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