/*
   * mm/rmap.c - physical to virtual reverse mappings
   *
   * Copyright 2001, Rik van Riel <riel@conectiva.com.br>
   * Released under the General Public License (GPL).
   *
   * Simple, low overhead reverse mapping scheme.
   * Please try to keep this thing as modular as possible.
   *
   * Provides methods for unmapping each kind of mapped page:
   * the anon methods track anonymous pages, and
   * the file methods track pages belonging to an inode.
   *
   * Original design by Rik van Riel <riel@conectiva.com.br> 2001
   * File methods by Dave McCracken <dmccr@us.ibm.com> 2003, 2004
   * Anonymous methods by Andrea Arcangeli <andrea@suse.de> 2004

   * Contributions by Hugh Dickins 2003, 2004

   */
  
  /*
   * Lock ordering in mm:
   *

   * inode->i_mutex	(while writing or truncating, not reading or faulting)

   *   mm->mmap_sem
   *     page->flags PG_locked (lock_page)

   *       mapping->i_mmap_mutex

   *         anon_vma->mutex

   *           mm->page_table_lock or pte_lock
   *             zone->lru_lock (in mark_page_accessed, isolate_lru_page)
   *             swap_lock (in swap_duplicate, swap_info_get)
   *               mmlist_lock (in mmput, drain_mmlist and others)
   *               mapping->private_lock (in __set_page_dirty_buffers)

   *               inode->i_lock (in set_page_dirty's __mark_inode_dirty)

   *               bdi.wb->list_lock (in set_page_dirty's __mark_inode_dirty)

   *                 sb_lock (within inode_lock in fs/fs-writeback.c)
   *                 mapping->tree_lock (widely used, in set_page_dirty,
   *                           in arch-dependent flush_dcache_mmap_lock,

   *                           within bdi.wb->list_lock in __sync_single_inode)

   *

   * anon_vma->mutex,mapping->i_mutex      (memory_failure, collect_procs_anon)
   *   ->tasklist_lock

   *     pte map lock

   */
  
  #include <linux/mm.h>
  #include <linux/pagemap.h>
  #include <linux/swap.h>
  #include <linux/swapops.h>
  #include <linux/slab.h>
  #include <linux/init.h>

  #include <linux/ksm.h>

  #include <linux/rmap.h>
  #include <linux/rcupdate.h>

  #include <linux/export.h>

  #include <linux/memcontrol.h>

  #include <linux/mmu_notifier.h>

  #include <linux/migrate.h>

  #include <linux/hugetlb.h>

  
  #include <asm/tlbflush.h>

  #include "internal.h"

  static struct kmem_cache *anon_vma_cachep;

  static struct kmem_cache *anon_vma_chain_cachep;

  
  static inline struct anon_vma *anon_vma_alloc(void)
  {

  	struct anon_vma *anon_vma;
  
  	anon_vma = kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL);
  	if (anon_vma) {
  		atomic_set(&anon_vma->refcount, 1);
  		/*
  		 * Initialise the anon_vma root to point to itself. If called
  		 * from fork, the root will be reset to the parents anon_vma.
  		 */
  		anon_vma->root = anon_vma;
  	}
  
  	return anon_vma;

  }

  static inline void anon_vma_free(struct anon_vma *anon_vma)

  {

  	VM_BUG_ON(atomic_read(&anon_vma->refcount));

  
  	/*
  	 * Synchronize against page_lock_anon_vma() such that
  	 * we can safely hold the lock without the anon_vma getting
  	 * freed.
  	 *
  	 * Relies on the full mb implied by the atomic_dec_and_test() from
  	 * put_anon_vma() against the acquire barrier implied by
  	 * mutex_trylock() from page_lock_anon_vma(). This orders:
  	 *
  	 * page_lock_anon_vma()		VS	put_anon_vma()
  	 *   mutex_trylock()			  atomic_dec_and_test()
  	 *   LOCK				  MB
  	 *   atomic_read()			  mutex_is_locked()
  	 *
  	 * LOCK should suffice since the actual taking of the lock must
  	 * happen _before_ what follows.
  	 */
  	if (mutex_is_locked(&anon_vma->root->mutex)) {
  		anon_vma_lock(anon_vma);
  		anon_vma_unlock(anon_vma);
  	}

  	kmem_cache_free(anon_vma_cachep, anon_vma);
  }

  static inline struct anon_vma_chain *anon_vma_chain_alloc(gfp_t gfp)

  {

  	return kmem_cache_alloc(anon_vma_chain_cachep, gfp);

  }

  static void anon_vma_chain_free(struct anon_vma_chain *anon_vma_chain)

  {
  	kmem_cache_free(anon_vma_chain_cachep, anon_vma_chain);
  }

  static void anon_vma_chain_link(struct vm_area_struct *vma,
  				struct anon_vma_chain *avc,
  				struct anon_vma *anon_vma)
  {
  	avc->vma = vma;
  	avc->anon_vma = anon_vma;
  	list_add(&avc->same_vma, &vma->anon_vma_chain);
  
  	/*
  	 * It's critical to add new vmas to the tail of the anon_vma,
  	 * see comment in huge_memory.c:__split_huge_page().
  	 */
  	list_add_tail(&avc->same_anon_vma, &anon_vma->head);
  }

  /**
   * anon_vma_prepare - attach an anon_vma to a memory region
   * @vma: the memory region in question
   *
   * This makes sure the memory mapping described by 'vma' has
   * an 'anon_vma' attached to it, so that we can associate the
   * anonymous pages mapped into it with that anon_vma.
   *
   * The common case will be that we already have one, but if

   * not we either need to find an adjacent mapping that we

   * can re-use the anon_vma from (very common when the only
   * reason for splitting a vma has been mprotect()), or we
   * allocate a new one.
   *
   * Anon-vma allocations are very subtle, because we may have
   * optimistically looked up an anon_vma in page_lock_anon_vma()
   * and that may actually touch the spinlock even in the newly
   * allocated vma (it depends on RCU to make sure that the
   * anon_vma isn't actually destroyed).
   *
   * As a result, we need to do proper anon_vma locking even
   * for the new allocation. At the same time, we do not want
   * to do any locking for the common case of already having
   * an anon_vma.
   *
   * This must be called with the mmap_sem held for reading.
   */

  int anon_vma_prepare(struct vm_area_struct *vma)
  {
  	struct anon_vma *anon_vma = vma->anon_vma;

  	struct anon_vma_chain *avc;

  
  	might_sleep();
  	if (unlikely(!anon_vma)) {
  		struct mm_struct *mm = vma->vm_mm;

  		struct anon_vma *allocated;

  		avc = anon_vma_chain_alloc(GFP_KERNEL);

  		if (!avc)
  			goto out_enomem;

  		anon_vma = find_mergeable_anon_vma(vma);

  		allocated = NULL;
  		if (!anon_vma) {

  			anon_vma = anon_vma_alloc();
  			if (unlikely(!anon_vma))

  				goto out_enomem_free_avc;

  			allocated = anon_vma;

  		}

  		anon_vma_lock(anon_vma);

  		/* page_table_lock to protect against threads */
  		spin_lock(&mm->page_table_lock);
  		if (likely(!vma->anon_vma)) {
  			vma->anon_vma = anon_vma;

  			anon_vma_chain_link(vma, avc, anon_vma);

  			allocated = NULL;

  			avc = NULL;

  		}
  		spin_unlock(&mm->page_table_lock);

  		anon_vma_unlock(anon_vma);

  
  		if (unlikely(allocated))

  			put_anon_vma(allocated);

  		if (unlikely(avc))

  			anon_vma_chain_free(avc);

  	}
  	return 0;

  
   out_enomem_free_avc:
  	anon_vma_chain_free(avc);
   out_enomem:
  	return -ENOMEM;

  }

  /*
   * This is a useful helper function for locking the anon_vma root as
   * we traverse the vma->anon_vma_chain, looping over anon_vma's that
   * have the same vma.
   *
   * Such anon_vma's should have the same root, so you'd expect to see
   * just a single mutex_lock for the whole traversal.
   */
  static inline struct anon_vma *lock_anon_vma_root(struct anon_vma *root, struct anon_vma *anon_vma)
  {
  	struct anon_vma *new_root = anon_vma->root;
  	if (new_root != root) {
  		if (WARN_ON_ONCE(root))
  			mutex_unlock(&root->mutex);
  		root = new_root;
  		mutex_lock(&root->mutex);
  	}
  	return root;
  }
  
  static inline void unlock_anon_vma_root(struct anon_vma *root)
  {
  	if (root)
  		mutex_unlock(&root->mutex);
  }

  /*
   * Attach the anon_vmas from src to dst.
   * Returns 0 on success, -ENOMEM on failure.
   */
  int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)

  {

  	struct anon_vma_chain *avc, *pavc;

  	struct anon_vma *root = NULL;

  	list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {

  		struct anon_vma *anon_vma;

  		avc = anon_vma_chain_alloc(GFP_NOWAIT | __GFP_NOWARN);
  		if (unlikely(!avc)) {
  			unlock_anon_vma_root(root);
  			root = NULL;
  			avc = anon_vma_chain_alloc(GFP_KERNEL);
  			if (!avc)
  				goto enomem_failure;
  		}

  		anon_vma = pavc->anon_vma;
  		root = lock_anon_vma_root(root, anon_vma);
  		anon_vma_chain_link(dst, avc, anon_vma);

  	}

  	unlock_anon_vma_root(root);

  	return 0;

   enomem_failure:
  	unlink_anon_vmas(dst);
  	return -ENOMEM;

  }

  /*

   * Some rmap walk that needs to find all ptes/hugepmds without false
   * negatives (like migrate and split_huge_page) running concurrent
   * with operations that copy or move pagetables (like mremap() and
   * fork()) to be safe. They depend on the anon_vma "same_anon_vma"
   * list to be in a certain order: the dst_vma must be placed after the
   * src_vma in the list. This is always guaranteed by fork() but
   * mremap() needs to call this function to enforce it in case the
   * dst_vma isn't newly allocated and chained with the anon_vma_clone()
   * function but just an extension of a pre-existing vma through
   * vma_merge.
   *
   * NOTE: the same_anon_vma list can still be changed by other
   * processes while mremap runs because mremap doesn't hold the
   * anon_vma mutex to prevent modifications to the list while it
   * runs. All we need to enforce is that the relative order of this
   * process vmas isn't changing (we don't care about other vmas
   * order). Each vma corresponds to an anon_vma_chain structure so
   * there's no risk that other processes calling anon_vma_moveto_tail()
   * and changing the same_anon_vma list under mremap() will screw with
   * the relative order of this process vmas in the list, because we
   * they can't alter the order of any vma that belongs to this
   * process. And there can't be another anon_vma_moveto_tail() running
   * concurrently with mremap() coming from this process because we hold
   * the mmap_sem for the whole mremap(). fork() ordering dependency
   * also shouldn't be affected because fork() only cares that the
   * parent vmas are placed in the list before the child vmas and
   * anon_vma_moveto_tail() won't reorder vmas from either the fork()
   * parent or child.
   */
  void anon_vma_moveto_tail(struct vm_area_struct *dst)
  {
  	struct anon_vma_chain *pavc;
  	struct anon_vma *root = NULL;
  
  	list_for_each_entry_reverse(pavc, &dst->anon_vma_chain, same_vma) {
  		struct anon_vma *anon_vma = pavc->anon_vma;
  		VM_BUG_ON(pavc->vma != dst);
  		root = lock_anon_vma_root(root, anon_vma);
  		list_del(&pavc->same_anon_vma);
  		list_add_tail(&pavc->same_anon_vma, &anon_vma->head);
  	}
  	unlock_anon_vma_root(root);
  }
  
  /*

   * Attach vma to its own anon_vma, as well as to the anon_vmas that
   * the corresponding VMA in the parent process is attached to.
   * Returns 0 on success, non-zero on failure.
   */
  int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)

  {

  	struct anon_vma_chain *avc;
  	struct anon_vma *anon_vma;

  	/* Don't bother if the parent process has no anon_vma here. */
  	if (!pvma->anon_vma)
  		return 0;
  
  	/*
  	 * First, attach the new VMA to the parent VMA's anon_vmas,
  	 * so rmap can find non-COWed pages in child processes.
  	 */
  	if (anon_vma_clone(vma, pvma))
  		return -ENOMEM;
  
  	/* Then add our own anon_vma. */
  	anon_vma = anon_vma_alloc();
  	if (!anon_vma)
  		goto out_error;

  	avc = anon_vma_chain_alloc(GFP_KERNEL);

  	if (!avc)
  		goto out_error_free_anon_vma;

  
  	/*
  	 * The root anon_vma's spinlock is the lock actually used when we
  	 * lock any of the anon_vmas in this anon_vma tree.
  	 */
  	anon_vma->root = pvma->anon_vma->root;

  	/*

  	 * With refcounts, an anon_vma can stay around longer than the
  	 * process it belongs to. The root anon_vma needs to be pinned until
  	 * this anon_vma is freed, because the lock lives in the root.

  	 */
  	get_anon_vma(anon_vma->root);

  	/* Mark this anon_vma as the one where our new (COWed) pages go. */
  	vma->anon_vma = anon_vma;

  	anon_vma_lock(anon_vma);

  	anon_vma_chain_link(vma, avc, anon_vma);

  	anon_vma_unlock(anon_vma);

  
  	return 0;
  
   out_error_free_anon_vma:

  	put_anon_vma(anon_vma);

   out_error:

  	unlink_anon_vmas(vma);

  	return -ENOMEM;

  }

  void unlink_anon_vmas(struct vm_area_struct *vma)
  {
  	struct anon_vma_chain *avc, *next;

  	struct anon_vma *root = NULL;

  	/*
  	 * Unlink each anon_vma chained to the VMA.  This list is ordered
  	 * from newest to oldest, ensuring the root anon_vma gets freed last.
  	 */

  	list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {

  		struct anon_vma *anon_vma = avc->anon_vma;
  
  		root = lock_anon_vma_root(root, anon_vma);
  		list_del(&avc->same_anon_vma);
  
  		/*
  		 * Leave empty anon_vmas on the list - we'll need
  		 * to free them outside the lock.
  		 */
  		if (list_empty(&anon_vma->head))
  			continue;
  
  		list_del(&avc->same_vma);
  		anon_vma_chain_free(avc);
  	}
  	unlock_anon_vma_root(root);
  
  	/*
  	 * Iterate the list once more, it now only contains empty and unlinked
  	 * anon_vmas, destroy them. Could not do before due to __put_anon_vma()
  	 * needing to acquire the anon_vma->root->mutex.
  	 */
  	list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
  		struct anon_vma *anon_vma = avc->anon_vma;
  
  		put_anon_vma(anon_vma);

  		list_del(&avc->same_vma);
  		anon_vma_chain_free(avc);
  	}
  }

  static void anon_vma_ctor(void *data)

  {

  	struct anon_vma *anon_vma = data;

  	mutex_init(&anon_vma->mutex);

  	atomic_set(&anon_vma->refcount, 0);

  	INIT_LIST_HEAD(&anon_vma->head);

  }
  
  void __init anon_vma_init(void)
  {
  	anon_vma_cachep = kmem_cache_create("anon_vma", sizeof(struct anon_vma),

  			0, SLAB_DESTROY_BY_RCU|SLAB_PANIC, anon_vma_ctor);

  	anon_vma_chain_cachep = KMEM_CACHE(anon_vma_chain, SLAB_PANIC);

  }
  
  /*

   * Getting a lock on a stable anon_vma from a page off the LRU is tricky!
   *
   * Since there is no serialization what so ever against page_remove_rmap()
   * the best this function can do is return a locked anon_vma that might
   * have been relevant to this page.
   *
   * The page might have been remapped to a different anon_vma or the anon_vma
   * returned may already be freed (and even reused).
   *

   * In case it was remapped to a different anon_vma, the new anon_vma will be a
   * child of the old anon_vma, and the anon_vma lifetime rules will therefore
   * ensure that any anon_vma obtained from the page will still be valid for as
   * long as we observe page_mapped() [ hence all those page_mapped() tests ].
   *

   * All users of this function must be very careful when walking the anon_vma
   * chain and verify that the page in question is indeed mapped in it
   * [ something equivalent to page_mapped_in_vma() ].
   *
   * Since anon_vma's slab is DESTROY_BY_RCU and we know from page_remove_rmap()
   * that the anon_vma pointer from page->mapping is valid if there is a
   * mapcount, we can dereference the anon_vma after observing those.

   */

  struct anon_vma *page_get_anon_vma(struct page *page)

  {

  	struct anon_vma *anon_vma = NULL;

  	unsigned long anon_mapping;
  
  	rcu_read_lock();

  	anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping);

  	if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)

  		goto out;
  	if (!page_mapped(page))
  		goto out;
  
  	anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);

  	if (!atomic_inc_not_zero(&anon_vma->refcount)) {
  		anon_vma = NULL;
  		goto out;
  	}

  
  	/*
  	 * If this page is still mapped, then its anon_vma cannot have been

  	 * freed.  But if it has been unmapped, we have no security against the
  	 * anon_vma structure being freed and reused (for another anon_vma:
  	 * SLAB_DESTROY_BY_RCU guarantees that - so the atomic_inc_not_zero()
  	 * above cannot corrupt).

  	 */

  	if (!page_mapped(page)) {
  		put_anon_vma(anon_vma);
  		anon_vma = NULL;
  	}

  out:
  	rcu_read_unlock();

  
  	return anon_vma;
  }

  /*
   * Similar to page_get_anon_vma() except it locks the anon_vma.
   *
   * Its a little more complex as it tries to keep the fast path to a single
   * atomic op -- the trylock. If we fail the trylock, we fall back to getting a
   * reference like with page_get_anon_vma() and then block on the mutex.
   */

  struct anon_vma *page_lock_anon_vma(struct page *page)
  {

  	struct anon_vma *anon_vma = NULL;

  	struct anon_vma *root_anon_vma;

  	unsigned long anon_mapping;

  	rcu_read_lock();
  	anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping);
  	if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
  		goto out;
  	if (!page_mapped(page))
  		goto out;
  
  	anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);

  	root_anon_vma = ACCESS_ONCE(anon_vma->root);
  	if (mutex_trylock(&root_anon_vma->mutex)) {

  		/*

  		 * If the page is still mapped, then this anon_vma is still
  		 * its anon_vma, and holding the mutex ensures that it will

  		 * not go away, see anon_vma_free().

  		 */

  		if (!page_mapped(page)) {
  			mutex_unlock(&root_anon_vma->mutex);

  			anon_vma = NULL;
  		}
  		goto out;
  	}

  	/* trylock failed, we got to sleep */
  	if (!atomic_inc_not_zero(&anon_vma->refcount)) {
  		anon_vma = NULL;
  		goto out;
  	}
  
  	if (!page_mapped(page)) {
  		put_anon_vma(anon_vma);
  		anon_vma = NULL;
  		goto out;
  	}
  
  	/* we pinned the anon_vma, its safe to sleep */
  	rcu_read_unlock();
  	anon_vma_lock(anon_vma);
  
  	if (atomic_dec_and_test(&anon_vma->refcount)) {
  		/*
  		 * Oops, we held the last refcount, release the lock
  		 * and bail -- can't simply use put_anon_vma() because
  		 * we'll deadlock on the anon_vma_lock() recursion.
  		 */
  		anon_vma_unlock(anon_vma);
  		__put_anon_vma(anon_vma);
  		anon_vma = NULL;
  	}
  
  	return anon_vma;
  
  out:
  	rcu_read_unlock();

  	return anon_vma;

  }

  void page_unlock_anon_vma(struct anon_vma *anon_vma)

  {

  	anon_vma_unlock(anon_vma);

  }
  
  /*

   * At what user virtual address is page expected in @vma?
   * Returns virtual address or -EFAULT if page's index/offset is not
   * within the range mapped the @vma.

   */

  inline unsigned long

  vma_address(struct page *page, struct vm_area_struct *vma)
  {
  	pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
  	unsigned long address;

  	if (unlikely(is_vm_hugetlb_page(vma)))
  		pgoff = page->index << huge_page_order(page_hstate(page));

  	address = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
  	if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {

  		/* page should be within @vma mapping range */

  		return -EFAULT;
  	}
  	return address;
  }
  
  /*

   * At what user virtual address is page expected in vma?

   * Caller should check the page is actually part of the vma.

   */
  unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma)
  {

  	if (PageAnon(page)) {

  		struct anon_vma *page__anon_vma = page_anon_vma(page);
  		/*
  		 * Note: swapoff's unuse_vma() is more efficient with this
  		 * check, and needs it to match anon_vma when KSM is active.
  		 */
  		if (!vma->anon_vma || !page__anon_vma ||
  		    vma->anon_vma->root != page__anon_vma->root)

  			return -EFAULT;
  	} else if (page->mapping && !(vma->vm_flags & VM_NONLINEAR)) {

  		if (!vma->vm_file ||
  		    vma->vm_file->f_mapping != page->mapping)

  			return -EFAULT;
  	} else
  		return -EFAULT;
  	return vma_address(page, vma);
  }
  
  /*

   * Check that @page is mapped at @address into @mm.
   *

   * If @sync is false, page_check_address may perform a racy check to avoid
   * the page table lock when the pte is not present (helpful when reclaiming
   * highly shared pages).
   *

   * On success returns with pte mapped and locked.

   */

  pte_t *__page_check_address(struct page *page, struct mm_struct *mm,

  			  unsigned long address, spinlock_t **ptlp, int sync)

  {
  	pgd_t *pgd;
  	pud_t *pud;
  	pmd_t *pmd;
  	pte_t *pte;

  	spinlock_t *ptl;

  	if (unlikely(PageHuge(page))) {
  		pte = huge_pte_offset(mm, address);
  		ptl = &mm->page_table_lock;
  		goto check;
  	}

  	pgd = pgd_offset(mm, address);

  	if (!pgd_present(*pgd))
  		return NULL;
  
  	pud = pud_offset(pgd, address);
  	if (!pud_present(*pud))
  		return NULL;
  
  	pmd = pmd_offset(pud, address);
  	if (!pmd_present(*pmd))
  		return NULL;

  	if (pmd_trans_huge(*pmd))
  		return NULL;

  
  	pte = pte_offset_map(pmd, address);
  	/* Make a quick check before getting the lock */

  	if (!sync && !pte_present(*pte)) {

  		pte_unmap(pte);
  		return NULL;
  	}

  	ptl = pte_lockptr(mm, pmd);

  check:

  	spin_lock(ptl);
  	if (pte_present(*pte) && page_to_pfn(page) == pte_pfn(*pte)) {
  		*ptlp = ptl;
  		return pte;

  	}

  	pte_unmap_unlock(pte, ptl);
  	return NULL;

  }

  /**
   * page_mapped_in_vma - check whether a page is really mapped in a VMA
   * @page: the page to test
   * @vma: the VMA to test
   *
   * Returns 1 if the page is mapped into the page tables of the VMA, 0
   * if the page is not mapped into the page tables of this VMA.  Only
   * valid for normal file or anonymous VMAs.
   */

  int page_mapped_in_vma(struct page *page, struct vm_area_struct *vma)

  {
  	unsigned long address;
  	pte_t *pte;
  	spinlock_t *ptl;
  
  	address = vma_address(page, vma);
  	if (address == -EFAULT)		/* out of vma range */
  		return 0;
  	pte = page_check_address(page, vma->vm_mm, address, &ptl, 1);
  	if (!pte)			/* the page is not in this mm */
  		return 0;
  	pte_unmap_unlock(pte, ptl);
  
  	return 1;
  }

  /*

   * Subfunctions of page_referenced: page_referenced_one called
   * repeatedly from either page_referenced_anon or page_referenced_file.
   */

  int page_referenced_one(struct page *page, struct vm_area_struct *vma,
  			unsigned long address, unsigned int *mapcount,
  			unsigned long *vm_flags)

  {
  	struct mm_struct *mm = vma->vm_mm;

  	int referenced = 0;

  	if (unlikely(PageTransHuge(page))) {
  		pmd_t *pmd;
  
  		spin_lock(&mm->page_table_lock);

  		/*
  		 * rmap might return false positives; we must filter
  		 * these out using page_check_address_pmd().
  		 */

  		pmd = page_check_address_pmd(page, mm, address,
  					     PAGE_CHECK_ADDRESS_PMD_FLAG);

  		if (!pmd) {
  			spin_unlock(&mm->page_table_lock);
  			goto out;
  		}
  
  		if (vma->vm_flags & VM_LOCKED) {
  			spin_unlock(&mm->page_table_lock);
  			*mapcount = 0;	/* break early from loop */
  			*vm_flags |= VM_LOCKED;
  			goto out;
  		}
  
  		/* go ahead even if the pmd is pmd_trans_splitting() */
  		if (pmdp_clear_flush_young_notify(vma, address, pmd))

  			referenced++;
  		spin_unlock(&mm->page_table_lock);
  	} else {
  		pte_t *pte;
  		spinlock_t *ptl;

  		/*
  		 * rmap might return false positives; we must filter
  		 * these out using page_check_address().
  		 */

  		pte = page_check_address(page, mm, address, &ptl, 0);
  		if (!pte)
  			goto out;

  		if (vma->vm_flags & VM_LOCKED) {
  			pte_unmap_unlock(pte, ptl);
  			*mapcount = 0;	/* break early from loop */
  			*vm_flags |= VM_LOCKED;
  			goto out;
  		}

  		if (ptep_clear_flush_young_notify(vma, address, pte)) {
  			/*
  			 * Don't treat a reference through a sequentially read
  			 * mapping as such.  If the page has been used in
  			 * another mapping, we will catch it; if this other
  			 * mapping is already gone, the unmap path will have
  			 * set PG_referenced or activated the page.
  			 */
  			if (likely(!VM_SequentialReadHint(vma)))
  				referenced++;
  		}
  		pte_unmap_unlock(pte, ptl);
  	}

  	(*mapcount)--;

  	if (referenced)
  		*vm_flags |= vma->vm_flags;

  out:

  	return referenced;
  }

  static int page_referenced_anon(struct page *page,

  				struct mem_cgroup *memcg,

  				unsigned long *vm_flags)

  {
  	unsigned int mapcount;
  	struct anon_vma *anon_vma;

  	struct anon_vma_chain *avc;

  	int referenced = 0;
  
  	anon_vma = page_lock_anon_vma(page);
  	if (!anon_vma)
  		return referenced;
  
  	mapcount = page_mapcount(page);

  	list_for_each_entry(avc, &anon_vma->head, same_anon_vma) {
  		struct vm_area_struct *vma = avc->vma;

  		unsigned long address = vma_address(page, vma);
  		if (address == -EFAULT)
  			continue;

  		/*
  		 * If we are reclaiming on behalf of a cgroup, skip
  		 * counting on behalf of references from different
  		 * cgroups
  		 */

  		if (memcg && !mm_match_cgroup(vma->vm_mm, memcg))

  			continue;

  		referenced += page_referenced_one(page, vma, address,

  						  &mapcount, vm_flags);

  		if (!mapcount)
  			break;
  	}

  
  	page_unlock_anon_vma(anon_vma);

  	return referenced;
  }
  
  /**
   * page_referenced_file - referenced check for object-based rmap
   * @page: the page we're checking references on.

   * @memcg: target memory control group

   * @vm_flags: collect encountered vma->vm_flags who actually referenced the page

   *
   * For an object-based mapped page, find all the places it is mapped and
   * check/clear the referenced flag.  This is done by following the page->mapping
   * pointer, then walking the chain of vmas it holds.  It returns the number
   * of references it found.
   *
   * This function is only called from page_referenced for object-based pages.
   */

  static int page_referenced_file(struct page *page,

  				struct mem_cgroup *memcg,

  				unsigned long *vm_flags)

  {
  	unsigned int mapcount;
  	struct address_space *mapping = page->mapping;
  	pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
  	struct vm_area_struct *vma;
  	struct prio_tree_iter iter;
  	int referenced = 0;
  
  	/*
  	 * The caller's checks on page->mapping and !PageAnon have made
  	 * sure that this is a file page: the check for page->mapping
  	 * excludes the case just before it gets set on an anon page.
  	 */
  	BUG_ON(PageAnon(page));
  
  	/*
  	 * The page lock not only makes sure that page->mapping cannot
  	 * suddenly be NULLified by truncation, it makes sure that the
  	 * structure at mapping cannot be freed and reused yet,

  	 * so we can safely take mapping->i_mmap_mutex.

  	 */
  	BUG_ON(!PageLocked(page));

  	mutex_lock(&mapping->i_mmap_mutex);

  
  	/*

  	 * i_mmap_mutex does not stabilize mapcount at all, but mapcount

  	 * is more likely to be accurate if we note it after spinning.
  	 */
  	mapcount = page_mapcount(page);
  
  	vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {

  		unsigned long address = vma_address(page, vma);
  		if (address == -EFAULT)
  			continue;

  		/*
  		 * If we are reclaiming on behalf of a cgroup, skip
  		 * counting on behalf of references from different
  		 * cgroups
  		 */

  		if (memcg && !mm_match_cgroup(vma->vm_mm, memcg))

  			continue;

  		referenced += page_referenced_one(page, vma, address,

  						  &mapcount, vm_flags);

  		if (!mapcount)
  			break;
  	}

  	mutex_unlock(&mapping->i_mmap_mutex);

  	return referenced;
  }
  
  /**
   * page_referenced - test if the page was referenced
   * @page: the page to test
   * @is_locked: caller holds lock on the page

   * @memcg: target memory cgroup

   * @vm_flags: collect encountered vma->vm_flags who actually referenced the page

   *
   * Quick test_and_clear_referenced for all mappings to a page,
   * returns the number of ptes which referenced the page.
   */

  int page_referenced(struct page *page,
  		    int is_locked,

  		    struct mem_cgroup *memcg,

  		    unsigned long *vm_flags)

  {
  	int referenced = 0;

  	int we_locked = 0;

  	*vm_flags = 0;

  	if (page_mapped(page) && page_rmapping(page)) {

  		if (!is_locked && (!PageAnon(page) || PageKsm(page))) {
  			we_locked = trylock_page(page);
  			if (!we_locked) {
  				referenced++;
  				goto out;
  			}
  		}
  		if (unlikely(PageKsm(page)))

  			referenced += page_referenced_ksm(page, memcg,

  								vm_flags);
  		else if (PageAnon(page))

  			referenced += page_referenced_anon(page, memcg,

  								vm_flags);

  		else if (page->mapping)

  			referenced += page_referenced_file(page, memcg,

  								vm_flags);

  		if (we_locked)

  			unlock_page(page);

  
  		if (page_test_and_clear_young(page_to_pfn(page)))
  			referenced++;

  	}

  out:

  	return referenced;
  }

  static int page_mkclean_one(struct page *page, struct vm_area_struct *vma,
  			    unsigned long address)

  {
  	struct mm_struct *mm = vma->vm_mm;

  	pte_t *pte;

  	spinlock_t *ptl;
  	int ret = 0;

  	pte = page_check_address(page, mm, address, &ptl, 1);

  	if (!pte)
  		goto out;

  	if (pte_dirty(*pte) || pte_write(*pte)) {
  		pte_t entry;

  		flush_cache_page(vma, address, pte_pfn(*pte));

  		entry = ptep_clear_flush_notify(vma, address, pte);

  		entry = pte_wrprotect(entry);
  		entry = pte_mkclean(entry);

  		set_pte_at(mm, address, pte, entry);

  		ret = 1;
  	}

  	pte_unmap_unlock(pte, ptl);
  out:
  	return ret;
  }
  
  static int page_mkclean_file(struct address_space *mapping, struct page *page)
  {
  	pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
  	struct vm_area_struct *vma;
  	struct prio_tree_iter iter;
  	int ret = 0;
  
  	BUG_ON(PageAnon(page));

  	mutex_lock(&mapping->i_mmap_mutex);

  	vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {

  		if (vma->vm_flags & VM_SHARED) {
  			unsigned long address = vma_address(page, vma);
  			if (address == -EFAULT)
  				continue;
  			ret += page_mkclean_one(page, vma, address);
  		}

  	}

  	mutex_unlock(&mapping->i_mmap_mutex);

  	return ret;
  }
  
  int page_mkclean(struct page *page)
  {
  	int ret = 0;
  
  	BUG_ON(!PageLocked(page));
  
  	if (page_mapped(page)) {
  		struct address_space *mapping = page_mapping(page);

  		if (mapping) {

  			ret = page_mkclean_file(mapping, page);

  			if (page_test_and_clear_dirty(page_to_pfn(page), 1))

  				ret = 1;

  		}

  	}
  
  	return ret;
  }

  EXPORT_SYMBOL_GPL(page_mkclean);

  /**

   * page_move_anon_rmap - move a page to our anon_vma
   * @page:	the page to move to our anon_vma
   * @vma:	the vma the page belongs to
   * @address:	the user virtual address mapped
   *
   * When a page belongs exclusively to one process after a COW event,
   * that page can be moved into the anon_vma that belongs to just that
   * process, so the rmap code will not search the parent or sibling
   * processes.
   */
  void page_move_anon_rmap(struct page *page,
  	struct vm_area_struct *vma, unsigned long address)
  {
  	struct anon_vma *anon_vma = vma->anon_vma;
  
  	VM_BUG_ON(!PageLocked(page));
  	VM_BUG_ON(!anon_vma);
  	VM_BUG_ON(page->index != linear_page_index(vma, address));
  
  	anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
  	page->mapping = (struct address_space *) anon_vma;
  }
  
  /**

   * __page_set_anon_rmap - set up new anonymous rmap
   * @page:	Page to add to rmap	
   * @vma:	VM area to add page to.
   * @address:	User virtual address of the mapping	

   * @exclusive:	the page is exclusively owned by the current process

   */
  static void __page_set_anon_rmap(struct page *page,

  	struct vm_area_struct *vma, unsigned long address, int exclusive)

  {

  	struct anon_vma *anon_vma = vma->anon_vma;

  	BUG_ON(!anon_vma);

  	if (PageAnon(page))
  		return;

  	/*

  	 * If the page isn't exclusively mapped into this vma,
  	 * we must use the _oldest_ possible anon_vma for the
  	 * page mapping!

  	 */

  	if (!exclusive)

  		anon_vma = anon_vma->root;

  	anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
  	page->mapping = (struct address_space *) anon_vma;

  	page->index = linear_page_index(vma, address);

  }
  
  /**

   * __page_check_anon_rmap - sanity check anonymous rmap addition

   * @page:	the page to add the mapping to
   * @vma:	the vm area in which the mapping is added
   * @address:	the user virtual address mapped
   */
  static void __page_check_anon_rmap(struct page *page,
  	struct vm_area_struct *vma, unsigned long address)
  {
  #ifdef CONFIG_DEBUG_VM
  	/*
  	 * The page's anon-rmap details (mapping and index) are guaranteed to
  	 * be set up correctly at this point.
  	 *
  	 * We have exclusion against page_add_anon_rmap because the caller
  	 * always holds the page locked, except if called from page_dup_rmap,
  	 * in which case the page is already known to be setup.
  	 *
  	 * We have exclusion against page_add_new_anon_rmap because those pages
  	 * are initially only visible via the pagetables, and the pte is locked
  	 * over the call to page_add_new_anon_rmap.
  	 */

  	BUG_ON(page_anon_vma(page)->root != vma->anon_vma->root);

  	BUG_ON(page->index != linear_page_index(vma, address));
  #endif
  }
  
  /**

   * page_add_anon_rmap - add pte mapping to an anonymous page
   * @page:	the page to add the mapping to
   * @vma:	the vm area in which the mapping is added
   * @address:	the user virtual address mapped
   *

   * The caller needs to hold the pte lock, and the page must be locked in

   * the anon_vma case: to serialize mapping,index checking after setting,
   * and to ensure that PageAnon is not being upgraded racily to PageKsm
   * (but PageKsm is never downgraded to PageAnon).

   */
  void page_add_anon_rmap(struct page *page,
  	struct vm_area_struct *vma, unsigned long address)
  {

  	do_page_add_anon_rmap(page, vma, address, 0);
  }
  
  /*
   * Special version of the above for do_swap_page, which often runs
   * into pages that are exclusively owned by the current process.
   * Everybody else should continue to use page_add_anon_rmap above.
   */
  void do_page_add_anon_rmap(struct page *page,
  	struct vm_area_struct *vma, unsigned long address, int exclusive)
  {

  	int first = atomic_inc_and_test(&page->_mapcount);

  	if (first) {
  		if (!PageTransHuge(page))
  			__inc_zone_page_state(page, NR_ANON_PAGES);
  		else
  			__inc_zone_page_state(page,
  					      NR_ANON_TRANSPARENT_HUGEPAGES);
  	}

  	if (unlikely(PageKsm(page)))
  		return;

  	VM_BUG_ON(!PageLocked(page));

  	/* address might be in next vma when migration races vma_adjust */

  	if (first)

  		__page_set_anon_rmap(page, vma, address, exclusive);

  	else

  		__page_check_anon_rmap(page, vma, address);

  }

  /**

   * page_add_new_anon_rmap - add pte mapping to a new anonymous page
   * @page:	the page to add the mapping to
   * @vma:	the vm area in which the mapping is added
   * @address:	the user virtual address mapped
   *
   * Same as page_add_anon_rmap but must only be called on *new* pages.
   * This means the inc-and-test can be bypassed.

   * Page does not have to be locked.

   */
  void page_add_new_anon_rmap(struct page *page,
  	struct vm_area_struct *vma, unsigned long address)
  {

  	VM_BUG_ON(address < vma->vm_start || address >= vma->vm_end);

  	SetPageSwapBacked(page);
  	atomic_set(&page->_mapcount, 0); /* increment count (starts at -1) */

  	if (!PageTransHuge(page))
  		__inc_zone_page_state(page, NR_ANON_PAGES);
  	else
  		__inc_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);

  	__page_set_anon_rmap(page, vma, address, 1);

  	if (page_evictable(page, vma))

  		lru_cache_add_lru(page, LRU_ACTIVE_ANON);

  	else
  		add_page_to_unevictable_list(page);

  }

  /**
   * page_add_file_rmap - add pte mapping to a file page
   * @page: the page to add the mapping to
   *

   * The caller needs to hold the pte lock.

   */
  void page_add_file_rmap(struct page *page)
  {

  	bool locked;
  	unsigned long flags;
  
  	mem_cgroup_begin_update_page_stat(page, &locked, &flags);

  	if (atomic_inc_and_test(&page->_mapcount)) {

  		__inc_zone_page_state(page, NR_FILE_MAPPED);

  		mem_cgroup_inc_page_stat(page, MEMCG_NR_FILE_MAPPED);

  	}

  	mem_cgroup_end_update_page_stat(page, &locked, &flags);

  }
  
  /**
   * page_remove_rmap - take down pte mapping from a page
   * @page: page to remove mapping from
   *

   * The caller needs to hold the pte lock.

   */

  void page_remove_rmap(struct page *page)

  {

  	bool anon = PageAnon(page);
  	bool locked;
  	unsigned long flags;
  
  	/*
  	 * The anon case has no mem_cgroup page_stat to update; but may
  	 * uncharge_page() below, where the lock ordering can deadlock if
  	 * we hold the lock against page_stat move: so avoid it on anon.
  	 */
  	if (!anon)
  		mem_cgroup_begin_update_page_stat(page, &locked, &flags);

  	/* page still mapped by someone else? */
  	if (!atomic_add_negative(-1, &page->_mapcount))

  		goto out;

  
  	/*
  	 * Now that the last pte has gone, s390 must transfer dirty
  	 * flag from storage key to struct page.  We can usually skip
  	 * this if the page is anon, so about to be freed; but perhaps
  	 * not if it's in swapcache - there might be another pte slot
  	 * containing the swap entry, but page not yet written to swap.
  	 */

  	if ((!anon || PageSwapCache(page)) &&

  	    page_test_and_clear_dirty(page_to_pfn(page), 1))

  		set_page_dirty(page);

  	/*
  	 * Hugepages are not counted in NR_ANON_PAGES nor NR_FILE_MAPPED
  	 * and not charged by memcg for now.
  	 */
  	if (unlikely(PageHuge(page)))

  		goto out;
  	if (anon) {

  		mem_cgroup_uncharge_page(page);

  		if (!PageTransHuge(page))
  			__dec_zone_page_state(page, NR_ANON_PAGES);
  		else
  			__dec_zone_page_state(page,
  					      NR_ANON_TRANSPARENT_HUGEPAGES);

  	} else {
  		__dec_zone_page_state(page, NR_FILE_MAPPED);

  		mem_cgroup_dec_page_stat(page, MEMCG_NR_FILE_MAPPED);

  	}

  	/*
  	 * It would be tidy to reset the PageAnon mapping here,
  	 * but that might overwrite a racing page_add_anon_rmap
  	 * which increments mapcount after us but sets mapping
  	 * before us: so leave the reset to free_hot_cold_page,
  	 * and remember that it's only reliable while mapped.
  	 * Leaving it set also helps swapoff to reinstate ptes
  	 * faster for those pages still in swapcache.
  	 */

  out:
  	if (!anon)
  		mem_cgroup_end_update_page_stat(page, &locked, &flags);

  }
  
  /*
   * Subfunctions of try_to_unmap: try_to_unmap_one called

   * repeatedly from try_to_unmap_ksm, try_to_unmap_anon or try_to_unmap_file.

   */

  int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
  		     unsigned long address, enum ttu_flags flags)

  {
  	struct mm_struct *mm = vma->vm_mm;

  	pte_t *pte;
  	pte_t pteval;

  	spinlock_t *ptl;

  	int ret = SWAP_AGAIN;

  	pte = page_check_address(page, mm, address, &ptl, 0);

  	if (!pte)

  		goto out;

  
  	/*
  	 * If the page is mlock()d, we cannot swap it out.
  	 * If it's recently referenced (perhaps page_referenced
  	 * skipped over this mm) then we should reactivate it.
  	 */

  	if (!(flags & TTU_IGNORE_MLOCK)) {

  		if (vma->vm_flags & VM_LOCKED)
  			goto out_mlock;

  		if (TTU_ACTION(flags) == TTU_MUNLOCK)

  			goto out_unmap;

  	}
  	if (!(flags & TTU_IGNORE_ACCESS)) {

  		if (ptep_clear_flush_young_notify(vma, address, pte)) {
  			ret = SWAP_FAIL;
  			goto out_unmap;
  		}
    	}

  	/* Nuke the page table entry. */
  	flush_cache_page(vma, address, page_to_pfn(page));

  	pteval = ptep_clear_flush_notify(vma, address, pte);

  
  	/* Move the dirty bit to the physical page now the pte is gone. */
  	if (pte_dirty(pteval))
  		set_page_dirty(page);

  	/* Update high watermark before we lower rss */
  	update_hiwater_rss(mm);

  	if (PageHWPoison(page) && !(flags & TTU_IGNORE_HWPOISON)) {
  		if (PageAnon(page))

  			dec_mm_counter(mm, MM_ANONPAGES);

  		else

  			dec_mm_counter(mm, MM_FILEPAGES);

  		set_pte_at(mm, address, pte,
  				swp_entry_to_pte(make_hwpoison_entry(page)));
  	} else if (PageAnon(page)) {

  		swp_entry_t entry = { .val = page_private(page) };

  
  		if (PageSwapCache(page)) {
  			/*
  			 * Store the swap location in the pte.
  			 * See handle_pte_fault() ...
  			 */

  			if (swap_duplicate(entry) < 0) {
  				set_pte_at(mm, address, pte, pteval);
  				ret = SWAP_FAIL;
  				goto out_unmap;
  			}

  			if (list_empty(&mm->mmlist)) {
  				spin_lock(&mmlist_lock);
  				if (list_empty(&mm->mmlist))
  					list_add(&mm->mmlist, &init_mm.mmlist);
  				spin_unlock(&mmlist_lock);
  			}

  			dec_mm_counter(mm, MM_ANONPAGES);

  			inc_mm_counter(mm, MM_SWAPENTS);

  		} else if (IS_ENABLED(CONFIG_MIGRATION)) {

  			/*
  			 * Store the pfn of the page in a special migration
  			 * pte. do_swap_page() will wait until the migration
  			 * pte is removed and then restart fault handling.
  			 */

  			BUG_ON(TTU_ACTION(flags) != TTU_MIGRATION);

  			entry = make_migration_entry(page, pte_write(pteval));

  		}
  		set_pte_at(mm, address, pte, swp_entry_to_pte(entry));
  		BUG_ON(pte_file(*pte));

  	} else if (IS_ENABLED(CONFIG_MIGRATION) &&
  		   (TTU_ACTION(flags) == TTU_MIGRATION)) {

  		/* Establish migration entry for a file page */
  		swp_entry_t entry;
  		entry = make_migration_entry(page, pte_write(pteval));
  		set_pte_at(mm, address, pte, swp_entry_to_pte(entry));
  	} else

  		dec_mm_counter(mm, MM_FILEPAGES);

  	page_remove_rmap(page);

  	page_cache_release(page);
  
  out_unmap:

  	pte_unmap_unlock(pte, ptl);

  out:
  	return ret;

  out_mlock:
  	pte_unmap_unlock(pte, ptl);
  
  
  	/*
  	 * We need mmap_sem locking, Otherwise VM_LOCKED check makes
  	 * unstable result and race. Plus, We can't wait here because

  	 * we now hold anon_vma->mutex or mapping->i_mmap_mutex.

  	 * if trylock failed, the page remain in evictable lru and later
  	 * vmscan could retry to move the page to unevictable lru if the
  	 * page is actually mlocked.
  	 */
  	if (down_read_trylock(&vma->vm_mm->mmap_sem)) {
  		if (vma->vm_flags & VM_LOCKED) {
  			mlock_vma_page(page);
  			ret = SWAP_MLOCK;

  		}

  		up_read(&vma->vm_mm->mmap_sem);

  	}

  	return ret;
  }
  
  /*
   * objrmap doesn't work for nonlinear VMAs because the assumption that
   * offset-into-file correlates with offset-into-virtual-addresses does not hold.
   * Consequently, given a particular page and its ->index, we cannot locate the
   * ptes which are mapping that page without an exhaustive linear search.
   *
   * So what this code does is a mini "virtual scan" of each nonlinear VMA which
   * maps the file to which the target page belongs.  The ->vm_private_data field
   * holds the current cursor into that scan.  Successive searches will circulate
   * around the vma's virtual address space.
   *
   * So as more replacement pressure is applied to the pages in a nonlinear VMA,
   * more scanning pressure is placed against them as well.   Eventually pages
   * will become fully unmapped and are eligible for eviction.
   *
   * For very sparsely populated VMAs this is a little inefficient - chances are
   * there there won't be many ptes located within the scan cluster.  In this case
   * maybe we could scan further - to the end of the pte page, perhaps.

   *
   * Mlocked pages:  check VM_LOCKED under mmap_sem held for read, if we can
   * acquire it without blocking.  If vma locked, mlock the pages in the cluster,
   * rather than unmapping them.  If we encounter the "check_page" that vmscan is
   * trying to unmap, return SWAP_MLOCK, else default SWAP_AGAIN.

   */
  #define CLUSTER_SIZE	min(32*PAGE_SIZE, PMD_SIZE)
  #define CLUSTER_MASK	(~(CLUSTER_SIZE - 1))

  static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
  		struct vm_area_struct *vma, struct page *check_page)

  {
  	struct mm_struct *mm = vma->vm_mm;
  	pgd_t *pgd;
  	pud_t *pud;
  	pmd_t *pmd;

  	pte_t *pte;

  	pte_t pteval;

  	spinlock_t *ptl;

  	struct page *page;
  	unsigned long address;
  	unsigned long end;

  	int ret = SWAP_AGAIN;
  	int locked_vma = 0;

  	address = (vma->vm_start + cursor) & CLUSTER_MASK;
  	end = address + CLUSTER_SIZE;
  	if (address < vma->vm_start)
  		address = vma->vm_start;
  	if (end > vma->vm_end)
  		end = vma->vm_end;
  
  	pgd = pgd_offset(mm, address);
  	if (!pgd_present(*pgd))

  		return ret;

  
  	pud = pud_offset(pgd, address);
  	if (!pud_present(*pud))

  		return ret;

  
  	pmd = pmd_offset(pud, address);
  	if (!pmd_present(*pmd))

  		return ret;
  
  	/*

  	 * If we can acquire the mmap_sem for read, and vma is VM_LOCKED,

  	 * keep the sem while scanning the cluster for mlocking pages.
  	 */

  	if (down_read_trylock(&vma->vm_mm->mmap_sem)) {

  		locked_vma = (vma->vm_flags & VM_LOCKED);
  		if (!locked_vma)
  			up_read(&vma->vm_mm->mmap_sem); /* don't need it */
  	}

  
  	pte = pte_offset_map_lock(mm, pmd, address, &ptl);

  	/* Update high watermark before we lower rss */
  	update_hiwater_rss(mm);

  	for (; address < end; pte++, address += PAGE_SIZE) {

  		if (!pte_present(*pte))
  			continue;

  		page = vm_normal_page(vma, address, *pte);
  		BUG_ON(!page || PageAnon(page));

  		if (locked_vma) {
  			mlock_vma_page(page);   /* no-op if already mlocked */
  			if (page == check_page)
  				ret = SWAP_MLOCK;
  			continue;	/* don't unmap */
  		}

  		if (ptep_clear_flush_young_notify(vma, address, pte))

  			continue;
  
  		/* Nuke the page table entry. */

  		flush_cache_page(vma, address, pte_pfn(*pte));

  		pteval = ptep_clear_flush_notify(vma, address, pte);

  
  		/* If nonlinear, store the file page offset in the pte. */
  		if (page->index != linear_page_index(vma, address))
  			set_pte_at(mm, address, pte, pgoff_to_pte(page->index));
  
  		/* Move the dirty bit to the physical page now the pte is gone. */
  		if (pte_dirty(pteval))
  			set_page_dirty(page);

  		page_remove_rmap(page);

  		page_cache_release(page);

  		dec_mm_counter(mm, MM_FILEPAGES);

  		(*mapcount)--;
  	}

  	pte_unmap_unlock(pte - 1, ptl);

  	if (locked_vma)
  		up_read(&vma->vm_mm->mmap_sem);
  	return ret;

  }

  bool is_vma_temporary_stack(struct vm_area_struct *vma)

  {
  	int maybe_stack = vma->vm_flags & (VM_GROWSDOWN | VM_GROWSUP);
  
  	if (!maybe_stack)
  		return false;
  
  	if ((vma->vm_flags & VM_STACK_INCOMPLETE_SETUP) ==
  						VM_STACK_INCOMPLETE_SETUP)
  		return true;
  
  	return false;
  }

  /**
   * try_to_unmap_anon - unmap or unlock anonymous page using the object-based
   * rmap method
   * @page: the page to unmap/unlock

   * @flags: action and flags

   *
   * Find all the mappings of a page using the mapping pointer and the vma chains
   * contained in the anon_vma struct it points to.
   *
   * This function is only called from try_to_unmap/try_to_munlock for
   * anonymous pages.
   * When called from try_to_munlock(), the mmap_sem of the mm containing the vma
   * where the page was found will be held for write.  So, we won't recheck
   * vm_flags for that VMA.  That should be OK, because that vma shouldn't be
   * 'LOCKED.
   */

  static int try_to_unmap_anon(struct page *page, enum ttu_flags flags)

  {
  	struct anon_vma *anon_vma;

  	struct anon_vma_chain *avc;

  	int ret = SWAP_AGAIN;

  	anon_vma = page_lock_anon_vma(page);
  	if (!anon_vma)
  		return ret;

  	list_for_each_entry(avc, &anon_vma->head, same_anon_vma) {
  		struct vm_area_struct *vma = avc->vma;

  		unsigned long address;
  
  		/*
  		 * During exec, a temporary VMA is setup and later moved.
  		 * The VMA is moved under the anon_vma lock but not the
  		 * page tables leading to a race where migration cannot
  		 * find the migration ptes. Rather than increasing the
  		 * locking requirements of exec(), migration skips
  		 * temporary VMAs until after exec() completes.
  		 */

  		if (IS_ENABLED(CONFIG_MIGRATION) && (flags & TTU_MIGRATION) &&

  				is_vma_temporary_stack(vma))
  			continue;
  
  		address = vma_address(page, vma);

  		if (address == -EFAULT)
  			continue;
  		ret = try_to_unmap_one(page, vma, address, flags);

  		if (ret != SWAP_AGAIN || !page_mapped(page))
  			break;

  	}

  
  	page_unlock_anon_vma(anon_vma);

  	return ret;
  }
  
  /**

   * try_to_unmap_file - unmap/unlock file page using the object-based rmap method
   * @page: the page to unmap/unlock

   * @flags: action and flags

   *
   * Find all the mappings of a page using the mapping pointer and the vma chains
   * contained in the address_space struct it points to.
   *

   * This function is only called from try_to_unmap/try_to_munlock for
   * object-based pages.
   * When called from try_to_munlock(), the mmap_sem of the mm containing the vma
   * where the page was found will be held for write.  So, we won't recheck
   * vm_flags for that VMA.  That should be OK, because that vma shouldn't be
   * 'LOCKED.

   */

  static int try_to_unmap_file(struct page *page, enum ttu_flags flags)

  {
  	struct address_space *mapping = page->mapping;
  	pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
  	struct vm_area_struct *vma;
  	struct prio_tree_iter iter;
  	int ret = SWAP_AGAIN;
  	unsigned long cursor;
  	unsigned long max_nl_cursor = 0;
  	unsigned long max_nl_size = 0;
  	unsigned int mapcount;

  	mutex_lock(&mapping->i_mmap_mutex);

  	vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {

  		unsigned long address = vma_address(page, vma);
  		if (address == -EFAULT)
  			continue;
  		ret = try_to_unmap_one(page, vma, address, flags);

  		if (ret != SWAP_AGAIN || !page_mapped(page))
  			goto out;

  	}
  
  	if (list_empty(&mapping->i_mmap_nonlinear))
  		goto out;

  	/*
  	 * We don't bother to try to find the munlocked page in nonlinears.
  	 * It's costly. Instead, later, page reclaim logic may call
  	 * try_to_unmap(TTU_MUNLOCK) and recover PG_mlocked lazily.
  	 */
  	if (TTU_ACTION(flags) == TTU_MUNLOCK)
  		goto out;

  	list_for_each_entry(vma, &mapping->i_mmap_nonlinear,
  						shared.vm_set.list) {

  		cursor = (unsigned long) vma->vm_private_data;
  		if (cursor > max_nl_cursor)
  			max_nl_cursor = cursor;
  		cursor = vma->vm_end - vma->vm_start;
  		if (cursor > max_nl_size)
  			max_nl_size = cursor;
  	}

  	if (max_nl_size == 0) {	/* all nonlinears locked or reserved ? */

  		ret = SWAP_FAIL;
  		goto out;
  	}
  
  	/*
  	 * We don't try to search for this page in the nonlinear vmas,
  	 * and page_referenced wouldn't have found it anyway.  Instead
  	 * just walk the nonlinear vmas trying to age and unmap some.
  	 * The mapcount of the page we came in with is irrelevant,
  	 * but even so use it as a guide to how hard we should try?
  	 */
  	mapcount = page_mapcount(page);
  	if (!mapcount)
  		goto out;

  	cond_resched();

  
  	max_nl_size = (max_nl_size + CLUSTER_SIZE - 1) & CLUSTER_MASK;
  	if (max_nl_cursor == 0)
  		max_nl_cursor = CLUSTER_SIZE;
  
  	do {
  		list_for_each_entry(vma, &mapping->i_mmap_nonlinear,
  						shared.vm_set.list) {

  			cursor = (unsigned long) vma->vm_private_data;

  			while ( cursor < max_nl_cursor &&

  				cursor < vma->vm_end - vma->vm_start) {

  				if (try_to_unmap_cluster(cursor, &mapcount,
  						vma, page) == SWAP_MLOCK)
  					ret = SWAP_MLOCK;

  				cursor += CLUSTER_SIZE;
  				vma->vm_private_data = (void *) cursor;
  				if ((int)mapcount <= 0)
  					goto out;
  			}
  			vma->vm_private_data = (void *) max_nl_cursor;
  		}

  		cond_resched();

  		max_nl_cursor += CLUSTER_SIZE;
  	} while (max_nl_cursor <= max_nl_size);
  
  	/*
  	 * Don't loop forever (perhaps all the remaining pages are
  	 * in locked vmas).  Reset cursor on all unreserved nonlinear
  	 * vmas, now forgetting on which ones it had fallen behind.
  	 */

  	list_for_each_entry(vma, &mapping->i_mmap_nonlinear, shared.vm_set.list)
  		vma->vm_private_data = NULL;

  out:

  	mutex_unlock(&mapping->i_mmap_mutex);

  	return ret;
  }
  
  /**
   * try_to_unmap - try to remove all page table mappings to a page
   * @page: the page to get unmapped

   * @flags: action and flags

   *
   * Tries to remove all the page table entries which are mapping this
   * page, used in the pageout path.  Caller must hold the page lock.
   * Return values are:
   *
   * SWAP_SUCCESS	- we succeeded in removing all mappings
   * SWAP_AGAIN	- we missed a mapping, try again later
   * SWAP_FAIL	- the page is unswappable

   * SWAP_MLOCK	- page is mlocked.

   */

  int try_to_unmap(struct page *page, enum ttu_flags flags)

  {
  	int ret;

  	BUG_ON(!PageLocked(page));

  	VM_BUG_ON(!PageHuge(page) && PageTransHuge(page));

  	if (unlikely(PageKsm(page)))
  		ret = try_to_unmap_ksm(page, flags);
  	else if (PageAnon(page))

  		ret = try_to_unmap_anon(page, flags);

  	else

  		ret = try_to_unmap_file(page, flags);

  	if (ret != SWAP_MLOCK && !page_mapped(page))

  		ret = SWAP_SUCCESS;
  	return ret;
  }

  /**
   * try_to_munlock - try to munlock a page
   * @page: the page to be munlocked
   *
   * Called from munlock code.  Checks all of the VMAs mapping the page
   * to make sure nobody else has this page mlocked. The page will be
   * returned with PG_mlocked cleared if no other vmas have it mlocked.
   *
   * Return values are:
   *

   * SWAP_AGAIN	- no vma is holding page mlocked, or,

   * SWAP_AGAIN	- page mapped in mlocked vma -- couldn't acquire mmap sem

   * SWAP_FAIL	- page cannot be located at present

   * SWAP_MLOCK	- page is now mlocked.
   */
  int try_to_munlock(struct page *page)
  {
  	VM_BUG_ON(!PageLocked(page) || PageLRU(page));

  	if (unlikely(PageKsm(page)))
  		return try_to_unmap_ksm(page, TTU_MUNLOCK);
  	else if (PageAnon(page))

  		return try_to_unmap_anon(page, TTU_MUNLOCK);

  	else

  		return try_to_unmap_file(page, TTU_MUNLOCK);

  }

  void __put_anon_vma(struct anon_vma *anon_vma)

  {

  	struct anon_vma *root = anon_vma->root;

  	if (root != anon_vma && atomic_dec_and_test(&root->refcount))
  		anon_vma_free(root);

  	anon_vma_free(anon_vma);

  }

  #ifdef CONFIG_MIGRATION
  /*
   * rmap_walk() and its helpers rmap_walk_anon() and rmap_walk_file():
   * Called by migrate.c to remove migration ptes, but might be used more later.
   */
  static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *,
  		struct vm_area_struct *, unsigned long, void *), void *arg)
  {
  	struct anon_vma *anon_vma;

  	struct anon_vma_chain *avc;

  	int ret = SWAP_AGAIN;
  
  	/*
  	 * Note: remove_migration_ptes() cannot use page_lock_anon_vma()
  	 * because that depends on page_mapped(); but not all its usages

  	 * are holding mmap_sem. Users without mmap_sem are required to
  	 * take a reference count to prevent the anon_vma disappearing

  	 */
  	anon_vma = page_anon_vma(page);
  	if (!anon_vma)
  		return ret;

  	anon_vma_lock(anon_vma);

  	list_for_each_entry(avc, &anon_vma->head, same_anon_vma) {
  		struct vm_area_struct *vma = avc->vma;

  		unsigned long address = vma_address(page, vma);
  		if (address == -EFAULT)
  			continue;
  		ret = rmap_one(page, vma, address, arg);
  		if (ret != SWAP_AGAIN)
  			break;
  	}

  	anon_vma_unlock(anon_vma);

  	return ret;
  }
  
  static int rmap_walk_file(struct page *page, int (*rmap_one)(struct page *,
  		struct vm_area_struct *, unsigned long, void *), void *arg)
  {
  	struct address_space *mapping = page->mapping;
  	pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
  	struct vm_area_struct *vma;
  	struct prio_tree_iter iter;
  	int ret = SWAP_AGAIN;
  
  	if (!mapping)
  		return ret;

  	mutex_lock(&mapping->i_mmap_mutex);

  	vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {
  		unsigned long address = vma_address(page, vma);
  		if (address == -EFAULT)
  			continue;
  		ret = rmap_one(page, vma, address, arg);
  		if (ret != SWAP_AGAIN)
  			break;
  	}
  	/*
  	 * No nonlinear handling: being always shared, nonlinear vmas
  	 * never contain migration ptes.  Decide what to do about this
  	 * limitation to linear when we need rmap_walk() on nonlinear.
  	 */

  	mutex_unlock(&mapping->i_mmap_mutex);

  	return ret;
  }
  
  int rmap_walk(struct page *page, int (*rmap_one)(struct page *,
  		struct vm_area_struct *, unsigned long, void *), void *arg)
  {
  	VM_BUG_ON(!PageLocked(page));
  
  	if (unlikely(PageKsm(page)))
  		return rmap_walk_ksm(page, rmap_one, arg);
  	else if (PageAnon(page))
  		return rmap_walk_anon(page, rmap_one, arg);
  	else
  		return rmap_walk_file(page, rmap_one, arg);
  }
  #endif /* CONFIG_MIGRATION */

  #ifdef CONFIG_HUGETLB_PAGE

  /*
   * The following three functions are for anonymous (private mapped) hugepages.
   * Unlike common anonymous pages, anonymous hugepages have no accounting code
   * and no lru code, because we handle hugepages differently from common pages.
   */
  static void __hugepage_set_anon_rmap(struct page *page,
  	struct vm_area_struct *vma, unsigned long address, int exclusive)
  {
  	struct anon_vma *anon_vma = vma->anon_vma;

  	BUG_ON(!anon_vma);

  
  	if (PageAnon(page))
  		return;
  	if (!exclusive)
  		anon_vma = anon_vma->root;

  	anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
  	page->mapping = (struct address_space *) anon_vma;
  	page->index = linear_page_index(vma, address);
  }
  
  void hugepage_add_anon_rmap(struct page *page,
  			    struct vm_area_struct *vma, unsigned long address)
  {
  	struct anon_vma *anon_vma = vma->anon_vma;
  	int first;

  
  	BUG_ON(!PageLocked(page));

  	BUG_ON(!anon_vma);

  	/* address might be in next vma when migration races vma_adjust */

  	first = atomic_inc_and_test(&page->_mapcount);
  	if (first)
  		__hugepage_set_anon_rmap(page, vma, address, 0);
  }
  
  void hugepage_add_new_anon_rmap(struct page *page,
  			struct vm_area_struct *vma, unsigned long address)
  {
  	BUG_ON(address < vma->vm_start || address >= vma->vm_end);
  	atomic_set(&page->_mapcount, 0);
  	__hugepage_set_anon_rmap(page, vma, address, 1);
  }

  #endif /* CONFIG_HUGETLB_PAGE */