mm/vmscan.c

   1 /*
   2  *  linux/mm/vmscan.c
   3  *
   4  *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
   5  *
   6  *  Swap reorganised 29.12.95, Stephen Tweedie.
   7  *  kswapd added: 7.1.96  sct
   8  *  Removed kswapd_ctl limits, and swap out as many pages as needed
   9  *  to bring the system back to freepages.high: 2.4.97, Rik van Riel.
  10  *  Zone aware kswapd started 02/00, Kanoj Sarcar (kanoj@sgi.com).
  11  *  Multiqueue VM started 5.8.00, Rik van Riel.
  12  */
  13
  14 #include <linux/mm.h>
  15 #include <linux/module.h>
  16 #include <linux/slab.h>
  17 #include <linux/kernel_stat.h>
  18 #include <linux/swap.h>
  19 #include <linux/pagemap.h>
  20 #include <linux/init.h>
  21 #include <linux/highmem.h>
  22 #include <linux/file.h>
  23 #include <linux/writeback.h>
  24 #include <linux/blkdev.h>
  25 #include <linux/buffer_head.h>  /* for try_to_release_page(),
  26                                         buffer_heads_over_limit */
  27 #include <linux/mm_inline.h>
  28 #include <linux/pagevec.h>
  29 #include <linux/backing-dev.h>
  30 #include <linux/rmap.h>
  31 #include <linux/topology.h>
  32 #include <linux/cpu.h>
  33 #include <linux/cpuset.h>
  34 #include <linux/notifier.h>
  35 #include <linux/rwsem.h>
  36
  37 #include <asm/tlbflush.h>
  38 #include <asm/div64.h>
  39
  40 #include <linux/swapops.h>
  41
  42 /* possible outcome of pageout() */
  43 typedef enum {
  44         /* failed to write page out, page is locked */
  45         PAGE_KEEP,
  46         /* move page to the active list, page is locked */
  47         PAGE_ACTIVATE,
  48         /* page has been sent to the disk successfully, page is unlocked */
  49         PAGE_SUCCESS,
  50         /* page is clean and locked */
  51         PAGE_CLEAN,
  52 } pageout_t;
  53
  54 struct scan_control {
  55         /* Ask refill_inactive_zone, or shrink_cache to scan this many pages */
  56         unsigned long nr_to_scan;
  57
  58         /* Incremented by the number of inactive pages that were scanned */
  59         unsigned long nr_scanned;
  60
  61         /* Incremented by the number of pages reclaimed */
  62         unsigned long nr_reclaimed;
  63
  64         unsigned long nr_mapped;        /* From page_state */
  65
  66         /* Ask shrink_caches, or shrink_zone to scan at this priority */
  67         unsigned int priority;
  68
  69         /* This context's GFP mask */
  70         gfp_t gfp_mask;
  71
  72         int may_writepage;
  73
  74         /* This context's SWAP_CLUSTER_MAX. If freeing memory for
  75          * suspend, we effectively ignore SWAP_CLUSTER_MAX.
  76          * In this context, it doesn't matter that we scan the
  77          * whole list at once. */
  78         int swap_cluster_max;
  79 };
  80
  81 /*
  82  * The list of shrinker callbacks used by to apply pressure to
  83  * ageable caches.
  84  */
  85 struct shrinker {
  86         shrinker_t              shrinker;
  87         struct list_head        list;
  88         int                     seeks;  /* seeks to recreate an obj */
  89         long                    nr;     /* objs pending delete */
  90 };
  91
  92 #define lru_to_page(_head) (list_entry((_head)->prev, struct page, lru))
  93
  94 #ifdef ARCH_HAS_PREFETCH
  95 #define prefetch_prev_lru_page(_page, _base, _field)                    \
  96         do {                                                            \
  97                 if ((_page)->lru.prev != _base) {                       \
  98                         struct page *prev;                              \
  99                                                                         \
 100                         prev = lru_to_page(&(_page->lru));              \
 101                         prefetch(&prev->_field);                        \
 102                 }                                                       \
 103         } while (0)
 104 #else
 105 #define prefetch_prev_lru_page(_page, _base, _field) do { } while (0)
 106 #endif
 107
 108 #ifdef ARCH_HAS_PREFETCHW
 109 #define prefetchw_prev_lru_page(_page, _base, _field)                   \
 110         do {                                                            \
 111                 if ((_page)->lru.prev != _base) {                       \
 112                         struct page *prev;                              \
 113                                                                         \
 114                         prev = lru_to_page(&(_page->lru));              \
 115                         prefetchw(&prev->_field);                       \
 116                 }                                                       \
 117         } while (0)
 118 #else
 119 #define prefetchw_prev_lru_page(_page, _base, _field) do { } while (0)
 120 #endif
 121
 122 /*
 123  * From 0 .. 100.  Higher means more swappy.
 124  */
 125 int vm_swappiness = 60;
 126 static long total_memory;
 127
 128 static LIST_HEAD(shrinker_list);
 129 static DECLARE_RWSEM(shrinker_rwsem);
 130
 131 /*
 132  * Add a shrinker callback to be called from the vm
 133  */
 134 struct shrinker *set_shrinker(int seeks, shrinker_t theshrinker)
 135 {
 136         struct shrinker *shrinker;
 137
 138         shrinker = kmalloc(sizeof(*shrinker), GFP_KERNEL);
 139         if (shrinker) {
 140                 shrinker->shrinker = theshrinker;
 141                 shrinker->seeks = seeks;
 142                 shrinker->nr = 0;
 143                 down_write(&shrinker_rwsem);
 144                 list_add_tail(&shrinker->list, &shrinker_list);
 145                 up_write(&shrinker_rwsem);
 146         }
 147         return shrinker;
 148 }
 149 EXPORT_SYMBOL(set_shrinker);
 150
 151 /*
 152  * Remove one
 153  */
 154 void remove_shrinker(struct shrinker *shrinker)
 155 {
 156         down_write(&shrinker_rwsem);
 157         list_del(&shrinker->list);
 158         up_write(&shrinker_rwsem);
 159         kfree(shrinker);
 160 }
 161 EXPORT_SYMBOL(remove_shrinker);
 162
 163 #define SHRINK_BATCH 128
 164 /*
 165  * Call the shrink functions to age shrinkable caches
 166  *
 167  * Here we assume it costs one seek to replace a lru page and that it also
 168  * takes a seek to recreate a cache object.  With this in mind we age equal
 169  * percentages of the lru and ageable caches.  This should balance the seeks
 170  * generated by these structures.
 171  *
 172  * If the vm encounted mapped pages on the LRU it increase the pressure on
 173  * slab to avoid swapping.
 174  *
 175  * We do weird things to avoid (scanned*seeks*entries) overflowing 32 bits.
 176  *
 177  * `lru_pages' represents the number of on-LRU pages in all the zones which
 178  * are eligible for the caller's allocation attempt.  It is used for balancing
 179  * slab reclaim versus page reclaim.
 180  *
 181  * Returns the number of slab objects which we shrunk.
 182  */
 183 int shrink_slab(unsigned long scanned, gfp_t gfp_mask, unsigned long lru_pages)
 184 {
 185         struct shrinker *shrinker;
 186         int ret = 0;
 187
 188         if (scanned == 0)
 189                 scanned = SWAP_CLUSTER_MAX;
 190
 191         if (!down_read_trylock(&shrinker_rwsem))
 192                 return 1;       /* Assume we'll be able to shrink next time */
 193
 194         list_for_each_entry(shrinker, &shrinker_list, list) {
 195                 unsigned long long delta;
 196                 unsigned long total_scan;
 197                 unsigned long max_pass = (*shrinker->shrinker)(0, gfp_mask);
 198
 199                 delta = (4 * scanned) / shrinker->seeks;
 200                 delta *= max_pass;
 201                 do_div(delta, lru_pages + 1);
 202                 shrinker->nr += delta;
 203                 if (shrinker->nr < 0) {
 204                         printk(KERN_ERR "%s: nr=%ld\n",
 205                                         __FUNCTION__, shrinker->nr);
 206                         shrinker->nr = max_pass;
 207                 }
 208
 209                 /*
 210                  * Avoid risking looping forever due to too large nr value:
 211                  * never try to free more than twice the estimate number of
 212                  * freeable entries.
 213                  */
 214                 if (shrinker->nr > max_pass * 2)
 215                         shrinker->nr = max_pass * 2;
 216
 217                 total_scan = shrinker->nr;
 218                 shrinker->nr = 0;
 219
 220                 while (total_scan >= SHRINK_BATCH) {
 221                         long this_scan = SHRINK_BATCH;
 222                         int shrink_ret;
 223                         int nr_before;
 224
 225                         nr_before = (*shrinker->shrinker)(0, gfp_mask);
 226                         shrink_ret = (*shrinker->shrinker)(this_scan, gfp_mask);
 227                         if (shrink_ret == -1)
 228                                 break;
 229                         if (shrink_ret < nr_before)
 230                                 ret += nr_before - shrink_ret;
 231                         mod_page_state(slabs_scanned, this_scan);
 232                         total_scan -= this_scan;
 233
 234                         cond_resched();
 235                 }
 236
 237                 shrinker->nr += total_scan;
 238         }
 239         up_read(&shrinker_rwsem);
 240         return ret;
 241 }
 242
 243 /* Called without lock on whether page is mapped, so answer is unstable */
 244 static inline int page_mapping_inuse(struct page *page)
 245 {
 246         struct address_space *mapping;
 247
 248         /* Page is in somebody's page tables. */
 249         if (page_mapped(page))
 250                 return 1;
 251
 252         /* Be more reluctant to reclaim swapcache than pagecache */
 253         if (PageSwapCache(page))
 254                 return 1;
 255
 256         mapping = page_mapping(page);
 257         if (!mapping)
 258                 return 0;
 259
 260         /* File is mmap'd by somebody? */
 261         return mapping_mapped(mapping);
 262 }
 263
 264 static inline int is_page_cache_freeable(struct page *page)
 265 {
 266         return page_count(page) - !!PagePrivate(page) == 2;
 267 }
 268
 269 static int may_write_to_queue(struct backing_dev_info *bdi)
 270 {
 271         if (current_is_kswapd())
 272                 return 1;
 273         if (current_is_pdflush())       /* This is unlikely, but why not... */
 274                 return 1;
 275         if (!bdi_write_congested(bdi))
 276                 return 1;
 277         if (bdi == current->backing_dev_info)
 278                 return 1;
 279         return 0;
 280 }
 281
 282 /*
 283  * We detected a synchronous write error writing a page out.  Probably
 284  * -ENOSPC.  We need to propagate that into the address_space for a subsequent
 285  * fsync(), msync() or close().
 286  *
 287  * The tricky part is that after writepage we cannot touch the mapping: nothing
 288  * prevents it from being freed up.  But we have a ref on the page and once
 289  * that page is locked, the mapping is pinned.
 290  *
 291  * We're allowed to run sleeping lock_page() here because we know the caller has
 292  * __GFP_FS.
 293  */
 294 static void handle_write_error(struct address_space *mapping,
 295                                 struct page *page, int error)
 296 {
 297         lock_page(page);
 298         if (page_mapping(page) == mapping) {
 299                 if (error == -ENOSPC)
 300                         set_bit(AS_ENOSPC, &mapping->flags);
 301                 else
 302                         set_bit(AS_EIO, &mapping->flags);
 303         }
 304         unlock_page(page);
 305 }
 306
 307 /*
 308  * pageout is called by shrink_list() for each dirty page. Calls ->writepage().
 309  */
 310 static pageout_t pageout(struct page *page, struct address_space *mapping)
 311 {
 312         /*
 313          * If the page is dirty, only perform writeback if that write
 314          * will be non-blocking.  To prevent this allocation from being
 315          * stalled by pagecache activity.  But note that there may be
 316          * stalls if we need to run get_block().  We could test
 317          * PagePrivate for that.
 318          *
 319          * If this process is currently in generic_file_write() against
 320          * this page's queue, we can perform writeback even if that
 321          * will block.
 322          *
 323          * If the page is swapcache, write it back even if that would
 324          * block, for some throttling. This happens by accident, because
 325          * swap_backing_dev_info is bust: it doesn't reflect the
 326          * congestion state of the swapdevs.  Easy to fix, if needed.
 327          * See swapfile.c:page_queue_congested().
 328          */
 329         if (!is_page_cache_freeable(page))
 330                 return PAGE_KEEP;
 331         if (!mapping) {
 332                 /*
 333                  * Some data journaling orphaned pages can have
 334                  * page->mapping == NULL while being dirty with clean buffers.
 335                  */
 336                 if (PagePrivate(page)) {
 337                         if (try_to_free_buffers(page)) {
 338                                 ClearPageDirty(page);
 339                                 printk("%s: orphaned page\n", __FUNCTION__);
 340                                 return PAGE_CLEAN;
 341                         }
 342                 }
 343                 return PAGE_KEEP;
 344         }
 345         if (mapping->a_ops->writepage == NULL)
 346                 return PAGE_ACTIVATE;
 347         if (!may_write_to_queue(mapping->backing_dev_info))
 348                 return PAGE_KEEP;
 349
 350         if (clear_page_dirty_for_io(page)) {
 351                 int res;
 352                 struct writeback_control wbc = {
 353                         .sync_mode = WB_SYNC_NONE,
 354                         .nr_to_write = SWAP_CLUSTER_MAX,
 355                         .nonblocking = 1,
 356                         .for_reclaim = 1,
 357                 };
 358
 359                 SetPageReclaim(page);
 360                 res = mapping->a_ops->writepage(page, &wbc);
 361                 if (res < 0)
 362                         handle_write_error(mapping, page, res);
 363                 if (res == AOP_WRITEPAGE_ACTIVATE) {
 364                         ClearPageReclaim(page);
 365                         return PAGE_ACTIVATE;
 366                 }
 367                 if (!PageWriteback(page)) {
 368                         /* synchronous write or broken a_ops? */
 369                         ClearPageReclaim(page);
 370                 }
 371
 372                 return PAGE_SUCCESS;
 373         }
 374
 375         return PAGE_CLEAN;
 376 }
 377
 378 /*
 379  * shrink_list adds the number of reclaimed pages to sc->nr_reclaimed
 380  */
 381 static int shrink_list(struct list_head *page_list, struct scan_control *sc)
 382 {
 383         LIST_HEAD(ret_pages);
 384         struct pagevec freed_pvec;
 385         int pgactivate = 0;
 386         int reclaimed = 0;
 387
 388         cond_resched();
 389
 390         pagevec_init(&freed_pvec, 1);
 391         while (!list_empty(page_list)) {
 392                 struct address_space *mapping;
 393                 struct page *page;
 394                 int may_enter_fs;
 395                 int referenced;
 396
 397                 cond_resched();
 398
 399                 page = lru_to_page(page_list);
 400                 list_del(&page->lru);
 401
 402                 if (TestSetPageLocked(page))
 403                         goto keep;
 404
 405                 BUG_ON(PageActive(page));
 406
 407                 sc->nr_scanned++;
 408                 /* Double the slab pressure for mapped and swapcache pages */
 409                 if (page_mapped(page) || PageSwapCache(page))
 410                         sc->nr_scanned++;
 411
 412                 if (PageWriteback(page))
 413                         goto keep_locked;
 414
 415                 referenced = page_referenced(page, 1);
 416                 /* In active use or really unfreeable?  Activate it. */
 417                 if (referenced && page_mapping_inuse(page))
 418                         goto activate_locked;
 419
 420 #ifdef CONFIG_SWAP
 421                 /*
 422                  * Anonymous process memory has backing store?
 423                  * Try to allocate it some swap space here.
 424                  */
 425                 if (PageAnon(page) && !PageSwapCache(page)) {
 426                         if (!add_to_swap(page))
 427                                 goto activate_locked;
 428                 }
 429 #endif /* CONFIG_SWAP */
 430
 431                 mapping = page_mapping(page);
 432                 may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
 433                         (PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));
 434
 435                 /*
 436                  * The page is mapped into the page tables of one or more
 437                  * processes. Try to unmap it here.
 438                  */
 439                 if (page_mapped(page) && mapping) {
 440                         switch (try_to_unmap(page)) {
 441                         case SWAP_FAIL:
 442                                 goto activate_locked;
 443                         case SWAP_AGAIN:
 444                                 goto keep_locked;
 445                         case SWAP_SUCCESS:
 446                                 ; /* try to free the page below */
 447                         }
 448                 }
 449
 450                 if (PageDirty(page)) {
 451                         if (referenced)
 452                                 goto keep_locked;
 453                         if (!may_enter_fs)
 454                                 goto keep_locked;
 455                         if (laptop_mode && !sc->may_writepage)
 456                                 goto keep_locked;
 457
 458                         /* Page is dirty, try to write it out here */
 459                         switch(pageout(page, mapping)) {
 460                         case PAGE_KEEP:
 461                                 goto keep_locked;
 462                         case PAGE_ACTIVATE:
 463                                 goto activate_locked;
 464                         case PAGE_SUCCESS:
 465                                 if (PageWriteback(page) || PageDirty(page))
 466                                         goto keep;
 467                                 /*
 468                                  * A synchronous write - probably a ramdisk.  Go
 469                                  * ahead and try to reclaim the page.
 470                                  */
 471                                 if (TestSetPageLocked(page))
 472                                         goto keep;
 473                                 if (PageDirty(page) || PageWriteback(page))
 474                                         goto keep_locked;
 475                                 mapping = page_mapping(page);
 476                         case PAGE_CLEAN:
 477                                 ; /* try to free the page below */
 478                         }
 479                 }
 480
 481                 /*
 482                  * If the page has buffers, try to free the buffer mappings
 483                  * associated with this page. If we succeed we try to free
 484                  * the page as well.
 485                  *
 486                  * We do this even if the page is PageDirty().
 487                  * try_to_release_page() does not perform I/O, but it is
 488                  * possible for a page to have PageDirty set, but it is actually
 489                  * clean (all its buffers are clean).  This happens if the
 490                  * buffers were written out directly, with submit_bh(). ext3
 491                  * will do this, as well as the blockdev mapping.
 492                  * try_to_release_page() will discover that cleanness and will
 493                  * drop the buffers and mark the page clean - it can be freed.
 494                  *
 495                  * Rarely, pages can have buffers and no ->mapping.  These are
 496                  * the pages which were not successfully invalidated in
 497                  * truncate_complete_page().  We try to drop those buffers here
 498                  * and if that worked, and the page is no longer mapped into
 499                  * process address space (page_count == 1) it can be freed.
 500                  * Otherwise, leave the page on the LRU so it is swappable.
 501                  */
 502                 if (PagePrivate(page)) {
 503                         if (!try_to_release_page(page, sc->gfp_mask))
 504                                 goto activate_locked;
 505                         if (!mapping && page_count(page) == 1)
 506                                 goto free_it;
 507                 }
 508
 509                 if (!mapping)
 510                         goto keep_locked;       /* truncate got there first */
 511
 512                 write_lock_irq(&mapping->tree_lock);
 513
 514                 /*
 515                  * The non-racy check for busy page.  It is critical to check
 516                  * PageDirty _after_ making sure that the page is freeable and
 517                  * not in use by anybody.       (pagecache + us == 2)
 518                  */
 519                 if (unlikely(page_count(page) != 2))
 520                         goto cannot_free;
 521                 smp_rmb();
 522                 if (unlikely(PageDirty(page)))
 523                         goto cannot_free;
 524
 525 #ifdef CONFIG_SWAP
 526                 if (PageSwapCache(page)) {
 527                         swp_entry_t swap = { .val = page_private(page) };
 528                         __delete_from_swap_cache(page);
 529                         write_unlock_irq(&mapping->tree_lock);
 530                         swap_free(swap);
 531                         __put_page(page);       /* The pagecache ref */
 532                         goto free_it;
 533                 }
 534 #endif /* CONFIG_SWAP */
 535
 536                 __remove_from_page_cache(page);
 537                 write_unlock_irq(&mapping->tree_lock);
 538                 __put_page(page);
 539
 540 free_it:
 541                 unlock_page(page);
 542                 reclaimed++;
 543                 if (!pagevec_add(&freed_pvec, page))
 544                         __pagevec_release_nonlru(&freed_pvec);
 545                 continue;
 546
 547 cannot_free:
 548                 write_unlock_irq(&mapping->tree_lock);
 549                 goto keep_locked;
 550
 551 activate_locked:
 552                 SetPageActive(page);
 553                 pgactivate++;
 554 keep_locked:
 555                 unlock_page(page);
 556 keep:
 557                 list_add(&page->lru, &ret_pages);
 558                 BUG_ON(PageLRU(page));
 559         }
 560         list_splice(&ret_pages, page_list);
 561         if (pagevec_count(&freed_pvec))
 562                 __pagevec_release_nonlru(&freed_pvec);
 563         mod_page_state(pgactivate, pgactivate);
 564         sc->nr_reclaimed += reclaimed;
 565         return reclaimed;
 566 }
 567
 568 /*
 569  * zone->lru_lock is heavily contended.  Some of the functions that
 570  * shrink the lists perform better by taking out a batch of pages
 571  * and working on them outside the LRU lock.
 572  *
 573  * For pagecache intensive workloads, this function is the hottest
 574  * spot in the kernel (apart from copy_*_user functions).
 575  *
 576  * Appropriate locks must be held before calling this function.
 577  *
 578  * @nr_to_scan: The number of pages to look through on the list.
 579  * @src:        The LRU list to pull pages off.
 580  * @dst:        The temp list to put pages on to.
 581  * @scanned:    The number of pages that were scanned.
 582  *
 583  * returns how many pages were moved onto *@dst.
 584  */
 585 static int isolate_lru_pages(int nr_to_scan, struct list_head *src,
 586                              struct list_head *dst, int *scanned)
 587 {
 588         int nr_taken = 0;
 589         struct page *page;
 590         int scan = 0;
 591
 592         while (scan++ < nr_to_scan && !list_empty(src)) {
 593                 page = lru_to_page(src);
 594                 prefetchw_prev_lru_page(page, src, flags);
 595
 596                 if (!TestClearPageLRU(page))
 597                         BUG();
 598                 list_del(&page->lru);
 599                 if (get_page_testone(page)) {
 600                         /*
 601                          * It is being freed elsewhere
 602                          */
 603                         __put_page(page);
 604                         SetPageLRU(page);
 605                         list_add(&page->lru, src);
 606                         continue;
 607                 } else {
 608                         list_add(&page->lru, dst);
 609                         nr_taken++;
 610                 }
 611         }
 612
 613         *scanned = scan;
 614         return nr_taken;
 615 }
 616
 617 /*
 618  * shrink_cache() adds the number of pages reclaimed to sc->nr_reclaimed
 619  */
 620 static void shrink_cache(struct zone *zone, struct scan_control *sc)
 621 {
 622         LIST_HEAD(page_list);
 623         struct pagevec pvec;
 624         int max_scan = sc->nr_to_scan;
 625
 626         pagevec_init(&pvec, 1);
 627
 628         lru_add_drain();
 629         spin_lock_irq(&zone->lru_lock);
 630         while (max_scan > 0) {
 631                 struct page *page;
 632                 int nr_taken;
 633                 int nr_scan;
 634                 int nr_freed;
 635
 636                 nr_taken = isolate_lru_pages(sc->swap_cluster_max,
 637                                              &zone->inactive_list,
 638                                              &page_list, &nr_scan);
 639                 zone->nr_inactive -= nr_taken;
 640                 zone->pages_scanned += nr_scan;
 641                 spin_unlock_irq(&zone->lru_lock);
 642
 643                 if (nr_taken == 0)
 644                         goto done;
 645
 646                 max_scan -= nr_scan;
 647                 nr_freed = shrink_list(&page_list, sc);
 648
 649                 local_irq_disable();
 650                 if (current_is_kswapd()) {
 651                         __mod_page_state_zone(zone, pgscan_kswapd, nr_scan);
 652                         __mod_page_state(kswapd_steal, nr_freed);
 653                 } else
 654                         __mod_page_state_zone(zone, pgscan_direct, nr_scan);
 655                 __mod_page_state_zone(zone, pgsteal, nr_freed);
 656
 657                 spin_lock(&zone->lru_lock);
 658                 /*
 659                  * Put back any unfreeable pages.
 660                  */
 661                 while (!list_empty(&page_list)) {
 662                         page = lru_to_page(&page_list);
 663                         if (TestSetPageLRU(page))
 664                                 BUG();
 665                         list_del(&page->lru);
 666                         if (PageActive(page))
 667                                 add_page_to_active_list(zone, page);
 668                         else
 669                                 add_page_to_inactive_list(zone, page);
 670                         if (!pagevec_add(&pvec, page)) {
 671                                 spin_unlock_irq(&zone->lru_lock);
 672                                 __pagevec_release(&pvec);
 673                                 spin_lock_irq(&zone->lru_lock);
 674                         }
 675                 }
 676         }
 677         spin_unlock_irq(&zone->lru_lock);
 678 done:
 679         pagevec_release(&pvec);
 680 }
 681
 682 /*
 683  * This moves pages from the active list to the inactive list.
 684  *
 685  * We move them the other way if the page is referenced by one or more
 686  * processes, from rmap.
 687  *
 688  * If the pages are mostly unmapped, the processing is fast and it is
 689  * appropriate to hold zone->lru_lock across the whole operation.  But if
 690  * the pages are mapped, the processing is slow (page_referenced()) so we
 691  * should drop zone->lru_lock around each page.  It's impossible to balance
 692  * this, so instead we remove the pages from the LRU while processing them.
 693  * It is safe to rely on PG_active against the non-LRU pages in here because
 694  * nobody will play with that bit on a non-LRU page.
 695  *
 696  * The downside is that we have to touch page->_count against each page.
 697  * But we had to alter page->flags anyway.
 698  */
 699 static void
 700 refill_inactive_zone(struct zone *zone, struct scan_control *sc)
 701 {
 702         int pgmoved;
 703         int pgdeactivate = 0;
 704         int pgscanned;
 705         int nr_pages = sc->nr_to_scan;
 706         LIST_HEAD(l_hold);      /* The pages which were snipped off */
 707         LIST_HEAD(l_inactive);  /* Pages to go onto the inactive_list */
 708         LIST_HEAD(l_active);    /* Pages to go onto the active_list */
 709         struct page *page;
 710         struct pagevec pvec;
 711         int reclaim_mapped = 0;
 712         long mapped_ratio;
 713         long distress;
 714         long swap_tendency;
 715
 716         lru_add_drain();
 717         spin_lock_irq(&zone->lru_lock);
 718         pgmoved = isolate_lru_pages(nr_pages, &zone->active_list,
 719                                     &l_hold, &pgscanned);
 720         zone->pages_scanned += pgscanned;
 721         zone->nr_active -= pgmoved;
 722         spin_unlock_irq(&zone->lru_lock);
 723
 724         /*
 725          * `distress' is a measure of how much trouble we're having reclaiming
 726          * pages.  0 -> no problems.  100 -> great trouble.
 727          */
 728         distress = 100 >> zone->prev_priority;
 729
 730         /*
 731          * The point of this algorithm is to decide when to start reclaiming
 732          * mapped memory instead of just pagecache.  Work out how much memory
 733          * is mapped.
 734          */
 735         mapped_ratio = (sc->nr_mapped * 100) / total_memory;
 736
 737         /*
 738          * Now decide how much we really want to unmap some pages.  The mapped
 739          * ratio is downgraded - just because there's a lot of mapped memory
 740          * doesn't necessarily mean that page reclaim isn't succeeding.
 741          *
 742          * The distress ratio is important - we don't want to start going oom.
 743          *
 744          * A 100% value of vm_swappiness overrides this algorithm altogether.
 745          */
 746         swap_tendency = mapped_ratio / 2 + distress + vm_swappiness;
 747
 748         /*
 749          * Now use this metric to decide whether to start moving mapped memory
 750          * onto the inactive list.
 751          */
 752         if (swap_tendency >= 100)
 753                 reclaim_mapped = 1;
 754
 755         while (!list_empty(&l_hold)) {
 756                 cond_resched();
 757                 page = lru_to_page(&l_hold);
 758                 list_del(&page->lru);
 759                 if (page_mapped(page)) {
 760                         if (!reclaim_mapped ||
 761                             (total_swap_pages == 0 && PageAnon(page)) ||
 762                             page_referenced(page, 0)) {
 763                                 list_add(&page->lru, &l_active);
 764                                 continue;
 765                         }
 766                 }
 767                 list_add(&page->lru, &l_inactive);
 768         }
 769
 770         pagevec_init(&pvec, 1);
 771         pgmoved = 0;
 772         spin_lock_irq(&zone->lru_lock);
 773         while (!list_empty(&l_inactive)) {
 774                 page = lru_to_page(&l_inactive);
 775                 prefetchw_prev_lru_page(page, &l_inactive, flags);
 776                 if (TestSetPageLRU(page))
 777                         BUG();
 778                 if (!TestClearPageActive(page))
 779                         BUG();
 780                 list_move(&page->lru, &zone->inactive_list);
 781                 pgmoved++;
 782                 if (!pagevec_add(&pvec, page)) {
 783                         zone->nr_inactive += pgmoved;
 784                         spin_unlock_irq(&zone->lru_lock);
 785                         pgdeactivate += pgmoved;
 786                         pgmoved = 0;
 787                         if (buffer_heads_over_limit)
 788                                 pagevec_strip(&pvec);
 789                         __pagevec_release(&pvec);
 790                         spin_lock_irq(&zone->lru_lock);
 791                 }
 792         }
 793         zone->nr_inactive += pgmoved;
 794         pgdeactivate += pgmoved;
 795         if (buffer_heads_over_limit) {
 796                 spin_unlock_irq(&zone->lru_lock);
 797                 pagevec_strip(&pvec);
 798                 spin_lock_irq(&zone->lru_lock);
 799         }
 800
 801         pgmoved = 0;
 802         while (!list_empty(&l_active)) {
 803                 page = lru_to_page(&l_active);
 804                 prefetchw_prev_lru_page(page, &l_active, flags);
 805                 if (TestSetPageLRU(page))
 806                         BUG();
 807                 BUG_ON(!PageActive(page));
 808                 list_move(&page->lru, &zone->active_list);
 809                 pgmoved++;
 810                 if (!pagevec_add(&pvec, page)) {
 811                         zone->nr_active += pgmoved;
 812                         pgmoved = 0;
 813                         spin_unlock_irq(&zone->lru_lock);
 814                         __pagevec_release(&pvec);
 815                         spin_lock_irq(&zone->lru_lock);
 816                 }
 817         }
 818         zone->nr_active += pgmoved;
 819         spin_unlock(&zone->lru_lock);
 820
 821         __mod_page_state_zone(zone, pgrefill, pgscanned);
 822         __mod_page_state(pgdeactivate, pgdeactivate);
 823         local_irq_enable();
 824
 825         pagevec_release(&pvec);
 826 }
 827
 828 /*
 829  * This is a basic per-zone page freer.  Used by both kswapd and direct reclaim.
 830  */
 831 static void
 832 shrink_zone(struct zone *zone, struct scan_control *sc)
 833 {
 834         unsigned long nr_active;
 835         unsigned long nr_inactive;
 836
 837         atomic_inc(&zone->reclaim_in_progress);
 838
 839         /*
 840          * Add one to `nr_to_scan' just to make sure that the kernel will
 841          * slowly sift through the active list.
 842          */
 843         zone->nr_scan_active += (zone->nr_active >> sc->priority) + 1;
 844         nr_active = zone->nr_scan_active;
 845         if (nr_active >= sc->swap_cluster_max)
 846                 zone->nr_scan_active = 0;
 847         else
 848                 nr_active = 0;
 849
 850         zone->nr_scan_inactive += (zone->nr_inactive >> sc->priority) + 1;
 851         nr_inactive = zone->nr_scan_inactive;
 852         if (nr_inactive >= sc->swap_cluster_max)
 853                 zone->nr_scan_inactive = 0;
 854         else
 855                 nr_inactive = 0;
 856
 857         while (nr_active || nr_inactive) {
 858                 if (nr_active) {
 859                         sc->nr_to_scan = min(nr_active,
 860                                         (unsigned long)sc->swap_cluster_max);
 861                         nr_active -= sc->nr_to_scan;
 862                         refill_inactive_zone(zone, sc);
 863                 }
 864
 865                 if (nr_inactive) {
 866                         sc->nr_to_scan = min(nr_inactive,
 867                                         (unsigned long)sc->swap_cluster_max);
 868                         nr_inactive -= sc->nr_to_scan;
 869                         shrink_cache(zone, sc);
 870                 }
 871         }
 872
 873         throttle_vm_writeout();
 874
 875         atomic_dec(&zone->reclaim_in_progress);
 876 }
 877
 878 /*
 879  * This is the direct reclaim path, for page-allocating processes.  We only
 880  * try to reclaim pages from zones which will satisfy the caller's allocation
 881  * request.
 882  *
 883  * We reclaim from a zone even if that zone is over pages_high.  Because:
 884  * a) The caller may be trying to free *extra* pages to satisfy a higher-order
 885  *    allocation or
 886  * b) The zones may be over pages_high but they must go *over* pages_high to
 887  *    satisfy the `incremental min' zone defense algorithm.
 888  *
 889  * Returns the number of reclaimed pages.
 890  *
 891  * If a zone is deemed to be full of pinned pages then just give it a light
 892  * scan then give up on it.
 893  */
 894 static void
 895 shrink_caches(struct zone **zones, struct scan_control *sc)
 896 {
 897         int i;
 898
 899         for (i = 0; zones[i] != NULL; i++) {
 900                 struct zone *zone = zones[i];
 901
 902                 if (!populated_zone(zone))
 903                         continue;
 904
 905                 if (!cpuset_zone_allowed(zone, __GFP_HARDWALL))
 906                         continue;
 907
 908                 zone->temp_priority = sc->priority;
 909                 if (zone->prev_priority > sc->priority)
 910                         zone->prev_priority = sc->priority;
 911
 912                 if (zone->all_unreclaimable && sc->priority != DEF_PRIORITY)
 913                         continue;       /* Let kswapd poll it */
 914
 915                 shrink_zone(zone, sc);
 916         }
 917 }
 918
 919 /*
 920  * This is the main entry point to direct page reclaim.
 921  *
 922  * If a full scan of the inactive list fails to free enough memory then we
 923  * are "out of memory" and something needs to be killed.
 924  *
 925  * If the caller is !__GFP_FS then the probability of a failure is reasonably
 926  * high - the zone may be full of dirty or under-writeback pages, which this
 927  * caller can't do much about.  We kick pdflush and take explicit naps in the
 928  * hope that some of these pages can be written.  But if the allocating task
 929  * holds filesystem locks which prevent writeout this might not work, and the
 930  * allocation attempt will fail.
 931  */
 932 int try_to_free_pages(struct zone **zones, gfp_t gfp_mask)
 933 {
 934         int priority;
 935         int ret = 0;
 936         int total_scanned = 0, total_reclaimed = 0;
 937         struct reclaim_state *reclaim_state = current->reclaim_state;
 938         struct scan_control sc;
 939         unsigned long lru_pages = 0;
 940         int i;
 941
 942         sc.gfp_mask = gfp_mask;
 943         sc.may_writepage = 0;
 944
 945         inc_page_state(allocstall);
 946
 947         for (i = 0; zones[i] != NULL; i++) {
 948                 struct zone *zone = zones[i];
 949
 950                 if (!cpuset_zone_allowed(zone, __GFP_HARDWALL))
 951                         continue;
 952
 953                 zone->temp_priority = DEF_PRIORITY;
 954                 lru_pages += zone->nr_active + zone->nr_inactive;
 955         }
 956
 957         for (priority = DEF_PRIORITY; priority >= 0; priority--) {
 958                 sc.nr_mapped = read_page_state(nr_mapped);
 959                 sc.nr_scanned = 0;
 960                 sc.nr_reclaimed = 0;
 961                 sc.priority = priority;
 962                 sc.swap_cluster_max = SWAP_CLUSTER_MAX;
 963                 if (!priority)
 964                         disable_swap_token();
 965                 shrink_caches(zones, &sc);
 966                 shrink_slab(sc.nr_scanned, gfp_mask, lru_pages);
 967                 if (reclaim_state) {
 968                         sc.nr_reclaimed += reclaim_state->reclaimed_slab;
 969                         reclaim_state->reclaimed_slab = 0;
 970                 }
 971                 total_scanned += sc.nr_scanned;
 972                 total_reclaimed += sc.nr_reclaimed;
 973                 if (total_reclaimed >= sc.swap_cluster_max) {
 974                         ret = 1;
 975                         goto out;
 976                 }
 977
 978                 /*
 979                  * Try to write back as many pages as we just scanned.  This
 980                  * tends to cause slow streaming writers to write data to the
 981                  * disk smoothly, at the dirtying rate, which is nice.   But
 982                  * that's undesirable in laptop mode, where we *want* lumpy
 983                  * writeout.  So in laptop mode, write out the whole world.
 984                  */
 985                 if (total_scanned > sc.swap_cluster_max + sc.swap_cluster_max/2) {
 986                         wakeup_pdflush(laptop_mode ? 0 : total_scanned);
 987                         sc.may_writepage = 1;
 988                 }
 989
 990                 /* Take a nap, wait for some writeback to complete */
 991                 if (sc.nr_scanned && priority < DEF_PRIORITY - 2)
 992                         blk_congestion_wait(WRITE, HZ/10);
 993         }
 994 out:
 995         for (i = 0; zones[i] != 0; i++) {
 996                 struct zone *zone = zones[i];
 997
 998                 if (!cpuset_zone_allowed(zone, __GFP_HARDWALL))
 999                         continue;
1000
1001                 zone->prev_priority = zone->temp_priority;
1002         }
1003         return ret;
1004 }
1005
1006 /*
1007  * For kswapd, balance_pgdat() will work across all this node's zones until
1008  * they are all at pages_high.
1009  *
1010  * If `nr_pages' is non-zero then it is the number of pages which are to be
1011  * reclaimed, regardless of the zone occupancies.  This is a software suspend
1012  * special.
1013  *
1014  * Returns the number of pages which were actually freed.
1015  *
1016  * There is special handling here for zones which are full of pinned pages.
1017  * This can happen if the pages are all mlocked, or if they are all used by
1018  * device drivers (say, ZONE_DMA).  Or if they are all in use by hugetlb.
1019  * What we do is to detect the case where all pages in the zone have been
1020  * scanned twice and there has been zero successful reclaim.  Mark the zone as
1021  * dead and from now on, only perform a short scan.  Basically we're polling
1022  * the zone for when the problem goes away.
1023  *
1024  * kswapd scans the zones in the highmem->normal->dma direction.  It skips
1025  * zones which have free_pages > pages_high, but once a zone is found to have
1026  * free_pages <= pages_high, we scan that zone and the lower zones regardless
1027  * of the number of free pages in the lower zones.  This interoperates with
1028  * the page allocator fallback scheme to ensure that aging of pages is balanced
1029  * across the zones.
1030  */
1031 static int balance_pgdat(pg_data_t *pgdat, int nr_pages, int order)
1032 {
1033         int to_free = nr_pages;
1034         int all_zones_ok;
1035         int priority;
1036         int i;
1037         int total_scanned, total_reclaimed;
1038         struct reclaim_state *reclaim_state = current->reclaim_state;
1039         struct scan_control sc;
1040
1041 loop_again:
1042         total_scanned = 0;
1043         total_reclaimed = 0;
1044         sc.gfp_mask = GFP_KERNEL;
1045         sc.may_writepage = 0;
1046         sc.nr_mapped = read_page_state(nr_mapped);
1047
1048         inc_page_state(pageoutrun);
1049
1050         for (i = 0; i < pgdat->nr_zones; i++) {
1051                 struct zone *zone = pgdat->node_zones + i;
1052
1053                 zone->temp_priority = DEF_PRIORITY;
1054         }
1055
1056         for (priority = DEF_PRIORITY; priority >= 0; priority--) {
1057                 int end_zone = 0;       /* Inclusive.  0 = ZONE_DMA */
1058                 unsigned long lru_pages = 0;
1059
1060                 /* The swap token gets in the way of swapout... */
1061                 if (!priority)
1062                         disable_swap_token();
1063
1064                 all_zones_ok = 1;
1065
1066                 if (nr_pages == 0) {
1067                         /*
1068                          * Scan in the highmem->dma direction for the highest
1069                          * zone which needs scanning
1070                          */
1071                         for (i = pgdat->nr_zones - 1; i >= 0; i--) {
1072                                 struct zone *zone = pgdat->node_zones + i;
1073
1074                                 if (!populated_zone(zone))
1075                                         continue;
1076
1077                                 if (zone->all_unreclaimable &&
1078                                                 priority != DEF_PRIORITY)
1079                                         continue;
1080
1081                                 if (!zone_watermark_ok(zone, order,
1082                                                 zone->pages_high, 0, 0)) {
1083                                         end_zone = i;
1084                                         goto scan;
1085                                 }
1086                         }
1087                         goto out;
1088                 } else {
1089                         end_zone = pgdat->nr_zones - 1;
1090                 }
1091 scan:
1092                 for (i = 0; i <= end_zone; i++) {
1093                         struct zone *zone = pgdat->node_zones + i;
1094
1095                         lru_pages += zone->nr_active + zone->nr_inactive;
1096                 }
1097
1098                 /*
1099                  * Now scan the zone in the dma->highmem direction, stopping
1100                  * at the last zone which needs scanning.
1101                  *
1102                  * We do this because the page allocator works in the opposite
1103                  * direction.  This prevents the page allocator from allocating
1104                  * pages behind kswapd's direction of progress, which would
1105                  * cause too much scanning of the lower zones.
1106                  */
1107                 for (i = 0; i <= end_zone; i++) {
1108                         struct zone *zone = pgdat->node_zones + i;
1109                         int nr_slab;
1110
1111                         if (!populated_zone(zone))
1112                                 continue;
1113
1114                         if (zone->all_unreclaimable && priority != DEF_PRIORITY)
1115                                 continue;
1116
1117                         if (nr_pages == 0) {    /* Not software suspend */
1118                                 if (!zone_watermark_ok(zone, order,
1119                                                 zone->pages_high, end_zone, 0))
1120                                         all_zones_ok = 0;
1121                         }
1122                         zone->temp_priority = priority;
1123                         if (zone->prev_priority > priority)
1124                                 zone->prev_priority = priority;
1125                         sc.nr_scanned = 0;
1126                         sc.nr_reclaimed = 0;
1127                         sc.priority = priority;
1128                         sc.swap_cluster_max = nr_pages? nr_pages : SWAP_CLUSTER_MAX;
1129                         atomic_inc(&zone->reclaim_in_progress);
1130                         shrink_zone(zone, &sc);
1131                         atomic_dec(&zone->reclaim_in_progress);
1132                         reclaim_state->reclaimed_slab = 0;
1133                         nr_slab = shrink_slab(sc.nr_scanned, GFP_KERNEL,
1134                                                 lru_pages);
1135                         sc.nr_reclaimed += reclaim_state->reclaimed_slab;
1136                         total_reclaimed += sc.nr_reclaimed;
1137                         total_scanned += sc.nr_scanned;
1138                         if (zone->all_unreclaimable)
1139                                 continue;
1140                         if (nr_slab == 0 && zone->pages_scanned >=
1141                                     (zone->nr_active + zone->nr_inactive) * 4)
1142                                 zone->all_unreclaimable = 1;
1143                         /*
1144                          * If we've done a decent amount of scanning and
1145                          * the reclaim ratio is low, start doing writepage
1146                          * even in laptop mode
1147                          */
1148                         if (total_scanned > SWAP_CLUSTER_MAX * 2 &&
1149                             total_scanned > total_reclaimed+total_reclaimed/2)
1150                                 sc.may_writepage = 1;
1151                 }
1152                 if (nr_pages && to_free > total_reclaimed)
1153                         continue;       /* swsusp: need to do more work */
1154                 if (all_zones_ok)
1155                         break;          /* kswapd: all done */
1156                 /*
1157                  * OK, kswapd is getting into trouble.  Take a nap, then take
1158                  * another pass across the zones.
1159                  */
1160                 if (total_scanned && priority < DEF_PRIORITY - 2)
1161                         blk_congestion_wait(WRITE, HZ/10);
1162
1163                 /*
1164                  * We do this so kswapd doesn't build up large priorities for
1165                  * example when it is freeing in parallel with allocators. It
1166                  * matches the direct reclaim path behaviour in terms of impact
1167                  * on zone->*_priority.
1168                  */
1169                 if ((total_reclaimed >= SWAP_CLUSTER_MAX) && (!nr_pages))
1170                         break;
1171         }
1172 out:
1173         for (i = 0; i < pgdat->nr_zones; i++) {
1174                 struct zone *zone = pgdat->node_zones + i;
1175
1176                 zone->prev_priority = zone->temp_priority;
1177         }
1178         if (!all_zones_ok) {
1179                 cond_resched();
1180                 goto loop_again;
1181         }
1182
1183         return total_reclaimed;
1184 }
1185
1186 /*
1187  * The background pageout daemon, started as a kernel thread
1188  * from the init process.
1189  *
1190  * This basically trickles out pages so that we have _some_
1191  * free memory available even if there is no other activity
1192  * that frees anything up. This is needed for things like routing
1193  * etc, where we otherwise might have all activity going on in
1194  * asynchronous contexts that cannot page things out.
1195  *
1196  * If there are applications that are active memory-allocators
1197  * (most normal use), this basically shouldn't matter.
1198  */
1199 static int kswapd(void *p)
1200 {
1201         unsigned long order;
1202         pg_data_t *pgdat = (pg_data_t*)p;
1203         struct task_struct *tsk = current;
1204         DEFINE_WAIT(wait);
1205         struct reclaim_state reclaim_state = {
1206                 .reclaimed_slab = 0,
1207         };
1208         cpumask_t cpumask;
1209
1210         daemonize("kswapd%d", pgdat->node_id);
1211         cpumask = node_to_cpumask(pgdat->node_id);
1212         if (!cpus_empty(cpumask))
1213                 set_cpus_allowed(tsk, cpumask);
1214         current->reclaim_state = &reclaim_state;
1215
1216         /*
1217          * Tell the memory management that we're a "memory allocator",
1218          * and that if we need more memory we should get access to it
1219          * regardless (see "__alloc_pages()"). "kswapd" should
1220          * never get caught in the normal page freeing logic.
1221          *
1222          * (Kswapd normally doesn't need memory anyway, but sometimes
1223          * you need a small amount of memory in order to be able to
1224          * page out something else, and this flag essentially protects
1225          * us from recursively trying to free more memory as we're
1226          * trying to free the first piece of memory in the first place).
1227          */
1228         tsk->flags |= PF_MEMALLOC|PF_KSWAPD;
1229
1230         order = 0;
1231         for ( ; ; ) {
1232                 unsigned long new_order;
1233
1234                 try_to_freeze();
1235
1236                 prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
1237                 new_order = pgdat->kswapd_max_order;
1238                 pgdat->kswapd_max_order = 0;
1239                 if (order < new_order) {
1240                         /*
1241                          * Don't sleep if someone wants a larger 'order'
1242                          * allocation
1243                          */
1244                         order = new_order;
1245                 } else {
1246                         schedule();
1247                         order = pgdat->kswapd_max_order;
1248                 }
1249                 finish_wait(&pgdat->kswapd_wait, &wait);
1250
1251                 balance_pgdat(pgdat, 0, order);
1252         }
1253         return 0;
1254 }
1255
1256 /*
1257  * A zone is low on free memory, so wake its kswapd task to service it.
1258  */
1259 void wakeup_kswapd(struct zone *zone, int order)
1260 {
1261         pg_data_t *pgdat;
1262
1263         if (!populated_zone(zone))
1264                 return;
1265
1266         pgdat = zone->zone_pgdat;
1267         if (zone_watermark_ok(zone, order, zone->pages_low, 0, 0))
1268                 return;
1269         if (pgdat->kswapd_max_order < order)
1270                 pgdat->kswapd_max_order = order;
1271         if (!cpuset_zone_allowed(zone, __GFP_HARDWALL))
1272                 return;
1273         if (!waitqueue_active(&pgdat->kswapd_wait))
1274                 return;
1275         wake_up_interruptible(&pgdat->kswapd_wait);
1276 }
1277
1278 #ifdef CONFIG_PM
1279 /*
1280  * Try to free `nr_pages' of memory, system-wide.  Returns the number of freed
1281  * pages.
1282  */
1283 int shrink_all_memory(int nr_pages)
1284 {
1285         pg_data_t *pgdat;
1286         int nr_to_free = nr_pages;
1287         int ret = 0;
1288         struct reclaim_state reclaim_state = {
1289                 .reclaimed_slab = 0,
1290         };
1291
1292         current->reclaim_state = &reclaim_state;
1293         for_each_pgdat(pgdat) {
1294                 int freed;
1295                 freed = balance_pgdat(pgdat, nr_to_free, 0);
1296                 ret += freed;
1297                 nr_to_free -= freed;
1298                 if (nr_to_free <= 0)
1299                         break;
1300         }
1301         current->reclaim_state = NULL;
1302         return ret;
1303 }
1304 #endif
1305
1306 #ifdef CONFIG_HOTPLUG_CPU
1307 /* It's optimal to keep kswapds on the same CPUs as their memory, but
1308    not required for correctness.  So if the last cpu in a node goes
1309    away, we get changed to run anywhere: as the first one comes back,
1310    restore their cpu bindings. */
1311 static int __devinit cpu_callback(struct notifier_block *nfb,
1312                                   unsigned long action,
1313                                   void *hcpu)
1314 {
1315         pg_data_t *pgdat;
1316         cpumask_t mask;
1317
1318         if (action == CPU_ONLINE) {
1319                 for_each_pgdat(pgdat) {
1320                         mask = node_to_cpumask(pgdat->node_id);
1321                         if (any_online_cpu(mask) != NR_CPUS)
1322                                 /* One of our CPUs online: restore mask */
1323                                 set_cpus_allowed(pgdat->kswapd, mask);
1324                 }
1325         }
1326         return NOTIFY_OK;
1327 }
1328 #endif /* CONFIG_HOTPLUG_CPU */
1329
1330 static int __init kswapd_init(void)
1331 {
1332         pg_data_t *pgdat;
1333         swap_setup();
1334         for_each_pgdat(pgdat)
1335                 pgdat->kswapd
1336                 = find_task_by_pid(kernel_thread(kswapd, pgdat, CLONE_KERNEL));
1337         total_memory = nr_free_pagecache_pages();
1338         hotcpu_notifier(cpu_callback, 0);
1339         return 0;
1340 }
1341
1342 module_init(kswapd_init)