Merge tag 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/rdma/rdma

[thirdparty/kernel/stable.git] / 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)
 *       hugetlbfs_i_mmap_rwsem_key (in huge_pmd_share)
 *         mapping->i_mmap_rwsem
 *           anon_vma->rwsem
 *             mm->page_table_lock or pte_lock
 *               pgdat->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)
 *                   mem_cgroup_{begin,end}_page_stat (memcg->move_lock)
 *                     i_pages lock (widely used)
 *                 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)
 *                   i_pages 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/sched/mm.h>
#include <linux/sched/task.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 <linux/page_idle.h>
#include <linux/memremap.h>
#include <linux/userfaultfd_k.h>

#include <asm/tlbflush.h>

#include <trace/events/tlb.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);
                anon_vma->degree = 1;   /* Reference for first vma */
                anon_vma->parent = anon_vma;
                /*
                 * 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.
         */
        might_sleep();
        if (rwsem_is_locked(&anon_vma->root->rwsem)) {
                anon_vma_lock_write(anon_vma);
                anon_vma_unlock_write(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, which
 * is handled inline by anon_vma_prepare(). 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 mm_struct *mm = vma->vm_mm;
        struct anon_vma *anon_vma, *allocated;
        struct anon_vma_chain *avc;

        might_sleep();

        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);
                /* vma reference or self-parent link for new root */
                anon_vma->degree++;
                allocated = NULL;
                avc = NULL;
        }
        spin_unlock(&mm->page_table_lock);
        anon_vma_unlock_write(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.
 *
 * If dst->anon_vma is NULL this function tries to find and reuse existing
 * anon_vma which has no vmas and only one child anon_vma. This prevents
 * degradation of anon_vma hierarchy to endless linear chain in case of
 * constantly forking task. On the other hand, an anon_vma with more than one
 * child isn't reused even if there was no alive vma, thus rmap walker has a
 * good chance of avoiding scanning the whole hierarchy when it searches where
 * page is mapped.
 */
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);

                /*
                 * Reuse existing anon_vma if its degree lower than two,
                 * that means it has no vma and only one anon_vma child.
                 *
                 * Do not chose parent anon_vma, otherwise first child
                 * will always reuse it. Root anon_vma is never reused:
                 * it has self-parent reference and at least one child.
                 */
                if (!dst->anon_vma && anon_vma != src->anon_vma &&
                                anon_vma->degree < 2)
                        dst->anon_vma = anon_vma;
        }
        if (dst->anon_vma)
                dst->anon_vma->degree++;
        unlock_anon_vma_root(root);
        return 0;

 enomem_failure:
        /*
         * dst->anon_vma is dropped here otherwise its degree can be incorrectly
         * decremented in unlink_anon_vmas().
         * We can safely do this because callers of anon_vma_clone() don't care
         * about dst->anon_vma if anon_vma_clone() failed.
         */
        dst->anon_vma = NULL;
        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;
        int error;

        /* Don't bother if the parent process has no anon_vma here. */
        if (!pvma->anon_vma)
                return 0;

        /* Drop inherited anon_vma, we'll reuse existing or allocate new. */
        vma->anon_vma = NULL;

        /*
         * First, attach the new VMA to the parent VMA's anon_vmas,
         * so rmap can find non-COWed pages in child processes.
         */
        error = anon_vma_clone(vma, pvma);
        if (error)
                return error;

        /* An existing anon_vma has been reused, all done then. */
        if (vma->anon_vma)
                return 0;

        /* 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;
        anon_vma->parent = pvma->anon_vma;
        /*
         * 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->parent->degree++;
        anon_vma_unlock_write(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.rb_root)) {
                        anon_vma->parent->degree--;
                        continue;
                }

                list_del(&avc->same_vma);
                anon_vma_chain_free(avc);
        }
        if (vma->anon_vma)
                vma->anon_vma->degree--;
        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;

                VM_WARN_ON(anon_vma->degree);
                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_CACHED;
}

void __init anon_vma_init(void)
{
        anon_vma_cachep = kmem_cache_create("anon_vma", sizeof(struct anon_vma),
                        0, SLAB_TYPESAFE_BY_RCU|SLAB_PANIC|SLAB_ACCOUNT,
                        anon_vma_ctor);
        anon_vma_chain_cachep = KMEM_CACHE(anon_vma_chain,
                        SLAB_PANIC|SLAB_ACCOUNT);
}

/*
 * 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)READ_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_TYPESAFE_BY_RCU guarantees that - so the atomic_inc_not_zero()
         * above cannot corrupt).
         */
        if (!page_mapped(page)) {
                rcu_read_unlock();
                put_anon_vma(anon_vma);
                return 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)READ_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 = READ_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)) {
                rcu_read_unlock();
                put_anon_vma(anon_vma);
                return NULL;
        }

        /* 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);
}

#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
/*
 * Flush TLB entries for recently unmapped pages from remote CPUs. It is
 * important if a PTE was dirty when it was unmapped that it's flushed
 * before any IO is initiated on the page to prevent lost writes. Similarly,
 * it must be flushed before freeing to prevent data leakage.
 */
void try_to_unmap_flush(void)
{
        struct tlbflush_unmap_batch *tlb_ubc = &current->tlb_ubc;

        if (!tlb_ubc->flush_required)
                return;

        arch_tlbbatch_flush(&tlb_ubc->arch);
        tlb_ubc->flush_required = false;
        tlb_ubc->writable = false;
}

/* Flush iff there are potentially writable TLB entries that can race with IO */
void try_to_unmap_flush_dirty(void)
{
        struct tlbflush_unmap_batch *tlb_ubc = &current->tlb_ubc;

        if (tlb_ubc->writable)
                try_to_unmap_flush();
}

static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable)
{
        struct tlbflush_unmap_batch *tlb_ubc = &current->tlb_ubc;

        arch_tlbbatch_add_mm(&tlb_ubc->arch, mm);
        tlb_ubc->flush_required = true;

        /*
         * Ensure compiler does not re-order the setting of tlb_flush_batched
         * before the PTE is cleared.
         */
        barrier();
        mm->tlb_flush_batched = true;

        /*
         * If the PTE was dirty then it's best to assume it's writable. The
         * caller must use try_to_unmap_flush_dirty() or try_to_unmap_flush()
         * before the page is queued for IO.
         */
        if (writable)
                tlb_ubc->writable = true;
}

/*
 * Returns true if the TLB flush should be deferred to the end of a batch of
 * unmap operations to reduce IPIs.
 */
static bool should_defer_flush(struct mm_struct *mm, enum ttu_flags flags)
{
        bool should_defer = false;

        if (!(flags & TTU_BATCH_FLUSH))
                return false;

        /* If remote CPUs need to be flushed then defer batch the flush */
        if (cpumask_any_but(mm_cpumask(mm), get_cpu()) < nr_cpu_ids)
                should_defer = true;
        put_cpu();

        return should_defer;
}

/*
 * Reclaim unmaps pages under the PTL but do not flush the TLB prior to
 * releasing the PTL if TLB flushes are batched. It's possible for a parallel
 * operation such as mprotect or munmap to race between reclaim unmapping
 * the page and flushing the page. If this race occurs, it potentially allows
 * access to data via a stale TLB entry. Tracking all mm's that have TLB
 * batching in flight would be expensive during reclaim so instead track
 * whether TLB batching occurred in the past and if so then do a flush here
 * if required. This will cost one additional flush per reclaim cycle paid
 * by the first operation at risk such as mprotect and mumap.
 *
 * This must be called under the PTL so that an access to tlb_flush_batched
 * that is potentially a "reclaim vs mprotect/munmap/etc" race will synchronise
 * via the PTL.
 */
void flush_tlb_batched_pending(struct mm_struct *mm)
{
        if (mm->tlb_flush_batched) {
                flush_tlb_mm(mm);

                /*
                 * Do not allow the compiler to re-order the clearing of
                 * tlb_flush_batched before the tlb is flushed.
                 */
                barrier();
                mm->tlb_flush_batched = false;
        }
}
#else
static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable)
{
}

static bool should_defer_flush(struct mm_struct *mm, enum ttu_flags flags)
{
        return false;
}
#endif /* CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH */

/*
 * 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) {
                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;
        p4d_t *p4d;
        pud_t *pud;
        pmd_t *pmd = NULL;
        pmd_t pmde;

        pgd = pgd_offset(mm, address);
        if (!pgd_present(*pgd))
                goto out;

        p4d = p4d_offset(pgd, address);
        if (!p4d_present(*p4d))
                goto out;

        pud = pud_offset(p4d, address);
        if (!pud_present(*pud))
                goto out;

        pmd = pmd_offset(pud, address);
        /*
         * Some THP functions use the sequence pmdp_huge_clear_flush(), set_pmd_at()
         * without holding anon_vma lock for write.  So when looking for a
         * genuine pmde (in which to find pte), test present and !THP together.
         */
        pmde = *pmd;
        barrier();
        if (!pmd_present(pmde) || pmd_trans_huge(pmde))
                pmd = NULL;
out:
        return pmd;
}

struct page_referenced_arg {
        int mapcount;
        int referenced;
        unsigned long vm_flags;
        struct mem_cgroup *memcg;
};
/*
 * arg: page_referenced_arg will be passed
 */
static bool page_referenced_one(struct page *page, struct vm_area_struct *vma,
                        unsigned long address, void *arg)
{
        struct page_referenced_arg *pra = arg;
        struct page_vma_mapped_walk pvmw = {
                .page = page,
                .vma = vma,
                .address = address,
        };
        int referenced = 0;

        while (page_vma_mapped_walk(&pvmw)) {
                address = pvmw.address;

                if (vma->vm_flags & VM_LOCKED) {
                        page_vma_mapped_walk_done(&pvmw);
                        pra->vm_flags |= VM_LOCKED;
                        return false; /* To break the loop */
                }

                if (pvmw.pte) {
                        if (ptep_clear_flush_young_notify(vma, address,
                                                pvmw.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(!(vma->vm_flags & VM_SEQ_READ)))
                                        referenced++;
                        }
                } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
                        if (pmdp_clear_flush_young_notify(vma, address,
                                                pvmw.pmd))
                                referenced++;
                } else {
                        /* unexpected pmd-mapped page? */
                        WARN_ON_ONCE(1);
                }

                pra->mapcount--;
        }

        if (referenced)
                clear_page_idle(page);
        if (test_and_clear_page_young(page))
                referenced++;

        if (referenced) {
                pra->referenced++;
                pra->vm_flags |= vma->vm_flags;
        }

        if (!pra->mapcount)
                return false; /* To break the loop */

        return true;
}

static bool invalid_page_referenced_vma(struct vm_area_struct *vma, void *arg)
{
        struct page_referenced_arg *pra = arg;
        struct mem_cgroup *memcg = pra->memcg;

        if (!mm_match_cgroup(vma->vm_mm, memcg))
                return true;

        return false;
}

/**
 * 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 we_locked = 0;
        struct page_referenced_arg pra = {
                .mapcount = total_mapcount(page),
                .memcg = memcg,
        };
        struct rmap_walk_control rwc = {
                .rmap_one = page_referenced_one,
                .arg = (void *)&pra,
                .anon_lock = page_lock_anon_vma_read,
        };

        *vm_flags = 0;
        if (!page_mapped(page))
                return 0;

        if (!page_rmapping(page))
                return 0;

        if (!is_locked && (!PageAnon(page) || PageKsm(page))) {
                we_locked = trylock_page(page);
                if (!we_locked)
                        return 1;
        }

        /*
         * If we are reclaiming on behalf of a cgroup, skip
         * counting on behalf of references from different
         * cgroups
         */
        if (memcg) {
                rwc.invalid_vma = invalid_page_referenced_vma;
        }

        rmap_walk(page, &rwc);
        *vm_flags = pra.vm_flags;

        if (we_locked)
                unlock_page(page);

        return pra.referenced;
}

static bool page_mkclean_one(struct page *page, struct vm_area_struct *vma,
                            unsigned long address, void *arg)
{
        struct page_vma_mapped_walk pvmw = {
                .page = page,
                .vma = vma,
                .address = address,
                .flags = PVMW_SYNC,
        };
        struct mmu_notifier_range range;
        int *cleaned = arg;

        /*
         * We have to assume the worse case ie pmd for invalidation. Note that
         * the page can not be free from this function.
         */
        mmu_notifier_range_init(&range, vma->vm_mm, address,
                                min(vma->vm_end, address +
                                    (PAGE_SIZE << compound_order(page))));
        mmu_notifier_invalidate_range_start(&range);

        while (page_vma_mapped_walk(&pvmw)) {
                unsigned long cstart;
                int ret = 0;

                cstart = address = pvmw.address;
                if (pvmw.pte) {
                        pte_t entry;
                        pte_t *pte = pvmw.pte;

                        if (!pte_dirty(*pte) && !pte_write(*pte))
                                continue;

                        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(vma->vm_mm, address, pte, entry);
                        ret = 1;
                } else {
#ifdef CONFIG_TRANSPARENT_HUGE_PAGECACHE
                        pmd_t *pmd = pvmw.pmd;
                        pmd_t entry;

                        if (!pmd_dirty(*pmd) && !pmd_write(*pmd))
                                continue;

                        flush_cache_page(vma, address, page_to_pfn(page));
                        entry = pmdp_huge_clear_flush(vma, address, pmd);
                        entry = pmd_wrprotect(entry);
                        entry = pmd_mkclean(entry);
                        set_pmd_at(vma->vm_mm, address, pmd, entry);
                        cstart &= PMD_MASK;
                        ret = 1;
#else
                        /* unexpected pmd-mapped page? */
                        WARN_ON_ONCE(1);
#endif
                }

                /*
                 * No need to call mmu_notifier_invalidate_range() as we are
                 * downgrading page table protection not changing it to point
                 * to a new page.
                 *
                 * See Documentation/vm/mmu_notifier.rst
                 */
                if (ret)
                        (*cleaned)++;
        }

        mmu_notifier_invalidate_range_end(&range);

        return true;
}

static bool invalid_mkclean_vma(struct vm_area_struct *vma, void *arg)
{
        if (vma->vm_flags & VM_SHARED)
                return false;

        return true;
}

int page_mkclean(struct page *page)
{
        int cleaned = 0;
        struct address_space *mapping;
        struct rmap_walk_control rwc = {
                .arg = (void *)&cleaned,
                .rmap_one = page_mkclean_one,
                .invalid_vma = invalid_mkclean_vma,
        };

        BUG_ON(!PageLocked(page));

        if (!page_mapped(page))
                return 0;

        mapping = page_mapping(page);
        if (!mapping)
                return 0;

        rmap_walk(page, &rwc);

        return cleaned;
}
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
 *
 * 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)
{
        struct anon_vma *anon_vma = vma->anon_vma;

        page = compound_head(page);

        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_VMA(!anon_vma, vma);

        anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
        /*
         * Ensure that anon_vma and the PAGE_MAPPING_ANON bit are written
         * simultaneously, so a concurrent reader (eg page_referenced()'s
         * PageAnon()) will not see one without the other.
         */
        WRITE_ONCE(page->mapping, (struct address_space *) anon_vma);
}

/**
 * __page_set_anon_rmap - set up new anonymous rmap
 * @page:       Page or Hugepage 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_to_pgoff(page) != 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
 * @compound:   charge the page as compound or small page
 *
 * 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, bool compound)
{
        do_page_add_anon_rmap(page, vma, address, compound ? RMAP_COMPOUND : 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 flags)
{
        bool compound = flags & RMAP_COMPOUND;
        bool first;

        if (compound) {
                atomic_t *mapcount;
                VM_BUG_ON_PAGE(!PageLocked(page), page);
                VM_BUG_ON_PAGE(!PageTransHuge(page), page);
                mapcount = compound_mapcount_ptr(page);
                first = atomic_inc_and_test(mapcount);
        } else {
                first = atomic_inc_and_test(&page->_mapcount);
        }

        if (first) {
                int nr = compound ? hpage_nr_pages(page) : 1;
                /*
                 * We use the irq-unsafe __{inc|mod}_zone_page_stat because
                 * these counters are not modified in interrupt context, and
                 * pte lock(a spinlock) is held, which implies preemption
                 * disabled.
                 */
                if (compound)
                        __inc_node_page_state(page, NR_ANON_THPS);
                __mod_node_page_state(page_pgdat(page), NR_ANON_MAPPED, nr);
        }
        if (unlikely(PageKsm(page)))
                return;

        VM_BUG_ON_PAGE(!PageLocked(page), page);

        /* address might be in next vma when migration races vma_adjust */
        if (first)
                __page_set_anon_rmap(page, vma, address,
                                flags & RMAP_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
 * @compound:   charge the page as compound or small page
 *
 * 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, bool compound)
{
        int nr = compound ? hpage_nr_pages(page) : 1;

        VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
        __SetPageSwapBacked(page);
        if (compound) {
                VM_BUG_ON_PAGE(!PageTransHuge(page), page);
                /* increment count (starts at -1) */
                atomic_set(compound_mapcount_ptr(page), 0);
                __inc_node_page_state(page, NR_ANON_THPS);
        } else {
                /* Anon THP always mapped first with PMD */
                VM_BUG_ON_PAGE(PageTransCompound(page), page);
                /* increment count (starts at -1) */
                atomic_set(&page->_mapcount, 0);
        }
        __mod_node_page_state(page_pgdat(page), NR_ANON_MAPPED, nr);
        __page_set_anon_rmap(page, vma, address, 1);
}

/**
 * page_add_file_rmap - add pte mapping to a file page
 * @page: the page to add the mapping to
 * @compound: charge the page as compound or small page
 *
 * The caller needs to hold the pte lock.
 */
void page_add_file_rmap(struct page *page, bool compound)
{
        int i, nr = 1;

        VM_BUG_ON_PAGE(compound && !PageTransHuge(page), page);
        lock_page_memcg(page);
        if (compound && PageTransHuge(page)) {
                for (i = 0, nr = 0; i < HPAGE_PMD_NR; i++) {
                        if (atomic_inc_and_test(&page[i]._mapcount))
                                nr++;
                }
                if (!atomic_inc_and_test(compound_mapcount_ptr(page)))
                        goto out;
                VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
                __inc_node_page_state(page, NR_SHMEM_PMDMAPPED);
        } else {
                if (PageTransCompound(page) && page_mapping(page)) {
                        VM_WARN_ON_ONCE(!PageLocked(page));

                        SetPageDoubleMap(compound_head(page));
                        if (PageMlocked(page))
                                clear_page_mlock(compound_head(page));
                }
                if (!atomic_inc_and_test(&page->_mapcount))
                        goto out;
        }
        __mod_lruvec_page_state(page, NR_FILE_MAPPED, nr);
out:
        unlock_page_memcg(page);
}

static void page_remove_file_rmap(struct page *page, bool compound)
{
        int i, nr = 1;

        VM_BUG_ON_PAGE(compound && !PageHead(page), page);
        lock_page_memcg(page);

        /* Hugepages are not counted in NR_FILE_MAPPED for now. */
        if (unlikely(PageHuge(page))) {
                /* hugetlb pages are always mapped with pmds */
                atomic_dec(compound_mapcount_ptr(page));
                goto out;
        }

        /* page still mapped by someone else? */
        if (compound && PageTransHuge(page)) {
                for (i = 0, nr = 0; i < HPAGE_PMD_NR; i++) {
                        if (atomic_add_negative(-1, &page[i]._mapcount))
                                nr++;
                }
                if (!atomic_add_negative(-1, compound_mapcount_ptr(page)))
                        goto out;
                VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
                __dec_node_page_state(page, NR_SHMEM_PMDMAPPED);
        } else {
                if (!atomic_add_negative(-1, &page->_mapcount))
                        goto out;
        }

        /*
         * We use the irq-unsafe __{inc|mod}_lruvec_page_state because
         * these counters are not modified in interrupt context, and
         * pte lock(a spinlock) is held, which implies preemption disabled.
         */
        __mod_lruvec_page_state(page, NR_FILE_MAPPED, -nr);

        if (unlikely(PageMlocked(page)))
                clear_page_mlock(page);
out:
        unlock_page_memcg(page);
}

static void page_remove_anon_compound_rmap(struct page *page)
{
        int i, nr;

        if (!atomic_add_negative(-1, compound_mapcount_ptr(page)))
                return;

        /* Hugepages are not counted in NR_ANON_PAGES for now. */
        if (unlikely(PageHuge(page)))
                return;

        if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
                return;

        __dec_node_page_state(page, NR_ANON_THPS);

        if (TestClearPageDoubleMap(page)) {
                /*
                 * Subpages can be mapped with PTEs too. Check how many of
                 * themi are still mapped.
                 */
                for (i = 0, nr = 0; i < HPAGE_PMD_NR; i++) {
                        if (atomic_add_negative(-1, &page[i]._mapcount))
                                nr++;
                }
        } else {
                nr = HPAGE_PMD_NR;
        }

        if (unlikely(PageMlocked(page)))
                clear_page_mlock(page);

        if (nr) {
                __mod_node_page_state(page_pgdat(page), NR_ANON_MAPPED, -nr);
                deferred_split_huge_page(page);
        }
}

/**
 * page_remove_rmap - take down pte mapping from a page
 * @page:       page to remove mapping from
 * @compound:   uncharge the page as compound or small page
 *
 * The caller needs to hold the pte lock.
 */
void page_remove_rmap(struct page *page, bool compound)
{
        if (!PageAnon(page))
                return page_remove_file_rmap(page, compound);

        if (compound)
                return page_remove_anon_compound_rmap(page);

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

        /*
         * We use the irq-unsafe __{inc|mod}_zone_page_stat because
         * these counters are not modified in interrupt context, and
         * pte lock(a spinlock) is held, which implies preemption disabled.
         */
        __dec_node_page_state(page, NR_ANON_MAPPED);

        if (unlikely(PageMlocked(page)))
                clear_page_mlock(page);

        if (PageTransCompound(page))
                deferred_split_huge_page(compound_head(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_unref_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.
         */
}

/*
 * @arg: enum ttu_flags will be passed to this argument
 */
static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
                     unsigned long address, void *arg)
{
        struct mm_struct *mm = vma->vm_mm;
        struct page_vma_mapped_walk pvmw = {
                .page = page,
                .vma = vma,
                .address = address,
        };
        pte_t pteval;
        struct page *subpage;
        bool ret = true;
        struct mmu_notifier_range range;
        enum ttu_flags flags = (enum ttu_flags)arg;

        /* munlock has nothing to gain from examining un-locked vmas */
        if ((flags & TTU_MUNLOCK) && !(vma->vm_flags & VM_LOCKED))
                return true;

        if (IS_ENABLED(CONFIG_MIGRATION) && (flags & TTU_MIGRATION) &&
            is_zone_device_page(page) && !is_device_private_page(page))
                return true;

        if (flags & TTU_SPLIT_HUGE_PMD) {
                split_huge_pmd_address(vma, address,
                                flags & TTU_SPLIT_FREEZE, page);
        }

        /*
         * For THP, we have to assume the worse case ie pmd for invalidation.
         * For hugetlb, it could be much worse if we need to do pud
         * invalidation in the case of pmd sharing.
         *
         * Note that the page can not be free in this function as call of
         * try_to_unmap() must hold a reference on the page.
         */
        mmu_notifier_range_init(&range, vma->vm_mm, address,
                                min(vma->vm_end, address +
                                    (PAGE_SIZE << compound_order(page))));
        if (PageHuge(page)) {
                /*
                 * If sharing is possible, start and end will be adjusted
                 * accordingly.
                 */
                adjust_range_if_pmd_sharing_possible(vma, &range.start,
                                                     &range.end);
        }
        mmu_notifier_invalidate_range_start(&range);

        while (page_vma_mapped_walk(&pvmw)) {
#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
                /* PMD-mapped THP migration entry */
                if (!pvmw.pte && (flags & TTU_MIGRATION)) {
                        VM_BUG_ON_PAGE(PageHuge(page) || !PageTransCompound(page), page);

                        set_pmd_migration_entry(&pvmw, page);
                        continue;
                }
#endif

                /*
                 * 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) {
                                /* PTE-mapped THP are never mlocked */
                                if (!PageTransCompound(page)) {
                                        /*
                                         * Holding pte lock, we do *not* need
                                         * mmap_sem here
                                         */
                                        mlock_vma_page(page);
                                }
                                ret = false;
                                page_vma_mapped_walk_done(&pvmw);
                                break;
                        }
                        if (flags & TTU_MUNLOCK)
                                continue;
                }

                /* Unexpected PMD-mapped THP? */
                VM_BUG_ON_PAGE(!pvmw.pte, page);

                subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte);
                address = pvmw.address;

                if (PageHuge(page)) {
                        if (huge_pmd_unshare(mm, &address, pvmw.pte)) {
                                /*
                                 * huge_pmd_unshare unmapped an entire PMD
                                 * page.  There is no way of knowing exactly
                                 * which PMDs may be cached for this mm, so
                                 * we must flush them all.  start/end were
                                 * already adjusted above to cover this range.
                                 */
                                flush_cache_range(vma, range.start, range.end);
                                flush_tlb_range(vma, range.start, range.end);
                                mmu_notifier_invalidate_range(mm, range.start,
                                                              range.end);

                                /*
                                 * The ref count of the PMD page was dropped
                                 * which is part of the way map counting
                                 * is done for shared PMDs.  Return 'true'
                                 * here.  When there is no other sharing,
                                 * huge_pmd_unshare returns false and we will
                                 * unmap the actual page and drop map count
                                 * to zero.
                                 */
                                page_vma_mapped_walk_done(&pvmw);
                                break;
                        }
                }

                if (IS_ENABLED(CONFIG_MIGRATION) &&
                    (flags & TTU_MIGRATION) &&
                    is_zone_device_page(page)) {
                        swp_entry_t entry;
                        pte_t swp_pte;

                        pteval = ptep_get_and_clear(mm, pvmw.address, pvmw.pte);

                        /*
                         * 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.
                         */
                        entry = make_migration_entry(page, 0);
                        swp_pte = swp_entry_to_pte(entry);
                        if (pte_soft_dirty(pteval))
                                swp_pte = pte_swp_mksoft_dirty(swp_pte);
                        set_pte_at(mm, pvmw.address, pvmw.pte, swp_pte);
                        /*
                         * No need to invalidate here it will synchronize on
                         * against the special swap migration pte.
                         */
                        goto discard;
                }

                if (!(flags & TTU_IGNORE_ACCESS)) {
                        if (ptep_clear_flush_young_notify(vma, address,
                                                pvmw.pte)) {
                                ret = false;
                                page_vma_mapped_walk_done(&pvmw);
                                break;
                        }
                }

                /* Nuke the page table entry. */
                flush_cache_page(vma, address, pte_pfn(*pvmw.pte));
                if (should_defer_flush(mm, flags)) {
                        /*
                         * We clear the PTE but do not flush so potentially
                         * a remote CPU could still be writing to the page.
                         * If the entry was previously clean then the
                         * architecture must guarantee that a clear->dirty
                         * transition on a cached TLB entry is written through
                         * and traps if the PTE is unmapped.
                         */
                        pteval = ptep_get_and_clear(mm, address, pvmw.pte);

                        set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
                } else {
                        pteval = ptep_clear_flush(vma, address, pvmw.pte);
                }

                /* Move the dirty bit to the 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)) {
                        pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
                        if (PageHuge(page)) {
                                int nr = 1 << compound_order(page);
                                hugetlb_count_sub(nr, mm);
                                set_huge_swap_pte_at(mm, address,
                                                     pvmw.pte, pteval,
                                                     vma_mmu_pagesize(vma));
                        } else {
                                dec_mm_counter(mm, mm_counter(page));
                                set_pte_at(mm, address, pvmw.pte, pteval);
                        }

                } else if (pte_unused(pteval) && !userfaultfd_armed(vma)) {
                        /*
                         * The guest indicated that the page content is of no
                         * interest anymore. Simply discard the pte, vmscan
                         * will take care of the rest.
                         * A future reference will then fault in a new zero
                         * page. When userfaultfd is active, we must not drop
                         * this page though, as its main user (postcopy
                         * migration) will not expect userfaults on already
                         * copied pages.
                         */
                        dec_mm_counter(mm, mm_counter(page));
                        /* We have to invalidate as we cleared the pte */
                        mmu_notifier_invalidate_range(mm, address,
                                                      address + PAGE_SIZE);
                } else if (IS_ENABLED(CONFIG_MIGRATION) &&
                                (flags & (TTU_MIGRATION|TTU_SPLIT_FREEZE))) {
                        swp_entry_t entry;
                        pte_t swp_pte;

                        if (arch_unmap_one(mm, vma, address, pteval) < 0) {
                                set_pte_at(mm, address, pvmw.pte, pteval);
                                ret = false;
                                page_vma_mapped_walk_done(&pvmw);
                                break;
                        }

                        /*
                         * 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.
                         */
                        entry = make_migration_entry(subpage,
                                        pte_write(pteval));
                        swp_pte = swp_entry_to_pte(entry);
                        if (pte_soft_dirty(pteval))
                                swp_pte = pte_swp_mksoft_dirty(swp_pte);
                        set_pte_at(mm, address, pvmw.pte, swp_pte);
                        /*
                         * No need to invalidate here it will synchronize on
                         * against the special swap migration pte.
                         */
                } else if (PageAnon(page)) {
                        swp_entry_t entry = { .val = page_private(subpage) };
                        pte_t swp_pte;
                        /*
                         * Store the swap location in the pte.
                         * See handle_pte_fault() ...
                         */
                        if (unlikely(PageSwapBacked(page) != PageSwapCache(page))) {
                                WARN_ON_ONCE(1);
                                ret = false;
                                /* We have to invalidate as we cleared the pte */
                                mmu_notifier_invalidate_range(mm, address,
                                                        address + PAGE_SIZE);
                                page_vma_mapped_walk_done(&pvmw);
                                break;
                        }

                        /* MADV_FREE page check */
                        if (!PageSwapBacked(page)) {
                                if (!PageDirty(page)) {
                                        /* Invalidate as we cleared the pte */
                                        mmu_notifier_invalidate_range(mm,
                                                address, address + PAGE_SIZE);
                                        dec_mm_counter(mm, MM_ANONPAGES);
                                        goto discard;
                                }

                                /*
                                 * If the page was redirtied, it cannot be
                                 * discarded. Remap the page to page table.
                                 */
                                set_pte_at(mm, address, pvmw.pte, pteval);
                                SetPageSwapBacked(page);
                                ret = false;
                                page_vma_mapped_walk_done(&pvmw);
                                break;
                        }

                        if (swap_duplicate(entry) < 0) {
                                set_pte_at(mm, address, pvmw.pte, pteval);
                                ret = false;
                                page_vma_mapped_walk_done(&pvmw);
                                break;
                        }
                        if (arch_unmap_one(mm, vma, address, pteval) < 0) {
                                set_pte_at(mm, address, pvmw.pte, pteval);
                                ret = false;
                                page_vma_mapped_walk_done(&pvmw);
                                break;
                        }
                        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);
                        swp_pte = swp_entry_to_pte(entry);
                        if (pte_soft_dirty(pteval))
                                swp_pte = pte_swp_mksoft_dirty(swp_pte);
                        set_pte_at(mm, address, pvmw.pte, swp_pte);
                        /* Invalidate as we cleared the pte */
                        mmu_notifier_invalidate_range(mm, address,
                                                      address + PAGE_SIZE);
                } else {
                        /*
                         * This is a locked file-backed page, thus it cannot
                         * be removed from the page cache and replaced by a new
                         * page before mmu_notifier_invalidate_range_end, so no
                         * concurrent thread might update its page table to
                         * point at new page while a device still is using this
                         * page.
                         *
                         * See Documentation/vm/mmu_notifier.rst
                         */
                        dec_mm_counter(mm, mm_counter_file(page));
                }
discard:
                /*
                 * No need to call mmu_notifier_invalidate_range() it has be
                 * done above for all cases requiring it to happen under page
                 * table lock before mmu_notifier_invalidate_range_end()
                 *
                 * See Documentation/vm/mmu_notifier.rst
                 */
                page_remove_rmap(subpage, PageHuge(page));
                put_page(page);
        }

        mmu_notifier_invalidate_range_end(&range);

        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;
}

static bool invalid_migration_vma(struct vm_area_struct *vma, void *arg)
{
        return is_vma_temporary_stack(vma);
}

static int page_mapcount_is_zero(struct page *page)
{
        return !total_mapcount(page);
}

/**
 * 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.
 *
 * If unmap is successful, return true. Otherwise, false.
 */
bool try_to_unmap(struct page *page, enum ttu_flags flags)
{
        struct rmap_walk_control rwc = {
                .rmap_one = try_to_unmap_one,
                .arg = (void *)flags,
                .done = page_mapcount_is_zero,
                .anon_lock = page_lock_anon_vma_read,
        };

        /*
         * 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 ((flags & (TTU_MIGRATION|TTU_SPLIT_FREEZE))
            && !PageKsm(page) && PageAnon(page))
                rwc.invalid_vma = invalid_migration_vma;

        if (flags & TTU_RMAP_LOCKED)
                rmap_walk_locked(page, &rwc);
        else
                rmap_walk(page, &rwc);

        return !page_mapcount(page) ? true : false;
}

static int page_not_mapped(struct page *page)
{
        return !page_mapped(page);
};

/**
 * 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.
 */

void try_to_munlock(struct page *page)
{
        struct rmap_walk_control rwc = {
                .rmap_one = try_to_unmap_one,
                .arg = (void *)TTU_MUNLOCK,
                .done = page_not_mapped,
                .anon_lock = page_lock_anon_vma_read,

        };

        VM_BUG_ON_PAGE(!PageLocked(page) || PageLRU(page), page);
        VM_BUG_ON_PAGE(PageCompound(page) && PageDoubleMap(page), page);

        rmap_walk(page, &rwc);
}

void __put_anon_vma(struct anon_vma *anon_vma)
{
        struct anon_vma *root = anon_vma->root;

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

static struct anon_vma *rmap_walk_anon_lock(struct page *page,
                                        struct rmap_walk_control *rwc)
{
        struct anon_vma *anon_vma;

        if (rwc->anon_lock)
                return rwc->anon_lock(page);

        /*
         * 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 NULL;

        anon_vma_lock_read(anon_vma);
        return anon_vma;
}

/*
 * rmap_walk_anon - do something to anonymous page using the object-based
 * rmap method
 * @page: the page to be handled
 * @rwc: control variable according to each walk type
 *
 * Find all the mappings of a page using the mapping pointer and the vma chains
 * contained in the anon_vma struct it points to.
 *
 * 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 void rmap_walk_anon(struct page *page, struct rmap_walk_control *rwc,
                bool locked)
{
        struct anon_vma *anon_vma;
        pgoff_t pgoff_start, pgoff_end;
        struct anon_vma_chain *avc;

        if (locked) {
                anon_vma = page_anon_vma(page);
                /* anon_vma disappear under us? */
                VM_BUG_ON_PAGE(!anon_vma, page);
        } else {
                anon_vma = rmap_walk_anon_lock(page, rwc);
        }
        if (!anon_vma)
                return;

        pgoff_start = page_to_pgoff(page);
        pgoff_end = pgoff_start + hpage_nr_pages(page) - 1;
        anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root,
                        pgoff_start, pgoff_end) {
                struct vm_area_struct *vma = avc->vma;
                unsigned long address = vma_address(page, vma);

                cond_resched();

                if (rwc->invalid_vma && rwc->invalid_vma(vma, rwc->arg))
                        continue;

                if (!rwc->rmap_one(page, vma, address, rwc->arg))
                        break;
                if (rwc->done && rwc->done(page))
                        break;
        }

        if (!locked)
                anon_vma_unlock_read(anon_vma);
}

/*
 * rmap_walk_file - do something to file page using the object-based rmap method
 * @page: the page to be handled
 * @rwc: control variable according to each walk type
 *
 * Find all the mappings of a page using the mapping pointer and the vma chains
 * contained in the address_space struct it points to.
 *
 * 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 void rmap_walk_file(struct page *page, struct rmap_walk_control *rwc,
                bool locked)
{
        struct address_space *mapping = page_mapping(page);
        pgoff_t pgoff_start, pgoff_end;
        struct vm_area_struct *vma;

        /*
         * 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_rwsem.
         */
        VM_BUG_ON_PAGE(!PageLocked(page), page);

        if (!mapping)
                return;

        pgoff_start = page_to_pgoff(page);
        pgoff_end = pgoff_start + hpage_nr_pages(page) - 1;
        if (!locked)
                i_mmap_lock_read(mapping);
        vma_interval_tree_foreach(vma, &mapping->i_mmap,
                        pgoff_start, pgoff_end) {
                unsigned long address = vma_address(page, vma);

                cond_resched();

                if (rwc->invalid_vma && rwc->invalid_vma(vma, rwc->arg))
                        continue;

                if (!rwc->rmap_one(page, vma, address, rwc->arg))
                        goto done;
                if (rwc->done && rwc->done(page))
                        goto done;
        }

done:
        if (!locked)
                i_mmap_unlock_read(mapping);
}

void rmap_walk(struct page *page, struct rmap_walk_control *rwc)
{
        if (unlikely(PageKsm(page)))
                rmap_walk_ksm(page, rwc);
        else if (PageAnon(page))
                rmap_walk_anon(page, rwc, false);
        else
                rmap_walk_file(page, rwc, false);
}

/* Like rmap_walk, but caller holds relevant rmap lock */
void rmap_walk_locked(struct page *page, struct rmap_walk_control *rwc)
{
        /* no ksm support for now */
        VM_BUG_ON_PAGE(PageKsm(page), page);
        if (PageAnon(page))
                rmap_walk_anon(page, rwc, true);
        else
                rmap_walk_file(page, rwc, true);
}

#ifdef CONFIG_HUGETLB_PAGE
/*
 * The following two 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.
 */
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(compound_mapcount_ptr(page));
        if (first)
                __page_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(compound_mapcount_ptr(page), 0);
        __page_set_anon_rmap(page, vma, address, 1);
}
#endif /* CONFIG_HUGETLB_PAGE */