linux/mm/backing-dev.c
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   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#include <trace/events/writeback.h>
  14
  15static atomic_long_t bdi_seq = ATOMIC_LONG_INIT(0);
  16
  17struct backing_dev_info default_backing_dev_info = {
  18        .name           = "default",
  19        .ra_pages       = VM_MAX_READAHEAD * 1024 / PAGE_CACHE_SIZE,
  20        .state          = 0,
  21        .capabilities   = BDI_CAP_MAP_COPY,
  22};
  23EXPORT_SYMBOL_GPL(default_backing_dev_info);
  24
  25struct backing_dev_info noop_backing_dev_info = {
  26        .name           = "noop",
  27        .capabilities   = BDI_CAP_NO_ACCT_AND_WRITEBACK,
  28};
  29EXPORT_SYMBOL_GPL(noop_backing_dev_info);
  30
  31static struct class *bdi_class;
  32
  33/*
  34 * bdi_lock protects updates to bdi_list. bdi_list has RCU reader side
  35 * locking.
  36 */
  37DEFINE_SPINLOCK(bdi_lock);
  38LIST_HEAD(bdi_list);
  39
  40/* bdi_wq serves all asynchronous writeback tasks */
  41struct workqueue_struct *bdi_wq;
  42
  43void bdi_lock_two(struct bdi_writeback *wb1, struct bdi_writeback *wb2)
  44{
  45        if (wb1 < wb2) {
  46                spin_lock(&wb1->list_lock);
  47                spin_lock_nested(&wb2->list_lock, 1);
  48        } else {
  49                spin_lock(&wb2->list_lock);
  50                spin_lock_nested(&wb1->list_lock, 1);
  51        }
  52}
  53
  54#ifdef CONFIG_DEBUG_FS
  55#include <linux/debugfs.h>
  56#include <linux/seq_file.h>
  57
  58static struct dentry *bdi_debug_root;
  59
  60static void bdi_debug_init(void)
  61{
  62        bdi_debug_root = debugfs_create_dir("bdi", NULL);
  63}
  64
  65static int bdi_debug_stats_show(struct seq_file *m, void *v)
  66{
  67        struct backing_dev_info *bdi = m->private;
  68        struct bdi_writeback *wb = &bdi->wb;
  69        unsigned long background_thresh;
  70        unsigned long dirty_thresh;
  71        unsigned long bdi_thresh;
  72        unsigned long nr_dirty, nr_io, nr_more_io;
  73        struct inode *inode;
  74
  75        nr_dirty = nr_io = nr_more_io = 0;
  76        spin_lock(&wb->list_lock);
  77        list_for_each_entry(inode, &wb->b_dirty, i_wb_list)
  78                nr_dirty++;
  79        list_for_each_entry(inode, &wb->b_io, i_wb_list)
  80                nr_io++;
  81        list_for_each_entry(inode, &wb->b_more_io, i_wb_list)
  82                nr_more_io++;
  83        spin_unlock(&wb->list_lock);
  84
  85        global_dirty_limits(&background_thresh, &dirty_thresh);
  86        bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
  87
  88#define K(x) ((x) << (PAGE_SHIFT - 10))
  89        seq_printf(m,
  90                   "BdiWriteback:       %10lu kB\n"
  91                   "BdiReclaimable:     %10lu kB\n"
  92                   "BdiDirtyThresh:     %10lu kB\n"
  93                   "DirtyThresh:        %10lu kB\n"
  94                   "BackgroundThresh:   %10lu kB\n"
  95                   "BdiDirtied:         %10lu kB\n"
  96                   "BdiWritten:         %10lu kB\n"
  97                   "BdiWriteBandwidth:  %10lu kBps\n"
  98                   "b_dirty:            %10lu\n"
  99                   "b_io:               %10lu\n"
 100                   "b_more_io:          %10lu\n"
 101                   "bdi_list:           %10u\n"
 102                   "state:              %10lx\n",
 103                   (unsigned long) K(bdi_stat(bdi, BDI_WRITEBACK)),
 104                   (unsigned long) K(bdi_stat(bdi, BDI_RECLAIMABLE)),
 105                   K(bdi_thresh),
 106                   K(dirty_thresh),
 107                   K(background_thresh),
 108                   (unsigned long) K(bdi_stat(bdi, BDI_DIRTIED)),
 109                   (unsigned long) K(bdi_stat(bdi, BDI_WRITTEN)),
 110                   (unsigned long) K(bdi->write_bandwidth),
 111                   nr_dirty,
 112                   nr_io,
 113                   nr_more_io,
 114                   !list_empty(&bdi->bdi_list), bdi->state);
 115#undef K
 116
 117        return 0;
 118}
 119
 120static int bdi_debug_stats_open(struct inode *inode, struct file *file)
 121{
 122        return single_open(file, bdi_debug_stats_show, inode->i_private);
 123}
 124
 125static const struct file_operations bdi_debug_stats_fops = {
 126        .open           = bdi_debug_stats_open,
 127        .read           = seq_read,
 128        .llseek         = seq_lseek,
 129        .release        = single_release,
 130};
 131
 132static void bdi_debug_register(struct backing_dev_info *bdi, const char *name)
 133{
 134        bdi->debug_dir = debugfs_create_dir(name, bdi_debug_root);
 135        bdi->debug_stats = debugfs_create_file("stats", 0444, bdi->debug_dir,
 136                                               bdi, &bdi_debug_stats_fops);
 137}
 138
 139static void bdi_debug_unregister(struct backing_dev_info *bdi)
 140{
 141        debugfs_remove(bdi->debug_stats);
 142        debugfs_remove(bdi->debug_dir);
 143}
 144#else
 145static inline void bdi_debug_init(void)
 146{
 147}
 148static inline void bdi_debug_register(struct backing_dev_info *bdi,
 149                                      const char *name)
 150{
 151}
 152static inline void bdi_debug_unregister(struct backing_dev_info *bdi)
 153{
 154}
 155#endif
 156
 157static ssize_t read_ahead_kb_store(struct device *dev,
 158                                  struct device_attribute *attr,
 159                                  const char *buf, size_t count)
 160{
 161        struct backing_dev_info *bdi = dev_get_drvdata(dev);
 162        unsigned long read_ahead_kb;
 163        ssize_t ret;
 164
 165        ret = kstrtoul(buf, 10, &read_ahead_kb);
 166        if (ret < 0)
 167                return ret;
 168
 169        bdi->ra_pages = read_ahead_kb >> (PAGE_SHIFT - 10);
 170
 171        return count;
 172}
 173
 174#define K(pages) ((pages) << (PAGE_SHIFT - 10))
 175
 176#define BDI_SHOW(name, expr)                                            \
 177static ssize_t name##_show(struct device *dev,                          \
 178                           struct device_attribute *attr, char *page)   \
 179{                                                                       \
 180        struct backing_dev_info *bdi = dev_get_drvdata(dev);            \
 181                                                                        \
 182        return snprintf(page, PAGE_SIZE-1, "%lld\n", (long long)expr);  \
 183}
 184
 185BDI_SHOW(read_ahead_kb, K(bdi->ra_pages))
 186
 187static ssize_t min_ratio_store(struct device *dev,
 188                struct device_attribute *attr, const char *buf, size_t count)
 189{
 190        struct backing_dev_info *bdi = dev_get_drvdata(dev);
 191        unsigned int ratio;
 192        ssize_t ret;
 193
 194        ret = kstrtouint(buf, 10, &ratio);
 195        if (ret < 0)
 196                return ret;
 197
 198        ret = bdi_set_min_ratio(bdi, ratio);
 199        if (!ret)
 200                ret = count;
 201
 202        return ret;
 203}
 204BDI_SHOW(min_ratio, bdi->min_ratio)
 205
 206static ssize_t max_ratio_store(struct device *dev,
 207                struct device_attribute *attr, const char *buf, size_t count)
 208{
 209        struct backing_dev_info *bdi = dev_get_drvdata(dev);
 210        unsigned int ratio;
 211        ssize_t ret;
 212
 213        ret = kstrtouint(buf, 10, &ratio);
 214        if (ret < 0)
 215                return ret;
 216
 217        ret = bdi_set_max_ratio(bdi, ratio);
 218        if (!ret)
 219                ret = count;
 220
 221        return ret;
 222}
 223BDI_SHOW(max_ratio, bdi->max_ratio)
 224
 225static ssize_t stable_pages_required_show(struct device *dev,
 226                                          struct device_attribute *attr,
 227                                          char *page)
 228{
 229        struct backing_dev_info *bdi = dev_get_drvdata(dev);
 230
 231        return snprintf(page, PAGE_SIZE-1, "%d\n",
 232                        bdi_cap_stable_pages_required(bdi) ? 1 : 0);
 233}
 234
 235#define __ATTR_RW(attr) __ATTR(attr, 0644, attr##_show, attr##_store)
 236
 237static struct device_attribute bdi_dev_attrs[] = {
 238        __ATTR_RW(read_ahead_kb),
 239        __ATTR_RW(min_ratio),
 240        __ATTR_RW(max_ratio),
 241        __ATTR_RO(stable_pages_required),
 242        __ATTR_NULL,
 243};
 244
 245static __init int bdi_class_init(void)
 246{
 247        bdi_class = class_create(THIS_MODULE, "bdi");
 248        if (IS_ERR(bdi_class))
 249                return PTR_ERR(bdi_class);
 250
 251        bdi_class->dev_attrs = bdi_dev_attrs;
 252        bdi_debug_init();
 253        return 0;
 254}
 255postcore_initcall(bdi_class_init);
 256
 257static int __init default_bdi_init(void)
 258{
 259        int err;
 260
 261        bdi_wq = alloc_workqueue("writeback", WQ_MEM_RECLAIM | WQ_FREEZABLE |
 262                                              WQ_UNBOUND | WQ_SYSFS, 0);
 263        if (!bdi_wq)
 264                return -ENOMEM;
 265
 266        err = bdi_init(&default_backing_dev_info);
 267        if (!err)
 268                bdi_register(&default_backing_dev_info, NULL, "default");
 269        err = bdi_init(&noop_backing_dev_info);
 270
 271        return err;
 272}
 273subsys_initcall(default_bdi_init);
 274
 275int bdi_has_dirty_io(struct backing_dev_info *bdi)
 276{
 277        return wb_has_dirty_io(&bdi->wb);
 278}
 279
 280/*
 281 * This function is used when the first inode for this bdi is marked dirty. It
 282 * wakes-up the corresponding bdi thread which should then take care of the
 283 * periodic background write-out of dirty inodes. Since the write-out would
 284 * starts only 'dirty_writeback_interval' centisecs from now anyway, we just
 285 * set up a timer which wakes the bdi thread up later.
 286 *
 287 * Note, we wouldn't bother setting up the timer, but this function is on the
 288 * fast-path (used by '__mark_inode_dirty()'), so we save few context switches
 289 * by delaying the wake-up.
 290 */
 291void bdi_wakeup_thread_delayed(struct backing_dev_info *bdi)
 292{
 293        unsigned long timeout;
 294
 295        timeout = msecs_to_jiffies(dirty_writeback_interval * 10);
 296        mod_delayed_work(bdi_wq, &bdi->wb.dwork, timeout);
 297}
 298
 299/*
 300 * Remove bdi from bdi_list, and ensure that it is no longer visible
 301 */
 302static void bdi_remove_from_list(struct backing_dev_info *bdi)
 303{
 304        spin_lock_bh(&bdi_lock);
 305        list_del_rcu(&bdi->bdi_list);
 306        spin_unlock_bh(&bdi_lock);
 307
 308        synchronize_rcu_expedited();
 309
 310        /* bdi_list is now unused, clear it to mark @bdi dying */
 311        INIT_LIST_HEAD(&bdi->bdi_list);
 312}
 313
 314int bdi_register(struct backing_dev_info *bdi, struct device *parent,
 315                const char *fmt, ...)
 316{
 317        va_list args;
 318        struct device *dev;
 319
 320        if (bdi->dev)   /* The driver needs to use separate queues per device */
 321                return 0;
 322
 323        va_start(args, fmt);
 324        dev = device_create_vargs(bdi_class, parent, MKDEV(0, 0), bdi, fmt, args);
 325        va_end(args);
 326        if (IS_ERR(dev))
 327                return PTR_ERR(dev);
 328
 329        bdi->dev = dev;
 330
 331        bdi_debug_register(bdi, dev_name(dev));
 332        set_bit(BDI_registered, &bdi->state);
 333
 334        spin_lock_bh(&bdi_lock);
 335        list_add_tail_rcu(&bdi->bdi_list, &bdi_list);
 336        spin_unlock_bh(&bdi_lock);
 337
 338        trace_writeback_bdi_register(bdi);
 339        return 0;
 340}
 341EXPORT_SYMBOL(bdi_register);
 342
 343int bdi_register_dev(struct backing_dev_info *bdi, dev_t dev)
 344{
 345        return bdi_register(bdi, NULL, "%u:%u", MAJOR(dev), MINOR(dev));
 346}
 347EXPORT_SYMBOL(bdi_register_dev);
 348
 349/*
 350 * Remove bdi from the global list and shutdown any threads we have running
 351 */
 352static void bdi_wb_shutdown(struct backing_dev_info *bdi)
 353{
 354        if (!bdi_cap_writeback_dirty(bdi))
 355                return;
 356
 357        /*
 358         * Make sure nobody finds us on the bdi_list anymore
 359         */
 360        bdi_remove_from_list(bdi);
 361
 362        /*
 363         * Drain work list and shutdown the delayed_work.  At this point,
 364         * @bdi->bdi_list is empty telling bdi_Writeback_workfn() that @bdi
 365         * is dying and its work_list needs to be drained no matter what.
 366         */
 367        mod_delayed_work(bdi_wq, &bdi->wb.dwork, 0);
 368        flush_delayed_work(&bdi->wb.dwork);
 369        WARN_ON(!list_empty(&bdi->work_list));
 370
 371        /*
 372         * This shouldn't be necessary unless @bdi for some reason has
 373         * unflushed dirty IO after work_list is drained.  Do it anyway
 374         * just in case.
 375         */
 376        cancel_delayed_work_sync(&bdi->wb.dwork);
 377}
 378
 379/*
 380 * This bdi is going away now, make sure that no super_blocks point to it
 381 */
 382static void bdi_prune_sb(struct backing_dev_info *bdi)
 383{
 384        struct super_block *sb;
 385
 386        spin_lock(&sb_lock);
 387        list_for_each_entry(sb, &super_blocks, s_list) {
 388                if (sb->s_bdi == bdi)
 389                        sb->s_bdi = &default_backing_dev_info;
 390        }
 391        spin_unlock(&sb_lock);
 392}
 393
 394void bdi_unregister(struct backing_dev_info *bdi)
 395{
 396        struct device *dev = bdi->dev;
 397
 398        if (dev) {
 399                bdi_set_min_ratio(bdi, 0);
 400                trace_writeback_bdi_unregister(bdi);
 401                bdi_prune_sb(bdi);
 402
 403                bdi_wb_shutdown(bdi);
 404                bdi_debug_unregister(bdi);
 405
 406                spin_lock_bh(&bdi->wb_lock);
 407                bdi->dev = NULL;
 408                spin_unlock_bh(&bdi->wb_lock);
 409
 410                device_unregister(dev);
 411        }
 412}
 413EXPORT_SYMBOL(bdi_unregister);
 414
 415static void bdi_wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi)
 416{
 417        memset(wb, 0, sizeof(*wb));
 418
 419        wb->bdi = bdi;
 420        wb->last_old_flush = jiffies;
 421        INIT_LIST_HEAD(&wb->b_dirty);
 422        INIT_LIST_HEAD(&wb->b_io);
 423        INIT_LIST_HEAD(&wb->b_more_io);
 424        spin_lock_init(&wb->list_lock);
 425        INIT_DELAYED_WORK(&wb->dwork, bdi_writeback_workfn);
 426}
 427
 428/*
 429 * Initial write bandwidth: 100 MB/s
 430 */
 431#define INIT_BW         (100 << (20 - PAGE_SHIFT))
 432
 433int bdi_init(struct backing_dev_info *bdi)
 434{
 435        int i, err;
 436
 437        bdi->dev = NULL;
 438
 439        bdi->min_ratio = 0;
 440        bdi->max_ratio = 100;
 441        bdi->max_prop_frac = FPROP_FRAC_BASE;
 442        spin_lock_init(&bdi->wb_lock);
 443        INIT_LIST_HEAD(&bdi->bdi_list);
 444        INIT_LIST_HEAD(&bdi->work_list);
 445
 446        bdi_wb_init(&bdi->wb, bdi);
 447
 448        for (i = 0; i < NR_BDI_STAT_ITEMS; i++) {
 449                err = percpu_counter_init(&bdi->bdi_stat[i], 0);
 450                if (err)
 451                        goto err;
 452        }
 453
 454        bdi->dirty_exceeded = 0;
 455
 456        bdi->bw_time_stamp = jiffies;
 457        bdi->written_stamp = 0;
 458
 459        bdi->balanced_dirty_ratelimit = INIT_BW;
 460        bdi->dirty_ratelimit = INIT_BW;
 461        bdi->write_bandwidth = INIT_BW;
 462        bdi->avg_write_bandwidth = INIT_BW;
 463
 464        err = fprop_local_init_percpu(&bdi->completions);
 465
 466        if (err) {
 467err:
 468                while (i--)
 469                        percpu_counter_destroy(&bdi->bdi_stat[i]);
 470        }
 471
 472        return err;
 473}
 474EXPORT_SYMBOL(bdi_init);
 475
 476void bdi_destroy(struct backing_dev_info *bdi)
 477{
 478        int i;
 479
 480        /*
 481         * Splice our entries to the default_backing_dev_info, if this
 482         * bdi disappears
 483         */
 484        if (bdi_has_dirty_io(bdi)) {
 485                struct bdi_writeback *dst = &default_backing_dev_info.wb;
 486
 487                bdi_lock_two(&bdi->wb, dst);
 488                list_splice(&bdi->wb.b_dirty, &dst->b_dirty);
 489                list_splice(&bdi->wb.b_io, &dst->b_io);
 490                list_splice(&bdi->wb.b_more_io, &dst->b_more_io);
 491                spin_unlock(&bdi->wb.list_lock);
 492                spin_unlock(&dst->list_lock);
 493        }
 494
 495        bdi_unregister(bdi);
 496
 497        /*
 498         * If bdi_unregister() had already been called earlier, the dwork
 499         * could still be pending because bdi_prune_sb() can race with the
 500         * bdi_wakeup_thread_delayed() calls from __mark_inode_dirty().
 501         */
 502        cancel_delayed_work_sync(&bdi->wb.dwork);
 503
 504        for (i = 0; i < NR_BDI_STAT_ITEMS; i++)
 505                percpu_counter_destroy(&bdi->bdi_stat[i]);
 506
 507        fprop_local_destroy_percpu(&bdi->completions);
 508}
 509EXPORT_SYMBOL(bdi_destroy);
 510
 511/*
 512 * For use from filesystems to quickly init and register a bdi associated
 513 * with dirty writeback
 514 */
 515int bdi_setup_and_register(struct backing_dev_info *bdi, char *name,
 516                           unsigned int cap)
 517{
 518        char tmp[32];
 519        int err;
 520
 521        bdi->name = name;
 522        bdi->capabilities = cap;
 523        err = bdi_init(bdi);
 524        if (err)
 525                return err;
 526
 527        sprintf(tmp, "%.28s%s", name, "-%d");
 528        err = bdi_register(bdi, NULL, tmp, atomic_long_inc_return(&bdi_seq));
 529        if (err) {
 530                bdi_destroy(bdi);
 531                return err;
 532        }
 533
 534        return 0;
 535}
 536EXPORT_SYMBOL(bdi_setup_and_register);
 537
 538static wait_queue_head_t congestion_wqh[2] = {
 539                __WAIT_QUEUE_HEAD_INITIALIZER(congestion_wqh[0]),
 540                __WAIT_QUEUE_HEAD_INITIALIZER(congestion_wqh[1])
 541        };
 542static atomic_t nr_bdi_congested[2];
 543
 544void clear_bdi_congested(struct backing_dev_info *bdi, int sync)
 545{
 546        enum bdi_state bit;
 547        wait_queue_head_t *wqh = &congestion_wqh[sync];
 548
 549        bit = sync ? BDI_sync_congested : BDI_async_congested;
 550        if (test_and_clear_bit(bit, &bdi->state))
 551                atomic_dec(&nr_bdi_congested[sync]);
 552        smp_mb__after_clear_bit();
 553        if (waitqueue_active(wqh))
 554                wake_up(wqh);
 555}
 556EXPORT_SYMBOL(clear_bdi_congested);
 557
 558void set_bdi_congested(struct backing_dev_info *bdi, int sync)
 559{
 560        enum bdi_state bit;
 561
 562        bit = sync ? BDI_sync_congested : BDI_async_congested;
 563        if (!test_and_set_bit(bit, &bdi->state))
 564                atomic_inc(&nr_bdi_congested[sync]);
 565}
 566EXPORT_SYMBOL(set_bdi_congested);
 567
 568/**
 569 * congestion_wait - wait for a backing_dev to become uncongested
 570 * @sync: SYNC or ASYNC IO
 571 * @timeout: timeout in jiffies
 572 *
 573 * Waits for up to @timeout jiffies for a backing_dev (any backing_dev) to exit
 574 * write congestion.  If no backing_devs are congested then just wait for the
 575 * next write to be completed.
 576 */
 577long congestion_wait(int sync, long timeout)
 578{
 579        long ret;
 580        unsigned long start = jiffies;
 581        DEFINE_WAIT(wait);
 582        wait_queue_head_t *wqh = &congestion_wqh[sync];
 583
 584        prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
 585        ret = io_schedule_timeout(timeout);
 586        finish_wait(wqh, &wait);
 587
 588        trace_writeback_congestion_wait(jiffies_to_usecs(timeout),
 589                                        jiffies_to_usecs(jiffies - start));
 590
 591        return ret;
 592}
 593EXPORT_SYMBOL(congestion_wait);
 594
 595/**
 596 * wait_iff_congested - Conditionally wait for a backing_dev to become uncongested or a zone to complete writes
 597 * @zone: A zone to check if it is heavily congested
 598 * @sync: SYNC or ASYNC IO
 599 * @timeout: timeout in jiffies
 600 *
 601 * In the event of a congested backing_dev (any backing_dev) and the given
 602 * @zone has experienced recent congestion, this waits for up to @timeout
 603 * jiffies for either a BDI to exit congestion of the given @sync queue
 604 * or a write to complete.
 605 *
 606 * In the absence of zone congestion, cond_resched() is called to yield
 607 * the processor if necessary but otherwise does not sleep.
 608 *
 609 * The return value is 0 if the sleep is for the full timeout. Otherwise,
 610 * it is the number of jiffies that were still remaining when the function
 611 * returned. return_value == timeout implies the function did not sleep.
 612 */
 613long wait_iff_congested(struct zone *zone, int sync, long timeout)
 614{
 615        long ret;
 616        unsigned long start = jiffies;
 617        DEFINE_WAIT(wait);
 618        wait_queue_head_t *wqh = &congestion_wqh[sync];
 619
 620        /*
 621         * If there is no congestion, or heavy congestion is not being
 622         * encountered in the current zone, yield if necessary instead
 623         * of sleeping on the congestion queue
 624         */
 625        if (atomic_read(&nr_bdi_congested[sync]) == 0 ||
 626                        !zone_is_reclaim_congested(zone)) {
 627                cond_resched();
 628
 629                /* In case we scheduled, work out time remaining */
 630                ret = timeout - (jiffies - start);
 631                if (ret < 0)
 632                        ret = 0;
 633
 634                goto out;
 635        }
 636
 637        /* Sleep until uncongested or a write happens */
 638        prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
 639        ret = io_schedule_timeout(timeout);
 640        finish_wait(wqh, &wait);
 641
 642out:
 643        trace_writeback_wait_iff_congested(jiffies_to_usecs(timeout),
 644                                        jiffies_to_usecs(jiffies - start));
 645
 646        return ret;
 647}
 648EXPORT_SYMBOL(wait_iff_congested);
 649
 650int pdflush_proc_obsolete(struct ctl_table *table, int write,
 651                        void __user *buffer, size_t *lenp, loff_t *ppos)
 652{
 653        char kbuf[] = "0\n";
 654
 655        if (*ppos) {
 656                *lenp = 0;
 657                return 0;
 658        }
 659
 660        if (copy_to_user(buffer, kbuf, sizeof(kbuf)))
 661                return -EFAULT;
 662        printk_once(KERN_WARNING "%s exported in /proc is scheduled for removal\n",
 663                        table->procname);
 664
 665        *lenp = 2;
 666        *ppos += *lenp;
 667        return 2;
 668}
 669
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