// SPDX-License-Identifier: GPL-2.0-only

  /*

   * 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

   */
  
  #include <linux/errno.h>

  #include <linux/mm.h>
  #include <linux/fs.h>

  #include <linux/mman.h>

  #include <linux/sched.h>

  #include <linux/sched/mm.h>

  #include <linux/sched/coredump.h>

  #include <linux/rwsem.h>
  #include <linux/pagemap.h>
  #include <linux/rmap.h>
  #include <linux/spinlock.h>

  #include <linux/xxhash.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/hashtable.h>

  #include <linux/freezer.h>

  #include <linux/oom.h>

  #include <linux/numa.h>

  #include <asm/tlbflush.h>

  #include "internal.h"

  #ifdef CONFIG_NUMA
  #define NUMA(x)		(x)
  #define DO_NUMA(x)	do { (x); } while (0)
  #else
  #define NUMA(x)		(0)
  #define DO_NUMA(x)	do { } while (0)
  #endif

  /**
   * DOC: Overview
   *

   * 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.
   *

   * The stable tree node includes information required for reverse
   * mapping from a KSM page to virtual addresses that map this page.
   *
   * In order to avoid large latencies of the rmap walks on KSM pages,
   * KSM maintains two types of nodes in the stable tree:
   *
   * * the regular nodes that keep the reverse mapping structures in a
   *   linked list
   * * the "chains" that link nodes ("dups") that represent the same
   *   write protected memory content, but each "dup" corresponds to a
   *   different KSM page copy of that content
   *
   * Internally, the regular nodes, "dups" and "chains" are represented
   * using the same :c:type:`struct stable_node` structure.
   *

   * 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.)

   *
   * If the merge_across_nodes tunable is unset, then KSM maintains multiple
   * stable trees and multiple unstable trees: one of each for each NUMA node.

   */
  
  /**
   * 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

   * @head: (overlaying parent) &migrate_nodes indicates temporarily on that list

   * @hlist_dup: linked into the stable_node->hlist with a stable_node chain

   * @list: linked into migrate_nodes, pending placement in the proper node tree

   * @hlist: hlist head of rmap_items using this ksm page

   * @kpfn: page frame number of this ksm page (perhaps temporarily on wrong nid)

   * @chain_prune_time: time of the last full garbage collection
   * @rmap_hlist_len: number of rmap_item entries in hlist or STABLE_NODE_CHAIN

   * @nid: NUMA node id of stable tree in which linked (may not match kpfn)

   */
  struct stable_node {

  	union {
  		struct rb_node node;	/* when node of stable tree */
  		struct {		/* when listed for migration */
  			struct list_head *head;

  			struct {
  				struct hlist_node hlist_dup;
  				struct list_head list;
  			};

  		};
  	};

  	struct hlist_head hlist;

  	union {
  		unsigned long kpfn;
  		unsigned long chain_prune_time;
  	};
  	/*
  	 * STABLE_NODE_CHAIN can be any negative number in
  	 * rmap_hlist_len negative range, but better not -1 to be able
  	 * to reliably detect underflows.
  	 */
  #define STABLE_NODE_CHAIN -1024
  	int rmap_hlist_len;

  #ifdef CONFIG_NUMA
  	int nid;
  #endif

  };
  
  /**

   * 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

   * @nid: NUMA node id of unstable tree in which linked (may not match page)

   * @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;

  	union {
  		struct anon_vma *anon_vma;	/* when stable */
  #ifdef CONFIG_NUMA
  		int nid;		/* when node of unstable tree */
  #endif
  	};

  	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 */

  #define KSM_FLAG_MASK	(SEQNR_MASK|UNSTABLE_FLAG|STABLE_FLAG)
  				/* to mask all the flags */

  
  /* The stable and unstable tree heads */

  static struct rb_root one_stable_tree[1] = { RB_ROOT };
  static struct rb_root one_unstable_tree[1] = { RB_ROOT };
  static struct rb_root *root_stable_tree = one_stable_tree;
  static struct rb_root *root_unstable_tree = one_unstable_tree;

  /* Recently migrated nodes of stable tree, pending proper placement */
  static LIST_HEAD(migrate_nodes);

  #define STABLE_NODE_DUP_HEAD ((struct list_head *)&migrate_nodes.prev)

  #define MM_SLOTS_HASH_BITS 10
  static DEFINE_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);

  
  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;

  /* The number of stable_node chains */
  static unsigned long ksm_stable_node_chains;
  
  /* The number of stable_node dups linked to the stable_node chains */
  static unsigned long ksm_stable_node_dups;
  
  /* Delay in pruning stale stable_node_dups in the stable_node_chains */
  static int ksm_stable_node_chains_prune_millisecs = 2000;
  
  /* Maximum number of page slots sharing a stable node */
  static int ksm_max_page_sharing = 256;

  /* 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;

  /* Checksum of an empty (zeroed) page */
  static unsigned int zero_checksum __read_mostly;
  
  /* Whether to merge empty (zeroed) pages with actual zero pages */
  static bool ksm_use_zero_pages __read_mostly;

  #ifdef CONFIG_NUMA

  /* Zeroed when merging across nodes is not allowed */
  static unsigned int ksm_merge_across_nodes = 1;

  static int ksm_nr_node_ids = 1;

  #else
  #define ksm_merge_across_nodes	1U

  #define ksm_nr_node_ids		1

  #endif

  #define KSM_RUN_STOP	0
  #define KSM_RUN_MERGE	1
  #define KSM_RUN_UNMERGE	2

  #define KSM_RUN_OFFLINE	4
  static unsigned long ksm_run = KSM_RUN_STOP;
  static void wait_while_offlining(void);

  
  static DECLARE_WAIT_QUEUE_HEAD(ksm_thread_wait);

  static DECLARE_WAIT_QUEUE_HEAD(ksm_iter_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 __always_inline bool is_stable_node_chain(struct stable_node *chain)
  {
  	return chain->rmap_hlist_len == STABLE_NODE_CHAIN;
  }
  
  static __always_inline bool is_stable_node_dup(struct stable_node *dup)
  {
  	return dup->head == STABLE_NODE_DUP_HEAD;
  }
  
  static inline void stable_node_chain_add_dup(struct stable_node *dup,
  					     struct stable_node *chain)
  {
  	VM_BUG_ON(is_stable_node_dup(dup));
  	dup->head = STABLE_NODE_DUP_HEAD;
  	VM_BUG_ON(!is_stable_node_chain(chain));
  	hlist_add_head(&dup->hlist_dup, &chain->hlist);
  	ksm_stable_node_dups++;
  }
  
  static inline void __stable_node_dup_del(struct stable_node *dup)
  {

  	VM_BUG_ON(!is_stable_node_dup(dup));

  	hlist_del(&dup->hlist_dup);
  	ksm_stable_node_dups--;
  }
  
  static inline void stable_node_dup_del(struct stable_node *dup)
  {
  	VM_BUG_ON(is_stable_node_chain(dup));
  	if (is_stable_node_dup(dup))
  		__stable_node_dup_del(dup);
  	else
  		rb_erase(&dup->node, root_stable_tree + NUMA(dup->nid));
  #ifdef CONFIG_DEBUG_VM
  	dup->head = NULL;
  #endif
  }

  static inline struct rmap_item *alloc_rmap_item(void)
  {

  	struct rmap_item *rmap_item;

  	rmap_item = kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL |
  						__GFP_NORETRY | __GFP_NOWARN);

  	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)
  {

  	/*
  	 * The allocation can take too long with GFP_KERNEL when memory is under
  	 * pressure, which may lead to hung task warnings.  Adding __GFP_HIGH
  	 * grants access to memory reserves, helping to avoid this problem.
  	 */
  	return kmem_cache_alloc(stable_node_cache, GFP_KERNEL | __GFP_HIGH);

  }
  
  static inline void free_stable_node(struct stable_node *stable_node)
  {

  	VM_BUG_ON(stable_node->rmap_hlist_len &&
  		  !is_stable_node_chain(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 *slot;

  	hash_for_each_possible(mm_slots_hash, slot, link, (unsigned long)mm)

  		if (slot->mm == mm)
  			return slot;

  	return NULL;
  }
  
  static void insert_to_mm_slots_hash(struct mm_struct *mm,
  				    struct mm_slot *mm_slot)
  {

  	mm_slot->mm = mm;

  	hash_add(mm_slots_hash, &mm_slot->link, (unsigned long)mm);

  }

  /*

   * 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(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.

   *
   * FAULT_FLAG/FOLL_REMOTE are because we do this outside the context
   * of the process that owns 'vma'.  We also do not want to enforce
   * protection keys here anyway.

   */

  static int break_ksm(struct vm_area_struct *vma, unsigned long addr)

  {
  	struct page *page;

  	vm_fault_t ret = 0;

  
  	do {
  		cond_resched();

  		page = follow_page(vma, addr,
  				FOLL_GET | FOLL_MIGRATION | FOLL_REMOTE);

  		if (IS_ERR_OR_NULL(page))

  			break;
  		if (PageKsm(page))

  			ret = handle_mm_fault(vma, addr,
  					FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE);

  		else
  			ret = VM_FAULT_WRITE;
  		put_page(page);

  	} while (!(ret & (VM_FAULT_WRITE | VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | 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 *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)) {

  		flush_anon_page(vma, page, addr);
  		flush_dcache_page(page);
  	} else {
  		put_page(page);

  out:
  		page = NULL;

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

  /*
   * This helper is used for getting right index into array of tree roots.
   * When merge_across_nodes knob is set to 1, there are only two rb-trees for
   * stable and unstable pages from all nodes with roots in index 0. Otherwise,
   * every node has its own stable and unstable tree.
   */
  static inline int get_kpfn_nid(unsigned long kpfn)
  {

  	return ksm_merge_across_nodes ? 0 : NUMA(pfn_to_nid(kpfn));

  }

  static struct stable_node *alloc_stable_node_chain(struct stable_node *dup,
  						   struct rb_root *root)
  {
  	struct stable_node *chain = alloc_stable_node();
  	VM_BUG_ON(is_stable_node_chain(dup));
  	if (likely(chain)) {
  		INIT_HLIST_HEAD(&chain->hlist);
  		chain->chain_prune_time = jiffies;
  		chain->rmap_hlist_len = STABLE_NODE_CHAIN;
  #if defined (CONFIG_DEBUG_VM) && defined(CONFIG_NUMA)

  		chain->nid = NUMA_NO_NODE; /* debug */

  #endif
  		ksm_stable_node_chains++;
  
  		/*
  		 * Put the stable node chain in the first dimension of
  		 * the stable tree and at the same time remove the old
  		 * stable node.
  		 */
  		rb_replace_node(&dup->node, &chain->node, root);
  
  		/*
  		 * Move the old stable node to the second dimension
  		 * queued in the hlist_dup. The invariant is that all
  		 * dup stable_nodes in the chain->hlist point to pages
  		 * that are wrprotected and have the exact same
  		 * content.
  		 */
  		stable_node_chain_add_dup(dup, chain);
  	}
  	return chain;
  }
  
  static inline void free_stable_node_chain(struct stable_node *chain,
  					  struct rb_root *root)
  {
  	rb_erase(&chain->node, root);
  	free_stable_node(chain);
  	ksm_stable_node_chains--;
  }

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

  	/* check it's not STABLE_NODE_CHAIN or negative */
  	BUG_ON(stable_node->rmap_hlist_len < 0);

  	hlist_for_each_entry(rmap_item, &stable_node->hlist, hlist) {

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

  		VM_BUG_ON(stable_node->rmap_hlist_len <= 0);
  		stable_node->rmap_hlist_len--;

  		put_anon_vma(rmap_item->anon_vma);

  		rmap_item->address &= PAGE_MASK;
  		cond_resched();
  	}

  	/*
  	 * We need the second aligned pointer of the migrate_nodes
  	 * list_head to stay clear from the rb_parent_color union
  	 * (aligned and different than any node) and also different
  	 * from &migrate_nodes. This will verify that future list.h changes

  	 * don't break STABLE_NODE_DUP_HEAD. Only recent gcc can handle it.

  	 */

  #if defined(GCC_VERSION) && GCC_VERSION >= 40903

  	BUILD_BUG_ON(STABLE_NODE_DUP_HEAD <= &migrate_nodes);
  	BUILD_BUG_ON(STABLE_NODE_DUP_HEAD >= &migrate_nodes + 1);
  #endif

  	if (stable_node->head == &migrate_nodes)
  		list_del(&stable_node->list);
  	else

  		stable_node_dup_del(stable_node);

  	free_stable_node(stable_node);
  }

  enum get_ksm_page_flags {
  	GET_KSM_PAGE_NOLOCK,
  	GET_KSM_PAGE_LOCK,
  	GET_KSM_PAGE_TRYLOCK
  };

  /*
   * 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.

   * But beware, the stable node's page might be being migrated.

   *
   * 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.

   * 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,
  				 enum get_ksm_page_flags flags)

  {
  	struct page *page;
  	void *expected_mapping;

  	unsigned long kpfn;

  	expected_mapping = (void *)((unsigned long)stable_node |
  					PAGE_MAPPING_KSM);

  again:

  	kpfn = READ_ONCE(stable_node->kpfn); /* Address dependency. */

  	page = pfn_to_page(kpfn);

  	if (READ_ONCE(page->mapping) != expected_mapping)

  		goto stale;

  
  	/*
  	 * We cannot do anything with the page while its refcount is 0.
  	 * Usually 0 means free, or tail of a higher-order page: in which
  	 * case this node is no longer referenced, and should be freed;

  	 * however, it might mean that the page is under page_ref_freeze().

  	 * The __remove_mapping() case is easy, again the node is now stale;

  	 * the same is in reuse_ksm_page() case; but if page is swapcache
  	 * in migrate_page_move_mapping(), it might still be our page,
  	 * in which case it's essential to keep the node.

  	 */
  	while (!get_page_unless_zero(page)) {
  		/*
  		 * Another check for page->mapping != expected_mapping would
  		 * work here too.  We have chosen the !PageSwapCache test to
  		 * optimize the common case, when the page is or is about to
  		 * be freed: PageSwapCache is cleared (under spin_lock_irq)

  		 * in the ref_freeze section of __remove_mapping(); but Anon

  		 * page->mapping reset to NULL later, in free_pages_prepare().
  		 */
  		if (!PageSwapCache(page))
  			goto stale;
  		cpu_relax();
  	}

  	if (READ_ONCE(page->mapping) != expected_mapping) {

  		put_page(page);
  		goto stale;
  	}

  	if (flags == GET_KSM_PAGE_TRYLOCK) {
  		if (!trylock_page(page)) {
  			put_page(page);
  			return ERR_PTR(-EBUSY);
  		}
  	} else if (flags == GET_KSM_PAGE_LOCK)

  		lock_page(page);

  
  	if (flags != GET_KSM_PAGE_NOLOCK) {

  		if (READ_ONCE(page->mapping) != expected_mapping) {

  			unlock_page(page);
  			put_page(page);
  			goto stale;
  		}
  	}

  	return page;

  stale:

  	/*
  	 * We come here from above when page->mapping or !PageSwapCache
  	 * suggests that the node is stale; but it might be under migration.
  	 * We need smp_rmb(), matching the smp_wmb() in ksm_migrate_page(),
  	 * before checking whether node->kpfn has been changed.
  	 */
  	smp_rmb();

  	if (READ_ONCE(stable_node->kpfn) != kpfn)

  		goto again;

  	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, GET_KSM_PAGE_LOCK);

  		if (!page)
  			goto out;

  		hlist_del(&rmap_item->hlist);

  		unlock_page(page);
  		put_page(page);

  		if (!hlist_empty(&stable_node->hlist))

  			ksm_pages_sharing--;
  		else

  			ksm_pages_shared--;

  		VM_BUG_ON(stable_node->rmap_hlist_len <= 0);
  		stable_node->rmap_hlist_len--;

  		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 + NUMA(rmap_item->nid));

  		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 rmap_items from stable tree 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;

  }

  static inline struct stable_node *page_stable_node(struct page *page)
  {
  	return PageKsm(page) ? page_rmapping(page) : NULL;
  }
  
  static inline void set_page_stable_node(struct page *page,
  					struct stable_node *stable_node)
  {
  	page->mapping = (void *)((unsigned long)stable_node | PAGE_MAPPING_KSM);
  }

  #ifdef CONFIG_SYSFS
  /*
   * Only called through the sysfs control interface:
   */

  static int remove_stable_node(struct stable_node *stable_node)
  {
  	struct page *page;
  	int err;

  	page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK);

  	if (!page) {
  		/*
  		 * get_ksm_page did remove_node_from_stable_tree itself.
  		 */
  		return 0;
  	}

  	if (WARN_ON_ONCE(page_mapped(page))) {
  		/*
  		 * This should not happen: but if it does, just refuse to let
  		 * merge_across_nodes be switched - there is no need to panic.
  		 */

  		err = -EBUSY;

  	} else {

  		/*

  		 * The stable node did not yet appear stale to get_ksm_page(),
  		 * since that allows for an unmapped ksm page to be recognized
  		 * right up until it is freed; but the node is safe to remove.

  		 * This page might be in a pagevec waiting to be freed,
  		 * or it might be PageSwapCache (perhaps under writeback),
  		 * or it might have been removed from swapcache a moment ago.
  		 */
  		set_page_stable_node(page, NULL);
  		remove_node_from_stable_tree(stable_node);
  		err = 0;
  	}
  
  	unlock_page(page);
  	put_page(page);
  	return err;
  }

  static int remove_stable_node_chain(struct stable_node *stable_node,
  				    struct rb_root *root)
  {
  	struct stable_node *dup;
  	struct hlist_node *hlist_safe;
  
  	if (!is_stable_node_chain(stable_node)) {
  		VM_BUG_ON(is_stable_node_dup(stable_node));
  		if (remove_stable_node(stable_node))
  			return true;
  		else
  			return false;
  	}
  
  	hlist_for_each_entry_safe(dup, hlist_safe,
  				  &stable_node->hlist, hlist_dup) {
  		VM_BUG_ON(!is_stable_node_dup(dup));
  		if (remove_stable_node(dup))
  			return true;
  	}
  	BUG_ON(!hlist_empty(&stable_node->hlist));
  	free_stable_node_chain(stable_node, root);
  	return false;
  }

  static int remove_all_stable_nodes(void)
  {

  	struct stable_node *stable_node, *next;

  	int nid;
  	int err = 0;

  	for (nid = 0; nid < ksm_nr_node_ids; nid++) {

  		while (root_stable_tree[nid].rb_node) {
  			stable_node = rb_entry(root_stable_tree[nid].rb_node,
  						struct stable_node, node);

  			if (remove_stable_node_chain(stable_node,
  						     root_stable_tree + nid)) {

  				err = -EBUSY;
  				break;	/* proceed to next nid */
  			}
  			cond_resched();
  		}
  	}

  	list_for_each_entry_safe(stable_node, next, &migrate_nodes, list) {

  		if (remove_stable_node(stable_node))
  			err = -EBUSY;
  		cond_resched();
  	}

  	return err;
  }

  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);

  		up_read(&mm->mmap_sem);

  
  		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)) {

  			hash_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);

  			mmdrop(mm);

  		} else

  			spin_unlock(&ksm_mmlist_lock);

  	}

  	/* Clean up stable nodes, but don't worry if some are still busy */
  	remove_all_stable_nodes();

  	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 = xxhash(addr, PAGE_SIZE, 0);

  	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;

  	struct page_vma_mapped_walk pvmw = {
  		.page = page,
  		.vma = vma,
  	};

  	int swapped;
  	int err = -EFAULT;

  	struct mmu_notifier_range range;

  	pvmw.address = page_address_in_vma(page, vma);
  	if (pvmw.address == -EFAULT)

  		goto out;

  	BUG_ON(PageTransCompound(page));

  	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, mm,

  				pvmw.address,

  				pvmw.address + PAGE_SIZE);
  	mmu_notifier_invalidate_range_start(&range);

  	if (!page_vma_mapped_walk(&pvmw))

  		goto out_mn;

  	if (WARN_ONCE(!pvmw.pte, "Unexpected PMD mapping?"))
  		goto out_unlock;

  	if (pte_write(*pvmw.pte) || pte_dirty(*pvmw.pte) ||

  	    (pte_protnone(*pvmw.pte) && pte_savedwrite(*pvmw.pte)) ||
  						mm_tlb_flush_pending(mm)) {

  		pte_t entry;
  
  		swapped = PageSwapCache(page);

  		flush_cache_page(vma, pvmw.address, 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.

  		 *
  		 * No need to notify as we are downgrading page table to read
  		 * only not changing it to point to a new page.
  		 *

  		 * See Documentation/vm/mmu_notifier.rst

  		 */

  		entry = ptep_clear_flush(vma, pvmw.address, pvmw.pte);

  		/*
  		 * 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, pvmw.address, pvmw.pte, entry);

  			goto out_unlock;
  		}

  		if (pte_dirty(entry))
  			set_page_dirty(page);

  
  		if (pte_protnone(entry))
  			entry = pte_mkclean(pte_clear_savedwrite(entry));
  		else
  			entry = pte_mkclean(pte_wrprotect(entry));

  		set_pte_at_notify(mm, pvmw.address, pvmw.pte, entry);

  	}

  	*orig_pte = *pvmw.pte;

  	err = 0;
  
  out_unlock:

  	page_vma_mapped_walk_done(&pvmw);

  out_mn:

  	mmu_notifier_invalidate_range_end(&range);

  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;

  	pte_t newpte;

  	spinlock_t *ptl;
  	unsigned long addr;

  	int err = -EFAULT;

  	struct mmu_notifier_range range;

  	addr = page_address_in_vma(page, vma);

  	if (addr == -EFAULT)
  		goto out;

  	pmd = mm_find_pmd(mm, addr);
  	if (!pmd)

  		goto out;

  	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, mm, addr,

  				addr + PAGE_SIZE);

  	mmu_notifier_invalidate_range_start(&range);

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

  		goto out_mn;

  	}

  	/*
  	 * No need to check ksm_use_zero_pages here: we can only have a
  	 * zero_page here if ksm_use_zero_pages was enabled alreaady.
  	 */
  	if (!is_zero_pfn(page_to_pfn(kpage))) {
  		get_page(kpage);
  		page_add_anon_rmap(kpage, vma, addr, false);
  		newpte = mk_pte(kpage, vma->vm_page_prot);
  	} else {
  		newpte = pte_mkspecial(pfn_pte(page_to_pfn(kpage),
  					       vma->vm_page_prot));

  		/*
  		 * We're replacing an anonymous page with a zero page, which is
  		 * not anonymous. We need to do proper accounting otherwise we
  		 * will get wrong values in /proc, and a BUG message in dmesg
  		 * when tearing down the mm.
  		 */
  		dec_mm_counter(mm, MM_ANONPAGES);

  	}

  
  	flush_cache_page(vma, addr, pte_pfn(*ptep));

  	/*
  	 * No need to notify as we are replacing a read only page with another
  	 * read only page with the same content.
  	 *

  	 * See Documentation/vm/mmu_notifier.rst

  	 */
  	ptep_clear_flush(vma, addr, ptep);

  	set_pte_at_notify(mm, addr, ptep, newpte);

  	page_remove_rmap(page, false);

  	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(&range);

  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 (!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 (PageTransCompound(page)) {

  		if (split_huge_page(page))

  			goto out_unlock;
  	}

  	/*
  	 * 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);

  			/*
  			 * Page reclaim just frees a clean page with no dirty
  			 * ptes: make sure that the ksm page would be swapped.
  			 */
  			if (!PageDirty(page))
  				SetPageDirty(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 */
  		}
  	}

  out_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);

  	vma = find_mergeable_vma(mm, rmap_item->address);
  	if (!vma)

  		goto out;

  	err = try_to_merge_one_page(vma, page, kpage);

  	if (err)
  		goto out;

  	/* Unstable nid is in union with stable anon_vma: remove first */
  	remove_rmap_item_from_tree(rmap_item);

  	/* 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;

  }

  static __always_inline
  bool __is_page_sharing_candidate(struct stable_node *stable_node, int offset)
  {
  	VM_BUG_ON(stable_node->rmap_hlist_len < 0);
  	/*
  	 * Check that at least one mapping still exists, otherwise
  	 * there's no much point to merge and share with this
  	 * stable_node, as the underlying tree_page of the other
  	 * sharer is going to be freed soon.
  	 */
  	return stable_node->rmap_hlist_len &&
  		stable_node->rmap_hlist_len + offset < ksm_max_page_sharing;
  }
  
  static __always_inline
  bool is_page_sharing_candidate(struct stable_node *stable_node)
  {
  	return __is_page_sharing_candidate(stable_node, 0);
  }

  static struct page *stable_node_dup(struct stable_node **_stable_node_dup,
  				    struct stable_node **_stable_node,
  				    struct rb_root *root,
  				    bool prune_stale_stable_nodes)

  {

  	struct stable_node *dup, *found = NULL, *stable_node = *_stable_node;

  	struct hlist_node *hlist_safe;

  	struct page *_tree_page, *tree_page = NULL;

  	int nr = 0;
  	int found_rmap_hlist_len;
  
  	if (!prune_stale_stable_nodes ||
  	    time_before(jiffies, stable_node->chain_prune_time +
  			msecs_to_jiffies(
  				ksm_stable_node_chains_prune_millisecs)))
  		prune_stale_stable_nodes = false;
  	else
  		stable_node->chain_prune_time = jiffies;
  
  	hlist_for_each_entry_safe(dup, hlist_safe,
  				  &stable_node->hlist, hlist_dup) {
  		cond_resched();
  		/*
  		 * We must walk all stable_node_dup to prune the stale
  		 * stable nodes during lookup.
  		 *
  		 * get_ksm_page can drop the nodes from the
  		 * stable_node->hlist if they point to freed pages
  		 * (that's why we do a _safe walk). The "dup"
  		 * stable_node parameter itself will be freed from
  		 * under us if it returns NULL.
  		 */

  		_tree_page = get_ksm_page(dup, GET_KSM_PAGE_NOLOCK);

  		if (!_tree_page)
  			continue;
  		nr += 1;
  		if (is_page_sharing_candidate(dup)) {
  			if (!found ||
  			    dup->rmap_hlist_len > found_rmap_hlist_len) {
  				if (found)

  					put_page(tree_page);

  				found = dup;
  				found_rmap_hlist_len = found->rmap_hlist_len;

  				tree_page = _tree_page;

  				/* skip put_page for found dup */

  				if (!prune_stale_stable_nodes)
  					break;

  				continue;
  			}
  		}
  		put_page(_tree_page);
  	}

  	if (found) {
  		/*
  		 * nr is counting all dups in the chain only if
  		 * prune_stale_stable_nodes is true, otherwise we may
  		 * break the loop at nr == 1 even if there are
  		 * multiple entries.
  		 */
  		if (prune_stale_stable_nodes && nr == 1) {

  			/*
  			 * If there's not just one entry it would
  			 * corrupt memory, better BUG_ON. In KSM
  			 * context with no lock held it's not even
  			 * fatal.
  			 */
  			BUG_ON(stable_node->hlist.first->next);
  
  			/*
  			 * There's just one entry and it is below the
  			 * deduplication limit so drop the chain.
  			 */
  			rb_replace_node(&stable_node->node, &found->node,
  					root);
  			free_stable_node(stable_node);
  			ksm_stable_node_chains--;
  			ksm_stable_node_dups--;

  			/*

  			 * NOTE: the caller depends on the stable_node
  			 * to be equal to stable_node_dup if the chain
  			 * was collapsed.

  			 */

  			*_stable_node = found;
  			/*
  			 * Just for robustneess as stable_node is
  			 * otherwise left as a stable pointer, the
  			 * compiler shall optimize it away at build
  			 * time.
  			 */
  			stable_node = NULL;

  		} else if (stable_node->hlist.first != &found->hlist_dup &&
  			   __is_page_sharing_candidate(found, 1)) {

  			/*

  			 * If the found stable_node dup can accept one
  			 * more future merge (in addition to the one
  			 * that is underway) and is not at the head of
  			 * the chain, put it there so next search will
  			 * be quicker in the !prune_stale_stable_nodes
  			 * case.
  			 *
  			 * NOTE: it would be inaccurate to use nr > 1
  			 * instead of checking the hlist.first pointer
  			 * directly, because in the
  			 * prune_stale_stable_nodes case "nr" isn't
  			 * the position of the found dup in the chain,
  			 * but the total number of dups in the chain.

  			 */
  			hlist_del(&found->hlist_dup);
  			hlist_add_head(&found->hlist_dup,
  				       &stable_node->hlist);
  		}
  	}

  	*_stable_node_dup = found;
  	return tree_page;

  }
  
  static struct stable_node *stable_node_dup_any(struct stable_node *stable_node,
  					       struct rb_root *root)
  {
  	if (!is_stable_node_chain(stable_node))
  		return stable_node;
  	if (hlist_empty(&stable_node->hlist)) {
  		free_stable_node_chain(stable_node, root);
  		return NULL;
  	}
  	return hlist_entry(stable_node->hlist.first,
  			   typeof(*stable_node), hlist_dup);
  }

  /*
   * Like for get_ksm_page, this function can free the *_stable_node and
   * *_stable_node_dup if the returned tree_page is NULL.
   *
   * It can also free and overwrite *_stable_node with the found
   * stable_node_dup if the chain is collapsed (in which case
   * *_stable_node will be equal to *_stable_node_dup like if the chain
   * never existed). It's up to the caller to verify tree_page is not
   * NULL before dereferencing *_stable_node or *_stable_node_dup.
   *
   * *_stable_node_dup is really a second output parameter of this
   * function and will be overwritten in all cases, the caller doesn't
   * need to initialize it.
   */
  static struct page *__stable_node_chain(struct stable_node **_stable_node_dup,
  					struct stable_node **_stable_node,
  					struct rb_root *root,
  					bool prune_stale_stable_nodes)

  {

  	struct stable_node *stable_node = *_stable_node;

  	if (!is_stable_node_chain(stable_node)) {
  		if (is_page_sharing_candidate(stable_node)) {

  			*_stable_node_dup = stable_node;

  			return get_ksm_page(stable_node, GET_KSM_PAGE_NOLOCK);

  		}

  		/*
  		 * _stable_node_dup set to NULL means the stable_node
  		 * reached the ksm_max_page_sharing limit.
  		 */
  		*_stable_node_dup = NULL;

  		return NULL;
  	}

  	return stable_node_dup(_stable_node_dup, _stable_node, root,

  			       prune_stale_stable_nodes);
  }

  static __always_inline struct page *chain_prune(struct stable_node **s_n_d,
  						struct stable_node **s_n,
  						struct rb_root *root)

  {

  	return __stable_node_chain(s_n_d, s_n, root, true);

  }

  static __always_inline struct page *chain(struct stable_node **s_n_d,
  					  struct stable_node *s_n,
  					  struct rb_root *root)

  {

  	struct stable_node *old_stable_node = s_n;
  	struct page *tree_page;
  
  	tree_page = __stable_node_chain(s_n_d, &s_n, root, false);
  	/* not pruning dups so s_n cannot have changed */
  	VM_BUG_ON(s_n != old_stable_node);
  	return tree_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)

  {

  	int nid;

  	struct rb_root *root;

  	struct rb_node **new;
  	struct rb_node *parent;

  	struct stable_node *stable_node, *stable_node_dup, *stable_node_any;

  	struct stable_node *page_node;

  	page_node = page_stable_node(page);
  	if (page_node && page_node->head != &migrate_nodes) {
  		/* ksm page forked */

  		get_page(page);

  		return page;

  	}

  	nid = get_kpfn_nid(page_to_pfn(page));

  	root = root_stable_tree + nid;

  again:

  	new = &root->rb_node;

  	parent = NULL;

  	while (*new) {

  		struct page *tree_page;

  		int ret;

  		cond_resched();

  		stable_node = rb_entry(*new, struct stable_node, node);

  		stable_node_any = NULL;

  		tree_page = chain_prune(&stable_node_dup, &stable_node,	root);

  		/*
  		 * NOTE: stable_node may have been freed by
  		 * chain_prune() if the returned stable_node_dup is
  		 * not NULL. stable_node_dup may have been inserted in
  		 * the rbtree instead as a regular stable_node (in
  		 * order to collapse the stable_node chain if a single

  		 * stable_node dup was found in it). In such case the
  		 * stable_node is overwritten by the calleee to point
  		 * to the stable_node_dup that was collapsed in the
  		 * stable rbtree and stable_node will be equal to
  		 * stable_node_dup like if the chain never existed.

  		 */

  		if (!stable_node_dup) {
  			/*
  			 * Either all stable_node dups were full in
  			 * this stable_node chain, or this chain was
  			 * empty and should be rb_erased.
  			 */
  			stable_node_any = stable_node_dup_any(stable_node,
  							      root);
  			if (!stable_node_any) {
  				/* rb_erase just run */
  				goto again;
  			}
  			/*
  			 * Take any of the stable_node dups page of
  			 * this stable_node chain to let the tree walk
  			 * continue. All KSM pages belonging to the
  			 * stable_node dups in a stable_node chain
  			 * have the same content and they're
  			 * wrprotected at all times. Any will work
  			 * fine to continue the walk.
  			 */

  			tree_page = get_ksm_page(stable_node_any,
  						 GET_KSM_PAGE_NOLOCK);

  		}
  		VM_BUG_ON(!stable_node_dup ^ !!stable_node_any);

  		if (!tree_page) {
  			/*
  			 * If we walked over a stale stable_node,
  			 * get_ksm_page() will call rb_erase() and it
  			 * may rebalance the tree from under us. So
  			 * restart the search from scratch. Returning
  			 * NULL would be safe too, but we'd generate
  			 * false negative insertions just because some
  			 * stable_node was stale.
  			 */
  			goto again;
  		}

  		ret = memcmp_pages(page, tree_page);

  		put_page(tree_page);

  		parent = *new;

  		if (ret < 0)

  			new = &parent->rb_left;

  		else if (ret > 0)

  			new = &parent->rb_right;

  		else {

  			if (page_node) {
  				VM_BUG_ON(page_node->head != &migrate_nodes);
  				/*
  				 * Test if the migrated page should be merged
  				 * into a stable node dup. If the mapcount is
  				 * 1 we can migrate it with another KSM page
  				 * without adding it to the chain.
  				 */
  				if (page_mapcount(page) > 1)
  					goto chain_append;
  			}
  
  			if (!stable_node_dup) {
  				/*
  				 * If the stable_node is a chain and
  				 * we got a payload match in memcmp
  				 * but we cannot merge the scanned
  				 * page in any of the existing
  				 * stable_node dups because they're
  				 * all full, we need to wait the
  				 * scanned page to find itself a match
  				 * in the unstable tree to create a
  				 * brand new KSM page to add later to
  				 * the dups of this stable_node.
  				 */
  				return NULL;
  			}

  			/*
  			 * Lock and unlock the stable_node's page (which
  			 * might already have been migrated) so that page
  			 * migration is sure to notice its raised count.
  			 * It would be more elegant to return stable_node
  			 * than kpage, but that involves more changes.
  			 */

  			tree_page = get_ksm_page(stable_node_dup,
  						 GET_KSM_PAGE_TRYLOCK);
  
  			if (PTR_ERR(tree_page) == -EBUSY)
  				return ERR_PTR(-EBUSY);

  			if (unlikely(!tree_page))
  				/*
  				 * The tree may have been rebalanced,
  				 * so re-evaluate parent and new.
  				 */

  				goto again;

  			unlock_page(tree_page);
  
  			if (get_kpfn_nid(stable_node_dup->kpfn) !=
  			    NUMA(stable_node_dup->nid)) {
  				put_page(tree_page);
  				goto replace;
  			}
  			return tree_page;

  		}

  	}

  	if (!page_node)
  		return NULL;
  
  	list_del(&page_node->list);
  	DO_NUMA(page_node->nid = nid);
  	rb_link_node(&page_node->node, parent, new);

  	rb_insert_color(&page_node->node, root);

  out:
  	if (is_page_sharing_candidate(page_node)) {
  		get_page(page);
  		return page;
  	} else
  		return NULL;

  
  replace:

  	/*
  	 * If stable_node was a chain and chain_prune collapsed it,

  	 * stable_node has been updated to be the new regular
  	 * stable_node. A collapse of the chain is indistinguishable
  	 * from the case there was no chain in the stable
  	 * rbtree. Otherwise stable_node is the chain and
  	 * stable_node_dup is the dup to replace.

  	 */

  	if (stable_node_dup == stable_node) {

  		VM_BUG_ON(is_stable_node_chain(stable_node_dup));
  		VM_BUG_ON(is_stable_node_dup(stable_node_dup));

  		/* there is no chain */
  		if (page_node) {
  			VM_BUG_ON(page_node->head != &migrate_nodes);
  			list_del(&page_node->list);
  			DO_NUMA(page_node->nid = nid);

  			rb_replace_node(&stable_node_dup->node,
  					&page_node->node,

  					root);
  			if (is_page_sharing_candidate(page_node))
  				get_page(page);
  			else
  				page = NULL;
  		} else {

  			rb_erase(&stable_node_dup->node, root);

  			page = NULL;
  		}

  	} else {

  		VM_BUG_ON(!is_stable_node_chain(stable_node));
  		__stable_node_dup_del(stable_node_dup);
  		if (page_node) {
  			VM_BUG_ON(page_node->head != &migrate_nodes);
  			list_del(&page_node->list);
  			DO_NUMA(page_node->nid = nid);
  			stable_node_chain_add_dup(page_node, stable_node);
  			if (is_page_sharing_candidate(page_node))
  				get_page(page);
  			else
  				page = NULL;
  		} else {
  			page = NULL;
  		}

  	}

  	stable_node_dup->head = &migrate_nodes;
  	list_add(&stable_node_dup->list, stable_node_dup->head);

  	return page;

  
  chain_append:
  	/* stable_node_dup could be null if it reached the limit */
  	if (!stable_node_dup)
  		stable_node_dup = stable_node_any;

  	/*
  	 * If stable_node was a chain and chain_prune collapsed it,

  	 * stable_node has been updated to be the new regular
  	 * stable_node. A collapse of the chain is indistinguishable
  	 * from the case there was no chain in the stable
  	 * rbtree. Otherwise stable_node is the chain and
  	 * stable_node_dup is the dup to replace.

  	 */

  	if (stable_node_dup == stable_node) {

  		VM_BUG_ON(is_stable_node_chain(stable_node_dup));
  		VM_BUG_ON(is_stable_node_dup(stable_node_dup));

  		/* chain is missing so create it */
  		stable_node = alloc_stable_node_chain(stable_node_dup,
  						      root);
  		if (!stable_node)
  			return NULL;
  	}
  	/*
  	 * Add this stable_node dup that was
  	 * migrated to the stable_node chain
  	 * of the current nid for this page
  	 * content.
  	 */

  	VM_BUG_ON(!is_stable_node_chain(stable_node));
  	VM_BUG_ON(!is_stable_node_dup(stable_node_dup));

  	VM_BUG_ON(page_node->head != &migrate_nodes);
  	list_del(&page_node->list);
  	DO_NUMA(page_node->nid = nid);
  	stable_node_chain_add_dup(page_node, stable_node);
  	goto out;

  }
  
  /*

   * stable_tree_insert - insert stable tree node 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)

  {

  	int nid;
  	unsigned long kpfn;

  	struct rb_root *root;

  	struct rb_node **new;

  	struct rb_node *parent;

  	struct stable_node *stable_node, *stable_node_dup, *stable_node_any;
  	bool need_chain = false;

  	kpfn = page_to_pfn(kpage);
  	nid = get_kpfn_nid(kpfn);

  	root = root_stable_tree + nid;

  again:
  	parent = NULL;

  	new = &root->rb_node;

  	while (*new) {

  		struct page *tree_page;

  		int ret;

  		cond_resched();

  		stable_node = rb_entry(*new, struct stable_node, node);

  		stable_node_any = NULL;

  		tree_page = chain(&stable_node_dup, stable_node, root);

  		if (!stable_node_dup) {
  			/*
  			 * Either all stable_node dups were full in
  			 * this stable_node chain, or this chain was
  			 * empty and should be rb_erased.
  			 */
  			stable_node_any = stable_node_dup_any(stable_node,
  							      root);
  			if (!stable_node_any) {
  				/* rb_erase just run */
  				goto again;
  			}
  			/*
  			 * Take any of the stable_node dups page of
  			 * this stable_node chain to let the tree walk
  			 * continue. All KSM pages belonging to the
  			 * stable_node dups in a stable_node chain
  			 * have the same content and they're
  			 * wrprotected at all times. Any will work
  			 * fine to continue the walk.
  			 */

  			tree_page = get_ksm_page(stable_node_any,
  						 GET_KSM_PAGE_NOLOCK);

  		}
  		VM_BUG_ON(!stable_node_dup ^ !!stable_node_any);

  		if (!tree_page) {
  			/*
  			 * If we walked over a stale stable_node,
  			 * get_ksm_page() will call rb_erase() and it
  			 * may rebalance the tree from under us. So
  			 * restart the search from scratch. Returning
  			 * NULL would be safe too, but we'd generate
  			 * false negative insertions just because some
  			 * stable_node was stale.
  			 */
  			goto again;
  		}