1// SPDX-License-Identifier: GPL-2.0-only
   2/*
   3 * Memory merging support.
   4 *
   5 * This code enables dynamic sharing of identical pages found in different
   6 * memory areas, even if they are not shared by fork()
   7 *
   8 * Copyright (C) 2008-2009 Red Hat, Inc.
   9 * Authors:
  10 *	Izik Eidus
  11 *	Andrea Arcangeli
  12 *	Chris Wright
  13 *	Hugh Dickins
  14 */
  15
  16#include <linux/errno.h>
  17#include <linux/mm.h>
  18#include <linux/fs.h>
  19#include <linux/mman.h>
  20#include <linux/sched.h>
  21#include <linux/sched/mm.h>
  22#include <linux/sched/coredump.h>
  23#include <linux/rwsem.h>
  24#include <linux/pagemap.h>
  25#include <linux/rmap.h>
  26#include <linux/spinlock.h>
  27#include <linux/xxhash.h>
  28#include <linux/delay.h>
  29#include <linux/kthread.h>
  30#include <linux/wait.h>
  31#include <linux/slab.h>
  32#include <linux/rbtree.h>
  33#include <linux/memory.h>
  34#include <linux/mmu_notifier.h>
  35#include <linux/swap.h>
  36#include <linux/ksm.h>
  37#include <linux/hashtable.h>
  38#include <linux/freezer.h>
  39#include <linux/oom.h>
  40#include <linux/numa.h>
  41
  42#include <asm/tlbflush.h>
  43#include "internal.h"
  44
  45#ifdef CONFIG_NUMA
  46#define NUMA(x)		(x)
  47#define DO_NUMA(x)	do { (x); } while (0)
  48#else
  49#define NUMA(x)		(0)
  50#define DO_NUMA(x)	do { } while (0)
  51#endif
  52
  53/**
  54 * DOC: Overview
  55 *
  56 * A few notes about the KSM scanning process,
  57 * to make it easier to understand the data structures below:
  58 *
  59 * In order to reduce excessive scanning, KSM sorts the memory pages by their
  60 * contents into a data structure that holds pointers to the pages' locations.
  61 *
  62 * Since the contents of the pages may change at any moment, KSM cannot just
  63 * insert the pages into a normal sorted tree and expect it to find anything.
  64 * Therefore KSM uses two data structures - the stable and the unstable tree.
  65 *
  66 * The stable tree holds pointers to all the merged pages (ksm pages), sorted
  67 * by their contents.  Because each such page is write-protected, searching on
  68 * this tree is fully assured to be working (except when pages are unmapped),
  69 * and therefore this tree is called the stable tree.
  70 *
  71 * The stable tree node includes information required for reverse
  72 * mapping from a KSM page to virtual addresses that map this page.
  73 *
  74 * In order to avoid large latencies of the rmap walks on KSM pages,
  75 * KSM maintains two types of nodes in the stable tree:
  76 *
  77 * * the regular nodes that keep the reverse mapping structures in a
  78 *   linked list
  79 * * the "chains" that link nodes ("dups") that represent the same
  80 *   write protected memory content, but each "dup" corresponds to a
  81 *   different KSM page copy of that content
  82 *
  83 * Internally, the regular nodes, "dups" and "chains" are represented
  84 * using the same :c:type:`struct stable_node` structure.
  85 *
  86 * In addition to the stable tree, KSM uses a second data structure called the
  87 * unstable tree: this tree holds pointers to pages which have been found to
  88 * be "unchanged for a period of time".  The unstable tree sorts these pages
  89 * by their contents, but since they are not write-protected, KSM cannot rely
  90 * upon the unstable tree to work correctly - the unstable tree is liable to
  91 * be corrupted as its contents are modified, and so it is called unstable.
  92 *
  93 * KSM solves this problem by several techniques:
  94 *
  95 * 1) The unstable tree is flushed every time KSM completes scanning all
  96 *    memory areas, and then the tree is rebuilt again from the beginning.
  97 * 2) KSM will only insert into the unstable tree, pages whose hash value
  98 *    has not changed since the previous scan of all memory areas.
  99 * 3) The unstable tree is a RedBlack Tree - so its balancing is based on the
 100 *    colors of the nodes and not on their contents, assuring that even when
 101 *    the tree gets "corrupted" it won't get out of balance, so scanning time
 102 *    remains the same (also, searching and inserting nodes in an rbtree uses
 103 *    the same algorithm, so we have no overhead when we flush and rebuild).
 104 * 4) KSM never flushes the stable tree, which means that even if it were to
 105 *    take 10 attempts to find a page in the unstable tree, once it is found,
 106 *    it is secured in the stable tree.  (When we scan a new page, we first
 107 *    compare it against the stable tree, and then against the unstable tree.)
 108 *
 109 * If the merge_across_nodes tunable is unset, then KSM maintains multiple
 110 * stable trees and multiple unstable trees: one of each for each NUMA node.
 111 */
 112
 113/**
 114 * struct mm_slot - ksm information per mm that is being scanned
 115 * @link: link to the mm_slots hash list
 116 * @mm_list: link into the mm_slots list, rooted in ksm_mm_head
 117 * @rmap_list: head for this mm_slot's singly-linked list of rmap_items
 118 * @mm: the mm that this information is valid for
 119 */
 120struct mm_slot {
 121	struct hlist_node link;
 122	struct list_head mm_list;
 123	struct rmap_item *rmap_list;
 124	struct mm_struct *mm;
 125};
 126
 127/**
 128 * struct ksm_scan - cursor for scanning
 129 * @mm_slot: the current mm_slot we are scanning
 130 * @address: the next address inside that to be scanned
 131 * @rmap_list: link to the next rmap to be scanned in the rmap_list
 132 * @seqnr: count of completed full scans (needed when removing unstable node)
 133 *
 134 * There is only the one ksm_scan instance of this cursor structure.
 135 */
 136struct ksm_scan {
 137	struct mm_slot *mm_slot;
 138	unsigned long address;
 139	struct rmap_item **rmap_list;
 140	unsigned long seqnr;
 141};
 142
 143/**
 144 * struct stable_node - node of the stable rbtree
 145 * @node: rb node of this ksm page in the stable tree
 146 * @head: (overlaying parent) &migrate_nodes indicates temporarily on that list
 147 * @hlist_dup: linked into the stable_node->hlist with a stable_node chain
 148 * @list: linked into migrate_nodes, pending placement in the proper node tree
 149 * @hlist: hlist head of rmap_items using this ksm page
 150 * @kpfn: page frame number of this ksm page (perhaps temporarily on wrong nid)
 151 * @chain_prune_time: time of the last full garbage collection
 152 * @rmap_hlist_len: number of rmap_item entries in hlist or STABLE_NODE_CHAIN
 153 * @nid: NUMA node id of stable tree in which linked (may not match kpfn)
 154 */
 155struct stable_node {
 156	union {
 157		struct rb_node node;	/* when node of stable tree */
 158		struct {		/* when listed for migration */
 159			struct list_head *head;
 160			struct {
 161				struct hlist_node hlist_dup;
 162				struct list_head list;
 163			};
 164		};
 165	};
 166	struct hlist_head hlist;
 167	union {
 168		unsigned long kpfn;
 169		unsigned long chain_prune_time;
 170	};
 171	/*
 172	 * STABLE_NODE_CHAIN can be any negative number in
 173	 * rmap_hlist_len negative range, but better not -1 to be able
 174	 * to reliably detect underflows.
 175	 */
 176#define STABLE_NODE_CHAIN -1024
 177	int rmap_hlist_len;
 178#ifdef CONFIG_NUMA
 179	int nid;
 180#endif
 181};
 182
 183/**
 184 * struct rmap_item - reverse mapping item for virtual addresses
 185 * @rmap_list: next rmap_item in mm_slot's singly-linked rmap_list
 186 * @anon_vma: pointer to anon_vma for this mm,address, when in stable tree
 187 * @nid: NUMA node id of unstable tree in which linked (may not match page)
 188 * @mm: the memory structure this rmap_item is pointing into
 189 * @address: the virtual address this rmap_item tracks (+ flags in low bits)
 190 * @oldchecksum: previous checksum of the page at that virtual address
 191 * @node: rb node of this rmap_item in the unstable tree
 192 * @head: pointer to stable_node heading this list in the stable tree
 193 * @hlist: link into hlist of rmap_items hanging off that stable_node
 194 */
 195struct rmap_item {
 196	struct rmap_item *rmap_list;
 197	union {
 198		struct anon_vma *anon_vma;	/* when stable */
 199#ifdef CONFIG_NUMA
 200		int nid;		/* when node of unstable tree */
 201#endif
 202	};
 203	struct mm_struct *mm;
 204	unsigned long address;		/* + low bits used for flags below */
 205	unsigned int oldchecksum;	/* when unstable */
 206	union {
 207		struct rb_node node;	/* when node of unstable tree */
 208		struct {		/* when listed from stable tree */
 209			struct stable_node *head;
 210			struct hlist_node hlist;
 211		};
 212	};
 213};
 214
 215#define SEQNR_MASK	0x0ff	/* low bits of unstable tree seqnr */
 216#define UNSTABLE_FLAG	0x100	/* is a node of the unstable tree */
 217#define STABLE_FLAG	0x200	/* is listed from the stable tree */
 218#define KSM_FLAG_MASK	(SEQNR_MASK|UNSTABLE_FLAG|STABLE_FLAG)
 219				/* to mask all the flags */
 220
 221/* The stable and unstable tree heads */
 222static struct rb_root one_stable_tree[1] = { RB_ROOT };
 223static struct rb_root one_unstable_tree[1] = { RB_ROOT };
 224static struct rb_root *root_stable_tree = one_stable_tree;
 225static struct rb_root *root_unstable_tree = one_unstable_tree;
 226
 227/* Recently migrated nodes of stable tree, pending proper placement */
 228static LIST_HEAD(migrate_nodes);
 229#define STABLE_NODE_DUP_HEAD ((struct list_head *)&migrate_nodes.prev)
 230
 231#define MM_SLOTS_HASH_BITS 10
 232static DEFINE_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);
 233
 234static struct mm_slot ksm_mm_head = {
 235	.mm_list = LIST_HEAD_INIT(ksm_mm_head.mm_list),
 236};
 237static struct ksm_scan ksm_scan = {
 238	.mm_slot = &ksm_mm_head,
 239};
 240
 241static struct kmem_cache *rmap_item_cache;
 242static struct kmem_cache *stable_node_cache;
 243static struct kmem_cache *mm_slot_cache;
 244
 245/* The number of nodes in the stable tree */
 246static unsigned long ksm_pages_shared;
 247
 248/* The number of page slots additionally sharing those nodes */
 249static unsigned long ksm_pages_sharing;
 250
 251/* The number of nodes in the unstable tree */
 252static unsigned long ksm_pages_unshared;
 253
 254/* The number of rmap_items in use: to calculate pages_volatile */
 255static unsigned long ksm_rmap_items;
 256
 257/* The number of stable_node chains */
 258static unsigned long ksm_stable_node_chains;
 259
 260/* The number of stable_node dups linked to the stable_node chains */
 261static unsigned long ksm_stable_node_dups;
 262
 263/* Delay in pruning stale stable_node_dups in the stable_node_chains */
 264static int ksm_stable_node_chains_prune_millisecs = 2000;
 265
 266/* Maximum number of page slots sharing a stable node */
 267static int ksm_max_page_sharing = 256;
 268
 269/* Number of pages ksmd should scan in one batch */
 270static unsigned int ksm_thread_pages_to_scan = 100;
 271
 272/* Milliseconds ksmd should sleep between batches */
 273static unsigned int ksm_thread_sleep_millisecs = 20;
 274
 275/* Checksum of an empty (zeroed) page */
 276static unsigned int zero_checksum __read_mostly;
 277
 278/* Whether to merge empty (zeroed) pages with actual zero pages */
 279static bool ksm_use_zero_pages __read_mostly;
 280
 281#ifdef CONFIG_NUMA
 282/* Zeroed when merging across nodes is not allowed */
 283static unsigned int ksm_merge_across_nodes = 1;
 284static int ksm_nr_node_ids = 1;
 285#else
 286#define ksm_merge_across_nodes	1U
 287#define ksm_nr_node_ids		1
 288#endif
 289
 290#define KSM_RUN_STOP	0
 291#define KSM_RUN_MERGE	1
 292#define KSM_RUN_UNMERGE	2
 293#define KSM_RUN_OFFLINE	4
 294static unsigned long ksm_run = KSM_RUN_STOP;
 295static void wait_while_offlining(void);
 296
 297static DECLARE_WAIT_QUEUE_HEAD(ksm_thread_wait);
 298static DECLARE_WAIT_QUEUE_HEAD(ksm_iter_wait);
 299static DEFINE_MUTEX(ksm_thread_mutex);
 300static DEFINE_SPINLOCK(ksm_mmlist_lock);
 301
 302#define KSM_KMEM_CACHE(__struct, __flags) kmem_cache_create("ksm_"#__struct,\
 303		sizeof(struct __struct), __alignof__(struct __struct),\
 304		(__flags), NULL)
 305
 306static int __init ksm_slab_init(void)
 307{
 308	rmap_item_cache = KSM_KMEM_CACHE(rmap_item, 0);
 309	if (!rmap_item_cache)
 310		goto out;
 311
 312	stable_node_cache = KSM_KMEM_CACHE(stable_node, 0);
 313	if (!stable_node_cache)
 314		goto out_free1;
 315
 316	mm_slot_cache = KSM_KMEM_CACHE(mm_slot, 0);
 317	if (!mm_slot_cache)
 318		goto out_free2;
 319
 320	return 0;
 321
 322out_free2:
 323	kmem_cache_destroy(stable_node_cache);
 324out_free1:
 325	kmem_cache_destroy(rmap_item_cache);
 326out:
 327	return -ENOMEM;
 328}
 329
 330static void __init ksm_slab_free(void)
 331{
 332	kmem_cache_destroy(mm_slot_cache);
 333	kmem_cache_destroy(stable_node_cache);
 334	kmem_cache_destroy(rmap_item_cache);
 335	mm_slot_cache = NULL;
 336}
 337
 338static __always_inline bool is_stable_node_chain(struct stable_node *chain)
 339{
 340	return chain->rmap_hlist_len == STABLE_NODE_CHAIN;
 341}
 342
 343static __always_inline bool is_stable_node_dup(struct stable_node *dup)
 344{
 345	return dup->head == STABLE_NODE_DUP_HEAD;
 346}
 347
 348static inline void stable_node_chain_add_dup(struct stable_node *dup,
 349					     struct stable_node *chain)
 350{
 351	VM_BUG_ON(is_stable_node_dup(dup));
 352	dup->head = STABLE_NODE_DUP_HEAD;
 353	VM_BUG_ON(!is_stable_node_chain(chain));
 354	hlist_add_head(&dup->hlist_dup, &chain->hlist);
 355	ksm_stable_node_dups++;
 356}
 357
 358static inline void __stable_node_dup_del(struct stable_node *dup)
 359{
 360	VM_BUG_ON(!is_stable_node_dup(dup));
 361	hlist_del(&dup->hlist_dup);
 362	ksm_stable_node_dups--;
 363}
 364
 365static inline void stable_node_dup_del(struct stable_node *dup)
 366{
 367	VM_BUG_ON(is_stable_node_chain(dup));
 368	if (is_stable_node_dup(dup))
 369		__stable_node_dup_del(dup);
 370	else
 371		rb_erase(&dup->node, root_stable_tree + NUMA(dup->nid));
 372#ifdef CONFIG_DEBUG_VM
 373	dup->head = NULL;
 374#endif
 375}
 376
 377static inline struct rmap_item *alloc_rmap_item(void)
 378{
 379	struct rmap_item *rmap_item;
 380
 381	rmap_item = kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL |
 382						__GFP_NORETRY | __GFP_NOWARN);
 383	if (rmap_item)
 384		ksm_rmap_items++;
 385	return rmap_item;
 386}
 387
 388static inline void free_rmap_item(struct rmap_item *rmap_item)
 389{
 390	ksm_rmap_items--;
 391	rmap_item->mm = NULL;	/* debug safety */
 392	kmem_cache_free(rmap_item_cache, rmap_item);
 393}
 394
 395static inline struct stable_node *alloc_stable_node(void)
 396{
 397	/*
 398	 * The allocation can take too long with GFP_KERNEL when memory is under
 399	 * pressure, which may lead to hung task warnings.  Adding __GFP_HIGH
 400	 * grants access to memory reserves, helping to avoid this problem.
 401	 */
 402	return kmem_cache_alloc(stable_node_cache, GFP_KERNEL | __GFP_HIGH);
 403}
 404
 405static inline void free_stable_node(struct stable_node *stable_node)
 406{
 407	VM_BUG_ON(stable_node->rmap_hlist_len &&
 408		  !is_stable_node_chain(stable_node));
 409	kmem_cache_free(stable_node_cache, stable_node);
 410}
 411
 412static inline struct mm_slot *alloc_mm_slot(void)
 413{
 414	if (!mm_slot_cache)	/* initialization failed */
 415		return NULL;
 416	return kmem_cache_zalloc(mm_slot_cache, GFP_KERNEL);
 417}
 418
 419static inline void free_mm_slot(struct mm_slot *mm_slot)
 420{
 421	kmem_cache_free(mm_slot_cache, mm_slot);
 422}
 423
 424static struct mm_slot *get_mm_slot(struct mm_struct *mm)
 425{
 426	struct mm_slot *slot;
 427
 428	hash_for_each_possible(mm_slots_hash, slot, link, (unsigned long)mm)
 429		if (slot->mm == mm)
 430			return slot;
 431
 432	return NULL;
 433}
 434
 435static void insert_to_mm_slots_hash(struct mm_struct *mm,
 436				    struct mm_slot *mm_slot)
 437{
 438	mm_slot->mm = mm;
 439	hash_add(mm_slots_hash, &mm_slot->link, (unsigned long)mm);
 440}
 441
 442/*
 443 * ksmd, and unmerge_and_remove_all_rmap_items(), must not touch an mm's
 444 * page tables after it has passed through ksm_exit() - which, if necessary,
 445 * takes mmap_sem briefly to serialize against them.  ksm_exit() does not set
 446 * a special flag: they can just back out as soon as mm_users goes to zero.
 447 * ksm_test_exit() is used throughout to make this test for exit: in some
 448 * places for correctness, in some places just to avoid unnecessary work.
 449 */
 450static inline bool ksm_test_exit(struct mm_struct *mm)
 451{
 452	return atomic_read(&mm->mm_users) == 0;
 453}
 454
 455/*
 456 * We use break_ksm to break COW on a ksm page: it's a stripped down
 457 *
 458 *	if (get_user_pages(addr, 1, 1, 1, &page, NULL) == 1)
 459 *		put_page(page);
 460 *
 461 * but taking great care only to touch a ksm page, in a VM_MERGEABLE vma,
 462 * in case the application has unmapped and remapped mm,addr meanwhile.
 463 * Could a ksm page appear anywhere else?  Actually yes, in a VM_PFNMAP
 464 * mmap of /dev/mem or /dev/kmem, where we would not want to touch it.
 465 *
 466 * FAULT_FLAG/FOLL_REMOTE are because we do this outside the context
 467 * of the process that owns 'vma'.  We also do not want to enforce
 468 * protection keys here anyway.
 469 */
 470static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
 471{
 472	struct page *page;
 473	vm_fault_t ret = 0;
 474
 475	do {
 476		cond_resched();
 477		page = follow_page(vma, addr,
 478				FOLL_GET | FOLL_MIGRATION | FOLL_REMOTE);
 479		if (IS_ERR_OR_NULL(page))
 480			break;
 481		if (PageKsm(page))
 482			ret = handle_mm_fault(vma, addr,
 483					FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE);
 
 484		else
 485			ret = VM_FAULT_WRITE;
 486		put_page(page);
 487	} while (!(ret & (VM_FAULT_WRITE | VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | VM_FAULT_OOM)));
 488	/*
 489	 * We must loop because handle_mm_fault() may back out if there's
 490	 * any difficulty e.g. if pte accessed bit gets updated concurrently.
 491	 *
 492	 * VM_FAULT_WRITE is what we have been hoping for: it indicates that
 493	 * COW has been broken, even if the vma does not permit VM_WRITE;
 494	 * but note that a concurrent fault might break PageKsm for us.
 495	 *
 496	 * VM_FAULT_SIGBUS could occur if we race with truncation of the
 497	 * backing file, which also invalidates anonymous pages: that's
 498	 * okay, that truncation will have unmapped the PageKsm for us.
 499	 *
 500	 * VM_FAULT_OOM: at the time of writing (late July 2009), setting
 501	 * aside mem_cgroup limits, VM_FAULT_OOM would only be set if the
 502	 * current task has TIF_MEMDIE set, and will be OOM killed on return
 503	 * to user; and ksmd, having no mm, would never be chosen for that.
 504	 *
 505	 * But if the mm is in a limited mem_cgroup, then the fault may fail
 506	 * with VM_FAULT_OOM even if the current task is not TIF_MEMDIE; and
 507	 * even ksmd can fail in this way - though it's usually breaking ksm
 508	 * just to undo a merge it made a moment before, so unlikely to oom.
 509	 *
 510	 * That's a pity: we might therefore have more kernel pages allocated
 511	 * than we're counting as nodes in the stable tree; but ksm_do_scan
 512	 * will retry to break_cow on each pass, so should recover the page
 513	 * in due course.  The important thing is to not let VM_MERGEABLE
 514	 * be cleared while any such pages might remain in the area.
 515	 */
 516	return (ret & VM_FAULT_OOM) ? -ENOMEM : 0;
 517}
 518
 519static struct vm_area_struct *find_mergeable_vma(struct mm_struct *mm,
 520		unsigned long addr)
 521{
 522	struct vm_area_struct *vma;
 523	if (ksm_test_exit(mm))
 524		return NULL;
 525	vma = find_vma(mm, addr);
 526	if (!vma || vma->vm_start > addr)
 527		return NULL;
 528	if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
 529		return NULL;
 530	return vma;
 531}
 532
 533static void break_cow(struct rmap_item *rmap_item)
 534{
 535	struct mm_struct *mm = rmap_item->mm;
 536	unsigned long addr = rmap_item->address;
 537	struct vm_area_struct *vma;
 538
 539	/*
 540	 * It is not an accident that whenever we want to break COW
 541	 * to undo, we also need to drop a reference to the anon_vma.
 542	 */
 543	put_anon_vma(rmap_item->anon_vma);
 544
 545	down_read(&mm->mmap_sem);
 546	vma = find_mergeable_vma(mm, addr);
 547	if (vma)
 548		break_ksm(vma, addr);
 549	up_read(&mm->mmap_sem);
 550}
 551
 552static struct page *get_mergeable_page(struct rmap_item *rmap_item)
 553{
 554	struct mm_struct *mm = rmap_item->mm;
 555	unsigned long addr = rmap_item->address;
 556	struct vm_area_struct *vma;
 557	struct page *page;
 558
 559	down_read(&mm->mmap_sem);
 560	vma = find_mergeable_vma(mm, addr);
 561	if (!vma)
 562		goto out;
 563
 564	page = follow_page(vma, addr, FOLL_GET);
 565	if (IS_ERR_OR_NULL(page))
 566		goto out;
 567	if (PageAnon(page)) {
 568		flush_anon_page(vma, page, addr);
 569		flush_dcache_page(page);
 570	} else {
 571		put_page(page);
 572out:
 573		page = NULL;
 574	}
 575	up_read(&mm->mmap_sem);
 576	return page;
 577}
 578
 579/*
 580 * This helper is used for getting right index into array of tree roots.
 581 * When merge_across_nodes knob is set to 1, there are only two rb-trees for
 582 * stable and unstable pages from all nodes with roots in index 0. Otherwise,
 583 * every node has its own stable and unstable tree.
 584 */
 585static inline int get_kpfn_nid(unsigned long kpfn)
 586{
 587	return ksm_merge_across_nodes ? 0 : NUMA(pfn_to_nid(kpfn));
 588}
 589
 590static struct stable_node *alloc_stable_node_chain(struct stable_node *dup,
 591						   struct rb_root *root)
 592{
 593	struct stable_node *chain = alloc_stable_node();
 594	VM_BUG_ON(is_stable_node_chain(dup));
 595	if (likely(chain)) {
 596		INIT_HLIST_HEAD(&chain->hlist);
 597		chain->chain_prune_time = jiffies;
 598		chain->rmap_hlist_len = STABLE_NODE_CHAIN;
 599#if defined (CONFIG_DEBUG_VM) && defined(CONFIG_NUMA)
 600		chain->nid = NUMA_NO_NODE; /* debug */
 601#endif
 602		ksm_stable_node_chains++;
 603
 604		/*
 605		 * Put the stable node chain in the first dimension of
 606		 * the stable tree and at the same time remove the old
 607		 * stable node.
 608		 */
 609		rb_replace_node(&dup->node, &chain->node, root);
 610
 611		/*
 612		 * Move the old stable node to the second dimension
 613		 * queued in the hlist_dup. The invariant is that all
 614		 * dup stable_nodes in the chain->hlist point to pages
 615		 * that are wrprotected and have the exact same
 616		 * content.
 617		 */
 618		stable_node_chain_add_dup(dup, chain);
 619	}
 620	return chain;
 621}
 622
 623static inline void free_stable_node_chain(struct stable_node *chain,
 624					  struct rb_root *root)
 625{
 626	rb_erase(&chain->node, root);
 627	free_stable_node(chain);
 628	ksm_stable_node_chains--;
 629}
 630
 631static void remove_node_from_stable_tree(struct stable_node *stable_node)
 632{
 633	struct rmap_item *rmap_item;
 634
 635	/* check it's not STABLE_NODE_CHAIN or negative */
 636	BUG_ON(stable_node->rmap_hlist_len < 0);
 637
 638	hlist_for_each_entry(rmap_item, &stable_node->hlist, hlist) {
 639		if (rmap_item->hlist.next)
 640			ksm_pages_sharing--;
 641		else
 642			ksm_pages_shared--;
 643		VM_BUG_ON(stable_node->rmap_hlist_len <= 0);
 644		stable_node->rmap_hlist_len--;
 645		put_anon_vma(rmap_item->anon_vma);
 646		rmap_item->address &= PAGE_MASK;
 647		cond_resched();
 648	}
 649
 650	/*
 651	 * We need the second aligned pointer of the migrate_nodes
 652	 * list_head to stay clear from the rb_parent_color union
 653	 * (aligned and different than any node) and also different
 654	 * from &migrate_nodes. This will verify that future list.h changes
 655	 * don't break STABLE_NODE_DUP_HEAD. Only recent gcc can handle it.
 656	 */
 657#if defined(GCC_VERSION) && GCC_VERSION >= 40903
 658	BUILD_BUG_ON(STABLE_NODE_DUP_HEAD <= &migrate_nodes);
 659	BUILD_BUG_ON(STABLE_NODE_DUP_HEAD >= &migrate_nodes + 1);
 660#endif
 661
 662	if (stable_node->head == &migrate_nodes)
 663		list_del(&stable_node->list);
 664	else
 665		stable_node_dup_del(stable_node);
 666	free_stable_node(stable_node);
 667}
 668
 669enum get_ksm_page_flags {
 670	GET_KSM_PAGE_NOLOCK,
 671	GET_KSM_PAGE_LOCK,
 672	GET_KSM_PAGE_TRYLOCK
 673};
 674
 675/*
 676 * get_ksm_page: checks if the page indicated by the stable node
 677 * is still its ksm page, despite having held no reference to it.
 678 * In which case we can trust the content of the page, and it
 679 * returns the gotten page; but if the page has now been zapped,
 680 * remove the stale node from the stable tree and return NULL.
 681 * But beware, the stable node's page might be being migrated.
 682 *
 683 * You would expect the stable_node to hold a reference to the ksm page.
 684 * But if it increments the page's count, swapping out has to wait for
 685 * ksmd to come around again before it can free the page, which may take
 686 * seconds or even minutes: much too unresponsive.  So instead we use a
 687 * "keyhole reference": access to the ksm page from the stable node peeps
 688 * out through its keyhole to see if that page still holds the right key,
 689 * pointing back to this stable node.  This relies on freeing a PageAnon
 690 * page to reset its page->mapping to NULL, and relies on no other use of
 691 * a page to put something that might look like our key in page->mapping.
 692 * is on its way to being freed; but it is an anomaly to bear in mind.
 693 */
 694static struct page *get_ksm_page(struct stable_node *stable_node,
 695				 enum get_ksm_page_flags flags)
 696{
 697	struct page *page;
 698	void *expected_mapping;
 699	unsigned long kpfn;
 700
 701	expected_mapping = (void *)((unsigned long)stable_node |
 702					PAGE_MAPPING_KSM);
 703again:
 704	kpfn = READ_ONCE(stable_node->kpfn); /* Address dependency. */
 705	page = pfn_to_page(kpfn);
 706	if (READ_ONCE(page->mapping) != expected_mapping)
 707		goto stale;
 708
 709	/*
 710	 * We cannot do anything with the page while its refcount is 0.
 711	 * Usually 0 means free, or tail of a higher-order page: in which
 712	 * case this node is no longer referenced, and should be freed;
 713	 * however, it might mean that the page is under page_ref_freeze().
 714	 * The __remove_mapping() case is easy, again the node is now stale;
 715	 * the same is in reuse_ksm_page() case; but if page is swapcache
 716	 * in migrate_page_move_mapping(), it might still be our page,
 717	 * in which case it's essential to keep the node.
 718	 */
 719	while (!get_page_unless_zero(page)) {
 720		/*
 721		 * Another check for page->mapping != expected_mapping would
 722		 * work here too.  We have chosen the !PageSwapCache test to
 723		 * optimize the common case, when the page is or is about to
 724		 * be freed: PageSwapCache is cleared (under spin_lock_irq)
 725		 * in the ref_freeze section of __remove_mapping(); but Anon
 726		 * page->mapping reset to NULL later, in free_pages_prepare().
 727		 */
 728		if (!PageSwapCache(page))
 729			goto stale;
 730		cpu_relax();
 731	}
 732
 733	if (READ_ONCE(page->mapping) != expected_mapping) {
 734		put_page(page);
 735		goto stale;
 736	}
 737
 738	if (flags == GET_KSM_PAGE_TRYLOCK) {
 739		if (!trylock_page(page)) {
 740			put_page(page);
 741			return ERR_PTR(-EBUSY);
 742		}
 743	} else if (flags == GET_KSM_PAGE_LOCK)
 744		lock_page(page);
 745
 746	if (flags != GET_KSM_PAGE_NOLOCK) {
 747		if (READ_ONCE(page->mapping) != expected_mapping) {
 748			unlock_page(page);
 749			put_page(page);
 750			goto stale;
 751		}
 752	}
 753	return page;
 754
 755stale:
 756	/*
 757	 * We come here from above when page->mapping or !PageSwapCache
 758	 * suggests that the node is stale; but it might be under migration.
 759	 * We need smp_rmb(), matching the smp_wmb() in ksm_migrate_page(),
 760	 * before checking whether node->kpfn has been changed.
 761	 */
 762	smp_rmb();
 763	if (READ_ONCE(stable_node->kpfn) != kpfn)
 764		goto again;
 765	remove_node_from_stable_tree(stable_node);
 766	return NULL;
 767}
 768
 769/*
 770 * Removing rmap_item from stable or unstable tree.
 771 * This function will clean the information from the stable/unstable tree.
 772 */
 773static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
 774{
 775	if (rmap_item->address & STABLE_FLAG) {
 776		struct stable_node *stable_node;
 777		struct page *page;
 778
 779		stable_node = rmap_item->head;
 780		page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK);
 781		if (!page)
 782			goto out;
 783
 784		hlist_del(&rmap_item->hlist);
 785		unlock_page(page);
 786		put_page(page);
 787
 788		if (!hlist_empty(&stable_node->hlist))
 789			ksm_pages_sharing--;
 790		else
 791			ksm_pages_shared--;
 792		VM_BUG_ON(stable_node->rmap_hlist_len <= 0);
 793		stable_node->rmap_hlist_len--;
 794
 795		put_anon_vma(rmap_item->anon_vma);
 796		rmap_item->address &= PAGE_MASK;
 797
 798	} else if (rmap_item->address & UNSTABLE_FLAG) {
 799		unsigned char age;
 800		/*
 801		 * Usually ksmd can and must skip the rb_erase, because
 802		 * root_unstable_tree was already reset to RB_ROOT.
 803		 * But be careful when an mm is exiting: do the rb_erase
 804		 * if this rmap_item was inserted by this scan, rather
 805		 * than left over from before.
 806		 */
 807		age = (unsigned char)(ksm_scan.seqnr - rmap_item->address);
 808		BUG_ON(age > 1);
 809		if (!age)
 810			rb_erase(&rmap_item->node,
 811				 root_unstable_tree + NUMA(rmap_item->nid));
 812		ksm_pages_unshared--;
 813		rmap_item->address &= PAGE_MASK;
 814	}
 815out:
 816	cond_resched();		/* we're called from many long loops */
 817}
 818
 819static void remove_trailing_rmap_items(struct mm_slot *mm_slot,
 820				       struct rmap_item **rmap_list)
 821{
 822	while (*rmap_list) {
 823		struct rmap_item *rmap_item = *rmap_list;
 824		*rmap_list = rmap_item->rmap_list;
 825		remove_rmap_item_from_tree(rmap_item);
 826		free_rmap_item(rmap_item);
 827	}
 828}
 829
 830/*
 831 * Though it's very tempting to unmerge rmap_items from stable tree rather
 832 * than check every pte of a given vma, the locking doesn't quite work for
 833 * that - an rmap_item is assigned to the stable tree after inserting ksm
 834 * page and upping mmap_sem.  Nor does it fit with the way we skip dup'ing
 835 * rmap_items from parent to child at fork time (so as not to waste time
 836 * if exit comes before the next scan reaches it).
 837 *
 838 * Similarly, although we'd like to remove rmap_items (so updating counts
 839 * and freeing memory) when unmerging an area, it's easier to leave that
 840 * to the next pass of ksmd - consider, for example, how ksmd might be
 841 * in cmp_and_merge_page on one of the rmap_items we would be removing.
 842 */
 843static int unmerge_ksm_pages(struct vm_area_struct *vma,
 844			     unsigned long start, unsigned long end)
 845{
 846	unsigned long addr;
 847	int err = 0;
 848
 849	for (addr = start; addr < end && !err; addr += PAGE_SIZE) {
 850		if (ksm_test_exit(vma->vm_mm))
 851			break;
 852		if (signal_pending(current))
 853			err = -ERESTARTSYS;
 854		else
 855			err = break_ksm(vma, addr);
 856	}
 857	return err;
 858}
 859
 860static inline struct stable_node *page_stable_node(struct page *page)
 861{
 862	return PageKsm(page) ? page_rmapping(page) : NULL;
 863}
 864
 865static inline void set_page_stable_node(struct page *page,
 866					struct stable_node *stable_node)
 867{
 868	page->mapping = (void *)((unsigned long)stable_node | PAGE_MAPPING_KSM);
 869}
 870
 871#ifdef CONFIG_SYSFS
 872/*
 873 * Only called through the sysfs control interface:
 874 */
 875static int remove_stable_node(struct stable_node *stable_node)
 876{
 877	struct page *page;
 878	int err;
 879
 880	page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK);
 881	if (!page) {
 882		/*
 883		 * get_ksm_page did remove_node_from_stable_tree itself.
 884		 */
 885		return 0;
 886	}
 887
 888	/*
 889	 * Page could be still mapped if this races with __mmput() running in
 890	 * between ksm_exit() and exit_mmap(). Just refuse to let
 891	 * merge_across_nodes/max_page_sharing be switched.
 892	 */
 893	err = -EBUSY;
 894	if (!page_mapped(page)) {
 895		/*
 896		 * The stable node did not yet appear stale to get_ksm_page(),
 897		 * since that allows for an unmapped ksm page to be recognized
 898		 * right up until it is freed; but the node is safe to remove.
 899		 * This page might be in a pagevec waiting to be freed,
 900		 * or it might be PageSwapCache (perhaps under writeback),
 901		 * or it might have been removed from swapcache a moment ago.
 902		 */
 903		set_page_stable_node(page, NULL);
 904		remove_node_from_stable_tree(stable_node);
 905		err = 0;
 906	}
 907
 908	unlock_page(page);
 909	put_page(page);
 910	return err;
 911}
 912
 913static int remove_stable_node_chain(struct stable_node *stable_node,
 914				    struct rb_root *root)
 915{
 916	struct stable_node *dup;
 917	struct hlist_node *hlist_safe;
 918
 919	if (!is_stable_node_chain(stable_node)) {
 920		VM_BUG_ON(is_stable_node_dup(stable_node));
 921		if (remove_stable_node(stable_node))
 922			return true;
 923		else
 924			return false;
 925	}
 926
 927	hlist_for_each_entry_safe(dup, hlist_safe,
 928				  &stable_node->hlist, hlist_dup) {
 929		VM_BUG_ON(!is_stable_node_dup(dup));
 930		if (remove_stable_node(dup))
 931			return true;
 932	}
 933	BUG_ON(!hlist_empty(&stable_node->hlist));
 934	free_stable_node_chain(stable_node, root);
 935	return false;
 936}
 937
 938static int remove_all_stable_nodes(void)
 939{
 940	struct stable_node *stable_node, *next;
 941	int nid;
 942	int err = 0;
 943
 944	for (nid = 0; nid < ksm_nr_node_ids; nid++) {
 945		while (root_stable_tree[nid].rb_node) {
 946			stable_node = rb_entry(root_stable_tree[nid].rb_node,
 947						struct stable_node, node);
 948			if (remove_stable_node_chain(stable_node,
 949						     root_stable_tree + nid)) {
 950				err = -EBUSY;
 951				break;	/* proceed to next nid */
 952			}
 953			cond_resched();
 954		}
 955	}
 956	list_for_each_entry_safe(stable_node, next, &migrate_nodes, list) {
 957		if (remove_stable_node(stable_node))
 958			err = -EBUSY;
 959		cond_resched();
 960	}
 961	return err;
 962}
 963
 964static int unmerge_and_remove_all_rmap_items(void)
 965{
 966	struct mm_slot *mm_slot;
 967	struct mm_struct *mm;
 968	struct vm_area_struct *vma;
 969	int err = 0;
 970
 971	spin_lock(&ksm_mmlist_lock);
 972	ksm_scan.mm_slot = list_entry(ksm_mm_head.mm_list.next,
 973						struct mm_slot, mm_list);
 974	spin_unlock(&ksm_mmlist_lock);
 975
 976	for (mm_slot = ksm_scan.mm_slot;
 977			mm_slot != &ksm_mm_head; mm_slot = ksm_scan.mm_slot) {
 978		mm = mm_slot->mm;
 979		down_read(&mm->mmap_sem);
 980		for (vma = mm->mmap; vma; vma = vma->vm_next) {
 981			if (ksm_test_exit(mm))
 982				break;
 983			if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
 984				continue;
 985			err = unmerge_ksm_pages(vma,
 986						vma->vm_start, vma->vm_end);
 987			if (err)
 988				goto error;
 989		}
 990
 991		remove_trailing_rmap_items(mm_slot, &mm_slot->rmap_list);
 992		up_read(&mm->mmap_sem);
 993
 994		spin_lock(&ksm_mmlist_lock);
 995		ksm_scan.mm_slot = list_entry(mm_slot->mm_list.next,
 996						struct mm_slot, mm_list);
 997		if (ksm_test_exit(mm)) {
 998			hash_del(&mm_slot->link);
 999			list_del(&mm_slot->mm_list);
1000			spin_unlock(&ksm_mmlist_lock);
1001
1002			free_mm_slot(mm_slot);
1003			clear_bit(MMF_VM_MERGEABLE, &mm->flags);
1004			mmdrop(mm);
1005		} else
1006			spin_unlock(&ksm_mmlist_lock);
1007	}
1008
1009	/* Clean up stable nodes, but don't worry if some are still busy */
1010	remove_all_stable_nodes();
1011	ksm_scan.seqnr = 0;
1012	return 0;
1013
1014error:
1015	up_read(&mm->mmap_sem);
1016	spin_lock(&ksm_mmlist_lock);
1017	ksm_scan.mm_slot = &ksm_mm_head;
1018	spin_unlock(&ksm_mmlist_lock);
1019	return err;
1020}
1021#endif /* CONFIG_SYSFS */
1022
1023static u32 calc_checksum(struct page *page)
1024{
1025	u32 checksum;
1026	void *addr = kmap_atomic(page);
1027	checksum = xxhash(addr, PAGE_SIZE, 0);
1028	kunmap_atomic(addr);
1029	return checksum;
1030}
1031
1032static int write_protect_page(struct vm_area_struct *vma, struct page *page,
1033			      pte_t *orig_pte)
1034{
1035	struct mm_struct *mm = vma->vm_mm;
1036	struct page_vma_mapped_walk pvmw = {
1037		.page = page,
1038		.vma = vma,
1039	};
1040	int swapped;
1041	int err = -EFAULT;
1042	struct mmu_notifier_range range;
1043
1044	pvmw.address = page_address_in_vma(page, vma);
1045	if (pvmw.address == -EFAULT)
1046		goto out;
1047
1048	BUG_ON(PageTransCompound(page));
1049
1050	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, mm,
1051				pvmw.address,
1052				pvmw.address + PAGE_SIZE);
1053	mmu_notifier_invalidate_range_start(&range);
1054
1055	if (!page_vma_mapped_walk(&pvmw))
1056		goto out_mn;
1057	if (WARN_ONCE(!pvmw.pte, "Unexpected PMD mapping?"))
1058		goto out_unlock;
1059
1060	if (pte_write(*pvmw.pte) || pte_dirty(*pvmw.pte) ||
1061	    (pte_protnone(*pvmw.pte) && pte_savedwrite(*pvmw.pte)) ||
1062						mm_tlb_flush_pending(mm)) {
1063		pte_t entry;
1064
1065		swapped = PageSwapCache(page);
1066		flush_cache_page(vma, pvmw.address, page_to_pfn(page));
1067		/*
1068		 * Ok this is tricky, when get_user_pages_fast() run it doesn't
1069		 * take any lock, therefore the check that we are going to make
1070		 * with the pagecount against the mapcount is racey and
1071		 * O_DIRECT can happen right after the check.
1072		 * So we clear the pte and flush the tlb before the check
1073		 * this assure us that no O_DIRECT can happen after the check
1074		 * or in the middle of the check.
1075		 *
1076		 * No need to notify as we are downgrading page table to read
1077		 * only not changing it to point to a new page.
1078		 *
1079		 * See Documentation/vm/mmu_notifier.rst
1080		 */
1081		entry = ptep_clear_flush(vma, pvmw.address, pvmw.pte);
1082		/*
1083		 * Check that no O_DIRECT or similar I/O is in progress on the
1084		 * page
1085		 */
1086		if (page_mapcount(page) + 1 + swapped != page_count(page)) {
1087			set_pte_at(mm, pvmw.address, pvmw.pte, entry);
1088			goto out_unlock;
1089		}
1090		if (pte_dirty(entry))
1091			set_page_dirty(page);
1092
1093		if (pte_protnone(entry))
1094			entry = pte_mkclean(pte_clear_savedwrite(entry));
1095		else
1096			entry = pte_mkclean(pte_wrprotect(entry));
1097		set_pte_at_notify(mm, pvmw.address, pvmw.pte, entry);
1098	}
1099	*orig_pte = *pvmw.pte;
1100	err = 0;
1101
1102out_unlock:
1103	page_vma_mapped_walk_done(&pvmw);
1104out_mn:
1105	mmu_notifier_invalidate_range_end(&range);
1106out:
1107	return err;
1108}
1109
1110/**
1111 * replace_page - replace page in vma by new ksm page
1112 * @vma:      vma that holds the pte pointing to page
1113 * @page:     the page we are replacing by kpage
1114 * @kpage:    the ksm page we replace page by
1115 * @orig_pte: the original value of the pte
1116 *
1117 * Returns 0 on success, -EFAULT on failure.
1118 */
1119static int replace_page(struct vm_area_struct *vma, struct page *page,
1120			struct page *kpage, pte_t orig_pte)
1121{
1122	struct mm_struct *mm = vma->vm_mm;
1123	pmd_t *pmd;
1124	pte_t *ptep;
1125	pte_t newpte;
1126	spinlock_t *ptl;
1127	unsigned long addr;
1128	int err = -EFAULT;
1129	struct mmu_notifier_range range;
1130
1131	addr = page_address_in_vma(page, vma);
1132	if (addr == -EFAULT)
1133		goto out;
1134
1135	pmd = mm_find_pmd(mm, addr);
1136	if (!pmd)
1137		goto out;
1138
1139	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, mm, addr,
1140				addr + PAGE_SIZE);
1141	mmu_notifier_invalidate_range_start(&range);
1142
1143	ptep = pte_offset_map_lock(mm, pmd, addr, &ptl);
1144	if (!pte_same(*ptep, orig_pte)) {
1145		pte_unmap_unlock(ptep, ptl);
1146		goto out_mn;
1147	}
1148
1149	/*
1150	 * No need to check ksm_use_zero_pages here: we can only have a
1151	 * zero_page here if ksm_use_zero_pages was enabled alreaady.
1152	 */
1153	if (!is_zero_pfn(page_to_pfn(kpage))) {
1154		get_page(kpage);
1155		page_add_anon_rmap(kpage, vma, addr, false);
1156		newpte = mk_pte(kpage, vma->vm_page_prot);
1157	} else {
1158		newpte = pte_mkspecial(pfn_pte(page_to_pfn(kpage),
1159					       vma->vm_page_prot));
1160		/*
1161		 * We're replacing an anonymous page with a zero page, which is
1162		 * not anonymous. We need to do proper accounting otherwise we
1163		 * will get wrong values in /proc, and a BUG message in dmesg
1164		 * when tearing down the mm.
1165		 */
1166		dec_mm_counter(mm, MM_ANONPAGES);
1167	}
1168
1169	flush_cache_page(vma, addr, pte_pfn(*ptep));
1170	/*
1171	 * No need to notify as we are replacing a read only page with another
1172	 * read only page with the same content.
1173	 *
1174	 * See Documentation/vm/mmu_notifier.rst
1175	 */
1176	ptep_clear_flush(vma, addr, ptep);
1177	set_pte_at_notify(mm, addr, ptep, newpte);
1178
1179	page_remove_rmap(page, false);
1180	if (!page_mapped(page))
1181		try_to_free_swap(page);
1182	put_page(page);
1183
1184	pte_unmap_unlock(ptep, ptl);
1185	err = 0;
1186out_mn:
1187	mmu_notifier_invalidate_range_end(&range);
1188out:
1189	return err;
1190}
1191
1192/*
1193 * try_to_merge_one_page - take two pages and merge them into one
1194 * @vma: the vma that holds the pte pointing to page
1195 * @page: the PageAnon page that we want to replace with kpage
1196 * @kpage: the PageKsm page that we want to map instead of page,
1197 *         or NULL the first time when we want to use page as kpage.
1198 *
1199 * This function returns 0 if the pages were merged, -EFAULT otherwise.
1200 */
1201static int try_to_merge_one_page(struct vm_area_struct *vma,
1202				 struct page *page, struct page *kpage)
1203{
1204	pte_t orig_pte = __pte(0);
1205	int err = -EFAULT;
1206
1207	if (page == kpage)			/* ksm page forked */
1208		return 0;
1209
1210	if (!PageAnon(page))
1211		goto out;
1212
1213	/*
1214	 * We need the page lock to read a stable PageSwapCache in
1215	 * write_protect_page().  We use trylock_page() instead of
1216	 * lock_page() because we don't want to wait here - we
1217	 * prefer to continue scanning and merging different pages,
1218	 * then come back to this page when it is unlocked.
1219	 */
1220	if (!trylock_page(page))
1221		goto out;
1222
1223	if (PageTransCompound(page)) {
1224		if (split_huge_page(page))
1225			goto out_unlock;
1226	}
1227
1228	/*
1229	 * If this anonymous page is mapped only here, its pte may need
1230	 * to be write-protected.  If it's mapped elsewhere, all of its
1231	 * ptes are necessarily already write-protected.  But in either
1232	 * case, we need to lock and check page_count is not raised.
1233	 */
1234	if (write_protect_page(vma, page, &orig_pte) == 0) {
1235		if (!kpage) {
1236			/*
1237			 * While we hold page lock, upgrade page from
1238			 * PageAnon+anon_vma to PageKsm+NULL stable_node:
1239			 * stable_tree_insert() will update stable_node.
1240			 */
1241			set_page_stable_node(page, NULL);
1242			mark_page_accessed(page);
1243			/*
1244			 * Page reclaim just frees a clean page with no dirty
1245			 * ptes: make sure that the ksm page would be swapped.
1246			 */
1247			if (!PageDirty(page))
1248				SetPageDirty(page);
1249			err = 0;
1250		} else if (pages_identical(page, kpage))
1251			err = replace_page(vma, page, kpage, orig_pte);
1252	}
1253
1254	if ((vma->vm_flags & VM_LOCKED) && kpage && !err) {
1255		munlock_vma_page(page);
1256		if (!PageMlocked(kpage)) {
1257			unlock_page(page);
1258			lock_page(kpage);
1259			mlock_vma_page(kpage);
1260			page = kpage;		/* for final unlock */
1261		}
1262	}
1263
1264out_unlock:
1265	unlock_page(page);
1266out:
1267	return err;
1268}
1269
1270/*
1271 * try_to_merge_with_ksm_page - like try_to_merge_two_pages,
1272 * but no new kernel page is allocated: kpage must already be a ksm page.
1273 *
1274 * This function returns 0 if the pages were merged, -EFAULT otherwise.
1275 */
1276static int try_to_merge_with_ksm_page(struct rmap_item *rmap_item,
1277				      struct page *page, struct page *kpage)
1278{
1279	struct mm_struct *mm = rmap_item->mm;
1280	struct vm_area_struct *vma;
1281	int err = -EFAULT;
1282
1283	down_read(&mm->mmap_sem);
1284	vma = find_mergeable_vma(mm, rmap_item->address);
1285	if (!vma)
1286		goto out;
1287
1288	err = try_to_merge_one_page(vma, page, kpage);
1289	if (err)
1290		goto out;
1291
1292	/* Unstable nid is in union with stable anon_vma: remove first */
1293	remove_rmap_item_from_tree(rmap_item);
1294
1295	/* Must get reference to anon_vma while still holding mmap_sem */
1296	rmap_item->anon_vma = vma->anon_vma;
1297	get_anon_vma(vma->anon_vma);
1298out:
1299	up_read(&mm->mmap_sem);
1300	return err;
1301}
1302
1303/*
1304 * try_to_merge_two_pages - take two identical pages and prepare them
1305 * to be merged into one page.
1306 *
1307 * This function returns the kpage if we successfully merged two identical
1308 * pages into one ksm page, NULL otherwise.
1309 *
1310 * Note that this function upgrades page to ksm page: if one of the pages
1311 * is already a ksm page, try_to_merge_with_ksm_page should be used.
1312 */
1313static struct page *try_to_merge_two_pages(struct rmap_item *rmap_item,
1314					   struct page *page,
1315					   struct rmap_item *tree_rmap_item,
1316					   struct page *tree_page)
1317{
1318	int err;
1319
1320	err = try_to_merge_with_ksm_page(rmap_item, page, NULL);
1321	if (!err) {
1322		err = try_to_merge_with_ksm_page(tree_rmap_item,
1323							tree_page, page);
1324		/*
1325		 * If that fails, we have a ksm page with only one pte
1326		 * pointing to it: so break it.
1327		 */
1328		if (err)
1329			break_cow(rmap_item);
1330	}
1331	return err ? NULL : page;
1332}
1333
1334static __always_inline
1335bool __is_page_sharing_candidate(struct stable_node *stable_node, int offset)
1336{
1337	VM_BUG_ON(stable_node->rmap_hlist_len < 0);
1338	/*
1339	 * Check that at least one mapping still exists, otherwise
1340	 * there's no much point to merge and share with this
1341	 * stable_node, as the underlying tree_page of the other
1342	 * sharer is going to be freed soon.
1343	 */
1344	return stable_node->rmap_hlist_len &&
1345		stable_node->rmap_hlist_len + offset < ksm_max_page_sharing;
1346}
1347
1348static __always_inline
1349bool is_page_sharing_candidate(struct stable_node *stable_node)
1350{
1351	return __is_page_sharing_candidate(stable_node, 0);
1352}
1353
1354static struct page *stable_node_dup(struct stable_node **_stable_node_dup,
1355				    struct stable_node **_stable_node,
1356				    struct rb_root *root,
1357				    bool prune_stale_stable_nodes)
1358{
1359	struct stable_node *dup, *found = NULL, *stable_node = *_stable_node;
1360	struct hlist_node *hlist_safe;
1361	struct page *_tree_page, *tree_page = NULL;
1362	int nr = 0;
1363	int found_rmap_hlist_len;
1364
1365	if (!prune_stale_stable_nodes ||
1366	    time_before(jiffies, stable_node->chain_prune_time +
1367			msecs_to_jiffies(
1368				ksm_stable_node_chains_prune_millisecs)))
1369		prune_stale_stable_nodes = false;
1370	else
1371		stable_node->chain_prune_time = jiffies;
1372
1373	hlist_for_each_entry_safe(dup, hlist_safe,
1374				  &stable_node->hlist, hlist_dup) {
1375		cond_resched();
1376		/*
1377		 * We must walk all stable_node_dup to prune the stale
1378		 * stable nodes during lookup.
1379		 *
1380		 * get_ksm_page can drop the nodes from the
1381		 * stable_node->hlist if they point to freed pages
1382		 * (that's why we do a _safe walk). The "dup"
1383		 * stable_node parameter itself will be freed from
1384		 * under us if it returns NULL.
1385		 */
1386		_tree_page = get_ksm_page(dup, GET_KSM_PAGE_NOLOCK);
1387		if (!_tree_page)
1388			continue;
1389		nr += 1;
1390		if (is_page_sharing_candidate(dup)) {
1391			if (!found ||
1392			    dup->rmap_hlist_len > found_rmap_hlist_len) {
1393				if (found)
1394					put_page(tree_page);
1395				found = dup;
1396				found_rmap_hlist_len = found->rmap_hlist_len;
1397				tree_page = _tree_page;
1398
1399				/* skip put_page for found dup */
1400				if (!prune_stale_stable_nodes)
1401					break;
1402				continue;
1403			}
1404		}
1405		put_page(_tree_page);
1406	}
1407
1408	if (found) {
1409		/*
1410		 * nr is counting all dups in the chain only if
1411		 * prune_stale_stable_nodes is true, otherwise we may
1412		 * break the loop at nr == 1 even if there are
1413		 * multiple entries.
1414		 */
1415		if (prune_stale_stable_nodes && nr == 1) {
1416			/*
1417			 * If there's not just one entry it would
1418			 * corrupt memory, better BUG_ON. In KSM
1419			 * context with no lock held it's not even
1420			 * fatal.
1421			 */
1422			BUG_ON(stable_node->hlist.first->next);
1423
1424			/*
1425			 * There's just one entry and it is below the
1426			 * deduplication limit so drop the chain.
1427			 */
1428			rb_replace_node(&stable_node->node, &found->node,
1429					root);
1430			free_stable_node(stable_node);
1431			ksm_stable_node_chains--;
1432			ksm_stable_node_dups--;
1433			/*
1434			 * NOTE: the caller depends on the stable_node
1435			 * to be equal to stable_node_dup if the chain
1436			 * was collapsed.
1437			 */
1438			*_stable_node = found;
1439			/*
1440			 * Just for robustneess as stable_node is
1441			 * otherwise left as a stable pointer, the
1442			 * compiler shall optimize it away at build
1443			 * time.
1444			 */
1445			stable_node = NULL;
1446		} else if (stable_node->hlist.first != &found->hlist_dup &&
1447			   __is_page_sharing_candidate(found, 1)) {
1448			/*
1449			 * If the found stable_node dup can accept one
1450			 * more future merge (in addition to the one
1451			 * that is underway) and is not at the head of
1452			 * the chain, put it there so next search will
1453			 * be quicker in the !prune_stale_stable_nodes
1454			 * case.
1455			 *
1456			 * NOTE: it would be inaccurate to use nr > 1
1457			 * instead of checking the hlist.first pointer
1458			 * directly, because in the
1459			 * prune_stale_stable_nodes case "nr" isn't
1460			 * the position of the found dup in the chain,
1461			 * but the total number of dups in the chain.
1462			 */
1463			hlist_del(&found->hlist_dup);
1464			hlist_add_head(&found->hlist_dup,
1465				       &stable_node->hlist);
1466		}
1467	}
1468
1469	*_stable_node_dup = found;
1470	return tree_page;
1471}
1472
1473static struct stable_node *stable_node_dup_any(struct stable_node *stable_node,
1474					       struct rb_root *root)
1475{
1476	if (!is_stable_node_chain(stable_node))
1477		return stable_node;
1478	if (hlist_empty(&stable_node->hlist)) {
1479		free_stable_node_chain(stable_node, root);
1480		return NULL;
1481	}
1482	return hlist_entry(stable_node->hlist.first,
1483			   typeof(*stable_node), hlist_dup);
1484}
1485
1486/*
1487 * Like for get_ksm_page, this function can free the *_stable_node and
1488 * *_stable_node_dup if the returned tree_page is NULL.
1489 *
1490 * It can also free and overwrite *_stable_node with the found
1491 * stable_node_dup if the chain is collapsed (in which case
1492 * *_stable_node will be equal to *_stable_node_dup like if the chain
1493 * never existed). It's up to the caller to verify tree_page is not
1494 * NULL before dereferencing *_stable_node or *_stable_node_dup.
1495 *
1496 * *_stable_node_dup is really a second output parameter of this
1497 * function and will be overwritten in all cases, the caller doesn't
1498 * need to initialize it.
1499 */
1500static struct page *__stable_node_chain(struct stable_node **_stable_node_dup,
1501					struct stable_node **_stable_node,
1502					struct rb_root *root,
1503					bool prune_stale_stable_nodes)
1504{
1505	struct stable_node *stable_node = *_stable_node;
1506	if (!is_stable_node_chain(stable_node)) {
1507		if (is_page_sharing_candidate(stable_node)) {
1508			*_stable_node_dup = stable_node;
1509			return get_ksm_page(stable_node, GET_KSM_PAGE_NOLOCK);
1510		}
1511		/*
1512		 * _stable_node_dup set to NULL means the stable_node
1513		 * reached the ksm_max_page_sharing limit.
1514		 */
1515		*_stable_node_dup = NULL;
1516		return NULL;
1517	}
1518	return stable_node_dup(_stable_node_dup, _stable_node, root,
1519			       prune_stale_stable_nodes);
1520}
1521
1522static __always_inline struct page *chain_prune(struct stable_node **s_n_d,
1523						struct stable_node **s_n,
1524						struct rb_root *root)
1525{
1526	return __stable_node_chain(s_n_d, s_n, root, true);
1527}
1528
1529static __always_inline struct page *chain(struct stable_node **s_n_d,
1530					  struct stable_node *s_n,
1531					  struct rb_root *root)
1532{
1533	struct stable_node *old_stable_node = s_n;
1534	struct page *tree_page;
1535
1536	tree_page = __stable_node_chain(s_n_d, &s_n, root, false);
1537	/* not pruning dups so s_n cannot have changed */
1538	VM_BUG_ON(s_n != old_stable_node);
1539	return tree_page;
1540}
1541
1542/*
1543 * stable_tree_search - search for page inside the stable tree
1544 *
1545 * This function checks if there is a page inside the stable tree
1546 * with identical content to the page that we are scanning right now.
1547 *
1548 * This function returns the stable tree node of identical content if found,
1549 * NULL otherwise.
1550 */
1551static struct page *stable_tree_search(struct page *page)
1552{
1553	int nid;
1554	struct rb_root *root;
1555	struct rb_node **new;
1556	struct rb_node *parent;
1557	struct stable_node *stable_node, *stable_node_dup, *stable_node_any;
1558	struct stable_node *page_node;
1559
1560	page_node = page_stable_node(page);
1561	if (page_node && page_node->head != &migrate_nodes) {
1562		/* ksm page forked */
1563		get_page(page);
1564		return page;
1565	}
1566
1567	nid = get_kpfn_nid(page_to_pfn(page));
1568	root = root_stable_tree + nid;
1569again:
1570	new = &root->rb_node;
1571	parent = NULL;
1572
1573	while (*new) {
1574		struct page *tree_page;
1575		int ret;
1576
1577		cond_resched();
1578		stable_node = rb_entry(*new, struct stable_node, node);
1579		stable_node_any = NULL;
1580		tree_page = chain_prune(&stable_node_dup, &stable_node,	root);
1581		/*
1582		 * NOTE: stable_node may have been freed by
1583		 * chain_prune() if the returned stable_node_dup is
1584		 * not NULL. stable_node_dup may have been inserted in
1585		 * the rbtree instead as a regular stable_node (in
1586		 * order to collapse the stable_node chain if a single
1587		 * stable_node dup was found in it). In such case the
1588		 * stable_node is overwritten by the calleee to point
1589		 * to the stable_node_dup that was collapsed in the
1590		 * stable rbtree and stable_node will be equal to
1591		 * stable_node_dup like if the chain never existed.
1592		 */
1593		if (!stable_node_dup) {
1594			/*
1595			 * Either all stable_node dups were full in
1596			 * this stable_node chain, or this chain was
1597			 * empty and should be rb_erased.
1598			 */
1599			stable_node_any = stable_node_dup_any(stable_node,
1600							      root);
1601			if (!stable_node_any) {
1602				/* rb_erase just run */
1603				goto again;
1604			}
1605			/*
1606			 * Take any of the stable_node dups page of
1607			 * this stable_node chain to let the tree walk
1608			 * continue. All KSM pages belonging to the
1609			 * stable_node dups in a stable_node chain
1610			 * have the same content and they're
1611			 * wrprotected at all times. Any will work
1612			 * fine to continue the walk.
1613			 */
1614			tree_page = get_ksm_page(stable_node_any,
1615						 GET_KSM_PAGE_NOLOCK);
1616		}
1617		VM_BUG_ON(!stable_node_dup ^ !!stable_node_any);
1618		if (!tree_page) {
1619			/*
1620			 * If we walked over a stale stable_node,
1621			 * get_ksm_page() will call rb_erase() and it
1622			 * may rebalance the tree from under us. So
1623			 * restart the search from scratch. Returning
1624			 * NULL would be safe too, but we'd generate
1625			 * false negative insertions just because some
1626			 * stable_node was stale.
1627			 */
1628			goto again;
1629		}
1630
1631		ret = memcmp_pages(page, tree_page);
1632		put_page(tree_page);
1633
1634		parent = *new;
1635		if (ret < 0)
1636			new = &parent->rb_left;
1637		else if (ret > 0)
1638			new = &parent->rb_right;
1639		else {
1640			if (page_node) {
1641				VM_BUG_ON(page_node->head != &migrate_nodes);
1642				/*
1643				 * Test if the migrated page should be merged
1644				 * into a stable node dup. If the mapcount is
1645				 * 1 we can migrate it with another KSM page
1646				 * without adding it to the chain.
1647				 */
1648				if (page_mapcount(page) > 1)
1649					goto chain_append;
1650			}
1651
1652			if (!stable_node_dup) {
1653				/*
1654				 * If the stable_node is a chain and
1655				 * we got a payload match in memcmp
1656				 * but we cannot merge the scanned
1657				 * page in any of the existing
1658				 * stable_node dups because they're
1659				 * all full, we need to wait the
1660				 * scanned page to find itself a match
1661				 * in the unstable tree to create a
1662				 * brand new KSM page to add later to
1663				 * the dups of this stable_node.
1664				 */
1665				return NULL;
1666			}
1667
1668			/*
1669			 * Lock and unlock the stable_node's page (which
1670			 * might already have been migrated) so that page
1671			 * migration is sure to notice its raised count.
1672			 * It would be more elegant to return stable_node
1673			 * than kpage, but that involves more changes.
1674			 */
1675			tree_page = get_ksm_page(stable_node_dup,
1676						 GET_KSM_PAGE_TRYLOCK);
1677
1678			if (PTR_ERR(tree_page) == -EBUSY)
1679				return ERR_PTR(-EBUSY);
1680
1681			if (unlikely(!tree_page))
1682				/*
1683				 * The tree may have been rebalanced,
1684				 * so re-evaluate parent and new.
1685				 */
1686				goto again;
1687			unlock_page(tree_page);
1688
1689			if (get_kpfn_nid(stable_node_dup->kpfn) !=
1690			    NUMA(stable_node_dup->nid)) {
1691				put_page(tree_page);
1692				goto replace;
1693			}
1694			return tree_page;
1695		}
1696	}
1697
1698	if (!page_node)
1699		return NULL;
1700
1701	list_del(&page_node->list);
1702	DO_NUMA(page_node->nid = nid);
1703	rb_link_node(&page_node->node, parent, new);
1704	rb_insert_color(&page_node->node, root);
1705out:
1706	if (is_page_sharing_candidate(page_node)) {
1707		get_page(page);
1708		return page;
1709	} else
1710		return NULL;
1711
1712replace:
1713	/*
1714	 * If stable_node was a chain and chain_prune collapsed it,
1715	 * stable_node has been updated to be the new regular
1716	 * stable_node. A collapse of the chain is indistinguishable
1717	 * from the case there was no chain in the stable
1718	 * rbtree. Otherwise stable_node is the chain and
1719	 * stable_node_dup is the dup to replace.
1720	 */
1721	if (stable_node_dup == stable_node) {
1722		VM_BUG_ON(is_stable_node_chain(stable_node_dup));
1723		VM_BUG_ON(is_stable_node_dup(stable_node_dup));
1724		/* there is no chain */
1725		if (page_node) {
1726			VM_BUG_ON(page_node->head != &migrate_nodes);
1727			list_del(&page_node->list);
1728			DO_NUMA(page_node->nid = nid);
1729			rb_replace_node(&stable_node_dup->node,
1730					&page_node->node,
1731					root);
1732			if (is_page_sharing_candidate(page_node))
1733				get_page(page);
1734			else
1735				page = NULL;
1736		} else {
1737			rb_erase(&stable_node_dup->node, root);
1738			page = NULL;
1739		}
1740	} else {
1741		VM_BUG_ON(!is_stable_node_chain(stable_node));
1742		__stable_node_dup_del(stable_node_dup);
1743		if (page_node) {
1744			VM_BUG_ON(page_node->head != &migrate_nodes);
1745			list_del(&page_node->list);
1746			DO_NUMA(page_node->nid = nid);
1747			stable_node_chain_add_dup(page_node, stable_node);
1748			if (is_page_sharing_candidate(page_node))
1749				get_page(page);
1750			else
1751				page = NULL;
1752		} else {
1753			page = NULL;
1754		}
1755	}
1756	stable_node_dup->head = &migrate_nodes;
1757	list_add(&stable_node_dup->list, stable_node_dup->head);
1758	return page;
1759
1760chain_append:
1761	/* stable_node_dup could be null if it reached the limit */
1762	if (!stable_node_dup)
1763		stable_node_dup = stable_node_any;
1764	/*
1765	 * If stable_node was a chain and chain_prune collapsed it,
1766	 * stable_node has been updated to be the new regular
1767	 * stable_node. A collapse of the chain is indistinguishable
1768	 * from the case there was no chain in the stable
1769	 * rbtree. Otherwise stable_node is the chain and
1770	 * stable_node_dup is the dup to replace.
1771	 */
1772	if (stable_node_dup == stable_node) {
1773		VM_BUG_ON(is_stable_node_chain(stable_node_dup));
1774		VM_BUG_ON(is_stable_node_dup(stable_node_dup));
1775		/* chain is missing so create it */
1776		stable_node = alloc_stable_node_chain(stable_node_dup,
1777						      root);
1778		if (!stable_node)
1779			return NULL;
1780	}
1781	/*
1782	 * Add this stable_node dup that was
1783	 * migrated to the stable_node chain
1784	 * of the current nid for this page
1785	 * content.
1786	 */
1787	VM_BUG_ON(!is_stable_node_chain(stable_node));
1788	VM_BUG_ON(!is_stable_node_dup(stable_node_dup));
1789	VM_BUG_ON(page_node->head != &migrate_nodes);
1790	list_del(&page_node->list);
1791	DO_NUMA(page_node->nid = nid);
1792	stable_node_chain_add_dup(page_node, stable_node);
1793	goto out;
1794}
1795
1796/*
1797 * stable_tree_insert - insert stable tree node pointing to new ksm page
1798 * into the stable tree.
1799 *
1800 * This function returns the stable tree node just allocated on success,
1801 * NULL otherwise.
1802 */
1803static struct stable_node *stable_tree_insert(struct page *kpage)
1804{
1805	int nid;
1806	unsigned long kpfn;
1807	struct rb_root *root;
1808	struct rb_node **new;
1809	struct rb_node *parent;
1810	struct stable_node *stable_node, *stable_node_dup, *stable_node_any;
1811	bool need_chain = false;
1812
1813	kpfn = page_to_pfn(kpage);
1814	nid = get_kpfn_nid(kpfn);
1815	root = root_stable_tree + nid;
1816again:
1817	parent = NULL;
1818	new = &root->rb_node;
1819
1820	while (*new) {
1821		struct page *tree_page;
1822		int ret;
1823
1824		cond_resched();
1825		stable_node = rb_entry(*new, struct stable_node, node);
1826		stable_node_any = NULL;
1827		tree_page = chain(&stable_node_dup, stable_node, root);
1828		if (!stable_node_dup) {
1829			/*
1830			 * Either all stable_node dups were full in
1831			 * this stable_node chain, or this chain was
1832			 * empty and should be rb_erased.
1833			 */
1834			stable_node_any = stable_node_dup_any(stable_node,
1835							      root);
1836			if (!stable_node_any) {
1837				/* rb_erase just run */
1838				goto again;
1839			}
1840			/*
1841			 * Take any of the stable_node dups page of
1842			 * this stable_node chain to let the tree walk
1843			 * continue. All KSM pages belonging to the
1844			 * stable_node dups in a stable_node chain
1845			 * have the same content and they're
1846			 * wrprotected at all times. Any will work
1847			 * fine to continue the walk.
1848			 */
1849			tree_page = get_ksm_page(stable_node_any,
1850						 GET_KSM_PAGE_NOLOCK);
1851		}
1852		VM_BUG_ON(!stable_node_dup ^ !!stable_node_any);
1853		if (!tree_page) {
1854			/*
1855			 * If we walked over a stale stable_node,
1856			 * get_ksm_page() will call rb_erase() and it
1857			 * may rebalance the tree from under us. So
1858			 * restart the search from scratch. Returning
1859			 * NULL would be safe too, but we'd generate
1860			 * false negative insertions just because some
1861			 * stable_node was stale.
1862			 */
1863			goto again;
1864		}
1865
1866		ret = memcmp_pages(kpage, tree_page);
1867		put_page(tree_page);
1868
1869		parent = *new;
1870		if (ret < 0)
1871			new = &parent->rb_left;
1872		else if (ret > 0)
1873			new = &parent->rb_right;
1874		else {
1875			need_chain = true;
1876			break;
1877		}
1878	}
1879
1880	stable_node_dup = alloc_stable_node();
1881	if (!stable_node_dup)
1882		return NULL;
1883
1884	INIT_HLIST_HEAD(&stable_node_dup->hlist);
1885	stable_node_dup->kpfn = kpfn;
1886	set_page_stable_node(kpage, stable_node_dup);
1887	stable_node_dup->rmap_hlist_len = 0;
1888	DO_NUMA(stable_node_dup->nid = nid);
1889	if (!need_chain) {
1890		rb_link_node(&stable_node_dup->node, parent, new);
1891		rb_insert_color(&stable_node_dup->node, root);
1892	} else {
1893		if (!is_stable_node_chain(stable_node)) {
1894			struct stable_node *orig = stable_node;
1895			/* chain is missing so create it */
1896			stable_node = alloc_stable_node_chain(orig, root);
1897			if (!stable_node) {
1898				free_stable_node(stable_node_dup);
1899				return NULL;
1900			}
1901		}
1902		stable_node_chain_add_dup(stable_node_dup, stable_node);
1903	}
1904
1905	return stable_node_dup;
1906}
1907
1908/*
1909 * unstable_tree_search_insert - search for identical page,
1910 * else insert rmap_item into the unstable tree.
1911 *
1912 * This function searches for a page in the unstable tree identical to the
1913 * page currently being scanned; and if no identical page is found in the
1914 * tree, we insert rmap_item as a new object into the unstable tree.
1915 *
1916 * This function returns pointer to rmap_item found to be identical
1917 * to the currently scanned page, NULL otherwise.
1918 *
1919 * This function does both searching and inserting, because they share
1920 * the same walking algorithm in an rbtree.
1921 */
1922static
1923struct rmap_item *unstable_tree_search_insert(struct rmap_item *rmap_item,
1924					      struct page *page,
1925					      struct page **tree_pagep)
1926{
1927	struct rb_node **new;
1928	struct rb_root *root;
1929	struct rb_node *parent = NULL;
1930	int nid;
1931
1932	nid = get_kpfn_nid(page_to_pfn(page));
1933	root = root_unstable_tree + nid;
1934	new = &root->rb_node;
1935
1936	while (*new) {
1937		struct rmap_item *tree_rmap_item;
1938		struct page *tree_page;
1939		int ret;
1940
1941		cond_resched();
1942		tree_rmap_item = rb_entry(*new, struct rmap_item, node);
1943		tree_page = get_mergeable_page(tree_rmap_item);
1944		if (!tree_page)
1945			return NULL;
1946
1947		/*
1948		 * Don't substitute a ksm page for a forked page.
1949		 */
1950		if (page == tree_page) {
1951			put_page(tree_page);
1952			return NULL;
1953		}
1954
1955		ret = memcmp_pages(page, tree_page);
1956
1957		parent = *new;
1958		if (ret < 0) {
1959			put_page(tree_page);
1960			new = &parent->rb_left;
1961		} else if (ret > 0) {
1962			put_page(tree_page);
1963			new = &parent->rb_right;
1964		} else if (!ksm_merge_across_nodes &&
1965			   page_to_nid(tree_page) != nid) {
1966			/*
1967			 * If tree_page has been migrated to another NUMA node,
1968			 * it will be flushed out and put in the right unstable
1969			 * tree next time: only merge with it when across_nodes.
1970			 */
1971			put_page(tree_page);
1972			return NULL;
1973		} else {
1974			*tree_pagep = tree_page;
1975			return tree_rmap_item;
1976		}
1977	}
1978
1979	rmap_item->address |= UNSTABLE_FLAG;
1980	rmap_item->address |= (ksm_scan.seqnr & SEQNR_MASK);
1981	DO_NUMA(rmap_item->nid = nid);
1982	rb_link_node(&rmap_item->node, parent, new);
1983	rb_insert_color(&rmap_item->node, root);
1984
1985	ksm_pages_unshared++;
1986	return NULL;
1987}
1988
1989/*
1990 * stable_tree_append - add another rmap_item to the linked list of
1991 * rmap_items hanging off a given node of the stable tree, all sharing
1992 * the same ksm page.
1993 */
1994static void stable_tree_append(struct rmap_item *rmap_item,
1995			       struct stable_node *stable_node,
1996			       bool max_page_sharing_bypass)
1997{
1998	/*
1999	 * rmap won't find this mapping if we don't insert the
2000	 * rmap_item in the right stable_node
2001	 * duplicate. page_migration could break later if rmap breaks,
2002	 * so we can as well crash here. We really need to check for
2003	 * rmap_hlist_len == STABLE_NODE_CHAIN, but we can as well check
2004	 * for other negative values as an undeflow if detected here
2005	 * for the first time (and not when decreasing rmap_hlist_len)
2006	 * would be sign of memory corruption in the stable_node.
2007	 */
2008	BUG_ON(stable_node->rmap_hlist_len < 0);
2009
2010	stable_node->rmap_hlist_len++;
2011	if (!max_page_sharing_bypass)
2012		/* possibly non fatal but unexpected overflow, only warn */
2013		WARN_ON_ONCE(stable_node->rmap_hlist_len >
2014			     ksm_max_page_sharing);
2015
2016	rmap_item->head = stable_node;
2017	rmap_item->address |= STABLE_FLAG;
2018	hlist_add_head(&rmap_item->hlist, &stable_node->hlist);
2019
2020	if (rmap_item->hlist.next)
2021		ksm_pages_sharing++;
2022	else
2023		ksm_pages_shared++;
2024}
2025
2026/*
2027 * cmp_and_merge_page - first see if page can be merged into the stable tree;
2028 * if not, compare checksum to previous and if it's the same, see if page can
2029 * be inserted into the unstable tree, or merged with a page already there and
2030 * both transferred to the stable tree.
2031 *
2032 * @page: the page that we are searching identical page to.
2033 * @rmap_item: the reverse mapping into the virtual address of this page
2034 */
2035static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
2036{
2037	struct mm_struct *mm = rmap_item->mm;
2038	struct rmap_item *tree_rmap_item;
2039	struct page *tree_page = NULL;
2040	struct stable_node *stable_node;
2041	struct page *kpage;
2042	unsigned int checksum;
2043	int err;
2044	bool max_page_sharing_bypass = false;
2045
2046	stable_node = page_stable_node(page);
2047	if (stable_node) {
2048		if (stable_node->head != &migrate_nodes &&
2049		    get_kpfn_nid(READ_ONCE(stable_node->kpfn)) !=
2050		    NUMA(stable_node->nid)) {
2051			stable_node_dup_del(stable_node);
2052			stable_node->head = &migrate_nodes;
2053			list_add(&stable_node->list, stable_node->head);
2054		}
2055		if (stable_node->head != &migrate_nodes &&
2056		    rmap_item->head == stable_node)
2057			return;
2058		/*
2059		 * If it's a KSM fork, allow it to go over the sharing limit
2060		 * without warnings.
2061		 */
2062		if (!is_page_sharing_candidate(stable_node))
2063			max_page_sharing_bypass = true;
2064	}
2065
2066	/* We first start with searching the page inside the stable tree */
2067	kpage = stable_tree_search(page);
2068	if (kpage == page && rmap_item->head == stable_node) {
2069		put_page(kpage);
2070		return;
2071	}
2072
2073	remove_rmap_item_from_tree(rmap_item);
2074
2075	if (kpage) {
2076		if (PTR_ERR(kpage) == -EBUSY)
2077			return;
2078
2079		err = try_to_merge_with_ksm_page(rmap_item, page, kpage);
2080		if (!err) {
2081			/*
2082			 * The page was successfully merged:
2083			 * add its rmap_item to the stable tree.
2084			 */
2085			lock_page(kpage);
2086			stable_tree_append(rmap_item, page_stable_node(kpage),
2087					   max_page_sharing_bypass);
2088			unlock_page(kpage);
2089		}
2090		put_page(kpage);
2091		return;
2092	}
2093
2094	/*
2095	 * If the hash value of the page has changed from the last time
2096	 * we calculated it, this page is changing frequently: therefore we
2097	 * don't want to insert it in the unstable tree, and we don't want
2098	 * to waste our time searching for something identical to it there.
2099	 */
2100	checksum = calc_checksum(page);
2101	if (rmap_item->oldchecksum != checksum) {
2102		rmap_item->oldchecksum = checksum;
2103		return;
2104	}
2105
2106	/*
2107	 * Same checksum as an empty page. We attempt to merge it with the
2108	 * appropriate zero page if the user enabled this via sysfs.
2109	 */
2110	if (ksm_use_zero_pages && (checksum == zero_checksum)) {
2111		struct vm_area_struct *vma;
2112
2113		down_read(&mm->mmap_sem);
2114		vma = find_mergeable_vma(mm, rmap_item->address);
2115		err = try_to_merge_one_page(vma, page,
2116					    ZERO_PAGE(rmap_item->address));
2117		up_read(&mm->mmap_sem);
 
 
 
 
 
 
 
 
2118		/*
2119		 * In case of failure, the page was not really empty, so we
2120		 * need to continue. Otherwise we're done.
2121		 */
2122		if (!err)
2123			return;
2124	}
2125	tree_rmap_item =
2126		unstable_tree_search_insert(rmap_item, page, &tree_page);
2127	if (tree_rmap_item) {
2128		bool split;
2129
2130		kpage = try_to_merge_two_pages(rmap_item, page,
2131						tree_rmap_item, tree_page);
2132		/*
2133		 * If both pages we tried to merge belong to the same compound
2134		 * page, then we actually ended up increasing the reference
2135		 * count of the same compound page twice, and split_huge_page
2136		 * failed.
2137		 * Here we set a flag if that happened, and we use it later to
2138		 * try split_huge_page again. Since we call put_page right
2139		 * afterwards, the reference count will be correct and
2140		 * split_huge_page should succeed.
2141		 */
2142		split = PageTransCompound(page)
2143			&& compound_head(page) == compound_head(tree_page);
2144		put_page(tree_page);
2145		if (kpage) {
2146			/*
2147			 * The pages were successfully merged: insert new
2148			 * node in the stable tree and add both rmap_items.
2149			 */
2150			lock_page(kpage);
2151			stable_node = stable_tree_insert(kpage);
2152			if (stable_node) {
2153				stable_tree_append(tree_rmap_item, stable_node,
2154						   false);
2155				stable_tree_append(rmap_item, stable_node,
2156						   false);
2157			}
2158			unlock_page(kpage);
2159
2160			/*
2161			 * If we fail to insert the page into the stable tree,
2162			 * we will have 2 virtual addresses that are pointing
2163			 * to a ksm page left outside the stable tree,
2164			 * in which case we need to break_cow on both.
2165			 */
2166			if (!stable_node) {
2167				break_cow(tree_rmap_item);
2168				break_cow(rmap_item);
2169			}
2170		} else if (split) {
2171			/*
2172			 * We are here if we tried to merge two pages and
2173			 * failed because they both belonged to the same
2174			 * compound page. We will split the page now, but no
2175			 * merging will take place.
2176			 * We do not want to add the cost of a full lock; if
2177			 * the page is locked, it is better to skip it and
2178			 * perhaps try again later.
2179			 */
2180			if (!trylock_page(page))
2181				return;
2182			split_huge_page(page);
2183			unlock_page(page);
2184		}
2185	}
2186}
2187
2188static struct rmap_item *get_next_rmap_item(struct mm_slot *mm_slot,
2189					    struct rmap_item **rmap_list,
2190					    unsigned long addr)
2191{
2192	struct rmap_item *rmap_item;
2193
2194	while (*rmap_list) {
2195		rmap_item = *rmap_list;
2196		if ((rmap_item->address & PAGE_MASK) == addr)
2197			return rmap_item;
2198		if (rmap_item->address > addr)
2199			break;
2200		*rmap_list = rmap_item->rmap_list;
2201		remove_rmap_item_from_tree(rmap_item);
2202		free_rmap_item(rmap_item);
2203	}
2204
2205	rmap_item = alloc_rmap_item();
2206	if (rmap_item) {
2207		/* It has already been zeroed */
2208		rmap_item->mm = mm_slot->mm;
2209		rmap_item->address = addr;
2210		rmap_item->rmap_list = *rmap_list;
2211		*rmap_list = rmap_item;
2212	}
2213	return rmap_item;
2214}
2215
2216static struct rmap_item *scan_get_next_rmap_item(struct page **page)
2217{
2218	struct mm_struct *mm;
2219	struct mm_slot *slot;
2220	struct vm_area_struct *vma;
2221	struct rmap_item *rmap_item;
2222	int nid;
2223
2224	if (list_empty(&ksm_mm_head.mm_list))
2225		return NULL;
2226
2227	slot = ksm_scan.mm_slot;
2228	if (slot == &ksm_mm_head) {
2229		/*
2230		 * A number of pages can hang around indefinitely on per-cpu
2231		 * pagevecs, raised page count preventing write_protect_page
2232		 * from merging them.  Though it doesn't really matter much,
2233		 * it is puzzling to see some stuck in pages_volatile until
2234		 * other activity jostles them out, and they also prevented
2235		 * LTP's KSM test from succeeding deterministically; so drain
2236		 * them here (here rather than on entry to ksm_do_scan(),
2237		 * so we don't IPI too often when pages_to_scan is set low).
2238		 */
2239		lru_add_drain_all();
2240
2241		/*
2242		 * Whereas stale stable_nodes on the stable_tree itself
2243		 * get pruned in the regular course of stable_tree_search(),
2244		 * those moved out to the migrate_nodes list can accumulate:
2245		 * so prune them once before each full scan.
2246		 */
2247		if (!ksm_merge_across_nodes) {
2248			struct stable_node *stable_node, *next;
2249			struct page *page;
2250
2251			list_for_each_entry_safe(stable_node, next,
2252						 &migrate_nodes, list) {
2253				page = get_ksm_page(stable_node,
2254						    GET_KSM_PAGE_NOLOCK);
2255				if (page)
2256					put_page(page);
2257				cond_resched();
2258			}
2259		}
2260
2261		for (nid = 0; nid < ksm_nr_node_ids; nid++)
2262			root_unstable_tree[nid] = RB_ROOT;
2263
2264		spin_lock(&ksm_mmlist_lock);
2265		slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list);
2266		ksm_scan.mm_slot = slot;
2267		spin_unlock(&ksm_mmlist_lock);
2268		/*
2269		 * Although we tested list_empty() above, a racing __ksm_exit
2270		 * of the last mm on the list may have removed it since then.
2271		 */
2272		if (slot == &ksm_mm_head)
2273			return NULL;
2274next_mm:
2275		ksm_scan.address = 0;
2276		ksm_scan.rmap_list = &slot->rmap_list;
2277	}
2278
2279	mm = slot->mm;
2280	down_read(&mm->mmap_sem);
2281	if (ksm_test_exit(mm))
2282		vma = NULL;
2283	else
2284		vma = find_vma(mm, ksm_scan.address);
2285
2286	for (; vma; vma = vma->vm_next) {
2287		if (!(vma->vm_flags & VM_MERGEABLE))
2288			continue;
2289		if (ksm_scan.address < vma->vm_start)
2290			ksm_scan.address = vma->vm_start;
2291		if (!vma->anon_vma)
2292			ksm_scan.address = vma->vm_end;
2293
2294		while (ksm_scan.address < vma->vm_end) {
2295			if (ksm_test_exit(mm))
2296				break;
2297			*page = follow_page(vma, ksm_scan.address, FOLL_GET);
2298			if (IS_ERR_OR_NULL(*page)) {
2299				ksm_scan.address += PAGE_SIZE;
2300				cond_resched();
2301				continue;
2302			}
2303			if (PageAnon(*page)) {
2304				flush_anon_page(vma, *page, ksm_scan.address);
2305				flush_dcache_page(*page);
2306				rmap_item = get_next_rmap_item(slot,
2307					ksm_scan.rmap_list, ksm_scan.address);
2308				if (rmap_item) {
2309					ksm_scan.rmap_list =
2310							&rmap_item->rmap_list;
2311					ksm_scan.address += PAGE_SIZE;
2312				} else
2313					put_page(*page);
2314				up_read(&mm->mmap_sem);
2315				return rmap_item;
2316			}
2317			put_page(*page);
2318			ksm_scan.address += PAGE_SIZE;
2319			cond_resched();
2320		}
2321	}
2322
2323	if (ksm_test_exit(mm)) {
2324		ksm_scan.address = 0;
2325		ksm_scan.rmap_list = &slot->rmap_list;
2326	}
2327	/*
2328	 * Nuke all the rmap_items that are above this current rmap:
2329	 * because there were no VM_MERGEABLE vmas with such addresses.
2330	 */
2331	remove_trailing_rmap_items(slot, ksm_scan.rmap_list);
2332
2333	spin_lock(&ksm_mmlist_lock);
2334	ksm_scan.mm_slot = list_entry(slot->mm_list.next,
2335						struct mm_slot, mm_list);
2336	if (ksm_scan.address == 0) {
2337		/*
2338		 * We've completed a full scan of all vmas, holding mmap_sem
2339		 * throughout, and found no VM_MERGEABLE: so do the same as
2340		 * __ksm_exit does to remove this mm from all our lists now.
2341		 * This applies either when cleaning up after __ksm_exit
2342		 * (but beware: we can reach here even before __ksm_exit),
2343		 * or when all VM_MERGEABLE areas have been unmapped (and
2344		 * mmap_sem then protects against race with MADV_MERGEABLE).
2345		 */
2346		hash_del(&slot->link);
2347		list_del(&slot->mm_list);
2348		spin_unlock(&ksm_mmlist_lock);
2349
2350		free_mm_slot(slot);
2351		clear_bit(MMF_VM_MERGEABLE, &mm->flags);
2352		up_read(&mm->mmap_sem);
2353		mmdrop(mm);
2354	} else {
2355		up_read(&mm->mmap_sem);
2356		/*
2357		 * up_read(&mm->mmap_sem) first because after
2358		 * spin_unlock(&ksm_mmlist_lock) run, the "mm" may
2359		 * already have been freed under us by __ksm_exit()
2360		 * because the "mm_slot" is still hashed and
2361		 * ksm_scan.mm_slot doesn't point to it anymore.
2362		 */
2363		spin_unlock(&ksm_mmlist_lock);
2364	}
2365
2366	/* Repeat until we've completed scanning the whole list */
2367	slot = ksm_scan.mm_slot;
2368	if (slot != &ksm_mm_head)
2369		goto next_mm;
2370
2371	ksm_scan.seqnr++;
2372	return NULL;
2373}
2374
2375/**
2376 * ksm_do_scan  - the ksm scanner main worker function.
2377 * @scan_npages:  number of pages we want to scan before we return.
2378 */
2379static void ksm_do_scan(unsigned int scan_npages)
2380{
2381	struct rmap_item *rmap_item;
2382	struct page *uninitialized_var(page);
2383
2384	while (scan_npages-- && likely(!freezing(current))) {
2385		cond_resched();
2386		rmap_item = scan_get_next_rmap_item(&page);
2387		if (!rmap_item)
2388			return;
2389		cmp_and_merge_page(page, rmap_item);
2390		put_page(page);
2391	}
2392}
2393
2394static int ksmd_should_run(void)
2395{
2396	return (ksm_run & KSM_RUN_MERGE) && !list_empty(&ksm_mm_head.mm_list);
2397}
2398
2399static int ksm_scan_thread(void *nothing)
2400{
2401	unsigned int sleep_ms;
2402
2403	set_freezable();
2404	set_user_nice(current, 5);
2405
2406	while (!kthread_should_stop()) {
2407		mutex_lock(&ksm_thread_mutex);
2408		wait_while_offlining();
2409		if (ksmd_should_run())
2410			ksm_do_scan(ksm_thread_pages_to_scan);
2411		mutex_unlock(&ksm_thread_mutex);
2412
2413		try_to_freeze();
2414
2415		if (ksmd_should_run()) {
2416			sleep_ms = READ_ONCE(ksm_thread_sleep_millisecs);
2417			wait_event_interruptible_timeout(ksm_iter_wait,
2418				sleep_ms != READ_ONCE(ksm_thread_sleep_millisecs),
2419				msecs_to_jiffies(sleep_ms));
2420		} else {
2421			wait_event_freezable(ksm_thread_wait,
2422				ksmd_should_run() || kthread_should_stop());
2423		}
2424	}
2425	return 0;
2426}
2427
2428int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
2429		unsigned long end, int advice, unsigned long *vm_flags)
2430{
2431	struct mm_struct *mm = vma->vm_mm;
2432	int err;
2433
2434	switch (advice) {
2435	case MADV_MERGEABLE:
2436		/*
2437		 * Be somewhat over-protective for now!
2438		 */
2439		if (*vm_flags & (VM_MERGEABLE | VM_SHARED  | VM_MAYSHARE   |
2440				 VM_PFNMAP    | VM_IO      | VM_DONTEXPAND |
2441				 VM_HUGETLB | VM_MIXEDMAP))
2442			return 0;		/* just ignore the advice */
2443
2444		if (vma_is_dax(vma))
2445			return 0;
2446
2447#ifdef VM_SAO
2448		if (*vm_flags & VM_SAO)
2449			return 0;
2450#endif
2451#ifdef VM_SPARC_ADI
2452		if (*vm_flags & VM_SPARC_ADI)
2453			return 0;
2454#endif
2455
2456		if (!test_bit(MMF_VM_MERGEABLE, &mm->flags)) {
2457			err = __ksm_enter(mm);
2458			if (err)
2459				return err;
2460		}
2461
2462		*vm_flags |= VM_MERGEABLE;
2463		break;
2464
2465	case MADV_UNMERGEABLE:
2466		if (!(*vm_flags & VM_MERGEABLE))
2467			return 0;		/* just ignore the advice */
2468
2469		if (vma->anon_vma) {
2470			err = unmerge_ksm_pages(vma, start, end);
2471			if (err)
2472				return err;
2473		}
2474
2475		*vm_flags &= ~VM_MERGEABLE;
2476		break;
2477	}
2478
2479	return 0;
2480}
 
2481
2482int __ksm_enter(struct mm_struct *mm)
2483{
2484	struct mm_slot *mm_slot;
2485	int needs_wakeup;
2486
2487	mm_slot = alloc_mm_slot();
2488	if (!mm_slot)
2489		return -ENOMEM;
2490
2491	/* Check ksm_run too?  Would need tighter locking */
2492	needs_wakeup = list_empty(&ksm_mm_head.mm_list);
2493
2494	spin_lock(&ksm_mmlist_lock);
2495	insert_to_mm_slots_hash(mm, mm_slot);
2496	/*
2497	 * When KSM_RUN_MERGE (or KSM_RUN_STOP),
2498	 * insert just behind the scanning cursor, to let the area settle
2499	 * down a little; when fork is followed by immediate exec, we don't
2500	 * want ksmd to waste time setting up and tearing down an rmap_list.
2501	 *
2502	 * But when KSM_RUN_UNMERGE, it's important to insert ahead of its
2503	 * scanning cursor, otherwise KSM pages in newly forked mms will be
2504	 * missed: then we might as well insert at the end of the list.
2505	 */
2506	if (ksm_run & KSM_RUN_UNMERGE)
2507		list_add_tail(&mm_slot->mm_list, &ksm_mm_head.mm_list);
2508	else
2509		list_add_tail(&mm_slot->mm_list, &ksm_scan.mm_slot->mm_list);
2510	spin_unlock(&ksm_mmlist_lock);
2511
2512	set_bit(MMF_VM_MERGEABLE, &mm->flags);
2513	mmgrab(mm);
2514
2515	if (needs_wakeup)
2516		wake_up_interruptible(&ksm_thread_wait);
2517
2518	return 0;
2519}
2520
2521void __ksm_exit(struct mm_struct *mm)
2522{
2523	struct mm_slot *mm_slot;
2524	int easy_to_free = 0;
2525
2526	/*
2527	 * This process is exiting: if it's straightforward (as is the
2528	 * case when ksmd was never running), free mm_slot immediately.
2529	 * But if it's at the cursor or has rmap_items linked to it, use
2530	 * mmap_sem to synchronize with any break_cows before pagetables
2531	 * are freed, and leave the mm_slot on the list for ksmd to free.
2532	 * Beware: ksm may already have noticed it exiting and freed the slot.
2533	 */
2534
2535	spin_lock(&ksm_mmlist_lock);
2536	mm_slot = get_mm_slot(mm);
2537	if (mm_slot && ksm_scan.mm_slot != mm_slot) {
2538		if (!mm_slot->rmap_list) {
2539			hash_del(&mm_slot->link);
2540			list_del(&mm_slot->mm_list);
2541			easy_to_free = 1;
2542		} else {
2543			list_move(&mm_slot->mm_list,
2544				  &ksm_scan.mm_slot->mm_list);
2545		}
2546	}
2547	spin_unlock(&ksm_mmlist_lock);
2548
2549	if (easy_to_free) {
2550		free_mm_slot(mm_slot);
2551		clear_bit(MMF_VM_MERGEABLE, &mm->flags);
2552		mmdrop(mm);
2553	} else if (mm_slot) {
2554		down_write(&mm->mmap_sem);
2555		up_write(&mm->mmap_sem);
2556	}
2557}
2558
2559struct page *ksm_might_need_to_copy(struct page *page,
2560			struct vm_area_struct *vma, unsigned long address)
2561{
2562	struct anon_vma *anon_vma = page_anon_vma(page);
2563	struct page *new_page;
2564
2565	if (PageKsm(page)) {
2566		if (page_stable_node(page) &&
2567		    !(ksm_run & KSM_RUN_UNMERGE))
2568			return page;	/* no need to copy it */
2569	} else if (!anon_vma) {
2570		return page;		/* no need to copy it */
2571	} else if (anon_vma->root == vma->anon_vma->root &&
2572		 page->index == linear_page_index(vma, address)) {
2573		return page;		/* still no need to copy it */
2574	}
2575	if (!PageUptodate(page))
2576		return page;		/* let do_swap_page report the error */
2577
2578	new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);
 
 
 
 
2579	if (new_page) {
2580		copy_user_highpage(new_page, page, address, vma);
2581
2582		SetPageDirty(new_page);
2583		__SetPageUptodate(new_page);
2584		__SetPageLocked(new_page);
2585	}
2586
2587	return new_page;
2588}
2589
2590void rmap_walk_ksm(struct page *page, struct rmap_walk_control *rwc)
2591{
2592	struct stable_node *stable_node;
2593	struct rmap_item *rmap_item;
2594	int search_new_forks = 0;
2595
2596	VM_BUG_ON_PAGE(!PageKsm(page), page);
2597
2598	/*
2599	 * Rely on the page lock to protect against concurrent modifications
2600	 * to that page's node of the stable tree.
2601	 */
2602	VM_BUG_ON_PAGE(!PageLocked(page), page);
2603
2604	stable_node = page_stable_node(page);
2605	if (!stable_node)
2606		return;
2607again:
2608	hlist_for_each_entry(rmap_item, &stable_node->hlist, hlist) {
2609		struct anon_vma *anon_vma = rmap_item->anon_vma;
2610		struct anon_vma_chain *vmac;
2611		struct vm_area_struct *vma;
2612
2613		cond_resched();
2614		anon_vma_lock_read(anon_vma);
2615		anon_vma_interval_tree_foreach(vmac, &anon_vma->rb_root,
2616					       0, ULONG_MAX) {
2617			unsigned long addr;
2618
2619			cond_resched();
2620			vma = vmac->vma;
2621
2622			/* Ignore the stable/unstable/sqnr flags */
2623			addr = rmap_item->address & ~KSM_FLAG_MASK;
2624
2625			if (addr < vma->vm_start || addr >= vma->vm_end)
2626				continue;
2627			/*
2628			 * Initially we examine only the vma which covers this
2629			 * rmap_item; but later, if there is still work to do,
2630			 * we examine covering vmas in other mms: in case they
2631			 * were forked from the original since ksmd passed.
2632			 */
2633			if ((rmap_item->mm == vma->vm_mm) == search_new_forks)
2634				continue;
2635
2636			if (rwc->invalid_vma && rwc->invalid_vma(vma, rwc->arg))
2637				continue;
2638
2639			if (!rwc->rmap_one(page, vma, addr, rwc->arg)) {
2640				anon_vma_unlock_read(anon_vma);
2641				return;
2642			}
2643			if (rwc->done && rwc->done(page)) {
2644				anon_vma_unlock_read(anon_vma);
2645				return;
2646			}
2647		}
2648		anon_vma_unlock_read(anon_vma);
2649	}
2650	if (!search_new_forks++)
2651		goto again;
2652}
2653
2654bool reuse_ksm_page(struct page *page,
2655		    struct vm_area_struct *vma,
2656		    unsigned long address)
2657{
2658#ifdef CONFIG_DEBUG_VM
2659	if (WARN_ON(is_zero_pfn(page_to_pfn(page))) ||
2660			WARN_ON(!page_mapped(page)) ||
2661			WARN_ON(!PageLocked(page))) {
2662		dump_page(page, "reuse_ksm_page");
2663		return false;
2664	}
2665#endif
2666
2667	if (PageSwapCache(page) || !page_stable_node(page))
2668		return false;
2669	/* Prohibit parallel get_ksm_page() */
2670	if (!page_ref_freeze(page, 1))
2671		return false;
2672
2673	page_move_anon_rmap(page, vma);
2674	page->index = linear_page_index(vma, address);
2675	page_ref_unfreeze(page, 1);
2676
2677	return true;
2678}
2679#ifdef CONFIG_MIGRATION
2680void ksm_migrate_page(struct page *newpage, struct page *oldpage)
2681{
2682	struct stable_node *stable_node;
2683
2684	VM_BUG_ON_PAGE(!PageLocked(oldpage), oldpage);
2685	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
2686	VM_BUG_ON_PAGE(newpage->mapping != oldpage->mapping, newpage);
2687
2688	stable_node = page_stable_node(newpage);
2689	if (stable_node) {
2690		VM_BUG_ON_PAGE(stable_node->kpfn != page_to_pfn(oldpage), oldpage);
2691		stable_node->kpfn = page_to_pfn(newpage);
2692		/*
2693		 * newpage->mapping was set in advance; now we need smp_wmb()
2694		 * to make sure that the new stable_node->kpfn is visible
2695		 * to get_ksm_page() before it can see that oldpage->mapping
2696		 * has gone stale (or that PageSwapCache has been cleared).
2697		 */
2698		smp_wmb();
2699		set_page_stable_node(oldpage, NULL);
2700	}
2701}
2702#endif /* CONFIG_MIGRATION */
2703
2704#ifdef CONFIG_MEMORY_HOTREMOVE
2705static void wait_while_offlining(void)
2706{
2707	while (ksm_run & KSM_RUN_OFFLINE) {
2708		mutex_unlock(&ksm_thread_mutex);
2709		wait_on_bit(&ksm_run, ilog2(KSM_RUN_OFFLINE),
2710			    TASK_UNINTERRUPTIBLE);
2711		mutex_lock(&ksm_thread_mutex);
2712	}
2713}
2714
2715static bool stable_node_dup_remove_range(struct stable_node *stable_node,
2716					 unsigned long start_pfn,
2717					 unsigned long end_pfn)
2718{
2719	if (stable_node->kpfn >= start_pfn &&
2720	    stable_node->kpfn < end_pfn) {
2721		/*
2722		 * Don't get_ksm_page, page has already gone:
2723		 * which is why we keep kpfn instead of page*
2724		 */
2725		remove_node_from_stable_tree(stable_node);
2726		return true;
2727	}
2728	return false;
2729}
2730
2731static bool stable_node_chain_remove_range(struct stable_node *stable_node,
2732					   unsigned long start_pfn,
2733					   unsigned long end_pfn,
2734					   struct rb_root *root)
2735{
2736	struct stable_node *dup;
2737	struct hlist_node *hlist_safe;
2738
2739	if (!is_stable_node_chain(stable_node)) {
2740		VM_BUG_ON(is_stable_node_dup(stable_node));
2741		return stable_node_dup_remove_range(stable_node, start_pfn,
2742						    end_pfn);
2743	}
2744
2745	hlist_for_each_entry_safe(dup, hlist_safe,
2746				  &stable_node->hlist, hlist_dup) {
2747		VM_BUG_ON(!is_stable_node_dup(dup));
2748		stable_node_dup_remove_range(dup, start_pfn, end_pfn);
2749	}
2750	if (hlist_empty(&stable_node->hlist)) {
2751		free_stable_node_chain(stable_node, root);
2752		return true; /* notify caller that tree was rebalanced */
2753	} else
2754		return false;
2755}
2756
2757static void ksm_check_stable_tree(unsigned long start_pfn,
2758				  unsigned long end_pfn)
2759{
2760	struct stable_node *stable_node, *next;
2761	struct rb_node *node;
2762	int nid;
2763
2764	for (nid = 0; nid < ksm_nr_node_ids; nid++) {
2765		node = rb_first(root_stable_tree + nid);
2766		while (node) {
2767			stable_node = rb_entry(node, struct stable_node, node);
2768			if (stable_node_chain_remove_range(stable_node,
2769							   start_pfn, end_pfn,
2770							   root_stable_tree +
2771							   nid))
2772				node = rb_first(root_stable_tree + nid);
2773			else
2774				node = rb_next(node);
2775			cond_resched();
2776		}
2777	}
2778	list_for_each_entry_safe(stable_node, next, &migrate_nodes, list) {
2779		if (stable_node->kpfn >= start_pfn &&
2780		    stable_node->kpfn < end_pfn)
2781			remove_node_from_stable_tree(stable_node);
2782		cond_resched();
2783	}
2784}
2785
2786static int ksm_memory_callback(struct notifier_block *self,
2787			       unsigned long action, void *arg)
2788{
2789	struct memory_notify *mn = arg;
2790
2791	switch (action) {
2792	case MEM_GOING_OFFLINE:
2793		/*
2794		 * Prevent ksm_do_scan(), unmerge_and_remove_all_rmap_items()
2795		 * and remove_all_stable_nodes() while memory is going offline:
2796		 * it is unsafe for them to touch the stable tree at this time.
2797		 * But unmerge_ksm_pages(), rmap lookups and other entry points
2798		 * which do not need the ksm_thread_mutex are all safe.
2799		 */
2800		mutex_lock(&ksm_thread_mutex);
2801		ksm_run |= KSM_RUN_OFFLINE;
2802		mutex_unlock(&ksm_thread_mutex);
2803		break;
2804
2805	case MEM_OFFLINE:
2806		/*
2807		 * Most of the work is done by page migration; but there might
2808		 * be a few stable_nodes left over, still pointing to struct
2809		 * pages which have been offlined: prune those from the tree,
2810		 * otherwise get_ksm_page() might later try to access a
2811		 * non-existent struct page.
2812		 */
2813		ksm_check_stable_tree(mn->start_pfn,
2814				      mn->start_pfn + mn->nr_pages);
2815		/* fallthrough */
2816
2817	case MEM_CANCEL_OFFLINE:
2818		mutex_lock(&ksm_thread_mutex);
2819		ksm_run &= ~KSM_RUN_OFFLINE;
2820		mutex_unlock(&ksm_thread_mutex);
2821
2822		smp_mb();	/* wake_up_bit advises this */
2823		wake_up_bit(&ksm_run, ilog2(KSM_RUN_OFFLINE));
2824		break;
2825	}
2826	return NOTIFY_OK;
2827}
2828#else
2829static void wait_while_offlining(void)
2830{
2831}
2832#endif /* CONFIG_MEMORY_HOTREMOVE */
2833
2834#ifdef CONFIG_SYSFS
2835/*
2836 * This all compiles without CONFIG_SYSFS, but is a waste of space.
2837 */
2838
2839#define KSM_ATTR_RO(_name) \
2840	static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
2841#define KSM_ATTR(_name) \
2842	static struct kobj_attribute _name##_attr = \
2843		__ATTR(_name, 0644, _name##_show, _name##_store)
2844
2845static ssize_t sleep_millisecs_show(struct kobject *kobj,
2846				    struct kobj_attribute *attr, char *buf)
2847{
2848	return sprintf(buf, "%u\n", ksm_thread_sleep_millisecs);
2849}
2850
2851static ssize_t sleep_millisecs_store(struct kobject *kobj,
2852				     struct kobj_attribute *attr,
2853				     const char *buf, size_t count)
2854{
2855	unsigned long msecs;
2856	int err;
2857
2858	err = kstrtoul(buf, 10, &msecs);
2859	if (err || msecs > UINT_MAX)
2860		return -EINVAL;
2861
2862	ksm_thread_sleep_millisecs = msecs;
2863	wake_up_interruptible(&ksm_iter_wait);
2864
2865	return count;
2866}
2867KSM_ATTR(sleep_millisecs);
2868
2869static ssize_t pages_to_scan_show(struct kobject *kobj,
2870				  struct kobj_attribute *attr, char *buf)
2871{
2872	return sprintf(buf, "%u\n", ksm_thread_pages_to_scan);
2873}
2874
2875static ssize_t pages_to_scan_store(struct kobject *kobj,
2876				   struct kobj_attribute *attr,
2877				   const char *buf, size_t count)
2878{
2879	int err;
2880	unsigned long nr_pages;
2881
2882	err = kstrtoul(buf, 10, &nr_pages);
2883	if (err || nr_pages > UINT_MAX)
2884		return -EINVAL;
2885
2886	ksm_thread_pages_to_scan = nr_pages;
2887
2888	return count;
2889}
2890KSM_ATTR(pages_to_scan);
2891
2892static ssize_t run_show(struct kobject *kobj, struct kobj_attribute *attr,
2893			char *buf)
2894{
2895	return sprintf(buf, "%lu\n", ksm_run);
2896}
2897
2898static ssize_t run_store(struct kobject *kobj, struct kobj_attribute *attr,
2899			 const char *buf, size_t count)
2900{
2901	int err;
2902	unsigned long flags;
2903
2904	err = kstrtoul(buf, 10, &flags);
2905	if (err || flags > UINT_MAX)
2906		return -EINVAL;
2907	if (flags > KSM_RUN_UNMERGE)
2908		return -EINVAL;
2909
2910	/*
2911	 * KSM_RUN_MERGE sets ksmd running, and 0 stops it running.
2912	 * KSM_RUN_UNMERGE stops it running and unmerges all rmap_items,
2913	 * breaking COW to free the pages_shared (but leaves mm_slots
2914	 * on the list for when ksmd may be set running again).
2915	 */
2916
2917	mutex_lock(&ksm_thread_mutex);
2918	wait_while_offlining();
2919	if (ksm_run != flags) {
2920		ksm_run = flags;
2921		if (flags & KSM_RUN_UNMERGE) {
2922			set_current_oom_origin();
2923			err = unmerge_and_remove_all_rmap_items();
2924			clear_current_oom_origin();
2925			if (err) {
2926				ksm_run = KSM_RUN_STOP;
2927				count = err;
2928			}
2929		}
2930	}
2931	mutex_unlock(&ksm_thread_mutex);
2932
2933	if (flags & KSM_RUN_MERGE)
2934		wake_up_interruptible(&ksm_thread_wait);
2935
2936	return count;
2937}
2938KSM_ATTR(run);
2939
2940#ifdef CONFIG_NUMA
2941static ssize_t merge_across_nodes_show(struct kobject *kobj,
2942				struct kobj_attribute *attr, char *buf)
2943{
2944	return sprintf(buf, "%u\n", ksm_merge_across_nodes);
2945}
2946
2947static ssize_t merge_across_nodes_store(struct kobject *kobj,
2948				   struct kobj_attribute *attr,
2949				   const char *buf, size_t count)
2950{
2951	int err;
2952	unsigned long knob;
2953
2954	err = kstrtoul(buf, 10, &knob);
2955	if (err)
2956		return err;
2957	if (knob > 1)
2958		return -EINVAL;
2959
2960	mutex_lock(&ksm_thread_mutex);
2961	wait_while_offlining();
2962	if (ksm_merge_across_nodes != knob) {
2963		if (ksm_pages_shared || remove_all_stable_nodes())
2964			err = -EBUSY;
2965		else if (root_stable_tree == one_stable_tree) {
2966			struct rb_root *buf;
2967			/*
2968			 * This is the first time that we switch away from the
2969			 * default of merging across nodes: must now allocate
2970			 * a buffer to hold as many roots as may be needed.
2971			 * Allocate stable and unstable together:
2972			 * MAXSMP NODES_SHIFT 10 will use 16kB.
2973			 */
2974			buf = kcalloc(nr_node_ids + nr_node_ids, sizeof(*buf),
2975				      GFP_KERNEL);
2976			/* Let us assume that RB_ROOT is NULL is zero */
2977			if (!buf)
2978				err = -ENOMEM;
2979			else {
2980				root_stable_tree = buf;
2981				root_unstable_tree = buf + nr_node_ids;
2982				/* Stable tree is empty but not the unstable */
2983				root_unstable_tree[0] = one_unstable_tree[0];
2984			}
2985		}
2986		if (!err) {
2987			ksm_merge_across_nodes = knob;
2988			ksm_nr_node_ids = knob ? 1 : nr_node_ids;
2989		}
2990	}
2991	mutex_unlock(&ksm_thread_mutex);
2992
2993	return err ? err : count;
2994}
2995KSM_ATTR(merge_across_nodes);
2996#endif
2997
2998static ssize_t use_zero_pages_show(struct kobject *kobj,
2999				struct kobj_attribute *attr, char *buf)
3000{
3001	return sprintf(buf, "%u\n", ksm_use_zero_pages);
3002}
3003static ssize_t use_zero_pages_store(struct kobject *kobj,
3004				   struct kobj_attribute *attr,
3005				   const char *buf, size_t count)
3006{
3007	int err;
3008	bool value;
3009
3010	err = kstrtobool(buf, &value);
3011	if (err)
3012		return -EINVAL;
3013
3014	ksm_use_zero_pages = value;
3015
3016	return count;
3017}
3018KSM_ATTR(use_zero_pages);
3019
3020static ssize_t max_page_sharing_show(struct kobject *kobj,
3021				     struct kobj_attribute *attr, char *buf)
3022{
3023	return sprintf(buf, "%u\n", ksm_max_page_sharing);
3024}
3025
3026static ssize_t max_page_sharing_store(struct kobject *kobj,
3027				      struct kobj_attribute *attr,
3028				      const char *buf, size_t count)
3029{
3030	int err;
3031	int knob;
3032
3033	err = kstrtoint(buf, 10, &knob);
3034	if (err)
3035		return err;
3036	/*
3037	 * When a KSM page is created it is shared by 2 mappings. This
3038	 * being a signed comparison, it implicitly verifies it's not
3039	 * negative.
3040	 */
3041	if (knob < 2)
3042		return -EINVAL;
3043
3044	if (READ_ONCE(ksm_max_page_sharing) == knob)
3045		return count;
3046
3047	mutex_lock(&ksm_thread_mutex);
3048	wait_while_offlining();
3049	if (ksm_max_page_sharing != knob) {
3050		if (ksm_pages_shared || remove_all_stable_nodes())
3051			err = -EBUSY;
3052		else
3053			ksm_max_page_sharing = knob;
3054	}
3055	mutex_unlock(&ksm_thread_mutex);
3056
3057	return err ? err : count;
3058}
3059KSM_ATTR(max_page_sharing);
3060
3061static ssize_t pages_shared_show(struct kobject *kobj,
3062				 struct kobj_attribute *attr, char *buf)
3063{
3064	return sprintf(buf, "%lu\n", ksm_pages_shared);
3065}
3066KSM_ATTR_RO(pages_shared);
3067
3068static ssize_t pages_sharing_show(struct kobject *kobj,
3069				  struct kobj_attribute *attr, char *buf)
3070{
3071	return sprintf(buf, "%lu\n", ksm_pages_sharing);
3072}
3073KSM_ATTR_RO(pages_sharing);
3074
3075static ssize_t pages_unshared_show(struct kobject *kobj,
3076				   struct kobj_attribute *attr, char *buf)
3077{
3078	return sprintf(buf, "%lu\n", ksm_pages_unshared);
3079}
3080KSM_ATTR_RO(pages_unshared);
3081
3082static ssize_t pages_volatile_show(struct kobject *kobj,
3083				   struct kobj_attribute *attr, char *buf)
3084{
3085	long ksm_pages_volatile;
3086
3087	ksm_pages_volatile = ksm_rmap_items - ksm_pages_shared
3088				- ksm_pages_sharing - ksm_pages_unshared;
3089	/*
3090	 * It was not worth any locking to calculate that statistic,
3091	 * but it might therefore sometimes be negative: conceal that.
3092	 */
3093	if (ksm_pages_volatile < 0)
3094		ksm_pages_volatile = 0;
3095	return sprintf(buf, "%ld\n", ksm_pages_volatile);
3096}
3097KSM_ATTR_RO(pages_volatile);
3098
3099static ssize_t stable_node_dups_show(struct kobject *kobj,
3100				     struct kobj_attribute *attr, char *buf)
3101{
3102	return sprintf(buf, "%lu\n", ksm_stable_node_dups);
3103}
3104KSM_ATTR_RO(stable_node_dups);
3105
3106static ssize_t stable_node_chains_show(struct kobject *kobj,
3107				       struct kobj_attribute *attr, char *buf)
3108{
3109	return sprintf(buf, "%lu\n", ksm_stable_node_chains);
3110}
3111KSM_ATTR_RO(stable_node_chains);
3112
3113static ssize_t
3114stable_node_chains_prune_millisecs_show(struct kobject *kobj,
3115					struct kobj_attribute *attr,
3116					char *buf)
3117{
3118	return sprintf(buf, "%u\n", ksm_stable_node_chains_prune_millisecs);
3119}
3120
3121static ssize_t
3122stable_node_chains_prune_millisecs_store(struct kobject *kobj,
3123					 struct kobj_attribute *attr,
3124					 const char *buf, size_t count)
3125{
3126	unsigned long msecs;
3127	int err;
3128
3129	err = kstrtoul(buf, 10, &msecs);
3130	if (err || msecs > UINT_MAX)
3131		return -EINVAL;
3132
3133	ksm_stable_node_chains_prune_millisecs = msecs;
3134
3135	return count;
3136}
3137KSM_ATTR(stable_node_chains_prune_millisecs);
3138
3139static ssize_t full_scans_show(struct kobject *kobj,
3140			       struct kobj_attribute *attr, char *buf)
3141{
3142	return sprintf(buf, "%lu\n", ksm_scan.seqnr);
3143}
3144KSM_ATTR_RO(full_scans);
3145
3146static struct attribute *ksm_attrs[] = {
3147	&sleep_millisecs_attr.attr,
3148	&pages_to_scan_attr.attr,
3149	&run_attr.attr,
3150	&pages_shared_attr.attr,
3151	&pages_sharing_attr.attr,
3152	&pages_unshared_attr.attr,
3153	&pages_volatile_attr.attr,
3154	&full_scans_attr.attr,
3155#ifdef CONFIG_NUMA
3156	&merge_across_nodes_attr.attr,
3157#endif
3158	&max_page_sharing_attr.attr,
3159	&stable_node_chains_attr.attr,
3160	&stable_node_dups_attr.attr,
3161	&stable_node_chains_prune_millisecs_attr.attr,
3162	&use_zero_pages_attr.attr,
3163	NULL,
3164};
3165
3166static const struct attribute_group ksm_attr_group = {
3167	.attrs = ksm_attrs,
3168	.name = "ksm",
3169};
3170#endif /* CONFIG_SYSFS */
3171
3172static int __init ksm_init(void)
3173{
3174	struct task_struct *ksm_thread;
3175	int err;
3176
3177	/* The correct value depends on page size and endianness */
3178	zero_checksum = calc_checksum(ZERO_PAGE(0));
3179	/* Default to false for backwards compatibility */
3180	ksm_use_zero_pages = false;
3181
3182	err = ksm_slab_init();
3183	if (err)
3184		goto out;
3185
3186	ksm_thread = kthread_run(ksm_scan_thread, NULL, "ksmd");
3187	if (IS_ERR(ksm_thread)) {
3188		pr_err("ksm: creating kthread failed\n");
3189		err = PTR_ERR(ksm_thread);
3190		goto out_free;
3191	}
3192
3193#ifdef CONFIG_SYSFS
3194	err = sysfs_create_group(mm_kobj, &ksm_attr_group);
3195	if (err) {
3196		pr_err("ksm: register sysfs failed\n");
3197		kthread_stop(ksm_thread);
3198		goto out_free;
3199	}
3200#else
3201	ksm_run = KSM_RUN_MERGE;	/* no way for user to start it */
3202
3203#endif /* CONFIG_SYSFS */
3204
3205#ifdef CONFIG_MEMORY_HOTREMOVE
3206	/* There is no significance to this priority 100 */
3207	hotplug_memory_notifier(ksm_memory_callback, 100);
3208#endif
3209	return 0;
3210
3211out_free:
3212	ksm_slab_free();
3213out:
3214	return err;
3215}
3216subsys_initcall(ksm_init);