mm/swap.c

   1 /*
   2  *  linux/mm/swap.c
   3  *
   4  *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
   5  */
   6
   7 /*
   8  * This file contains the default values for the operation of the
   9  * Linux VM subsystem. Fine-tuning documentation can be found in
  10  * Documentation/sysctl/vm.txt.
  11  * Started 18.12.91
  12  * Swap aging added 23.2.95, Stephen Tweedie.
  13  * Buffermem limits added 12.3.98, Rik van Riel.
  14  */
  15
  16 #include <linux/mm.h>
  17 #include <linux/sched.h>
  18 #include <linux/kernel_stat.h>
  19 #include <linux/swap.h>
  20 #include <linux/mman.h>
  21 #include <linux/pagemap.h>
  22 #include <linux/pagevec.h>
  23 #include <linux/init.h>
  24 #include <linux/module.h>
  25 #include <linux/mm_inline.h>
  26 #include <linux/buffer_head.h>  /* for try_to_release_page() */
  27 #include <linux/percpu_counter.h>
  28 #include <linux/percpu.h>
  29 #include <linux/cpu.h>
  30 #include <linux/notifier.h>
  31 #include <linux/backing-dev.h>
  32 #include <linux/memcontrol.h>
  33 #include <linux/gfp.h>
  34
  35 #include "internal.h"
  36
  37 /* How many pages do we try to swap or page in/out together? */
  38 int page_cluster;
  39
  40 static DEFINE_PER_CPU(struct pagevec[NR_LRU_LISTS], lru_add_pvecs);
  41 static DEFINE_PER_CPU(struct pagevec, lru_rotate_pvecs);
  42 static DEFINE_PER_CPU(struct pagevec, lru_deactivate_pvecs);
  43
  44 /*
  45  * This path almost never happens for VM activity - pages are normally
  46  * freed via pagevecs.  But it gets used by networking.
  47  */
  48 static void __page_cache_release(struct page *page)
  49 {
  50         if (PageLRU(page)) {
  51                 unsigned long flags;
  52                 struct zone *zone = page_zone(page);
  53
  54                 spin_lock_irqsave(&zone->lru_lock, flags);
  55                 VM_BUG_ON(!PageLRU(page));
  56                 __ClearPageLRU(page);
  57                 del_page_from_lru(zone, page);
  58                 spin_unlock_irqrestore(&zone->lru_lock, flags);
  59         }
  60 }
  61
  62 static void __put_single_page(struct page *page)
  63 {
  64         __page_cache_release(page);
  65         free_hot_cold_page(page, 0);
  66 }
  67
  68 static void __put_compound_page(struct page *page)
  69 {
  70         compound_page_dtor *dtor;
  71
  72         __page_cache_release(page);
  73         dtor = get_compound_page_dtor(page);
  74         (*dtor)(page);
  75 }
  76
  77 static void put_compound_page(struct page *page)
  78 {
  79         if (unlikely(PageTail(page))) {
  80                 /* __split_huge_page_refcount can run under us */
  81                 struct page *page_head = page->first_page;
  82                 smp_rmb();
  83                 /*
  84                  * If PageTail is still set after smp_rmb() we can be sure
  85                  * that the page->first_page we read wasn't a dangling pointer.
  86                  * See __split_huge_page_refcount() smp_wmb().
  87                  */
  88                 if (likely(PageTail(page) && get_page_unless_zero(page_head))) {
  89                         unsigned long flags;
  90                         /*
  91                          * Verify that our page_head wasn't converted
  92                          * to a a regular page before we got a
  93                          * reference on it.
  94                          */
  95                         if (unlikely(!PageHead(page_head))) {
  96                                 /* PageHead is cleared after PageTail */
  97                                 smp_rmb();
  98                                 VM_BUG_ON(PageTail(page));
  99                                 goto out_put_head;
 100                         }
 101                         /*
 102                          * Only run compound_lock on a valid PageHead,
 103                          * after having it pinned with
 104                          * get_page_unless_zero() above.
 105                          */
 106                         smp_mb();
 107                         /* page_head wasn't a dangling pointer */
 108                         flags = compound_lock_irqsave(page_head);
 109                         if (unlikely(!PageTail(page))) {
 110                                 /* __split_huge_page_refcount run before us */
 111                                 compound_unlock_irqrestore(page_head, flags);
 112                                 VM_BUG_ON(PageHead(page_head));
 113                         out_put_head:
 114                                 if (put_page_testzero(page_head))
 115                                         __put_single_page(page_head);
 116                         out_put_single:
 117                                 if (put_page_testzero(page))
 118                                         __put_single_page(page);
 119                                 return;
 120                         }
 121                         VM_BUG_ON(page_head != page->first_page);
 122                         /*
 123                          * We can release the refcount taken by
 124                          * get_page_unless_zero now that
 125                          * split_huge_page_refcount is blocked on the
 126                          * compound_lock.
 127                          */
 128                         if (put_page_testzero(page_head))
 129                                 VM_BUG_ON(1);
 130                         /* __split_huge_page_refcount will wait now */
 131                         VM_BUG_ON(atomic_read(&page->_count) <= 0);
 132                         atomic_dec(&page->_count);
 133                         VM_BUG_ON(atomic_read(&page_head->_count) <= 0);
 134                         compound_unlock_irqrestore(page_head, flags);
 135                         if (put_page_testzero(page_head)) {
 136                                 if (PageHead(page_head))
 137                                         __put_compound_page(page_head);
 138                                 else
 139                                         __put_single_page(page_head);
 140                         }
 141                 } else {
 142                         /* page_head is a dangling pointer */
 143                         VM_BUG_ON(PageTail(page));
 144                         goto out_put_single;
 145                 }
 146         } else if (put_page_testzero(page)) {
 147                 if (PageHead(page))
 148                         __put_compound_page(page);
 149                 else
 150                         __put_single_page(page);
 151         }
 152 }
 153
 154 void put_page(struct page *page)
 155 {
 156         if (unlikely(PageCompound(page)))
 157                 put_compound_page(page);
 158         else if (put_page_testzero(page))
 159                 __put_single_page(page);
 160 }
 161 EXPORT_SYMBOL(put_page);
 162
 163 /**
 164  * put_pages_list() - release a list of pages
 165  * @pages: list of pages threaded on page->lru
 166  *
 167  * Release a list of pages which are strung together on page.lru.  Currently
 168  * used by read_cache_pages() and related error recovery code.
 169  */
 170 void put_pages_list(struct list_head *pages)
 171 {
 172         while (!list_empty(pages)) {
 173                 struct page *victim;
 174
 175                 victim = list_entry(pages->prev, struct page, lru);
 176                 list_del(&victim->lru);
 177                 page_cache_release(victim);
 178         }
 179 }
 180 EXPORT_SYMBOL(put_pages_list);
 181
 182 static void pagevec_lru_move_fn(struct pagevec *pvec,
 183                                 void (*move_fn)(struct page *page, void *arg),
 184                                 void *arg)
 185 {
 186         int i;
 187         struct zone *zone = NULL;
 188         unsigned long flags = 0;
 189
 190         for (i = 0; i < pagevec_count(pvec); i++) {
 191                 struct page *page = pvec->pages[i];
 192                 struct zone *pagezone = page_zone(page);
 193
 194                 if (pagezone != zone) {
 195                         if (zone)
 196                                 spin_unlock_irqrestore(&zone->lru_lock, flags);
 197                         zone = pagezone;
 198                         spin_lock_irqsave(&zone->lru_lock, flags);
 199                 }
 200
 201                 (*move_fn)(page, arg);
 202         }
 203         if (zone)
 204                 spin_unlock_irqrestore(&zone->lru_lock, flags);
 205         release_pages(pvec->pages, pvec->nr, pvec->cold);
 206         pagevec_reinit(pvec);
 207 }
 208
 209 static void pagevec_move_tail_fn(struct page *page, void *arg)
 210 {
 211         int *pgmoved = arg;
 212         struct zone *zone = page_zone(page);
 213
 214         if (PageLRU(page) && !PageActive(page) && !PageUnevictable(page)) {
 215                 enum lru_list lru = page_lru_base_type(page);
 216                 list_move_tail(&page->lru, &zone->lru[lru].list);
 217                 mem_cgroup_rotate_reclaimable_page(page);
 218                 (*pgmoved)++;
 219         }
 220 }
 221
 222 /*
 223  * pagevec_move_tail() must be called with IRQ disabled.
 224  * Otherwise this may cause nasty races.
 225  */
 226 static void pagevec_move_tail(struct pagevec *pvec)
 227 {
 228         int pgmoved = 0;
 229
 230         pagevec_lru_move_fn(pvec, pagevec_move_tail_fn, &pgmoved);
 231         __count_vm_events(PGROTATED, pgmoved);
 232 }
 233
 234 /*
 235  * Writeback is about to end against a page which has been marked for immediate
 236  * reclaim.  If it still appears to be reclaimable, move it to the tail of the
 237  * inactive list.
 238  */
 239 void rotate_reclaimable_page(struct page *page)
 240 {
 241         if (!PageLocked(page) && !PageDirty(page) && !PageActive(page) &&
 242             !PageUnevictable(page) && PageLRU(page)) {
 243                 struct pagevec *pvec;
 244                 unsigned long flags;
 245
 246                 page_cache_get(page);
 247                 local_irq_save(flags);
 248                 pvec = &__get_cpu_var(lru_rotate_pvecs);
 249                 if (!pagevec_add(pvec, page))
 250                         pagevec_move_tail(pvec);
 251                 local_irq_restore(flags);
 252         }
 253 }
 254
 255 static void update_page_reclaim_stat(struct zone *zone, struct page *page,
 256                                      int file, int rotated)
 257 {
 258         struct zone_reclaim_stat *reclaim_stat = &zone->reclaim_stat;
 259         struct zone_reclaim_stat *memcg_reclaim_stat;
 260
 261         memcg_reclaim_stat = mem_cgroup_get_reclaim_stat_from_page(page);
 262
 263         reclaim_stat->recent_scanned[file]++;
 264         if (rotated)
 265                 reclaim_stat->recent_rotated[file]++;
 266
 267         if (!memcg_reclaim_stat)
 268                 return;
 269
 270         memcg_reclaim_stat->recent_scanned[file]++;
 271         if (rotated)
 272                 memcg_reclaim_stat->recent_rotated[file]++;
 273 }
 274
 275 /*
 276  * FIXME: speed this up?
 277  */
 278 void activate_page(struct page *page)
 279 {
 280         struct zone *zone = page_zone(page);
 281
 282         spin_lock_irq(&zone->lru_lock);
 283         if (PageLRU(page) && !PageActive(page) && !PageUnevictable(page)) {
 284                 int file = page_is_file_cache(page);
 285                 int lru = page_lru_base_type(page);
 286                 del_page_from_lru_list(zone, page, lru);
 287
 288                 SetPageActive(page);
 289                 lru += LRU_ACTIVE;
 290                 add_page_to_lru_list(zone, page, lru);
 291                 __count_vm_event(PGACTIVATE);
 292
 293                 update_page_reclaim_stat(zone, page, file, 1);
 294         }
 295         spin_unlock_irq(&zone->lru_lock);
 296 }
 297
 298 /*
 299  * Mark a page as having seen activity.
 300  *
 301  * inactive,unreferenced        ->      inactive,referenced
 302  * inactive,referenced          ->      active,unreferenced
 303  * active,unreferenced          ->      active,referenced
 304  */
 305 void mark_page_accessed(struct page *page)
 306 {
 307         if (!PageActive(page) && !PageUnevictable(page) &&
 308                         PageReferenced(page) && PageLRU(page)) {
 309                 activate_page(page);
 310                 ClearPageReferenced(page);
 311         } else if (!PageReferenced(page)) {
 312                 SetPageReferenced(page);
 313         }
 314 }
 315
 316 EXPORT_SYMBOL(mark_page_accessed);
 317
 318 void __lru_cache_add(struct page *page, enum lru_list lru)
 319 {
 320         struct pagevec *pvec = &get_cpu_var(lru_add_pvecs)[lru];
 321
 322         page_cache_get(page);
 323         if (!pagevec_add(pvec, page))
 324                 ____pagevec_lru_add(pvec, lru);
 325         put_cpu_var(lru_add_pvecs);
 326 }
 327 EXPORT_SYMBOL(__lru_cache_add);
 328
 329 /**
 330  * lru_cache_add_lru - add a page to a page list
 331  * @page: the page to be added to the LRU.
 332  * @lru: the LRU list to which the page is added.
 333  */
 334 void lru_cache_add_lru(struct page *page, enum lru_list lru)
 335 {
 336         if (PageActive(page)) {
 337                 VM_BUG_ON(PageUnevictable(page));
 338                 ClearPageActive(page);
 339         } else if (PageUnevictable(page)) {
 340                 VM_BUG_ON(PageActive(page));
 341                 ClearPageUnevictable(page);
 342         }
 343
 344         VM_BUG_ON(PageLRU(page) || PageActive(page) || PageUnevictable(page));
 345         __lru_cache_add(page, lru);
 346 }
 347
 348 /**
 349  * add_page_to_unevictable_list - add a page to the unevictable list
 350  * @page:  the page to be added to the unevictable list
 351  *
 352  * Add page directly to its zone's unevictable list.  To avoid races with
 353  * tasks that might be making the page evictable, through eg. munlock,
 354  * munmap or exit, while it's not on the lru, we want to add the page
 355  * while it's locked or otherwise "invisible" to other tasks.  This is
 356  * difficult to do when using the pagevec cache, so bypass that.
 357  */
 358 void add_page_to_unevictable_list(struct page *page)
 359 {
 360         struct zone *zone = page_zone(page);
 361
 362         spin_lock_irq(&zone->lru_lock);
 363         SetPageUnevictable(page);
 364         SetPageLRU(page);
 365         add_page_to_lru_list(zone, page, LRU_UNEVICTABLE);
 366         spin_unlock_irq(&zone->lru_lock);
 367 }
 368
 369 /*
 370  * If the page can not be invalidated, it is moved to the
 371  * inactive list to speed up its reclaim.  It is moved to the
 372  * head of the list, rather than the tail, to give the flusher
 373  * threads some time to write it out, as this is much more
 374  * effective than the single-page writeout from reclaim.
 375  *
 376  * If the page isn't page_mapped and dirty/writeback, the page
 377  * could reclaim asap using PG_reclaim.
 378  *
 379  * 1. active, mapped page -> none
 380  * 2. active, dirty/writeback page -> inactive, head, PG_reclaim
 381  * 3. inactive, mapped page -> none
 382  * 4. inactive, dirty/writeback page -> inactive, head, PG_reclaim
 383  * 5. inactive, clean -> inactive, tail
 384  * 6. Others -> none
 385  *
 386  * In 4, why it moves inactive's head, the VM expects the page would
 387  * be write it out by flusher threads as this is much more effective
 388  * than the single-page writeout from reclaim.
 389  */
 390 static void lru_deactivate_fn(struct page *page, void *arg)
 391 {
 392         int lru, file;
 393         bool active;
 394         struct zone *zone = page_zone(page);
 395
 396         if (!PageLRU(page))
 397                 return;
 398
 399         if (PageUnevictable(page))
 400                 return;
 401
 402         /* Some processes are using the page */
 403         if (page_mapped(page))
 404                 return;
 405
 406         active = PageActive(page);
 407
 408         file = page_is_file_cache(page);
 409         lru = page_lru_base_type(page);
 410         del_page_from_lru_list(zone, page, lru + active);
 411         ClearPageActive(page);
 412         ClearPageReferenced(page);
 413         add_page_to_lru_list(zone, page, lru);
 414
 415         if (PageWriteback(page) || PageDirty(page)) {
 416                 /*
 417                  * PG_reclaim could be raced with end_page_writeback
 418                  * It can make readahead confusing.  But race window
 419                  * is _really_ small and  it's non-critical problem.
 420                  */
 421                 SetPageReclaim(page);
 422         } else {
 423                 /*
 424                  * The page's writeback ends up during pagevec
 425                  * We moves tha page into tail of inactive.
 426                  */
 427                 list_move_tail(&page->lru, &zone->lru[lru].list);
 428                 mem_cgroup_rotate_reclaimable_page(page);
 429                 __count_vm_event(PGROTATED);
 430         }
 431
 432         if (active)
 433                 __count_vm_event(PGDEACTIVATE);
 434         update_page_reclaim_stat(zone, page, file, 0);
 435 }
 436
 437 /*
 438  * Drain pages out of the cpu's pagevecs.
 439  * Either "cpu" is the current CPU, and preemption has already been
 440  * disabled; or "cpu" is being hot-unplugged, and is already dead.
 441  */
 442 static void drain_cpu_pagevecs(int cpu)
 443 {
 444         struct pagevec *pvecs = per_cpu(lru_add_pvecs, cpu);
 445         struct pagevec *pvec;
 446         int lru;
 447
 448         for_each_lru(lru) {
 449                 pvec = &pvecs[lru - LRU_BASE];
 450                 if (pagevec_count(pvec))
 451                         ____pagevec_lru_add(pvec, lru);
 452         }
 453
 454         pvec = &per_cpu(lru_rotate_pvecs, cpu);
 455         if (pagevec_count(pvec)) {
 456                 unsigned long flags;
 457
 458                 /* No harm done if a racing interrupt already did this */
 459                 local_irq_save(flags);
 460                 pagevec_move_tail(pvec);
 461                 local_irq_restore(flags);
 462         }
 463
 464         pvec = &per_cpu(lru_deactivate_pvecs, cpu);
 465         if (pagevec_count(pvec))
 466                 pagevec_lru_move_fn(pvec, lru_deactivate_fn, NULL);
 467 }
 468
 469 /**
 470  * deactivate_page - forcefully deactivate a page
 471  * @page: page to deactivate
 472  *
 473  * This function hints the VM that @page is a good reclaim candidate,
 474  * for example if its invalidation fails due to the page being dirty
 475  * or under writeback.
 476  */
 477 void deactivate_page(struct page *page)
 478 {
 479         /*
 480          * In a workload with many unevictable page such as mprotect, unevictable
 481          * page deactivation for accelerating reclaim is pointless.
 482          */
 483         if (PageUnevictable(page))
 484                 return;
 485
 486         if (likely(get_page_unless_zero(page))) {
 487                 struct pagevec *pvec = &get_cpu_var(lru_deactivate_pvecs);
 488
 489                 if (!pagevec_add(pvec, page))
 490                         pagevec_lru_move_fn(pvec, lru_deactivate_fn, NULL);
 491                 put_cpu_var(lru_deactivate_pvecs);
 492         }
 493 }
 494
 495 void lru_add_drain(void)
 496 {
 497         drain_cpu_pagevecs(get_cpu());
 498         put_cpu();
 499 }
 500
 501 static void lru_add_drain_per_cpu(struct work_struct *dummy)
 502 {
 503         lru_add_drain();
 504 }
 505
 506 /*
 507  * Returns 0 for success
 508  */
 509 int lru_add_drain_all(void)
 510 {
 511         return schedule_on_each_cpu(lru_add_drain_per_cpu);
 512 }
 513
 514 /*
 515  * Batched page_cache_release().  Decrement the reference count on all the
 516  * passed pages.  If it fell to zero then remove the page from the LRU and
 517  * free it.
 518  *
 519  * Avoid taking zone->lru_lock if possible, but if it is taken, retain it
 520  * for the remainder of the operation.
 521  *
 522  * The locking in this function is against shrink_inactive_list(): we recheck
 523  * the page count inside the lock to see whether shrink_inactive_list()
 524  * grabbed the page via the LRU.  If it did, give up: shrink_inactive_list()
 525  * will free it.
 526  */
 527 void release_pages(struct page **pages, int nr, int cold)
 528 {
 529         int i;
 530         struct pagevec pages_to_free;
 531         struct zone *zone = NULL;
 532         unsigned long uninitialized_var(flags);
 533
 534         pagevec_init(&pages_to_free, cold);
 535         for (i = 0; i < nr; i++) {
 536                 struct page *page = pages[i];
 537
 538                 if (unlikely(PageCompound(page))) {
 539                         if (zone) {
 540                                 spin_unlock_irqrestore(&zone->lru_lock, flags);
 541                                 zone = NULL;
 542                         }
 543                         put_compound_page(page);
 544                         continue;
 545                 }
 546
 547                 if (!put_page_testzero(page))
 548                         continue;
 549
 550                 if (PageLRU(page)) {
 551                         struct zone *pagezone = page_zone(page);
 552
 553                         if (pagezone != zone) {
 554                                 if (zone)
 555                                         spin_unlock_irqrestore(&zone->lru_lock,
 556                                                                         flags);
 557                                 zone = pagezone;
 558                                 spin_lock_irqsave(&zone->lru_lock, flags);
 559                         }
 560                         VM_BUG_ON(!PageLRU(page));
 561                         __ClearPageLRU(page);
 562                         del_page_from_lru(zone, page);
 563                 }
 564
 565                 if (!pagevec_add(&pages_to_free, page)) {
 566                         if (zone) {
 567                                 spin_unlock_irqrestore(&zone->lru_lock, flags);
 568                                 zone = NULL;
 569                         }
 570                         __pagevec_free(&pages_to_free);
 571                         pagevec_reinit(&pages_to_free);
 572                 }
 573         }
 574         if (zone)
 575                 spin_unlock_irqrestore(&zone->lru_lock, flags);
 576
 577         pagevec_free(&pages_to_free);
 578 }
 579 EXPORT_SYMBOL(release_pages);
 580
 581 /*
 582  * The pages which we're about to release may be in the deferred lru-addition
 583  * queues.  That would prevent them from really being freed right now.  That's
 584  * OK from a correctness point of view but is inefficient - those pages may be
 585  * cache-warm and we want to give them back to the page allocator ASAP.
 586  *
 587  * So __pagevec_release() will drain those queues here.  __pagevec_lru_add()
 588  * and __pagevec_lru_add_active() call release_pages() directly to avoid
 589  * mutual recursion.
 590  */
 591 void __pagevec_release(struct pagevec *pvec)
 592 {
 593         lru_add_drain();
 594         release_pages(pvec->pages, pagevec_count(pvec), pvec->cold);
 595         pagevec_reinit(pvec);
 596 }
 597
 598 EXPORT_SYMBOL(__pagevec_release);
 599
 600 /* used by __split_huge_page_refcount() */
 601 void lru_add_page_tail(struct zone* zone,
 602                        struct page *page, struct page *page_tail)
 603 {
 604         int active;
 605         enum lru_list lru;
 606         const int file = 0;
 607         struct list_head *head;
 608
 609         VM_BUG_ON(!PageHead(page));
 610         VM_BUG_ON(PageCompound(page_tail));
 611         VM_BUG_ON(PageLRU(page_tail));
 612         VM_BUG_ON(!spin_is_locked(&zone->lru_lock));
 613
 614         SetPageLRU(page_tail);
 615
 616         if (page_evictable(page_tail, NULL)) {
 617                 if (PageActive(page)) {
 618                         SetPageActive(page_tail);
 619                         active = 1;
 620                         lru = LRU_ACTIVE_ANON;
 621                 } else {
 622                         active = 0;
 623                         lru = LRU_INACTIVE_ANON;
 624                 }
 625                 update_page_reclaim_stat(zone, page_tail, file, active);
 626                 if (likely(PageLRU(page)))
 627                         head = page->lru.prev;
 628                 else
 629                         head = &zone->lru[lru].list;
 630                 __add_page_to_lru_list(zone, page_tail, lru, head);
 631         } else {
 632                 SetPageUnevictable(page_tail);
 633                 add_page_to_lru_list(zone, page_tail, LRU_UNEVICTABLE);
 634         }
 635 }
 636
 637 static void ____pagevec_lru_add_fn(struct page *page, void *arg)
 638 {
 639         enum lru_list lru = (enum lru_list)arg;
 640         struct zone *zone = page_zone(page);
 641         int file = is_file_lru(lru);
 642         int active = is_active_lru(lru);
 643
 644         VM_BUG_ON(PageActive(page));
 645         VM_BUG_ON(PageUnevictable(page));
 646         VM_BUG_ON(PageLRU(page));
 647
 648         SetPageLRU(page);
 649         if (active)
 650                 SetPageActive(page);
 651         update_page_reclaim_stat(zone, page, file, active);
 652         add_page_to_lru_list(zone, page, lru);
 653 }
 654
 655 /*
 656  * Add the passed pages to the LRU, then drop the caller's refcount
 657  * on them.  Reinitialises the caller's pagevec.
 658  */
 659 void ____pagevec_lru_add(struct pagevec *pvec, enum lru_list lru)
 660 {
 661         VM_BUG_ON(is_unevictable_lru(lru));
 662
 663         pagevec_lru_move_fn(pvec, ____pagevec_lru_add_fn, (void *)lru);
 664 }
 665
 666 EXPORT_SYMBOL(____pagevec_lru_add);
 667
 668 /*
 669  * Try to drop buffers from the pages in a pagevec
 670  */
 671 void pagevec_strip(struct pagevec *pvec)
 672 {
 673         int i;
 674
 675         for (i = 0; i < pagevec_count(pvec); i++) {
 676                 struct page *page = pvec->pages[i];
 677
 678                 if (page_has_private(page) && trylock_page(page)) {
 679                         if (page_has_private(page))
 680                                 try_to_release_page(page, 0);
 681                         unlock_page(page);
 682                 }
 683         }
 684 }
 685
 686 /**
 687  * pagevec_lookup - gang pagecache lookup
 688  * @pvec:       Where the resulting pages are placed
 689  * @mapping:    The address_space to search
 690  * @start:      The starting page index
 691  * @nr_pages:   The maximum number of pages
 692  *
 693  * pagevec_lookup() will search for and return a group of up to @nr_pages pages
 694  * in the mapping.  The pages are placed in @pvec.  pagevec_lookup() takes a
 695  * reference against the pages in @pvec.
 696  *
 697  * The search returns a group of mapping-contiguous pages with ascending
 698  * indexes.  There may be holes in the indices due to not-present pages.
 699  *
 700  * pagevec_lookup() returns the number of pages which were found.
 701  */
 702 unsigned pagevec_lookup(struct pagevec *pvec, struct address_space *mapping,
 703                 pgoff_t start, unsigned nr_pages)
 704 {
 705         pvec->nr = find_get_pages(mapping, start, nr_pages, pvec->pages);
 706         return pagevec_count(pvec);
 707 }
 708
 709 EXPORT_SYMBOL(pagevec_lookup);
 710
 711 unsigned pagevec_lookup_tag(struct pagevec *pvec, struct address_space *mapping,
 712                 pgoff_t *index, int tag, unsigned nr_pages)
 713 {
 714         pvec->nr = find_get_pages_tag(mapping, index, tag,
 715                                         nr_pages, pvec->pages);
 716         return pagevec_count(pvec);
 717 }
 718
 719 EXPORT_SYMBOL(pagevec_lookup_tag);
 720
 721 /*
 722  * Perform any setup for the swap system
 723  */
 724 void __init swap_setup(void)
 725 {
 726         unsigned long megs = totalram_pages >> (20 - PAGE_SHIFT);
 727
 728 #ifdef CONFIG_SWAP
 729         bdi_init(swapper_space.backing_dev_info);
 730 #endif
 731
 732         /* Use a smaller cluster for small-memory machines */
 733         if (megs < 16)
 734                 page_cluster = 2;
 735         else
 736                 page_cluster = 3;
 737         /*
 738          * Right now other parts of the system means that we
 739          * _really_ don't want to cluster much more
 740          */
 741 }