FreeBSD/Linux Kernel Cross Reference
sys/mm/rmap.c

     /*
      * 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->rwsem
     *           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->rwsem,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 <linux/backing-dev.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_read() 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
             * down_read_trylock() from page_lock_anon_vma_read(). This orders:
             *
             * page_lock_anon_vma_read()    VS      put_anon_vma()
             *   down_read_trylock()                  atomic_dec_and_test()
            *   LOCK                                 MB
            *   atomic_read()                        rwsem_is_locked()
            *
            * LOCK should suffice since the actual taking of the lock must
            * happen _before_ what follows.
            */
           if (rwsem_is_locked(&anon_vma->root->rwsem)) {
                   anon_vma_lock_write(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);
           anon_vma_interval_tree_insert(avc, &anon_vma->rb_root);
   }
   
   /**
    * 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_read()
    * 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_write(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))
                           up_write(&root->rwsem);
                   root = new_root;
                   down_write(&root->rwsem);
           }
           return root;
   }
   
   static inline void unlock_anon_vma_root(struct anon_vma *root)
   {
           if (root)
                   up_write(&root->rwsem);
   }
   
   /*
    * 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;
   }
   
   /*
    * 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_write(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);
                   anon_vma_interval_tree_remove(avc, &anon_vma->rb_root);
   
                   /*
                    * Leave empty anon_vmas on the list - we'll need
                    * to free them outside the lock.
                    */
                   if (RB_EMPTY_ROOT(&anon_vma->rb_root))
                           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 write-acquire the anon_vma->root->rwsem.
            */
           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;
   
           init_rwsem(&anon_vma->rwsem);
           atomic_set(&anon_vma->refcount, 0);
           anon_vma->rb_root = RB_ROOT;
   }
   
   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_read(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 (down_read_trylock(&root_anon_vma->rwsem)) {
                   /*
                    * 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)) {
                           up_read(&root_anon_vma->rwsem);
                           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_read(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_write() recursion.
                    */
                   anon_vma_unlock_read(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_read(struct anon_vma *anon_vma)
   {
           anon_vma_unlock_read(anon_vma);
   }
   
   /*
    * At what user virtual address is page expected in @vma?
    */
   static inline unsigned long
   __vma_address(struct page *page, struct vm_area_struct *vma)
   {
           pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
   
           if (unlikely(is_vm_hugetlb_page(vma)))
                   pgoff = page->index << huge_page_order(page_hstate(page));
   
           return vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
   }
   
   inline unsigned long
   vma_address(struct page *page, struct vm_area_struct *vma)
   {
           unsigned long address = __vma_address(page, vma);
   
           /* page should be within @vma mapping range */
           VM_BUG_ON(address < vma->vm_start || address >= vma->vm_end);
   
           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)
   {
           unsigned long address;
           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;
           address = __vma_address(page, vma);
           if (unlikely(address < vma->vm_start || address >= vma->vm_end))
                   return -EFAULT;
           return address;
   }
   
   pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address)
   {
           pgd_t *pgd;
           pud_t *pud;
           pmd_t *pmd = NULL;
   
           pgd = pgd_offset(mm, address);
           if (!pgd_present(*pgd))
                   goto out;
   
           pud = pud_offset(pgd, address);
           if (!pud_present(*pud))
                   goto out;
   
           pmd = pmd_offset(pud, address);
           if (!pmd_present(*pmd))
                   pmd = NULL;
   out:
           return pmd;
   }
   
   /*
    * 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)
   {
           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;
           }
   
           pmd = mm_find_pmd(mm, address);
           if (!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 (unlikely(address < vma->vm_start || address >= vma->vm_end))
                   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;
           pgoff_t pgoff;
           struct anon_vma_chain *avc;
           int referenced = 0;
   
           anon_vma = page_lock_anon_vma_read(page);
           if (!anon_vma)
                   return referenced;
   
           mapcount = page_mapcount(page);
           pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
           anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root, pgoff, pgoff) {
                   struct vm_area_struct *vma = avc->vma;
                   unsigned long address = vma_address(page, vma);
                   /*
                    * 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_read(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;
           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_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
                   unsigned long address = vma_address(page, vma);
                   /*
                    * 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(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);
   
           if (ret)
                   mmu_notifier_invalidate_page(mm, address);
   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;
           int ret = 0;
   
           BUG_ON(PageAnon(page));
   
           mutex_lock(&mapping->i_mmap_mutex);
           vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
                   if (vma->vm_flags & VM_SHARED) {
                           unsigned long address = vma_address(page, vma);
                           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);
           }
   
           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 (!mlocked_vma_newpage(vma, page))
                  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)
  {
          struct address_space *mapping = page_mapping(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.
           *
           * And we can skip it on file pages, so long as the filesystem
           * participates in dirty tracking (note that this is not only an
           * optimization but also solves problems caused by dirty flag in
           * storage key getting set by a write from inside kernel); but need to
           * catch shm and tmpfs and ramfs pages which have been modified since
           * creation by read fault.
           *
           * Note that mapping must be decided above, before decrementing
           * mapcount (which luckily provides a barrier): once page is unmapped,
           * it could be truncated and page->mapping reset to NULL at any moment.
           * Note also that we are relying on page_mapping(page) to set mapping
           * to &swapper_space when PageSwapCache(page).
           */
          if (mapping && !mapping_cap_account_dirty(mapping) &&
              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);
                  mem_cgroup_end_update_page_stat(page, &locked, &flags);
          }
          if (unlikely(PageMlocked(page)))
                  clear_page_mlock(page);
          /*
           * 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.
           */
          return;
  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(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 (!PageHuge(page)) {
                          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);
          if (ret != SWAP_FAIL)
                  mmu_notifier_invalidate_page(mm, address);
  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->rwsem 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;
          pmd_t *pmd;
          pte_t *pte;
          pte_t pteval;
          spinlock_t *ptl;
          struct page *page;
          unsigned long address;
          unsigned long mmun_start;       /* For mmu_notifiers */
          unsigned long mmun_end;         /* For mmu_notifiers */
          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;
  
          pmd = mm_find_pmd(mm, address);
          if (!pmd)
                  return ret;
  
          mmun_start = address;
          mmun_end   = end;
          mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
  
          /*
           * 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(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);
          mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
          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;
          pgoff_t pgoff;
          struct anon_vma_chain *avc;
          int ret = SWAP_AGAIN;
  
          anon_vma = page_lock_anon_vma_read(page);
          if (!anon_vma)
                  return ret;
  
          pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
          anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root, pgoff, pgoff) {
                  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);
                  ret = try_to_unmap_one(page, vma, address, flags);
                  if (ret != SWAP_AGAIN || !page_mapped(page))
                          break;
          }
  
          page_unlock_anon_vma_read(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;
          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_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
                  unsigned long address = vma_address(page, vma);
                  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.nonlinear) {
                  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.nonlinear) {
                          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.nonlinear)
                  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;
          pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
          struct anon_vma_chain *avc;
          int ret = SWAP_AGAIN;
  
          /*
           * Note: remove_migration_ptes() cannot use page_lock_anon_vma_read()
           * 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_read(anon_vma);
          anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root, pgoff, pgoff) {
                  struct vm_area_struct *vma = avc->vma;
                  unsigned long address = vma_address(page, vma);
                  ret = rmap_one(page, vma, address, arg);
                  if (ret != SWAP_AGAIN)
                          break;
          }
          anon_vma_unlock_read(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;
          int ret = SWAP_AGAIN;
  
          if (!mapping)
                  return ret;
          mutex_lock(&mapping->i_mmap_mutex);
          vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
                  unsigned long address = vma_address(page, vma);
                  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 */

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