mm: migration: allow the migration of PageSwapCache pages

[linux-2.6.git] / mm / migrate.c

/*
 * Memory Migration functionality - linux/mm/migration.c
 *
 * Copyright (C) 2006 Silicon Graphics, Inc., Christoph Lameter
 *
 * Page migration was first developed in the context of the memory hotplug
 * project. The main authors of the migration code are:
 *
 * IWAMOTO Toshihiro <iwamoto@valinux.co.jp>
 * Hirokazu Takahashi <taka@valinux.co.jp>
 * Dave Hansen <haveblue@us.ibm.com>
 * Christoph Lameter
 */

#include <linux/migrate.h>
#include <linux/module.h>
#include <linux/swap.h>
#include <linux/swapops.h>
#include <linux/pagemap.h>
#include <linux/buffer_head.h>
#include <linux/mm_inline.h>
#include <linux/nsproxy.h>
#include <linux/pagevec.h>
#include <linux/ksm.h>
#include <linux/rmap.h>
#include <linux/topology.h>
#include <linux/cpu.h>
#include <linux/cpuset.h>
#include <linux/writeback.h>
#include <linux/mempolicy.h>
#include <linux/vmalloc.h>
#include <linux/security.h>
#include <linux/memcontrol.h>
#include <linux/syscalls.h>
#include <linux/gfp.h>

#include "internal.h"

#define lru_to_page(_head) (list_entry((_head)->prev, struct page, lru))

/*
 * migrate_prep() needs to be called before we start compiling a list of pages
 * to be migrated using isolate_lru_page().
 */
int migrate_prep(void)
{
        /*
         * Clear the LRU lists so pages can be isolated.
         * Note that pages may be moved off the LRU after we have
         * drained them. Those pages will fail to migrate like other
         * pages that may be busy.
         */
        lru_add_drain_all();

        return 0;
}

/*
 * Add isolated pages on the list back to the LRU under page lock
 * to avoid leaking evictable pages back onto unevictable list.
 */
void putback_lru_pages(struct list_head *l)
{
        struct page *page;
        struct page *page2;

        list_for_each_entry_safe(page, page2, l, lru) {
                list_del(&page->lru);
                dec_zone_page_state(page, NR_ISOLATED_ANON +
                                page_is_file_cache(page));
                putback_lru_page(page);
        }
}

/*
 * Restore a potential migration pte to a working pte entry
 */
static int remove_migration_pte(struct page *new, struct vm_area_struct *vma,
                                 unsigned long addr, void *old)
{
        struct mm_struct *mm = vma->vm_mm;
        swp_entry_t entry;
        pgd_t *pgd;
        pud_t *pud;
        pmd_t *pmd;
        pte_t *ptep, pte;
        spinlock_t *ptl;

        pgd = pgd_offset(mm, addr);
        if (!pgd_present(*pgd))
                goto out;

        pud = pud_offset(pgd, addr);
        if (!pud_present(*pud))
                goto out;

        pmd = pmd_offset(pud, addr);
        if (!pmd_present(*pmd))
                goto out;

        ptep = pte_offset_map(pmd, addr);

        if (!is_swap_pte(*ptep)) {
                pte_unmap(ptep);
                goto out;
        }

        ptl = pte_lockptr(mm, pmd);
        spin_lock(ptl);
        pte = *ptep;
        if (!is_swap_pte(pte))
                goto unlock;

        entry = pte_to_swp_entry(pte);

        if (!is_migration_entry(entry) ||
            migration_entry_to_page(entry) != old)
                goto unlock;

        get_page(new);
        pte = pte_mkold(mk_pte(new, vma->vm_page_prot));
        if (is_write_migration_entry(entry))
                pte = pte_mkwrite(pte);
        flush_cache_page(vma, addr, pte_pfn(pte));
        set_pte_at(mm, addr, ptep, pte);

        if (PageAnon(new))
                page_add_anon_rmap(new, vma, addr);
        else
                page_add_file_rmap(new);

        /* No need to invalidate - it was non-present before */
        update_mmu_cache(vma, addr, ptep);
unlock:
        pte_unmap_unlock(ptep, ptl);
out:
        return SWAP_AGAIN;
}

/*
 * Get rid of all migration entries and replace them by
 * references to the indicated page.
 */
static void remove_migration_ptes(struct page *old, struct page *new)
{
        rmap_walk(new, remove_migration_pte, old);
}

/*
 * Something used the pte of a page under migration. We need to
 * get to the page and wait until migration is finished.
 * When we return from this function the fault will be retried.
 *
 * This function is called from do_swap_page().
 */
void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
                                unsigned long address)
{
        pte_t *ptep, pte;
        spinlock_t *ptl;
        swp_entry_t entry;
        struct page *page;

        ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
        pte = *ptep;
        if (!is_swap_pte(pte))
                goto out;

        entry = pte_to_swp_entry(pte);
        if (!is_migration_entry(entry))
                goto out;

        page = migration_entry_to_page(entry);

        /*
         * Once radix-tree replacement of page migration started, page_count
         * *must* be zero. And, we don't want to call wait_on_page_locked()
         * against a page without get_page().
         * So, we use get_page_unless_zero(), here. Even failed, page fault
         * will occur again.
         */
        if (!get_page_unless_zero(page))
                goto out;
        pte_unmap_unlock(ptep, ptl);
        wait_on_page_locked(page);
        put_page(page);
        return;
out:
        pte_unmap_unlock(ptep, ptl);
}

/*
 * Replace the page in the mapping.
 *
 * The number of remaining references must be:
 * 1 for anonymous pages without a mapping
 * 2 for pages with a mapping
 * 3 for pages with a mapping and PagePrivate/PagePrivate2 set.
 */
static int migrate_page_move_mapping(struct address_space *mapping,
                struct page *newpage, struct page *page)
{
        int expected_count;
        void **pslot;

        if (!mapping) {
                /* Anonymous page without mapping */
                if (page_count(page) != 1)
                        return -EAGAIN;
                return 0;
        }

        spin_lock_irq(&mapping->tree_lock);

        pslot = radix_tree_lookup_slot(&mapping->page_tree,
                                        page_index(page));

        expected_count = 2 + page_has_private(page);
        if (page_count(page) != expected_count ||
                        (struct page *)radix_tree_deref_slot(pslot) != page) {
                spin_unlock_irq(&mapping->tree_lock);
                return -EAGAIN;
        }

        if (!page_freeze_refs(page, expected_count)) {
                spin_unlock_irq(&mapping->tree_lock);
                return -EAGAIN;
        }

        /*
         * Now we know that no one else is looking at the page.
         */
        get_page(newpage);      /* add cache reference */
        if (PageSwapCache(page)) {
                SetPageSwapCache(newpage);
                set_page_private(newpage, page_private(page));
        }

        radix_tree_replace_slot(pslot, newpage);

        page_unfreeze_refs(page, expected_count);
        /*
         * Drop cache reference from old page.
         * We know this isn't the last reference.
         */
        __put_page(page);

        /*
         * If moved to a different zone then also account
         * the page for that zone. Other VM counters will be
         * taken care of when we establish references to the
         * new page and drop references to the old page.
         *
         * Note that anonymous pages are accounted for
         * via NR_FILE_PAGES and NR_ANON_PAGES if they
         * are mapped to swap space.
         */
        __dec_zone_page_state(page, NR_FILE_PAGES);
        __inc_zone_page_state(newpage, NR_FILE_PAGES);
        if (PageSwapBacked(page)) {
                __dec_zone_page_state(page, NR_SHMEM);
                __inc_zone_page_state(newpage, NR_SHMEM);
        }
        spin_unlock_irq(&mapping->tree_lock);

        return 0;
}

/*
 * Copy the page to its new location
 */
static void migrate_page_copy(struct page *newpage, struct page *page)
{
        copy_highpage(newpage, page);

        if (PageError(page))
                SetPageError(newpage);
        if (PageReferenced(page))
                SetPageReferenced(newpage);
        if (PageUptodate(page))
                SetPageUptodate(newpage);
        if (TestClearPageActive(page)) {
                VM_BUG_ON(PageUnevictable(page));
                SetPageActive(newpage);
        } else if (TestClearPageUnevictable(page))
                SetPageUnevictable(newpage);
        if (PageChecked(page))
                SetPageChecked(newpage);
        if (PageMappedToDisk(page))
                SetPageMappedToDisk(newpage);

        if (PageDirty(page)) {
                clear_page_dirty_for_io(page);
                /*
                 * Want to mark the page and the radix tree as dirty, and
                 * redo the accounting that clear_page_dirty_for_io undid,
                 * but we can't use set_page_dirty because that function
                 * is actually a signal that all of the page has become dirty.
                 * Wheras only part of our page may be dirty.
                 */
                __set_page_dirty_nobuffers(newpage);
        }

        mlock_migrate_page(newpage, page);
        ksm_migrate_page(newpage, page);

        ClearPageSwapCache(page);
        ClearPagePrivate(page);
        set_page_private(page, 0);
        page->mapping = NULL;

        /*
         * If any waiters have accumulated on the new page then
         * wake them up.
         */
        if (PageWriteback(newpage))
                end_page_writeback(newpage);
}

/************************************************************
 *                    Migration functions
 ***********************************************************/

/* Always fail migration. Used for mappings that are not movable */
int fail_migrate_page(struct address_space *mapping,
                        struct page *newpage, struct page *page)
{
        return -EIO;
}
EXPORT_SYMBOL(fail_migrate_page);

/*
 * Common logic to directly migrate a single page suitable for
 * pages that do not use PagePrivate/PagePrivate2.
 *
 * Pages are locked upon entry and exit.
 */
int migrate_page(struct address_space *mapping,
                struct page *newpage, struct page *page)
{
        int rc;

        BUG_ON(PageWriteback(page));    /* Writeback must be complete */

        rc = migrate_page_move_mapping(mapping, newpage, page);

        if (rc)
                return rc;

        migrate_page_copy(newpage, page);
        return 0;
}
EXPORT_SYMBOL(migrate_page);

#ifdef CONFIG_BLOCK
/*
 * Migration function for pages with buffers. This function can only be used
 * if the underlying filesystem guarantees that no other references to "page"
 * exist.
 */
int buffer_migrate_page(struct address_space *mapping,
                struct page *newpage, struct page *page)
{
        struct buffer_head *bh, *head;
        int rc;

        if (!page_has_buffers(page))
                return migrate_page(mapping, newpage, page);

        head = page_buffers(page);

        rc = migrate_page_move_mapping(mapping, newpage, page);

        if (rc)
                return rc;

        bh = head;
        do {
                get_bh(bh);
                lock_buffer(bh);
                bh = bh->b_this_page;

        } while (bh != head);

        ClearPagePrivate(page);
        set_page_private(newpage, page_private(page));
        set_page_private(page, 0);
        put_page(page);
        get_page(newpage);

        bh = head;
        do {
                set_bh_page(bh, newpage, bh_offset(bh));
                bh = bh->b_this_page;

        } while (bh != head);

        SetPagePrivate(newpage);

        migrate_page_copy(newpage, page);

        bh = head;
        do {
                unlock_buffer(bh);
                put_bh(bh);
                bh = bh->b_this_page;

        } while (bh != head);

        return 0;
}
EXPORT_SYMBOL(buffer_migrate_page);
#endif

/*
 * Writeback a page to clean the dirty state
 */
static int writeout(struct address_space *mapping, struct page *page)
{
        struct writeback_control wbc = {
                .sync_mode = WB_SYNC_NONE,
                .nr_to_write = 1,
                .range_start = 0,
                .range_end = LLONG_MAX,
                .nonblocking = 1,
                .for_reclaim = 1
        };
        int rc;

        if (!mapping->a_ops->writepage)
                /* No write method for the address space */
                return -EINVAL;

        if (!clear_page_dirty_for_io(page))
                /* Someone else already triggered a write */
                return -EAGAIN;

        /*
         * A dirty page may imply that the underlying filesystem has
         * the page on some queue. So the page must be clean for
         * migration. Writeout may mean we loose the lock and the
         * page state is no longer what we checked for earlier.
         * At this point we know that the migration attempt cannot
         * be successful.
         */
        remove_migration_ptes(page, page);

        rc = mapping->a_ops->writepage(page, &wbc);

        if (rc != AOP_WRITEPAGE_ACTIVATE)
                /* unlocked. Relock */
                lock_page(page);

        return (rc < 0) ? -EIO : -EAGAIN;
}

/*
 * Default handling if a filesystem does not provide a migration function.
 */
static int fallback_migrate_page(struct address_space *mapping,
        struct page *newpage, struct page *page)
{
        if (PageDirty(page))
                return writeout(mapping, page);

        /*
         * Buffers may be managed in a filesystem specific way.
         * We must have no buffers or drop them.
         */
        if (page_has_private(page) &&
            !try_to_release_page(page, GFP_KERNEL))
                return -EAGAIN;

        return migrate_page(mapping, newpage, page);
}

/*
 * Move a page to a newly allocated page
 * The page is locked and all ptes have been successfully removed.
 *
 * The new page will have replaced the old page if this function
 * is successful.
 *
 * Return value:
 *   < 0 - error code
 *  == 0 - success
 */
static int move_to_new_page(struct page *newpage, struct page *page,
                                                int remap_swapcache)
{
        struct address_space *mapping;
        int rc;

        /*
         * Block others from accessing the page when we get around to
         * establishing additional references. We are the only one
         * holding a reference to the new page at this point.
         */
        if (!trylock_page(newpage))
                BUG();

        /* Prepare mapping for the new page.*/
        newpage->index = page->index;
        newpage->mapping = page->mapping;
        if (PageSwapBacked(page))
                SetPageSwapBacked(newpage);

        mapping = page_mapping(page);
        if (!mapping)
                rc = migrate_page(mapping, newpage, page);
        else if (mapping->a_ops->migratepage)
                /*
                 * Most pages have a mapping and most filesystems
                 * should provide a migration function. Anonymous
                 * pages are part of swap space which also has its
                 * own migration function. This is the most common
                 * path for page migration.
                 */
                rc = mapping->a_ops->migratepage(mapping,
                                                newpage, page);
        else
                rc = fallback_migrate_page(mapping, newpage, page);

        if (rc) {
                newpage->mapping = NULL;
        } else {
                if (remap_swapcache)
                        remove_migration_ptes(page, newpage);
        }

        unlock_page(newpage);

        return rc;
}

/*
 * Obtain the lock on page, remove all ptes and migrate the page
 * to the newly allocated page in newpage.
 */
static int unmap_and_move(new_page_t get_new_page, unsigned long private,
                        struct page *page, int force, int offlining)
{
        int rc = 0;
        int *result = NULL;
        struct page *newpage = get_new_page(page, private, &result);
        int remap_swapcache = 1;
        int rcu_locked = 0;
        int charge = 0;
        struct mem_cgroup *mem = NULL;
        struct anon_vma *anon_vma = NULL;

        if (!newpage)
                return -ENOMEM;

        if (page_count(page) == 1) {
                /* page was freed from under us. So we are done. */
                goto move_newpage;
        }

        /* prepare cgroup just returns 0 or -ENOMEM */
        rc = -EAGAIN;

        if (!trylock_page(page)) {
                if (!force)
                        goto move_newpage;
                lock_page(page);
        }

        /*
         * Only memory hotplug's offline_pages() caller has locked out KSM,
         * and can safely migrate a KSM page.  The other cases have skipped
         * PageKsm along with PageReserved - but it is only now when we have
         * the page lock that we can be certain it will not go KSM beneath us
         * (KSM will not upgrade a page from PageAnon to PageKsm when it sees
         * its pagecount raised, but only here do we take the page lock which
         * serializes that).
         */
        if (PageKsm(page) && !offlining) {
                rc = -EBUSY;
                goto unlock;
        }

        /* charge against new page */
        charge = mem_cgroup_prepare_migration(page, &mem);
        if (charge == -ENOMEM) {
                rc = -ENOMEM;
                goto unlock;
        }
        BUG_ON(charge);

        if (PageWriteback(page)) {
                if (!force)
                        goto uncharge;
                wait_on_page_writeback(page);
        }
        /*
         * By try_to_unmap(), page->mapcount goes down to 0 here. In this case,
         * we cannot notice that anon_vma is freed while we migrates a page.
         * This rcu_read_lock() delays freeing anon_vma pointer until the end
         * of migration. File cache pages are no problem because of page_lock()
         * File Caches may use write_page() or lock_page() in migration, then,
         * just care Anon page here.
         */
        if (PageAnon(page)) {
                rcu_read_lock();
                rcu_locked = 1;

                /* Determine how to safely use anon_vma */
                if (!page_mapped(page)) {
                        if (!PageSwapCache(page))
                                goto rcu_unlock;

                        /*
                         * We cannot be sure that the anon_vma of an unmapped
                         * swapcache page is safe to use because we don't
                         * know in advance if the VMA that this page belonged
                         * to still exists. If the VMA and others sharing the
                         * data have been freed, then the anon_vma could
                         * already be invalid.
                         *
                         * To avoid this possibility, swapcache pages get
                         * migrated but are not remapped when migration
                         * completes
                         */
                        remap_swapcache = 0;
                } else {
                        /*
                         * Take a reference count on the anon_vma if the
                         * page is mapped so that it is guaranteed to
                         * exist when the page is remapped later
                         */
                        anon_vma = page_anon_vma(page);
                        atomic_inc(&anon_vma->external_refcount);
                }
        }

        /*
         * Corner case handling:
         * 1. When a new swap-cache page is read into, it is added to the LRU
         * and treated as swapcache but it has no rmap yet.
         * Calling try_to_unmap() against a page->mapping==NULL page will
         * trigger a BUG.  So handle it here.
         * 2. An orphaned page (see truncate_complete_page) might have
         * fs-private metadata. The page can be picked up due to memory
         * offlining.  Everywhere else except page reclaim, the page is
         * invisible to the vm, so the page can not be migrated.  So try to
         * free the metadata, so the page can be freed.
         */
        if (!page->mapping) {
                if (!PageAnon(page) && page_has_private(page)) {
                        /*
                         * Go direct to try_to_free_buffers() here because
                         * a) that's what try_to_release_page() would do anyway
                         * b) we may be under rcu_read_lock() here, so we can't
                         *    use GFP_KERNEL which is what try_to_release_page()
                         *    needs to be effective.
                         */
                        try_to_free_buffers(page);
                        goto rcu_unlock;
                }
                goto skip_unmap;
        }

        /* Establish migration ptes or remove ptes */
        try_to_unmap(page, TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);

skip_unmap:
        if (!page_mapped(page))
                rc = move_to_new_page(newpage, page, remap_swapcache);

        if (rc && remap_swapcache)
                remove_migration_ptes(page, page);
rcu_unlock:

        /* Drop an anon_vma reference if we took one */
        if (anon_vma && atomic_dec_and_lock(&anon_vma->external_refcount, &anon_vma->lock)) {
                int empty = list_empty(&anon_vma->head);
                spin_unlock(&anon_vma->lock);
                if (empty)
                        anon_vma_free(anon_vma);
        }

        if (rcu_locked)
                rcu_read_unlock();
uncharge:
        if (!charge)
                mem_cgroup_end_migration(mem, page, newpage);
unlock:
        unlock_page(page);

        if (rc != -EAGAIN) {
                /*
                 * A page that has been migrated has all references
                 * removed and will be freed. A page that has not been
                 * migrated will have kepts its references and be
                 * restored.
                 */
                list_del(&page->lru);
                dec_zone_page_state(page, NR_ISOLATED_ANON +
                                page_is_file_cache(page));
                putback_lru_page(page);
        }

move_newpage:

        /*
         * Move the new page to the LRU. If migration was not successful
         * then this will free the page.
         */
        putback_lru_page(newpage);

        if (result) {
                if (rc)
                        *result = rc;
                else
                        *result = page_to_nid(newpage);
        }
        return rc;
}

/*
 * migrate_pages
 *
 * The function takes one list of pages to migrate and a function
 * that determines from the page to be migrated and the private data
 * the target of the move and allocates the page.
 *
 * The function returns after 10 attempts or if no pages
 * are movable anymore because to has become empty
 * or no retryable pages exist anymore. All pages will be
 * returned to the LRU or freed.
 *
 * Return: Number of pages not migrated or error code.
 */
int migrate_pages(struct list_head *from,
                new_page_t get_new_page, unsigned long private, int offlining)
{
        int retry = 1;
        int nr_failed = 0;
        int pass = 0;
        struct page *page;
        struct page *page2;
        int swapwrite = current->flags & PF_SWAPWRITE;
        int rc;

        if (!swapwrite)
                current->flags |= PF_SWAPWRITE;

        for(pass = 0; pass < 10 && retry; pass++) {
                retry = 0;

                list_for_each_entry_safe(page, page2, from, lru) {
                        cond_resched();

                        rc = unmap_and_move(get_new_page, private,
                                                page, pass > 2, offlining);

                        switch(rc) {
                        case -ENOMEM:
                                goto out;
                        case -EAGAIN:
                                retry++;
                                break;
                        case 0:
                                break;
                        default:
                                /* Permanent failure */
                                nr_failed++;
                                break;
                        }
                }
        }
        rc = 0;
out:
        if (!swapwrite)
                current->flags &= ~PF_SWAPWRITE;

        putback_lru_pages(from);

        if (rc)
                return rc;

        return nr_failed + retry;
}

#ifdef CONFIG_NUMA
/*
 * Move a list of individual pages
 */
struct page_to_node {
        unsigned long addr;
        struct page *page;
        int node;
        int status;
};

static struct page *new_page_node(struct page *p, unsigned long private,
                int **result)
{
        struct page_to_node *pm = (struct page_to_node *)private;

        while (pm->node != MAX_NUMNODES && pm->page != p)
                pm++;

        if (pm->node == MAX_NUMNODES)
                return NULL;

        *result = &pm->status;

        return alloc_pages_exact_node(pm->node,
                                GFP_HIGHUSER_MOVABLE | GFP_THISNODE, 0);
}

/*
 * Move a set of pages as indicated in the pm array. The addr
 * field must be set to the virtual address of the page to be moved
 * and the node number must contain a valid target node.
 * The pm array ends with node = MAX_NUMNODES.
 */
static int do_move_page_to_node_array(struct mm_struct *mm,
                                      struct page_to_node *pm,
                                      int migrate_all)
{
        int err;
        struct page_to_node *pp;
        LIST_HEAD(pagelist);

        down_read(&mm->mmap_sem);

        /*
         * Build a list of pages to migrate
         */
        for (pp = pm; pp->node != MAX_NUMNODES; pp++) {
                struct vm_area_struct *vma;
                struct page *page;

                err = -EFAULT;
                vma = find_vma(mm, pp->addr);
                if (!vma || !vma_migratable(vma))
                        goto set_status;

                page = follow_page(vma, pp->addr, FOLL_GET);

                err = PTR_ERR(page);
                if (IS_ERR(page))
                        goto set_status;

                err = -ENOENT;
                if (!page)
                        goto set_status;

                /* Use PageReserved to check for zero page */
                if (PageReserved(page) || PageKsm(page))
                        goto put_and_set;

                pp->page = page;
                err = page_to_nid(page);

                if (err == pp->node)
                        /*
                         * Node already in the right place
                         */
                        goto put_and_set;

                err = -EACCES;
                if (page_mapcount(page) > 1 &&
                                !migrate_all)
                        goto put_and_set;

                err = isolate_lru_page(page);
                if (!err) {
                        list_add_tail(&page->lru, &pagelist);
                        inc_zone_page_state(page, NR_ISOLATED_ANON +
                                            page_is_file_cache(page));
                }
put_and_set:
                /*
                 * Either remove the duplicate refcount from
                 * isolate_lru_page() or drop the page ref if it was
                 * not isolated.
                 */
                put_page(page);
set_status:
                pp->status = err;
        }

        err = 0;
        if (!list_empty(&pagelist))
                err = migrate_pages(&pagelist, new_page_node,
                                (unsigned long)pm, 0);

        up_read(&mm->mmap_sem);
        return err;
}

/*
 * Migrate an array of page address onto an array of nodes and fill
 * the corresponding array of status.
 */
static int do_pages_move(struct mm_struct *mm, struct task_struct *task,
                         unsigned long nr_pages,
                         const void __user * __user *pages,
                         const int __user *nodes,
                         int __user *status, int flags)
{
        struct page_to_node *pm;
        nodemask_t task_nodes;
        unsigned long chunk_nr_pages;
        unsigned long chunk_start;
        int err;

        task_nodes = cpuset_mems_allowed(task);

        err = -ENOMEM;
        pm = (struct page_to_node *)__get_free_page(GFP_KERNEL);
        if (!pm)
                goto out;

        migrate_prep();

        /*
         * Store a chunk of page_to_node array in a page,
         * but keep the last one as a marker
         */
        chunk_nr_pages = (PAGE_SIZE / sizeof(struct page_to_node)) - 1;

        for (chunk_start = 0;
             chunk_start < nr_pages;
             chunk_start += chunk_nr_pages) {
                int j;

                if (chunk_start + chunk_nr_pages > nr_pages)
                        chunk_nr_pages = nr_pages - chunk_start;

                /* fill the chunk pm with addrs and nodes from user-space */
                for (j = 0; j < chunk_nr_pages; j++) {
                        const void __user *p;
                        int node;

                        err = -EFAULT;
                        if (get_user(p, pages + j + chunk_start))
                                goto out_pm;
                        pm[j].addr = (unsigned long) p;

                        if (get_user(node, nodes + j + chunk_start))
                                goto out_pm;

                        err = -ENODEV;
                        if (node < 0 || node >= MAX_NUMNODES)
                                goto out_pm;

                        if (!node_state(node, N_HIGH_MEMORY))
                                goto out_pm;

                        err = -EACCES;
                        if (!node_isset(node, task_nodes))
                                goto out_pm;

                        pm[j].node = node;
                }

                /* End marker for this chunk */
                pm[chunk_nr_pages].node = MAX_NUMNODES;

                /* Migrate this chunk */
                err = do_move_page_to_node_array(mm, pm,
                                                 flags & MPOL_MF_MOVE_ALL);
                if (err < 0)
                        goto out_pm;

                /* Return status information */
                for (j = 0; j < chunk_nr_pages; j++)
                        if (put_user(pm[j].status, status + j + chunk_start)) {
                                err = -EFAULT;
                                goto out_pm;
                        }
        }
        err = 0;

out_pm:
        free_page((unsigned long)pm);
out:
        return err;
}

/*
 * Determine the nodes of an array of pages and store it in an array of status.
 */
static void do_pages_stat_array(struct mm_struct *mm, unsigned long nr_pages,
                                const void __user **pages, int *status)
{
        unsigned long i;

        down_read(&mm->mmap_sem);

        for (i = 0; i < nr_pages; i++) {
                unsigned long addr = (unsigned long)(*pages);
                struct vm_area_struct *vma;
                struct page *page;
                int err = -EFAULT;

                vma = find_vma(mm, addr);
                if (!vma)
                        goto set_status;

                page = follow_page(vma, addr, 0);

                err = PTR_ERR(page);
                if (IS_ERR(page))
                        goto set_status;

                err = -ENOENT;
                /* Use PageReserved to check for zero page */
                if (!page || PageReserved(page) || PageKsm(page))
                        goto set_status;

                err = page_to_nid(page);
set_status:
                *status = err;

                pages++;
                status++;
        }

        up_read(&mm->mmap_sem);
}

/*
 * Determine the nodes of a user array of pages and store it in
 * a user array of status.
 */
static int do_pages_stat(struct mm_struct *mm, unsigned long nr_pages,
                         const void __user * __user *pages,
                         int __user *status)
{
#define DO_PAGES_STAT_CHUNK_NR 16
        const void __user *chunk_pages[DO_PAGES_STAT_CHUNK_NR];
        int chunk_status[DO_PAGES_STAT_CHUNK_NR];

        while (nr_pages) {
                unsigned long chunk_nr;

                chunk_nr = nr_pages;
                if (chunk_nr > DO_PAGES_STAT_CHUNK_NR)
                        chunk_nr = DO_PAGES_STAT_CHUNK_NR;

                if (copy_from_user(chunk_pages, pages, chunk_nr * sizeof(*chunk_pages)))
                        break;

                do_pages_stat_array(mm, chunk_nr, chunk_pages, chunk_status);

                if (copy_to_user(status, chunk_status, chunk_nr * sizeof(*status)))
                        break;

                pages += chunk_nr;
                status += chunk_nr;
                nr_pages -= chunk_nr;
        }
        return nr_pages ? -EFAULT : 0;
}

/*
 * Move a list of pages in the address space of the currently executing
 * process.
 */
SYSCALL_DEFINE6(move_pages, pid_t, pid, unsigned long, nr_pages,
                const void __user * __user *, pages,
                const int __user *, nodes,
                int __user *, status, int, flags)
{
        const struct cred *cred = current_cred(), *tcred;
        struct task_struct *task;
        struct mm_struct *mm;
        int err;

        /* Check flags */
        if (flags & ~(MPOL_MF_MOVE|MPOL_MF_MOVE_ALL))
                return -EINVAL;

        if ((flags & MPOL_MF_MOVE_ALL) && !capable(CAP_SYS_NICE))
                return -EPERM;

        /* Find the mm_struct */
        read_lock(&tasklist_lock);
        task = pid ? find_task_by_vpid(pid) : current;
        if (!task) {
                read_unlock(&tasklist_lock);
                return -ESRCH;
        }
        mm = get_task_mm(task);
        read_unlock(&tasklist_lock);

        if (!mm)
                return -EINVAL;

        /*
         * Check if this process has the right to modify the specified
         * process. The right exists if the process has administrative
         * capabilities, superuser privileges or the same
         * userid as the target process.
         */
        rcu_read_lock();
        tcred = __task_cred(task);
        if (cred->euid != tcred->suid && cred->euid != tcred->uid &&
            cred->uid  != tcred->suid && cred->uid  != tcred->uid &&
            !capable(CAP_SYS_NICE)) {
                rcu_read_unlock();
                err = -EPERM;
                goto out;
        }
        rcu_read_unlock();

        err = security_task_movememory(task);
        if (err)
                goto out;

        if (nodes) {
                err = do_pages_move(mm, task, nr_pages, pages, nodes, status,
                                    flags);
        } else {
                err = do_pages_stat(mm, nr_pages, pages, status);
        }

out:
        mmput(mm);
        return err;
}

/*
 * Call migration functions in the vma_ops that may prepare
 * memory in a vm for migration. migration functions may perform
 * the migration for vmas that do not have an underlying page struct.
 */
int migrate_vmas(struct mm_struct *mm, const nodemask_t *to,
        const nodemask_t *from, unsigned long flags)
{
        struct vm_area_struct *vma;
        int err = 0;

        for (vma = mm->mmap; vma && !err; vma = vma->vm_next) {
                if (vma->vm_ops && vma->vm_ops->migrate) {
                        err = vma->vm_ops->migrate(vma, to, from, flags);
                        if (err)
                                break;
                }
        }
        return err;
}
#endif