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 // 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 */

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