migrate.c

Go to the documentation of this file.

/*
 * 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 <[email protected]>
 * Hirokazu Takahashi <[email protected]>
 * Dave Hansen <[email protected]>
 * Christoph Lameter
 */

#include <linux/migrate.h>
#include <linux/export.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/hugetlb.h>
#include <linux/hugetlb_cgroup.h>
#include <linux/gfp.h>

#include <asm/tlbflush.h>

#include "internal.h"

/*
 * migrate_prep() needs to be called before we start compiling a list of pages
 * to be migrated using isolate_lru_page(). If scheduling work on other CPUs is
 * undesirable, use migrate_prep_local()
 */
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;
}

/* Do the necessary work of migrate_prep but not if it involves other CPUs */
int migrate_prep_local(void)
{
    lru_add_drain();

    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;

    if (unlikely(PageHuge(new))) {
        ptep = huge_pte_offset(mm, addr);
        if (!ptep)
            goto out;
        ptl = &mm->page_table_lock;
    } else {
        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_trans_huge(*pmd))
            goto out;
        if (!pmd_present(*pmd))
            goto out;

        ptep = pte_offset_map(pmd, addr);

        /*
         * Peek to check is_swap_pte() before taking ptlock?  No, we
         * can race mremap's move_ptes(), which skips anon_vma lock.
         */

        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);
#ifdef CONFIG_HUGETLB_PAGE
    if (PageHuge(new))
        pte = pte_mkhuge(pte);
#endif
    flush_cache_page(vma, addr, pte_pfn(pte));
    set_pte_at(mm, addr, ptep, pte);

    if (PageHuge(new)) {
        if (PageAnon(new))
            hugepage_add_anon_rmap(new, vma, addr);
        else
            page_dup_rmap(new);
    } else 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.
 */
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);
}

#ifdef CONFIG_BLOCK
/* Returns true if all buffers are successfully locked */
static bool buffer_migrate_lock_buffers(struct buffer_head *head,
                            enum migrate_mode mode)
{
    struct buffer_head *bh = head;

    /* Simple case, sync compaction */
    if (mode != MIGRATE_ASYNC) {
        do {
            get_bh(bh);
            lock_buffer(bh);
            bh = bh->b_this_page;

        } while (bh != head);

        return true;
    }

    /* async case, we cannot block on lock_buffer so use trylock_buffer */
    do {
        get_bh(bh);
        if (!trylock_buffer(bh)) {
            /*
             * We failed to lock the buffer and cannot stall in
             * async migration. Release the taken locks
             */
            struct buffer_head *failed_bh = bh;
            put_bh(failed_bh);
            bh = head;
            while (bh != failed_bh) {
                unlock_buffer(bh);
                put_bh(bh);
                bh = bh->b_this_page;
            }
            return false;
        }

        bh = bh->b_this_page;
    } while (bh != head);
    return true;
}
#else
static inline bool buffer_migrate_lock_buffers(struct buffer_head *head,
                            enum migrate_mode mode)
{
    return true;
}
#endif /* CONFIG_BLOCK */

/*
 * 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,
        struct buffer_head *head, enum migrate_mode mode)
{
    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 ||
        radix_tree_deref_slot_protected(pslot, &mapping->tree_lock) != page) {
        spin_unlock_irq(&mapping->tree_lock);
        return -EAGAIN;
    }

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

    /*
     * In the async migration case of moving a page with buffers, lock the
     * buffers using trylock before the mapping is moved. If the mapping
     * was moved, we later failed to lock the buffers and could not move
     * the mapping back due to an elevated page count, we would have to
     * block waiting on other references to be dropped.
     */
    if (mode == MIGRATE_ASYNC && head &&
            !buffer_migrate_lock_buffers(head, mode)) {
        page_unfreeze_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);

    /*
     * Drop cache reference from old page by unfreezing
     * to one less reference.
     * We know this isn't the last reference.
     */
    page_unfreeze_refs(page, expected_count - 1);

    /*
     * 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 (!PageSwapCache(page) && PageSwapBacked(page)) {
        __dec_zone_page_state(page, NR_SHMEM);
        __inc_zone_page_state(newpage, NR_SHMEM);
    }
    spin_unlock_irq(&mapping->tree_lock);

    return 0;
}

/*
 * The expected number of remaining references is the same as that
 * of migrate_page_move_mapping().
 */
int migrate_huge_page_move_mapping(struct address_space *mapping,
                   struct page *newpage, struct page *page)
{
    int expected_count;
    void **pslot;

    if (!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 ||
        radix_tree_deref_slot_protected(pslot, &mapping->tree_lock) != page) {
        spin_unlock_irq(&mapping->tree_lock);
        return -EAGAIN;
    }

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

    get_page(newpage);

    radix_tree_replace_slot(pslot, newpage);

    page_unfreeze_refs(page, expected_count - 1);

    spin_unlock_irq(&mapping->tree_lock);
    return 0;
}

/*
 * Copy the page to its new location
 */
void migrate_page_copy(struct page *newpage, struct page *page)
{
    if (PageHuge(page))
        copy_huge_page(newpage, page);
    else
        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.
         * Whereas only part of our page may be dirty.
         */
        if (PageSwapBacked(page))
            SetPageDirty(newpage);
        else
            __set_page_dirty_nobuffers(newpage);
    }

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

    ClearPageSwapCache(page);
    ClearPagePrivate(page);
    set_page_private(page, 0);

    /*
     * 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,
        enum migrate_mode mode)
{
    int rc;

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

    rc = migrate_page_move_mapping(mapping, newpage, page, NULL, mode);

    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, enum migrate_mode mode)
{
    struct buffer_head *bh, *head;
    int rc;

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

    head = page_buffers(page);

    rc = migrate_page_move_mapping(mapping, newpage, page, head, mode);

    if (rc)
        return rc;

    /*
     * In the async case, migrate_page_move_mapping locked the buffers
     * with an IRQ-safe spinlock held. In the sync case, the buffers
     * need to be locked now
     */
    if (mode != MIGRATE_ASYNC)
        BUG_ON(!buffer_migrate_lock_buffers(head, mode));

    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,
        .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, enum migrate_mode mode)
{
    if (PageDirty(page)) {
        /* Only writeback pages in full synchronous migration */
        if (mode != MIGRATE_SYNC)
            return -EBUSY;
        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, mode);
}

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

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

    unlock_page(newpage);

    return rc;
}

static int __unmap_and_move(struct page *page, struct page *newpage,
            int force, bool offlining, enum migrate_mode mode)
{
    int rc = -EAGAIN;
    int remap_swapcache = 1;
    struct mem_cgroup *mem;
    struct anon_vma *anon_vma = NULL;

    if (!trylock_page(page)) {
        if (!force || mode == MIGRATE_ASYNC)
            goto out;

        /*
         * It's not safe for direct compaction to call lock_page.
         * For example, during page readahead pages are added locked
         * to the LRU. Later, when the IO completes the pages are
         * marked uptodate and unlocked. However, the queueing
         * could be merging multiple pages for one bio (e.g.
         * mpage_readpages). If an allocation happens for the
         * second or third page, the process can end up locking
         * the same page twice and deadlocking. Rather than
         * trying to be clever about what pages can be locked,
         * avoid the use of lock_page for direct compaction
         * altogether.
         */
        if (current->flags & PF_MEMALLOC)
            goto out;

        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 */
    mem_cgroup_prepare_migration(page, newpage, &mem);

    if (PageWriteback(page)) {
        /*
         * Only in the case of a full syncronous migration is it
         * necessary to wait for PageWriteback. In the async case,
         * the retry loop is too short and in the sync-light case,
         * the overhead of stalling is too much
         */
        if (mode != MIGRATE_SYNC) {
            rc = -EBUSY;
            goto uncharge;
        }
        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 get_anon_vma() 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)) {
        /*
         * Only page_lock_anon_vma() understands the subtleties of
         * getting a hold on an anon_vma from outside one of its mms.
         */
        anon_vma = page_get_anon_vma(page);
        if (anon_vma) {
            /*
             * Anon page
             */
        } else if (PageSwapCache(page)) {
            /*
             * 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 {
            goto uncharge;
        }
    }

    /*
     * 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) {
        VM_BUG_ON(PageAnon(page));
        if (page_has_private(page)) {
            try_to_free_buffers(page);
            goto uncharge;
        }
        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, mode);

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

    /* Drop an anon_vma reference if we took one */
    if (anon_vma)
        put_anon_vma(anon_vma);

uncharge:
    mem_cgroup_end_migration(mem, page, newpage, rc == 0);
unlock:
    unlock_page(page);
out:
    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, bool offlining,
            enum migrate_mode mode)
{
    int rc = 0;
    int *result = NULL;
    struct page *newpage = get_new_page(page, private, &result);

    if (!newpage)
        return -ENOMEM;

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

    if (unlikely(PageTransHuge(page)))
        if (unlikely(split_huge_page(page)))
            goto out;

    rc = __unmap_and_move(page, newpage, force, offlining, mode);
out:
    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 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;
}

/*
 * Counterpart of unmap_and_move_page() for hugepage migration.
 *
 * This function doesn't wait the completion of hugepage I/O
 * because there is no race between I/O and migration for hugepage.
 * Note that currently hugepage I/O occurs only in direct I/O
 * where no lock is held and PG_writeback is irrelevant,
 * and writeback status of all subpages are counted in the reference
 * count of the head page (i.e. if all subpages of a 2MB hugepage are
 * under direct I/O, the reference of the head page is 512 and a bit more.)
 * This means that when we try to migrate hugepage whose subpages are
 * doing direct I/O, some references remain after try_to_unmap() and
 * hugepage migration fails without data corruption.
 *
 * There is also no race when direct I/O is issued on the page under migration,
 * because then pte is replaced with migration swap entry and direct I/O code
 * will wait in the page fault for migration to complete.
 */
static int unmap_and_move_huge_page(new_page_t get_new_page,
                unsigned long private, struct page *hpage,
                int force, bool offlining,
                enum migrate_mode mode)
{
    int rc = 0;
    int *result = NULL;
    struct page *new_hpage = get_new_page(hpage, private, &result);
    struct anon_vma *anon_vma = NULL;

    if (!new_hpage)
        return -ENOMEM;

    rc = -EAGAIN;

    if (!trylock_page(hpage)) {
        if (!force || mode != MIGRATE_SYNC)
            goto out;
        lock_page(hpage);
    }

    if (PageAnon(hpage))
        anon_vma = page_get_anon_vma(hpage);

    try_to_unmap(hpage, TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);

    if (!page_mapped(hpage))
        rc = move_to_new_page(new_hpage, hpage, 1, mode);

    if (rc)
        remove_migration_ptes(hpage, hpage);

    if (anon_vma)
        put_anon_vma(anon_vma);

    if (!rc)
        hugetlb_cgroup_migrate(hpage, new_hpage);

    unlock_page(hpage);
out:
    put_page(new_hpage);
    if (result) {
        if (rc)
            *result = rc;
        else
            *result = page_to_nid(new_hpage);
    }
    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.
 * Caller should call putback_lru_pages to return pages to the LRU
 * or free list only if ret != 0.
 *
 * 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, bool offlining,
        enum migrate_mode mode)
{
    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,
                        mode);

            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;

    if (rc)
        return rc;

    return nr_failed + retry;
}

int migrate_huge_page(struct page *hpage, new_page_t get_new_page,
              unsigned long private, bool offlining,
              enum migrate_mode mode)
{
    int pass, rc;

    for (pass = 0; pass < 10; pass++) {
        rc = unmap_and_move_huge_page(get_new_page,
                          private, hpage, pass > 2, offlining,
                          mode);
        switch (rc) {
        case -ENOMEM:
            goto out;
        case -EAGAIN:
            /* try again */
            cond_resched();
            break;
        case 0:
            goto out;
        default:
            rc = -EIO;
            goto out;
        }
    }
out:
    return rc;
}

#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 || pp->addr < vma->vm_start || !vma_migratable(vma))
            goto set_status;

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

        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, MIGRATE_SYNC);
        if (err)
            putback_lru_pages(&pagelist);
    }

    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, nodemask_t task_nodes,
             unsigned long nr_pages,
             const void __user * __user *pages,
             const int __user *nodes,
             int __user *status, int flags)
{
    struct page_to_node *pm;
    unsigned long chunk_nr_pages;
    unsigned long chunk_start;
    int err;

    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 || addr < vma->vm_start)
            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;
    nodemask_t task_nodes;

    /* 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 */
    rcu_read_lock();
    task = pid ? find_task_by_vpid(pid) : current;
    if (!task) {
        rcu_read_unlock();
        return -ESRCH;
    }
    get_task_struct(task);

    /*
     * 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.
     */
    tcred = __task_cred(task);
    if (!uid_eq(cred->euid, tcred->suid) && !uid_eq(cred->euid, tcred->uid) &&
        !uid_eq(cred->uid,  tcred->suid) && !uid_eq(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;

    task_nodes = cpuset_mems_allowed(task);
    mm = get_task_mm(task);
    put_task_struct(task);

    if (!mm)
        return -EINVAL;

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

    mmput(mm);
    return err;

out:
    put_task_struct(task);
    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