2 * mm/rmap.c - physical to virtual reverse mappings
4 * Copyright 2001, Rik van Riel <riel@conectiva.com.br>
5 * Released under the General Public License (GPL).
7 * Simple, low overhead reverse mapping scheme.
8 * Please try to keep this thing as modular as possible.
10 * Provides methods for unmapping each kind of mapped page:
11 * the anon methods track anonymous pages, and
12 * the file methods track pages belonging to an inode.
14 * Original design by Rik van Riel <riel@conectiva.com.br> 2001
15 * File methods by Dave McCracken <dmccr@us.ibm.com> 2003, 2004
16 * Anonymous methods by Andrea Arcangeli <andrea@suse.de> 2004
17 * Contributions by Hugh Dickins <hugh@veritas.com> 2003, 2004
21 * Lock ordering in mm:
23 * inode->i_mutex (while writing or truncating, not reading or faulting)
26 * When a page fault occurs in writing from user to file, down_read
27 * of mmap_sem nests within i_mutex; in sys_msync, i_mutex nests within
28 * down_read of mmap_sem; i_mutex and down_write of mmap_sem are never
29 * taken together; in truncation, i_mutex is taken outermost.
32 * page->flags PG_locked (lock_page)
33 * mapping->i_mmap_lock
35 * mm->page_table_lock or pte_lock
36 * zone->lru_lock (in mark_page_accessed, isolate_lru_page)
37 * swap_lock (in swap_duplicate, swap_info_get)
38 * mmlist_lock (in mmput, drain_mmlist and others)
39 * mapping->private_lock (in __set_page_dirty_buffers)
40 * inode_lock (in set_page_dirty's __mark_inode_dirty)
41 * sb_lock (within inode_lock in fs/fs-writeback.c)
42 * mapping->tree_lock (widely used, in set_page_dirty,
43 * in arch-dependent flush_dcache_mmap_lock,
44 * within inode_lock in __sync_single_inode)
48 #include <linux/pagemap.h>
49 #include <linux/swap.h>
50 #include <linux/swapops.h>
51 #include <linux/slab.h>
52 #include <linux/init.h>
53 #include <linux/rmap.h>
54 #include <linux/rcupdate.h>
55 #include <linux/module.h>
56 #include <linux/vs_memory.h>
58 #include <asm/tlbflush.h>
60 struct kmem_cache *anon_vma_cachep;
62 static inline void validate_anon_vma(struct vm_area_struct *find_vma)
64 #ifdef CONFIG_DEBUG_VM
65 struct anon_vma *anon_vma = find_vma->anon_vma;
66 struct vm_area_struct *vma;
67 unsigned int mapcount = 0;
70 list_for_each_entry(vma, &anon_vma->head, anon_vma_node) {
72 BUG_ON(mapcount > 100000);
80 /* This must be called under the mmap_sem. */
81 int anon_vma_prepare(struct vm_area_struct *vma)
83 struct anon_vma *anon_vma = vma->anon_vma;
86 if (unlikely(!anon_vma)) {
87 struct mm_struct *mm = vma->vm_mm;
88 struct anon_vma *allocated, *locked;
90 anon_vma = find_mergeable_anon_vma(vma);
94 spin_lock(&locked->lock);
96 anon_vma = anon_vma_alloc();
97 if (unlikely(!anon_vma))
103 /* page_table_lock to protect against threads */
104 spin_lock(&mm->page_table_lock);
105 if (likely(!vma->anon_vma)) {
106 vma->anon_vma = anon_vma;
107 list_add(&vma->anon_vma_node, &anon_vma->head);
110 spin_unlock(&mm->page_table_lock);
113 spin_unlock(&locked->lock);
114 if (unlikely(allocated))
115 anon_vma_free(allocated);
120 void __anon_vma_merge(struct vm_area_struct *vma, struct vm_area_struct *next)
122 BUG_ON(vma->anon_vma != next->anon_vma);
123 list_del(&next->anon_vma_node);
126 void __anon_vma_link(struct vm_area_struct *vma)
128 struct anon_vma *anon_vma = vma->anon_vma;
131 list_add(&vma->anon_vma_node, &anon_vma->head);
132 validate_anon_vma(vma);
136 void anon_vma_link(struct vm_area_struct *vma)
138 struct anon_vma *anon_vma = vma->anon_vma;
141 spin_lock(&anon_vma->lock);
142 list_add(&vma->anon_vma_node, &anon_vma->head);
143 validate_anon_vma(vma);
144 spin_unlock(&anon_vma->lock);
148 void anon_vma_unlink(struct vm_area_struct *vma)
150 struct anon_vma *anon_vma = vma->anon_vma;
156 spin_lock(&anon_vma->lock);
157 validate_anon_vma(vma);
158 list_del(&vma->anon_vma_node);
160 /* We must garbage collect the anon_vma if it's empty */
161 empty = list_empty(&anon_vma->head);
162 spin_unlock(&anon_vma->lock);
165 anon_vma_free(anon_vma);
168 static void anon_vma_ctor(void *data, struct kmem_cache *cachep,
171 if ((flags & (SLAB_CTOR_VERIFY|SLAB_CTOR_CONSTRUCTOR)) ==
172 SLAB_CTOR_CONSTRUCTOR) {
173 struct anon_vma *anon_vma = data;
175 spin_lock_init(&anon_vma->lock);
176 INIT_LIST_HEAD(&anon_vma->head);
180 void __init anon_vma_init(void)
182 anon_vma_cachep = kmem_cache_create("anon_vma", sizeof(struct anon_vma),
183 0, SLAB_DESTROY_BY_RCU|SLAB_PANIC, anon_vma_ctor, NULL);
187 * Getting a lock on a stable anon_vma from a page off the LRU is
188 * tricky: page_lock_anon_vma rely on RCU to guard against the races.
190 static struct anon_vma *page_lock_anon_vma(struct page *page)
192 struct anon_vma *anon_vma = NULL;
193 unsigned long anon_mapping;
196 anon_mapping = (unsigned long) page->mapping;
197 if (!(anon_mapping & PAGE_MAPPING_ANON))
199 if (!page_mapped(page))
202 anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
203 spin_lock(&anon_vma->lock);
209 #ifdef CONFIG_MIGRATION
211 * Remove an anonymous page from swap replacing the swap pte's
212 * through real pte's pointing to valid pages and then releasing
213 * the page from the swap cache.
215 * Must hold page lock on page and mmap_sem of one vma that contains
218 void remove_from_swap(struct page *page)
220 struct anon_vma *anon_vma;
221 struct vm_area_struct *vma;
222 unsigned long mapping;
224 if (!PageSwapCache(page))
227 mapping = (unsigned long)page->mapping;
229 if (!mapping || (mapping & PAGE_MAPPING_ANON) == 0)
233 * We hold the mmap_sem lock. So no need to call page_lock_anon_vma.
235 anon_vma = (struct anon_vma *) (mapping - PAGE_MAPPING_ANON);
236 spin_lock(&anon_vma->lock);
238 list_for_each_entry(vma, &anon_vma->head, anon_vma_node)
239 remove_vma_swap(vma, page);
241 spin_unlock(&anon_vma->lock);
242 delete_from_swap_cache(page);
244 EXPORT_SYMBOL(remove_from_swap);
248 * At what user virtual address is page expected in vma?
250 static inline unsigned long
251 vma_address(struct page *page, struct vm_area_struct *vma)
253 pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
254 unsigned long address;
256 address = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
257 if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
258 /* page should be within any vma from prio_tree_next */
259 BUG_ON(!PageAnon(page));
266 * At what user virtual address is page expected in vma? checking that the
267 * page matches the vma: currently only used on anon pages, by unuse_vma;
269 unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma)
271 if (PageAnon(page)) {
272 if ((void *)vma->anon_vma !=
273 (void *)page->mapping - PAGE_MAPPING_ANON)
275 } else if (page->mapping && !(vma->vm_flags & VM_NONLINEAR)) {
277 vma->vm_file->f_mapping != page->mapping)
281 return vma_address(page, vma);
285 * Check that @page is mapped at @address into @mm.
287 * On success returns with pte mapped and locked.
289 pte_t *page_check_address(struct page *page, struct mm_struct *mm,
290 unsigned long address, spinlock_t **ptlp)
298 pgd = pgd_offset(mm, address);
299 if (!pgd_present(*pgd))
302 pud = pud_offset(pgd, address);
303 if (!pud_present(*pud))
306 pmd = pmd_offset(pud, address);
307 if (!pmd_present(*pmd))
310 pte = pte_offset_map(pmd, address);
311 /* Make a quick check before getting the lock */
312 if (!pte_present(*pte)) {
317 ptl = pte_lockptr(mm, pmd);
319 if (pte_present(*pte) && page_to_pfn(page) == pte_pfn(*pte)) {
323 pte_unmap_unlock(pte, ptl);
328 * Subfunctions of page_referenced: page_referenced_one called
329 * repeatedly from either page_referenced_anon or page_referenced_file.
331 static int page_referenced_one(struct page *page,
332 struct vm_area_struct *vma, unsigned int *mapcount)
334 struct mm_struct *mm = vma->vm_mm;
335 unsigned long address;
340 address = vma_address(page, vma);
341 if (address == -EFAULT)
344 pte = page_check_address(page, mm, address, &ptl);
348 if (ptep_clear_flush_young(vma, address, pte))
351 /* Pretend the page is referenced if the task has the
352 swap token and is in the middle of a page fault. */
353 if (mm != current->mm && has_swap_token(mm) &&
354 rwsem_is_locked(&mm->mmap_sem))
358 pte_unmap_unlock(pte, ptl);
363 static int page_referenced_anon(struct page *page)
365 unsigned int mapcount;
366 struct anon_vma *anon_vma;
367 struct vm_area_struct *vma;
370 anon_vma = page_lock_anon_vma(page);
374 mapcount = page_mapcount(page);
375 list_for_each_entry(vma, &anon_vma->head, anon_vma_node) {
376 referenced += page_referenced_one(page, vma, &mapcount);
380 spin_unlock(&anon_vma->lock);
385 * page_referenced_file - referenced check for object-based rmap
386 * @page: the page we're checking references on.
388 * For an object-based mapped page, find all the places it is mapped and
389 * check/clear the referenced flag. This is done by following the page->mapping
390 * pointer, then walking the chain of vmas it holds. It returns the number
391 * of references it found.
393 * This function is only called from page_referenced for object-based pages.
395 static int page_referenced_file(struct page *page)
397 unsigned int mapcount;
398 struct address_space *mapping = page->mapping;
399 pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
400 struct vm_area_struct *vma;
401 struct prio_tree_iter iter;
405 * The caller's checks on page->mapping and !PageAnon have made
406 * sure that this is a file page: the check for page->mapping
407 * excludes the case just before it gets set on an anon page.
409 BUG_ON(PageAnon(page));
412 * The page lock not only makes sure that page->mapping cannot
413 * suddenly be NULLified by truncation, it makes sure that the
414 * structure at mapping cannot be freed and reused yet,
415 * so we can safely take mapping->i_mmap_lock.
417 BUG_ON(!PageLocked(page));
419 spin_lock(&mapping->i_mmap_lock);
422 * i_mmap_lock does not stabilize mapcount at all, but mapcount
423 * is more likely to be accurate if we note it after spinning.
425 mapcount = page_mapcount(page);
427 vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {
428 if ((vma->vm_flags & (VM_LOCKED|VM_MAYSHARE))
429 == (VM_LOCKED|VM_MAYSHARE)) {
433 referenced += page_referenced_one(page, vma, &mapcount);
438 spin_unlock(&mapping->i_mmap_lock);
443 * page_referenced - test if the page was referenced
444 * @page: the page to test
445 * @is_locked: caller holds lock on the page
447 * Quick test_and_clear_referenced for all mappings to a page,
448 * returns the number of ptes which referenced the page.
450 int page_referenced(struct page *page, int is_locked)
454 if (page_test_and_clear_young(page))
457 if (TestClearPageReferenced(page))
460 if (page_mapped(page) && page->mapping) {
462 referenced += page_referenced_anon(page);
464 referenced += page_referenced_file(page);
465 else if (TestSetPageLocked(page))
469 referenced += page_referenced_file(page);
477 * page_set_anon_rmap - setup new anonymous rmap
478 * @page: the page to add the mapping to
479 * @vma: the vm area in which the mapping is added
480 * @address: the user virtual address mapped
482 static void __page_set_anon_rmap(struct page *page,
483 struct vm_area_struct *vma, unsigned long address)
485 struct anon_vma *anon_vma = vma->anon_vma;
488 anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
489 page->mapping = (struct address_space *) anon_vma;
491 page->index = linear_page_index(vma, address);
494 * nr_mapped state can be updated without turning off
495 * interrupts because it is not modified via interrupt.
497 __inc_page_state(nr_mapped);
501 * page_add_anon_rmap - add pte mapping to an anonymous page
502 * @page: the page to add the mapping to
503 * @vma: the vm area in which the mapping is added
504 * @address: the user virtual address mapped
506 * The caller needs to hold the pte lock.
508 void page_add_anon_rmap(struct page *page,
509 struct vm_area_struct *vma, unsigned long address)
511 if (atomic_inc_and_test(&page->_mapcount))
512 __page_set_anon_rmap(page, vma, address);
513 /* else checking page index and mapping is racy */
517 * page_add_new_anon_rmap - add pte mapping to a new anonymous page
518 * @page: the page to add the mapping to
519 * @vma: the vm area in which the mapping is added
520 * @address: the user virtual address mapped
522 * Same as page_add_anon_rmap but must only be called on *new* pages.
523 * This means the inc-and-test can be bypassed.
525 void page_add_new_anon_rmap(struct page *page,
526 struct vm_area_struct *vma, unsigned long address)
528 atomic_set(&page->_mapcount, 0); /* elevate count by 1 (starts at -1) */
529 __page_set_anon_rmap(page, vma, address);
533 * page_add_file_rmap - add pte mapping to a file page
534 * @page: the page to add the mapping to
536 * The caller needs to hold the pte lock.
538 void page_add_file_rmap(struct page *page)
540 if (atomic_inc_and_test(&page->_mapcount))
541 __inc_page_state(nr_mapped);
545 * page_remove_rmap - take down pte mapping from a page
546 * @page: page to remove mapping from
548 * The caller needs to hold the pte lock.
550 void page_remove_rmap(struct page *page)
552 if (atomic_add_negative(-1, &page->_mapcount)) {
553 #ifdef CONFIG_DEBUG_VM
554 if (unlikely(page_mapcount(page) < 0)) {
555 printk (KERN_EMERG "Eeek! page_mapcount(page) went negative! (%d)\n", page_mapcount(page));
556 printk (KERN_EMERG " page->flags = %lx\n", page->flags);
557 printk (KERN_EMERG " page->count = %x\n", page_count(page));
558 printk (KERN_EMERG " page->mapping = %p\n", page->mapping);
561 BUG_ON(page_mapcount(page) < 0);
563 * It would be tidy to reset the PageAnon mapping here,
564 * but that might overwrite a racing page_add_anon_rmap
565 * which increments mapcount after us but sets mapping
566 * before us: so leave the reset to free_hot_cold_page,
567 * and remember that it's only reliable while mapped.
568 * Leaving it set also helps swapoff to reinstate ptes
569 * faster for those pages still in swapcache.
571 if (page_test_and_clear_dirty(page))
572 set_page_dirty(page);
573 __dec_page_state(nr_mapped);
578 * Subfunctions of try_to_unmap: try_to_unmap_one called
579 * repeatedly from either try_to_unmap_anon or try_to_unmap_file.
581 static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
584 struct mm_struct *mm = vma->vm_mm;
585 unsigned long address;
589 int ret = SWAP_AGAIN;
591 address = vma_address(page, vma);
592 if (address == -EFAULT)
595 pte = page_check_address(page, mm, address, &ptl);
600 * If the page is mlock()d, we cannot swap it out.
601 * If it's recently referenced (perhaps page_referenced
602 * skipped over this mm) then we should reactivate it.
604 if ((vma->vm_flags & VM_LOCKED) ||
605 (ptep_clear_flush_young(vma, address, pte)
611 /* Nuke the page table entry. */
612 flush_cache_page(vma, address, page_to_pfn(page));
613 pteval = ptep_clear_flush(vma, address, pte);
615 /* Move the dirty bit to the physical page now the pte is gone. */
616 if (pte_dirty(pteval))
617 set_page_dirty(page);
619 /* Update high watermark before we lower rss */
620 update_hiwater_rss(mm);
622 if (PageAnon(page)) {
623 swp_entry_t entry = { .val = page_private(page) };
625 * Store the swap location in the pte.
626 * See handle_pte_fault() ...
628 BUG_ON(!PageSwapCache(page));
629 swap_duplicate(entry);
630 if (list_empty(&mm->mmlist)) {
631 spin_lock(&mmlist_lock);
632 if (list_empty(&mm->mmlist))
633 list_add(&mm->mmlist, &init_mm.mmlist);
634 spin_unlock(&mmlist_lock);
636 set_pte_at(mm, address, pte, swp_entry_to_pte(entry));
637 BUG_ON(pte_file(*pte));
638 dec_mm_counter(mm, anon_rss);
640 dec_mm_counter(mm, file_rss);
642 page_remove_rmap(page);
643 page_cache_release(page);
646 pte_unmap_unlock(pte, ptl);
652 * objrmap doesn't work for nonlinear VMAs because the assumption that
653 * offset-into-file correlates with offset-into-virtual-addresses does not hold.
654 * Consequently, given a particular page and its ->index, we cannot locate the
655 * ptes which are mapping that page without an exhaustive linear search.
657 * So what this code does is a mini "virtual scan" of each nonlinear VMA which
658 * maps the file to which the target page belongs. The ->vm_private_data field
659 * holds the current cursor into that scan. Successive searches will circulate
660 * around the vma's virtual address space.
662 * So as more replacement pressure is applied to the pages in a nonlinear VMA,
663 * more scanning pressure is placed against them as well. Eventually pages
664 * will become fully unmapped and are eligible for eviction.
666 * For very sparsely populated VMAs this is a little inefficient - chances are
667 * there there won't be many ptes located within the scan cluster. In this case
668 * maybe we could scan further - to the end of the pte page, perhaps.
670 #define CLUSTER_SIZE min(32*PAGE_SIZE, PMD_SIZE)
671 #define CLUSTER_MASK (~(CLUSTER_SIZE - 1))
673 static void try_to_unmap_cluster(unsigned long cursor,
674 unsigned int *mapcount, struct vm_area_struct *vma)
676 struct mm_struct *mm = vma->vm_mm;
684 unsigned long address;
687 address = (vma->vm_start + cursor) & CLUSTER_MASK;
688 end = address + CLUSTER_SIZE;
689 if (address < vma->vm_start)
690 address = vma->vm_start;
691 if (end > vma->vm_end)
694 pgd = pgd_offset(mm, address);
695 if (!pgd_present(*pgd))
698 pud = pud_offset(pgd, address);
699 if (!pud_present(*pud))
702 pmd = pmd_offset(pud, address);
703 if (!pmd_present(*pmd))
706 pte = pte_offset_map_lock(mm, pmd, address, &ptl);
708 /* Update high watermark before we lower rss */
709 update_hiwater_rss(mm);
711 for (; address < end; pte++, address += PAGE_SIZE) {
712 if (!pte_present(*pte))
714 page = vm_normal_page(vma, address, *pte);
715 BUG_ON(!page || PageAnon(page));
717 if (ptep_clear_flush_young(vma, address, pte))
720 /* Nuke the page table entry. */
721 flush_cache_page(vma, address, pte_pfn(*pte));
722 pteval = ptep_clear_flush(vma, address, pte);
724 /* If nonlinear, store the file page offset in the pte. */
725 if (page->index != linear_page_index(vma, address))
726 set_pte_at(mm, address, pte, pgoff_to_pte(page->index));
728 /* Move the dirty bit to the physical page now the pte is gone. */
729 if (pte_dirty(pteval))
730 set_page_dirty(page);
732 page_remove_rmap(page);
733 page_cache_release(page);
734 dec_mm_counter(mm, file_rss);
737 pte_unmap_unlock(pte - 1, ptl);
740 static int try_to_unmap_anon(struct page *page, int ignore_refs)
742 struct anon_vma *anon_vma;
743 struct vm_area_struct *vma;
744 int ret = SWAP_AGAIN;
746 anon_vma = page_lock_anon_vma(page);
750 list_for_each_entry(vma, &anon_vma->head, anon_vma_node) {
751 ret = try_to_unmap_one(page, vma, ignore_refs);
752 if (ret == SWAP_FAIL || !page_mapped(page))
755 spin_unlock(&anon_vma->lock);
760 * try_to_unmap_file - unmap file page using the object-based rmap method
761 * @page: the page to unmap
763 * Find all the mappings of a page using the mapping pointer and the vma chains
764 * contained in the address_space struct it points to.
766 * This function is only called from try_to_unmap for object-based pages.
768 static int try_to_unmap_file(struct page *page, int ignore_refs)
770 struct address_space *mapping = page->mapping;
771 pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
772 struct vm_area_struct *vma;
773 struct prio_tree_iter iter;
774 int ret = SWAP_AGAIN;
775 unsigned long cursor;
776 unsigned long max_nl_cursor = 0;
777 unsigned long max_nl_size = 0;
778 unsigned int mapcount;
780 spin_lock(&mapping->i_mmap_lock);
781 vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {
782 ret = try_to_unmap_one(page, vma, ignore_refs);
783 if (ret == SWAP_FAIL || !page_mapped(page))
787 if (list_empty(&mapping->i_mmap_nonlinear))
790 list_for_each_entry(vma, &mapping->i_mmap_nonlinear,
791 shared.vm_set.list) {
792 if (vma->vm_flags & VM_LOCKED)
794 cursor = (unsigned long) vma->vm_private_data;
795 if (cursor > max_nl_cursor)
796 max_nl_cursor = cursor;
797 cursor = vma->vm_end - vma->vm_start;
798 if (cursor > max_nl_size)
799 max_nl_size = cursor;
802 if (max_nl_size == 0) { /* any nonlinears locked or reserved */
808 * We don't try to search for this page in the nonlinear vmas,
809 * and page_referenced wouldn't have found it anyway. Instead
810 * just walk the nonlinear vmas trying to age and unmap some.
811 * The mapcount of the page we came in with is irrelevant,
812 * but even so use it as a guide to how hard we should try?
814 mapcount = page_mapcount(page);
817 cond_resched_lock(&mapping->i_mmap_lock);
819 max_nl_size = (max_nl_size + CLUSTER_SIZE - 1) & CLUSTER_MASK;
820 if (max_nl_cursor == 0)
821 max_nl_cursor = CLUSTER_SIZE;
824 list_for_each_entry(vma, &mapping->i_mmap_nonlinear,
825 shared.vm_set.list) {
826 if (vma->vm_flags & VM_LOCKED)
828 cursor = (unsigned long) vma->vm_private_data;
829 while ( cursor < max_nl_cursor &&
830 cursor < vma->vm_end - vma->vm_start) {
831 try_to_unmap_cluster(cursor, &mapcount, vma);
832 cursor += CLUSTER_SIZE;
833 vma->vm_private_data = (void *) cursor;
834 if ((int)mapcount <= 0)
837 vma->vm_private_data = (void *) max_nl_cursor;
839 cond_resched_lock(&mapping->i_mmap_lock);
840 max_nl_cursor += CLUSTER_SIZE;
841 } while (max_nl_cursor <= max_nl_size);
844 * Don't loop forever (perhaps all the remaining pages are
845 * in locked vmas). Reset cursor on all unreserved nonlinear
846 * vmas, now forgetting on which ones it had fallen behind.
848 list_for_each_entry(vma, &mapping->i_mmap_nonlinear, shared.vm_set.list)
849 vma->vm_private_data = NULL;
851 spin_unlock(&mapping->i_mmap_lock);
856 * try_to_unmap - try to remove all page table mappings to a page
857 * @page: the page to get unmapped
859 * Tries to remove all the page table entries which are mapping this
860 * page, used in the pageout path. Caller must hold the page lock.
863 * SWAP_SUCCESS - we succeeded in removing all mappings
864 * SWAP_AGAIN - we missed a mapping, try again later
865 * SWAP_FAIL - the page is unswappable
867 int try_to_unmap(struct page *page, int ignore_refs)
871 BUG_ON(!PageLocked(page));
874 ret = try_to_unmap_anon(page, ignore_refs);
876 ret = try_to_unmap_file(page, ignore_refs);
878 if (!page_mapped(page))