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 /* The number of nodes in the unstable tree */
159 static unsigned long ksm_pages_unshared;
161 /* The number of rmap_items in use: to calculate pages_volatile */
162 static unsigned long ksm_rmap_items;
164 /* Limit on the number of unswappable pages used */
165 static unsigned long ksm_max_kernel_pages;
167 /* Number of pages ksmd should scan in one batch */
168 static unsigned int ksm_thread_pages_to_scan;
170 /* Milliseconds ksmd should sleep between batches */
171 static unsigned int ksm_thread_sleep_millisecs;
173 #define KSM_RUN_STOP 0
174 #define KSM_RUN_MERGE 1
175 #define KSM_RUN_UNMERGE 2
176 static unsigned int ksm_run;
178 static DECLARE_WAIT_QUEUE_HEAD(ksm_thread_wait);
179 static DEFINE_MUTEX(ksm_thread_mutex);
180 static DEFINE_SPINLOCK(ksm_mmlist_lock);
182 #define KSM_KMEM_CACHE(__struct, __flags) kmem_cache_create("ksm_"#__struct,\
183 sizeof(struct __struct), __alignof__(struct __struct),\
186 static int __init ksm_slab_init(void)
188 rmap_item_cache = KSM_KMEM_CACHE(rmap_item, 0);
189 if (!rmap_item_cache)
192 mm_slot_cache = KSM_KMEM_CACHE(mm_slot, 0);
199 kmem_cache_destroy(rmap_item_cache);
204 static void __init ksm_slab_free(void)
206 kmem_cache_destroy(mm_slot_cache);
207 kmem_cache_destroy(rmap_item_cache);
208 mm_slot_cache = NULL;
211 static inline struct rmap_item *alloc_rmap_item(void)
213 struct rmap_item *rmap_item;
215 rmap_item = kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL);
221 static inline void free_rmap_item(struct rmap_item *rmap_item)
224 rmap_item->mm = NULL; /* debug safety */
225 kmem_cache_free(rmap_item_cache, rmap_item);
228 static inline struct mm_slot *alloc_mm_slot(void)
230 if (!mm_slot_cache) /* initialization failed */
232 return kmem_cache_zalloc(mm_slot_cache, GFP_KERNEL);
235 static inline void free_mm_slot(struct mm_slot *mm_slot)
237 kmem_cache_free(mm_slot_cache, mm_slot);
240 static int __init mm_slots_hash_init(void)
242 mm_slots_hash = kzalloc(MM_SLOTS_HASH_HEADS * sizeof(struct hlist_head),
249 static void __init mm_slots_hash_free(void)
251 kfree(mm_slots_hash);
254 static struct mm_slot *get_mm_slot(struct mm_struct *mm)
256 struct mm_slot *mm_slot;
257 struct hlist_head *bucket;
258 struct hlist_node *node;
260 bucket = &mm_slots_hash[((unsigned long)mm / sizeof(struct mm_struct))
261 % MM_SLOTS_HASH_HEADS];
262 hlist_for_each_entry(mm_slot, node, bucket, link) {
263 if (mm == mm_slot->mm)
269 static void insert_to_mm_slots_hash(struct mm_struct *mm,
270 struct mm_slot *mm_slot)
272 struct hlist_head *bucket;
274 bucket = &mm_slots_hash[((unsigned long)mm / sizeof(struct mm_struct))
275 % MM_SLOTS_HASH_HEADS];
277 INIT_LIST_HEAD(&mm_slot->rmap_list);
278 hlist_add_head(&mm_slot->link, bucket);
281 static inline int in_stable_tree(struct rmap_item *rmap_item)
283 return rmap_item->address & STABLE_FLAG;
287 * We use break_ksm to break COW on a ksm page: it's a stripped down
289 * if (get_user_pages(current, mm, addr, 1, 1, 1, &page, NULL) == 1)
292 * but taking great care only to touch a ksm page, in a VM_MERGEABLE vma,
293 * in case the application has unmapped and remapped mm,addr meanwhile.
294 * Could a ksm page appear anywhere else? Actually yes, in a VM_PFNMAP
295 * mmap of /dev/mem or /dev/kmem, where we would not want to touch it.
297 static void break_ksm(struct vm_area_struct *vma, unsigned long addr)
304 page = follow_page(vma, addr, FOLL_GET);
308 ret = handle_mm_fault(vma->vm_mm, vma, addr,
311 ret = VM_FAULT_WRITE;
313 } while (!(ret & (VM_FAULT_WRITE | VM_FAULT_SIGBUS)));
315 /* Which leaves us looping there if VM_FAULT_OOM: hmmm... */
318 static void __break_cow(struct mm_struct *mm, unsigned long addr)
320 struct vm_area_struct *vma;
322 vma = find_vma(mm, addr);
323 if (!vma || vma->vm_start > addr)
325 if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
327 break_ksm(vma, addr);
330 static void break_cow(struct mm_struct *mm, unsigned long addr)
332 down_read(&mm->mmap_sem);
333 __break_cow(mm, addr);
334 up_read(&mm->mmap_sem);
337 static struct page *get_mergeable_page(struct rmap_item *rmap_item)
339 struct mm_struct *mm = rmap_item->mm;
340 unsigned long addr = rmap_item->address;
341 struct vm_area_struct *vma;
344 down_read(&mm->mmap_sem);
345 vma = find_vma(mm, addr);
346 if (!vma || vma->vm_start > addr)
348 if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
351 page = follow_page(vma, addr, FOLL_GET);
354 if (PageAnon(page)) {
355 flush_anon_page(vma, page, addr);
356 flush_dcache_page(page);
361 up_read(&mm->mmap_sem);
366 * get_ksm_page: checks if the page at the virtual address in rmap_item
367 * is still PageKsm, in which case we can trust the content of the page,
368 * and it returns the gotten page; but NULL if the page has been zapped.
370 static struct page *get_ksm_page(struct rmap_item *rmap_item)
374 page = get_mergeable_page(rmap_item);
375 if (page && !PageKsm(page)) {
383 * Removing rmap_item from stable or unstable tree.
384 * This function will clean the information from the stable/unstable tree.
386 static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
388 if (in_stable_tree(rmap_item)) {
389 struct rmap_item *next_item = rmap_item->next;
391 if (rmap_item->address & NODE_FLAG) {
393 rb_replace_node(&rmap_item->node,
396 next_item->address |= NODE_FLAG;
399 rb_erase(&rmap_item->node, &root_stable_tree);
403 struct rmap_item *prev_item = rmap_item->prev;
405 BUG_ON(prev_item->next != rmap_item);
406 prev_item->next = next_item;
408 BUG_ON(next_item->prev != rmap_item);
409 next_item->prev = rmap_item->prev;
414 rmap_item->next = NULL;
416 } else if (rmap_item->address & NODE_FLAG) {
419 * ksm_thread can and must skip the rb_erase, because
420 * root_unstable_tree was already reset to RB_ROOT.
421 * But __ksm_exit has to be careful: do the rb_erase
422 * if it's interrupting a scan, and this rmap_item was
423 * inserted by this scan rather than left from before.
425 * Because of the case in which remove_mm_from_lists
426 * increments seqnr before removing rmaps, unstable_nr
427 * may even be 2 behind seqnr, but should never be
428 * further behind. Yes, I did have trouble with this!
430 age = (unsigned char)(ksm_scan.seqnr - rmap_item->address);
433 rb_erase(&rmap_item->node, &root_unstable_tree);
434 ksm_pages_unshared--;
437 rmap_item->address &= PAGE_MASK;
439 cond_resched(); /* we're called from many long loops */
442 static void remove_all_slot_rmap_items(struct mm_slot *mm_slot)
444 struct rmap_item *rmap_item, *node;
446 list_for_each_entry_safe(rmap_item, node, &mm_slot->rmap_list, link) {
447 remove_rmap_item_from_tree(rmap_item);
448 list_del(&rmap_item->link);
449 free_rmap_item(rmap_item);
453 static void remove_trailing_rmap_items(struct mm_slot *mm_slot,
454 struct list_head *cur)
456 struct rmap_item *rmap_item;
458 while (cur != &mm_slot->rmap_list) {
459 rmap_item = list_entry(cur, struct rmap_item, link);
461 remove_rmap_item_from_tree(rmap_item);
462 list_del(&rmap_item->link);
463 free_rmap_item(rmap_item);
468 * Though it's very tempting to unmerge in_stable_tree(rmap_item)s rather
469 * than check every pte of a given vma, the locking doesn't quite work for
470 * that - an rmap_item is assigned to the stable tree after inserting ksm
471 * page and upping mmap_sem. Nor does it fit with the way we skip dup'ing
472 * rmap_items from parent to child at fork time (so as not to waste time
473 * if exit comes before the next scan reaches it).
475 static void unmerge_ksm_pages(struct vm_area_struct *vma,
476 unsigned long start, unsigned long end)
480 for (addr = start; addr < end; addr += PAGE_SIZE)
481 break_ksm(vma, addr);
484 static void unmerge_and_remove_all_rmap_items(void)
486 struct mm_slot *mm_slot;
487 struct mm_struct *mm;
488 struct vm_area_struct *vma;
490 list_for_each_entry(mm_slot, &ksm_mm_head.mm_list, mm_list) {
492 down_read(&mm->mmap_sem);
493 for (vma = mm->mmap; vma; vma = vma->vm_next) {
494 if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
496 unmerge_ksm_pages(vma, vma->vm_start, vma->vm_end);
498 remove_all_slot_rmap_items(mm_slot);
499 up_read(&mm->mmap_sem);
502 spin_lock(&ksm_mmlist_lock);
503 if (ksm_scan.mm_slot != &ksm_mm_head) {
504 ksm_scan.mm_slot = &ksm_mm_head;
507 spin_unlock(&ksm_mmlist_lock);
510 static void remove_mm_from_lists(struct mm_struct *mm)
512 struct mm_slot *mm_slot;
514 spin_lock(&ksm_mmlist_lock);
515 mm_slot = get_mm_slot(mm);
518 * This mm_slot is always at the scanning cursor when we're
519 * called from scan_get_next_rmap_item; but it's a special
520 * case when we're called from __ksm_exit.
522 if (ksm_scan.mm_slot == mm_slot) {
523 ksm_scan.mm_slot = list_entry(
524 mm_slot->mm_list.next, struct mm_slot, mm_list);
525 ksm_scan.address = 0;
526 ksm_scan.rmap_item = list_entry(
527 &ksm_scan.mm_slot->rmap_list, struct rmap_item, link);
528 if (ksm_scan.mm_slot == &ksm_mm_head)
532 hlist_del(&mm_slot->link);
533 list_del(&mm_slot->mm_list);
534 spin_unlock(&ksm_mmlist_lock);
536 remove_all_slot_rmap_items(mm_slot);
537 free_mm_slot(mm_slot);
538 clear_bit(MMF_VM_MERGEABLE, &mm->flags);
541 static u32 calc_checksum(struct page *page)
544 void *addr = kmap_atomic(page, KM_USER0);
545 checksum = jhash2(addr, PAGE_SIZE / 4, 17);
546 kunmap_atomic(addr, KM_USER0);
550 static int memcmp_pages(struct page *page1, struct page *page2)
555 addr1 = kmap_atomic(page1, KM_USER0);
556 addr2 = kmap_atomic(page2, KM_USER1);
557 ret = memcmp(addr1, addr2, PAGE_SIZE);
558 kunmap_atomic(addr2, KM_USER1);
559 kunmap_atomic(addr1, KM_USER0);
563 static inline int pages_identical(struct page *page1, struct page *page2)
565 return !memcmp_pages(page1, page2);
568 static int write_protect_page(struct vm_area_struct *vma, struct page *page,
571 struct mm_struct *mm = vma->vm_mm;
578 addr = page_address_in_vma(page, vma);
582 ptep = page_check_address(page, mm, addr, &ptl, 0);
586 if (pte_write(*ptep)) {
589 swapped = PageSwapCache(page);
590 flush_cache_page(vma, addr, page_to_pfn(page));
592 * Ok this is tricky, when get_user_pages_fast() run it doesnt
593 * take any lock, therefore the check that we are going to make
594 * with the pagecount against the mapcount is racey and
595 * O_DIRECT can happen right after the check.
596 * So we clear the pte and flush the tlb before the check
597 * this assure us that no O_DIRECT can happen after the check
598 * or in the middle of the check.
600 entry = ptep_clear_flush(vma, addr, ptep);
602 * Check that no O_DIRECT or similar I/O is in progress on the
605 if ((page_mapcount(page) + 2 + swapped) != page_count(page)) {
606 set_pte_at_notify(mm, addr, ptep, entry);
609 entry = pte_wrprotect(entry);
610 set_pte_at_notify(mm, addr, ptep, entry);
616 pte_unmap_unlock(ptep, ptl);
622 * replace_page - replace page in vma by new ksm page
623 * @vma: vma that holds the pte pointing to oldpage
624 * @oldpage: the page we are replacing by newpage
625 * @newpage: the ksm page we replace oldpage by
626 * @orig_pte: the original value of the pte
628 * Returns 0 on success, -EFAULT on failure.
630 static int replace_page(struct vm_area_struct *vma, struct page *oldpage,
631 struct page *newpage, pte_t orig_pte)
633 struct mm_struct *mm = vma->vm_mm;
643 prot = vm_get_page_prot(vma->vm_flags & ~VM_WRITE);
645 addr = page_address_in_vma(oldpage, vma);
649 pgd = pgd_offset(mm, addr);
650 if (!pgd_present(*pgd))
653 pud = pud_offset(pgd, addr);
654 if (!pud_present(*pud))
657 pmd = pmd_offset(pud, addr);
658 if (!pmd_present(*pmd))
661 ptep = pte_offset_map_lock(mm, pmd, addr, &ptl);
662 if (!pte_same(*ptep, orig_pte)) {
663 pte_unmap_unlock(ptep, ptl);
668 page_add_ksm_rmap(newpage);
670 flush_cache_page(vma, addr, pte_pfn(*ptep));
671 ptep_clear_flush(vma, addr, ptep);
672 set_pte_at_notify(mm, addr, ptep, mk_pte(newpage, prot));
674 page_remove_rmap(oldpage);
677 pte_unmap_unlock(ptep, ptl);
684 * try_to_merge_one_page - take two pages and merge them into one
685 * @vma: the vma that hold the pte pointing into oldpage
686 * @oldpage: the page that we want to replace with newpage
687 * @newpage: the page that we want to map instead of oldpage
690 * oldpage should be a PageAnon page, while newpage should be a PageKsm page,
691 * or a newly allocated kernel page which page_add_ksm_rmap will make PageKsm.
693 * This function returns 0 if the pages were merged, -EFAULT otherwise.
695 static int try_to_merge_one_page(struct vm_area_struct *vma,
696 struct page *oldpage,
697 struct page *newpage)
699 pte_t orig_pte = __pte(0);
702 if (!(vma->vm_flags & VM_MERGEABLE))
705 if (!PageAnon(oldpage))
712 * We need the page lock to read a stable PageSwapCache in
713 * write_protect_page(). We use trylock_page() instead of
714 * lock_page() because we don't want to wait here - we
715 * prefer to continue scanning and merging different pages,
716 * then come back to this page when it is unlocked.
718 if (!trylock_page(oldpage))
721 * If this anonymous page is mapped only here, its pte may need
722 * to be write-protected. If it's mapped elsewhere, all of its
723 * ptes are necessarily already write-protected. But in either
724 * case, we need to lock and check page_count is not raised.
726 if (write_protect_page(vma, oldpage, &orig_pte)) {
727 unlock_page(oldpage);
730 unlock_page(oldpage);
732 if (pages_identical(oldpage, newpage))
733 err = replace_page(vma, oldpage, newpage, orig_pte);
743 * try_to_merge_two_pages - take two identical pages and prepare them
744 * to be merged into one page.
746 * This function returns 0 if we successfully mapped two identical pages
747 * into one page, -EFAULT otherwise.
749 * Note that this function allocates a new kernel page: if one of the pages
750 * is already a ksm page, try_to_merge_with_ksm_page should be used.
752 static int try_to_merge_two_pages(struct mm_struct *mm1, unsigned long addr1,
753 struct page *page1, struct mm_struct *mm2,
754 unsigned long addr2, struct page *page2)
756 struct vm_area_struct *vma;
761 * The number of nodes in the stable tree
762 * is the number of kernel pages that we hold.
764 if (ksm_max_kernel_pages &&
765 ksm_max_kernel_pages <= ksm_pages_shared)
768 kpage = alloc_page(GFP_HIGHUSER);
772 down_read(&mm1->mmap_sem);
773 vma = find_vma(mm1, addr1);
774 if (!vma || vma->vm_start > addr1) {
776 up_read(&mm1->mmap_sem);
780 copy_user_highpage(kpage, page1, addr1, vma);
781 err = try_to_merge_one_page(vma, page1, kpage);
782 up_read(&mm1->mmap_sem);
785 down_read(&mm2->mmap_sem);
786 vma = find_vma(mm2, addr2);
787 if (!vma || vma->vm_start > addr2) {
789 up_read(&mm2->mmap_sem);
790 break_cow(mm1, addr1);
794 err = try_to_merge_one_page(vma, page2, kpage);
795 up_read(&mm2->mmap_sem);
798 * If the second try_to_merge_one_page failed, we have a
799 * ksm page with just one pte pointing to it, so break it.
802 break_cow(mm1, addr1);
810 * try_to_merge_with_ksm_page - like try_to_merge_two_pages,
811 * but no new kernel page is allocated: kpage must already be a ksm page.
813 static int try_to_merge_with_ksm_page(struct mm_struct *mm1,
818 struct vm_area_struct *vma;
821 down_read(&mm1->mmap_sem);
822 vma = find_vma(mm1, addr1);
823 if (!vma || vma->vm_start > addr1) {
824 up_read(&mm1->mmap_sem);
828 err = try_to_merge_one_page(vma, page1, kpage);
829 up_read(&mm1->mmap_sem);
835 * stable_tree_search - search page inside the stable tree
836 * @page: the page that we are searching identical pages to.
837 * @page2: pointer into identical page that we are holding inside the stable
838 * tree that we have found.
839 * @rmap_item: the reverse mapping item
841 * This function checks if there is a page inside the stable tree
842 * with identical content to the page that we are scanning right now.
844 * This function return rmap_item pointer to the identical item if found,
847 static struct rmap_item *stable_tree_search(struct page *page,
849 struct rmap_item *rmap_item)
851 struct rb_node *node = root_stable_tree.rb_node;
854 struct rmap_item *tree_rmap_item, *next_rmap_item;
857 tree_rmap_item = rb_entry(node, struct rmap_item, node);
858 while (tree_rmap_item) {
859 BUG_ON(!in_stable_tree(tree_rmap_item));
861 page2[0] = get_ksm_page(tree_rmap_item);
864 next_rmap_item = tree_rmap_item->next;
865 remove_rmap_item_from_tree(tree_rmap_item);
866 tree_rmap_item = next_rmap_item;
871 ret = memcmp_pages(page, page2[0]);
875 node = node->rb_left;
876 } else if (ret > 0) {
878 node = node->rb_right;
880 return tree_rmap_item;
888 * stable_tree_insert - insert rmap_item pointing to new ksm page
889 * into the stable tree.
891 * @page: the page that we are searching identical page to inside the stable
893 * @rmap_item: pointer to the reverse mapping item.
895 * This function returns rmap_item if success, NULL otherwise.
897 static struct rmap_item *stable_tree_insert(struct page *page,
898 struct rmap_item *rmap_item)
900 struct rb_node **new = &root_stable_tree.rb_node;
901 struct rb_node *parent = NULL;
904 struct rmap_item *tree_rmap_item, *next_rmap_item;
905 struct page *tree_page;
908 tree_rmap_item = rb_entry(*new, struct rmap_item, node);
909 while (tree_rmap_item) {
910 BUG_ON(!in_stable_tree(tree_rmap_item));
912 tree_page = get_ksm_page(tree_rmap_item);
915 next_rmap_item = tree_rmap_item->next;
916 remove_rmap_item_from_tree(tree_rmap_item);
917 tree_rmap_item = next_rmap_item;
922 ret = memcmp_pages(page, tree_page);
927 new = &parent->rb_left;
929 new = &parent->rb_right;
932 * It is not a bug that stable_tree_search() didn't
933 * find this node: because at that time our page was
934 * not yet write-protected, so may have changed since.
940 rmap_item->address |= NODE_FLAG | STABLE_FLAG;
941 rmap_item->next = NULL;
942 rb_link_node(&rmap_item->node, parent, new);
943 rb_insert_color(&rmap_item->node, &root_stable_tree);
950 * unstable_tree_search_insert - search and insert items into the unstable tree.
952 * @page: the page that we are going to search for identical page or to insert
953 * into the unstable tree
954 * @page2: pointer into identical page that was found inside the unstable tree
955 * @rmap_item: the reverse mapping item of page
957 * This function searches for a page in the unstable tree identical to the
958 * page currently being scanned; and if no identical page is found in the
959 * tree, we insert rmap_item as a new object into the unstable tree.
961 * This function returns pointer to rmap_item found to be identical
962 * to the currently scanned page, NULL otherwise.
964 * This function does both searching and inserting, because they share
965 * the same walking algorithm in an rbtree.
967 static struct rmap_item *unstable_tree_search_insert(struct page *page,
969 struct rmap_item *rmap_item)
971 struct rb_node **new = &root_unstable_tree.rb_node;
972 struct rb_node *parent = NULL;
975 struct rmap_item *tree_rmap_item;
978 tree_rmap_item = rb_entry(*new, struct rmap_item, node);
979 page2[0] = get_mergeable_page(tree_rmap_item);
984 * Don't substitute an unswappable ksm page
985 * just for one good swappable forked page.
987 if (page == page2[0]) {
992 ret = memcmp_pages(page, page2[0]);
997 new = &parent->rb_left;
998 } else if (ret > 0) {
1000 new = &parent->rb_right;
1002 return tree_rmap_item;
1006 rmap_item->address |= NODE_FLAG;
1007 rmap_item->address |= (ksm_scan.seqnr & SEQNR_MASK);
1008 rb_link_node(&rmap_item->node, parent, new);
1009 rb_insert_color(&rmap_item->node, &root_unstable_tree);
1011 ksm_pages_unshared++;
1016 * stable_tree_append - add another rmap_item to the linked list of
1017 * rmap_items hanging off a given node of the stable tree, all sharing
1018 * the same ksm page.
1020 static void stable_tree_append(struct rmap_item *rmap_item,
1021 struct rmap_item *tree_rmap_item)
1023 rmap_item->next = tree_rmap_item->next;
1024 rmap_item->prev = tree_rmap_item;
1026 if (tree_rmap_item->next)
1027 tree_rmap_item->next->prev = rmap_item;
1029 tree_rmap_item->next = rmap_item;
1030 rmap_item->address |= STABLE_FLAG;
1032 ksm_pages_sharing++;
1036 * cmp_and_merge_page - take a page computes its hash value and check if there
1037 * is similar hash value to different page,
1038 * in case we find that there is similar hash to different page we call to
1039 * try_to_merge_two_pages().
1041 * @page: the page that we are searching identical page to.
1042 * @rmap_item: the reverse mapping into the virtual address of this page
1044 static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
1046 struct page *page2[1];
1047 struct rmap_item *tree_rmap_item;
1048 unsigned int checksum;
1051 if (in_stable_tree(rmap_item))
1052 remove_rmap_item_from_tree(rmap_item);
1054 /* We first start with searching the page inside the stable tree */
1055 tree_rmap_item = stable_tree_search(page, page2, rmap_item);
1056 if (tree_rmap_item) {
1057 if (page == page2[0]) /* forked */
1060 err = try_to_merge_with_ksm_page(rmap_item->mm,
1067 * The page was successfully merged:
1068 * add its rmap_item to the stable tree.
1070 stable_tree_append(rmap_item, tree_rmap_item);
1076 * A ksm page might have got here by fork, but its other
1077 * references have already been removed from the stable tree.
1080 break_cow(rmap_item->mm, rmap_item->address);
1083 * In case the hash value of the page was changed from the last time we
1084 * have calculated it, this page to be changed frequely, therefore we
1085 * don't want to insert it to the unstable tree, and we don't want to
1086 * waste our time to search if there is something identical to it there.
1088 checksum = calc_checksum(page);
1089 if (rmap_item->oldchecksum != checksum) {
1090 rmap_item->oldchecksum = checksum;
1094 tree_rmap_item = unstable_tree_search_insert(page, page2, rmap_item);
1095 if (tree_rmap_item) {
1096 err = try_to_merge_two_pages(rmap_item->mm,
1097 rmap_item->address, page,
1099 tree_rmap_item->address, page2[0]);
1101 * As soon as we merge this page, we want to remove the
1102 * rmap_item of the page we have merged with from the unstable
1103 * tree, and insert it instead as new node in the stable tree.
1106 rb_erase(&tree_rmap_item->node, &root_unstable_tree);
1107 tree_rmap_item->address &= ~NODE_FLAG;
1108 ksm_pages_unshared--;
1111 * If we fail to insert the page into the stable tree,
1112 * we will have 2 virtual addresses that are pointing
1113 * to a ksm page left outside the stable tree,
1114 * in which case we need to break_cow on both.
1116 if (stable_tree_insert(page2[0], tree_rmap_item))
1117 stable_tree_append(rmap_item, tree_rmap_item);
1119 break_cow(tree_rmap_item->mm,
1120 tree_rmap_item->address);
1121 break_cow(rmap_item->mm, rmap_item->address);
1129 static struct rmap_item *get_next_rmap_item(struct mm_slot *mm_slot,
1130 struct list_head *cur,
1133 struct rmap_item *rmap_item;
1135 while (cur != &mm_slot->rmap_list) {
1136 rmap_item = list_entry(cur, struct rmap_item, link);
1137 if ((rmap_item->address & PAGE_MASK) == addr) {
1138 if (!in_stable_tree(rmap_item))
1139 remove_rmap_item_from_tree(rmap_item);
1142 if (rmap_item->address > addr)
1145 remove_rmap_item_from_tree(rmap_item);
1146 list_del(&rmap_item->link);
1147 free_rmap_item(rmap_item);
1150 rmap_item = alloc_rmap_item();
1152 /* It has already been zeroed */
1153 rmap_item->mm = mm_slot->mm;
1154 rmap_item->address = addr;
1155 list_add_tail(&rmap_item->link, cur);
1160 static struct rmap_item *scan_get_next_rmap_item(struct page **page)
1162 struct mm_struct *mm;
1163 struct mm_slot *slot;
1164 struct vm_area_struct *vma;
1165 struct rmap_item *rmap_item;
1167 if (list_empty(&ksm_mm_head.mm_list))
1170 slot = ksm_scan.mm_slot;
1171 if (slot == &ksm_mm_head) {
1172 root_unstable_tree = RB_ROOT;
1174 spin_lock(&ksm_mmlist_lock);
1175 slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list);
1176 ksm_scan.mm_slot = slot;
1177 spin_unlock(&ksm_mmlist_lock);
1179 ksm_scan.address = 0;
1180 ksm_scan.rmap_item = list_entry(&slot->rmap_list,
1181 struct rmap_item, link);
1185 down_read(&mm->mmap_sem);
1186 for (vma = find_vma(mm, ksm_scan.address); vma; vma = vma->vm_next) {
1187 if (!(vma->vm_flags & VM_MERGEABLE))
1189 if (ksm_scan.address < vma->vm_start)
1190 ksm_scan.address = vma->vm_start;
1192 ksm_scan.address = vma->vm_end;
1194 while (ksm_scan.address < vma->vm_end) {
1195 *page = follow_page(vma, ksm_scan.address, FOLL_GET);
1196 if (*page && PageAnon(*page)) {
1197 flush_anon_page(vma, *page, ksm_scan.address);
1198 flush_dcache_page(*page);
1199 rmap_item = get_next_rmap_item(slot,
1200 ksm_scan.rmap_item->link.next,
1203 ksm_scan.rmap_item = rmap_item;
1204 ksm_scan.address += PAGE_SIZE;
1207 up_read(&mm->mmap_sem);
1212 ksm_scan.address += PAGE_SIZE;
1217 if (!ksm_scan.address) {
1219 * We've completed a full scan of all vmas, holding mmap_sem
1220 * throughout, and found no VM_MERGEABLE: so do the same as
1221 * __ksm_exit does to remove this mm from all our lists now.
1223 remove_mm_from_lists(mm);
1224 up_read(&mm->mmap_sem);
1225 slot = ksm_scan.mm_slot;
1226 if (slot != &ksm_mm_head)
1232 * Nuke all the rmap_items that are above this current rmap:
1233 * because there were no VM_MERGEABLE vmas with such addresses.
1235 remove_trailing_rmap_items(slot, ksm_scan.rmap_item->link.next);
1236 up_read(&mm->mmap_sem);
1238 spin_lock(&ksm_mmlist_lock);
1239 slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list);
1240 ksm_scan.mm_slot = slot;
1241 spin_unlock(&ksm_mmlist_lock);
1243 /* Repeat until we've completed scanning the whole list */
1244 if (slot != &ksm_mm_head)
1248 * Bump seqnr here rather than at top, so that __ksm_exit
1249 * can skip rb_erase on unstable tree until we run again.
1256 * ksm_do_scan - the ksm scanner main worker function.
1257 * @scan_npages - number of pages we want to scan before we return.
1259 static void ksm_do_scan(unsigned int scan_npages)
1261 struct rmap_item *rmap_item;
1264 while (scan_npages--) {
1266 rmap_item = scan_get_next_rmap_item(&page);
1269 if (!PageKsm(page) || !in_stable_tree(rmap_item))
1270 cmp_and_merge_page(page, rmap_item);
1271 else if (page_mapcount(page) == 1) {
1273 * Replace now-unshared ksm page by ordinary page.
1275 break_cow(rmap_item->mm, rmap_item->address);
1276 remove_rmap_item_from_tree(rmap_item);
1277 rmap_item->oldchecksum = calc_checksum(page);
1283 static int ksm_scan_thread(void *nothing)
1285 set_user_nice(current, 5);
1287 while (!kthread_should_stop()) {
1288 if (ksm_run & KSM_RUN_MERGE) {
1289 mutex_lock(&ksm_thread_mutex);
1290 ksm_do_scan(ksm_thread_pages_to_scan);
1291 mutex_unlock(&ksm_thread_mutex);
1292 schedule_timeout_interruptible(
1293 msecs_to_jiffies(ksm_thread_sleep_millisecs));
1295 wait_event_interruptible(ksm_thread_wait,
1296 (ksm_run & KSM_RUN_MERGE) ||
1297 kthread_should_stop());
1303 int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
1304 unsigned long end, int advice, unsigned long *vm_flags)
1306 struct mm_struct *mm = vma->vm_mm;
1309 case MADV_MERGEABLE:
1311 * Be somewhat over-protective for now!
1313 if (*vm_flags & (VM_MERGEABLE | VM_SHARED | VM_MAYSHARE |
1314 VM_PFNMAP | VM_IO | VM_DONTEXPAND |
1315 VM_RESERVED | VM_HUGETLB | VM_INSERTPAGE |
1316 VM_MIXEDMAP | VM_SAO))
1317 return 0; /* just ignore the advice */
1319 if (!test_bit(MMF_VM_MERGEABLE, &mm->flags))
1320 if (__ksm_enter(mm) < 0)
1323 *vm_flags |= VM_MERGEABLE;
1326 case MADV_UNMERGEABLE:
1327 if (!(*vm_flags & VM_MERGEABLE))
1328 return 0; /* just ignore the advice */
1331 unmerge_ksm_pages(vma, start, end);
1333 *vm_flags &= ~VM_MERGEABLE;
1340 int __ksm_enter(struct mm_struct *mm)
1342 struct mm_slot *mm_slot = alloc_mm_slot();
1346 spin_lock(&ksm_mmlist_lock);
1347 insert_to_mm_slots_hash(mm, mm_slot);
1349 * Insert just behind the scanning cursor, to let the area settle
1350 * down a little; when fork is followed by immediate exec, we don't
1351 * want ksmd to waste time setting up and tearing down an rmap_list.
1353 list_add_tail(&mm_slot->mm_list, &ksm_scan.mm_slot->mm_list);
1354 spin_unlock(&ksm_mmlist_lock);
1356 set_bit(MMF_VM_MERGEABLE, &mm->flags);
1360 void __ksm_exit(struct mm_struct *mm)
1363 * This process is exiting: doesn't hold and doesn't need mmap_sem;
1364 * but we do need to exclude ksmd and other exiters while we modify
1365 * the various lists and trees.
1367 mutex_lock(&ksm_thread_mutex);
1368 remove_mm_from_lists(mm);
1369 mutex_unlock(&ksm_thread_mutex);
1372 #define KSM_ATTR_RO(_name) \
1373 static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
1374 #define KSM_ATTR(_name) \
1375 static struct kobj_attribute _name##_attr = \
1376 __ATTR(_name, 0644, _name##_show, _name##_store)
1378 static ssize_t sleep_millisecs_show(struct kobject *kobj,
1379 struct kobj_attribute *attr, char *buf)
1381 return sprintf(buf, "%u\n", ksm_thread_sleep_millisecs);
1384 static ssize_t sleep_millisecs_store(struct kobject *kobj,
1385 struct kobj_attribute *attr,
1386 const char *buf, size_t count)
1388 unsigned long msecs;
1391 err = strict_strtoul(buf, 10, &msecs);
1392 if (err || msecs > UINT_MAX)
1395 ksm_thread_sleep_millisecs = msecs;
1399 KSM_ATTR(sleep_millisecs);
1401 static ssize_t pages_to_scan_show(struct kobject *kobj,
1402 struct kobj_attribute *attr, char *buf)
1404 return sprintf(buf, "%u\n", ksm_thread_pages_to_scan);
1407 static ssize_t pages_to_scan_store(struct kobject *kobj,
1408 struct kobj_attribute *attr,
1409 const char *buf, size_t count)
1412 unsigned long nr_pages;
1414 err = strict_strtoul(buf, 10, &nr_pages);
1415 if (err || nr_pages > UINT_MAX)
1418 ksm_thread_pages_to_scan = nr_pages;
1422 KSM_ATTR(pages_to_scan);
1424 static ssize_t run_show(struct kobject *kobj, struct kobj_attribute *attr,
1427 return sprintf(buf, "%u\n", ksm_run);
1430 static ssize_t run_store(struct kobject *kobj, struct kobj_attribute *attr,
1431 const char *buf, size_t count)
1434 unsigned long flags;
1436 err = strict_strtoul(buf, 10, &flags);
1437 if (err || flags > UINT_MAX)
1439 if (flags > KSM_RUN_UNMERGE)
1443 * KSM_RUN_MERGE sets ksmd running, and 0 stops it running.
1444 * KSM_RUN_UNMERGE stops it running and unmerges all rmap_items,
1445 * breaking COW to free the unswappable pages_shared (but leaves
1446 * mm_slots on the list for when ksmd may be set running again).
1449 mutex_lock(&ksm_thread_mutex);
1450 if (ksm_run != flags) {
1452 if (flags & KSM_RUN_UNMERGE)
1453 unmerge_and_remove_all_rmap_items();
1455 mutex_unlock(&ksm_thread_mutex);
1457 if (flags & KSM_RUN_MERGE)
1458 wake_up_interruptible(&ksm_thread_wait);
1464 static ssize_t max_kernel_pages_store(struct kobject *kobj,
1465 struct kobj_attribute *attr,
1466 const char *buf, size_t count)
1469 unsigned long nr_pages;
1471 err = strict_strtoul(buf, 10, &nr_pages);
1475 ksm_max_kernel_pages = nr_pages;
1480 static ssize_t max_kernel_pages_show(struct kobject *kobj,
1481 struct kobj_attribute *attr, char *buf)
1483 return sprintf(buf, "%lu\n", ksm_max_kernel_pages);
1485 KSM_ATTR(max_kernel_pages);
1487 static ssize_t pages_shared_show(struct kobject *kobj,
1488 struct kobj_attribute *attr, char *buf)
1490 return sprintf(buf, "%lu\n", ksm_pages_shared);
1492 KSM_ATTR_RO(pages_shared);
1494 static ssize_t pages_sharing_show(struct kobject *kobj,
1495 struct kobj_attribute *attr, char *buf)
1497 return sprintf(buf, "%lu\n", ksm_pages_sharing);
1499 KSM_ATTR_RO(pages_sharing);
1501 static ssize_t pages_unshared_show(struct kobject *kobj,
1502 struct kobj_attribute *attr, char *buf)
1504 return sprintf(buf, "%lu\n", ksm_pages_unshared);
1506 KSM_ATTR_RO(pages_unshared);
1508 static ssize_t pages_volatile_show(struct kobject *kobj,
1509 struct kobj_attribute *attr, char *buf)
1511 long ksm_pages_volatile;
1513 ksm_pages_volatile = ksm_rmap_items - ksm_pages_shared
1514 - ksm_pages_sharing - ksm_pages_unshared;
1516 * It was not worth any locking to calculate that statistic,
1517 * but it might therefore sometimes be negative: conceal that.
1519 if (ksm_pages_volatile < 0)
1520 ksm_pages_volatile = 0;
1521 return sprintf(buf, "%ld\n", ksm_pages_volatile);
1523 KSM_ATTR_RO(pages_volatile);
1525 static ssize_t full_scans_show(struct kobject *kobj,
1526 struct kobj_attribute *attr, char *buf)
1528 return sprintf(buf, "%lu\n", ksm_scan.seqnr);
1530 KSM_ATTR_RO(full_scans);
1532 static struct attribute *ksm_attrs[] = {
1533 &sleep_millisecs_attr.attr,
1534 &pages_to_scan_attr.attr,
1536 &max_kernel_pages_attr.attr,
1537 &pages_shared_attr.attr,
1538 &pages_sharing_attr.attr,
1539 &pages_unshared_attr.attr,
1540 &pages_volatile_attr.attr,
1541 &full_scans_attr.attr,
1545 static struct attribute_group ksm_attr_group = {
1550 static int __init ksm_init(void)
1552 struct task_struct *ksm_thread;
1555 err = ksm_slab_init();
1559 err = mm_slots_hash_init();
1563 ksm_thread = kthread_run(ksm_scan_thread, NULL, "ksmd");
1564 if (IS_ERR(ksm_thread)) {
1565 printk(KERN_ERR "ksm: creating kthread failed\n");
1566 err = PTR_ERR(ksm_thread);
1570 err = sysfs_create_group(mm_kobj, &ksm_attr_group);
1572 printk(KERN_ERR "ksm: register sysfs failed\n");
1579 kthread_stop(ksm_thread);
1581 mm_slots_hash_free();
1587 module_init(ksm_init)