KVM: PPC: Book3S HV: Handle 1GB pages in radix page fault handler

[linux/fpc-iii.git] / mm / migrate.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Memory Migration functionality - linux/mm/migrate.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/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/backing-dev.h>
#include <linux/compaction.h>
#include <linux/syscalls.h>
#include <linux/hugetlb.h>
#include <linux/hugetlb_cgroup.h>
#include <linux/gfp.h>
#include <linux/pfn_t.h>
#include <linux/memremap.h>
#include <linux/userfaultfd_k.h>
#include <linux/balloon_compaction.h>
#include <linux/mmu_notifier.h>
#include <linux/page_idle.h>
#include <linux/page_owner.h>
#include <linux/sched/mm.h>
#include <linux/ptrace.h>

#include <asm/tlbflush.h>

#define CREATE_TRACE_POINTS
#include <trace/events/migrate.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;
}

int isolate_movable_page(struct page *page, isolate_mode_t mode)
{
        struct address_space *mapping;

        /*
         * Avoid burning cycles with pages that are yet under __free_pages(),
         * or just got freed under us.
         *
         * In case we 'win' a race for a movable page being freed under us and
         * raise its refcount preventing __free_pages() from doing its job
         * the put_page() at the end of this block will take care of
         * release this page, thus avoiding a nasty leakage.
         */
        if (unlikely(!get_page_unless_zero(page)))
                goto out;

        /*
         * Check PageMovable before holding a PG_lock because page's owner
         * assumes anybody doesn't touch PG_lock of newly allocated page
         * so unconditionally grapping the lock ruins page's owner side.
         */
        if (unlikely(!__PageMovable(page)))
                goto out_putpage;
        /*
         * As movable pages are not isolated from LRU lists, concurrent
         * compaction threads can race against page migration functions
         * as well as race against the releasing a page.
         *
         * In order to avoid having an already isolated movable page
         * being (wrongly) re-isolated while it is under migration,
         * or to avoid attempting to isolate pages being released,
         * lets be sure we have the page lock
         * before proceeding with the movable page isolation steps.
         */
        if (unlikely(!trylock_page(page)))
                goto out_putpage;

        if (!PageMovable(page) || PageIsolated(page))
                goto out_no_isolated;

        mapping = page_mapping(page);
        VM_BUG_ON_PAGE(!mapping, page);

        if (!mapping->a_ops->isolate_page(page, mode))
                goto out_no_isolated;

        /* Driver shouldn't use PG_isolated bit of page->flags */
        WARN_ON_ONCE(PageIsolated(page));
        __SetPageIsolated(page);
        unlock_page(page);

        return 0;

out_no_isolated:
        unlock_page(page);
out_putpage:
        put_page(page);
out:
        return -EBUSY;
}

/* It should be called on page which is PG_movable */
void putback_movable_page(struct page *page)
{
        struct address_space *mapping;

        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_PAGE(!PageMovable(page), page);
        VM_BUG_ON_PAGE(!PageIsolated(page), page);

        mapping = page_mapping(page);
        mapping->a_ops->putback_page(page);
        __ClearPageIsolated(page);
}

/*
 * Put previously isolated pages back onto the appropriate lists
 * from where they were once taken off for compaction/migration.
 *
 * This function shall be used whenever the isolated pageset has been
 * built from lru, balloon, hugetlbfs page. See isolate_migratepages_range()
 * and isolate_huge_page().
 */
void putback_movable_pages(struct list_head *l)
{
        struct page *page;
        struct page *page2;

        list_for_each_entry_safe(page, page2, l, lru) {
                if (unlikely(PageHuge(page))) {
                        putback_active_hugepage(page);
                        continue;
                }
                list_del(&page->lru);
                /*
                 * We isolated non-lru movable page so here we can use
                 * __PageMovable because LRU page's mapping cannot have
                 * PAGE_MAPPING_MOVABLE.
                 */
                if (unlikely(__PageMovable(page))) {
                        VM_BUG_ON_PAGE(!PageIsolated(page), page);
                        lock_page(page);
                        if (PageMovable(page))
                                putback_movable_page(page);
                        else
                                __ClearPageIsolated(page);
                        unlock_page(page);
                        put_page(page);
                } else {
                        mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON +
                                        page_is_file_cache(page), -hpage_nr_pages(page));
                        putback_lru_page(page);
                }
        }
}

/*
 * Restore a potential migration pte to a working pte entry
 */
static bool remove_migration_pte(struct page *page, struct vm_area_struct *vma,
                                 unsigned long addr, void *old)
{
        struct page_vma_mapped_walk pvmw = {
                .page = old,
                .vma = vma,
                .address = addr,
                .flags = PVMW_SYNC | PVMW_MIGRATION,
        };
        struct page *new;
        pte_t pte;
        swp_entry_t entry;

        VM_BUG_ON_PAGE(PageTail(page), page);
        while (page_vma_mapped_walk(&pvmw)) {
                if (PageKsm(page))
                        new = page;
                else
                        new = page - pvmw.page->index +
                                linear_page_index(vma, pvmw.address);

#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
                /* PMD-mapped THP migration entry */
                if (!pvmw.pte) {
                        VM_BUG_ON_PAGE(PageHuge(page) || !PageTransCompound(page), page);
                        remove_migration_pmd(&pvmw, new);
                        continue;
                }
#endif

                get_page(new);
                pte = pte_mkold(mk_pte(new, READ_ONCE(vma->vm_page_prot)));
                if (pte_swp_soft_dirty(*pvmw.pte))
                        pte = pte_mksoft_dirty(pte);

                /*
                 * Recheck VMA as permissions can change since migration started
                 */
                entry = pte_to_swp_entry(*pvmw.pte);
                if (is_write_migration_entry(entry))
                        pte = maybe_mkwrite(pte, vma);

                if (unlikely(is_zone_device_page(new))) {
                        if (is_device_private_page(new)) {
                                entry = make_device_private_entry(new, pte_write(pte));
                                pte = swp_entry_to_pte(entry);
                        } else if (is_device_public_page(new)) {
                                pte = pte_mkdevmap(pte);
                                flush_dcache_page(new);
                        }
                } else
                        flush_dcache_page(new);

#ifdef CONFIG_HUGETLB_PAGE
                if (PageHuge(new)) {
                        pte = pte_mkhuge(pte);
                        pte = arch_make_huge_pte(pte, vma, new, 0);
                        set_huge_pte_at(vma->vm_mm, pvmw.address, pvmw.pte, pte);
                        if (PageAnon(new))
                                hugepage_add_anon_rmap(new, vma, pvmw.address);
                        else
                                page_dup_rmap(new, true);
                } else
#endif
                {
                        set_pte_at(vma->vm_mm, pvmw.address, pvmw.pte, pte);

                        if (PageAnon(new))
                                page_add_anon_rmap(new, vma, pvmw.address, false);
                        else
                                page_add_file_rmap(new, false);
                }
                if (vma->vm_flags & VM_LOCKED && !PageTransCompound(new))
                        mlock_vma_page(new);

                /* No need to invalidate - it was non-present before */
                update_mmu_cache(vma, pvmw.address, pvmw.pte);
        }

        return true;
}

/*
 * Get rid of all migration entries and replace them by
 * references to the indicated page.
 */
void remove_migration_ptes(struct page *old, struct page *new, bool locked)
{
        struct rmap_walk_control rwc = {
                .rmap_one = remove_migration_pte,
                .arg = old,
        };

        if (locked)
                rmap_walk_locked(new, &rwc);
        else
                rmap_walk(new, &rwc);
}

/*
 * 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, pte_t *ptep,
                                spinlock_t *ptl)
{
        pte_t pte;
        swp_entry_t entry;
        struct page *page;

        spin_lock(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);
}

void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
                                unsigned long address)
{
        spinlock_t *ptl = pte_lockptr(mm, pmd);
        pte_t *ptep = pte_offset_map(pmd, address);
        __migration_entry_wait(mm, ptep, ptl);
}

void migration_entry_wait_huge(struct vm_area_struct *vma,
                struct mm_struct *mm, pte_t *pte)
{
        spinlock_t *ptl = huge_pte_lockptr(hstate_vma(vma), mm, pte);
        __migration_entry_wait(mm, pte, ptl);
}

#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
void pmd_migration_entry_wait(struct mm_struct *mm, pmd_t *pmd)
{
        spinlock_t *ptl;
        struct page *page;

        ptl = pmd_lock(mm, pmd);
        if (!is_pmd_migration_entry(*pmd))
                goto unlock;
        page = migration_entry_to_page(pmd_to_swp_entry(*pmd));
        if (!get_page_unless_zero(page))
                goto unlock;
        spin_unlock(ptl);
        wait_on_page_locked(page);
        put_page(page);
        return;
unlock:
        spin_unlock(ptl);
}
#endif

#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.
 */
int migrate_page_move_mapping(struct address_space *mapping,
                struct page *newpage, struct page *page,
                struct buffer_head *head, enum migrate_mode mode,
                int extra_count)
{
        struct zone *oldzone, *newzone;
        int dirty;
        int expected_count = 1 + extra_count;
        void **pslot;

        /*
         * Device public or private pages have an extra refcount as they are
         * ZONE_DEVICE pages.
         */
        expected_count += is_device_private_page(page);
        expected_count += is_device_public_page(page);

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

                /* No turning back from here */
                newpage->index = page->index;
                newpage->mapping = page->mapping;
                if (PageSwapBacked(page))
                        __SetPageSwapBacked(newpage);

                return MIGRATEPAGE_SUCCESS;
        }

        oldzone = page_zone(page);
        newzone = page_zone(newpage);

        spin_lock_irq(&mapping->tree_lock);

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

        expected_count += 1 + 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_ref_freeze(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_ref_unfreeze(page, expected_count);
                spin_unlock_irq(&mapping->tree_lock);
                return -EAGAIN;
        }

        /*
         * Now we know that no one else is looking at the page:
         * no turning back from here.
         */
        newpage->index = page->index;
        newpage->mapping = page->mapping;
        get_page(newpage);      /* add cache reference */
        if (PageSwapBacked(page)) {
                __SetPageSwapBacked(newpage);
                if (PageSwapCache(page)) {
                        SetPageSwapCache(newpage);
                        set_page_private(newpage, page_private(page));
                }
        } else {
                VM_BUG_ON_PAGE(PageSwapCache(page), page);
        }

        /* Move dirty while page refs frozen and newpage not yet exposed */
        dirty = PageDirty(page);
        if (dirty) {
                ClearPageDirty(page);
                SetPageDirty(newpage);
        }

        radix_tree_replace_slot(&mapping->page_tree, pslot, newpage);

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

        spin_unlock(&mapping->tree_lock);
        /* Leave irq disabled to prevent preemption while updating stats */

        /*
         * 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_MAPPED if they
         * are mapped to swap space.
         */
        if (newzone != oldzone) {
                __dec_node_state(oldzone->zone_pgdat, NR_FILE_PAGES);
                __inc_node_state(newzone->zone_pgdat, NR_FILE_PAGES);
                if (PageSwapBacked(page) && !PageSwapCache(page)) {
                        __dec_node_state(oldzone->zone_pgdat, NR_SHMEM);
                        __inc_node_state(newzone->zone_pgdat, NR_SHMEM);
                }
                if (dirty && mapping_cap_account_dirty(mapping)) {
                        __dec_node_state(oldzone->zone_pgdat, NR_FILE_DIRTY);
                        __dec_zone_state(oldzone, NR_ZONE_WRITE_PENDING);
                        __inc_node_state(newzone->zone_pgdat, NR_FILE_DIRTY);
                        __inc_zone_state(newzone, NR_ZONE_WRITE_PENDING);
                }
        }
        local_irq_enable();

        return MIGRATEPAGE_SUCCESS;
}
EXPORT_SYMBOL(migrate_page_move_mapping);

/*
 * 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;

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

        newpage->index = page->index;
        newpage->mapping = page->mapping;

        get_page(newpage);

        radix_tree_replace_slot(&mapping->page_tree, pslot, newpage);

        page_ref_unfreeze(page, expected_count - 1);

        spin_unlock_irq(&mapping->tree_lock);

        return MIGRATEPAGE_SUCCESS;
}

/*
 * Gigantic pages are so large that we do not guarantee that page++ pointer
 * arithmetic will work across the entire page.  We need something more
 * specialized.
 */
static void __copy_gigantic_page(struct page *dst, struct page *src,
                                int nr_pages)
{
        int i;
        struct page *dst_base = dst;
        struct page *src_base = src;

        for (i = 0; i < nr_pages; ) {
                cond_resched();
                copy_highpage(dst, src);

                i++;
                dst = mem_map_next(dst, dst_base, i);
                src = mem_map_next(src, src_base, i);
        }
}

static void copy_huge_page(struct page *dst, struct page *src)
{
        int i;
        int nr_pages;

        if (PageHuge(src)) {
                /* hugetlbfs page */
                struct hstate *h = page_hstate(src);
                nr_pages = pages_per_huge_page(h);

                if (unlikely(nr_pages > MAX_ORDER_NR_PAGES)) {
                        __copy_gigantic_page(dst, src, nr_pages);
                        return;
                }
        } else {
                /* thp page */
                BUG_ON(!PageTransHuge(src));
                nr_pages = hpage_nr_pages(src);
        }

        for (i = 0; i < nr_pages; i++) {
                cond_resched();
                copy_highpage(dst + i, src + i);
        }
}

/*
 * Copy the page to its new location
 */
void migrate_page_states(struct page *newpage, struct page *page)
{
        int cpupid;

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

        /* Move dirty on pages not done by migrate_page_move_mapping() */
        if (PageDirty(page))
                SetPageDirty(newpage);

        if (page_is_young(page))
                set_page_young(newpage);
        if (page_is_idle(page))
                set_page_idle(newpage);

        /*
         * Copy NUMA information to the new page, to prevent over-eager
         * future migrations of this same page.
         */
        cpupid = page_cpupid_xchg_last(page, -1);
        page_cpupid_xchg_last(newpage, cpupid);

        ksm_migrate_page(newpage, page);
        /*
         * Please do not reorder this without considering how mm/ksm.c's
         * get_ksm_page() depends upon ksm_migrate_page() and PageSwapCache().
         */
        if (PageSwapCache(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);

        copy_page_owner(page, newpage);

        mem_cgroup_migrate(page, newpage);
}
EXPORT_SYMBOL(migrate_page_states);

void migrate_page_copy(struct page *newpage, struct page *page)
{
        if (PageHuge(page) || PageTransHuge(page))
                copy_huge_page(newpage, page);
        else
                copy_highpage(newpage, page);

        migrate_page_states(newpage, page);
}
EXPORT_SYMBOL(migrate_page_copy);

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

/*
 * Common logic to directly migrate a single LRU 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, 0);

        if (rc != MIGRATEPAGE_SUCCESS)
                return rc;

        if (mode != MIGRATE_SYNC_NO_COPY)
                migrate_page_copy(newpage, page);
        else
                migrate_page_states(newpage, page);
        return MIGRATEPAGE_SUCCESS;
}
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, 0);

        if (rc != MIGRATEPAGE_SUCCESS)
                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);

        if (mode != MIGRATE_SYNC_NO_COPY)
                migrate_page_copy(newpage, page);
        else
                migrate_page_states(newpage, page);

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

        } while (bh != head);

        return MIGRATEPAGE_SUCCESS;
}
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, false);

        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 */
                switch (mode) {
                case MIGRATE_SYNC:
                case MIGRATE_SYNC_NO_COPY:
                        break;
                default:
                        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
 *  MIGRATEPAGE_SUCCESS - success
 */
static int move_to_new_page(struct page *newpage, struct page *page,
                                enum migrate_mode mode)
{
        struct address_space *mapping;
        int rc = -EAGAIN;
        bool is_lru = !__PageMovable(page);

        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);

        mapping = page_mapping(page);

        if (likely(is_lru)) {
                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);
        } else {
                /*
                 * In case of non-lru page, it could be released after
                 * isolation step. In that case, we shouldn't try migration.
                 */
                VM_BUG_ON_PAGE(!PageIsolated(page), page);
                if (!PageMovable(page)) {
                        rc = MIGRATEPAGE_SUCCESS;
                        __ClearPageIsolated(page);
                        goto out;
                }

                rc = mapping->a_ops->migratepage(mapping, newpage,
                                                page, mode);
                WARN_ON_ONCE(rc == MIGRATEPAGE_SUCCESS &&
                        !PageIsolated(page));
        }

        /*
         * When successful, old pagecache page->mapping must be cleared before
         * page is freed; but stats require that PageAnon be left as PageAnon.
         */
        if (rc == MIGRATEPAGE_SUCCESS) {
                if (__PageMovable(page)) {
                        VM_BUG_ON_PAGE(!PageIsolated(page), page);

                        /*
                         * We clear PG_movable under page_lock so any compactor
                         * cannot try to migrate this page.
                         */
                        __ClearPageIsolated(page);
                }

                /*
                 * Anonymous and movable page->mapping will be cleard by
                 * free_pages_prepare so don't reset it here for keeping
                 * the type to work PageAnon, for example.
                 */
                if (!PageMappingFlags(page))
                        page->mapping = NULL;
        }
out:
        return rc;
}

static int __unmap_and_move(struct page *page, struct page *newpage,
                                int force, enum migrate_mode mode)
{
        int rc = -EAGAIN;
        int page_was_mapped = 0;
        struct anon_vma *anon_vma = NULL;
        bool is_lru = !__PageMovable(page);

        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);
        }

        if (PageWriteback(page)) {
                /*
                 * Only in the case of a full synchronous 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
                 */
                switch (mode) {
                case MIGRATE_SYNC:
                case MIGRATE_SYNC_NO_COPY:
                        break;
                default:
                        rc = -EBUSY;
                        goto out_unlock;
                }
                if (!force)
                        goto out_unlock;
                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.
         *
         * Only page_get_anon_vma() understands the subtleties of
         * getting a hold on an anon_vma from outside one of its mms.
         * But if we cannot get anon_vma, then we won't need it anyway,
         * because that implies that the anon page is no longer mapped
         * (and cannot be remapped so long as we hold the page lock).
         */
        if (PageAnon(page) && !PageKsm(page))
                anon_vma = page_get_anon_vma(page);

        /*
         * Block others from accessing the new page when we get around to
         * establishing additional references. We are usually the only one
         * holding a reference to newpage at this point. We used to have a BUG
         * here if trylock_page(newpage) fails, but would like to allow for
         * cases where there might be a race with the previous use of newpage.
         * This is much like races on refcount of oldpage: just don't BUG().
         */
        if (unlikely(!trylock_page(newpage)))
                goto out_unlock;

        if (unlikely(!is_lru)) {
                rc = move_to_new_page(newpage, page, mode);
                goto out_unlock_both;
        }

        /*
         * 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_PAGE(PageAnon(page), page);
                if (page_has_private(page)) {
                        try_to_free_buffers(page);
                        goto out_unlock_both;
                }
        } else if (page_mapped(page)) {
                /* Establish migration ptes */
                VM_BUG_ON_PAGE(PageAnon(page) && !PageKsm(page) && !anon_vma,
                                page);
                try_to_unmap(page,
                        TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
                page_was_mapped = 1;
        }

        if (!page_mapped(page))
                rc = move_to_new_page(newpage, page, mode);

        if (page_was_mapped)
                remove_migration_ptes(page,
                        rc == MIGRATEPAGE_SUCCESS ? newpage : page, false);

out_unlock_both:
        unlock_page(newpage);
out_unlock:
        /* Drop an anon_vma reference if we took one */
        if (anon_vma)
                put_anon_vma(anon_vma);
        unlock_page(page);
out:
        /*
         * If migration is successful, decrease refcount of the newpage
         * which will not free the page because new page owner increased
         * refcounter. As well, if it is LRU page, add the page to LRU
         * list in here.
         */
        if (rc == MIGRATEPAGE_SUCCESS) {
                if (unlikely(__PageMovable(newpage)))
                        put_page(newpage);
                else
                        putback_lru_page(newpage);
        }

        return rc;
}

/*
 * gcc 4.7 and 4.8 on arm get an ICEs when inlining unmap_and_move().  Work
 * around it.
 */
#if (GCC_VERSION >= 40700 && GCC_VERSION < 40900) && defined(CONFIG_ARM)
#define ICE_noinline noinline
#else
#define ICE_noinline
#endif

/*
 * Obtain the lock on page, remove all ptes and migrate the page
 * to the newly allocated page in newpage.
 */
static ICE_noinline int unmap_and_move(new_page_t get_new_page,
                                   free_page_t put_new_page,
                                   unsigned long private, struct page *page,
                                   int force, enum migrate_mode mode,
                                   enum migrate_reason reason)
{
        int rc = MIGRATEPAGE_SUCCESS;
        int *result = NULL;
        struct page *newpage;

        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. */
                ClearPageActive(page);
                ClearPageUnevictable(page);
                if (unlikely(__PageMovable(page))) {
                        lock_page(page);
                        if (!PageMovable(page))
                                __ClearPageIsolated(page);
                        unlock_page(page);
                }
                if (put_new_page)
                        put_new_page(newpage, private);
                else
                        put_page(newpage);
                goto out;
        }

        if (unlikely(PageTransHuge(page) && !PageTransHuge(newpage))) {
                lock_page(page);
                rc = split_huge_page(page);
                unlock_page(page);
                if (rc)
                        goto out;
        }

        rc = __unmap_and_move(page, newpage, force, mode);
        if (rc == MIGRATEPAGE_SUCCESS)
                set_page_owner_migrate_reason(newpage, reason);

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);

                /*
                 * Compaction can migrate also non-LRU pages which are
                 * not accounted to NR_ISOLATED_*. They can be recognized
                 * as __PageMovable
                 */
                if (likely(!__PageMovable(page)))
                        mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON +
                                        page_is_file_cache(page), -hpage_nr_pages(page));
        }

        /*
         * If migration is successful, releases reference grabbed during
         * isolation. Otherwise, restore the page to right list unless
         * we want to retry.
         */
        if (rc == MIGRATEPAGE_SUCCESS) {
                put_page(page);
                if (reason == MR_MEMORY_FAILURE) {
                        /*
                         * Set PG_HWPoison on just freed page
                         * intentionally. Although it's rather weird,
                         * it's how HWPoison flag works at the moment.
                         */
                        if (!test_set_page_hwpoison(page))
                                num_poisoned_pages_inc();
                }
        } else {
                if (rc != -EAGAIN) {
                        if (likely(!__PageMovable(page))) {
                                putback_lru_page(page);
                                goto put_new;
                        }

                        lock_page(page);
                        if (PageMovable(page))
                                putback_movable_page(page);
                        else
                                __ClearPageIsolated(page);
                        unlock_page(page);
                        put_page(page);
                }
put_new:
                if (put_new_page)
                        put_new_page(newpage, private);
                else
                        put_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,
                                free_page_t put_new_page, unsigned long private,
                                struct page *hpage, int force,
                                enum migrate_mode mode, int reason)
{
        int rc = -EAGAIN;
        int *result = NULL;
        int page_was_mapped = 0;
        struct page *new_hpage;
        struct anon_vma *anon_vma = NULL;

        /*
         * Movability of hugepages depends on architectures and hugepage size.
         * This check is necessary because some callers of hugepage migration
         * like soft offline and memory hotremove don't walk through page
         * tables or check whether the hugepage is pmd-based or not before
         * kicking migration.
         */
        if (!hugepage_migration_supported(page_hstate(hpage))) {
                putback_active_hugepage(hpage);
                return -ENOSYS;
        }

        new_hpage = get_new_page(hpage, private, &result);
        if (!new_hpage)
                return -ENOMEM;

        if (!trylock_page(hpage)) {
                if (!force)
                        goto out;
                switch (mode) {
                case MIGRATE_SYNC:
                case MIGRATE_SYNC_NO_COPY:
                        break;
                default:
                        goto out;
                }
                lock_page(hpage);
        }

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

        if (unlikely(!trylock_page(new_hpage)))
                goto put_anon;

        if (page_mapped(hpage)) {
                try_to_unmap(hpage,
                        TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
                page_was_mapped = 1;
        }

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

        if (page_was_mapped)
                remove_migration_ptes(hpage,
                        rc == MIGRATEPAGE_SUCCESS ? new_hpage : hpage, false);

        unlock_page(new_hpage);

put_anon:
        if (anon_vma)
                put_anon_vma(anon_vma);

        if (rc == MIGRATEPAGE_SUCCESS) {
                move_hugetlb_state(hpage, new_hpage, reason);
                put_new_page = NULL;
        }

        unlock_page(hpage);
out:
        if (rc != -EAGAIN)
                putback_active_hugepage(hpage);
        if (reason == MR_MEMORY_FAILURE && !test_set_page_hwpoison(hpage))
                num_poisoned_pages_inc();

        /*
         * If migration was not successful and there's a freeing callback, use
         * it.  Otherwise, put_page() will drop the reference grabbed during
         * isolation.
         */
        if (put_new_page)
                put_new_page(new_hpage, private);
        else
                putback_active_hugepage(new_hpage);

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

/*
 * migrate_pages - migrate the pages specified in a list, to the free pages
 *                 supplied as the target for the page migration
 *
 * @from:               The list of pages to be migrated.
 * @get_new_page:       The function used to allocate free pages to be used
 *                      as the target of the page migration.
 * @put_new_page:       The function used to free target pages if migration
 *                      fails, or NULL if no special handling is necessary.
 * @private:            Private data to be passed on to get_new_page()
 * @mode:               The migration mode that specifies the constraints for
 *                      page migration, if any.
 * @reason:             The reason for page migration.
 *
 * The function returns after 10 attempts or if no pages are movable any more
 * because the list has become empty or no retryable pages exist any more.
 * The caller should call putback_movable_pages() to return pages to the LRU
 * or free list only if ret != 0.
 *
 * Returns the number of pages that were not migrated, or an error code.
 */
int migrate_pages(struct list_head *from, new_page_t get_new_page,
                free_page_t put_new_page, unsigned long private,
                enum migrate_mode mode, int reason)
{
        int retry = 1;
        int nr_failed = 0;
        int nr_succeeded = 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();

                        if (PageHuge(page))
                                rc = unmap_and_move_huge_page(get_new_page,
                                                put_new_page, private, page,
                                                pass > 2, mode, reason);
                        else
                                rc = unmap_and_move(get_new_page, put_new_page,
                                                private, page, pass > 2, mode,
                                                reason);

                        switch(rc) {
                        case -ENOMEM:
                                nr_failed++;
                                goto out;
                        case -EAGAIN:
                                retry++;
                                break;
                        case MIGRATEPAGE_SUCCESS:
                                nr_succeeded++;
                                break;
                        default:
                                /*
                                 * Permanent failure (-EBUSY, -ENOSYS, etc.):
                                 * unlike -EAGAIN case, the failed page is
                                 * removed from migration page list and not
                                 * retried in the next outer loop.
                                 */
                                nr_failed++;
                                break;
                        }
                }
        }
        nr_failed += retry;
        rc = nr_failed;
out:
        if (nr_succeeded)
                count_vm_events(PGMIGRATE_SUCCESS, nr_succeeded);
        if (nr_failed)
                count_vm_events(PGMIGRATE_FAIL, nr_failed);
        trace_mm_migrate_pages(nr_succeeded, nr_failed, mode, reason);

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

        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;

        if (PageHuge(p))
                return alloc_huge_page_node(page_hstate(compound_head(p)),
                                        pm->node);
        else if (thp_migration_supported() && PageTransHuge(p)) {
                struct page *thp;

                thp = alloc_pages_node(pm->node,
                        (GFP_TRANSHUGE | __GFP_THISNODE) & ~__GFP_RECLAIM,
                        HPAGE_PMD_ORDER);
                if (!thp)
                        return NULL;
                prep_transhuge_page(thp);
                return thp;
        } else
                return __alloc_pages_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;
                struct page *head;
                unsigned int follflags;

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

                /* FOLL_DUMP to ignore special (like zero) pages */
                follflags = FOLL_GET | FOLL_DUMP;
                if (!thp_migration_supported())
                        follflags |= FOLL_SPLIT;
                page = follow_page(vma, pp->addr, follflags);

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

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

                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;

                if (PageHuge(page)) {
                        if (PageHead(page)) {
                                isolate_huge_page(page, &pagelist);
                                err = 0;
                                pp->page = page;
                        }
                        goto put_and_set;
                }

                pp->page = compound_head(page);
                head = compound_head(page);
                err = isolate_lru_page(head);
                if (!err) {
                        list_add_tail(&head->lru, &pagelist);
                        mod_node_page_state(page_pgdat(head),
                                NR_ISOLATED_ANON + page_is_file_cache(head),
                                hpage_nr_pages(head));
                }
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, NULL,
                                (unsigned long)pm, MIGRATE_SYNC, MR_SYSCALL);
                if (err)
                        putback_movable_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_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;

                /* FOLL_DUMP to ignore special (like zero) pages */
                page = follow_page(vma, addr, FOLL_DUMP);

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

                err = page ? page_to_nid(page) : -ENOENT;
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)
{
        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. Use the regular "ptrace_may_access()" checks.
         */
        if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
                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;
}

#ifdef CONFIG_NUMA_BALANCING
/*
 * Returns true if this is a safe migration target node for misplaced NUMA
 * pages. Currently it only checks the watermarks which crude
 */
static bool migrate_balanced_pgdat(struct pglist_data *pgdat,
                                   unsigned long nr_migrate_pages)
{
        int z;

        for (z = pgdat->nr_zones - 1; z >= 0; z--) {
                struct zone *zone = pgdat->node_zones + z;

                if (!populated_zone(zone))
                        continue;

                /* Avoid waking kswapd by allocating pages_to_migrate pages. */
                if (!zone_watermark_ok(zone, 0,
                                       high_wmark_pages(zone) +
                                       nr_migrate_pages,
                                       0, 0))
                        continue;
                return true;
        }
        return false;
}

static struct page *alloc_misplaced_dst_page(struct page *page,
                                           unsigned long data,
                                           int **result)
{
        int nid = (int) data;
        struct page *newpage;

        newpage = __alloc_pages_node(nid,
                                         (GFP_HIGHUSER_MOVABLE |
                                          __GFP_THISNODE | __GFP_NOMEMALLOC |
                                          __GFP_NORETRY | __GFP_NOWARN) &
                                         ~__GFP_RECLAIM, 0);

        return newpage;
}

/*
 * page migration rate limiting control.
 * Do not migrate more than @pages_to_migrate in a @migrate_interval_millisecs
 * window of time. Default here says do not migrate more than 1280M per second.
 */
static unsigned int migrate_interval_millisecs __read_mostly = 100;
static unsigned int ratelimit_pages __read_mostly = 128 << (20 - PAGE_SHIFT);

/* Returns true if the node is migrate rate-limited after the update */
static bool numamigrate_update_ratelimit(pg_data_t *pgdat,
                                        unsigned long nr_pages)
{
        /*
         * Rate-limit the amount of data that is being migrated to a node.
         * Optimal placement is no good if the memory bus is saturated and
         * all the time is being spent migrating!
         */
        if (time_after(jiffies, pgdat->numabalancing_migrate_next_window)) {
                spin_lock(&pgdat->numabalancing_migrate_lock);
                pgdat->numabalancing_migrate_nr_pages = 0;
                pgdat->numabalancing_migrate_next_window = jiffies +
                        msecs_to_jiffies(migrate_interval_millisecs);
                spin_unlock(&pgdat->numabalancing_migrate_lock);
        }
        if (pgdat->numabalancing_migrate_nr_pages > ratelimit_pages) {
                trace_mm_numa_migrate_ratelimit(current, pgdat->node_id,
                                                                nr_pages);
                return true;
        }

        /*
         * This is an unlocked non-atomic update so errors are possible.
         * The consequences are failing to migrate when we potentiall should
         * have which is not severe enough to warrant locking. If it is ever
         * a problem, it can be converted to a per-cpu counter.
         */
        pgdat->numabalancing_migrate_nr_pages += nr_pages;
        return false;
}

static int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)
{
        int page_lru;

        VM_BUG_ON_PAGE(compound_order(page) && !PageTransHuge(page), page);

        /* Avoid migrating to a node that is nearly full */
        if (!migrate_balanced_pgdat(pgdat, 1UL << compound_order(page)))
                return 0;

        if (isolate_lru_page(page))
                return 0;

        /*
         * migrate_misplaced_transhuge_page() skips page migration's usual
         * check on page_count(), so we must do it here, now that the page
         * has been isolated: a GUP pin, or any other pin, prevents migration.
         * The expected page count is 3: 1 for page's mapcount and 1 for the
         * caller's pin and 1 for the reference taken by isolate_lru_page().
         */
        if (PageTransHuge(page) && page_count(page) != 3) {
                putback_lru_page(page);
                return 0;
        }

        page_lru = page_is_file_cache(page);
        mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON + page_lru,
                                hpage_nr_pages(page));

        /*
         * Isolating the page has taken another reference, so the
         * caller's reference can be safely dropped without the page
         * disappearing underneath us during migration.
         */
        put_page(page);
        return 1;
}

bool pmd_trans_migrating(pmd_t pmd)
{
        struct page *page = pmd_page(pmd);
        return PageLocked(page);
}

/*
 * Attempt to migrate a misplaced page to the specified destination
 * node. Caller is expected to have an elevated reference count on
 * the page that will be dropped by this function before returning.
 */
int migrate_misplaced_page(struct page *page, struct vm_area_struct *vma,
                           int node)
{
        pg_data_t *pgdat = NODE_DATA(node);
        int isolated;
        int nr_remaining;
        LIST_HEAD(migratepages);

        /*
         * Don't migrate file pages that are mapped in multiple processes
         * with execute permissions as they are probably shared libraries.
         */
        if (page_mapcount(page) != 1 && page_is_file_cache(page) &&
            (vma->vm_flags & VM_EXEC))
                goto out;

        /*
         * Rate-limit the amount of data that is being migrated to a node.
         * Optimal placement is no good if the memory bus is saturated and
         * all the time is being spent migrating!
         */
        if (numamigrate_update_ratelimit(pgdat, 1))
                goto out;

        isolated = numamigrate_isolate_page(pgdat, page);
        if (!isolated)
                goto out;

        list_add(&page->lru, &migratepages);
        nr_remaining = migrate_pages(&migratepages, alloc_misplaced_dst_page,
                                     NULL, node, MIGRATE_ASYNC,
                                     MR_NUMA_MISPLACED);
        if (nr_remaining) {
                if (!list_empty(&migratepages)) {
                        list_del(&page->lru);
                        dec_node_page_state(page, NR_ISOLATED_ANON +
                                        page_is_file_cache(page));
                        putback_lru_page(page);
                }
                isolated = 0;
        } else
                count_vm_numa_event(NUMA_PAGE_MIGRATE);
        BUG_ON(!list_empty(&migratepages));
        return isolated;

out:
        put_page(page);
        return 0;
}
#endif /* CONFIG_NUMA_BALANCING */

#if defined(CONFIG_NUMA_BALANCING) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
/*
 * Migrates a THP to a given target node. page must be locked and is unlocked
 * before returning.
 */
int migrate_misplaced_transhuge_page(struct mm_struct *mm,
                                struct vm_area_struct *vma,
                                pmd_t *pmd, pmd_t entry,
                                unsigned long address,
                                struct page *page, int node)
{
        spinlock_t *ptl;
        pg_data_t *pgdat = NODE_DATA(node);
        int isolated = 0;
        struct page *new_page = NULL;
        int page_lru = page_is_file_cache(page);
        unsigned long mmun_start = address & HPAGE_PMD_MASK;
        unsigned long mmun_end = mmun_start + HPAGE_PMD_SIZE;

        /*
         * Rate-limit the amount of data that is being migrated to a node.
         * Optimal placement is no good if the memory bus is saturated and
         * all the time is being spent migrating!
         */
        if (numamigrate_update_ratelimit(pgdat, HPAGE_PMD_NR))
                goto out_dropref;

        new_page = alloc_pages_node(node,
                (GFP_TRANSHUGE_LIGHT | __GFP_THISNODE),
                HPAGE_PMD_ORDER);
        if (!new_page)
                goto out_fail;
        prep_transhuge_page(new_page);

        isolated = numamigrate_isolate_page(pgdat, page);
        if (!isolated) {
                put_page(new_page);
                goto out_fail;
        }

        /* Prepare a page as a migration target */
        __SetPageLocked(new_page);
        if (PageSwapBacked(page))
                __SetPageSwapBacked(new_page);

        /* anon mapping, we can simply copy page->mapping to the new page: */
        new_page->mapping = page->mapping;
        new_page->index = page->index;
        migrate_page_copy(new_page, page);
        WARN_ON(PageLRU(new_page));

        /* Recheck the target PMD */
        mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
        ptl = pmd_lock(mm, pmd);
        if (unlikely(!pmd_same(*pmd, entry) || !page_ref_freeze(page, 2))) {
                spin_unlock(ptl);
                mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);

                /* Reverse changes made by migrate_page_copy() */
                if (TestClearPageActive(new_page))
                        SetPageActive(page);
                if (TestClearPageUnevictable(new_page))
                        SetPageUnevictable(page);

                unlock_page(new_page);
                put_page(new_page);             /* Free it */

                /* Retake the callers reference and putback on LRU */
                get_page(page);
                putback_lru_page(page);
                mod_node_page_state(page_pgdat(page),
                         NR_ISOLATED_ANON + page_lru, -HPAGE_PMD_NR);

                goto out_unlock;
        }

        entry = mk_huge_pmd(new_page, vma->vm_page_prot);
        entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);

        /*
         * Clear the old entry under pagetable lock and establish the new PTE.
         * Any parallel GUP will either observe the old page blocking on the
         * page lock, block on the page table lock or observe the new page.
         * The SetPageUptodate on the new page and page_add_new_anon_rmap
         * guarantee the copy is visible before the pagetable update.
         */
        flush_cache_range(vma, mmun_start, mmun_end);
        page_add_anon_rmap(new_page, vma, mmun_start, true);
        pmdp_huge_clear_flush_notify(vma, mmun_start, pmd);
        set_pmd_at(mm, mmun_start, pmd, entry);
        update_mmu_cache_pmd(vma, address, &entry);

        page_ref_unfreeze(page, 2);
        mlock_migrate_page(new_page, page);
        page_remove_rmap(page, true);
        set_page_owner_migrate_reason(new_page, MR_NUMA_MISPLACED);

        spin_unlock(ptl);
        /*
         * No need to double call mmu_notifier->invalidate_range() callback as
         * the above pmdp_huge_clear_flush_notify() did already call it.
         */
        mmu_notifier_invalidate_range_only_end(mm, mmun_start, mmun_end);

        /* Take an "isolate" reference and put new page on the LRU. */
        get_page(new_page);
        putback_lru_page(new_page);

        unlock_page(new_page);
        unlock_page(page);
        put_page(page);                 /* Drop the rmap reference */
        put_page(page);                 /* Drop the LRU isolation reference */

        count_vm_events(PGMIGRATE_SUCCESS, HPAGE_PMD_NR);
        count_vm_numa_events(NUMA_PAGE_MIGRATE, HPAGE_PMD_NR);

        mod_node_page_state(page_pgdat(page),
                        NR_ISOLATED_ANON + page_lru,
                        -HPAGE_PMD_NR);
        return isolated;

out_fail:
        count_vm_events(PGMIGRATE_FAIL, HPAGE_PMD_NR);
out_dropref:
        ptl = pmd_lock(mm, pmd);
        if (pmd_same(*pmd, entry)) {
                entry = pmd_modify(entry, vma->vm_page_prot);
                set_pmd_at(mm, mmun_start, pmd, entry);
                update_mmu_cache_pmd(vma, address, &entry);
        }
        spin_unlock(ptl);

out_unlock:
        unlock_page(page);
        put_page(page);
        return 0;
}
#endif /* CONFIG_NUMA_BALANCING */

#endif /* CONFIG_NUMA */

#if defined(CONFIG_MIGRATE_VMA_HELPER)
struct migrate_vma {
        struct vm_area_struct   *vma;
        unsigned long           *dst;
        unsigned long           *src;
        unsigned long           cpages;
        unsigned long           npages;
        unsigned long           start;
        unsigned long           end;
};

static int migrate_vma_collect_hole(unsigned long start,
                                    unsigned long end,
                                    struct mm_walk *walk)
{
        struct migrate_vma *migrate = walk->private;
        unsigned long addr;

        for (addr = start & PAGE_MASK; addr < end; addr += PAGE_SIZE) {
                migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE;
                migrate->dst[migrate->npages] = 0;
                migrate->npages++;
                migrate->cpages++;
        }

        return 0;
}

static int migrate_vma_collect_skip(unsigned long start,
                                    unsigned long end,
                                    struct mm_walk *walk)
{
        struct migrate_vma *migrate = walk->private;
        unsigned long addr;

        for (addr = start & PAGE_MASK; addr < end; addr += PAGE_SIZE) {
                migrate->dst[migrate->npages] = 0;
                migrate->src[migrate->npages++] = 0;
        }

        return 0;
}

static int migrate_vma_collect_pmd(pmd_t *pmdp,
                                   unsigned long start,
                                   unsigned long end,
                                   struct mm_walk *walk)
{
        struct migrate_vma *migrate = walk->private;
        struct vm_area_struct *vma = walk->vma;
        struct mm_struct *mm = vma->vm_mm;
        unsigned long addr = start, unmapped = 0;
        spinlock_t *ptl;
        pte_t *ptep;

again:
        if (pmd_none(*pmdp))
                return migrate_vma_collect_hole(start, end, walk);

        if (pmd_trans_huge(*pmdp)) {
                struct page *page;

                ptl = pmd_lock(mm, pmdp);
                if (unlikely(!pmd_trans_huge(*pmdp))) {
                        spin_unlock(ptl);
                        goto again;
                }

                page = pmd_page(*pmdp);
                if (is_huge_zero_page(page)) {
                        spin_unlock(ptl);
                        split_huge_pmd(vma, pmdp, addr);
                        if (pmd_trans_unstable(pmdp))
                                return migrate_vma_collect_skip(start, end,
                                                                walk);
                } else {
                        int ret;

                        get_page(page);
                        spin_unlock(ptl);
                        if (unlikely(!trylock_page(page)))
                                return migrate_vma_collect_skip(start, end,
                                                                walk);
                        ret = split_huge_page(page);
                        unlock_page(page);
                        put_page(page);
                        if (ret)
                                return migrate_vma_collect_skip(start, end,
                                                                walk);
                        if (pmd_none(*pmdp))
                                return migrate_vma_collect_hole(start, end,
                                                                walk);
                }
        }

        if (unlikely(pmd_bad(*pmdp)))
                return migrate_vma_collect_skip(start, end, walk);

        ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
        arch_enter_lazy_mmu_mode();

        for (; addr < end; addr += PAGE_SIZE, ptep++) {
                unsigned long mpfn, pfn;
                struct page *page;
                swp_entry_t entry;
                pte_t pte;

                pte = *ptep;
                pfn = pte_pfn(pte);

                if (pte_none(pte)) {
                        mpfn = MIGRATE_PFN_MIGRATE;
                        migrate->cpages++;
                        pfn = 0;
                        goto next;
                }

                if (!pte_present(pte)) {
                        mpfn = pfn = 0;

                        /*
                         * Only care about unaddressable device page special
                         * page table entry. Other special swap entries are not
                         * migratable, and we ignore regular swapped page.
                         */
                        entry = pte_to_swp_entry(pte);
                        if (!is_device_private_entry(entry))
                                goto next;

                        page = device_private_entry_to_page(entry);
                        mpfn = migrate_pfn(page_to_pfn(page))|
                                MIGRATE_PFN_DEVICE | MIGRATE_PFN_MIGRATE;
                        if (is_write_device_private_entry(entry))
                                mpfn |= MIGRATE_PFN_WRITE;
                } else {
                        if (is_zero_pfn(pfn)) {
                                mpfn = MIGRATE_PFN_MIGRATE;
                                migrate->cpages++;
                                pfn = 0;
                                goto next;
                        }
                        page = _vm_normal_page(migrate->vma, addr, pte, true);
                        mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
                        mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
                }

                /* FIXME support THP */
                if (!page || !page->mapping || PageTransCompound(page)) {
                        mpfn = pfn = 0;
                        goto next;
                }
                pfn = page_to_pfn(page);

                /*
                 * By getting a reference on the page we pin it and that blocks
                 * any kind of migration. Side effect is that it "freezes" the
                 * pte.
                 *
                 * We drop this reference after isolating the page from the lru
                 * for non device page (device page are not on the lru and thus
                 * can't be dropped from it).
                 */
                get_page(page);
                migrate->cpages++;

                /*
                 * Optimize for the common case where page is only mapped once
                 * in one process. If we can lock the page, then we can safely
                 * set up a special migration page table entry now.
                 */
                if (trylock_page(page)) {
                        pte_t swp_pte;

                        mpfn |= MIGRATE_PFN_LOCKED;
                        ptep_get_and_clear(mm, addr, ptep);

                        /* Setup special migration page table entry */
                        entry = make_migration_entry(page, pte_write(pte));
                        swp_pte = swp_entry_to_pte(entry);
                        if (pte_soft_dirty(pte))
                                swp_pte = pte_swp_mksoft_dirty(swp_pte);
                        set_pte_at(mm, addr, ptep, swp_pte);

                        /*
                         * This is like regular unmap: we remove the rmap and
                         * drop page refcount. Page won't be freed, as we took
                         * a reference just above.
                         */
                        page_remove_rmap(page, false);
                        put_page(page);

                        if (pte_present(pte))
                                unmapped++;
                }

next:
                migrate->dst[migrate->npages] = 0;
                migrate->src[migrate->npages++] = mpfn;
        }
        arch_leave_lazy_mmu_mode();
        pte_unmap_unlock(ptep - 1, ptl);

        /* Only flush the TLB if we actually modified any entries */
        if (unmapped)
                flush_tlb_range(walk->vma, start, end);

        return 0;
}

/*
 * migrate_vma_collect() - collect pages over a range of virtual addresses
 * @migrate: migrate struct containing all migration information
 *
 * This will walk the CPU page table. For each virtual address backed by a
 * valid page, it updates the src array and takes a reference on the page, in
 * order to pin the page until we lock it and unmap it.
 */
static void migrate_vma_collect(struct migrate_vma *migrate)
{
        struct mm_walk mm_walk;

        mm_walk.pmd_entry = migrate_vma_collect_pmd;
        mm_walk.pte_entry = NULL;
        mm_walk.pte_hole = migrate_vma_collect_hole;
        mm_walk.hugetlb_entry = NULL;
        mm_walk.test_walk = NULL;
        mm_walk.vma = migrate->vma;
        mm_walk.mm = migrate->vma->vm_mm;
        mm_walk.private = migrate;

        mmu_notifier_invalidate_range_start(mm_walk.mm,
                                            migrate->start,
                                            migrate->end);
        walk_page_range(migrate->start, migrate->end, &mm_walk);
        mmu_notifier_invalidate_range_end(mm_walk.mm,
                                          migrate->start,
                                          migrate->end);

        migrate->end = migrate->start + (migrate->npages << PAGE_SHIFT);
}

/*
 * migrate_vma_check_page() - check if page is pinned or not
 * @page: struct page to check
 *
 * Pinned pages cannot be migrated. This is the same test as in
 * migrate_page_move_mapping(), except that here we allow migration of a
 * ZONE_DEVICE page.
 */
static bool migrate_vma_check_page(struct page *page)
{
        /*
         * One extra ref because caller holds an extra reference, either from
         * isolate_lru_page() for a regular page, or migrate_vma_collect() for
         * a device page.
         */
        int extra = 1;

        /*
         * FIXME support THP (transparent huge page), it is bit more complex to
         * check them than regular pages, because they can be mapped with a pmd
         * or with a pte (split pte mapping).
         */
        if (PageCompound(page))
                return false;

        /* Page from ZONE_DEVICE have one extra reference */
        if (is_zone_device_page(page)) {
                /*
                 * Private page can never be pin as they have no valid pte and
                 * GUP will fail for those. Yet if there is a pending migration
                 * a thread might try to wait on the pte migration entry and
                 * will bump the page reference count. Sadly there is no way to
                 * differentiate a regular pin from migration wait. Hence to
                 * avoid 2 racing thread trying to migrate back to CPU to enter
                 * infinite loop (one stoping migration because the other is
                 * waiting on pte migration entry). We always return true here.
                 *
                 * FIXME proper solution is to rework migration_entry_wait() so
                 * it does not need to take a reference on page.
                 */
                if (is_device_private_page(page))
                        return true;

                /*
                 * Only allow device public page to be migrated and account for
                 * the extra reference count imply by ZONE_DEVICE pages.
                 */
                if (!is_device_public_page(page))
                        return false;
                extra++;
        }

        /* For file back page */
        if (page_mapping(page))
                extra += 1 + page_has_private(page);

        if ((page_count(page) - extra) > page_mapcount(page))
                return false;

        return true;
}

/*
 * migrate_vma_prepare() - lock pages and isolate them from the lru
 * @migrate: migrate struct containing all migration information
 *
 * This locks pages that have been collected by migrate_vma_collect(). Once each
 * page is locked it is isolated from the lru (for non-device pages). Finally,
 * the ref taken by migrate_vma_collect() is dropped, as locked pages cannot be
 * migrated by concurrent kernel threads.
 */
static void migrate_vma_prepare(struct migrate_vma *migrate)
{
        const unsigned long npages = migrate->npages;
        const unsigned long start = migrate->start;
        unsigned long addr, i, restore = 0;
        bool allow_drain = true;

        lru_add_drain();

        for (i = 0; (i < npages) && migrate->cpages; i++) {
                struct page *page = migrate_pfn_to_page(migrate->src[i]);
                bool remap = true;

                if (!page)
                        continue;

                if (!(migrate->src[i] & MIGRATE_PFN_LOCKED)) {
                        /*
                         * Because we are migrating several pages there can be
                         * a deadlock between 2 concurrent migration where each
                         * are waiting on each other page lock.
                         *
                         * Make migrate_vma() a best effort thing and backoff
                         * for any page we can not lock right away.
                         */
                        if (!trylock_page(page)) {
                                migrate->src[i] = 0;
                                migrate->cpages--;
                                put_page(page);
                                continue;
                        }
                        remap = false;
                        migrate->src[i] |= MIGRATE_PFN_LOCKED;
                }

                /* ZONE_DEVICE pages are not on LRU */
                if (!is_zone_device_page(page)) {
                        if (!PageLRU(page) && allow_drain) {
                                /* Drain CPU's pagevec */
                                lru_add_drain_all();
                                allow_drain = false;
                        }

                        if (isolate_lru_page(page)) {
                                if (remap) {
                                        migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
                                        migrate->cpages--;
                                        restore++;
                                } else {
                                        migrate->src[i] = 0;
                                        unlock_page(page);
                                        migrate->cpages--;
                                        put_page(page);
                                }
                                continue;
                        }

                        /* Drop the reference we took in collect */
                        put_page(page);
                }

                if (!migrate_vma_check_page(page)) {
                        if (remap) {
                                migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
                                migrate->cpages--;
                                restore++;

                                if (!is_zone_device_page(page)) {
                                        get_page(page);
                                        putback_lru_page(page);
                                }
                        } else {
                                migrate->src[i] = 0;
                                unlock_page(page);
                                migrate->cpages--;

                                if (!is_zone_device_page(page))
                                        putback_lru_page(page);
                                else
                                        put_page(page);
                        }
                }
        }

        for (i = 0, addr = start; i < npages && restore; i++, addr += PAGE_SIZE) {
                struct page *page = migrate_pfn_to_page(migrate->src[i]);

                if (!page || (migrate->src[i] & MIGRATE_PFN_MIGRATE))
                        continue;

                remove_migration_pte(page, migrate->vma, addr, page);

                migrate->src[i] = 0;
                unlock_page(page);
                put_page(page);
                restore--;
        }
}

/*
 * migrate_vma_unmap() - replace page mapping with special migration pte entry
 * @migrate: migrate struct containing all migration information
 *
 * Replace page mapping (CPU page table pte) with a special migration pte entry
 * and check again if it has been pinned. Pinned pages are restored because we
 * cannot migrate them.
 *
 * This is the last step before we call the device driver callback to allocate
 * destination memory and copy contents of original page over to new page.
 */
static void migrate_vma_unmap(struct migrate_vma *migrate)
{
        int flags = TTU_MIGRATION | TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS;
        const unsigned long npages = migrate->npages;
        const unsigned long start = migrate->start;
        unsigned long addr, i, restore = 0;

        for (i = 0; i < npages; i++) {
                struct page *page = migrate_pfn_to_page(migrate->src[i]);

                if (!page || !(migrate->src[i] & MIGRATE_PFN_MIGRATE))
                        continue;

                if (page_mapped(page)) {
                        try_to_unmap(page, flags);
                        if (page_mapped(page))
                                goto restore;
                }

                if (migrate_vma_check_page(page))
                        continue;

restore:
                migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
                migrate->cpages--;
                restore++;
        }

        for (addr = start, i = 0; i < npages && restore; addr += PAGE_SIZE, i++) {
                struct page *page = migrate_pfn_to_page(migrate->src[i]);

                if (!page || (migrate->src[i] & MIGRATE_PFN_MIGRATE))
                        continue;

                remove_migration_ptes(page, page, false);

                migrate->src[i] = 0;
                unlock_page(page);
                restore--;

                if (is_zone_device_page(page))
                        put_page(page);
                else
                        putback_lru_page(page);
        }
}

static void migrate_vma_insert_page(struct migrate_vma *migrate,
                                    unsigned long addr,
                                    struct page *page,
                                    unsigned long *src,
                                    unsigned long *dst)
{
        struct vm_area_struct *vma = migrate->vma;
        struct mm_struct *mm = vma->vm_mm;
        struct mem_cgroup *memcg;
        bool flush = false;
        spinlock_t *ptl;
        pte_t entry;
        pgd_t *pgdp;
        p4d_t *p4dp;
        pud_t *pudp;
        pmd_t *pmdp;
        pte_t *ptep;

        /* Only allow populating anonymous memory */
        if (!vma_is_anonymous(vma))
                goto abort;

        pgdp = pgd_offset(mm, addr);
        p4dp = p4d_alloc(mm, pgdp, addr);
        if (!p4dp)
                goto abort;
        pudp = pud_alloc(mm, p4dp, addr);
        if (!pudp)
                goto abort;
        pmdp = pmd_alloc(mm, pudp, addr);
        if (!pmdp)
                goto abort;

        if (pmd_trans_huge(*pmdp) || pmd_devmap(*pmdp))
                goto abort;

        /*
         * Use pte_alloc() instead of pte_alloc_map().  We can't run
         * pte_offset_map() on pmds where a huge pmd might be created
         * from a different thread.
         *
         * pte_alloc_map() is safe to use under down_write(mmap_sem) or when
         * parallel threads are excluded by other means.
         *
         * Here we only have down_read(mmap_sem).
         */
        if (pte_alloc(mm, pmdp, addr))
                goto abort;

        /* See the comment in pte_alloc_one_map() */
        if (unlikely(pmd_trans_unstable(pmdp)))
                goto abort;

        if (unlikely(anon_vma_prepare(vma)))
                goto abort;
        if (mem_cgroup_try_charge(page, vma->vm_mm, GFP_KERNEL, &memcg, false))
                goto abort;

        /*
         * The memory barrier inside __SetPageUptodate makes sure that
         * preceding stores to the page contents become visible before
         * the set_pte_at() write.
         */
        __SetPageUptodate(page);

        if (is_zone_device_page(page)) {
                if (is_device_private_page(page)) {
                        swp_entry_t swp_entry;

                        swp_entry = make_device_private_entry(page, vma->vm_flags & VM_WRITE);
                        entry = swp_entry_to_pte(swp_entry);
                } else if (is_device_public_page(page)) {
                        entry = pte_mkold(mk_pte(page, READ_ONCE(vma->vm_page_prot)));
                        if (vma->vm_flags & VM_WRITE)
                                entry = pte_mkwrite(pte_mkdirty(entry));
                        entry = pte_mkdevmap(entry);
                }
        } else {
                entry = mk_pte(page, vma->vm_page_prot);
                if (vma->vm_flags & VM_WRITE)
                        entry = pte_mkwrite(pte_mkdirty(entry));
        }

        ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);

        if (pte_present(*ptep)) {
                unsigned long pfn = pte_pfn(*ptep);

                if (!is_zero_pfn(pfn)) {
                        pte_unmap_unlock(ptep, ptl);
                        mem_cgroup_cancel_charge(page, memcg, false);
                        goto abort;
                }
                flush = true;
        } else if (!pte_none(*ptep)) {
                pte_unmap_unlock(ptep, ptl);
                mem_cgroup_cancel_charge(page, memcg, false);
                goto abort;
        }

        /*
         * Check for usefaultfd but do not deliver the fault. Instead,
         * just back off.
         */
        if (userfaultfd_missing(vma)) {
                pte_unmap_unlock(ptep, ptl);
                mem_cgroup_cancel_charge(page, memcg, false);
                goto abort;
        }

        inc_mm_counter(mm, MM_ANONPAGES);
        page_add_new_anon_rmap(page, vma, addr, false);
        mem_cgroup_commit_charge(page, memcg, false, false);
        if (!is_zone_device_page(page))
                lru_cache_add_active_or_unevictable(page, vma);
        get_page(page);

        if (flush) {
                flush_cache_page(vma, addr, pte_pfn(*ptep));
                ptep_clear_flush_notify(vma, addr, ptep);
                set_pte_at_notify(mm, addr, ptep, entry);
                update_mmu_cache(vma, addr, ptep);
        } else {
                /* No need to invalidate - it was non-present before */
                set_pte_at(mm, addr, ptep, entry);
                update_mmu_cache(vma, addr, ptep);
        }

        pte_unmap_unlock(ptep, ptl);
        *src = MIGRATE_PFN_MIGRATE;
        return;

abort:
        *src &= ~MIGRATE_PFN_MIGRATE;
}

/*
 * migrate_vma_pages() - migrate meta-data from src page to dst page
 * @migrate: migrate struct containing all migration information
 *
 * This migrates struct page meta-data from source struct page to destination
 * struct page. This effectively finishes the migration from source page to the
 * destination page.
 */
static void migrate_vma_pages(struct migrate_vma *migrate)
{
        const unsigned long npages = migrate->npages;
        const unsigned long start = migrate->start;
        struct vm_area_struct *vma = migrate->vma;
        struct mm_struct *mm = vma->vm_mm;
        unsigned long addr, i, mmu_start;
        bool notified = false;

        for (i = 0, addr = start; i < npages; addr += PAGE_SIZE, i++) {
                struct page *newpage = migrate_pfn_to_page(migrate->dst[i]);
                struct page *page = migrate_pfn_to_page(migrate->src[i]);
                struct address_space *mapping;
                int r;

                if (!newpage) {
                        migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
                        continue;
                }

                if (!page) {
                        if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE)) {
                                continue;
                        }
                        if (!notified) {
                                mmu_start = addr;
                                notified = true;
                                mmu_notifier_invalidate_range_start(mm,
                                                                mmu_start,
                                                                migrate->end);
                        }
                        migrate_vma_insert_page(migrate, addr, newpage,
                                                &migrate->src[i],
                                                &migrate->dst[i]);
                        continue;
                }

                mapping = page_mapping(page);

                if (is_zone_device_page(newpage)) {
                        if (is_device_private_page(newpage)) {
                                /*
                                 * For now only support private anonymous when
                                 * migrating to un-addressable device memory.
                                 */
                                if (mapping) {
                                        migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
                                        continue;
                                }
                        } else if (!is_device_public_page(newpage)) {
                                /*
                                 * Other types of ZONE_DEVICE page are not
                                 * supported.
                                 */
                                migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
                                continue;
                        }
                }

                r = migrate_page(mapping, newpage, page, MIGRATE_SYNC_NO_COPY);
                if (r != MIGRATEPAGE_SUCCESS)
                        migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
        }

        /*
         * No need to double call mmu_notifier->invalidate_range() callback as
         * the above ptep_clear_flush_notify() inside migrate_vma_insert_page()
         * did already call it.
         */
        if (notified)
                mmu_notifier_invalidate_range_only_end(mm, mmu_start,
                                                       migrate->end);
}

/*
 * migrate_vma_finalize() - restore CPU page table entry
 * @migrate: migrate struct containing all migration information
 *
 * This replaces the special migration pte entry with either a mapping to the
 * new page if migration was successful for that page, or to the original page
 * otherwise.
 *
 * This also unlocks the pages and puts them back on the lru, or drops the extra
 * refcount, for device pages.
 */
static void migrate_vma_finalize(struct migrate_vma *migrate)
{
        const unsigned long npages = migrate->npages;
        unsigned long i;

        for (i = 0; i < npages; i++) {
                struct page *newpage = migrate_pfn_to_page(migrate->dst[i]);
                struct page *page = migrate_pfn_to_page(migrate->src[i]);

                if (!page) {
                        if (newpage) {
                                unlock_page(newpage);
                                put_page(newpage);
                        }
                        continue;
                }

                if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE) || !newpage) {
                        if (newpage) {
                                unlock_page(newpage);
                                put_page(newpage);
                        }
                        newpage = page;
                }

                remove_migration_ptes(page, newpage, false);
                unlock_page(page);
                migrate->cpages--;

                if (is_zone_device_page(page))
                        put_page(page);
                else
                        putback_lru_page(page);

                if (newpage != page) {
                        unlock_page(newpage);
                        if (is_zone_device_page(newpage))
                                put_page(newpage);
                        else
                                putback_lru_page(newpage);
                }
        }
}

/*
 * migrate_vma() - migrate a range of memory inside vma
 *
 * @ops: migration callback for allocating destination memory and copying
 * @vma: virtual memory area containing the range to be migrated
 * @start: start address of the range to migrate (inclusive)
 * @end: end address of the range to migrate (exclusive)
 * @src: array of hmm_pfn_t containing source pfns
 * @dst: array of hmm_pfn_t containing destination pfns
 * @private: pointer passed back to each of the callback
 * Returns: 0 on success, error code otherwise
 *
 * This function tries to migrate a range of memory virtual address range, using
 * callbacks to allocate and copy memory from source to destination. First it
 * collects all the pages backing each virtual address in the range, saving this
 * inside the src array. Then it locks those pages and unmaps them. Once the pages
 * are locked and unmapped, it checks whether each page is pinned or not. Pages
 * that aren't pinned have the MIGRATE_PFN_MIGRATE flag set (by this function)
 * in the corresponding src array entry. It then restores any pages that are
 * pinned, by remapping and unlocking those pages.
 *
 * At this point it calls the alloc_and_copy() callback. For documentation on
 * what is expected from that callback, see struct migrate_vma_ops comments in
 * include/linux/migrate.h
 *
 * After the alloc_and_copy() callback, this function goes over each entry in
 * the src array that has the MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE flag
 * set. If the corresponding entry in dst array has MIGRATE_PFN_VALID flag set,
 * then the function tries to migrate struct page information from the source
 * struct page to the destination struct page. If it fails to migrate the struct
 * page information, then it clears the MIGRATE_PFN_MIGRATE flag in the src
 * array.
 *
 * At this point all successfully migrated pages have an entry in the src
 * array with MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE flag set and the dst
 * array entry with MIGRATE_PFN_VALID flag set.
 *
 * It then calls the finalize_and_map() callback. See comments for "struct
 * migrate_vma_ops", in include/linux/migrate.h for details about
 * finalize_and_map() behavior.
 *
 * After the finalize_and_map() callback, for successfully migrated pages, this
 * function updates the CPU page table to point to new pages, otherwise it
 * restores the CPU page table to point to the original source pages.
 *
 * Function returns 0 after the above steps, even if no pages were migrated
 * (The function only returns an error if any of the arguments are invalid.)
 *
 * Both src and dst array must be big enough for (end - start) >> PAGE_SHIFT
 * unsigned long entries.
 */
int migrate_vma(const struct migrate_vma_ops *ops,
                struct vm_area_struct *vma,
                unsigned long start,
                unsigned long end,
                unsigned long *src,
                unsigned long *dst,
                void *private)
{
        struct migrate_vma migrate;

        /* Sanity check the arguments */
        start &= PAGE_MASK;
        end &= PAGE_MASK;
        if (!vma || is_vm_hugetlb_page(vma) || (vma->vm_flags & VM_SPECIAL))
                return -EINVAL;
        if (start < vma->vm_start || start >= vma->vm_end)
                return -EINVAL;
        if (end <= vma->vm_start || end > vma->vm_end)
                return -EINVAL;
        if (!ops || !src || !dst || start >= end)
                return -EINVAL;

        memset(src, 0, sizeof(*src) * ((end - start) >> PAGE_SHIFT));
        migrate.src = src;
        migrate.dst = dst;
        migrate.start = start;
        migrate.npages = 0;
        migrate.cpages = 0;
        migrate.end = end;
        migrate.vma = vma;

        /* Collect, and try to unmap source pages */
        migrate_vma_collect(&migrate);
        if (!migrate.cpages)
                return 0;

        /* Lock and isolate page */
        migrate_vma_prepare(&migrate);
        if (!migrate.cpages)
                return 0;

        /* Unmap pages */
        migrate_vma_unmap(&migrate);
        if (!migrate.cpages)
                return 0;

        /*
         * At this point pages are locked and unmapped, and thus they have
         * stable content and can safely be copied to destination memory that
         * is allocated by the callback.
         *
         * Note that migration can fail in migrate_vma_struct_page() for each
         * individual page.
         */
        ops->alloc_and_copy(vma, src, dst, start, end, private);

        /* This does the real migration of struct page */
        migrate_vma_pages(&migrate);

        ops->finalize_and_map(vma, src, dst, start, end, private);

        /* Unlock and remap pages */
        migrate_vma_finalize(&migrate);

        return 0;
}
EXPORT_SYMBOL(migrate_vma);
#endif /* defined(MIGRATE_VMA_HELPER) */