Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/s390/linux

[firefly-linux-kernel-4.4.55.git] / mm / ksm.c

/*
 * 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 - ksm information per mm that is being scanned
 * @link: link to the mm_slots hash list
 * @mm_list: link into the mm_slots list, rooted in ksm_mm_head
 * @rmap_list: head for this mm_slot's singly-linked list of rmap_items
 * @mm: the mm that this information is valid for
 */
struct mm_slot {
        struct hlist_node link;
        struct list_head mm_list;
        struct rmap_item *rmap_list;
        struct mm_struct *mm;
};

/**
 * struct ksm_scan - cursor for scanning
 * @mm_slot: the current mm_slot we are scanning
 * @address: the next address inside that to be scanned
 * @rmap_list: link to the next rmap to be scanned in the rmap_list
 * @seqnr: count of completed full scans (needed when removing unstable node)
 *
 * There is only the one ksm_scan instance of this cursor structure.
 */
struct ksm_scan {
        struct mm_slot *mm_slot;
        unsigned long address;
        struct rmap_item **rmap_list;
        unsigned long seqnr;
};

/**
 * struct stable_node - node of the stable rbtree
 * @node: rb node of this ksm page in the stable tree
 * @hlist: hlist head of rmap_items using this ksm page
 * @kpfn: page frame number of this ksm page
 */
struct stable_node {
        struct rb_node node;
        struct hlist_head hlist;
        unsigned long kpfn;
};

/**
 * struct rmap_item - reverse mapping item for virtual addresses
 * @rmap_list: next rmap_item in mm_slot's singly-linked rmap_list
 * @anon_vma: pointer to anon_vma for this mm,address, when in stable tree
 * @mm: the memory structure this rmap_item is pointing into
 * @address: the virtual address this rmap_item tracks (+ flags in low bits)
 * @oldchecksum: previous checksum of the page at that virtual address
 * @node: rb node of this rmap_item in the unstable tree
 * @head: pointer to stable_node heading this list in the stable tree
 * @hlist: link into hlist of rmap_items hanging off that stable_node
 */
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;
}

/**
 * replace_page - replace page in vma by new ksm page
 * @vma:      vma that holds the pte pointing to page
 * @page:     the page we are replacing by kpage
 * @kpage:    the ksm page we replace page by
 * @orig_pte: the original value of the pte
 *
 * Returns 0 on success, -EFAULT on failure.
 */
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;
        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;

        pmd = mm_find_pmd(mm, addr);
        if (!pmd)
                goto out;
        BUG_ON(pmd_trans_huge(*pmd));

        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;
}

/**
 * ksm_do_scan  - the ksm scanner main worker function.
 * @scan_npages - number of pages we want to scan before we return.
 */
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) {
                        set_current_oom_origin();
                        err = unmerge_and_remove_all_rmap_items();
                        clear_current_oom_origin();
                        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)