2 * Memory merging support.
4 * This code enables dynamic sharing of identical pages found in different
5 * memory areas, even if they are not shared by fork()
7 * Copyright (C) 2008-2009 Red Hat, Inc.
14 * This work is licensed under the terms of the GNU GPL, version 2.
17 #include <linux/errno.h>
20 #include <linux/mman.h>
21 #include <linux/sched.h>
22 #include <linux/rwsem.h>
23 #include <linux/pagemap.h>
24 #include <linux/rmap.h>
25 #include <linux/spinlock.h>
26 #include <linux/jhash.h>
27 #include <linux/delay.h>
28 #include <linux/kthread.h>
29 #include <linux/wait.h>
30 #include <linux/slab.h>
31 #include <linux/rbtree.h>
32 #include <linux/mmu_notifier.h>
33 #include <linux/ksm.h>
35 #include <asm/tlbflush.h>
38 * A few notes about the KSM scanning process,
39 * to make it easier to understand the data structures below:
41 * In order to reduce excessive scanning, KSM sorts the memory pages by their
42 * contents into a data structure that holds pointers to the pages' locations.
44 * Since the contents of the pages may change at any moment, KSM cannot just
45 * insert the pages into a normal sorted tree and expect it to find anything.
46 * Therefore KSM uses two data structures - the stable and the unstable tree.
48 * The stable tree holds pointers to all the merged pages (ksm pages), sorted
49 * by their contents. Because each such page is write-protected, searching on
50 * this tree is fully assured to be working (except when pages are unmapped),
51 * and therefore this tree is called the stable tree.
53 * In addition to the stable tree, KSM uses a second data structure called the
54 * unstable tree: this tree holds pointers to pages which have been found to
55 * be "unchanged for a period of time". The unstable tree sorts these pages
56 * by their contents, but since they are not write-protected, KSM cannot rely
57 * upon the unstable tree to work correctly - the unstable tree is liable to
58 * be corrupted as its contents are modified, and so it is called unstable.
60 * KSM solves this problem by several techniques:
62 * 1) The unstable tree is flushed every time KSM completes scanning all
63 * memory areas, and then the tree is rebuilt again from the beginning.
64 * 2) KSM will only insert into the unstable tree, pages whose hash value
65 * has not changed since the previous scan of all memory areas.
66 * 3) The unstable tree is a RedBlack Tree - so its balancing is based on the
67 * colors of the nodes and not on their contents, assuring that even when
68 * the tree gets "corrupted" it won't get out of balance, so scanning time
69 * remains the same (also, searching and inserting nodes in an rbtree uses
70 * the same algorithm, so we have no overhead when we flush and rebuild).
71 * 4) KSM never flushes the stable tree, which means that even if it were to
72 * take 10 attempts to find a page in the unstable tree, once it is found,
73 * it is secured in the stable tree. (When we scan a new page, we first
74 * compare it against the stable tree, and then against the unstable tree.)
78 * struct mm_slot - ksm information per mm that is being scanned
79 * @link: link to the mm_slots hash list
80 * @mm_list: link into the mm_slots list, rooted in ksm_mm_head
81 * @rmap_list: head for this mm_slot's list of rmap_items
82 * @mm: the mm that this information is valid for
85 struct hlist_node link;
86 struct list_head mm_list;
87 struct list_head rmap_list;
92 * struct ksm_scan - cursor for scanning
93 * @mm_slot: the current mm_slot we are scanning
94 * @address: the next address inside that to be scanned
95 * @rmap_item: the current rmap that we are scanning inside the rmap_list
96 * @seqnr: count of completed full scans (needed when removing unstable node)
98 * There is only the one ksm_scan instance of this cursor structure.
101 struct mm_slot *mm_slot;
102 unsigned long address;
103 struct rmap_item *rmap_item;
108 * struct rmap_item - reverse mapping item for virtual addresses
109 * @link: link into mm_slot's rmap_list (rmap_list is per mm)
110 * @mm: the memory structure this rmap_item is pointing into
111 * @address: the virtual address this rmap_item tracks (+ flags in low bits)
112 * @oldchecksum: previous checksum of the page at that virtual address
113 * @node: rb_node of this rmap_item in either unstable or stable tree
114 * @next: next rmap_item hanging off the same node of the stable tree
115 * @prev: previous rmap_item hanging off the same node of the stable tree
118 struct list_head link;
119 struct mm_struct *mm;
120 unsigned long address; /* + low bits used for flags below */
122 unsigned int oldchecksum; /* when unstable */
123 struct rmap_item *next; /* when stable */
126 struct rb_node node; /* when tree node */
127 struct rmap_item *prev; /* in stable list */
131 #define SEQNR_MASK 0x0ff /* low bits of unstable tree seqnr */
132 #define NODE_FLAG 0x100 /* is a node of unstable or stable tree */
133 #define STABLE_FLAG 0x200 /* is a node or list item of stable tree */
135 /* The stable and unstable tree heads */
136 static struct rb_root root_stable_tree = RB_ROOT;
137 static struct rb_root root_unstable_tree = RB_ROOT;
139 #define MM_SLOTS_HASH_HEADS 1024
140 static struct hlist_head *mm_slots_hash;
142 static struct mm_slot ksm_mm_head = {
143 .mm_list = LIST_HEAD_INIT(ksm_mm_head.mm_list),
145 static struct ksm_scan ksm_scan = {
146 .mm_slot = &ksm_mm_head,
149 static struct kmem_cache *rmap_item_cache;
150 static struct kmem_cache *mm_slot_cache;
152 /* The number of nodes in the stable tree */
153 static unsigned long ksm_pages_shared;
155 /* The number of page slots additionally sharing those nodes */
156 static unsigned long ksm_pages_sharing;
158 /* Limit on the number of unswappable pages used */
159 static unsigned long ksm_max_kernel_pages;
161 /* Number of pages ksmd should scan in one batch */
162 static unsigned int ksm_thread_pages_to_scan;
164 /* Milliseconds ksmd should sleep between batches */
165 static unsigned int ksm_thread_sleep_millisecs;
167 #define KSM_RUN_STOP 0
168 #define KSM_RUN_MERGE 1
169 #define KSM_RUN_UNMERGE 2
170 static unsigned int ksm_run;
172 static DECLARE_WAIT_QUEUE_HEAD(ksm_thread_wait);
173 static DEFINE_MUTEX(ksm_thread_mutex);
174 static DEFINE_SPINLOCK(ksm_mmlist_lock);
176 #define KSM_KMEM_CACHE(__struct, __flags) kmem_cache_create("ksm_"#__struct,\
177 sizeof(struct __struct), __alignof__(struct __struct),\
180 static int __init ksm_slab_init(void)
182 rmap_item_cache = KSM_KMEM_CACHE(rmap_item, 0);
183 if (!rmap_item_cache)
186 mm_slot_cache = KSM_KMEM_CACHE(mm_slot, 0);
193 kmem_cache_destroy(rmap_item_cache);
198 static void __init ksm_slab_free(void)
200 kmem_cache_destroy(mm_slot_cache);
201 kmem_cache_destroy(rmap_item_cache);
202 mm_slot_cache = NULL;
205 static inline struct rmap_item *alloc_rmap_item(void)
207 return kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL);
210 static inline void free_rmap_item(struct rmap_item *rmap_item)
212 rmap_item->mm = NULL; /* debug safety */
213 kmem_cache_free(rmap_item_cache, rmap_item);
216 static inline struct mm_slot *alloc_mm_slot(void)
218 if (!mm_slot_cache) /* initialization failed */
220 return kmem_cache_zalloc(mm_slot_cache, GFP_KERNEL);
223 static inline void free_mm_slot(struct mm_slot *mm_slot)
225 kmem_cache_free(mm_slot_cache, mm_slot);
228 static int __init mm_slots_hash_init(void)
230 mm_slots_hash = kzalloc(MM_SLOTS_HASH_HEADS * sizeof(struct hlist_head),
237 static void __init mm_slots_hash_free(void)
239 kfree(mm_slots_hash);
242 static struct mm_slot *get_mm_slot(struct mm_struct *mm)
244 struct mm_slot *mm_slot;
245 struct hlist_head *bucket;
246 struct hlist_node *node;
248 bucket = &mm_slots_hash[((unsigned long)mm / sizeof(struct mm_struct))
249 % MM_SLOTS_HASH_HEADS];
250 hlist_for_each_entry(mm_slot, node, bucket, link) {
251 if (mm == mm_slot->mm)
257 static void insert_to_mm_slots_hash(struct mm_struct *mm,
258 struct mm_slot *mm_slot)
260 struct hlist_head *bucket;
262 bucket = &mm_slots_hash[((unsigned long)mm / sizeof(struct mm_struct))
263 % MM_SLOTS_HASH_HEADS];
265 INIT_LIST_HEAD(&mm_slot->rmap_list);
266 hlist_add_head(&mm_slot->link, bucket);
269 static inline int in_stable_tree(struct rmap_item *rmap_item)
271 return rmap_item->address & STABLE_FLAG;
275 * We use break_ksm to break COW on a ksm page: it's a stripped down
277 * if (get_user_pages(current, mm, addr, 1, 1, 1, &page, NULL) == 1)
280 * but taking great care only to touch a ksm page, in a VM_MERGEABLE vma,
281 * in case the application has unmapped and remapped mm,addr meanwhile.
282 * Could a ksm page appear anywhere else? Actually yes, in a VM_PFNMAP
283 * mmap of /dev/mem or /dev/kmem, where we would not want to touch it.
285 static void break_ksm(struct vm_area_struct *vma, unsigned long addr)
292 page = follow_page(vma, addr, FOLL_GET);
296 ret = handle_mm_fault(vma->vm_mm, vma, addr,
299 ret = VM_FAULT_WRITE;
301 } while (!(ret & (VM_FAULT_WRITE | VM_FAULT_SIGBUS)));
303 /* Which leaves us looping there if VM_FAULT_OOM: hmmm... */
306 static void __break_cow(struct mm_struct *mm, unsigned long addr)
308 struct vm_area_struct *vma;
310 vma = find_vma(mm, addr);
311 if (!vma || vma->vm_start > addr)
313 if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
315 break_ksm(vma, addr);
318 static void break_cow(struct mm_struct *mm, unsigned long addr)
320 down_read(&mm->mmap_sem);
321 __break_cow(mm, addr);
322 up_read(&mm->mmap_sem);
325 static struct page *get_mergeable_page(struct rmap_item *rmap_item)
327 struct mm_struct *mm = rmap_item->mm;
328 unsigned long addr = rmap_item->address;
329 struct vm_area_struct *vma;
332 down_read(&mm->mmap_sem);
333 vma = find_vma(mm, addr);
334 if (!vma || vma->vm_start > addr)
336 if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
339 page = follow_page(vma, addr, FOLL_GET);
342 if (PageAnon(page)) {
343 flush_anon_page(vma, page, addr);
344 flush_dcache_page(page);
349 up_read(&mm->mmap_sem);
354 * get_ksm_page: checks if the page at the virtual address in rmap_item
355 * is still PageKsm, in which case we can trust the content of the page,
356 * and it returns the gotten page; but NULL if the page has been zapped.
358 static struct page *get_ksm_page(struct rmap_item *rmap_item)
362 page = get_mergeable_page(rmap_item);
363 if (page && !PageKsm(page)) {
371 * Removing rmap_item from stable or unstable tree.
372 * This function will clean the information from the stable/unstable tree.
374 static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
376 if (in_stable_tree(rmap_item)) {
377 struct rmap_item *next_item = rmap_item->next;
379 if (rmap_item->address & NODE_FLAG) {
381 rb_replace_node(&rmap_item->node,
384 next_item->address |= NODE_FLAG;
387 rb_erase(&rmap_item->node, &root_stable_tree);
391 struct rmap_item *prev_item = rmap_item->prev;
393 BUG_ON(prev_item->next != rmap_item);
394 prev_item->next = next_item;
396 BUG_ON(next_item->prev != rmap_item);
397 next_item->prev = rmap_item->prev;
402 rmap_item->next = NULL;
404 } else if (rmap_item->address & NODE_FLAG) {
407 * ksm_thread can and must skip the rb_erase, because
408 * root_unstable_tree was already reset to RB_ROOT.
409 * But __ksm_exit has to be careful: do the rb_erase
410 * if it's interrupting a scan, and this rmap_item was
411 * inserted by this scan rather than left from before.
413 * Because of the case in which remove_mm_from_lists
414 * increments seqnr before removing rmaps, unstable_nr
415 * may even be 2 behind seqnr, but should never be
416 * further behind. Yes, I did have trouble with this!
418 age = (unsigned char)(ksm_scan.seqnr - rmap_item->address);
421 rb_erase(&rmap_item->node, &root_unstable_tree);
424 rmap_item->address &= PAGE_MASK;
426 cond_resched(); /* we're called from many long loops */
429 static void remove_all_slot_rmap_items(struct mm_slot *mm_slot)
431 struct rmap_item *rmap_item, *node;
433 list_for_each_entry_safe(rmap_item, node, &mm_slot->rmap_list, link) {
434 remove_rmap_item_from_tree(rmap_item);
435 list_del(&rmap_item->link);
436 free_rmap_item(rmap_item);
440 static void remove_trailing_rmap_items(struct mm_slot *mm_slot,
441 struct list_head *cur)
443 struct rmap_item *rmap_item;
445 while (cur != &mm_slot->rmap_list) {
446 rmap_item = list_entry(cur, struct rmap_item, link);
448 remove_rmap_item_from_tree(rmap_item);
449 list_del(&rmap_item->link);
450 free_rmap_item(rmap_item);
455 * Though it's very tempting to unmerge in_stable_tree(rmap_item)s rather
456 * than check every pte of a given vma, the locking doesn't quite work for
457 * that - an rmap_item is assigned to the stable tree after inserting ksm
458 * page and upping mmap_sem. Nor does it fit with the way we skip dup'ing
459 * rmap_items from parent to child at fork time (so as not to waste time
460 * if exit comes before the next scan reaches it).
462 static void unmerge_ksm_pages(struct vm_area_struct *vma,
463 unsigned long start, unsigned long end)
467 for (addr = start; addr < end; addr += PAGE_SIZE)
468 break_ksm(vma, addr);
471 static void unmerge_and_remove_all_rmap_items(void)
473 struct mm_slot *mm_slot;
474 struct mm_struct *mm;
475 struct vm_area_struct *vma;
477 list_for_each_entry(mm_slot, &ksm_mm_head.mm_list, mm_list) {
479 down_read(&mm->mmap_sem);
480 for (vma = mm->mmap; vma; vma = vma->vm_next) {
481 if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
483 unmerge_ksm_pages(vma, vma->vm_start, vma->vm_end);
485 remove_all_slot_rmap_items(mm_slot);
486 up_read(&mm->mmap_sem);
489 spin_lock(&ksm_mmlist_lock);
490 if (ksm_scan.mm_slot != &ksm_mm_head) {
491 ksm_scan.mm_slot = &ksm_mm_head;
494 spin_unlock(&ksm_mmlist_lock);
497 static void remove_mm_from_lists(struct mm_struct *mm)
499 struct mm_slot *mm_slot;
501 spin_lock(&ksm_mmlist_lock);
502 mm_slot = get_mm_slot(mm);
505 * This mm_slot is always at the scanning cursor when we're
506 * called from scan_get_next_rmap_item; but it's a special
507 * case when we're called from __ksm_exit.
509 if (ksm_scan.mm_slot == mm_slot) {
510 ksm_scan.mm_slot = list_entry(
511 mm_slot->mm_list.next, struct mm_slot, mm_list);
512 ksm_scan.address = 0;
513 ksm_scan.rmap_item = list_entry(
514 &ksm_scan.mm_slot->rmap_list, struct rmap_item, link);
515 if (ksm_scan.mm_slot == &ksm_mm_head)
519 hlist_del(&mm_slot->link);
520 list_del(&mm_slot->mm_list);
521 spin_unlock(&ksm_mmlist_lock);
523 remove_all_slot_rmap_items(mm_slot);
524 free_mm_slot(mm_slot);
525 clear_bit(MMF_VM_MERGEABLE, &mm->flags);
528 static u32 calc_checksum(struct page *page)
531 void *addr = kmap_atomic(page, KM_USER0);
532 checksum = jhash2(addr, PAGE_SIZE / 4, 17);
533 kunmap_atomic(addr, KM_USER0);
537 static int memcmp_pages(struct page *page1, struct page *page2)
542 addr1 = kmap_atomic(page1, KM_USER0);
543 addr2 = kmap_atomic(page2, KM_USER1);
544 ret = memcmp(addr1, addr2, PAGE_SIZE);
545 kunmap_atomic(addr2, KM_USER1);
546 kunmap_atomic(addr1, KM_USER0);
550 static inline int pages_identical(struct page *page1, struct page *page2)
552 return !memcmp_pages(page1, page2);
555 static int write_protect_page(struct vm_area_struct *vma, struct page *page,
558 struct mm_struct *mm = vma->vm_mm;
565 addr = page_address_in_vma(page, vma);
569 ptep = page_check_address(page, mm, addr, &ptl, 0);
573 if (pte_write(*ptep)) {
576 swapped = PageSwapCache(page);
577 flush_cache_page(vma, addr, page_to_pfn(page));
579 * Ok this is tricky, when get_user_pages_fast() run it doesnt
580 * take any lock, therefore the check that we are going to make
581 * with the pagecount against the mapcount is racey and
582 * O_DIRECT can happen right after the check.
583 * So we clear the pte and flush the tlb before the check
584 * this assure us that no O_DIRECT can happen after the check
585 * or in the middle of the check.
587 entry = ptep_clear_flush(vma, addr, ptep);
589 * Check that no O_DIRECT or similar I/O is in progress on the
592 if ((page_mapcount(page) + 2 + swapped) != page_count(page)) {
593 set_pte_at_notify(mm, addr, ptep, entry);
596 entry = pte_wrprotect(entry);
597 set_pte_at_notify(mm, addr, ptep, entry);
603 pte_unmap_unlock(ptep, ptl);
609 * replace_page - replace page in vma by new ksm page
610 * @vma: vma that holds the pte pointing to oldpage
611 * @oldpage: the page we are replacing by newpage
612 * @newpage: the ksm page we replace oldpage by
613 * @orig_pte: the original value of the pte
615 * Returns 0 on success, -EFAULT on failure.
617 static int replace_page(struct vm_area_struct *vma, struct page *oldpage,
618 struct page *newpage, pte_t orig_pte)
620 struct mm_struct *mm = vma->vm_mm;
630 prot = vm_get_page_prot(vma->vm_flags & ~VM_WRITE);
632 addr = page_address_in_vma(oldpage, vma);
636 pgd = pgd_offset(mm, addr);
637 if (!pgd_present(*pgd))
640 pud = pud_offset(pgd, addr);
641 if (!pud_present(*pud))
644 pmd = pmd_offset(pud, addr);
645 if (!pmd_present(*pmd))
648 ptep = pte_offset_map_lock(mm, pmd, addr, &ptl);
649 if (!pte_same(*ptep, orig_pte)) {
650 pte_unmap_unlock(ptep, ptl);
655 page_add_ksm_rmap(newpage);
657 flush_cache_page(vma, addr, pte_pfn(*ptep));
658 ptep_clear_flush(vma, addr, ptep);
659 set_pte_at_notify(mm, addr, ptep, mk_pte(newpage, prot));
661 page_remove_rmap(oldpage);
664 pte_unmap_unlock(ptep, ptl);
671 * try_to_merge_one_page - take two pages and merge them into one
672 * @vma: the vma that hold the pte pointing into oldpage
673 * @oldpage: the page that we want to replace with newpage
674 * @newpage: the page that we want to map instead of oldpage
677 * oldpage should be a PageAnon page, while newpage should be a PageKsm page,
678 * or a newly allocated kernel page which page_add_ksm_rmap will make PageKsm.
680 * This function returns 0 if the pages were merged, -EFAULT otherwise.
682 static int try_to_merge_one_page(struct vm_area_struct *vma,
683 struct page *oldpage,
684 struct page *newpage)
686 pte_t orig_pte = __pte(0);
689 if (!(vma->vm_flags & VM_MERGEABLE))
692 if (!PageAnon(oldpage))
699 * We need the page lock to read a stable PageSwapCache in
700 * write_protect_page(). We use trylock_page() instead of
701 * lock_page() because we don't want to wait here - we
702 * prefer to continue scanning and merging different pages,
703 * then come back to this page when it is unlocked.
705 if (!trylock_page(oldpage))
708 * If this anonymous page is mapped only here, its pte may need
709 * to be write-protected. If it's mapped elsewhere, all of its
710 * ptes are necessarily already write-protected. But in either
711 * case, we need to lock and check page_count is not raised.
713 if (write_protect_page(vma, oldpage, &orig_pte)) {
714 unlock_page(oldpage);
717 unlock_page(oldpage);
719 if (pages_identical(oldpage, newpage))
720 err = replace_page(vma, oldpage, newpage, orig_pte);
730 * try_to_merge_two_pages - take two identical pages and prepare them
731 * to be merged into one page.
733 * This function returns 0 if we successfully mapped two identical pages
734 * into one page, -EFAULT otherwise.
736 * Note that this function allocates a new kernel page: if one of the pages
737 * is already a ksm page, try_to_merge_with_ksm_page should be used.
739 static int try_to_merge_two_pages(struct mm_struct *mm1, unsigned long addr1,
740 struct page *page1, struct mm_struct *mm2,
741 unsigned long addr2, struct page *page2)
743 struct vm_area_struct *vma;
748 * The number of nodes in the stable tree
749 * is the number of kernel pages that we hold.
751 if (ksm_max_kernel_pages &&
752 ksm_max_kernel_pages <= ksm_pages_shared)
755 kpage = alloc_page(GFP_HIGHUSER);
759 down_read(&mm1->mmap_sem);
760 vma = find_vma(mm1, addr1);
761 if (!vma || vma->vm_start > addr1) {
763 up_read(&mm1->mmap_sem);
767 copy_user_highpage(kpage, page1, addr1, vma);
768 err = try_to_merge_one_page(vma, page1, kpage);
769 up_read(&mm1->mmap_sem);
772 down_read(&mm2->mmap_sem);
773 vma = find_vma(mm2, addr2);
774 if (!vma || vma->vm_start > addr2) {
776 up_read(&mm2->mmap_sem);
777 break_cow(mm1, addr1);
781 err = try_to_merge_one_page(vma, page2, kpage);
782 up_read(&mm2->mmap_sem);
785 * If the second try_to_merge_one_page failed, we have a
786 * ksm page with just one pte pointing to it, so break it.
789 break_cow(mm1, addr1);
797 * try_to_merge_with_ksm_page - like try_to_merge_two_pages,
798 * but no new kernel page is allocated: kpage must already be a ksm page.
800 static int try_to_merge_with_ksm_page(struct mm_struct *mm1,
805 struct vm_area_struct *vma;
808 down_read(&mm1->mmap_sem);
809 vma = find_vma(mm1, addr1);
810 if (!vma || vma->vm_start > addr1) {
811 up_read(&mm1->mmap_sem);
815 err = try_to_merge_one_page(vma, page1, kpage);
816 up_read(&mm1->mmap_sem);
822 * stable_tree_search - search page inside the stable tree
823 * @page: the page that we are searching identical pages to.
824 * @page2: pointer into identical page that we are holding inside the stable
825 * tree that we have found.
826 * @rmap_item: the reverse mapping item
828 * This function checks if there is a page inside the stable tree
829 * with identical content to the page that we are scanning right now.
831 * This function return rmap_item pointer to the identical item if found,
834 static struct rmap_item *stable_tree_search(struct page *page,
836 struct rmap_item *rmap_item)
838 struct rb_node *node = root_stable_tree.rb_node;
841 struct rmap_item *tree_rmap_item, *next_rmap_item;
844 tree_rmap_item = rb_entry(node, struct rmap_item, node);
845 while (tree_rmap_item) {
846 BUG_ON(!in_stable_tree(tree_rmap_item));
848 page2[0] = get_ksm_page(tree_rmap_item);
851 next_rmap_item = tree_rmap_item->next;
852 remove_rmap_item_from_tree(tree_rmap_item);
853 tree_rmap_item = next_rmap_item;
858 ret = memcmp_pages(page, page2[0]);
862 node = node->rb_left;
863 } else if (ret > 0) {
865 node = node->rb_right;
867 return tree_rmap_item;
875 * stable_tree_insert - insert rmap_item pointing to new ksm page
876 * into the stable tree.
878 * @page: the page that we are searching identical page to inside the stable
880 * @rmap_item: pointer to the reverse mapping item.
882 * This function returns rmap_item if success, NULL otherwise.
884 static struct rmap_item *stable_tree_insert(struct page *page,
885 struct rmap_item *rmap_item)
887 struct rb_node **new = &root_stable_tree.rb_node;
888 struct rb_node *parent = NULL;
891 struct rmap_item *tree_rmap_item, *next_rmap_item;
892 struct page *tree_page;
895 tree_rmap_item = rb_entry(*new, struct rmap_item, node);
896 while (tree_rmap_item) {
897 BUG_ON(!in_stable_tree(tree_rmap_item));
899 tree_page = get_ksm_page(tree_rmap_item);
902 next_rmap_item = tree_rmap_item->next;
903 remove_rmap_item_from_tree(tree_rmap_item);
904 tree_rmap_item = next_rmap_item;
909 ret = memcmp_pages(page, tree_page);
914 new = &parent->rb_left;
916 new = &parent->rb_right;
919 * It is not a bug that stable_tree_search() didn't
920 * find this node: because at that time our page was
921 * not yet write-protected, so may have changed since.
927 rmap_item->address |= NODE_FLAG | STABLE_FLAG;
928 rmap_item->next = NULL;
929 rb_link_node(&rmap_item->node, parent, new);
930 rb_insert_color(&rmap_item->node, &root_stable_tree);
937 * unstable_tree_search_insert - search and insert items into the unstable tree.
939 * @page: the page that we are going to search for identical page or to insert
940 * into the unstable tree
941 * @page2: pointer into identical page that was found inside the unstable tree
942 * @rmap_item: the reverse mapping item of page
944 * This function searches for a page in the unstable tree identical to the
945 * page currently being scanned; and if no identical page is found in the
946 * tree, we insert rmap_item as a new object into the unstable tree.
948 * This function returns pointer to rmap_item found to be identical
949 * to the currently scanned page, NULL otherwise.
951 * This function does both searching and inserting, because they share
952 * the same walking algorithm in an rbtree.
954 static struct rmap_item *unstable_tree_search_insert(struct page *page,
956 struct rmap_item *rmap_item)
958 struct rb_node **new = &root_unstable_tree.rb_node;
959 struct rb_node *parent = NULL;
962 struct rmap_item *tree_rmap_item;
965 tree_rmap_item = rb_entry(*new, struct rmap_item, node);
966 page2[0] = get_mergeable_page(tree_rmap_item);
971 * Don't substitute an unswappable ksm page
972 * just for one good swappable forked page.
974 if (page == page2[0]) {
979 ret = memcmp_pages(page, page2[0]);
984 new = &parent->rb_left;
985 } else if (ret > 0) {
987 new = &parent->rb_right;
989 return tree_rmap_item;
993 rmap_item->address |= NODE_FLAG;
994 rmap_item->address |= (ksm_scan.seqnr & SEQNR_MASK);
995 rb_link_node(&rmap_item->node, parent, new);
996 rb_insert_color(&rmap_item->node, &root_unstable_tree);
1002 * stable_tree_append - add another rmap_item to the linked list of
1003 * rmap_items hanging off a given node of the stable tree, all sharing
1004 * the same ksm page.
1006 static void stable_tree_append(struct rmap_item *rmap_item,
1007 struct rmap_item *tree_rmap_item)
1009 rmap_item->next = tree_rmap_item->next;
1010 rmap_item->prev = tree_rmap_item;
1012 if (tree_rmap_item->next)
1013 tree_rmap_item->next->prev = rmap_item;
1015 tree_rmap_item->next = rmap_item;
1016 rmap_item->address |= STABLE_FLAG;
1018 ksm_pages_sharing++;
1022 * cmp_and_merge_page - take a page computes its hash value and check if there
1023 * is similar hash value to different page,
1024 * in case we find that there is similar hash to different page we call to
1025 * try_to_merge_two_pages().
1027 * @page: the page that we are searching identical page to.
1028 * @rmap_item: the reverse mapping into the virtual address of this page
1030 static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
1032 struct page *page2[1];
1033 struct rmap_item *tree_rmap_item;
1034 unsigned int checksum;
1037 if (in_stable_tree(rmap_item))
1038 remove_rmap_item_from_tree(rmap_item);
1040 /* We first start with searching the page inside the stable tree */
1041 tree_rmap_item = stable_tree_search(page, page2, rmap_item);
1042 if (tree_rmap_item) {
1043 if (page == page2[0]) /* forked */
1046 err = try_to_merge_with_ksm_page(rmap_item->mm,
1053 * The page was successfully merged:
1054 * add its rmap_item to the stable tree.
1056 stable_tree_append(rmap_item, tree_rmap_item);
1062 * A ksm page might have got here by fork, but its other
1063 * references have already been removed from the stable tree.
1066 break_cow(rmap_item->mm, rmap_item->address);
1069 * In case the hash value of the page was changed from the last time we
1070 * have calculated it, this page to be changed frequely, therefore we
1071 * don't want to insert it to the unstable tree, and we don't want to
1072 * waste our time to search if there is something identical to it there.
1074 checksum = calc_checksum(page);
1075 if (rmap_item->oldchecksum != checksum) {
1076 rmap_item->oldchecksum = checksum;
1080 tree_rmap_item = unstable_tree_search_insert(page, page2, rmap_item);
1081 if (tree_rmap_item) {
1082 err = try_to_merge_two_pages(rmap_item->mm,
1083 rmap_item->address, page,
1085 tree_rmap_item->address, page2[0]);
1087 * As soon as we merge this page, we want to remove the
1088 * rmap_item of the page we have merged with from the unstable
1089 * tree, and insert it instead as new node in the stable tree.
1092 rb_erase(&tree_rmap_item->node, &root_unstable_tree);
1093 tree_rmap_item->address &= ~NODE_FLAG;
1095 * If we fail to insert the page into the stable tree,
1096 * we will have 2 virtual addresses that are pointing
1097 * to a ksm page left outside the stable tree,
1098 * in which case we need to break_cow on both.
1100 if (stable_tree_insert(page2[0], tree_rmap_item))
1101 stable_tree_append(rmap_item, tree_rmap_item);
1103 break_cow(tree_rmap_item->mm,
1104 tree_rmap_item->address);
1105 break_cow(rmap_item->mm, rmap_item->address);
1113 static struct rmap_item *get_next_rmap_item(struct mm_slot *mm_slot,
1114 struct list_head *cur,
1117 struct rmap_item *rmap_item;
1119 while (cur != &mm_slot->rmap_list) {
1120 rmap_item = list_entry(cur, struct rmap_item, link);
1121 if ((rmap_item->address & PAGE_MASK) == addr) {
1122 if (!in_stable_tree(rmap_item))
1123 remove_rmap_item_from_tree(rmap_item);
1126 if (rmap_item->address > addr)
1129 remove_rmap_item_from_tree(rmap_item);
1130 list_del(&rmap_item->link);
1131 free_rmap_item(rmap_item);
1134 rmap_item = alloc_rmap_item();
1136 /* It has already been zeroed */
1137 rmap_item->mm = mm_slot->mm;
1138 rmap_item->address = addr;
1139 list_add_tail(&rmap_item->link, cur);
1144 static struct rmap_item *scan_get_next_rmap_item(struct page **page)
1146 struct mm_struct *mm;
1147 struct mm_slot *slot;
1148 struct vm_area_struct *vma;
1149 struct rmap_item *rmap_item;
1151 if (list_empty(&ksm_mm_head.mm_list))
1154 slot = ksm_scan.mm_slot;
1155 if (slot == &ksm_mm_head) {
1156 root_unstable_tree = RB_ROOT;
1158 spin_lock(&ksm_mmlist_lock);
1159 slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list);
1160 ksm_scan.mm_slot = slot;
1161 spin_unlock(&ksm_mmlist_lock);
1163 ksm_scan.address = 0;
1164 ksm_scan.rmap_item = list_entry(&slot->rmap_list,
1165 struct rmap_item, link);
1169 down_read(&mm->mmap_sem);
1170 for (vma = find_vma(mm, ksm_scan.address); vma; vma = vma->vm_next) {
1171 if (!(vma->vm_flags & VM_MERGEABLE))
1173 if (ksm_scan.address < vma->vm_start)
1174 ksm_scan.address = vma->vm_start;
1176 ksm_scan.address = vma->vm_end;
1178 while (ksm_scan.address < vma->vm_end) {
1179 *page = follow_page(vma, ksm_scan.address, FOLL_GET);
1180 if (*page && PageAnon(*page)) {
1181 flush_anon_page(vma, *page, ksm_scan.address);
1182 flush_dcache_page(*page);
1183 rmap_item = get_next_rmap_item(slot,
1184 ksm_scan.rmap_item->link.next,
1187 ksm_scan.rmap_item = rmap_item;
1188 ksm_scan.address += PAGE_SIZE;
1191 up_read(&mm->mmap_sem);
1196 ksm_scan.address += PAGE_SIZE;
1201 if (!ksm_scan.address) {
1203 * We've completed a full scan of all vmas, holding mmap_sem
1204 * throughout, and found no VM_MERGEABLE: so do the same as
1205 * __ksm_exit does to remove this mm from all our lists now.
1207 remove_mm_from_lists(mm);
1208 up_read(&mm->mmap_sem);
1209 slot = ksm_scan.mm_slot;
1210 if (slot != &ksm_mm_head)
1216 * Nuke all the rmap_items that are above this current rmap:
1217 * because there were no VM_MERGEABLE vmas with such addresses.
1219 remove_trailing_rmap_items(slot, ksm_scan.rmap_item->link.next);
1220 up_read(&mm->mmap_sem);
1222 spin_lock(&ksm_mmlist_lock);
1223 slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list);
1224 ksm_scan.mm_slot = slot;
1225 spin_unlock(&ksm_mmlist_lock);
1227 /* Repeat until we've completed scanning the whole list */
1228 if (slot != &ksm_mm_head)
1232 * Bump seqnr here rather than at top, so that __ksm_exit
1233 * can skip rb_erase on unstable tree until we run again.
1240 * ksm_do_scan - the ksm scanner main worker function.
1241 * @scan_npages - number of pages we want to scan before we return.
1243 static void ksm_do_scan(unsigned int scan_npages)
1245 struct rmap_item *rmap_item;
1248 while (scan_npages--) {
1250 rmap_item = scan_get_next_rmap_item(&page);
1253 if (!PageKsm(page) || !in_stable_tree(rmap_item))
1254 cmp_and_merge_page(page, rmap_item);
1259 static int ksm_scan_thread(void *nothing)
1261 set_user_nice(current, 5);
1263 while (!kthread_should_stop()) {
1264 if (ksm_run & KSM_RUN_MERGE) {
1265 mutex_lock(&ksm_thread_mutex);
1266 ksm_do_scan(ksm_thread_pages_to_scan);
1267 mutex_unlock(&ksm_thread_mutex);
1268 schedule_timeout_interruptible(
1269 msecs_to_jiffies(ksm_thread_sleep_millisecs));
1271 wait_event_interruptible(ksm_thread_wait,
1272 (ksm_run & KSM_RUN_MERGE) ||
1273 kthread_should_stop());
1279 int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
1280 unsigned long end, int advice, unsigned long *vm_flags)
1282 struct mm_struct *mm = vma->vm_mm;
1285 case MADV_MERGEABLE:
1287 * Be somewhat over-protective for now!
1289 if (*vm_flags & (VM_MERGEABLE | VM_SHARED | VM_MAYSHARE |
1290 VM_PFNMAP | VM_IO | VM_DONTEXPAND |
1291 VM_RESERVED | VM_HUGETLB | VM_INSERTPAGE |
1292 VM_MIXEDMAP | VM_SAO))
1293 return 0; /* just ignore the advice */
1295 if (!test_bit(MMF_VM_MERGEABLE, &mm->flags))
1296 if (__ksm_enter(mm) < 0)
1299 *vm_flags |= VM_MERGEABLE;
1302 case MADV_UNMERGEABLE:
1303 if (!(*vm_flags & VM_MERGEABLE))
1304 return 0; /* just ignore the advice */
1307 unmerge_ksm_pages(vma, start, end);
1309 *vm_flags &= ~VM_MERGEABLE;
1316 int __ksm_enter(struct mm_struct *mm)
1318 struct mm_slot *mm_slot = alloc_mm_slot();
1322 spin_lock(&ksm_mmlist_lock);
1323 insert_to_mm_slots_hash(mm, mm_slot);
1325 * Insert just behind the scanning cursor, to let the area settle
1326 * down a little; when fork is followed by immediate exec, we don't
1327 * want ksmd to waste time setting up and tearing down an rmap_list.
1329 list_add_tail(&mm_slot->mm_list, &ksm_scan.mm_slot->mm_list);
1330 spin_unlock(&ksm_mmlist_lock);
1332 set_bit(MMF_VM_MERGEABLE, &mm->flags);
1336 void __ksm_exit(struct mm_struct *mm)
1339 * This process is exiting: doesn't hold and doesn't need mmap_sem;
1340 * but we do need to exclude ksmd and other exiters while we modify
1341 * the various lists and trees.
1343 mutex_lock(&ksm_thread_mutex);
1344 remove_mm_from_lists(mm);
1345 mutex_unlock(&ksm_thread_mutex);
1348 #define KSM_ATTR_RO(_name) \
1349 static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
1350 #define KSM_ATTR(_name) \
1351 static struct kobj_attribute _name##_attr = \
1352 __ATTR(_name, 0644, _name##_show, _name##_store)
1354 static ssize_t sleep_millisecs_show(struct kobject *kobj,
1355 struct kobj_attribute *attr, char *buf)
1357 return sprintf(buf, "%u\n", ksm_thread_sleep_millisecs);
1360 static ssize_t sleep_millisecs_store(struct kobject *kobj,
1361 struct kobj_attribute *attr,
1362 const char *buf, size_t count)
1364 unsigned long msecs;
1367 err = strict_strtoul(buf, 10, &msecs);
1368 if (err || msecs > UINT_MAX)
1371 ksm_thread_sleep_millisecs = msecs;
1375 KSM_ATTR(sleep_millisecs);
1377 static ssize_t pages_to_scan_show(struct kobject *kobj,
1378 struct kobj_attribute *attr, char *buf)
1380 return sprintf(buf, "%u\n", ksm_thread_pages_to_scan);
1383 static ssize_t pages_to_scan_store(struct kobject *kobj,
1384 struct kobj_attribute *attr,
1385 const char *buf, size_t count)
1388 unsigned long nr_pages;
1390 err = strict_strtoul(buf, 10, &nr_pages);
1391 if (err || nr_pages > UINT_MAX)
1394 ksm_thread_pages_to_scan = nr_pages;
1398 KSM_ATTR(pages_to_scan);
1400 static ssize_t run_show(struct kobject *kobj, struct kobj_attribute *attr,
1403 return sprintf(buf, "%u\n", ksm_run);
1406 static ssize_t run_store(struct kobject *kobj, struct kobj_attribute *attr,
1407 const char *buf, size_t count)
1410 unsigned long flags;
1412 err = strict_strtoul(buf, 10, &flags);
1413 if (err || flags > UINT_MAX)
1415 if (flags > KSM_RUN_UNMERGE)
1419 * KSM_RUN_MERGE sets ksmd running, and 0 stops it running.
1420 * KSM_RUN_UNMERGE stops it running and unmerges all rmap_items,
1421 * breaking COW to free the unswappable pages_shared (but leaves
1422 * mm_slots on the list for when ksmd may be set running again).
1425 mutex_lock(&ksm_thread_mutex);
1426 if (ksm_run != flags) {
1428 if (flags & KSM_RUN_UNMERGE)
1429 unmerge_and_remove_all_rmap_items();
1431 mutex_unlock(&ksm_thread_mutex);
1433 if (flags & KSM_RUN_MERGE)
1434 wake_up_interruptible(&ksm_thread_wait);
1440 static ssize_t max_kernel_pages_store(struct kobject *kobj,
1441 struct kobj_attribute *attr,
1442 const char *buf, size_t count)
1445 unsigned long nr_pages;
1447 err = strict_strtoul(buf, 10, &nr_pages);
1451 ksm_max_kernel_pages = nr_pages;
1456 static ssize_t max_kernel_pages_show(struct kobject *kobj,
1457 struct kobj_attribute *attr, char *buf)
1459 return sprintf(buf, "%lu\n", ksm_max_kernel_pages);
1461 KSM_ATTR(max_kernel_pages);
1463 static ssize_t pages_shared_show(struct kobject *kobj,
1464 struct kobj_attribute *attr, char *buf)
1466 return sprintf(buf, "%lu\n", ksm_pages_shared);
1468 KSM_ATTR_RO(pages_shared);
1470 static ssize_t pages_sharing_show(struct kobject *kobj,
1471 struct kobj_attribute *attr, char *buf)
1473 return sprintf(buf, "%lu\n", ksm_pages_sharing);
1475 KSM_ATTR_RO(pages_sharing);
1477 static struct attribute *ksm_attrs[] = {
1478 &sleep_millisecs_attr.attr,
1479 &pages_to_scan_attr.attr,
1481 &max_kernel_pages_attr.attr,
1482 &pages_shared_attr.attr,
1483 &pages_sharing_attr.attr,
1487 static struct attribute_group ksm_attr_group = {
1492 static int __init ksm_init(void)
1494 struct task_struct *ksm_thread;
1497 err = ksm_slab_init();
1501 err = mm_slots_hash_init();
1505 ksm_thread = kthread_run(ksm_scan_thread, NULL, "ksmd");
1506 if (IS_ERR(ksm_thread)) {
1507 printk(KERN_ERR "ksm: creating kthread failed\n");
1508 err = PTR_ERR(ksm_thread);
1512 err = sysfs_create_group(mm_kobj, &ksm_attr_group);
1514 printk(KERN_ERR "ksm: register sysfs failed\n");
1521 kthread_stop(ksm_thread);
1523 mm_slots_hash_free();
1529 module_init(ksm_init)