Linux 6.9-rc1

[linux-2.6-microblaze.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/compat.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/page_idle.h>
#include <linux/page_owner.h>
#include <linux/sched/mm.h>
#include <linux/ptrace.h>
#include <linux/oom.h>
#include <linux/memory.h>
#include <linux/random.h>
#include <linux/sched/sysctl.h>

#include <asm/tlbflush.h>

#include <trace/events/migrate.h>

#include "internal.h"

int isolate_movable_page(struct page *page, isolate_mode_t mode)
{
        const struct movable_operations *mops;

        /*
         * 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 grabbing 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;

        mops = page_movable_ops(page);
        VM_BUG_ON_PAGE(!mops, page);

        if (!mops->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;
}

static void putback_movable_page(struct page *page)
{
        const struct movable_operations *mops = page_movable_ops(page);

        mops->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_hugetlb().
 */
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_lru(page), -thp_nr_pages(page));
                        putback_lru_page(page);
                }
        }
}

/*
 * Restore a potential migration pte to a working pte entry
 */
static bool remove_migration_pte(struct folio *folio,
                struct vm_area_struct *vma, unsigned long addr, void *old)
{
        DEFINE_FOLIO_VMA_WALK(pvmw, old, vma, addr, PVMW_SYNC | PVMW_MIGRATION);

        while (page_vma_mapped_walk(&pvmw)) {
                rmap_t rmap_flags = RMAP_NONE;
                pte_t pte;
                swp_entry_t entry;
                struct page *new;
                unsigned long idx = 0;

                /* pgoff is invalid for ksm pages, but they are never large */
                if (folio_test_large(folio) && !folio_test_hugetlb(folio))
                        idx = linear_page_index(vma, pvmw.address) - pvmw.pgoff;
                new = folio_page(folio, idx);

#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
                /* PMD-mapped THP migration entry */
                if (!pvmw.pte) {
                        VM_BUG_ON_FOLIO(folio_test_hugetlb(folio) ||
                                        !folio_test_pmd_mappable(folio), folio);
                        remove_migration_pmd(&pvmw, new);
                        continue;
                }
#endif

                folio_get(folio);
                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_writable_migration_entry(entry))
                        pte = maybe_mkwrite(pte, vma);
                else if (pte_swp_uffd_wp(*pvmw.pte))
                        pte = pte_mkuffd_wp(pte);

                if (folio_test_anon(folio) && !is_readable_migration_entry(entry))
                        rmap_flags |= RMAP_EXCLUSIVE;

                if (unlikely(is_device_private_page(new))) {
                        if (pte_write(pte))
                                entry = make_writable_device_private_entry(
                                                        page_to_pfn(new));
                        else
                                entry = make_readable_device_private_entry(
                                                        page_to_pfn(new));
                        pte = swp_entry_to_pte(entry);
                        if (pte_swp_soft_dirty(*pvmw.pte))
                                pte = pte_swp_mksoft_dirty(pte);
                        if (pte_swp_uffd_wp(*pvmw.pte))
                                pte = pte_swp_mkuffd_wp(pte);
                }

#ifdef CONFIG_HUGETLB_PAGE
                if (folio_test_hugetlb(folio)) {
                        unsigned int shift = huge_page_shift(hstate_vma(vma));

                        pte = pte_mkhuge(pte);
                        pte = arch_make_huge_pte(pte, shift, vma->vm_flags);
                        if (folio_test_anon(folio))
                                hugepage_add_anon_rmap(new, vma, pvmw.address,
                                                       rmap_flags);
                        else
                                page_dup_file_rmap(new, true);
                        set_huge_pte_at(vma->vm_mm, pvmw.address, pvmw.pte, pte);
                } else
#endif
                {
                        if (folio_test_anon(folio))
                                page_add_anon_rmap(new, vma, pvmw.address,
                                                   rmap_flags);
                        else
                                page_add_file_rmap(new, vma, false);
                        set_pte_at(vma->vm_mm, pvmw.address, pvmw.pte, pte);
                }
                if (vma->vm_flags & VM_LOCKED)
                        mlock_page_drain_local();

                trace_remove_migration_pte(pvmw.address, pte_val(pte),
                                           compound_order(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 folio *src, struct folio *dst, bool locked)
{
        struct rmap_walk_control rwc = {
                .rmap_one = remove_migration_pte,
                .arg = src,
        };

        if (locked)
                rmap_walk_locked(dst, &rwc);
        else
                rmap_walk(dst, &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;

        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;

        migration_entry_wait_on_locked(entry, ptep, ptl);
        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);
}

#ifdef CONFIG_HUGETLB_PAGE
void __migration_entry_wait_huge(pte_t *ptep, spinlock_t *ptl)
{
        pte_t pte;

        spin_lock(ptl);
        pte = huge_ptep_get(ptep);

        if (unlikely(!is_hugetlb_entry_migration(pte)))
                spin_unlock(ptl);
        else
                migration_entry_wait_on_locked(pte_to_swp_entry(pte), NULL, ptl);
}

void migration_entry_wait_huge(struct vm_area_struct *vma, pte_t *pte)
{
        spinlock_t *ptl = huge_pte_lockptr(hstate_vma(vma), vma->vm_mm, pte);

        __migration_entry_wait_huge(pte, ptl);
}
#endif

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

        ptl = pmd_lock(mm, pmd);
        if (!is_pmd_migration_entry(*pmd))
                goto unlock;
        migration_entry_wait_on_locked(pmd_to_swp_entry(*pmd), NULL, ptl);
        return;
unlock:
        spin_unlock(ptl);
}
#endif

static int folio_expected_refs(struct address_space *mapping,
                struct folio *folio)
{
        int refs = 1;
        if (!mapping)
                return refs;

        refs += folio_nr_pages(folio);
        if (folio_test_private(folio))
                refs++;

        return refs;
}

/*
 * 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 folio_migrate_mapping(struct address_space *mapping,
                struct folio *newfolio, struct folio *folio, int extra_count)
{
        XA_STATE(xas, &mapping->i_pages, folio_index(folio));
        struct zone *oldzone, *newzone;
        int dirty;
        int expected_count = folio_expected_refs(mapping, folio) + extra_count;
        long nr = folio_nr_pages(folio);

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

                /* No turning back from here */
                newfolio->index = folio->index;
                newfolio->mapping = folio->mapping;
                if (folio_test_swapbacked(folio))
                        __folio_set_swapbacked(newfolio);

                return MIGRATEPAGE_SUCCESS;
        }

        oldzone = folio_zone(folio);
        newzone = folio_zone(newfolio);

        xas_lock_irq(&xas);
        if (!folio_ref_freeze(folio, expected_count)) {
                xas_unlock_irq(&xas);
                return -EAGAIN;
        }

        /*
         * Now we know that no one else is looking at the folio:
         * no turning back from here.
         */
        newfolio->index = folio->index;
        newfolio->mapping = folio->mapping;
        folio_ref_add(newfolio, nr); /* add cache reference */
        if (folio_test_swapbacked(folio)) {
                __folio_set_swapbacked(newfolio);
                if (folio_test_swapcache(folio)) {
                        folio_set_swapcache(newfolio);
                        newfolio->private = folio_get_private(folio);
                }
        } else {
                VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
        }

        /* Move dirty while page refs frozen and newpage not yet exposed */
        dirty = folio_test_dirty(folio);
        if (dirty) {
                folio_clear_dirty(folio);
                folio_set_dirty(newfolio);
        }

        xas_store(&xas, newfolio);

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

        xas_unlock(&xas);
        /* 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) {
                struct lruvec *old_lruvec, *new_lruvec;
                struct mem_cgroup *memcg;

                memcg = folio_memcg(folio);
                old_lruvec = mem_cgroup_lruvec(memcg, oldzone->zone_pgdat);
                new_lruvec = mem_cgroup_lruvec(memcg, newzone->zone_pgdat);

                __mod_lruvec_state(old_lruvec, NR_FILE_PAGES, -nr);
                __mod_lruvec_state(new_lruvec, NR_FILE_PAGES, nr);
                if (folio_test_swapbacked(folio) && !folio_test_swapcache(folio)) {
                        __mod_lruvec_state(old_lruvec, NR_SHMEM, -nr);
                        __mod_lruvec_state(new_lruvec, NR_SHMEM, nr);
                }
#ifdef CONFIG_SWAP
                if (folio_test_swapcache(folio)) {
                        __mod_lruvec_state(old_lruvec, NR_SWAPCACHE, -nr);
                        __mod_lruvec_state(new_lruvec, NR_SWAPCACHE, nr);
                }
#endif
                if (dirty && mapping_can_writeback(mapping)) {
                        __mod_lruvec_state(old_lruvec, NR_FILE_DIRTY, -nr);
                        __mod_zone_page_state(oldzone, NR_ZONE_WRITE_PENDING, -nr);
                        __mod_lruvec_state(new_lruvec, NR_FILE_DIRTY, nr);
                        __mod_zone_page_state(newzone, NR_ZONE_WRITE_PENDING, nr);
                }
        }
        local_irq_enable();

        return MIGRATEPAGE_SUCCESS;
}
EXPORT_SYMBOL(folio_migrate_mapping);

/*
 * The expected number of remaining references is the same as that
 * of folio_migrate_mapping().
 */
int migrate_huge_page_move_mapping(struct address_space *mapping,
                                   struct folio *dst, struct folio *src)
{
        XA_STATE(xas, &mapping->i_pages, folio_index(src));
        int expected_count;

        xas_lock_irq(&xas);
        expected_count = 2 + folio_has_private(src);
        if (!folio_ref_freeze(src, expected_count)) {
                xas_unlock_irq(&xas);
                return -EAGAIN;
        }

        dst->index = src->index;
        dst->mapping = src->mapping;

        folio_get(dst);

        xas_store(&xas, dst);

        folio_ref_unfreeze(src, expected_count - 1);

        xas_unlock_irq(&xas);

        return MIGRATEPAGE_SUCCESS;
}

/*
 * Copy the flags and some other ancillary information
 */
void folio_migrate_flags(struct folio *newfolio, struct folio *folio)
{
        int cpupid;

        if (folio_test_error(folio))
                folio_set_error(newfolio);
        if (folio_test_referenced(folio))
                folio_set_referenced(newfolio);
        if (folio_test_uptodate(folio))
                folio_mark_uptodate(newfolio);
        if (folio_test_clear_active(folio)) {
                VM_BUG_ON_FOLIO(folio_test_unevictable(folio), folio);
                folio_set_active(newfolio);
        } else if (folio_test_clear_unevictable(folio))
                folio_set_unevictable(newfolio);
        if (folio_test_workingset(folio))
                folio_set_workingset(newfolio);
        if (folio_test_checked(folio))
                folio_set_checked(newfolio);
        /*
         * PG_anon_exclusive (-> PG_mappedtodisk) is always migrated via
         * migration entries. We can still have PG_anon_exclusive set on an
         * effectively unmapped and unreferenced first sub-pages of an
         * anonymous THP: we can simply copy it here via PG_mappedtodisk.
         */
        if (folio_test_mappedtodisk(folio))
                folio_set_mappedtodisk(newfolio);

        /* Move dirty on pages not done by folio_migrate_mapping() */
        if (folio_test_dirty(folio))
                folio_set_dirty(newfolio);

        if (folio_test_young(folio))
                folio_set_young(newfolio);
        if (folio_test_idle(folio))
                folio_set_idle(newfolio);

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

        folio_migrate_ksm(newfolio, folio);
        /*
         * Please do not reorder this without considering how mm/ksm.c's
         * get_ksm_page() depends upon ksm_migrate_page() and PageSwapCache().
         */
        if (folio_test_swapcache(folio))
                folio_clear_swapcache(folio);
        folio_clear_private(folio);

        /* page->private contains hugetlb specific flags */
        if (!folio_test_hugetlb(folio))
                folio->private = NULL;

        /*
         * If any waiters have accumulated on the new page then
         * wake them up.
         */
        if (folio_test_writeback(newfolio))
                folio_end_writeback(newfolio);

        /*
         * PG_readahead shares the same bit with PG_reclaim.  The above
         * end_page_writeback() may clear PG_readahead mistakenly, so set the
         * bit after that.
         */
        if (folio_test_readahead(folio))
                folio_set_readahead(newfolio);

        folio_copy_owner(newfolio, folio);

        if (!folio_test_hugetlb(folio))
                mem_cgroup_migrate(folio, newfolio);
}
EXPORT_SYMBOL(folio_migrate_flags);

void folio_migrate_copy(struct folio *newfolio, struct folio *folio)
{
        folio_copy(newfolio, folio);
        folio_migrate_flags(newfolio, folio);
}
EXPORT_SYMBOL(folio_migrate_copy);

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

/**
 * migrate_folio() - Simple folio migration.
 * @mapping: The address_space containing the folio.
 * @dst: The folio to migrate the data to.
 * @src: The folio containing the current data.
 * @mode: How to migrate the page.
 *
 * Common logic to directly migrate a single LRU folio suitable for
 * folios that do not use PagePrivate/PagePrivate2.
 *
 * Folios are locked upon entry and exit.
 */
int migrate_folio(struct address_space *mapping, struct folio *dst,
                struct folio *src, enum migrate_mode mode)
{
        int rc;

        BUG_ON(folio_test_writeback(src));      /* Writeback must be complete */

        rc = folio_migrate_mapping(mapping, dst, src, 0);

        if (rc != MIGRATEPAGE_SUCCESS)
                return rc;

        if (mode != MIGRATE_SYNC_NO_COPY)
                folio_migrate_copy(dst, src);
        else
                folio_migrate_flags(dst, src);
        return MIGRATEPAGE_SUCCESS;
}
EXPORT_SYMBOL(migrate_folio);

#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 {
                        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 {
                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;
                        bh = head;
                        while (bh != failed_bh) {
                                unlock_buffer(bh);
                                bh = bh->b_this_page;
                        }
                        return false;
                }

                bh = bh->b_this_page;
        } while (bh != head);
        return true;
}

static int __buffer_migrate_folio(struct address_space *mapping,
                struct folio *dst, struct folio *src, enum migrate_mode mode,
                bool check_refs)
{
        struct buffer_head *bh, *head;
        int rc;
        int expected_count;

        head = folio_buffers(src);
        if (!head)
                return migrate_folio(mapping, dst, src, mode);

        /* Check whether page does not have extra refs before we do more work */
        expected_count = folio_expected_refs(mapping, src);
        if (folio_ref_count(src) != expected_count)
                return -EAGAIN;

        if (!buffer_migrate_lock_buffers(head, mode))
                return -EAGAIN;

        if (check_refs) {
                bool busy;
                bool invalidated = false;

recheck_buffers:
                busy = false;
                spin_lock(&mapping->private_lock);
                bh = head;
                do {
                        if (atomic_read(&bh->b_count)) {
                                busy = true;
                                break;
                        }
                        bh = bh->b_this_page;
                } while (bh != head);
                if (busy) {
                        if (invalidated) {
                                rc = -EAGAIN;
                                goto unlock_buffers;
                        }
                        spin_unlock(&mapping->private_lock);
                        invalidate_bh_lrus();
                        invalidated = true;
                        goto recheck_buffers;
                }
        }

        rc = folio_migrate_mapping(mapping, dst, src, 0);
        if (rc != MIGRATEPAGE_SUCCESS)
                goto unlock_buffers;

        folio_attach_private(dst, folio_detach_private(src));

        bh = head;
        do {
                set_bh_page(bh, &dst->page, bh_offset(bh));
                bh = bh->b_this_page;
        } while (bh != head);

        if (mode != MIGRATE_SYNC_NO_COPY)
                folio_migrate_copy(dst, src);
        else
                folio_migrate_flags(dst, src);

        rc = MIGRATEPAGE_SUCCESS;
unlock_buffers:
        if (check_refs)
                spin_unlock(&mapping->private_lock);
        bh = head;
        do {
                unlock_buffer(bh);
                bh = bh->b_this_page;
        } while (bh != head);

        return rc;
}

/**
 * buffer_migrate_folio() - Migration function for folios with buffers.
 * @mapping: The address space containing @src.
 * @dst: The folio to migrate to.
 * @src: The folio to migrate from.
 * @mode: How to migrate the folio.
 *
 * This function can only be used if the underlying filesystem guarantees
 * that no other references to @src exist. For example attached buffer
 * heads are accessed only under the folio lock.  If your filesystem cannot
 * provide this guarantee, buffer_migrate_folio_norefs() may be more
 * appropriate.
 *
 * Return: 0 on success or a negative errno on failure.
 */
int buffer_migrate_folio(struct address_space *mapping,
                struct folio *dst, struct folio *src, enum migrate_mode mode)
{
        return __buffer_migrate_folio(mapping, dst, src, mode, false);
}
EXPORT_SYMBOL(buffer_migrate_folio);

/**
 * buffer_migrate_folio_norefs() - Migration function for folios with buffers.
 * @mapping: The address space containing @src.
 * @dst: The folio to migrate to.
 * @src: The folio to migrate from.
 * @mode: How to migrate the folio.
 *
 * Like buffer_migrate_folio() except that this variant is more careful
 * and checks that there are also no buffer head references. This function
 * is the right one for mappings where buffer heads are directly looked
 * up and referenced (such as block device mappings).
 *
 * Return: 0 on success or a negative errno on failure.
 */
int buffer_migrate_folio_norefs(struct address_space *mapping,
                struct folio *dst, struct folio *src, enum migrate_mode mode)
{
        return __buffer_migrate_folio(mapping, dst, src, mode, true);
}
#endif

int filemap_migrate_folio(struct address_space *mapping,
                struct folio *dst, struct folio *src, enum migrate_mode mode)
{
        int ret;

        ret = folio_migrate_mapping(mapping, dst, src, 0);
        if (ret != MIGRATEPAGE_SUCCESS)
                return ret;

        if (folio_get_private(src))
                folio_attach_private(dst, folio_detach_private(src));

        if (mode != MIGRATE_SYNC_NO_COPY)
                folio_migrate_copy(dst, src);
        else
                folio_migrate_flags(dst, src);
        return MIGRATEPAGE_SUCCESS;
}
EXPORT_SYMBOL_GPL(filemap_migrate_folio);

/*
 * Writeback a folio to clean the dirty state
 */
static int writeout(struct address_space *mapping, struct folio *folio)
{
        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 (!folio_clear_dirty_for_io(folio))
                /* Someone else already triggered a write */
                return -EAGAIN;

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

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

        if (rc != AOP_WRITEPAGE_ACTIVATE)
                /* unlocked. Relock */
                folio_lock(folio);

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

/*
 * Default handling if a filesystem does not provide a migration function.
 */
static int fallback_migrate_folio(struct address_space *mapping,
                struct folio *dst, struct folio *src, enum migrate_mode mode)
{
        if (folio_test_dirty(src)) {
                /* Only writeback folios in full synchronous migration */
                switch (mode) {
                case MIGRATE_SYNC:
                case MIGRATE_SYNC_NO_COPY:
                        break;
                default:
                        return -EBUSY;
                }
                return writeout(mapping, src);
        }

        /*
         * Buffers may be managed in a filesystem specific way.
         * We must have no buffers or drop them.
         */
        if (folio_test_private(src) &&
            !filemap_release_folio(src, GFP_KERNEL))
                return mode == MIGRATE_SYNC ? -EAGAIN : -EBUSY;

        return migrate_folio(mapping, dst, src, 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_folio(struct folio *dst, struct folio *src,
                                enum migrate_mode mode)
{
        int rc = -EAGAIN;
        bool is_lru = !__PageMovable(&src->page);

        VM_BUG_ON_FOLIO(!folio_test_locked(src), src);
        VM_BUG_ON_FOLIO(!folio_test_locked(dst), dst);

        if (likely(is_lru)) {
                struct address_space *mapping = folio_mapping(src);

                if (!mapping)
                        rc = migrate_folio(mapping, dst, src, mode);
                else if (mapping->a_ops->migrate_folio)
                        /*
                         * Most folios have a mapping and most filesystems
                         * provide a migrate_folio callback. Anonymous folios
                         * are part of swap space which also has its own
                         * migrate_folio callback. This is the most common path
                         * for page migration.
                         */
                        rc = mapping->a_ops->migrate_folio(mapping, dst, src,
                                                                mode);
                else
                        rc = fallback_migrate_folio(mapping, dst, src, mode);
        } else {
                const struct movable_operations *mops;

                /*
                 * In case of non-lru page, it could be released after
                 * isolation step. In that case, we shouldn't try migration.
                 */
                VM_BUG_ON_FOLIO(!folio_test_isolated(src), src);
                if (!folio_test_movable(src)) {
                        rc = MIGRATEPAGE_SUCCESS;
                        folio_clear_isolated(src);
                        goto out;
                }

                mops = page_movable_ops(&src->page);
                rc = mops->migrate_page(&dst->page, &src->page, mode);
                WARN_ON_ONCE(rc == MIGRATEPAGE_SUCCESS &&
                                !folio_test_isolated(src));
        }

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

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

                /*
                 * Anonymous and movable src->mapping will be cleared by
                 * free_pages_prepare so don't reset it here for keeping
                 * the type to work PageAnon, for example.
                 */
                if (!folio_mapping_flags(src))
                        src->mapping = NULL;

                if (likely(!folio_is_zone_device(dst)))
                        flush_dcache_folio(dst);
        }
out:
        return rc;
}

static int __unmap_and_move(struct page *page, struct page *newpage,
                                int force, enum migrate_mode mode)
{
        struct folio *folio = page_folio(page);
        struct folio *dst = page_folio(newpage);
        int rc = -EAGAIN;
        bool page_was_mapped = false;
        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_readahead). 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_migrate(), 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_folio(dst, folio, 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_cleanup_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(folio);
                        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_migrate(folio, 0);
                page_was_mapped = true;
        }

        if (!page_mapped(page))
                rc = move_to_new_folio(dst, folio, mode);

        /*
         * When successful, push newpage to LRU immediately: so that if it
         * turns out to be an mlocked page, remove_migration_ptes() will
         * automatically build up the correct newpage->mlock_count for it.
         *
         * We would like to do something similar for the old page, when
         * unsuccessful, and other cases when a page has been temporarily
         * isolated from the unevictable LRU: but this case is the easiest.
         */
        if (rc == MIGRATEPAGE_SUCCESS) {
                lru_cache_add(newpage);
                if (page_was_mapped)
                        lru_add_drain();
        }

        if (page_was_mapped)
                remove_migration_ptes(folio,
                        rc == MIGRATEPAGE_SUCCESS ? dst : folio, 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.
         */
        if (rc == MIGRATEPAGE_SUCCESS)
                put_page(newpage);

        return rc;
}

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

        if (!thp_migration_supported() && PageTransHuge(page))
                return -ENOSYS;

        if (page_count(page) == 1) {
                /* Page was freed from under us. So we are done. */
                ClearPageActive(page);
                ClearPageUnevictable(page);
                /* free_pages_prepare() will clear PG_isolated. */
                goto out;
        }

        newpage = get_new_page(page, private);
        if (!newpage)
                return -ENOMEM;

        newpage->private = 0;
        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 kept its references and be restored.
                 */
                list_del(&page->lru);
        }

        /*
         * 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) {
                /*
                 * 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_lru(page), -thp_nr_pages(page));

                if (reason != MR_MEMORY_FAILURE)
                        /*
                         * We release the page in page_handle_poison.
                         */
                        put_page(page);
        } else {
                if (rc != -EAGAIN)
                        list_add_tail(&page->lru, ret);

                if (put_new_page)
                        put_new_page(newpage, private);
                else
                        put_page(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,
                                struct list_head *ret)
{
        struct folio *dst, *src = page_folio(hpage);
        int rc = -EAGAIN;
        int page_was_mapped = 0;
        struct page *new_hpage;
        struct anon_vma *anon_vma = NULL;
        struct address_space *mapping = NULL;

        /*
         * Migratability of hugepages depends on architectures and their 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))) {
                list_move_tail(&hpage->lru, ret);
                return -ENOSYS;
        }

        if (page_count(hpage) == 1) {
                /* page was freed from under us. So we are done. */
                putback_active_hugepage(hpage);
                return MIGRATEPAGE_SUCCESS;
        }

        new_hpage = get_new_page(hpage, private);
        if (!new_hpage)
                return -ENOMEM;
        dst = page_folio(new_hpage);

        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);
        }

        /*
         * Check for pages which are in the process of being freed.  Without
         * page_mapping() set, hugetlbfs specific move page routine will not
         * be called and we could leak usage counts for subpools.
         */
        if (hugetlb_page_subpool(hpage) && !page_mapping(hpage)) {
                rc = -EBUSY;
                goto out_unlock;
        }

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

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

        if (page_mapped(hpage)) {
                enum ttu_flags ttu = 0;

                if (!PageAnon(hpage)) {
                        /*
                         * In shared mappings, try_to_unmap could potentially
                         * call huge_pmd_unshare.  Because of this, take
                         * semaphore in write mode here and set TTU_RMAP_LOCKED
                         * to let lower levels know we have taken the lock.
                         */
                        mapping = hugetlb_page_mapping_lock_write(hpage);
                        if (unlikely(!mapping))
                                goto unlock_put_anon;

                        ttu = TTU_RMAP_LOCKED;
                }

                try_to_migrate(src, ttu);
                page_was_mapped = 1;

                if (ttu & TTU_RMAP_LOCKED)
                        i_mmap_unlock_write(mapping);
        }

        if (!page_mapped(hpage))
                rc = move_to_new_folio(dst, src, mode);

        if (page_was_mapped)
                remove_migration_ptes(src,
                        rc == MIGRATEPAGE_SUCCESS ? dst : src, false);

unlock_put_anon:
        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;
        }

out_unlock:
        unlock_page(hpage);
out:
        if (rc == MIGRATEPAGE_SUCCESS)
                putback_active_hugepage(hpage);
        else if (rc != -EAGAIN)
                list_move_tail(&hpage->lru, ret);

        /*
         * 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);

        return rc;
}

static inline int try_split_thp(struct page *page, struct page **page2,
                                struct list_head *from)
{
        int rc = 0;

        lock_page(page);
        rc = split_huge_page_to_list(page, from);
        unlock_page(page);
        if (!rc)
                list_safe_reset_next(page, *page2, lru);

        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.
 * @ret_succeeded:      Set to the number of normal pages migrated successfully if
 *                      the caller passes a non-NULL pointer.
 *
 * 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.
 * It is caller's responsibility to call putback_movable_pages() to return pages
 * to the LRU or free list only if ret != 0.
 *
 * Returns the number of {normal page, THP, hugetlb} that were not migrated, or
 * an error code. The number of THP splits will be considered as the number of
 * non-migrated THP, no matter how many subpages of the THP are migrated successfully.
 */
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, unsigned int *ret_succeeded)
{
        int retry = 1;
        int thp_retry = 1;
        int nr_failed = 0;
        int nr_failed_pages = 0;
        int nr_succeeded = 0;
        int nr_thp_succeeded = 0;
        int nr_thp_failed = 0;
        int nr_thp_split = 0;
        int pass = 0;
        bool is_thp = false;
        struct page *page;
        struct page *page2;
        int rc, nr_subpages;
        LIST_HEAD(ret_pages);
        LIST_HEAD(thp_split_pages);
        bool nosplit = (reason == MR_NUMA_MISPLACED);
        bool no_subpage_counting = false;

        trace_mm_migrate_pages_start(mode, reason);

thp_subpage_migration:
        for (pass = 0; pass < 10 && (retry || thp_retry); pass++) {
                retry = 0;
                thp_retry = 0;

                list_for_each_entry_safe(page, page2, from, lru) {
retry:
                        /*
                         * THP statistics is based on the source huge page.
                         * Capture required information that might get lost
                         * during migration.
                         */
                        is_thp = PageTransHuge(page) && !PageHuge(page);
                        nr_subpages = compound_nr(page);
                        cond_resched();

                        if (PageHuge(page))
                                rc = unmap_and_move_huge_page(get_new_page,
                                                put_new_page, private, page,
                                                pass > 2, mode, reason,
                                                &ret_pages);
                        else
                                rc = unmap_and_move(get_new_page, put_new_page,
                                                private, page, pass > 2, mode,
                                                reason, &ret_pages);
                        /*
                         * The rules are:
                         *      Success: non hugetlb page will be freed, hugetlb
                         *               page will be put back
                         *      -EAGAIN: stay on the from list
                         *      -ENOMEM: stay on the from list
                         *      Other errno: put on ret_pages list then splice to
                         *                   from list
                         */
                        switch(rc) {
                        /*
                         * THP migration might be unsupported or the
                         * allocation could've failed so we should
                         * retry on the same page with the THP split
                         * to base pages.
                         *
                         * Head page is retried immediately and tail
                         * pages are added to the tail of the list so
                         * we encounter them after the rest of the list
                         * is processed.
                         */
                        case -ENOSYS:
                                /* THP migration is unsupported */
                                if (is_thp) {
                                        nr_thp_failed++;
                                        if (!try_split_thp(page, &page2, &thp_split_pages)) {
                                                nr_thp_split++;
                                                goto retry;
                                        }
                                /* Hugetlb migration is unsupported */
                                } else if (!no_subpage_counting) {
                                        nr_failed++;
                                }

                                nr_failed_pages += nr_subpages;
                                break;
                        case -ENOMEM:
                                /*
                                 * When memory is low, don't bother to try to migrate
                                 * other pages, just exit.
                                 * THP NUMA faulting doesn't split THP to retry.
                                 */
                                if (is_thp && !nosplit) {
                                        nr_thp_failed++;
                                        if (!try_split_thp(page, &page2, &thp_split_pages)) {
                                                nr_thp_split++;
                                                goto retry;
                                        }
                                } else if (!no_subpage_counting) {
                                        nr_failed++;
                                }

                                nr_failed_pages += nr_subpages;
                                /*
                                 * There might be some subpages of fail-to-migrate THPs
                                 * left in thp_split_pages list. Move them back to migration
                                 * list so that they could be put back to the right list by
                                 * the caller otherwise the page refcnt will be leaked.
                                 */
                                list_splice_init(&thp_split_pages, from);
                                nr_thp_failed += thp_retry;
                                goto out;
                        case -EAGAIN:
                                if (is_thp)
                                        thp_retry++;
                                else
                                        retry++;
                                break;
                        case MIGRATEPAGE_SUCCESS:
                                nr_succeeded += nr_subpages;
                                if (is_thp)
                                        nr_thp_succeeded++;
                                break;
                        default:
                                /*
                                 * Permanent failure (-EBUSY, etc.):
                                 * unlike -EAGAIN case, the failed page is
                                 * removed from migration page list and not
                                 * retried in the next outer loop.
                                 */
                                if (is_thp)
                                        nr_thp_failed++;
                                else if (!no_subpage_counting)
                                        nr_failed++;

                                nr_failed_pages += nr_subpages;
                                break;
                        }
                }
        }
        nr_failed += retry;
        nr_thp_failed += thp_retry;
        /*
         * Try to migrate subpages of fail-to-migrate THPs, no nr_failed
         * counting in this round, since all subpages of a THP is counted
         * as 1 failure in the first round.
         */
        if (!list_empty(&thp_split_pages)) {
                /*
                 * Move non-migrated pages (after 10 retries) to ret_pages
                 * to avoid migrating them again.
                 */
                list_splice_init(from, &ret_pages);
                list_splice_init(&thp_split_pages, from);
                no_subpage_counting = true;
                retry = 1;
                goto thp_subpage_migration;
        }

        rc = nr_failed + nr_thp_failed;
out:
        /*
         * Put the permanent failure page back to migration list, they
         * will be put back to the right list by the caller.
         */
        list_splice(&ret_pages, from);

        count_vm_events(PGMIGRATE_SUCCESS, nr_succeeded);
        count_vm_events(PGMIGRATE_FAIL, nr_failed_pages);
        count_vm_events(THP_MIGRATION_SUCCESS, nr_thp_succeeded);
        count_vm_events(THP_MIGRATION_FAIL, nr_thp_failed);
        count_vm_events(THP_MIGRATION_SPLIT, nr_thp_split);
        trace_mm_migrate_pages(nr_succeeded, nr_failed_pages, nr_thp_succeeded,
                               nr_thp_failed, nr_thp_split, mode, reason);

        if (ret_succeeded)
                *ret_succeeded = nr_succeeded;

        return rc;
}

struct page *alloc_migration_target(struct page *page, unsigned long private)
{
        struct folio *folio = page_folio(page);
        struct migration_target_control *mtc;
        gfp_t gfp_mask;
        unsigned int order = 0;
        struct folio *new_folio = NULL;
        int nid;
        int zidx;

        mtc = (struct migration_target_control *)private;
        gfp_mask = mtc->gfp_mask;
        nid = mtc->nid;
        if (nid == NUMA_NO_NODE)
                nid = folio_nid(folio);

        if (folio_test_hugetlb(folio)) {
                struct hstate *h = page_hstate(&folio->page);

                gfp_mask = htlb_modify_alloc_mask(h, gfp_mask);
                return alloc_huge_page_nodemask(h, nid, mtc->nmask, gfp_mask);
        }

        if (folio_test_large(folio)) {
                /*
                 * clear __GFP_RECLAIM to make the migration callback
                 * consistent with regular THP allocations.
                 */
                gfp_mask &= ~__GFP_RECLAIM;
                gfp_mask |= GFP_TRANSHUGE;
                order = folio_order(folio);
        }
        zidx = zone_idx(folio_zone(folio));
        if (is_highmem_idx(zidx) || zidx == ZONE_MOVABLE)
                gfp_mask |= __GFP_HIGHMEM;

        new_folio = __folio_alloc(gfp_mask, order, nid, mtc->nmask);

        return &new_folio->page;
}

#ifdef CONFIG_NUMA

static int store_status(int __user *status, int start, int value, int nr)
{
        while (nr-- > 0) {
                if (put_user(value, status + start))
                        return -EFAULT;
                start++;
        }

        return 0;
}

static int do_move_pages_to_node(struct mm_struct *mm,
                struct list_head *pagelist, int node)
{
        int err;
        struct migration_target_control mtc = {
                .nid = node,
                .gfp_mask = GFP_HIGHUSER_MOVABLE | __GFP_THISNODE,
        };

        err = migrate_pages(pagelist, alloc_migration_target, NULL,
                (unsigned long)&mtc, MIGRATE_SYNC, MR_SYSCALL, NULL);
        if (err)
                putback_movable_pages(pagelist);
        return err;
}

/*
 * Resolves the given address to a struct page, isolates it from the LRU and
 * puts it to the given pagelist.
 * Returns:
 *     errno - if the page cannot be found/isolated
 *     0 - when it doesn't have to be migrated because it is already on the
 *         target node
 *     1 - when it has been queued
 */
static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
                int node, struct list_head *pagelist, bool migrate_all)
{
        struct vm_area_struct *vma;
        struct page *page;
        int err;

        mmap_read_lock(mm);
        err = -EFAULT;
        vma = vma_lookup(mm, addr);
        if (!vma || !vma_migratable(vma))
                goto out;

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

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

        err = -ENOENT;
        if (!page || is_zone_device_page(page))
                goto out;

        err = 0;
        if (page_to_nid(page) == node)
                goto out_putpage;

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

        if (PageHuge(page)) {
                if (PageHead(page)) {
                        err = isolate_hugetlb(page, pagelist);
                        if (!err)
                                err = 1;
                }
        } else {
                struct page *head;

                head = compound_head(page);
                err = isolate_lru_page(head);
                if (err)
                        goto out_putpage;

                err = 1;
                list_add_tail(&head->lru, pagelist);
                mod_node_page_state(page_pgdat(head),
                        NR_ISOLATED_ANON + page_is_file_lru(head),
                        thp_nr_pages(head));
        }
out_putpage:
        /*
         * Either remove the duplicate refcount from
         * isolate_lru_page() or drop the page ref if it was
         * not isolated.
         */
        put_page(page);
out:
        mmap_read_unlock(mm);
        return err;
}

static int move_pages_and_store_status(struct mm_struct *mm, int node,
                struct list_head *pagelist, int __user *status,
                int start, int i, unsigned long nr_pages)
{
        int err;

        if (list_empty(pagelist))
                return 0;

        err = do_move_pages_to_node(mm, pagelist, node);
        if (err) {
                /*
                 * Positive err means the number of failed
                 * pages to migrate.  Since we are going to
                 * abort and return the number of non-migrated
                 * pages, so need to include the rest of the
                 * nr_pages that have not been attempted as
                 * well.
                 */
                if (err > 0)
                        err += nr_pages - i - 1;
                return err;
        }
        return store_status(status, start, node, i - start);
}

/*
 * 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)
{
        int current_node = NUMA_NO_NODE;
        LIST_HEAD(pagelist);
        int start, i;
        int err = 0, err1;

        lru_cache_disable();

        for (i = start = 0; i < nr_pages; i++) {
                const void __user *p;
                unsigned long addr;
                int node;

                err = -EFAULT;
                if (get_user(p, pages + i))
                        goto out_flush;
                if (get_user(node, nodes + i))
                        goto out_flush;
                addr = (unsigned long)untagged_addr(p);

                err = -ENODEV;
                if (node < 0 || node >= MAX_NUMNODES)
                        goto out_flush;
                if (!node_state(node, N_MEMORY))
                        goto out_flush;

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

                if (current_node == NUMA_NO_NODE) {
                        current_node = node;
                        start = i;
                } else if (node != current_node) {
                        err = move_pages_and_store_status(mm, current_node,
                                        &pagelist, status, start, i, nr_pages);
                        if (err)
                                goto out;
                        start = i;
                        current_node = node;
                }

                /*
                 * Errors in the page lookup or isolation are not fatal and we simply
                 * report them via status
                 */
                err = add_page_for_migration(mm, addr, current_node,
                                &pagelist, flags & MPOL_MF_MOVE_ALL);

                if (err > 0) {
                        /* The page is successfully queued for migration */
                        continue;
                }

                /*
                 * The move_pages() man page does not have an -EEXIST choice, so
                 * use -EFAULT instead.
                 */
                if (err == -EEXIST)
                        err = -EFAULT;

                /*
                 * If the page is already on the target node (!err), store the
                 * node, otherwise, store the err.
                 */
                err = store_status(status, i, err ? : current_node, 1);
                if (err)
                        goto out_flush;

                err = move_pages_and_store_status(mm, current_node, &pagelist,
                                status, start, i, nr_pages);
                if (err)
                        goto out;
                current_node = NUMA_NO_NODE;
        }
out_flush:
        /* Make sure we do not overwrite the existing error */
        err1 = move_pages_and_store_status(mm, current_node, &pagelist,
                                status, start, i, nr_pages);
        if (err >= 0)
                err = err1;
out:
        lru_cache_enable();
        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;

        mmap_read_lock(mm);

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

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

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

                if (page && !is_zone_device_page(page)) {
                        err = page_to_nid(page);
                        put_page(page);
                } else {
                        err = -ENOENT;
                }
set_status:
                *status = err;

                pages++;
                status++;
        }

        mmap_read_unlock(mm);
}

static int get_compat_pages_array(const void __user *chunk_pages[],
                                  const void __user * __user *pages,
                                  unsigned long chunk_nr)
{
        compat_uptr_t __user *pages32 = (compat_uptr_t __user *)pages;
        compat_uptr_t p;
        int i;

        for (i = 0; i < chunk_nr; i++) {
                if (get_user(p, pages32 + i))
                        return -EFAULT;
                chunk_pages[i] = compat_ptr(p);
        }

        return 0;
}

/*
 * 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 16UL
        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 = min(nr_pages, DO_PAGES_STAT_CHUNK_NR);

                if (in_compat_syscall()) {
                        if (get_compat_pages_array(chunk_pages, pages,
                                                   chunk_nr))
                                break;
                } else {
                        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;
}

static struct mm_struct *find_mm_struct(pid_t pid, nodemask_t *mem_nodes)
{
        struct task_struct *task;
        struct mm_struct *mm;

        /*
         * There is no need to check if current process has the right to modify
         * the specified process when they are same.
         */
        if (!pid) {
                mmget(current->mm);
                *mem_nodes = cpuset_mems_allowed(current);
                return current->mm;
        }

        /* Find the mm_struct */
        rcu_read_lock();
        task = find_task_by_vpid(pid);
        if (!task) {
                rcu_read_unlock();
                return ERR_PTR(-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();
                mm = ERR_PTR(-EPERM);
                goto out;
        }
        rcu_read_unlock();

        mm = ERR_PTR(security_task_movememory(task));
        if (IS_ERR(mm))
                goto out;
        *mem_nodes = cpuset_mems_allowed(task);
        mm = get_task_mm(task);
out:
        put_task_struct(task);
        if (!mm)
                mm = ERR_PTR(-EINVAL);
        return mm;
}

/*
 * Move a list of pages in the address space of the currently executing
 * process.
 */
static int kernel_move_pages(pid_t pid, unsigned long nr_pages,
                             const void __user * __user *pages,
                             const int __user *nodes,
                             int __user *status, int flags)
{
        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;

        mm = find_mm_struct(pid, &task_nodes);
        if (IS_ERR(mm))
                return PTR_ERR(mm);

        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;
}

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)
{
        return kernel_move_pages(pid, nr_pages, pages, nodes, status, flags);
}

#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 is 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 (!managed_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,
                                       ZONE_MOVABLE, 0))
                        continue;
                return true;
        }
        return false;
}

static struct page *alloc_misplaced_dst_page(struct page *page,
                                           unsigned long data)
{
        int nid = (int) data;
        int order = compound_order(page);
        gfp_t gfp = __GFP_THISNODE;
        struct folio *new;

        if (order > 0)
                gfp |= GFP_TRANSHUGE_LIGHT;
        else {
                gfp |= GFP_HIGHUSER_MOVABLE | __GFP_NOMEMALLOC | __GFP_NORETRY |
                        __GFP_NOWARN;
                gfp &= ~__GFP_RECLAIM;
        }
        new = __folio_alloc_node(gfp, order, nid);

        return &new->page;
}

static int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)
{
        int nr_pages = thp_nr_pages(page);
        int order = compound_order(page);

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

        /* Do not migrate THP mapped by multiple processes */
        if (PageTransHuge(page) && total_mapcount(page) > 1)
                return 0;

        /* Avoid migrating to a node that is nearly full */
        if (!migrate_balanced_pgdat(pgdat, nr_pages)) {
                int z;

                if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING))
                        return 0;
                for (z = pgdat->nr_zones - 1; z >= 0; z--) {
                        if (managed_zone(pgdat->node_zones + z))
                                break;
                }
                wakeup_kswapd(pgdat->node_zones + z, 0, order, ZONE_MOVABLE);
                return 0;
        }

        if (isolate_lru_page(page))
                return 0;

        mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON + page_is_file_lru(page),
                            nr_pages);

        /*
         * 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;
}

/*
 * 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;
        unsigned int nr_succeeded;
        LIST_HEAD(migratepages);
        int nr_pages = thp_nr_pages(page);

        /*
         * 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_lru(page) &&
            (vma->vm_flags & VM_EXEC))
                goto out;

        /*
         * Also do not migrate dirty pages as not all filesystems can move
         * dirty pages in MIGRATE_ASYNC mode which is a waste of cycles.
         */
        if (page_is_file_lru(page) && PageDirty(page))
                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, &nr_succeeded);
        if (nr_remaining) {
                if (!list_empty(&migratepages)) {
                        list_del(&page->lru);
                        mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON +
                                        page_is_file_lru(page), -nr_pages);
                        putback_lru_page(page);
                }
                isolated = 0;
        }
        if (nr_succeeded) {
                count_vm_numa_events(NUMA_PAGE_MIGRATE, nr_succeeded);
                if (!node_is_toptier(page_to_nid(page)) && node_is_toptier(node))
                        mod_node_page_state(pgdat, PGPROMOTE_SUCCESS,
                                            nr_succeeded);
        }
        BUG_ON(!list_empty(&migratepages));
        return isolated;

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

/*
 * node_demotion[] example:
 *
 * Consider a system with two sockets.  Each socket has
 * three classes of memory attached: fast, medium and slow.
 * Each memory class is placed in its own NUMA node.  The
 * CPUs are placed in the node with the "fast" memory.  The
 * 6 NUMA nodes (0-5) might be split among the sockets like
 * this:
 *
 *      Socket A: 0, 1, 2
 *      Socket B: 3, 4, 5
 *
 * When Node 0 fills up, its memory should be migrated to
 * Node 1.  When Node 1 fills up, it should be migrated to
 * Node 2.  The migration path start on the nodes with the
 * processors (since allocations default to this node) and
 * fast memory, progress through medium and end with the
 * slow memory:
 *
 *      0 -> 1 -> 2 -> stop
 *      3 -> 4 -> 5 -> stop
 *
 * This is represented in the node_demotion[] like this:
 *
 *      {  nr=1, nodes[0]=1 }, // Node 0 migrates to 1
 *      {  nr=1, nodes[0]=2 }, // Node 1 migrates to 2
 *      {  nr=0, nodes[0]=-1 }, // Node 2 does not migrate
 *      {  nr=1, nodes[0]=4 }, // Node 3 migrates to 4
 *      {  nr=1, nodes[0]=5 }, // Node 4 migrates to 5
 *      {  nr=0, nodes[0]=-1 }, // Node 5 does not migrate
 *
 * Moreover some systems may have multiple slow memory nodes.
 * Suppose a system has one socket with 3 memory nodes, node 0
 * is fast memory type, and node 1/2 both are slow memory
 * type, and the distance between fast memory node and slow
 * memory node is same. So the migration path should be:
 *
 *      0 -> 1/2 -> stop
 *
 * This is represented in the node_demotion[] like this:
 *      { nr=2, {nodes[0]=1, nodes[1]=2} }, // Node 0 migrates to node 1 and node 2
 *      { nr=0, nodes[0]=-1, }, // Node 1 dose not migrate
 *      { nr=0, nodes[0]=-1, }, // Node 2 does not migrate
 */

/*
 * Writes to this array occur without locking.  Cycles are
 * not allowed: Node X demotes to Y which demotes to X...
 *
 * If multiple reads are performed, a single rcu_read_lock()
 * must be held over all reads to ensure that no cycles are
 * observed.
 */
#define DEFAULT_DEMOTION_TARGET_NODES 15

#if MAX_NUMNODES < DEFAULT_DEMOTION_TARGET_NODES
#define DEMOTION_TARGET_NODES   (MAX_NUMNODES - 1)
#else
#define DEMOTION_TARGET_NODES   DEFAULT_DEMOTION_TARGET_NODES
#endif

struct demotion_nodes {
        unsigned short nr;
        short nodes[DEMOTION_TARGET_NODES];
};

static struct demotion_nodes *node_demotion __read_mostly;

/**
 * next_demotion_node() - Get the next node in the demotion path
 * @node: The starting node to lookup the next node
 *
 * Return: node id for next memory node in the demotion path hierarchy
 * from @node; NUMA_NO_NODE if @node is terminal.  This does not keep
 * @node online or guarantee that it *continues* to be the next demotion
 * target.
 */
int next_demotion_node(int node)
{
        struct demotion_nodes *nd;
        unsigned short target_nr, index;
        int target;

        if (!node_demotion)
                return NUMA_NO_NODE;

        nd = &node_demotion[node];

        /*
         * node_demotion[] is updated without excluding this
         * function from running.  RCU doesn't provide any
         * compiler barriers, so the READ_ONCE() is required
         * to avoid compiler reordering or read merging.
         *
         * Make sure to use RCU over entire code blocks if
         * node_demotion[] reads need to be consistent.
         */
        rcu_read_lock();
        target_nr = READ_ONCE(nd->nr);

        switch (target_nr) {
        case 0:
                target = NUMA_NO_NODE;
                goto out;
        case 1:
                index = 0;
                break;
        default:
                /*
                 * If there are multiple target nodes, just select one
                 * target node randomly.
                 *
                 * In addition, we can also use round-robin to select
                 * target node, but we should introduce another variable
                 * for node_demotion[] to record last selected target node,
                 * that may cause cache ping-pong due to the changing of
                 * last target node. Or introducing per-cpu data to avoid
                 * caching issue, which seems more complicated. So selecting
                 * target node randomly seems better until now.
                 */
                index = get_random_int() % target_nr;
                break;
        }

        target = READ_ONCE(nd->nodes[index]);

out:
        rcu_read_unlock();
        return target;
}

/* Disable reclaim-based migration. */
static void __disable_all_migrate_targets(void)
{
        int node, i;

        if (!node_demotion)
                return;

        for_each_online_node(node) {
                node_demotion[node].nr = 0;
                for (i = 0; i < DEMOTION_TARGET_NODES; i++)
                        node_demotion[node].nodes[i] = NUMA_NO_NODE;
        }
}

static void disable_all_migrate_targets(void)
{
        __disable_all_migrate_targets();

        /*
         * Ensure that the "disable" is visible across the system.
         * Readers will see either a combination of before+disable
         * state or disable+after.  They will never see before and
         * after state together.
         *
         * The before+after state together might have cycles and
         * could cause readers to do things like loop until this
         * function finishes.  This ensures they can only see a
         * single "bad" read and would, for instance, only loop
         * once.
         */
        synchronize_rcu();
}

/*
 * Find an automatic demotion target for 'node'.
 * Failing here is OK.  It might just indicate
 * being at the end of a chain.
 */
static int establish_migrate_target(int node, nodemask_t *used,
                                    int best_distance)
{
        int migration_target, index, val;
        struct demotion_nodes *nd;

        if (!node_demotion)
                return NUMA_NO_NODE;

        nd = &node_demotion[node];

        migration_target = find_next_best_node(node, used);
        if (migration_target == NUMA_NO_NODE)
                return NUMA_NO_NODE;

        /*
         * If the node has been set a migration target node before,
         * which means it's the best distance between them. Still
         * check if this node can be demoted to other target nodes
         * if they have a same best distance.
         */
        if (best_distance != -1) {
                val = node_distance(node, migration_target);
                if (val > best_distance)
                        goto out_clear;
        }

        index = nd->nr;
        if (WARN_ONCE(index >= DEMOTION_TARGET_NODES,
                      "Exceeds maximum demotion target nodes\n"))
                goto out_clear;

        nd->nodes[index] = migration_target;
        nd->nr++;

        return migration_target;
out_clear:
        node_clear(migration_target, *used);
        return NUMA_NO_NODE;
}

/*
 * When memory fills up on a node, memory contents can be
 * automatically migrated to another node instead of
 * discarded at reclaim.
 *
 * Establish a "migration path" which will start at nodes
 * with CPUs and will follow the priorities used to build the
 * page allocator zonelists.
 *
 * The difference here is that cycles must be avoided.  If
 * node0 migrates to node1, then neither node1, nor anything
 * node1 migrates to can migrate to node0. Also one node can
 * be migrated to multiple nodes if the target nodes all have
 * a same best-distance against the source node.
 *
 * This function can run simultaneously with readers of
 * node_demotion[].  However, it can not run simultaneously
 * with itself.  Exclusion is provided by memory hotplug events
 * being single-threaded.
 */
static void __set_migration_target_nodes(void)
{
        nodemask_t next_pass;
        nodemask_t this_pass;
        nodemask_t used_targets = NODE_MASK_NONE;
        int node, best_distance;

        /*
         * Avoid any oddities like cycles that could occur
         * from changes in the topology.  This will leave
         * a momentary gap when migration is disabled.
         */
        disable_all_migrate_targets();

        /*
         * Allocations go close to CPUs, first.  Assume that
         * the migration path starts at the nodes with CPUs.
         */
        next_pass = node_states[N_CPU];
again:
        this_pass = next_pass;
        next_pass = NODE_MASK_NONE;
        /*
         * To avoid cycles in the migration "graph", ensure
         * that migration sources are not future targets by
         * setting them in 'used_targets'.  Do this only
         * once per pass so that multiple source nodes can
         * share a target node.
         *
         * 'used_targets' will become unavailable in future
         * passes.  This limits some opportunities for
         * multiple source nodes to share a destination.
         */
        nodes_or(used_targets, used_targets, this_pass);

        for_each_node_mask(node, this_pass) {
                best_distance = -1;

                /*
                 * Try to set up the migration path for the node, and the target
                 * migration nodes can be multiple, so doing a loop to find all
                 * the target nodes if they all have a best node distance.
                 */
                do {
                        int target_node =
                                establish_migrate_target(node, &used_targets,
                                                         best_distance);

                        if (target_node == NUMA_NO_NODE)
                                break;

                        if (best_distance == -1)
                                best_distance = node_distance(node, target_node);

                        /*
                         * Visit targets from this pass in the next pass.
                         * Eventually, every node will have been part of
                         * a pass, and will become set in 'used_targets'.
                         */
                        node_set(target_node, next_pass);
                } while (1);
        }
        /*
         * 'next_pass' contains nodes which became migration
         * targets in this pass.  Make additional passes until
         * no more migrations targets are available.
         */
        if (!nodes_empty(next_pass))
                goto again;
}

/*
 * For callers that do not hold get_online_mems() already.
 */
void set_migration_target_nodes(void)
{
        get_online_mems();
        __set_migration_target_nodes();
        put_online_mems();
}

/*
 * This leaves migrate-on-reclaim transiently disabled between
 * the MEM_GOING_OFFLINE and MEM_OFFLINE events.  This runs
 * whether reclaim-based migration is enabled or not, which
 * ensures that the user can turn reclaim-based migration at
 * any time without needing to recalculate migration targets.
 *
 * These callbacks already hold get_online_mems().  That is why
 * __set_migration_target_nodes() can be used as opposed to
 * set_migration_target_nodes().
 */
#ifdef CONFIG_MEMORY_HOTPLUG
static int __meminit migrate_on_reclaim_callback(struct notifier_block *self,
                                                 unsigned long action, void *_arg)
{
        struct memory_notify *arg = _arg;

        /*
         * Only update the node migration order when a node is
         * changing status, like online->offline.  This avoids
         * the overhead of synchronize_rcu() in most cases.
         */
        if (arg->status_change_nid < 0)
                return notifier_from_errno(0);

        switch (action) {
        case MEM_GOING_OFFLINE:
                /*
                 * Make sure there are not transient states where
                 * an offline node is a migration target.  This
                 * will leave migration disabled until the offline
                 * completes and the MEM_OFFLINE case below runs.
                 */
                disable_all_migrate_targets();
                break;
        case MEM_OFFLINE:
        case MEM_ONLINE:
                /*
                 * Recalculate the target nodes once the node
                 * reaches its final state (online or offline).
                 */
                __set_migration_target_nodes();
                break;
        case MEM_CANCEL_OFFLINE:
                /*
                 * MEM_GOING_OFFLINE disabled all the migration
                 * targets.  Reenable them.
                 */
                __set_migration_target_nodes();
                break;
        case MEM_GOING_ONLINE:
        case MEM_CANCEL_ONLINE:
                break;
        }

        return notifier_from_errno(0);
}
#endif

void __init migrate_on_reclaim_init(void)
{
        node_demotion = kcalloc(nr_node_ids,
                                sizeof(struct demotion_nodes),
                                GFP_KERNEL);
        WARN_ON(!node_demotion);
#ifdef CONFIG_MEMORY_HOTPLUG
        hotplug_memory_notifier(migrate_on_reclaim_callback, 100);
#endif
        /*
         * At this point, all numa nodes with memory/CPus have their state
         * properly set, so we can build the demotion order now.
         * Let us hold the cpu_hotplug lock just, as we could possibily have
         * CPU hotplug events during boot.
         */
        cpus_read_lock();
        set_migration_target_nodes();
        cpus_read_unlock();
}

bool numa_demotion_enabled = false;

#ifdef CONFIG_SYSFS
static ssize_t numa_demotion_enabled_show(struct kobject *kobj,
                                          struct kobj_attribute *attr, char *buf)
{
        return sysfs_emit(buf, "%s\n",
                          numa_demotion_enabled ? "true" : "false");
}

static ssize_t numa_demotion_enabled_store(struct kobject *kobj,
                                           struct kobj_attribute *attr,
                                           const char *buf, size_t count)
{
        ssize_t ret;

        ret = kstrtobool(buf, &numa_demotion_enabled);
        if (ret)
                return ret;

        return count;
}

static struct kobj_attribute numa_demotion_enabled_attr =
        __ATTR(demotion_enabled, 0644, numa_demotion_enabled_show,
               numa_demotion_enabled_store);

static struct attribute *numa_attrs[] = {
        &numa_demotion_enabled_attr.attr,
        NULL,
};

static const struct attribute_group numa_attr_group = {
        .attrs = numa_attrs,
};

static int __init numa_init_sysfs(void)
{
        int err;
        struct kobject *numa_kobj;

        numa_kobj = kobject_create_and_add("numa", mm_kobj);
        if (!numa_kobj) {
                pr_err("failed to create numa kobject\n");
                return -ENOMEM;
        }
        err = sysfs_create_group(numa_kobj, &numa_attr_group);
        if (err) {
                pr_err("failed to register numa group\n");
                goto delete_obj;
        }
        return 0;

delete_obj:
        kobject_put(numa_kobj);
        return err;
}
subsys_initcall(numa_init_sysfs);
#endif /* CONFIG_SYSFS */
#endif /* CONFIG_NUMA */