mm/backing-dev.c

   1
   2 #include <linux/wait.h>
   3 #include <linux/backing-dev.h>
   4 #include <linux/kthread.h>
   5 #include <linux/freezer.h>
   6 #include <linux/fs.h>
   7 #include <linux/pagemap.h>
   8 #include <linux/mm.h>
   9 #include <linux/sched.h>
  10 #include <linux/module.h>
  11 #include <linux/writeback.h>
  12 #include <linux/device.h>
  13
  14 void default_unplug_io_fn(struct backing_dev_info *bdi, struct page *page)
  15 {
  16 }
  17 EXPORT_SYMBOL(default_unplug_io_fn);
  18
  19 struct backing_dev_info default_backing_dev_info = {
  20         .name           = "default",
  21         .ra_pages       = VM_MAX_READAHEAD * 1024 / PAGE_CACHE_SIZE,
  22         .state          = 0,
  23         .capabilities   = BDI_CAP_MAP_COPY,
  24         .unplug_io_fn   = default_unplug_io_fn,
  25 };
  26 EXPORT_SYMBOL_GPL(default_backing_dev_info);
  27
  28 static struct class *bdi_class;
  29 DEFINE_SPINLOCK(bdi_lock);
  30 LIST_HEAD(bdi_list);
  31 LIST_HEAD(bdi_pending_list);
  32
  33 static struct task_struct *sync_supers_tsk;
  34 static struct timer_list sync_supers_timer;
  35
  36 static int bdi_sync_supers(void *);
  37 static void sync_supers_timer_fn(unsigned long);
  38 static void arm_supers_timer(void);
  39
  40 static void bdi_add_default_flusher_task(struct backing_dev_info *bdi);
  41
  42 #ifdef CONFIG_DEBUG_FS
  43 #include <linux/debugfs.h>
  44 #include <linux/seq_file.h>
  45
  46 static struct dentry *bdi_debug_root;
  47
  48 static void bdi_debug_init(void)
  49 {
  50         bdi_debug_root = debugfs_create_dir("bdi", NULL);
  51 }
  52
  53 static int bdi_debug_stats_show(struct seq_file *m, void *v)
  54 {
  55         struct backing_dev_info *bdi = m->private;
  56         struct bdi_writeback *wb;
  57         unsigned long background_thresh;
  58         unsigned long dirty_thresh;
  59         unsigned long bdi_thresh;
  60         unsigned long nr_dirty, nr_io, nr_more_io, nr_wb;
  61         struct inode *inode;
  62
  63         /*
  64          * inode lock is enough here, the bdi->wb_list is protected by
  65          * RCU on the reader side
  66          */
  67         nr_wb = nr_dirty = nr_io = nr_more_io = 0;
  68         spin_lock(&inode_lock);
  69         list_for_each_entry(wb, &bdi->wb_list, list) {
  70                 nr_wb++;
  71                 list_for_each_entry(inode, &wb->b_dirty, i_list)
  72                         nr_dirty++;
  73                 list_for_each_entry(inode, &wb->b_io, i_list)
  74                         nr_io++;
  75                 list_for_each_entry(inode, &wb->b_more_io, i_list)
  76                         nr_more_io++;
  77         }
  78         spin_unlock(&inode_lock);
  79
  80         get_dirty_limits(&background_thresh, &dirty_thresh, &bdi_thresh, bdi);
  81
  82 #define K(x) ((x) << (PAGE_SHIFT - 10))
  83         seq_printf(m,
  84                    "BdiWriteback:     %8lu kB\n"
  85                    "BdiReclaimable:   %8lu kB\n"
  86                    "BdiDirtyThresh:   %8lu kB\n"
  87                    "DirtyThresh:      %8lu kB\n"
  88                    "BackgroundThresh: %8lu kB\n"
  89                    "WriteBack threads:%8lu\n"
  90                    "b_dirty:          %8lu\n"
  91                    "b_io:             %8lu\n"
  92                    "b_more_io:        %8lu\n"
  93                    "bdi_list:         %8u\n"
  94                    "state:            %8lx\n"
  95                    "wb_mask:          %8lx\n"
  96                    "wb_list:          %8u\n"
  97                    "wb_cnt:           %8u\n",
  98                    (unsigned long) K(bdi_stat(bdi, BDI_WRITEBACK)),
  99                    (unsigned long) K(bdi_stat(bdi, BDI_RECLAIMABLE)),
 100                    K(bdi_thresh), K(dirty_thresh),
 101                    K(background_thresh), nr_wb, nr_dirty, nr_io, nr_more_io,
 102                    !list_empty(&bdi->bdi_list), bdi->state, bdi->wb_mask,
 103                    !list_empty(&bdi->wb_list), bdi->wb_cnt);
 104 #undef K
 105
 106         return 0;
 107 }
 108
 109 static int bdi_debug_stats_open(struct inode *inode, struct file *file)
 110 {
 111         return single_open(file, bdi_debug_stats_show, inode->i_private);
 112 }
 113
 114 static const struct file_operations bdi_debug_stats_fops = {
 115         .open           = bdi_debug_stats_open,
 116         .read           = seq_read,
 117         .llseek         = seq_lseek,
 118         .release        = single_release,
 119 };
 120
 121 static void bdi_debug_register(struct backing_dev_info *bdi, const char *name)
 122 {
 123         bdi->debug_dir = debugfs_create_dir(name, bdi_debug_root);
 124         bdi->debug_stats = debugfs_create_file("stats", 0444, bdi->debug_dir,
 125                                                bdi, &bdi_debug_stats_fops);
 126 }
 127
 128 static void bdi_debug_unregister(struct backing_dev_info *bdi)
 129 {
 130         debugfs_remove(bdi->debug_stats);
 131         debugfs_remove(bdi->debug_dir);
 132 }
 133 #else
 134 static inline void bdi_debug_init(void)
 135 {
 136 }
 137 static inline void bdi_debug_register(struct backing_dev_info *bdi,
 138                                       const char *name)
 139 {
 140 }
 141 static inline void bdi_debug_unregister(struct backing_dev_info *bdi)
 142 {
 143 }
 144 #endif
 145
 146 static ssize_t read_ahead_kb_store(struct device *dev,
 147                                   struct device_attribute *attr,
 148                                   const char *buf, size_t count)
 149 {
 150         struct backing_dev_info *bdi = dev_get_drvdata(dev);
 151         char *end;
 152         unsigned long read_ahead_kb;
 153         ssize_t ret = -EINVAL;
 154
 155         read_ahead_kb = simple_strtoul(buf, &end, 10);
 156         if (*buf && (end[0] == '\0' || (end[0] == '\n' && end[1] == '\0'))) {
 157                 bdi->ra_pages = read_ahead_kb >> (PAGE_SHIFT - 10);
 158                 ret = count;
 159         }
 160         return ret;
 161 }
 162
 163 #define K(pages) ((pages) << (PAGE_SHIFT - 10))
 164
 165 #define BDI_SHOW(name, expr)                                            \
 166 static ssize_t name##_show(struct device *dev,                          \
 167                            struct device_attribute *attr, char *page)   \
 168 {                                                                       \
 169         struct backing_dev_info *bdi = dev_get_drvdata(dev);            \
 170                                                                         \
 171         return snprintf(page, PAGE_SIZE-1, "%lld\n", (long long)expr);  \
 172 }
 173
 174 BDI_SHOW(read_ahead_kb, K(bdi->ra_pages))
 175
 176 static ssize_t min_ratio_store(struct device *dev,
 177                 struct device_attribute *attr, const char *buf, size_t count)
 178 {
 179         struct backing_dev_info *bdi = dev_get_drvdata(dev);
 180         char *end;
 181         unsigned int ratio;
 182         ssize_t ret = -EINVAL;
 183
 184         ratio = simple_strtoul(buf, &end, 10);
 185         if (*buf && (end[0] == '\0' || (end[0] == '\n' && end[1] == '\0'))) {
 186                 ret = bdi_set_min_ratio(bdi, ratio);
 187                 if (!ret)
 188                         ret = count;
 189         }
 190         return ret;
 191 }
 192 BDI_SHOW(min_ratio, bdi->min_ratio)
 193
 194 static ssize_t max_ratio_store(struct device *dev,
 195                 struct device_attribute *attr, const char *buf, size_t count)
 196 {
 197         struct backing_dev_info *bdi = dev_get_drvdata(dev);
 198         char *end;
 199         unsigned int ratio;
 200         ssize_t ret = -EINVAL;
 201
 202         ratio = simple_strtoul(buf, &end, 10);
 203         if (*buf && (end[0] == '\0' || (end[0] == '\n' && end[1] == '\0'))) {
 204                 ret = bdi_set_max_ratio(bdi, ratio);
 205                 if (!ret)
 206                         ret = count;
 207         }
 208         return ret;
 209 }
 210 BDI_SHOW(max_ratio, bdi->max_ratio)
 211
 212 #define __ATTR_RW(attr) __ATTR(attr, 0644, attr##_show, attr##_store)
 213
 214 static struct device_attribute bdi_dev_attrs[] = {
 215         __ATTR_RW(read_ahead_kb),
 216         __ATTR_RW(min_ratio),
 217         __ATTR_RW(max_ratio),
 218         __ATTR_NULL,
 219 };
 220
 221 static __init int bdi_class_init(void)
 222 {
 223         bdi_class = class_create(THIS_MODULE, "bdi");
 224         bdi_class->dev_attrs = bdi_dev_attrs;
 225         bdi_debug_init();
 226         return 0;
 227 }
 228 postcore_initcall(bdi_class_init);
 229
 230 static int __init default_bdi_init(void)
 231 {
 232         int err;
 233
 234         sync_supers_tsk = kthread_run(bdi_sync_supers, NULL, "sync_supers");
 235         BUG_ON(IS_ERR(sync_supers_tsk));
 236
 237         init_timer(&sync_supers_timer);
 238         setup_timer(&sync_supers_timer, sync_supers_timer_fn, 0);
 239         arm_supers_timer();
 240
 241         err = bdi_init(&default_backing_dev_info);
 242         if (!err)
 243                 bdi_register(&default_backing_dev_info, NULL, "default");
 244
 245         return err;
 246 }
 247 subsys_initcall(default_bdi_init);
 248
 249 static void bdi_wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi)
 250 {
 251         memset(wb, 0, sizeof(*wb));
 252
 253         wb->bdi = bdi;
 254         wb->last_old_flush = jiffies;
 255         INIT_LIST_HEAD(&wb->b_dirty);
 256         INIT_LIST_HEAD(&wb->b_io);
 257         INIT_LIST_HEAD(&wb->b_more_io);
 258 }
 259
 260 static void bdi_task_init(struct backing_dev_info *bdi,
 261                           struct bdi_writeback *wb)
 262 {
 263         struct task_struct *tsk = current;
 264
 265         spin_lock(&bdi->wb_lock);
 266         list_add_tail_rcu(&wb->list, &bdi->wb_list);
 267         spin_unlock(&bdi->wb_lock);
 268
 269         tsk->flags |= PF_FLUSHER | PF_SWAPWRITE;
 270         set_freezable();
 271
 272         /*
 273          * Our parent may run at a different priority, just set us to normal
 274          */
 275         set_user_nice(tsk, 0);
 276 }
 277
 278 static int bdi_start_fn(void *ptr)
 279 {
 280         struct bdi_writeback *wb = ptr;
 281         struct backing_dev_info *bdi = wb->bdi;
 282         int ret;
 283
 284         /*
 285          * Add us to the active bdi_list
 286          */
 287         spin_lock(&bdi_lock);
 288         list_add(&bdi->bdi_list, &bdi_list);
 289         spin_unlock(&bdi_lock);
 290
 291         bdi_task_init(bdi, wb);
 292
 293         /*
 294          * Clear pending bit and wakeup anybody waiting to tear us down
 295          */
 296         clear_bit(BDI_pending, &bdi->state);
 297         smp_mb__after_clear_bit();
 298         wake_up_bit(&bdi->state, BDI_pending);
 299
 300         ret = bdi_writeback_task(wb);
 301
 302         /*
 303          * Remove us from the list
 304          */
 305         spin_lock(&bdi->wb_lock);
 306         list_del_rcu(&wb->list);
 307         spin_unlock(&bdi->wb_lock);
 308
 309         /*
 310          * Flush any work that raced with us exiting. No new work
 311          * will be added, since this bdi isn't discoverable anymore.
 312          */
 313         if (!list_empty(&bdi->work_list))
 314                 wb_do_writeback(wb, 1);
 315
 316         wb->task = NULL;
 317         return ret;
 318 }
 319
 320 int bdi_has_dirty_io(struct backing_dev_info *bdi)
 321 {
 322         return wb_has_dirty_io(&bdi->wb);
 323 }
 324
 325 static void bdi_flush_io(struct backing_dev_info *bdi)
 326 {
 327         struct writeback_control wbc = {
 328                 .bdi                    = bdi,
 329                 .sync_mode              = WB_SYNC_NONE,
 330                 .older_than_this        = NULL,
 331                 .range_cyclic           = 1,
 332                 .nr_to_write            = 1024,
 333         };
 334
 335         writeback_inodes_wbc(&wbc);
 336 }
 337
 338 /*
 339  * kupdated() used to do this. We cannot do it from the bdi_forker_task()
 340  * or we risk deadlocking on ->s_umount. The longer term solution would be
 341  * to implement sync_supers_bdi() or similar and simply do it from the
 342  * bdi writeback tasks individually.
 343  */
 344 static int bdi_sync_supers(void *unused)
 345 {
 346         set_user_nice(current, 0);
 347
 348         while (!kthread_should_stop()) {
 349                 set_current_state(TASK_INTERRUPTIBLE);
 350                 schedule();
 351
 352                 /*
 353                  * Do this periodically, like kupdated() did before.
 354                  */
 355                 sync_supers();
 356         }
 357
 358         return 0;
 359 }
 360
 361 static void arm_supers_timer(void)
 362 {
 363         unsigned long next;
 364
 365         next = msecs_to_jiffies(dirty_writeback_interval * 10) + jiffies;
 366         mod_timer(&sync_supers_timer, round_jiffies_up(next));
 367 }
 368
 369 static void sync_supers_timer_fn(unsigned long unused)
 370 {
 371         wake_up_process(sync_supers_tsk);
 372         arm_supers_timer();
 373 }
 374
 375 static int bdi_forker_task(void *ptr)
 376 {
 377         struct bdi_writeback *me = ptr;
 378
 379         bdi_task_init(me->bdi, me);
 380
 381         for (;;) {
 382                 struct backing_dev_info *bdi, *tmp;
 383                 struct bdi_writeback *wb;
 384
 385                 /*
 386                  * Temporary measure, we want to make sure we don't see
 387                  * dirty data on the default backing_dev_info
 388                  */
 389                 if (wb_has_dirty_io(me) || !list_empty(&me->bdi->work_list))
 390                         wb_do_writeback(me, 0);
 391
 392                 spin_lock(&bdi_lock);
 393
 394                 /*
 395                  * Check if any existing bdi's have dirty data without
 396                  * a thread registered. If so, set that up.
 397                  */
 398                 list_for_each_entry_safe(bdi, tmp, &bdi_list, bdi_list) {
 399                         if (bdi->wb.task)
 400                                 continue;
 401                         if (list_empty(&bdi->work_list) &&
 402                             !bdi_has_dirty_io(bdi))
 403                                 continue;
 404
 405                         bdi_add_default_flusher_task(bdi);
 406                 }
 407
 408                 set_current_state(TASK_INTERRUPTIBLE);
 409
 410                 if (list_empty(&bdi_pending_list)) {
 411                         unsigned long wait;
 412
 413                         spin_unlock(&bdi_lock);
 414                         wait = msecs_to_jiffies(dirty_writeback_interval * 10);
 415                         schedule_timeout(wait);
 416                         try_to_freeze();
 417                         continue;
 418                 }
 419
 420                 __set_current_state(TASK_RUNNING);
 421
 422                 /*
 423                  * This is our real job - check for pending entries in
 424                  * bdi_pending_list, and create the tasks that got added
 425                  */
 426                 bdi = list_entry(bdi_pending_list.next, struct backing_dev_info,
 427                                  bdi_list);
 428                 list_del_init(&bdi->bdi_list);
 429                 spin_unlock(&bdi_lock);
 430
 431                 wb = &bdi->wb;
 432                 wb->task = kthread_run(bdi_start_fn, wb, "flush-%s",
 433                                         dev_name(bdi->dev));
 434                 /*
 435                  * If task creation fails, then readd the bdi to
 436                  * the pending list and force writeout of the bdi
 437                  * from this forker thread. That will free some memory
 438                  * and we can try again.
 439                  */
 440                 if (IS_ERR(wb->task)) {
 441                         wb->task = NULL;
 442
 443                         /*
 444                          * Add this 'bdi' to the back, so we get
 445                          * a chance to flush other bdi's to free
 446                          * memory.
 447                          */
 448                         spin_lock(&bdi_lock);
 449                         list_add_tail(&bdi->bdi_list, &bdi_pending_list);
 450                         spin_unlock(&bdi_lock);
 451
 452                         bdi_flush_io(bdi);
 453                 }
 454         }
 455
 456         return 0;
 457 }
 458
 459 /*
 460  * Add the default flusher task that gets created for any bdi
 461  * that has dirty data pending writeout
 462  */
 463 void static bdi_add_default_flusher_task(struct backing_dev_info *bdi)
 464 {
 465         if (!bdi_cap_writeback_dirty(bdi))
 466                 return;
 467
 468         /*
 469          * Check with the helper whether to proceed adding a task. Will only
 470          * abort if we two or more simultanous calls to
 471          * bdi_add_default_flusher_task() occured, further additions will block
 472          * waiting for previous additions to finish.
 473          */
 474         if (!test_and_set_bit(BDI_pending, &bdi->state)) {
 475                 list_move_tail(&bdi->bdi_list, &bdi_pending_list);
 476
 477                 /*
 478                  * We are now on the pending list, wake up bdi_forker_task()
 479                  * to finish the job and add us back to the active bdi_list
 480                  */
 481                 wake_up_process(default_backing_dev_info.wb.task);
 482         }
 483 }
 484
 485 int bdi_register(struct backing_dev_info *bdi, struct device *parent,
 486                 const char *fmt, ...)
 487 {
 488         va_list args;
 489         int ret = 0;
 490         struct device *dev;
 491
 492         if (bdi->dev)   /* The driver needs to use separate queues per device */
 493                 goto exit;
 494
 495         va_start(args, fmt);
 496         dev = device_create_vargs(bdi_class, parent, MKDEV(0, 0), bdi, fmt, args);
 497         va_end(args);
 498         if (IS_ERR(dev)) {
 499                 ret = PTR_ERR(dev);
 500                 goto exit;
 501         }
 502
 503         spin_lock(&bdi_lock);
 504         list_add_tail(&bdi->bdi_list, &bdi_list);
 505         spin_unlock(&bdi_lock);
 506
 507         bdi->dev = dev;
 508
 509         /*
 510          * Just start the forker thread for our default backing_dev_info,
 511          * and add other bdi's to the list. They will get a thread created
 512          * on-demand when they need it.
 513          */
 514         if (bdi_cap_flush_forker(bdi)) {
 515                 struct bdi_writeback *wb = &bdi->wb;
 516
 517                 wb->task = kthread_run(bdi_forker_task, wb, "bdi-%s",
 518                                                 dev_name(dev));
 519                 if (IS_ERR(wb->task)) {
 520                         wb->task = NULL;
 521                         ret = -ENOMEM;
 522
 523                         spin_lock(&bdi_lock);
 524                         list_del(&bdi->bdi_list);
 525                         spin_unlock(&bdi_lock);
 526                         goto exit;
 527                 }
 528         }
 529
 530         bdi_debug_register(bdi, dev_name(dev));
 531 exit:
 532         return ret;
 533 }
 534 EXPORT_SYMBOL(bdi_register);
 535
 536 int bdi_register_dev(struct backing_dev_info *bdi, dev_t dev)
 537 {
 538         return bdi_register(bdi, NULL, "%u:%u", MAJOR(dev), MINOR(dev));
 539 }
 540 EXPORT_SYMBOL(bdi_register_dev);
 541
 542 /*
 543  * Remove bdi from the global list and shutdown any threads we have running
 544  */
 545 static void bdi_wb_shutdown(struct backing_dev_info *bdi)
 546 {
 547         struct bdi_writeback *wb;
 548
 549         if (!bdi_cap_writeback_dirty(bdi))
 550                 return;
 551
 552         /*
 553          * If setup is pending, wait for that to complete first
 554          */
 555         wait_on_bit(&bdi->state, BDI_pending, bdi_sched_wait,
 556                         TASK_UNINTERRUPTIBLE);
 557
 558         /*
 559          * Make sure nobody finds us on the bdi_list anymore
 560          */
 561         spin_lock(&bdi_lock);
 562         list_del(&bdi->bdi_list);
 563         spin_unlock(&bdi_lock);
 564
 565         /*
 566          * Finally, kill the kernel threads. We don't need to be RCU
 567          * safe anymore, since the bdi is gone from visibility.
 568          */
 569         list_for_each_entry(wb, &bdi->wb_list, list)
 570                 kthread_stop(wb->task);
 571 }
 572
 573 void bdi_unregister(struct backing_dev_info *bdi)
 574 {
 575         if (bdi->dev) {
 576                 if (!bdi_cap_flush_forker(bdi))
 577                         bdi_wb_shutdown(bdi);
 578                 bdi_debug_unregister(bdi);
 579                 device_unregister(bdi->dev);
 580                 bdi->dev = NULL;
 581         }
 582 }
 583 EXPORT_SYMBOL(bdi_unregister);
 584
 585 int bdi_init(struct backing_dev_info *bdi)
 586 {
 587         int i, err;
 588
 589         bdi->dev = NULL;
 590
 591         bdi->min_ratio = 0;
 592         bdi->max_ratio = 100;
 593         bdi->max_prop_frac = PROP_FRAC_BASE;
 594         spin_lock_init(&bdi->wb_lock);
 595         INIT_LIST_HEAD(&bdi->bdi_list);
 596         INIT_LIST_HEAD(&bdi->wb_list);
 597         INIT_LIST_HEAD(&bdi->work_list);
 598
 599         bdi_wb_init(&bdi->wb, bdi);
 600
 601         /*
 602          * Just one thread support for now, hard code mask and count
 603          */
 604         bdi->wb_mask = 1;
 605         bdi->wb_cnt = 1;
 606
 607         for (i = 0; i < NR_BDI_STAT_ITEMS; i++) {
 608                 err = percpu_counter_init(&bdi->bdi_stat[i], 0);
 609                 if (err)
 610                         goto err;
 611         }
 612
 613         bdi->dirty_exceeded = 0;
 614         err = prop_local_init_percpu(&bdi->completions);
 615
 616         if (err) {
 617 err:
 618                 while (i--)
 619                         percpu_counter_destroy(&bdi->bdi_stat[i]);
 620         }
 621
 622         return err;
 623 }
 624 EXPORT_SYMBOL(bdi_init);
 625
 626 void bdi_destroy(struct backing_dev_info *bdi)
 627 {
 628         int i;
 629
 630         WARN_ON(bdi_has_dirty_io(bdi));
 631
 632         bdi_unregister(bdi);
 633
 634         for (i = 0; i < NR_BDI_STAT_ITEMS; i++)
 635                 percpu_counter_destroy(&bdi->bdi_stat[i]);
 636
 637         prop_local_destroy_percpu(&bdi->completions);
 638 }
 639 EXPORT_SYMBOL(bdi_destroy);
 640
 641 static wait_queue_head_t congestion_wqh[2] = {
 642                 __WAIT_QUEUE_HEAD_INITIALIZER(congestion_wqh[0]),
 643                 __WAIT_QUEUE_HEAD_INITIALIZER(congestion_wqh[1])
 644         };
 645
 646 void clear_bdi_congested(struct backing_dev_info *bdi, int sync)
 647 {
 648         enum bdi_state bit;
 649         wait_queue_head_t *wqh = &congestion_wqh[sync];
 650
 651         bit = sync ? BDI_sync_congested : BDI_async_congested;
 652         clear_bit(bit, &bdi->state);
 653         smp_mb__after_clear_bit();
 654         if (waitqueue_active(wqh))
 655                 wake_up(wqh);
 656 }
 657 EXPORT_SYMBOL(clear_bdi_congested);
 658
 659 void set_bdi_congested(struct backing_dev_info *bdi, int sync)
 660 {
 661         enum bdi_state bit;
 662
 663         bit = sync ? BDI_sync_congested : BDI_async_congested;
 664         set_bit(bit, &bdi->state);
 665 }
 666 EXPORT_SYMBOL(set_bdi_congested);
 667
 668 /**
 669  * congestion_wait - wait for a backing_dev to become uncongested
 670  * @sync: SYNC or ASYNC IO
 671  * @timeout: timeout in jiffies
 672  *
 673  * Waits for up to @timeout jiffies for a backing_dev (any backing_dev) to exit
 674  * write congestion.  If no backing_devs are congested then just wait for the
 675  * next write to be completed.
 676  */
 677 long congestion_wait(int sync, long timeout)
 678 {
 679         long ret;
 680         DEFINE_WAIT(wait);
 681         wait_queue_head_t *wqh = &congestion_wqh[sync];
 682
 683         prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
 684         ret = io_schedule_timeout(timeout);
 685         finish_wait(wqh, &wait);
 686         return ret;
 687 }
 688 EXPORT_SYMBOL(congestion_wait);
 689