mm: count swap usage

[linux-flexiantxendom0-natty.git] / mm / rmap.c

/*
 * mm/rmap.c - physical to virtual reverse mappings
 *
 * Copyright 2001, Rik van Riel <riel@conectiva.com.br>
 * Released under the General Public License (GPL).
 *
 * Simple, low overhead reverse mapping scheme.
 * Please try to keep this thing as modular as possible.
 *
 * Provides methods for unmapping each kind of mapped page:
 * the anon methods track anonymous pages, and
 * the file methods track pages belonging to an inode.
 *
 * Original design by Rik van Riel <riel@conectiva.com.br> 2001
 * File methods by Dave McCracken <dmccr@us.ibm.com> 2003, 2004
 * Anonymous methods by Andrea Arcangeli <andrea@suse.de> 2004
 * Contributions by Hugh Dickins 2003, 2004
 */

/*
 * Lock ordering in mm:
 *
 * inode->i_mutex       (while writing or truncating, not reading or faulting)
 *   inode->i_alloc_sem (vmtruncate_range)
 *   mm->mmap_sem
 *     page->flags PG_locked (lock_page)
 *       mapping->i_mmap_lock
 *         anon_vma->lock
 *           mm->page_table_lock or pte_lock
 *             zone->lru_lock (in mark_page_accessed, isolate_lru_page)
 *             swap_lock (in swap_duplicate, swap_info_get)
 *               mmlist_lock (in mmput, drain_mmlist and others)
 *               mapping->private_lock (in __set_page_dirty_buffers)
 *               inode_lock (in set_page_dirty's __mark_inode_dirty)
 *                 sb_lock (within inode_lock in fs/fs-writeback.c)
 *                 mapping->tree_lock (widely used, in set_page_dirty,
 *                           in arch-dependent flush_dcache_mmap_lock,
 *                           within inode_lock in __sync_single_inode)
 *
 * (code doesn't rely on that order so it could be switched around)
 * ->tasklist_lock
 *   anon_vma->lock      (memory_failure, collect_procs_anon)
 *     pte map lock
 */

#include <linux/mm.h>
#include <linux/pagemap.h>
#include <linux/swap.h>
#include <linux/swapops.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/ksm.h>
#include <linux/rmap.h>
#include <linux/rcupdate.h>
#include <linux/module.h>
#include <linux/memcontrol.h>
#include <linux/mmu_notifier.h>
#include <linux/migrate.h>

#include <asm/tlbflush.h>

#include "internal.h"

static struct kmem_cache *anon_vma_cachep;

static inline struct anon_vma *anon_vma_alloc(void)
{
        return kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL);
}

void anon_vma_free(struct anon_vma *anon_vma)
{
        kmem_cache_free(anon_vma_cachep, anon_vma);
}

/**
 * anon_vma_prepare - attach an anon_vma to a memory region
 * @vma: the memory region in question
 *
 * This makes sure the memory mapping described by 'vma' has
 * an 'anon_vma' attached to it, so that we can associate the
 * anonymous pages mapped into it with that anon_vma.
 *
 * The common case will be that we already have one, but if
 * if not we either need to find an adjacent mapping that we
 * can re-use the anon_vma from (very common when the only
 * reason for splitting a vma has been mprotect()), or we
 * allocate a new one.
 *
 * Anon-vma allocations are very subtle, because we may have
 * optimistically looked up an anon_vma in page_lock_anon_vma()
 * and that may actually touch the spinlock even in the newly
 * allocated vma (it depends on RCU to make sure that the
 * anon_vma isn't actually destroyed).
 *
 * As a result, we need to do proper anon_vma locking even
 * for the new allocation. At the same time, we do not want
 * to do any locking for the common case of already having
 * an anon_vma.
 *
 * This must be called with the mmap_sem held for reading.
 */
int anon_vma_prepare(struct vm_area_struct *vma)
{
        struct anon_vma *anon_vma = vma->anon_vma;

        might_sleep();
        if (unlikely(!anon_vma)) {
                struct mm_struct *mm = vma->vm_mm;
                struct anon_vma *allocated;

                anon_vma = find_mergeable_anon_vma(vma);
                allocated = NULL;
                if (!anon_vma) {
                        anon_vma = anon_vma_alloc();
                        if (unlikely(!anon_vma))
                                return -ENOMEM;
                        allocated = anon_vma;
                }
                spin_lock(&anon_vma->lock);

                /* page_table_lock to protect against threads */
                spin_lock(&mm->page_table_lock);
                if (likely(!vma->anon_vma)) {
                        vma->anon_vma = anon_vma;
                        list_add_tail(&vma->anon_vma_node, &anon_vma->head);
                        allocated = NULL;
                }
                spin_unlock(&mm->page_table_lock);

                spin_unlock(&anon_vma->lock);
                if (unlikely(allocated))
                        anon_vma_free(allocated);
        }
        return 0;
}

void __anon_vma_merge(struct vm_area_struct *vma, struct vm_area_struct *next)
{
        BUG_ON(vma->anon_vma != next->anon_vma);
        list_del(&next->anon_vma_node);
}

void __anon_vma_link(struct vm_area_struct *vma)
{
        struct anon_vma *anon_vma = vma->anon_vma;

        if (anon_vma)
                list_add_tail(&vma->anon_vma_node, &anon_vma->head);
}

void anon_vma_link(struct vm_area_struct *vma)
{
        struct anon_vma *anon_vma = vma->anon_vma;

        if (anon_vma) {
                spin_lock(&anon_vma->lock);
                list_add_tail(&vma->anon_vma_node, &anon_vma->head);
                spin_unlock(&anon_vma->lock);
        }
}

void anon_vma_unlink(struct vm_area_struct *vma)
{
        struct anon_vma *anon_vma = vma->anon_vma;
        int empty;

        if (!anon_vma)
                return;

        spin_lock(&anon_vma->lock);
        list_del(&vma->anon_vma_node);

        /* We must garbage collect the anon_vma if it's empty */
        empty = list_empty(&anon_vma->head) && !ksm_refcount(anon_vma);
        spin_unlock(&anon_vma->lock);

        if (empty)
                anon_vma_free(anon_vma);
}

static void anon_vma_ctor(void *data)
{
        struct anon_vma *anon_vma = data;

        spin_lock_init(&anon_vma->lock);
        ksm_refcount_init(anon_vma);
        INIT_LIST_HEAD(&anon_vma->head);
}

void __init anon_vma_init(void)
{
        anon_vma_cachep = kmem_cache_create("anon_vma", sizeof(struct anon_vma),
                        0, SLAB_DESTROY_BY_RCU|SLAB_PANIC, anon_vma_ctor);
}

/*
 * Getting a lock on a stable anon_vma from a page off the LRU is
 * tricky: page_lock_anon_vma rely on RCU to guard against the races.
 */
struct anon_vma *page_lock_anon_vma(struct page *page)
{
        struct anon_vma *anon_vma;
        unsigned long anon_mapping;

        rcu_read_lock();
        anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping);
        if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
                goto out;
        if (!page_mapped(page))
                goto out;

        anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
        spin_lock(&anon_vma->lock);
        return anon_vma;
out:
        rcu_read_unlock();
        return NULL;
}

void page_unlock_anon_vma(struct anon_vma *anon_vma)
{
        spin_unlock(&anon_vma->lock);
        rcu_read_unlock();
}

/*
 * At what user virtual address is page expected in @vma?
 * Returns virtual address or -EFAULT if page's index/offset is not
 * within the range mapped the @vma.
 */
static inline unsigned long
vma_address(struct page *page, struct vm_area_struct *vma)
{
        pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
        unsigned long address;

        address = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
        if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
                /* page should be within @vma mapping range */
                return -EFAULT;
        }
        return address;
}

/*
 * At what user virtual address is page expected in vma?
 * checking that the page matches the vma.
 */
unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma)
{
        if (PageAnon(page)) {
                if (vma->anon_vma != page_anon_vma(page))
                        return -EFAULT;
        } else if (page->mapping && !(vma->vm_flags & VM_NONLINEAR)) {
                if (!vma->vm_file ||
                    vma->vm_file->f_mapping != page->mapping)
                        return -EFAULT;
        } else
                return -EFAULT;
        return vma_address(page, vma);
}

/*
 * Check that @page is mapped at @address into @mm.
 *
 * If @sync is false, page_check_address may perform a racy check to avoid
 * the page table lock when the pte is not present (helpful when reclaiming
 * highly shared pages).
 *
 * On success returns with pte mapped and locked.
 */
pte_t *page_check_address(struct page *page, struct mm_struct *mm,
                          unsigned long address, spinlock_t **ptlp, int sync)
{
        pgd_t *pgd;
        pud_t *pud;
        pmd_t *pmd;
        pte_t *pte;
        spinlock_t *ptl;

        pgd = pgd_offset(mm, address);
        if (!pgd_present(*pgd))
                return NULL;

        pud = pud_offset(pgd, address);
        if (!pud_present(*pud))
                return NULL;

        pmd = pmd_offset(pud, address);
        if (!pmd_present(*pmd))
                return NULL;

        pte = pte_offset_map(pmd, address);
        /* Make a quick check before getting the lock */
        if (!sync && !pte_present(*pte)) {
                pte_unmap(pte);
                return NULL;
        }

        ptl = pte_lockptr(mm, pmd);
        spin_lock(ptl);
        if (pte_present(*pte) && page_to_pfn(page) == pte_pfn(*pte)) {
                *ptlp = ptl;
                return pte;
        }
        pte_unmap_unlock(pte, ptl);
        return NULL;
}

/**
 * page_mapped_in_vma - check whether a page is really mapped in a VMA
 * @page: the page to test
 * @vma: the VMA to test
 *
 * Returns 1 if the page is mapped into the page tables of the VMA, 0
 * if the page is not mapped into the page tables of this VMA.  Only
 * valid for normal file or anonymous VMAs.
 */
int page_mapped_in_vma(struct page *page, struct vm_area_struct *vma)
{
        unsigned long address;
        pte_t *pte;
        spinlock_t *ptl;

        address = vma_address(page, vma);
        if (address == -EFAULT)         /* out of vma range */
                return 0;
        pte = page_check_address(page, vma->vm_mm, address, &ptl, 1);
        if (!pte)                       /* the page is not in this mm */
                return 0;
        pte_unmap_unlock(pte, ptl);

        return 1;
}

/*
 * Subfunctions of page_referenced: page_referenced_one called
 * repeatedly from either page_referenced_anon or page_referenced_file.
 */
int page_referenced_one(struct page *page, struct vm_area_struct *vma,
                        unsigned long address, unsigned int *mapcount,
                        unsigned long *vm_flags)
{
        struct mm_struct *mm = vma->vm_mm;
        pte_t *pte;
        spinlock_t *ptl;
        int referenced = 0;

        pte = page_check_address(page, mm, address, &ptl, 0);
        if (!pte)
                goto out;

        /*
         * Don't want to elevate referenced for mlocked page that gets this far,
         * in order that it progresses to try_to_unmap and is moved to the
         * unevictable list.
         */
        if (vma->vm_flags & VM_LOCKED) {
                *mapcount = 1;  /* break early from loop */
                *vm_flags |= VM_LOCKED;
                goto out_unmap;
        }

        if (ptep_clear_flush_young_notify(vma, address, pte)) {
                /*
                 * Don't treat a reference through a sequentially read
                 * mapping as such.  If the page has been used in
                 * another mapping, we will catch it; if this other
                 * mapping is already gone, the unmap path will have
                 * set PG_referenced or activated the page.
                 */
                if (likely(!VM_SequentialReadHint(vma)))
                        referenced++;
        }

        /* Pretend the page is referenced if the task has the
           swap token and is in the middle of a page fault. */
        if (mm != current->mm && has_swap_token(mm) &&
                        rwsem_is_locked(&mm->mmap_sem))
                referenced++;

out_unmap:
        (*mapcount)--;
        pte_unmap_unlock(pte, ptl);

        if (referenced)
                *vm_flags |= vma->vm_flags;
out:
        return referenced;
}

static int page_referenced_anon(struct page *page,
                                struct mem_cgroup *mem_cont,
                                unsigned long *vm_flags)
{
        unsigned int mapcount;
        struct anon_vma *anon_vma;
        struct vm_area_struct *vma;
        int referenced = 0;

        anon_vma = page_lock_anon_vma(page);
        if (!anon_vma)
                return referenced;

        mapcount = page_mapcount(page);
        list_for_each_entry(vma, &anon_vma->head, anon_vma_node) {
                unsigned long address = vma_address(page, vma);
                if (address == -EFAULT)
                        continue;
                /*
                 * If we are reclaiming on behalf of a cgroup, skip
                 * counting on behalf of references from different
                 * cgroups
                 */
                if (mem_cont && !mm_match_cgroup(vma->vm_mm, mem_cont))
                        continue;
                referenced += page_referenced_one(page, vma, address,
                                                  &mapcount, vm_flags);
                if (!mapcount)
                        break;
        }

        page_unlock_anon_vma(anon_vma);
        return referenced;
}

/**
 * page_referenced_file - referenced check for object-based rmap
 * @page: the page we're checking references on.
 * @mem_cont: target memory controller
 * @vm_flags: collect encountered vma->vm_flags who actually referenced the page
 *
 * For an object-based mapped page, find all the places it is mapped and
 * check/clear the referenced flag.  This is done by following the page->mapping
 * pointer, then walking the chain of vmas it holds.  It returns the number
 * of references it found.
 *
 * This function is only called from page_referenced for object-based pages.
 */
static int page_referenced_file(struct page *page,
                                struct mem_cgroup *mem_cont,
                                unsigned long *vm_flags)
{
        unsigned int mapcount;
        struct address_space *mapping = page->mapping;
        pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
        struct vm_area_struct *vma;
        struct prio_tree_iter iter;
        int referenced = 0;

        /*
         * The caller's checks on page->mapping and !PageAnon have made
         * sure that this is a file page: the check for page->mapping
         * excludes the case just before it gets set on an anon page.
         */
        BUG_ON(PageAnon(page));

        /*
         * The page lock not only makes sure that page->mapping cannot
         * suddenly be NULLified by truncation, it makes sure that the
         * structure at mapping cannot be freed and reused yet,
         * so we can safely take mapping->i_mmap_lock.
         */
        BUG_ON(!PageLocked(page));

        spin_lock(&mapping->i_mmap_lock);

        /*
         * i_mmap_lock does not stabilize mapcount at all, but mapcount
         * is more likely to be accurate if we note it after spinning.
         */
        mapcount = page_mapcount(page);

        vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {
                unsigned long address = vma_address(page, vma);
                if (address == -EFAULT)
                        continue;
                /*
                 * If we are reclaiming on behalf of a cgroup, skip
                 * counting on behalf of references from different
                 * cgroups
                 */
                if (mem_cont && !mm_match_cgroup(vma->vm_mm, mem_cont))
                        continue;
                referenced += page_referenced_one(page, vma, address,
                                                  &mapcount, vm_flags);
                if (!mapcount)
                        break;
        }

        spin_unlock(&mapping->i_mmap_lock);
        return referenced;
}

/**
 * page_referenced - test if the page was referenced
 * @page: the page to test
 * @is_locked: caller holds lock on the page
 * @mem_cont: target memory controller
 * @vm_flags: collect encountered vma->vm_flags who actually referenced the page
 *
 * Quick test_and_clear_referenced for all mappings to a page,
 * returns the number of ptes which referenced the page.
 */
int page_referenced(struct page *page,
                    int is_locked,
                    struct mem_cgroup *mem_cont,
                    unsigned long *vm_flags)
{
        int referenced = 0;
        int we_locked = 0;

        if (TestClearPageReferenced(page))
                referenced++;

        *vm_flags = 0;
        if (page_mapped(page) && page_rmapping(page)) {
                if (!is_locked && (!PageAnon(page) || PageKsm(page))) {
                        we_locked = trylock_page(page);
                        if (!we_locked) {
                                referenced++;
                                goto out;
                        }
                }
                if (unlikely(PageKsm(page)))
                        referenced += page_referenced_ksm(page, mem_cont,
                                                                vm_flags);
                else if (PageAnon(page))
                        referenced += page_referenced_anon(page, mem_cont,
                                                                vm_flags);
                else if (page->mapping)
                        referenced += page_referenced_file(page, mem_cont,
                                                                vm_flags);
                if (we_locked)
                        unlock_page(page);
        }
out:
        if (page_test_and_clear_young(page))
                referenced++;

        return referenced;
}

static int page_mkclean_one(struct page *page, struct vm_area_struct *vma,
                            unsigned long address)
{
        struct mm_struct *mm = vma->vm_mm;
        pte_t *pte;
        spinlock_t *ptl;
        int ret = 0;

        pte = page_check_address(page, mm, address, &ptl, 1);
        if (!pte)
                goto out;

        if (pte_dirty(*pte) || pte_write(*pte)) {
                pte_t entry;

                flush_cache_page(vma, address, pte_pfn(*pte));
                entry = ptep_clear_flush_notify(vma, address, pte);
                entry = pte_wrprotect(entry);
                entry = pte_mkclean(entry);
                set_pte_at(mm, address, pte, entry);
                ret = 1;
        }

        pte_unmap_unlock(pte, ptl);
out:
        return ret;
}

static int page_mkclean_file(struct address_space *mapping, struct page *page)
{
        pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
        struct vm_area_struct *vma;
        struct prio_tree_iter iter;
        int ret = 0;

        BUG_ON(PageAnon(page));

        spin_lock(&mapping->i_mmap_lock);
        vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {
                if (vma->vm_flags & VM_SHARED) {
                        unsigned long address = vma_address(page, vma);
                        if (address == -EFAULT)
                                continue;
                        ret += page_mkclean_one(page, vma, address);
                }
        }
        spin_unlock(&mapping->i_mmap_lock);
        return ret;
}

int page_mkclean(struct page *page)
{
        int ret = 0;

        BUG_ON(!PageLocked(page));

        if (page_mapped(page)) {
                struct address_space *mapping = page_mapping(page);
                if (mapping) {
                        ret = page_mkclean_file(mapping, page);
                        if (page_test_dirty(page)) {
                                page_clear_dirty(page);
                                ret = 1;
                        }
                }
        }

        return ret;
}
EXPORT_SYMBOL_GPL(page_mkclean);

/**
 * __page_set_anon_rmap - setup new anonymous rmap
 * @page:       the page to add the mapping to
 * @vma:        the vm area in which the mapping is added
 * @address:    the user virtual address mapped
 */
static void __page_set_anon_rmap(struct page *page,
        struct vm_area_struct *vma, unsigned long address)
{
        struct anon_vma *anon_vma = vma->anon_vma;

        BUG_ON(!anon_vma);
        anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
        page->mapping = (struct address_space *) anon_vma;
        page->index = linear_page_index(vma, address);
}

/**
 * __page_check_anon_rmap - sanity check anonymous rmap addition
 * @page:       the page to add the mapping to
 * @vma:        the vm area in which the mapping is added
 * @address:    the user virtual address mapped
 */
static void __page_check_anon_rmap(struct page *page,
        struct vm_area_struct *vma, unsigned long address)
{
#ifdef CONFIG_DEBUG_VM
        /*
         * The page's anon-rmap details (mapping and index) are guaranteed to
         * be set up correctly at this point.
         *
         * We have exclusion against page_add_anon_rmap because the caller
         * always holds the page locked, except if called from page_dup_rmap,
         * in which case the page is already known to be setup.
         *
         * We have exclusion against page_add_new_anon_rmap because those pages
         * are initially only visible via the pagetables, and the pte is locked
         * over the call to page_add_new_anon_rmap.
         */
        struct anon_vma *anon_vma = vma->anon_vma;
        anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
        BUG_ON(page->mapping != (struct address_space *)anon_vma);
        BUG_ON(page->index != linear_page_index(vma, address));
#endif
}

/**
 * page_add_anon_rmap - add pte mapping to an anonymous page
 * @page:       the page to add the mapping to
 * @vma:        the vm area in which the mapping is added
 * @address:    the user virtual address mapped
 *
 * The caller needs to hold the pte lock, and the page must be locked in
 * the anon_vma case: to serialize mapping,index checking after setting,
 * and to ensure that PageAnon is not being upgraded racily to PageKsm
 * (but PageKsm is never downgraded to PageAnon).
 */
void page_add_anon_rmap(struct page *page,
        struct vm_area_struct *vma, unsigned long address)
{
        int first = atomic_inc_and_test(&page->_mapcount);
        if (first)
                __inc_zone_page_state(page, NR_ANON_PAGES);
        if (unlikely(PageKsm(page)))
                return;

        VM_BUG_ON(!PageLocked(page));
        VM_BUG_ON(address < vma->vm_start || address >= vma->vm_end);
        if (first)
                __page_set_anon_rmap(page, vma, address);
        else
                __page_check_anon_rmap(page, vma, address);
}

/**
 * page_add_new_anon_rmap - add pte mapping to a new anonymous page
 * @page:       the page to add the mapping to
 * @vma:        the vm area in which the mapping is added
 * @address:    the user virtual address mapped
 *
 * Same as page_add_anon_rmap but must only be called on *new* pages.
 * This means the inc-and-test can be bypassed.
 * Page does not have to be locked.
 */
void page_add_new_anon_rmap(struct page *page,
        struct vm_area_struct *vma, unsigned long address)
{
        VM_BUG_ON(address < vma->vm_start || address >= vma->vm_end);
        SetPageSwapBacked(page);
        atomic_set(&page->_mapcount, 0); /* increment count (starts at -1) */
        __inc_zone_page_state(page, NR_ANON_PAGES);
        __page_set_anon_rmap(page, vma, address);
        if (page_evictable(page, vma))
                lru_cache_add_lru(page, LRU_ACTIVE_ANON);
        else
                add_page_to_unevictable_list(page);
}

/**
 * page_add_file_rmap - add pte mapping to a file page
 * @page: the page to add the mapping to
 *
 * The caller needs to hold the pte lock.
 */
void page_add_file_rmap(struct page *page)
{
        if (atomic_inc_and_test(&page->_mapcount)) {
                __inc_zone_page_state(page, NR_FILE_MAPPED);
                mem_cgroup_update_file_mapped(page, 1);
        }
}

/**
 * page_remove_rmap - take down pte mapping from a page
 * @page: page to remove mapping from
 *
 * The caller needs to hold the pte lock.
 */
void page_remove_rmap(struct page *page)
{
        /* page still mapped by someone else? */
        if (!atomic_add_negative(-1, &page->_mapcount))
                return;

        /*
         * Now that the last pte has gone, s390 must transfer dirty
         * flag from storage key to struct page.  We can usually skip
         * this if the page is anon, so about to be freed; but perhaps
         * not if it's in swapcache - there might be another pte slot
         * containing the swap entry, but page not yet written to swap.
         */
        if ((!PageAnon(page) || PageSwapCache(page)) && page_test_dirty(page)) {
                page_clear_dirty(page);
                set_page_dirty(page);
        }
        if (PageAnon(page)) {
                mem_cgroup_uncharge_page(page);
                __dec_zone_page_state(page, NR_ANON_PAGES);
        } else {
                __dec_zone_page_state(page, NR_FILE_MAPPED);
                mem_cgroup_update_file_mapped(page, -1);
        }
        /*
         * It would be tidy to reset the PageAnon mapping here,
         * but that might overwrite a racing page_add_anon_rmap
         * which increments mapcount after us but sets mapping
         * before us: so leave the reset to free_hot_cold_page,
         * and remember that it's only reliable while mapped.
         * Leaving it set also helps swapoff to reinstate ptes
         * faster for those pages still in swapcache.
         */
}

/*
 * Subfunctions of try_to_unmap: try_to_unmap_one called
 * repeatedly from either try_to_unmap_anon or try_to_unmap_file.
 */
int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
                     unsigned long address, enum ttu_flags flags)
{
        struct mm_struct *mm = vma->vm_mm;
        pte_t *pte;
        pte_t pteval;
        spinlock_t *ptl;
        int ret = SWAP_AGAIN;

        pte = page_check_address(page, mm, address, &ptl, 0);
        if (!pte)
                goto out;

        /*
         * If the page is mlock()d, we cannot swap it out.
         * If it's recently referenced (perhaps page_referenced
         * skipped over this mm) then we should reactivate it.
         */
        if (!(flags & TTU_IGNORE_MLOCK)) {
                if (vma->vm_flags & VM_LOCKED)
                        goto out_mlock;

                if (TTU_ACTION(flags) == TTU_MUNLOCK)
                        goto out_unmap;
        }
        if (!(flags & TTU_IGNORE_ACCESS)) {
                if (ptep_clear_flush_young_notify(vma, address, pte)) {
                        ret = SWAP_FAIL;
                        goto out_unmap;
                }
        }

        /* Nuke the page table entry. */
        flush_cache_page(vma, address, page_to_pfn(page));
        pteval = ptep_clear_flush_notify(vma, address, pte);

        /* Move the dirty bit to the physical page now the pte is gone. */
        if (pte_dirty(pteval))
                set_page_dirty(page);

        /* Update high watermark before we lower rss */
        update_hiwater_rss(mm);

        if (PageHWPoison(page) && !(flags & TTU_IGNORE_HWPOISON)) {
                if (PageAnon(page))
                        dec_mm_counter(mm, MM_ANONPAGES);
                else
                        dec_mm_counter(mm, MM_FILEPAGES);
                set_pte_at(mm, address, pte,
                                swp_entry_to_pte(make_hwpoison_entry(page)));
        } else if (PageAnon(page)) {
                swp_entry_t entry = { .val = page_private(page) };

                if (PageSwapCache(page)) {
                        /*
                         * Store the swap location in the pte.
                         * See handle_pte_fault() ...
                         */
                        if (swap_duplicate(entry) < 0) {
                                set_pte_at(mm, address, pte, pteval);
                                ret = SWAP_FAIL;
                                goto out_unmap;
                        }
                        if (list_empty(&mm->mmlist)) {
                                spin_lock(&mmlist_lock);
                                if (list_empty(&mm->mmlist))
                                        list_add(&mm->mmlist, &init_mm.mmlist);
                                spin_unlock(&mmlist_lock);
                        }
                        dec_mm_counter(mm, MM_ANONPAGES);
                        inc_mm_counter(mm, MM_SWAPENTS);
                } else if (PAGE_MIGRATION) {
                        /*
                         * Store the pfn of the page in a special migration
                         * pte. do_swap_page() will wait until the migration
                         * pte is removed and then restart fault handling.
                         */
                        BUG_ON(TTU_ACTION(flags) != TTU_MIGRATION);
                        entry = make_migration_entry(page, pte_write(pteval));
                }
                set_pte_at(mm, address, pte, swp_entry_to_pte(entry));
                BUG_ON(pte_file(*pte));
        } else if (PAGE_MIGRATION && (TTU_ACTION(flags) == TTU_MIGRATION)) {
                /* Establish migration entry for a file page */
                swp_entry_t entry;
                entry = make_migration_entry(page, pte_write(pteval));
                set_pte_at(mm, address, pte, swp_entry_to_pte(entry));
        } else
                dec_mm_counter(mm, MM_FILEPAGES);

        page_remove_rmap(page);
        page_cache_release(page);

out_unmap:
        pte_unmap_unlock(pte, ptl);
out:
        return ret;

out_mlock:
        pte_unmap_unlock(pte, ptl);


        /*
         * We need mmap_sem locking, Otherwise VM_LOCKED check makes
         * unstable result and race. Plus, We can't wait here because
         * we now hold anon_vma->lock or mapping->i_mmap_lock.
         * if trylock failed, the page remain in evictable lru and later
         * vmscan could retry to move the page to unevictable lru if the
         * page is actually mlocked.
         */
        if (down_read_trylock(&vma->vm_mm->mmap_sem)) {
                if (vma->vm_flags & VM_LOCKED) {
                        mlock_vma_page(page);
                        ret = SWAP_MLOCK;
                }
                up_read(&vma->vm_mm->mmap_sem);
        }
        return ret;
}

/*
 * objrmap doesn't work for nonlinear VMAs because the assumption that
 * offset-into-file correlates with offset-into-virtual-addresses does not hold.
 * Consequently, given a particular page and its ->index, we cannot locate the
 * ptes which are mapping that page without an exhaustive linear search.
 *
 * So what this code does is a mini "virtual scan" of each nonlinear VMA which
 * maps the file to which the target page belongs.  The ->vm_private_data field
 * holds the current cursor into that scan.  Successive searches will circulate
 * around the vma's virtual address space.
 *
 * So as more replacement pressure is applied to the pages in a nonlinear VMA,
 * more scanning pressure is placed against them as well.   Eventually pages
 * will become fully unmapped and are eligible for eviction.
 *
 * For very sparsely populated VMAs this is a little inefficient - chances are
 * there there won't be many ptes located within the scan cluster.  In this case
 * maybe we could scan further - to the end of the pte page, perhaps.
 *
 * Mlocked pages:  check VM_LOCKED under mmap_sem held for read, if we can
 * acquire it without blocking.  If vma locked, mlock the pages in the cluster,
 * rather than unmapping them.  If we encounter the "check_page" that vmscan is
 * trying to unmap, return SWAP_MLOCK, else default SWAP_AGAIN.
 */
#define CLUSTER_SIZE    min(32*PAGE_SIZE, PMD_SIZE)
#define CLUSTER_MASK    (~(CLUSTER_SIZE - 1))

static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
                struct vm_area_struct *vma, struct page *check_page)
{
        struct mm_struct *mm = vma->vm_mm;
        pgd_t *pgd;
        pud_t *pud;
        pmd_t *pmd;
        pte_t *pte;
        pte_t pteval;
        spinlock_t *ptl;
        struct page *page;
        unsigned long address;
        unsigned long end;
        int ret = SWAP_AGAIN;
        int locked_vma = 0;

        address = (vma->vm_start + cursor) & CLUSTER_MASK;
        end = address + CLUSTER_SIZE;
        if (address < vma->vm_start)
                address = vma->vm_start;
        if (end > vma->vm_end)
                end = vma->vm_end;

        pgd = pgd_offset(mm, address);
        if (!pgd_present(*pgd))
                return ret;

        pud = pud_offset(pgd, address);
        if (!pud_present(*pud))
                return ret;

        pmd = pmd_offset(pud, address);
        if (!pmd_present(*pmd))
                return ret;

        /*
         * If we can acquire the mmap_sem for read, and vma is VM_LOCKED,
         * keep the sem while scanning the cluster for mlocking pages.
         */
        if (down_read_trylock(&vma->vm_mm->mmap_sem)) {
                locked_vma = (vma->vm_flags & VM_LOCKED);
                if (!locked_vma)
                        up_read(&vma->vm_mm->mmap_sem); /* don't need it */
        }

        pte = pte_offset_map_lock(mm, pmd, address, &ptl);

        /* Update high watermark before we lower rss */
        update_hiwater_rss(mm);

        for (; address < end; pte++, address += PAGE_SIZE) {
                if (!pte_present(*pte))
                        continue;
                page = vm_normal_page(vma, address, *pte);
                BUG_ON(!page || PageAnon(page));

                if (locked_vma) {
                        mlock_vma_page(page);   /* no-op if already mlocked */
                        if (page == check_page)
                                ret = SWAP_MLOCK;
                        continue;       /* don't unmap */
                }

                if (ptep_clear_flush_young_notify(vma, address, pte))
                        continue;

                /* Nuke the page table entry. */
                flush_cache_page(vma, address, pte_pfn(*pte));
                pteval = ptep_clear_flush_notify(vma, address, pte);

                /* If nonlinear, store the file page offset in the pte. */
                if (page->index != linear_page_index(vma, address))
                        set_pte_at(mm, address, pte, pgoff_to_pte(page->index));

                /* Move the dirty bit to the physical page now the pte is gone. */
                if (pte_dirty(pteval))
                        set_page_dirty(page);

                page_remove_rmap(page);
                page_cache_release(page);
                dec_mm_counter(mm, MM_FILEPAGES);
                (*mapcount)--;
        }
        pte_unmap_unlock(pte - 1, ptl);
        if (locked_vma)
                up_read(&vma->vm_mm->mmap_sem);
        return ret;
}

/**
 * try_to_unmap_anon - unmap or unlock anonymous page using the object-based
 * rmap method
 * @page: the page to unmap/unlock
 * @flags: action and flags
 *
 * Find all the mappings of a page using the mapping pointer and the vma chains
 * contained in the anon_vma struct it points to.
 *
 * This function is only called from try_to_unmap/try_to_munlock for
 * anonymous pages.
 * When called from try_to_munlock(), the mmap_sem of the mm containing the vma
 * where the page was found will be held for write.  So, we won't recheck
 * vm_flags for that VMA.  That should be OK, because that vma shouldn't be
 * 'LOCKED.
 */
static int try_to_unmap_anon(struct page *page, enum ttu_flags flags)
{
        struct anon_vma *anon_vma;
        struct vm_area_struct *vma;
        int ret = SWAP_AGAIN;

        anon_vma = page_lock_anon_vma(page);
        if (!anon_vma)
                return ret;

        list_for_each_entry(vma, &anon_vma->head, anon_vma_node) {
                unsigned long address = vma_address(page, vma);
                if (address == -EFAULT)
                        continue;
                ret = try_to_unmap_one(page, vma, address, flags);
                if (ret != SWAP_AGAIN || !page_mapped(page))
                        break;
        }

        page_unlock_anon_vma(anon_vma);
        return ret;
}

/**
 * try_to_unmap_file - unmap/unlock file page using the object-based rmap method
 * @page: the page to unmap/unlock
 * @flags: action and flags
 *
 * Find all the mappings of a page using the mapping pointer and the vma chains
 * contained in the address_space struct it points to.
 *
 * This function is only called from try_to_unmap/try_to_munlock for
 * object-based pages.
 * When called from try_to_munlock(), the mmap_sem of the mm containing the vma
 * where the page was found will be held for write.  So, we won't recheck
 * vm_flags for that VMA.  That should be OK, because that vma shouldn't be
 * 'LOCKED.
 */
static int try_to_unmap_file(struct page *page, enum ttu_flags flags)
{
        struct address_space *mapping = page->mapping;
        pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
        struct vm_area_struct *vma;
        struct prio_tree_iter iter;
        int ret = SWAP_AGAIN;
        unsigned long cursor;
        unsigned long max_nl_cursor = 0;
        unsigned long max_nl_size = 0;
        unsigned int mapcount;

        spin_lock(&mapping->i_mmap_lock);
        vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {
                unsigned long address = vma_address(page, vma);
                if (address == -EFAULT)
                        continue;
                ret = try_to_unmap_one(page, vma, address, flags);
                if (ret != SWAP_AGAIN || !page_mapped(page))
                        goto out;
        }

        if (list_empty(&mapping->i_mmap_nonlinear))
                goto out;

        /*
         * We don't bother to try to find the munlocked page in nonlinears.
         * It's costly. Instead, later, page reclaim logic may call
         * try_to_unmap(TTU_MUNLOCK) and recover PG_mlocked lazily.
         */
        if (TTU_ACTION(flags) == TTU_MUNLOCK)
                goto out;

        list_for_each_entry(vma, &mapping->i_mmap_nonlinear,
                                                shared.vm_set.list) {
                cursor = (unsigned long) vma->vm_private_data;
                if (cursor > max_nl_cursor)
                        max_nl_cursor = cursor;
                cursor = vma->vm_end - vma->vm_start;
                if (cursor > max_nl_size)
                        max_nl_size = cursor;
        }

        if (max_nl_size == 0) { /* all nonlinears locked or reserved ? */
                ret = SWAP_FAIL;
                goto out;
        }

        /*
         * We don't try to search for this page in the nonlinear vmas,
         * and page_referenced wouldn't have found it anyway.  Instead
         * just walk the nonlinear vmas trying to age and unmap some.
         * The mapcount of the page we came in with is irrelevant,
         * but even so use it as a guide to how hard we should try?
         */
        mapcount = page_mapcount(page);
        if (!mapcount)
                goto out;
        cond_resched_lock(&mapping->i_mmap_lock);

        max_nl_size = (max_nl_size + CLUSTER_SIZE - 1) & CLUSTER_MASK;
        if (max_nl_cursor == 0)
                max_nl_cursor = CLUSTER_SIZE;

        do {
                list_for_each_entry(vma, &mapping->i_mmap_nonlinear,
                                                shared.vm_set.list) {
                        cursor = (unsigned long) vma->vm_private_data;
                        while ( cursor < max_nl_cursor &&
                                cursor < vma->vm_end - vma->vm_start) {
                                if (try_to_unmap_cluster(cursor, &mapcount,
                                                vma, page) == SWAP_MLOCK)
                                        ret = SWAP_MLOCK;
                                cursor += CLUSTER_SIZE;
                                vma->vm_private_data = (void *) cursor;
                                if ((int)mapcount <= 0)
                                        goto out;
                        }
                        vma->vm_private_data = (void *) max_nl_cursor;
                }
                cond_resched_lock(&mapping->i_mmap_lock);
                max_nl_cursor += CLUSTER_SIZE;
        } while (max_nl_cursor <= max_nl_size);

        /*
         * Don't loop forever (perhaps all the remaining pages are
         * in locked vmas).  Reset cursor on all unreserved nonlinear
         * vmas, now forgetting on which ones it had fallen behind.
         */
        list_for_each_entry(vma, &mapping->i_mmap_nonlinear, shared.vm_set.list)
                vma->vm_private_data = NULL;
out:
        spin_unlock(&mapping->i_mmap_lock);
        return ret;
}

/**
 * try_to_unmap - try to remove all page table mappings to a page
 * @page: the page to get unmapped
 * @flags: action and flags
 *
 * Tries to remove all the page table entries which are mapping this
 * page, used in the pageout path.  Caller must hold the page lock.
 * Return values are:
 *
 * SWAP_SUCCESS - we succeeded in removing all mappings
 * SWAP_AGAIN   - we missed a mapping, try again later
 * SWAP_FAIL    - the page is unswappable
 * SWAP_MLOCK   - page is mlocked.
 */
int try_to_unmap(struct page *page, enum ttu_flags flags)
{
        int ret;

        BUG_ON(!PageLocked(page));

        if (unlikely(PageKsm(page)))
                ret = try_to_unmap_ksm(page, flags);
        else if (PageAnon(page))
                ret = try_to_unmap_anon(page, flags);
        else
                ret = try_to_unmap_file(page, flags);
        if (ret != SWAP_MLOCK && !page_mapped(page))
                ret = SWAP_SUCCESS;
        return ret;
}

/**
 * try_to_munlock - try to munlock a page
 * @page: the page to be munlocked
 *
 * Called from munlock code.  Checks all of the VMAs mapping the page
 * to make sure nobody else has this page mlocked. The page will be
 * returned with PG_mlocked cleared if no other vmas have it mlocked.
 *
 * Return values are:
 *
 * SWAP_AGAIN   - no vma is holding page mlocked, or,
 * SWAP_AGAIN   - page mapped in mlocked vma -- couldn't acquire mmap sem
 * SWAP_FAIL    - page cannot be located at present
 * SWAP_MLOCK   - page is now mlocked.
 */
int try_to_munlock(struct page *page)
{
        VM_BUG_ON(!PageLocked(page) || PageLRU(page));

        if (unlikely(PageKsm(page)))
                return try_to_unmap_ksm(page, TTU_MUNLOCK);
        else if (PageAnon(page))
                return try_to_unmap_anon(page, TTU_MUNLOCK);
        else
                return try_to_unmap_file(page, TTU_MUNLOCK);
}

#ifdef CONFIG_MIGRATION
/*
 * rmap_walk() and its helpers rmap_walk_anon() and rmap_walk_file():
 * Called by migrate.c to remove migration ptes, but might be used more later.
 */
static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *,
                struct vm_area_struct *, unsigned long, void *), void *arg)
{
        struct anon_vma *anon_vma;
        struct vm_area_struct *vma;
        int ret = SWAP_AGAIN;

        /*
         * Note: remove_migration_ptes() cannot use page_lock_anon_vma()
         * because that depends on page_mapped(); but not all its usages
         * are holding mmap_sem, which also gave the necessary guarantee
         * (that this anon_vma's slab has not already been destroyed).
         * This needs to be reviewed later: avoiding page_lock_anon_vma()
         * is risky, and currently limits the usefulness of rmap_walk().
         */
        anon_vma = page_anon_vma(page);
        if (!anon_vma)
                return ret;
        spin_lock(&anon_vma->lock);
        list_for_each_entry(vma, &anon_vma->head, anon_vma_node) {
                unsigned long address = vma_address(page, vma);
                if (address == -EFAULT)
                        continue;
                ret = rmap_one(page, vma, address, arg);
                if (ret != SWAP_AGAIN)
                        break;
        }
        spin_unlock(&anon_vma->lock);
        return ret;
}

static int rmap_walk_file(struct page *page, int (*rmap_one)(struct page *,
                struct vm_area_struct *, unsigned long, void *), void *arg)
{
        struct address_space *mapping = page->mapping;
        pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
        struct vm_area_struct *vma;
        struct prio_tree_iter iter;
        int ret = SWAP_AGAIN;

        if (!mapping)
                return ret;
        spin_lock(&mapping->i_mmap_lock);
        vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {
                unsigned long address = vma_address(page, vma);
                if (address == -EFAULT)
                        continue;
                ret = rmap_one(page, vma, address, arg);
                if (ret != SWAP_AGAIN)
                        break;
        }
        /*
         * No nonlinear handling: being always shared, nonlinear vmas
         * never contain migration ptes.  Decide what to do about this
         * limitation to linear when we need rmap_walk() on nonlinear.
         */
        spin_unlock(&mapping->i_mmap_lock);
        return ret;
}

int rmap_walk(struct page *page, int (*rmap_one)(struct page *,
                struct vm_area_struct *, unsigned long, void *), void *arg)
{
        VM_BUG_ON(!PageLocked(page));

        if (unlikely(PageKsm(page)))
                return rmap_walk_ksm(page, rmap_one, arg);
        else if (PageAnon(page))
                return rmap_walk_anon(page, rmap_one, arg);
        else
                return rmap_walk_file(page, rmap_one, arg);
}
#endif /* CONFIG_MIGRATION */