[NET]: Avoid pointless allocation casts in BSD compression module

[linux-2.6/linux-acpi-2.6/ibm-acpi-2.6.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 <hugh@veritas.com> 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)
 */

#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/rmap.h>
#include <linux/rcupdate.h>
#include <linux/module.h>
#include <linux/kallsyms.h>

#include <asm/tlbflush.h>

struct kmem_cache *anon_vma_cachep;

/* This must be called under the mmap_sem. */
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, *locked;

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

                /* 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);

                if (locked)
                        spin_unlock(&locked->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);
        spin_unlock(&anon_vma->lock);

        if (empty)
                anon_vma_free(anon_vma);
}

static void anon_vma_ctor(void *data, struct kmem_cache *cachep,
                          unsigned long flags)
{
        struct anon_vma *anon_vma = data;

        spin_lock_init(&anon_vma->lock);
        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.
 */
static 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) page->mapping;
        if (!(anon_mapping & 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;
}

static 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?
 */
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 any vma from prio_tree_next */
                BUG_ON(!PageAnon(page));
                return -EFAULT;
        }
        return address;
}

/*
 * At what user virtual address is page expected in vma? checking that the
 * page matches the vma: currently only used on anon pages, by unuse_vma;
 */
unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma)
{
        if (PageAnon(page)) {
                if ((void *)vma->anon_vma !=
                    (void *)page->mapping - PAGE_MAPPING_ANON)
                        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.
 *
 * 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)
{
        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 (!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;
}

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

        address = vma_address(page, vma);
        if (address == -EFAULT)
                goto out;

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

        if (ptep_clear_flush_young(vma, address, pte))
                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++;

        (*mapcount)--;
        pte_unmap_unlock(pte, ptl);
out:
        return referenced;
}

static int page_referenced_anon(struct page *page)
{
        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) {
                referenced += page_referenced_one(page, vma, &mapcount);
                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.
 *
 * 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)
{
        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) {
                if ((vma->vm_flags & (VM_LOCKED|VM_MAYSHARE))
                                  == (VM_LOCKED|VM_MAYSHARE)) {
                        referenced++;
                        break;
                }
                referenced += page_referenced_one(page, vma, &mapcount);
                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
 *
 * 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)
{
        int referenced = 0;

        if (page_test_and_clear_young(page))
                referenced++;

        if (TestClearPageReferenced(page))
                referenced++;

        if (page_mapped(page) && page->mapping) {
                if (PageAnon(page))
                        referenced += page_referenced_anon(page);
                else if (is_locked)
                        referenced += page_referenced_file(page);
                else if (TestSetPageLocked(page))
                        referenced++;
                else {
                        if (page->mapping)
                                referenced += page_referenced_file(page);
                        unlock_page(page);
                }
        }
        return referenced;
}

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

        address = vma_address(page, vma);
        if (address == -EFAULT)
                goto out;

        pte = page_check_address(page, mm, address, &ptl);
        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(vma, address, pte);
                entry = pte_wrprotect(entry);
                entry = pte_mkclean(entry);
                set_pte_at(mm, address, pte, entry);
                lazy_mmu_prot_update(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)
                        ret += page_mkclean_one(page, vma);
        }
        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);

        /*
         * nr_mapped state can be updated without turning off
         * interrupts because it is not modified via interrupt.
         */
        __inc_zone_page_state(page, NR_ANON_PAGES);
}

/**
 * page_set_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.
 */
void page_add_anon_rmap(struct page *page,
        struct vm_area_struct *vma, unsigned long address)
{
        VM_BUG_ON(!PageLocked(page));
        VM_BUG_ON(address < vma->vm_start || address >= vma->vm_end);
        if (atomic_inc_and_test(&page->_mapcount))
                __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)
{
        BUG_ON(address < vma->vm_start || address >= vma->vm_end);
        atomic_set(&page->_mapcount, 0); /* elevate count by 1 (starts at -1) */
        __page_set_anon_rmap(page, vma, address);
}

/**
 * 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);
}

#ifdef CONFIG_DEBUG_VM
/**
 * page_dup_rmap - duplicate pte mapping to a page
 * @page:       the page to add the mapping to
 *
 * For copy_page_range only: minimal extract from page_add_file_rmap /
 * page_add_anon_rmap, avoiding unnecessary tests (already checked) so it's
 * quicker.
 *
 * The caller needs to hold the pte lock.
 */
void page_dup_rmap(struct page *page, struct vm_area_struct *vma, unsigned long address)
{
        BUG_ON(page_mapcount(page) == 0);
        if (PageAnon(page))
                __page_check_anon_rmap(page, vma, address);
        atomic_inc(&page->_mapcount);
}
#endif

/**
 * 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, struct vm_area_struct *vma)
{
        if (atomic_add_negative(-1, &page->_mapcount)) {
                if (unlikely(page_mapcount(page) < 0)) {
                        printk (KERN_EMERG "Eeek! page_mapcount(page) went negative! (%d)\n", page_mapcount(page));
                        printk (KERN_EMERG "  page pfn = %lx\n", page_to_pfn(page));
                        printk (KERN_EMERG "  page->flags = %lx\n", page->flags);
                        printk (KERN_EMERG "  page->count = %x\n", page_count(page));
                        printk (KERN_EMERG "  page->mapping = %p\n", page->mapping);
                        print_symbol (KERN_EMERG "  vma->vm_ops = %s\n", (unsigned long)vma->vm_ops);
                        if (vma->vm_ops) {
                                print_symbol (KERN_EMERG "  vma->vm_ops->nopage = %s\n", (unsigned long)vma->vm_ops->nopage);
                                print_symbol (KERN_EMERG "  vma->vm_ops->fault = %s\n", (unsigned long)vma->vm_ops->fault);
                        }
                        if (vma->vm_file && vma->vm_file->f_op)
                                print_symbol (KERN_EMERG "  vma->vm_file->f_op->mmap = %s\n", (unsigned long)vma->vm_file->f_op->mmap);
                        BUG();
                }

                /*
                 * 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.
                 */
                if (page_test_dirty(page)) {
                        page_clear_dirty(page);
                        set_page_dirty(page);
                }
                __dec_zone_page_state(page,
                                PageAnon(page) ? NR_ANON_PAGES : NR_FILE_MAPPED);
        }
}

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

        address = vma_address(page, vma);
        if (address == -EFAULT)
                goto out;

        pte = page_check_address(page, mm, address, &ptl);
        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 (!migration && ((vma->vm_flags & VM_LOCKED) ||
                        (ptep_clear_flush_young(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(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 (PageAnon(page)) {
                swp_entry_t entry = { .val = page_private(page) };

                if (PageSwapCache(page)) {
                        /*
                         * Store the swap location in the pte.
                         * See handle_pte_fault() ...
                         */
                        swap_duplicate(entry);
                        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, anon_rss);
#ifdef CONFIG_MIGRATION
                } else {
                        /*
                         * 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(!migration);
                        entry = make_migration_entry(page, pte_write(pteval));
#endif
                }
                set_pte_at(mm, address, pte, swp_entry_to_pte(entry));
                BUG_ON(pte_file(*pte));
        } else
#ifdef CONFIG_MIGRATION
        if (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
#endif
                dec_mm_counter(mm, file_rss);


        page_remove_rmap(page, vma);
        page_cache_release(page);

out_unmap:
        pte_unmap_unlock(pte, ptl);
out:
        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.
 */
#define CLUSTER_SIZE    min(32*PAGE_SIZE, PMD_SIZE)
#define CLUSTER_MASK    (~(CLUSTER_SIZE - 1))

static void try_to_unmap_cluster(unsigned long cursor,
        unsigned int *mapcount, struct vm_area_struct *vma)
{
        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;

        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;

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

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

        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 (ptep_clear_flush_young(vma, address, pte))
                        continue;

                /* Nuke the page table entry. */
                flush_cache_page(vma, address, pte_pfn(*pte));
                pteval = ptep_clear_flush(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, vma);
                page_cache_release(page);
                dec_mm_counter(mm, file_rss);
                (*mapcount)--;
        }
        pte_unmap_unlock(pte - 1, ptl);
}

static int try_to_unmap_anon(struct page *page, int migration)
{
        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) {
                ret = try_to_unmap_one(page, vma, migration);
                if (ret == SWAP_FAIL || !page_mapped(page))
                        break;
        }

        page_unlock_anon_vma(anon_vma);
        return ret;
}

/**
 * try_to_unmap_file - unmap file page using the object-based rmap method
 * @page: the page to unmap
 *
 * 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 for object-based pages.
 */
static int try_to_unmap_file(struct page *page, int migration)
{
        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) {
                ret = try_to_unmap_one(page, vma, migration);
                if (ret == SWAP_FAIL || !page_mapped(page))
                        goto out;
        }

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

        list_for_each_entry(vma, &mapping->i_mmap_nonlinear,
                                                shared.vm_set.list) {
                if ((vma->vm_flags & VM_LOCKED) && !migration)
                        continue;
                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) { /* any 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) {
                        if ((vma->vm_flags & VM_LOCKED) && !migration)
                                continue;
                        cursor = (unsigned long) vma->vm_private_data;
                        while ( cursor < max_nl_cursor &&
                                cursor < vma->vm_end - vma->vm_start) {
                                try_to_unmap_cluster(cursor, &mapcount, vma);
                                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
 *
 * 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
 */
int try_to_unmap(struct page *page, int migration)
{
        int ret;

        BUG_ON(!PageLocked(page));

        if (PageAnon(page))
                ret = try_to_unmap_anon(page, migration);
        else
                ret = try_to_unmap_file(page, migration);

        if (!page_mapped(page))
                ret = SWAP_SUCCESS;
        return ret;
}