
 478         if (!zone_sectors)
 479                 return true;
 480
 481         /*
 482          * Check partition start and size alignement. If the drive has a
 483          * smaller last runt zone, ignore it and allow the partition to
 484          * use it. Check the zone size too: it should be a power of 2 number
 485          * of sectors.
 486          */
 487         if (WARN_ON_ONCE(!is_power_of_2(zone_sectors))) {
 488                 u32 rem;
 489
 490                 div_u64_rem(from, zone_sectors, &rem);
 491                 if (rem)
 492                         return false;
 493                 if ((from + size) < get_capacity(disk)) {
 494                         div_u64_rem(size, zone_sectors, &rem);
 495                         if (rem)
 496                                 return false;
 497                 }
 498
 499         } else {
 500
 501                 if (from & (zone_sectors - 1))
 502                         return false;
 503                 if ((from + size) < get_capacity(disk) &&
 504                     (size & (zone_sectors - 1)))
 505                         return false;
 506
 507         }
 508
 509         return true;
 510 }
 511
 512 int rescan_partitions(struct gendisk *disk, struct block_device *bdev)
 513 {
 514         struct parsed_partitions *state = NULL;
 515         struct hd_struct *part;
 516         int p, highest, res;
 517 rescan:
 518         if (state && !IS_ERR(state)) {
 519                 free_partitions(state);
 520                 state = NULL;
 521         }
 522
 523         res = drop_partitions(disk, bdev);
 524         if (res)
 525                 return res;
 526
 527         if (disk->fops->revalidate_disk)
 528                 disk->fops->revalidate_disk(disk);
 529         check_disk_size_change(disk, bdev, true);
 530         bdev->bd_invalidated = 0;
 531         if (!get_capacity(disk) || !(state = check_partition(disk, bdev)))
 532                 return 0;
 533         if (IS_ERR(state)) {
 534                 /*
 535                  * I/O error reading the partition table.  If any
 536                  * partition code tried to read beyond EOD, retry
 537                  * after unlocking native capacity.
 538                  */
 539                 if (PTR_ERR(state) == -ENOSPC) {
 540                         printk(KERN_WARNING "%s: partition table beyond EOD, ",
 541                                disk->disk_name);
 542                         if (disk_unlock_native_capacity(disk))
 543                                 goto rescan;
 544                 }
 545                 return -EIO;
 546         }
 547         /*
 548          * If any partition code tried to read beyond EOD, try
 549          * unlocking native capacity even if partition table is
 550          * successfully read as we could be missing some partitions.
 551          */
 552         if (state->access_beyond_eod) {
 553                 printk(KERN_WARNING
 554                        "%s: partition table partially beyond EOD, ",
 555                        disk->disk_name);
 556                 if (disk_unlock_native_capacity(disk))
 557                         goto rescan;
 558         }
 559
 560         /* tell userspace that the media / partition table may have changed */
 561         kobject_uevent(&disk_to_dev(disk)->kobj, KOBJ_CHANGE);
 562
 563         /* Detect the highest partition number and preallocate
 564          * disk->part_tbl.  This is an optimization and not strictly
 565          * necessary.
 566          */
 567         for (p = 1, highest = 0; p < state->limit; p++)
 568                 if (state->parts[p].size)
 569                         highest = p;
 570
 571         disk_expand_part_tbl(disk, highest);
 572
 573         /* add partitions */
 574         for (p = 1; p < state->limit; p++) {
 575                 sector_t size, from;
 576
 577                 size = state->parts[p].size;
 578                 if (!size)
 579                         continue;
 580
 581                 from = state->parts[p].from;
 582                 if (from >= get_capacity(disk)) {
 583                         printk(KERN_WARNING
 584                                "%s: p%d start %llu is beyond EOD, ",
 585                                disk->disk_name, p, (unsigned long long) from);
 586                         if (disk_unlock_native_capacity(disk))
 587                                 goto rescan;
 588                         continue;
 589                 }
 590
 591                 if (from + size > get_capacity(disk)) {
 592                         printk(KERN_WARNING
 593                                "%s: p%d size %llu extends beyond EOD, ",
 594                                disk->disk_name, p, (unsigned long long) size);
 595
 596                         if (disk_unlock_native_capacity(disk)) {
 597                                 /* free state and restart */
 598                                 goto rescan;
 599                         } else {
 600                                 /*
 601                                  * we can not ignore partitions of broken tables
 602                                  * created by for example camera firmware, but
 603                                  * we limit them to the end of the disk to avoid
 604                                  * creating invalid block devices
 605                                  */
 606                                 size = get_capacity(disk) - from;
 607                         }
 608                 }
 609
 610                 /*
 611                  * On a zoned block device, partitions should be aligned on the
 612                  * device zone size (i.e. zone boundary crossing not allowed).
 613                  * Otherwise, resetting the write pointer of the last zone of
 614                  * one partition may impact the following partition.
 615                  */
 616                 if (bdev_is_zoned(bdev) &&
 617                     !part_zone_aligned(disk, bdev, from, size)) {
 618                         printk(KERN_WARNING
 619                                "%s: p%d start %llu+%llu is not zone aligned\n",
 620                                disk->disk_name, p, (unsigned long long) from,
 621                                (unsigned long long) size);
 622                         continue;
 623                 }
 624
 625                 part = add_partition(disk, p, from, size,
 626                                      state->parts[p].flags,
 627                                      &state->parts[p].info);
 628                 if (IS_ERR(part)) {
 629                         printk(KERN_ERR " %s: p%d could not be added: %ld\n",
 630                                disk->disk_name, p, -PTR_ERR(part));
 631                         continue;
 632                 }
 633 #ifdef CONFIG_BLK_DEV_MD
 634                 if (state->parts[p].flags & ADDPART_FLAG_RAID)
 635                         md_autodetect_dev(part_to_dev(part)->devt);
 636 #endif
 637         }
 638         free_partitions(state);
 639         return 0;
 640 }
 641
 642 int invalidate_partitions(struct gendisk *disk, struct block_device *bdev)
 643 {
 644         int res;
 645
 646         if (!bdev->bd_invalidated)
 647                 return 0;
 648
 649         res = drop_partitions(disk, bdev);
 650         if (res)
 651                 return res;
 652
 653         set_capacity(disk, 0);
 654         check_disk_size_change(disk, bdev, false);
 655         bdev->bd_invalidated = 0;
 656         /* tell userspace that the media / partition table may have changed */
 657         kobject_uevent(&disk_to_dev(disk)->kobj, KOBJ_CHANGE);
 658
 659         return 0;
 660 }
 661
 662 unsigned char *read_dev_sector(struct block_device *bdev, sector_t n, Sector *p)
 663 {
 664         struct address_space *mapping = bdev->bd_inode->i_mapping;
 665         struct page *page;
 666
 667         page = read_mapping_page(mapping, (pgoff_t)(n >> (PAGE_SHIFT-9)), NULL);
 668         if (!IS_ERR(page)) {
 669                 if (PageError(page))
 670                         goto fail;
 671                 p->v = page;
 672                 return (unsigned char *)page_address(page) +  ((n & ((1 << (PAGE_SHIFT - 9)) - 1)) << 9);
 673 fail:
 674                 put_page(page);
 675         }
 676         p->v = NULL;
 677         return NULL;
 678 }
 679
 680 EXPORT_SYMBOL(read_dev_sector);