/*

   * 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 <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_HEADS 1024
  static struct hlist_head *mm_slots_hash;
  
  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 int __init mm_slots_hash_init(void)
  {
  	mm_slots_hash = kzalloc(MM_SLOTS_HASH_HEADS * sizeof(struct hlist_head),
  				GFP_KERNEL);
  	if (!mm_slots_hash)
  		return -ENOMEM;
  	return 0;
  }
  
  static void __init mm_slots_hash_free(void)
  {
  	kfree(mm_slots_hash);
  }
  
  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[((unsigned long)mm / sizeof(struct mm_struct))
  				% MM_SLOTS_HASH_HEADS];
  	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[((unsigned long)mm / sizeof(struct mm_struct))
  				% MM_SLOTS_HASH_HEADS];
  	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;
  }

  static void hold_anon_vma(struct rmap_item *rmap_item,
  			  struct anon_vma *anon_vma)
  {
  	rmap_item->anon_vma = anon_vma;
  	atomic_inc(&anon_vma->ksm_refcount);
  }
  
  static void drop_anon_vma(struct rmap_item *rmap_item)
  {
  	struct anon_vma *anon_vma = rmap_item->anon_vma;
  
  	if (atomic_dec_and_lock(&anon_vma->ksm_refcount, &anon_vma->lock)) {
  		int empty = list_empty(&anon_vma->head);
  		spin_unlock(&anon_vma->lock);
  		if (empty)
  			anon_vma_free(anon_vma);
  	}
  }

  /*

   * 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 (!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 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.
  	 */
  	drop_anon_vma(rmap_item);

  	down_read(&mm->mmap_sem);

  	if (ksm_test_exit(mm))
  		goto out;

  	vma = find_vma(mm, addr);
  	if (!vma || vma->vm_start > addr)

  		goto out;

  	if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)

  		goto out;

  	break_ksm(vma, addr);

  out:

  	up_read(&mm->mmap_sem);
  }
  
  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);

  	if (ksm_test_exit(mm))
  		goto out;

  	vma = find_vma(mm, addr);
  	if (!vma || vma->vm_start > addr)
  		goto out;
  	if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
  		goto out;
  
  	page = follow_page(vma, addr, FOLL_GET);
  	if (!page)
  		goto out;
  	if (PageAnon(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--;
  		drop_anon_vma(rmap_item);
  		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--;

  		drop_anon_vma(rmap_item);

  		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, KM_USER0);
  	checksum = jhash2(addr, PAGE_SIZE / 4, 17);
  	kunmap_atomic(addr, KM_USER0);
  	return checksum;
  }
  
  static int memcmp_pages(struct page *page1, struct page *page2)
  {
  	char *addr1, *addr2;
  	int ret;
  
  	addr1 = kmap_atomic(page1, KM_USER0);
  	addr2 = kmap_atomic(page2, KM_USER1);
  	ret = memcmp(addr1, addr2, PAGE_SIZE);
  	kunmap_atomic(addr2, KM_USER1);
  	kunmap_atomic(addr1, KM_USER0);
  	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;
  
  	addr = page_address_in_vma(page, vma);
  	if (addr == -EFAULT)
  		goto out;
  
  	ptep = page_check_address(page, mm, addr, &ptl, 0);
  	if (!ptep)
  		goto out;
  
  	if (pte_write(*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 doesnt
  		 * 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;
  		}
  		entry = pte_wrprotect(entry);
  		set_pte_at_notify(mm, addr, ptep, entry);
  	}
  	*orig_pte = *ptep;
  	err = 0;
  
  out_unlock:
  	pte_unmap_unlock(ptep, ptl);
  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;
  	pgd_t *pgd;
  	pud_t *pud;
  	pmd_t *pmd;
  	pte_t *ptep;
  	spinlock_t *ptl;
  	unsigned long addr;

  	int err = -EFAULT;

  	addr = page_address_in_vma(page, vma);

  	if (addr == -EFAULT)
  		goto out;
  
  	pgd = pgd_offset(mm, addr);
  	if (!pgd_present(*pgd))
  		goto out;
  
  	pud = pud_offset(pgd, addr);
  	if (!pud_present(*pud))
  		goto out;
  
  	pmd = pmd_offset(pud, addr);
  	if (!pmd_present(*pmd))
  		goto out;
  
  	ptep = pte_offset_map_lock(mm, pmd, addr, &ptl);
  	if (!pte_same(*ptep, orig_pte)) {
  		pte_unmap_unlock(ptep, ptl);
  		goto out;
  	}

  	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);
  	put_page(page);

  
  	pte_unmap_unlock(ptep, ptl);
  	err = 0;
  out:
  	return err;
  }
  
  /*
   * 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 (!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 */
  	hold_anon_vma(rmap_item, 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 (!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) {
  		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);
  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 (*page && PageAnon(*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;
  			}
  			if (*page)
  				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 *page;
  
  	while (scan_npages--) {
  		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_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);
  
  		if (ksmd_should_run()) {

  			schedule_timeout_interruptible(
  				msecs_to_jiffies(ksm_thread_sleep_millisecs));
  		} else {
  			wait_event_interruptible(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_RESERVED  | VM_HUGETLB | VM_INSERTPAGE |

  				 VM_NONLINEAR | VM_MIXEDMAP | VM_SAO))

  			return 0;		/* just ignore the advice */

  		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;
  
  	unlock_page(page);	/* any racers will COW it, not modify it */
  
  	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 (page_evictable(new_page, vma))
  			lru_cache_add_lru(new_page, LRU_ACTIVE_ANON);
  		else
  			add_page_to_unevictable_list(new_page);
  	}
  
  	page_cache_release(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;

  		spin_lock(&anon_vma->lock);

  		list_for_each_entry(vmac, &anon_vma->head, same_anon_vma) {
  			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;
  		}
  		spin_unlock(&anon_vma->lock);

  		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;

  		spin_lock(&anon_vma->lock);

  		list_for_each_entry(vmac, &anon_vma->head, same_anon_vma) {
  			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)) {
  				spin_unlock(&anon_vma->lock);
  				goto out;
  			}
  		}
  		spin_unlock(&anon_vma->lock);

  	}

  	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;
  
  		spin_lock(&anon_vma->lock);

  		list_for_each_entry(vmac, &anon_vma->head, same_anon_vma) {
  			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) {
  				spin_unlock(&anon_vma->lock);
  				goto out;
  			}
  		}
  		spin_unlock(&anon_vma->lock);
  	}
  	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(&ksm_thread_mutex);
  		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
  ", 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
  ", 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
  ", 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) {

  			current->flags |= PF_OOM_ORIGIN;

  			err = unmerge_and_remove_all_rmap_items();

  			current->flags &= ~PF_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
  ", 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
  ", 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
  ", 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
  ", 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
  ", 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;
  
  	err = mm_slots_hash_init();
  	if (err)
  		goto out_free1;
  
  	ksm_thread = kthread_run(ksm_scan_thread, NULL, "ksmd");
  	if (IS_ERR(ksm_thread)) {
  		printk(KERN_ERR "ksm: creating kthread failed
  ");
  		err = PTR_ERR(ksm_thread);
  		goto out_free2;
  	}

  #ifdef CONFIG_SYSFS

  	err = sysfs_create_group(mm_kobj, &ksm_attr_group);
  	if (err) {
  		printk(KERN_ERR "ksm: register sysfs failed
  ");

  		kthread_stop(ksm_thread);
  		goto out_free2;

  	}

  #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_free2:
  	mm_slots_hash_free();
  out_free1:
  	ksm_slab_free();
  out:
  	return err;

  }

  module_init(ksm_init)