ksm.c

Go to the documentation of this file.

/*
 * Memory merging support.
 *
 * This code enables dynamic sharing of identical pages found in different
 * memory areas, even if they are not shared by fork()
 *
 * Copyright (C) 2008-2009 Red Hat, Inc.
 * Authors:
 *  Izik Eidus
 *  Andrea Arcangeli
 *  Chris Wright
 *  Hugh Dickins
 *
 * This work is licensed under the terms of the GNU GPL, version 2.
 */

#include <linux/errno.h>
#include <linux/mm.h>
#include <linux/fs.h>
#include <linux/mman.h>
#include <linux/sched.h>
#include <linux/rwsem.h>
#include <linux/pagemap.h>
#include <linux/rmap.h>
#include <linux/spinlock.h>
#include <linux/jhash.h>
#include <linux/delay.h>
#include <linux/kthread.h>
#include <linux/wait.h>
#include <linux/slab.h>
#include <linux/rbtree.h>
#include <linux/memory.h>
#include <linux/mmu_notifier.h>
#include <linux/swap.h>
#include <linux/ksm.h>
#include <linux/hash.h>
#include <linux/freezer.h>
#include <linux/oom.h>

#include <asm/tlbflush.h>
#include "internal.h"

/*
 * A few notes about the KSM scanning process,
 * to make it easier to understand the data structures below:
 *
 * In order to reduce excessive scanning, KSM sorts the memory pages by their
 * contents into a data structure that holds pointers to the pages' locations.
 *
 * Since the contents of the pages may change at any moment, KSM cannot just
 * insert the pages into a normal sorted tree and expect it to find anything.
 * Therefore KSM uses two data structures - the stable and the unstable tree.
 *
 * The stable tree holds pointers to all the merged pages (ksm pages), sorted
 * by their contents.  Because each such page is write-protected, searching on
 * this tree is fully assured to be working (except when pages are unmapped),
 * and therefore this tree is called the stable tree.
 *
 * In addition to the stable tree, KSM uses a second data structure called the
 * unstable tree: this tree holds pointers to pages which have been found to
 * be "unchanged for a period of time".  The unstable tree sorts these pages
 * by their contents, but since they are not write-protected, KSM cannot rely
 * upon the unstable tree to work correctly - the unstable tree is liable to
 * be corrupted as its contents are modified, and so it is called unstable.
 *
 * KSM solves this problem by several techniques:
 *
 * 1) The unstable tree is flushed every time KSM completes scanning all
 *    memory areas, and then the tree is rebuilt again from the beginning.
 * 2) KSM will only insert into the unstable tree, pages whose hash value
 *    has not changed since the previous scan of all memory areas.
 * 3) The unstable tree is a RedBlack Tree - so its balancing is based on the
 *    colors of the nodes and not on their contents, assuring that even when
 *    the tree gets "corrupted" it won't get out of balance, so scanning time
 *    remains the same (also, searching and inserting nodes in an rbtree uses
 *    the same algorithm, so we have no overhead when we flush and rebuild).
 * 4) KSM never flushes the stable tree, which means that even if it were to
 *    take 10 attempts to find a page in the unstable tree, once it is found,
 *    it is secured in the stable tree.  (When we scan a new page, we first
 *    compare it against the stable tree, and then against the unstable tree.)
 */

struct mm_slot {
    struct hlist_node link;
    struct list_head mm_list;
    struct rmap_item *rmap_list;
    struct mm_struct *mm;
};

struct ksm_scan {
    struct mm_slot *mm_slot;
    unsigned long address;
    struct rmap_item **rmap_list;
    unsigned long seqnr;
};

struct stable_node {
    struct rb_node node;
    struct hlist_head hlist;
    unsigned long kpfn;
};

struct rmap_item {
    struct rmap_item *rmap_list;
    struct anon_vma *anon_vma;  /* when stable */
    struct mm_struct *mm;
    unsigned long address;      /* + low bits used for flags below */
    unsigned int oldchecksum;   /* when unstable */
    union {
        struct rb_node node;    /* when node of unstable tree */
        struct {        /* when listed from stable tree */
            struct stable_node *head;
            struct hlist_node hlist;
        };
    };
};

#define SEQNR_MASK  0x0ff   /* low bits of unstable tree seqnr */
#define UNSTABLE_FLAG   0x100   /* is a node of the unstable tree */
#define STABLE_FLAG 0x200   /* is listed from the stable tree */

/* The stable and unstable tree heads */
static struct rb_root root_stable_tree = RB_ROOT;
static struct rb_root root_unstable_tree = RB_ROOT;

#define MM_SLOTS_HASH_SHIFT 10
#define MM_SLOTS_HASH_HEADS (1 << MM_SLOTS_HASH_SHIFT)
static struct hlist_head mm_slots_hash[MM_SLOTS_HASH_HEADS];

static struct mm_slot ksm_mm_head = {
    .mm_list = LIST_HEAD_INIT(ksm_mm_head.mm_list),
};
static struct ksm_scan ksm_scan = {
    .mm_slot = &ksm_mm_head,
};

static struct kmem_cache *rmap_item_cache;
static struct kmem_cache *stable_node_cache;
static struct kmem_cache *mm_slot_cache;

/* The number of nodes in the stable tree */
static unsigned long ksm_pages_shared;

/* The number of page slots additionally sharing those nodes */
static unsigned long ksm_pages_sharing;

/* The number of nodes in the unstable tree */
static unsigned long ksm_pages_unshared;

/* The number of rmap_items in use: to calculate pages_volatile */
static unsigned long ksm_rmap_items;

/* Number of pages ksmd should scan in one batch */
static unsigned int ksm_thread_pages_to_scan = 100;

/* Milliseconds ksmd should sleep between batches */
static unsigned int ksm_thread_sleep_millisecs = 20;

#define KSM_RUN_STOP    0
#define KSM_RUN_MERGE   1
#define KSM_RUN_UNMERGE 2
static unsigned int ksm_run = KSM_RUN_STOP;

static DECLARE_WAIT_QUEUE_HEAD(ksm_thread_wait);
static DEFINE_MUTEX(ksm_thread_mutex);
static DEFINE_SPINLOCK(ksm_mmlist_lock);

#define KSM_KMEM_CACHE(__struct, __flags) kmem_cache_create("ksm_"#__struct,\
        sizeof(struct __struct), __alignof__(struct __struct),\
        (__flags), NULL)

static int __init ksm_slab_init(void)
{
    rmap_item_cache = KSM_KMEM_CACHE(rmap_item, 0);
    if (!rmap_item_cache)
        goto out;

    stable_node_cache = KSM_KMEM_CACHE(stable_node, 0);
    if (!stable_node_cache)
        goto out_free1;

    mm_slot_cache = KSM_KMEM_CACHE(mm_slot, 0);
    if (!mm_slot_cache)
        goto out_free2;

    return 0;

out_free2:
    kmem_cache_destroy(stable_node_cache);
out_free1:
    kmem_cache_destroy(rmap_item_cache);
out:
    return -ENOMEM;
}

static void __init ksm_slab_free(void)
{
    kmem_cache_destroy(mm_slot_cache);
    kmem_cache_destroy(stable_node_cache);
    kmem_cache_destroy(rmap_item_cache);
    mm_slot_cache = NULL;
}

static inline struct rmap_item *alloc_rmap_item(void)
{
    struct rmap_item *rmap_item;

    rmap_item = kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL);
    if (rmap_item)
        ksm_rmap_items++;
    return rmap_item;
}

static inline void free_rmap_item(struct rmap_item *rmap_item)
{
    ksm_rmap_items--;
    rmap_item->mm = NULL;   /* debug safety */
    kmem_cache_free(rmap_item_cache, rmap_item);
}

static inline struct stable_node *alloc_stable_node(void)
{
    return kmem_cache_alloc(stable_node_cache, GFP_KERNEL);
}

static inline void free_stable_node(struct stable_node *stable_node)
{
    kmem_cache_free(stable_node_cache, stable_node);
}

static inline struct mm_slot *alloc_mm_slot(void)
{
    if (!mm_slot_cache) /* initialization failed */
        return NULL;
    return kmem_cache_zalloc(mm_slot_cache, GFP_KERNEL);
}

static inline void free_mm_slot(struct mm_slot *mm_slot)
{
    kmem_cache_free(mm_slot_cache, mm_slot);
}

static struct mm_slot *get_mm_slot(struct mm_struct *mm)
{
    struct mm_slot *mm_slot;
    struct hlist_head *bucket;
    struct hlist_node *node;

    bucket = &mm_slots_hash[hash_ptr(mm, MM_SLOTS_HASH_SHIFT)];
    hlist_for_each_entry(mm_slot, node, bucket, link) {
        if (mm == mm_slot->mm)
            return mm_slot;
    }
    return NULL;
}

static void insert_to_mm_slots_hash(struct mm_struct *mm,
                    struct mm_slot *mm_slot)
{
    struct hlist_head *bucket;

    bucket = &mm_slots_hash[hash_ptr(mm, MM_SLOTS_HASH_SHIFT)];
    mm_slot->mm = mm;
    hlist_add_head(&mm_slot->link, bucket);
}

static inline int in_stable_tree(struct rmap_item *rmap_item)
{
    return rmap_item->address & STABLE_FLAG;
}

/*
 * ksmd, and unmerge_and_remove_all_rmap_items(), must not touch an mm's
 * page tables after it has passed through ksm_exit() - which, if necessary,
 * takes mmap_sem briefly to serialize against them.  ksm_exit() does not set
 * a special flag: they can just back out as soon as mm_users goes to zero.
 * ksm_test_exit() is used throughout to make this test for exit: in some
 * places for correctness, in some places just to avoid unnecessary work.
 */
static inline bool ksm_test_exit(struct mm_struct *mm)
{
    return atomic_read(&mm->mm_users) == 0;
}

/*
 * We use break_ksm to break COW on a ksm page: it's a stripped down
 *
 *  if (get_user_pages(current, mm, addr, 1, 1, 1, &page, NULL) == 1)
 *      put_page(page);
 *
 * but taking great care only to touch a ksm page, in a VM_MERGEABLE vma,
 * in case the application has unmapped and remapped mm,addr meanwhile.
 * Could a ksm page appear anywhere else?  Actually yes, in a VM_PFNMAP
 * mmap of /dev/mem or /dev/kmem, where we would not want to touch it.
 */
static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
{
    struct page *page;
    int ret = 0;

    do {
        cond_resched();
        page = follow_page(vma, addr, FOLL_GET);
        if (IS_ERR_OR_NULL(page))
            break;
        if (PageKsm(page))
            ret = handle_mm_fault(vma->vm_mm, vma, addr,
                            FAULT_FLAG_WRITE);
        else
            ret = VM_FAULT_WRITE;
        put_page(page);
    } while (!(ret & (VM_FAULT_WRITE | VM_FAULT_SIGBUS | VM_FAULT_OOM)));
    /*
     * We must loop because handle_mm_fault() may back out if there's
     * any difficulty e.g. if pte accessed bit gets updated concurrently.
     *
     * VM_FAULT_WRITE is what we have been hoping for: it indicates that
     * COW has been broken, even if the vma does not permit VM_WRITE;
     * but note that a concurrent fault might break PageKsm for us.
     *
     * VM_FAULT_SIGBUS could occur if we race with truncation of the
     * backing file, which also invalidates anonymous pages: that's
     * okay, that truncation will have unmapped the PageKsm for us.
     *
     * VM_FAULT_OOM: at the time of writing (late July 2009), setting
     * aside mem_cgroup limits, VM_FAULT_OOM would only be set if the
     * current task has TIF_MEMDIE set, and will be OOM killed on return
     * to user; and ksmd, having no mm, would never be chosen for that.
     *
     * But if the mm is in a limited mem_cgroup, then the fault may fail
     * with VM_FAULT_OOM even if the current task is not TIF_MEMDIE; and
     * even ksmd can fail in this way - though it's usually breaking ksm
     * just to undo a merge it made a moment before, so unlikely to oom.
     *
     * That's a pity: we might therefore have more kernel pages allocated
     * than we're counting as nodes in the stable tree; but ksm_do_scan
     * will retry to break_cow on each pass, so should recover the page
     * in due course.  The important thing is to not let VM_MERGEABLE
     * be cleared while any such pages might remain in the area.
     */
    return (ret & VM_FAULT_OOM) ? -ENOMEM : 0;
}

static struct vm_area_struct *find_mergeable_vma(struct mm_struct *mm,
        unsigned long addr)
{
    struct vm_area_struct *vma;
    if (ksm_test_exit(mm))
        return NULL;
    vma = find_vma(mm, addr);
    if (!vma || vma->vm_start > addr)
        return NULL;
    if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
        return NULL;
    return vma;
}

static void break_cow(struct rmap_item *rmap_item)
{
    struct mm_struct *mm = rmap_item->mm;
    unsigned long addr = rmap_item->address;
    struct vm_area_struct *vma;

    /*
     * It is not an accident that whenever we want to break COW
     * to undo, we also need to drop a reference to the anon_vma.
     */
    put_anon_vma(rmap_item->anon_vma);

    down_read(&mm->mmap_sem);
    vma = find_mergeable_vma(mm, addr);
    if (vma)
        break_ksm(vma, addr);
    up_read(&mm->mmap_sem);
}

static struct page *page_trans_compound_anon(struct page *page)
{
    if (PageTransCompound(page)) {
        struct page *head = compound_trans_head(page);
        /*
         * head may actually be splitted and freed from under
         * us but it's ok here.
         */
        if (PageAnon(head))
            return head;
    }
    return NULL;
}

static struct page *get_mergeable_page(struct rmap_item *rmap_item)
{
    struct mm_struct *mm = rmap_item->mm;
    unsigned long addr = rmap_item->address;
    struct vm_area_struct *vma;
    struct page *page;

    down_read(&mm->mmap_sem);
    vma = find_mergeable_vma(mm, addr);
    if (!vma)
        goto out;

    page = follow_page(vma, addr, FOLL_GET);
    if (IS_ERR_OR_NULL(page))
        goto out;
    if (PageAnon(page) || page_trans_compound_anon(page)) {
        flush_anon_page(vma, page, addr);
        flush_dcache_page(page);
    } else {
        put_page(page);
out:        page = NULL;
    }
    up_read(&mm->mmap_sem);
    return page;
}

static void remove_node_from_stable_tree(struct stable_node *stable_node)
{
    struct rmap_item *rmap_item;
    struct hlist_node *hlist;

    hlist_for_each_entry(rmap_item, hlist, &stable_node->hlist, hlist) {
        if (rmap_item->hlist.next)
            ksm_pages_sharing--;
        else
            ksm_pages_shared--;
        put_anon_vma(rmap_item->anon_vma);
        rmap_item->address &= PAGE_MASK;
        cond_resched();
    }

    rb_erase(&stable_node->node, &root_stable_tree);
    free_stable_node(stable_node);
}

/*
 * get_ksm_page: checks if the page indicated by the stable node
 * is still its ksm page, despite having held no reference to it.
 * In which case we can trust the content of the page, and it
 * returns the gotten page; but if the page has now been zapped,
 * remove the stale node from the stable tree and return NULL.
 *
 * You would expect the stable_node to hold a reference to the ksm page.
 * But if it increments the page's count, swapping out has to wait for
 * ksmd to come around again before it can free the page, which may take
 * seconds or even minutes: much too unresponsive.  So instead we use a
 * "keyhole reference": access to the ksm page from the stable node peeps
 * out through its keyhole to see if that page still holds the right key,
 * pointing back to this stable node.  This relies on freeing a PageAnon
 * page to reset its page->mapping to NULL, and relies on no other use of
 * a page to put something that might look like our key in page->mapping.
 *
 * include/linux/pagemap.h page_cache_get_speculative() is a good reference,
 * but this is different - made simpler by ksm_thread_mutex being held, but
 * interesting for assuming that no other use of the struct page could ever
 * put our expected_mapping into page->mapping (or a field of the union which
 * coincides with page->mapping).  The RCU calls are not for KSM at all, but
 * to keep the page_count protocol described with page_cache_get_speculative.
 *
 * Note: it is possible that get_ksm_page() will return NULL one moment,
 * then page the next, if the page is in between page_freeze_refs() and
 * page_unfreeze_refs(): this shouldn't be a problem anywhere, the page
 * is on its way to being freed; but it is an anomaly to bear in mind.
 */
static struct page *get_ksm_page(struct stable_node *stable_node)
{
    struct page *page;
    void *expected_mapping;

    page = pfn_to_page(stable_node->kpfn);
    expected_mapping = (void *)stable_node +
                (PAGE_MAPPING_ANON | PAGE_MAPPING_KSM);
    rcu_read_lock();
    if (page->mapping != expected_mapping)
        goto stale;
    if (!get_page_unless_zero(page))
        goto stale;
    if (page->mapping != expected_mapping) {
        put_page(page);
        goto stale;
    }
    rcu_read_unlock();
    return page;
stale:
    rcu_read_unlock();
    remove_node_from_stable_tree(stable_node);
    return NULL;
}

/*
 * Removing rmap_item from stable or unstable tree.
 * This function will clean the information from the stable/unstable tree.
 */
static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
{
    if (rmap_item->address & STABLE_FLAG) {
        struct stable_node *stable_node;
        struct page *page;

        stable_node = rmap_item->head;
        page = get_ksm_page(stable_node);
        if (!page)
            goto out;

        lock_page(page);
        hlist_del(&rmap_item->hlist);
        unlock_page(page);
        put_page(page);

        if (stable_node->hlist.first)
            ksm_pages_sharing--;
        else
            ksm_pages_shared--;

        put_anon_vma(rmap_item->anon_vma);
        rmap_item->address &= PAGE_MASK;

    } else if (rmap_item->address & UNSTABLE_FLAG) {
        unsigned char age;
        /*
         * Usually ksmd can and must skip the rb_erase, because
         * root_unstable_tree was already reset to RB_ROOT.
         * But be careful when an mm is exiting: do the rb_erase
         * if this rmap_item was inserted by this scan, rather
         * than left over from before.
         */
        age = (unsigned char)(ksm_scan.seqnr - rmap_item->address);
        BUG_ON(age > 1);
        if (!age)
            rb_erase(&rmap_item->node, &root_unstable_tree);

        ksm_pages_unshared--;
        rmap_item->address &= PAGE_MASK;
    }
out:
    cond_resched();     /* we're called from many long loops */
}

static void remove_trailing_rmap_items(struct mm_slot *mm_slot,
                       struct rmap_item **rmap_list)
{
    while (*rmap_list) {
        struct rmap_item *rmap_item = *rmap_list;
        *rmap_list = rmap_item->rmap_list;
        remove_rmap_item_from_tree(rmap_item);
        free_rmap_item(rmap_item);
    }
}

/*
 * Though it's very tempting to unmerge in_stable_tree(rmap_item)s rather
 * than check every pte of a given vma, the locking doesn't quite work for
 * that - an rmap_item is assigned to the stable tree after inserting ksm
 * page and upping mmap_sem.  Nor does it fit with the way we skip dup'ing
 * rmap_items from parent to child at fork time (so as not to waste time
 * if exit comes before the next scan reaches it).
 *
 * Similarly, although we'd like to remove rmap_items (so updating counts
 * and freeing memory) when unmerging an area, it's easier to leave that
 * to the next pass of ksmd - consider, for example, how ksmd might be
 * in cmp_and_merge_page on one of the rmap_items we would be removing.
 */
static int unmerge_ksm_pages(struct vm_area_struct *vma,
                 unsigned long start, unsigned long end)
{
    unsigned long addr;
    int err = 0;

    for (addr = start; addr < end && !err; addr += PAGE_SIZE) {
        if (ksm_test_exit(vma->vm_mm))
            break;
        if (signal_pending(current))
            err = -ERESTARTSYS;
        else
            err = break_ksm(vma, addr);
    }
    return err;
}

#ifdef CONFIG_SYSFS
/*
 * Only called through the sysfs control interface:
 */
static int unmerge_and_remove_all_rmap_items(void)
{
    struct mm_slot *mm_slot;
    struct mm_struct *mm;
    struct vm_area_struct *vma;
    int err = 0;

    spin_lock(&ksm_mmlist_lock);
    ksm_scan.mm_slot = list_entry(ksm_mm_head.mm_list.next,
                        struct mm_slot, mm_list);
    spin_unlock(&ksm_mmlist_lock);

    for (mm_slot = ksm_scan.mm_slot;
            mm_slot != &ksm_mm_head; mm_slot = ksm_scan.mm_slot) {
        mm = mm_slot->mm;
        down_read(&mm->mmap_sem);
        for (vma = mm->mmap; vma; vma = vma->vm_next) {
            if (ksm_test_exit(mm))
                break;
            if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
                continue;
            err = unmerge_ksm_pages(vma,
                        vma->vm_start, vma->vm_end);
            if (err)
                goto error;
        }

        remove_trailing_rmap_items(mm_slot, &mm_slot->rmap_list);

        spin_lock(&ksm_mmlist_lock);
        ksm_scan.mm_slot = list_entry(mm_slot->mm_list.next,
                        struct mm_slot, mm_list);
        if (ksm_test_exit(mm)) {
            hlist_del(&mm_slot->link);
            list_del(&mm_slot->mm_list);
            spin_unlock(&ksm_mmlist_lock);

            free_mm_slot(mm_slot);
            clear_bit(MMF_VM_MERGEABLE, &mm->flags);
            up_read(&mm->mmap_sem);
            mmdrop(mm);
        } else {
            spin_unlock(&ksm_mmlist_lock);
            up_read(&mm->mmap_sem);
        }
    }

    ksm_scan.seqnr = 0;
    return 0;

error:
    up_read(&mm->mmap_sem);
    spin_lock(&ksm_mmlist_lock);
    ksm_scan.mm_slot = &ksm_mm_head;
    spin_unlock(&ksm_mmlist_lock);
    return err;
}
#endif /* CONFIG_SYSFS */

static u32 calc_checksum(struct page *page)
{
    u32 checksum;
    void *addr = kmap_atomic(page);
    checksum = jhash2(addr, PAGE_SIZE / 4, 17);
    kunmap_atomic(addr);
    return checksum;
}

static int memcmp_pages(struct page *page1, struct page *page2)
{
    char *addr1, *addr2;
    int ret;

    addr1 = kmap_atomic(page1);
    addr2 = kmap_atomic(page2);
    ret = memcmp(addr1, addr2, PAGE_SIZE);
    kunmap_atomic(addr2);
    kunmap_atomic(addr1);
    return ret;
}

static inline int pages_identical(struct page *page1, struct page *page2)
{
    return !memcmp_pages(page1, page2);
}

static int write_protect_page(struct vm_area_struct *vma, struct page *page,
                  pte_t *orig_pte)
{
    struct mm_struct *mm = vma->vm_mm;
    unsigned long addr;
    pte_t *ptep;
    spinlock_t *ptl;
    int swapped;
    int err = -EFAULT;
    unsigned long mmun_start;   /* For mmu_notifiers */
    unsigned long mmun_end;     /* For mmu_notifiers */

    addr = page_address_in_vma(page, vma);
    if (addr == -EFAULT)
        goto out;

    BUG_ON(PageTransCompound(page));

    mmun_start = addr;
    mmun_end   = addr + PAGE_SIZE;
    mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);

    ptep = page_check_address(page, mm, addr, &ptl, 0);
    if (!ptep)
        goto out_mn;

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

        swapped = PageSwapCache(page);
        flush_cache_page(vma, addr, page_to_pfn(page));
        /*
         * Ok this is tricky, when get_user_pages_fast() run it doesn't
         * take any lock, therefore the check that we are going to make
         * with the pagecount against the mapcount is racey and
         * O_DIRECT can happen right after the check.
         * So we clear the pte and flush the tlb before the check
         * this assure us that no O_DIRECT can happen after the check
         * or in the middle of the check.
         */
        entry = ptep_clear_flush(vma, addr, ptep);
        /*
         * Check that no O_DIRECT or similar I/O is in progress on the
         * page
         */
        if (page_mapcount(page) + 1 + swapped != page_count(page)) {
            set_pte_at(mm, addr, ptep, entry);
            goto out_unlock;
        }
        if (pte_dirty(entry))
            set_page_dirty(page);
        entry = pte_mkclean(pte_wrprotect(entry));
        set_pte_at_notify(mm, addr, ptep, entry);
    }
    *orig_pte = *ptep;
    err = 0;

out_unlock:
    pte_unmap_unlock(ptep, ptl);
out_mn:
    mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
out:
    return err;
}

static int replace_page(struct vm_area_struct *vma, struct page *page,
            struct page *kpage, pte_t orig_pte)
{
    struct mm_struct *mm = vma->vm_mm;
    pgd_t *pgd;
    pud_t *pud;
    pmd_t *pmd;
    pte_t *ptep;
    spinlock_t *ptl;
    unsigned long addr;
    int err = -EFAULT;
    unsigned long mmun_start;   /* For mmu_notifiers */
    unsigned long mmun_end;     /* For mmu_notifiers */

    addr = page_address_in_vma(page, vma);
    if (addr == -EFAULT)
        goto out;

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

    pud = pud_offset(pgd, addr);
    if (!pud_present(*pud))
        goto out;

    pmd = pmd_offset(pud, addr);
    BUG_ON(pmd_trans_huge(*pmd));
    if (!pmd_present(*pmd))
        goto out;

    mmun_start = addr;
    mmun_end   = addr + PAGE_SIZE;
    mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);

    ptep = pte_offset_map_lock(mm, pmd, addr, &ptl);
    if (!pte_same(*ptep, orig_pte)) {
        pte_unmap_unlock(ptep, ptl);
        goto out_mn;
    }

    get_page(kpage);
    page_add_anon_rmap(kpage, vma, addr);

    flush_cache_page(vma, addr, pte_pfn(*ptep));
    ptep_clear_flush(vma, addr, ptep);
    set_pte_at_notify(mm, addr, ptep, mk_pte(kpage, vma->vm_page_prot));

    page_remove_rmap(page);
    if (!page_mapped(page))
        try_to_free_swap(page);
    put_page(page);

    pte_unmap_unlock(ptep, ptl);
    err = 0;
out_mn:
    mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
out:
    return err;
}

static int page_trans_compound_anon_split(struct page *page)
{
    int ret = 0;
    struct page *transhuge_head = page_trans_compound_anon(page);
    if (transhuge_head) {
        /* Get the reference on the head to split it. */
        if (get_page_unless_zero(transhuge_head)) {
            /*
             * Recheck we got the reference while the head
             * was still anonymous.
             */
            if (PageAnon(transhuge_head))
                ret = split_huge_page(transhuge_head);
            else
                /*
                 * Retry later if split_huge_page run
                 * from under us.
                 */
                ret = 1;
            put_page(transhuge_head);
        } else
            /* Retry later if split_huge_page run from under us. */
            ret = 1;
    }
    return ret;
}

/*
 * try_to_merge_one_page - take two pages and merge them into one
 * @vma: the vma that holds the pte pointing to page
 * @page: the PageAnon page that we want to replace with kpage
 * @kpage: the PageKsm page that we want to map instead of page,
 *         or NULL the first time when we want to use page as kpage.
 *
 * This function returns 0 if the pages were merged, -EFAULT otherwise.
 */
static int try_to_merge_one_page(struct vm_area_struct *vma,
                 struct page *page, struct page *kpage)
{
    pte_t orig_pte = __pte(0);
    int err = -EFAULT;

    if (page == kpage)          /* ksm page forked */
        return 0;

    if (!(vma->vm_flags & VM_MERGEABLE))
        goto out;
    if (PageTransCompound(page) && page_trans_compound_anon_split(page))
        goto out;
    BUG_ON(PageTransCompound(page));
    if (!PageAnon(page))
        goto out;

    /*
     * We need the page lock to read a stable PageSwapCache in
     * write_protect_page().  We use trylock_page() instead of
     * lock_page() because we don't want to wait here - we
     * prefer to continue scanning and merging different pages,
     * then come back to this page when it is unlocked.
     */
    if (!trylock_page(page))
        goto out;
    /*
     * If this anonymous page is mapped only here, its pte may need
     * to be write-protected.  If it's mapped elsewhere, all of its
     * ptes are necessarily already write-protected.  But in either
     * case, we need to lock and check page_count is not raised.
     */
    if (write_protect_page(vma, page, &orig_pte) == 0) {
        if (!kpage) {
            /*
             * While we hold page lock, upgrade page from
             * PageAnon+anon_vma to PageKsm+NULL stable_node:
             * stable_tree_insert() will update stable_node.
             */
            set_page_stable_node(page, NULL);
            mark_page_accessed(page);
            err = 0;
        } else if (pages_identical(page, kpage))
            err = replace_page(vma, page, kpage, orig_pte);
    }

    if ((vma->vm_flags & VM_LOCKED) && kpage && !err) {
        munlock_vma_page(page);
        if (!PageMlocked(kpage)) {
            unlock_page(page);
            lock_page(kpage);
            mlock_vma_page(kpage);
            page = kpage;       /* for final unlock */
        }
    }

    unlock_page(page);
out:
    return err;
}

/*
 * try_to_merge_with_ksm_page - like try_to_merge_two_pages,
 * but no new kernel page is allocated: kpage must already be a ksm page.
 *
 * This function returns 0 if the pages were merged, -EFAULT otherwise.
 */
static int try_to_merge_with_ksm_page(struct rmap_item *rmap_item,
                      struct page *page, struct page *kpage)
{
    struct mm_struct *mm = rmap_item->mm;
    struct vm_area_struct *vma;
    int err = -EFAULT;

    down_read(&mm->mmap_sem);
    if (ksm_test_exit(mm))
        goto out;
    vma = find_vma(mm, rmap_item->address);
    if (!vma || vma->vm_start > rmap_item->address)
        goto out;

    err = try_to_merge_one_page(vma, page, kpage);
    if (err)
        goto out;

    /* Must get reference to anon_vma while still holding mmap_sem */
    rmap_item->anon_vma = vma->anon_vma;
    get_anon_vma(vma->anon_vma);
out:
    up_read(&mm->mmap_sem);
    return err;
}

/*
 * try_to_merge_two_pages - take two identical pages and prepare them
 * to be merged into one page.
 *
 * This function returns the kpage if we successfully merged two identical
 * pages into one ksm page, NULL otherwise.
 *
 * Note that this function upgrades page to ksm page: if one of the pages
 * is already a ksm page, try_to_merge_with_ksm_page should be used.
 */
static struct page *try_to_merge_two_pages(struct rmap_item *rmap_item,
                       struct page *page,
                       struct rmap_item *tree_rmap_item,
                       struct page *tree_page)
{
    int err;

    err = try_to_merge_with_ksm_page(rmap_item, page, NULL);
    if (!err) {
        err = try_to_merge_with_ksm_page(tree_rmap_item,
                            tree_page, page);
        /*
         * If that fails, we have a ksm page with only one pte
         * pointing to it: so break it.
         */
        if (err)
            break_cow(rmap_item);
    }
    return err ? NULL : page;
}

/*
 * stable_tree_search - search for page inside the stable tree
 *
 * This function checks if there is a page inside the stable tree
 * with identical content to the page that we are scanning right now.
 *
 * This function returns the stable tree node of identical content if found,
 * NULL otherwise.
 */
static struct page *stable_tree_search(struct page *page)
{
    struct rb_node *node = root_stable_tree.rb_node;
    struct stable_node *stable_node;

    stable_node = page_stable_node(page);
    if (stable_node) {          /* ksm page forked */
        get_page(page);
        return page;
    }

    while (node) {
        struct page *tree_page;
        int ret;

        cond_resched();
        stable_node = rb_entry(node, struct stable_node, node);
        tree_page = get_ksm_page(stable_node);
        if (!tree_page)
            return NULL;

        ret = memcmp_pages(page, tree_page);

        if (ret < 0) {
            put_page(tree_page);
            node = node->rb_left;
        } else if (ret > 0) {
            put_page(tree_page);
            node = node->rb_right;
        } else
            return tree_page;
    }

    return NULL;
}

/*
 * stable_tree_insert - insert rmap_item pointing to new ksm page
 * into the stable tree.
 *
 * This function returns the stable tree node just allocated on success,
 * NULL otherwise.
 */
static struct stable_node *stable_tree_insert(struct page *kpage)
{
    struct rb_node **new = &root_stable_tree.rb_node;
    struct rb_node *parent = NULL;
    struct stable_node *stable_node;

    while (*new) {
        struct page *tree_page;
        int ret;

        cond_resched();
        stable_node = rb_entry(*new, struct stable_node, node);
        tree_page = get_ksm_page(stable_node);
        if (!tree_page)
            return NULL;

        ret = memcmp_pages(kpage, tree_page);
        put_page(tree_page);

        parent = *new;
        if (ret < 0)
            new = &parent->rb_left;
        else if (ret > 0)
            new = &parent->rb_right;
        else {
            /*
             * It is not a bug that stable_tree_search() didn't
             * find this node: because at that time our page was
             * not yet write-protected, so may have changed since.
             */
            return NULL;
        }
    }

    stable_node = alloc_stable_node();
    if (!stable_node)
        return NULL;

    rb_link_node(&stable_node->node, parent, new);
    rb_insert_color(&stable_node->node, &root_stable_tree);

    INIT_HLIST_HEAD(&stable_node->hlist);

    stable_node->kpfn = page_to_pfn(kpage);
    set_page_stable_node(kpage, stable_node);

    return stable_node;
}

/*
 * unstable_tree_search_insert - search for identical page,
 * else insert rmap_item into the unstable tree.
 *
 * This function searches for a page in the unstable tree identical to the
 * page currently being scanned; and if no identical page is found in the
 * tree, we insert rmap_item as a new object into the unstable tree.
 *
 * This function returns pointer to rmap_item found to be identical
 * to the currently scanned page, NULL otherwise.
 *
 * This function does both searching and inserting, because they share
 * the same walking algorithm in an rbtree.
 */
static
struct rmap_item *unstable_tree_search_insert(struct rmap_item *rmap_item,
                          struct page *page,
                          struct page **tree_pagep)

{
    struct rb_node **new = &root_unstable_tree.rb_node;
    struct rb_node *parent = NULL;

    while (*new) {
        struct rmap_item *tree_rmap_item;
        struct page *tree_page;
        int ret;

        cond_resched();
        tree_rmap_item = rb_entry(*new, struct rmap_item, node);
        tree_page = get_mergeable_page(tree_rmap_item);
        if (IS_ERR_OR_NULL(tree_page))
            return NULL;

        /*
         * Don't substitute a ksm page for a forked page.
         */
        if (page == tree_page) {
            put_page(tree_page);
            return NULL;
        }

        ret = memcmp_pages(page, tree_page);

        parent = *new;
        if (ret < 0) {
            put_page(tree_page);
            new = &parent->rb_left;
        } else if (ret > 0) {
            put_page(tree_page);
            new = &parent->rb_right;
        } else {
            *tree_pagep = tree_page;
            return tree_rmap_item;
        }
    }

    rmap_item->address |= UNSTABLE_FLAG;
    rmap_item->address |= (ksm_scan.seqnr & SEQNR_MASK);
    rb_link_node(&rmap_item->node, parent, new);
    rb_insert_color(&rmap_item->node, &root_unstable_tree);

    ksm_pages_unshared++;
    return NULL;
}

/*
 * stable_tree_append - add another rmap_item to the linked list of
 * rmap_items hanging off a given node of the stable tree, all sharing
 * the same ksm page.
 */
static void stable_tree_append(struct rmap_item *rmap_item,
                   struct stable_node *stable_node)
{
    rmap_item->head = stable_node;
    rmap_item->address |= STABLE_FLAG;
    hlist_add_head(&rmap_item->hlist, &stable_node->hlist);

    if (rmap_item->hlist.next)
        ksm_pages_sharing++;
    else
        ksm_pages_shared++;
}

/*
 * cmp_and_merge_page - first see if page can be merged into the stable tree;
 * if not, compare checksum to previous and if it's the same, see if page can
 * be inserted into the unstable tree, or merged with a page already there and
 * both transferred to the stable tree.
 *
 * @page: the page that we are searching identical page to.
 * @rmap_item: the reverse mapping into the virtual address of this page
 */
static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
{
    struct rmap_item *tree_rmap_item;
    struct page *tree_page = NULL;
    struct stable_node *stable_node;
    struct page *kpage;
    unsigned int checksum;
    int err;

    remove_rmap_item_from_tree(rmap_item);

    /* We first start with searching the page inside the stable tree */
    kpage = stable_tree_search(page);
    if (kpage) {
        err = try_to_merge_with_ksm_page(rmap_item, page, kpage);
        if (!err) {
            /*
             * The page was successfully merged:
             * add its rmap_item to the stable tree.
             */
            lock_page(kpage);
            stable_tree_append(rmap_item, page_stable_node(kpage));
            unlock_page(kpage);
        }
        put_page(kpage);
        return;
    }

    /*
     * If the hash value of the page has changed from the last time
     * we calculated it, this page is changing frequently: therefore we
     * don't want to insert it in the unstable tree, and we don't want
     * to waste our time searching for something identical to it there.
     */
    checksum = calc_checksum(page);
    if (rmap_item->oldchecksum != checksum) {
        rmap_item->oldchecksum = checksum;
        return;
    }

    tree_rmap_item =
        unstable_tree_search_insert(rmap_item, page, &tree_page);
    if (tree_rmap_item) {
        kpage = try_to_merge_two_pages(rmap_item, page,
                        tree_rmap_item, tree_page);
        put_page(tree_page);
        /*
         * As soon as we merge this page, we want to remove the
         * rmap_item of the page we have merged with from the unstable
         * tree, and insert it instead as new node in the stable tree.
         */
        if (kpage) {
            remove_rmap_item_from_tree(tree_rmap_item);

            lock_page(kpage);
            stable_node = stable_tree_insert(kpage);
            if (stable_node) {
                stable_tree_append(tree_rmap_item, stable_node);
                stable_tree_append(rmap_item, stable_node);
            }
            unlock_page(kpage);

            /*
             * If we fail to insert the page into the stable tree,
             * we will have 2 virtual addresses that are pointing
             * to a ksm page left outside the stable tree,
             * in which case we need to break_cow on both.
             */
            if (!stable_node) {
                break_cow(tree_rmap_item);
                break_cow(rmap_item);
            }
        }
    }
}

static struct rmap_item *get_next_rmap_item(struct mm_slot *mm_slot,
                        struct rmap_item **rmap_list,
                        unsigned long addr)
{
    struct rmap_item *rmap_item;

    while (*rmap_list) {
        rmap_item = *rmap_list;
        if ((rmap_item->address & PAGE_MASK) == addr)
            return rmap_item;
        if (rmap_item->address > addr)
            break;
        *rmap_list = rmap_item->rmap_list;
        remove_rmap_item_from_tree(rmap_item);
        free_rmap_item(rmap_item);
    }

    rmap_item = alloc_rmap_item();
    if (rmap_item) {
        /* It has already been zeroed */
        rmap_item->mm = mm_slot->mm;
        rmap_item->address = addr;
        rmap_item->rmap_list = *rmap_list;
        *rmap_list = rmap_item;
    }
    return rmap_item;
}

static struct rmap_item *scan_get_next_rmap_item(struct page **page)
{
    struct mm_struct *mm;
    struct mm_slot *slot;
    struct vm_area_struct *vma;
    struct rmap_item *rmap_item;

    if (list_empty(&ksm_mm_head.mm_list))
        return NULL;

    slot = ksm_scan.mm_slot;
    if (slot == &ksm_mm_head) {
        /*
         * A number of pages can hang around indefinitely on per-cpu
         * pagevecs, raised page count preventing write_protect_page
         * from merging them.  Though it doesn't really matter much,
         * it is puzzling to see some stuck in pages_volatile until
         * other activity jostles them out, and they also prevented
         * LTP's KSM test from succeeding deterministically; so drain
         * them here (here rather than on entry to ksm_do_scan(),
         * so we don't IPI too often when pages_to_scan is set low).
         */
        lru_add_drain_all();

        root_unstable_tree = RB_ROOT;

        spin_lock(&ksm_mmlist_lock);
        slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list);
        ksm_scan.mm_slot = slot;
        spin_unlock(&ksm_mmlist_lock);
        /*
         * Although we tested list_empty() above, a racing __ksm_exit
         * of the last mm on the list may have removed it since then.
         */
        if (slot == &ksm_mm_head)
            return NULL;
next_mm:
        ksm_scan.address = 0;
        ksm_scan.rmap_list = &slot->rmap_list;
    }

    mm = slot->mm;
    down_read(&mm->mmap_sem);
    if (ksm_test_exit(mm))
        vma = NULL;
    else
        vma = find_vma(mm, ksm_scan.address);

    for (; vma; vma = vma->vm_next) {
        if (!(vma->vm_flags & VM_MERGEABLE))
            continue;
        if (ksm_scan.address < vma->vm_start)
            ksm_scan.address = vma->vm_start;
        if (!vma->anon_vma)
            ksm_scan.address = vma->vm_end;

        while (ksm_scan.address < vma->vm_end) {
            if (ksm_test_exit(mm))
                break;
            *page = follow_page(vma, ksm_scan.address, FOLL_GET);
            if (IS_ERR_OR_NULL(*page)) {
                ksm_scan.address += PAGE_SIZE;
                cond_resched();
                continue;
            }
            if (PageAnon(*page) ||
                page_trans_compound_anon(*page)) {
                flush_anon_page(vma, *page, ksm_scan.address);
                flush_dcache_page(*page);
                rmap_item = get_next_rmap_item(slot,
                    ksm_scan.rmap_list, ksm_scan.address);
                if (rmap_item) {
                    ksm_scan.rmap_list =
                            &rmap_item->rmap_list;
                    ksm_scan.address += PAGE_SIZE;
                } else
                    put_page(*page);
                up_read(&mm->mmap_sem);
                return rmap_item;
            }
            put_page(*page);
            ksm_scan.address += PAGE_SIZE;
            cond_resched();
        }
    }

    if (ksm_test_exit(mm)) {
        ksm_scan.address = 0;
        ksm_scan.rmap_list = &slot->rmap_list;
    }
    /*
     * Nuke all the rmap_items that are above this current rmap:
     * because there were no VM_MERGEABLE vmas with such addresses.
     */
    remove_trailing_rmap_items(slot, ksm_scan.rmap_list);

    spin_lock(&ksm_mmlist_lock);
    ksm_scan.mm_slot = list_entry(slot->mm_list.next,
                        struct mm_slot, mm_list);
    if (ksm_scan.address == 0) {
        /*
         * We've completed a full scan of all vmas, holding mmap_sem
         * throughout, and found no VM_MERGEABLE: so do the same as
         * __ksm_exit does to remove this mm from all our lists now.
         * This applies either when cleaning up after __ksm_exit
         * (but beware: we can reach here even before __ksm_exit),
         * or when all VM_MERGEABLE areas have been unmapped (and
         * mmap_sem then protects against race with MADV_MERGEABLE).
         */
        hlist_del(&slot->link);
        list_del(&slot->mm_list);
        spin_unlock(&ksm_mmlist_lock);

        free_mm_slot(slot);
        clear_bit(MMF_VM_MERGEABLE, &mm->flags);
        up_read(&mm->mmap_sem);
        mmdrop(mm);
    } else {
        spin_unlock(&ksm_mmlist_lock);
        up_read(&mm->mmap_sem);
    }

    /* Repeat until we've completed scanning the whole list */
    slot = ksm_scan.mm_slot;
    if (slot != &ksm_mm_head)
        goto next_mm;

    ksm_scan.seqnr++;
    return NULL;
}

static void ksm_do_scan(unsigned int scan_npages)
{
    struct rmap_item *rmap_item;
    struct page *uninitialized_var(page);

    while (scan_npages-- && likely(!freezing(current))) {
        cond_resched();
        rmap_item = scan_get_next_rmap_item(&page);
        if (!rmap_item)
            return;
        if (!PageKsm(page) || !in_stable_tree(rmap_item))
            cmp_and_merge_page(page, rmap_item);
        put_page(page);
    }
}

static int ksmd_should_run(void)
{
    return (ksm_run & KSM_RUN_MERGE) && !list_empty(&ksm_mm_head.mm_list);
}

static int ksm_scan_thread(void *nothing)
{
    set_freezable();
    set_user_nice(current, 5);

    while (!kthread_should_stop()) {
        mutex_lock(&ksm_thread_mutex);
        if (ksmd_should_run())
            ksm_do_scan(ksm_thread_pages_to_scan);
        mutex_unlock(&ksm_thread_mutex);

        try_to_freeze();

        if (ksmd_should_run()) {
            schedule_timeout_interruptible(
                msecs_to_jiffies(ksm_thread_sleep_millisecs));
        } else {
            wait_event_freezable(ksm_thread_wait,
                ksmd_should_run() || kthread_should_stop());
        }
    }
    return 0;
}

int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
        unsigned long end, int advice, unsigned long *vm_flags)
{
    struct mm_struct *mm = vma->vm_mm;
    int err;

    switch (advice) {
    case MADV_MERGEABLE:
        /*
         * Be somewhat over-protective for now!
         */
        if (*vm_flags & (VM_MERGEABLE | VM_SHARED  | VM_MAYSHARE   |
                 VM_PFNMAP    | VM_IO      | VM_DONTEXPAND |
                 VM_HUGETLB | VM_NONLINEAR | VM_MIXEDMAP))
            return 0;       /* just ignore the advice */

#ifdef VM_SAO
        if (*vm_flags & VM_SAO)
            return 0;
#endif

        if (!test_bit(MMF_VM_MERGEABLE, &mm->flags)) {
            err = __ksm_enter(mm);
            if (err)
                return err;
        }

        *vm_flags |= VM_MERGEABLE;
        break;

    case MADV_UNMERGEABLE:
        if (!(*vm_flags & VM_MERGEABLE))
            return 0;       /* just ignore the advice */

        if (vma->anon_vma) {
            err = unmerge_ksm_pages(vma, start, end);
            if (err)
                return err;
        }

        *vm_flags &= ~VM_MERGEABLE;
        break;
    }

    return 0;
}

int __ksm_enter(struct mm_struct *mm)
{
    struct mm_slot *mm_slot;
    int needs_wakeup;

    mm_slot = alloc_mm_slot();
    if (!mm_slot)
        return -ENOMEM;

    /* Check ksm_run too?  Would need tighter locking */
    needs_wakeup = list_empty(&ksm_mm_head.mm_list);

    spin_lock(&ksm_mmlist_lock);
    insert_to_mm_slots_hash(mm, mm_slot);
    /*
     * Insert just behind the scanning cursor, to let the area settle
     * down a little; when fork is followed by immediate exec, we don't
     * want ksmd to waste time setting up and tearing down an rmap_list.
     */
    list_add_tail(&mm_slot->mm_list, &ksm_scan.mm_slot->mm_list);
    spin_unlock(&ksm_mmlist_lock);

    set_bit(MMF_VM_MERGEABLE, &mm->flags);
    atomic_inc(&mm->mm_count);

    if (needs_wakeup)
        wake_up_interruptible(&ksm_thread_wait);

    return 0;
}

void __ksm_exit(struct mm_struct *mm)
{
    struct mm_slot *mm_slot;
    int easy_to_free = 0;

    /*
     * This process is exiting: if it's straightforward (as is the
     * case when ksmd was never running), free mm_slot immediately.
     * But if it's at the cursor or has rmap_items linked to it, use
     * mmap_sem to synchronize with any break_cows before pagetables
     * are freed, and leave the mm_slot on the list for ksmd to free.
     * Beware: ksm may already have noticed it exiting and freed the slot.
     */

    spin_lock(&ksm_mmlist_lock);
    mm_slot = get_mm_slot(mm);
    if (mm_slot && ksm_scan.mm_slot != mm_slot) {
        if (!mm_slot->rmap_list) {
            hlist_del(&mm_slot->link);
            list_del(&mm_slot->mm_list);
            easy_to_free = 1;
        } else {
            list_move(&mm_slot->mm_list,
                  &ksm_scan.mm_slot->mm_list);
        }
    }
    spin_unlock(&ksm_mmlist_lock);

    if (easy_to_free) {
        free_mm_slot(mm_slot);
        clear_bit(MMF_VM_MERGEABLE, &mm->flags);
        mmdrop(mm);
    } else if (mm_slot) {
        down_write(&mm->mmap_sem);
        up_write(&mm->mmap_sem);
    }
}

struct page *ksm_does_need_to_copy(struct page *page,
            struct vm_area_struct *vma, unsigned long address)
{
    struct page *new_page;

    new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);
    if (new_page) {
        copy_user_highpage(new_page, page, address, vma);

        SetPageDirty(new_page);
        __SetPageUptodate(new_page);
        SetPageSwapBacked(new_page);
        __set_page_locked(new_page);

        if (!mlocked_vma_newpage(vma, new_page))
            lru_cache_add_lru(new_page, LRU_ACTIVE_ANON);
        else
            add_page_to_unevictable_list(new_page);
    }

    return new_page;
}

int page_referenced_ksm(struct page *page, struct mem_cgroup *memcg,
            unsigned long *vm_flags)
{
    struct stable_node *stable_node;
    struct rmap_item *rmap_item;
    struct hlist_node *hlist;
    unsigned int mapcount = page_mapcount(page);
    int referenced = 0;
    int search_new_forks = 0;

    VM_BUG_ON(!PageKsm(page));
    VM_BUG_ON(!PageLocked(page));

    stable_node = page_stable_node(page);
    if (!stable_node)
        return 0;
again:
    hlist_for_each_entry(rmap_item, hlist, &stable_node->hlist, hlist) {
        struct anon_vma *anon_vma = rmap_item->anon_vma;
        struct anon_vma_chain *vmac;
        struct vm_area_struct *vma;

        anon_vma_lock(anon_vma);
        anon_vma_interval_tree_foreach(vmac, &anon_vma->rb_root,
                           0, ULONG_MAX) {
            vma = vmac->vma;
            if (rmap_item->address < vma->vm_start ||
                rmap_item->address >= vma->vm_end)
                continue;
            /*
             * Initially we examine only the vma which covers this
             * rmap_item; but later, if there is still work to do,
             * we examine covering vmas in other mms: in case they
             * were forked from the original since ksmd passed.
             */
            if ((rmap_item->mm == vma->vm_mm) == search_new_forks)
                continue;

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

            referenced += page_referenced_one(page, vma,
                rmap_item->address, &mapcount, vm_flags);
            if (!search_new_forks || !mapcount)
                break;
        }
        anon_vma_unlock(anon_vma);
        if (!mapcount)
            goto out;
    }
    if (!search_new_forks++)
        goto again;
out:
    return referenced;
}

int try_to_unmap_ksm(struct page *page, enum ttu_flags flags)
{
    struct stable_node *stable_node;
    struct hlist_node *hlist;
    struct rmap_item *rmap_item;
    int ret = SWAP_AGAIN;
    int search_new_forks = 0;

    VM_BUG_ON(!PageKsm(page));
    VM_BUG_ON(!PageLocked(page));

    stable_node = page_stable_node(page);
    if (!stable_node)
        return SWAP_FAIL;
again:
    hlist_for_each_entry(rmap_item, hlist, &stable_node->hlist, hlist) {
        struct anon_vma *anon_vma = rmap_item->anon_vma;
        struct anon_vma_chain *vmac;
        struct vm_area_struct *vma;

        anon_vma_lock(anon_vma);
        anon_vma_interval_tree_foreach(vmac, &anon_vma->rb_root,
                           0, ULONG_MAX) {
            vma = vmac->vma;
            if (rmap_item->address < vma->vm_start ||
                rmap_item->address >= vma->vm_end)
                continue;
            /*
             * Initially we examine only the vma which covers this
             * rmap_item; but later, if there is still work to do,
             * we examine covering vmas in other mms: in case they
             * were forked from the original since ksmd passed.
             */
            if ((rmap_item->mm == vma->vm_mm) == search_new_forks)
                continue;

            ret = try_to_unmap_one(page, vma,
                    rmap_item->address, flags);
            if (ret != SWAP_AGAIN || !page_mapped(page)) {
                anon_vma_unlock(anon_vma);
                goto out;
            }
        }
        anon_vma_unlock(anon_vma);
    }
    if (!search_new_forks++)
        goto again;
out:
    return ret;
}

#ifdef CONFIG_MIGRATION
int rmap_walk_ksm(struct page *page, int (*rmap_one)(struct page *,
          struct vm_area_struct *, unsigned long, void *), void *arg)
{
    struct stable_node *stable_node;
    struct hlist_node *hlist;
    struct rmap_item *rmap_item;
    int ret = SWAP_AGAIN;
    int search_new_forks = 0;

    VM_BUG_ON(!PageKsm(page));
    VM_BUG_ON(!PageLocked(page));

    stable_node = page_stable_node(page);
    if (!stable_node)
        return ret;
again:
    hlist_for_each_entry(rmap_item, hlist, &stable_node->hlist, hlist) {
        struct anon_vma *anon_vma = rmap_item->anon_vma;
        struct anon_vma_chain *vmac;
        struct vm_area_struct *vma;

        anon_vma_lock(anon_vma);
        anon_vma_interval_tree_foreach(vmac, &anon_vma->rb_root,
                           0, ULONG_MAX) {
            vma = vmac->vma;
            if (rmap_item->address < vma->vm_start ||
                rmap_item->address >= vma->vm_end)
                continue;
            /*
             * Initially we examine only the vma which covers this
             * rmap_item; but later, if there is still work to do,
             * we examine covering vmas in other mms: in case they
             * were forked from the original since ksmd passed.
             */
            if ((rmap_item->mm == vma->vm_mm) == search_new_forks)
                continue;

            ret = rmap_one(page, vma, rmap_item->address, arg);
            if (ret != SWAP_AGAIN) {
                anon_vma_unlock(anon_vma);
                goto out;
            }
        }
        anon_vma_unlock(anon_vma);
    }
    if (!search_new_forks++)
        goto again;
out:
    return ret;
}

void ksm_migrate_page(struct page *newpage, struct page *oldpage)
{
    struct stable_node *stable_node;

    VM_BUG_ON(!PageLocked(oldpage));
    VM_BUG_ON(!PageLocked(newpage));
    VM_BUG_ON(newpage->mapping != oldpage->mapping);

    stable_node = page_stable_node(newpage);
    if (stable_node) {
        VM_BUG_ON(stable_node->kpfn != page_to_pfn(oldpage));
        stable_node->kpfn = page_to_pfn(newpage);
    }
}
#endif /* CONFIG_MIGRATION */

#ifdef CONFIG_MEMORY_HOTREMOVE
static struct stable_node *ksm_check_stable_tree(unsigned long start_pfn,
                         unsigned long end_pfn)
{
    struct rb_node *node;

    for (node = rb_first(&root_stable_tree); node; node = rb_next(node)) {
        struct stable_node *stable_node;

        stable_node = rb_entry(node, struct stable_node, node);
        if (stable_node->kpfn >= start_pfn &&
            stable_node->kpfn < end_pfn)
            return stable_node;
    }
    return NULL;
}

static int ksm_memory_callback(struct notifier_block *self,
                   unsigned long action, void *arg)
{
    struct memory_notify *mn = arg;
    struct stable_node *stable_node;

    switch (action) {
    case MEM_GOING_OFFLINE:
        /*
         * Keep it very simple for now: just lock out ksmd and
         * MADV_UNMERGEABLE while any memory is going offline.
         * mutex_lock_nested() is necessary because lockdep was alarmed
         * that here we take ksm_thread_mutex inside notifier chain
         * mutex, and later take notifier chain mutex inside
         * ksm_thread_mutex to unlock it.   But that's safe because both
         * are inside mem_hotplug_mutex.
         */
        mutex_lock_nested(&ksm_thread_mutex, SINGLE_DEPTH_NESTING);
        break;

    case MEM_OFFLINE:
        /*
         * Most of the work is done by page migration; but there might
         * be a few stable_nodes left over, still pointing to struct
         * pages which have been offlined: prune those from the tree.
         */
        while ((stable_node = ksm_check_stable_tree(mn->start_pfn,
                    mn->start_pfn + mn->nr_pages)) != NULL)
            remove_node_from_stable_tree(stable_node);
        /* fallthrough */

    case MEM_CANCEL_OFFLINE:
        mutex_unlock(&ksm_thread_mutex);
        break;
    }
    return NOTIFY_OK;
}
#endif /* CONFIG_MEMORY_HOTREMOVE */

#ifdef CONFIG_SYSFS
/*
 * This all compiles without CONFIG_SYSFS, but is a waste of space.
 */

#define KSM_ATTR_RO(_name) \
    static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
#define KSM_ATTR(_name) \
    static struct kobj_attribute _name##_attr = \
        __ATTR(_name, 0644, _name##_show, _name##_store)

static ssize_t sleep_millisecs_show(struct kobject *kobj,
                    struct kobj_attribute *attr, char *buf)
{
    return sprintf(buf, "%u\n", ksm_thread_sleep_millisecs);
}

static ssize_t sleep_millisecs_store(struct kobject *kobj,
                     struct kobj_attribute *attr,
                     const char *buf, size_t count)
{
    unsigned long msecs;
    int err;

    err = strict_strtoul(buf, 10, &msecs);
    if (err || msecs > UINT_MAX)
        return -EINVAL;

    ksm_thread_sleep_millisecs = msecs;

    return count;
}
KSM_ATTR(sleep_millisecs);

static ssize_t pages_to_scan_show(struct kobject *kobj,
                  struct kobj_attribute *attr, char *buf)
{
    return sprintf(buf, "%u\n", ksm_thread_pages_to_scan);
}

static ssize_t pages_to_scan_store(struct kobject *kobj,
                   struct kobj_attribute *attr,
                   const char *buf, size_t count)
{
    int err;
    unsigned long nr_pages;

    err = strict_strtoul(buf, 10, &nr_pages);
    if (err || nr_pages > UINT_MAX)
        return -EINVAL;

    ksm_thread_pages_to_scan = nr_pages;

    return count;
}
KSM_ATTR(pages_to_scan);

static ssize_t run_show(struct kobject *kobj, struct kobj_attribute *attr,
            char *buf)
{
    return sprintf(buf, "%u\n", ksm_run);
}

static ssize_t run_store(struct kobject *kobj, struct kobj_attribute *attr,
             const char *buf, size_t count)
{
    int err;
    unsigned long flags;

    err = strict_strtoul(buf, 10, &flags);
    if (err || flags > UINT_MAX)
        return -EINVAL;
    if (flags > KSM_RUN_UNMERGE)
        return -EINVAL;

    /*
     * KSM_RUN_MERGE sets ksmd running, and 0 stops it running.
     * KSM_RUN_UNMERGE stops it running and unmerges all rmap_items,
     * breaking COW to free the pages_shared (but leaves mm_slots
     * on the list for when ksmd may be set running again).
     */

    mutex_lock(&ksm_thread_mutex);
    if (ksm_run != flags) {
        ksm_run = flags;
        if (flags & KSM_RUN_UNMERGE) {
            int oom_score_adj;

            oom_score_adj = test_set_oom_score_adj(OOM_SCORE_ADJ_MAX);
            err = unmerge_and_remove_all_rmap_items();
            compare_swap_oom_score_adj(OOM_SCORE_ADJ_MAX,
                                oom_score_adj);
            if (err) {
                ksm_run = KSM_RUN_STOP;
                count = err;
            }
        }
    }
    mutex_unlock(&ksm_thread_mutex);

    if (flags & KSM_RUN_MERGE)
        wake_up_interruptible(&ksm_thread_wait);

    return count;
}
KSM_ATTR(run);

static ssize_t pages_shared_show(struct kobject *kobj,
                 struct kobj_attribute *attr, char *buf)
{
    return sprintf(buf, "%lu\n", ksm_pages_shared);
}
KSM_ATTR_RO(pages_shared);

static ssize_t pages_sharing_show(struct kobject *kobj,
                  struct kobj_attribute *attr, char *buf)
{
    return sprintf(buf, "%lu\n", ksm_pages_sharing);
}
KSM_ATTR_RO(pages_sharing);

static ssize_t pages_unshared_show(struct kobject *kobj,
                   struct kobj_attribute *attr, char *buf)
{
    return sprintf(buf, "%lu\n", ksm_pages_unshared);
}
KSM_ATTR_RO(pages_unshared);

static ssize_t pages_volatile_show(struct kobject *kobj,
                   struct kobj_attribute *attr, char *buf)
{
    long ksm_pages_volatile;

    ksm_pages_volatile = ksm_rmap_items - ksm_pages_shared
                - ksm_pages_sharing - ksm_pages_unshared;
    /*
     * It was not worth any locking to calculate that statistic,
     * but it might therefore sometimes be negative: conceal that.
     */
    if (ksm_pages_volatile < 0)
        ksm_pages_volatile = 0;
    return sprintf(buf, "%ld\n", ksm_pages_volatile);
}
KSM_ATTR_RO(pages_volatile);

static ssize_t full_scans_show(struct kobject *kobj,
                   struct kobj_attribute *attr, char *buf)
{
    return sprintf(buf, "%lu\n", ksm_scan.seqnr);
}
KSM_ATTR_RO(full_scans);

static struct attribute *ksm_attrs[] = {
    &sleep_millisecs_attr.attr,
    &pages_to_scan_attr.attr,
    &run_attr.attr,
    &pages_shared_attr.attr,
    &pages_sharing_attr.attr,
    &pages_unshared_attr.attr,
    &pages_volatile_attr.attr,
    &full_scans_attr.attr,
    NULL,
};

static struct attribute_group ksm_attr_group = {
    .attrs = ksm_attrs,
    .name = "ksm",
};
#endif /* CONFIG_SYSFS */

static int __init ksm_init(void)
{
    struct task_struct *ksm_thread;
    int err;

    err = ksm_slab_init();
    if (err)
        goto out;

    ksm_thread = kthread_run(ksm_scan_thread, NULL, "ksmd");
    if (IS_ERR(ksm_thread)) {
        printk(KERN_ERR "ksm: creating kthread failed\n");
        err = PTR_ERR(ksm_thread);
        goto out_free;
    }

#ifdef CONFIG_SYSFS
    err = sysfs_create_group(mm_kobj, &ksm_attr_group);
    if (err) {
        printk(KERN_ERR "ksm: register sysfs failed\n");
        kthread_stop(ksm_thread);
        goto out_free;
    }
#else
    ksm_run = KSM_RUN_MERGE;    /* no way for user to start it */

#endif /* CONFIG_SYSFS */

#ifdef CONFIG_MEMORY_HOTREMOVE
    /*
     * Choose a high priority since the callback takes ksm_thread_mutex:
     * later callbacks could only be taking locks which nest within that.
     */
    hotplug_memory_notifier(ksm_memory_callback, 100);
#endif
    return 0;

out_free:
    ksm_slab_free();
out:
    return err;
}
module_init(ksm_init)