FreeBSD/Linux Kernel Cross Reference
sys/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_read(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_read(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_read(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_read(anon_vma);
                                  goto out;
                          }
                  }
                  anon_vma_unlock_read(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_read(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_read(anon_vma);
                                  goto out;
                          }
                  }
                  anon_vma_unlock_read(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)

Cache object: a3d071b154f0ade2f66c70f06b4f24e0