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 int 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 | VM_FAULT_OOM)));
315 * We must loop because handle_mm_fault() may back out if there's
316 * any difficulty e.g. if pte accessed bit gets updated concurrently.
318 * VM_FAULT_WRITE is what we have been hoping for: it indicates that
319 * COW has been broken, even if the vma does not permit VM_WRITE;
320 * but note that a concurrent fault might break PageKsm for us.
322 * VM_FAULT_SIGBUS could occur if we race with truncation of the
323 * backing file, which also invalidates anonymous pages: that's
324 * okay, that truncation will have unmapped the PageKsm for us.
326 * VM_FAULT_OOM: at the time of writing (late July 2009), setting
327 * aside mem_cgroup limits, VM_FAULT_OOM would only be set if the
328 * current task has TIF_MEMDIE set, and will be OOM killed on return
329 * to user; and ksmd, having no mm, would never be chosen for that.
331 * But if the mm is in a limited mem_cgroup, then the fault may fail
332 * with VM_FAULT_OOM even if the current task is not TIF_MEMDIE; and
333 * even ksmd can fail in this way - though it's usually breaking ksm
334 * just to undo a merge it made a moment before, so unlikely to oom.
336 * That's a pity: we might therefore have more kernel pages allocated
337 * than we're counting as nodes in the stable tree; but ksm_do_scan
338 * will retry to break_cow on each pass, so should recover the page
339 * in due course. The important thing is to not let VM_MERGEABLE
340 * be cleared while any such pages might remain in the area.
342 return (ret & VM_FAULT_OOM) ? -ENOMEM : 0;
345 static void break_cow(struct mm_struct *mm, unsigned long addr)
347 struct vm_area_struct *vma;
349 down_read(&mm->mmap_sem);
350 vma = find_vma(mm, addr);
351 if (!vma || vma->vm_start > addr)
353 if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
355 break_ksm(vma, addr);
357 up_read(&mm->mmap_sem);
360 static struct page *get_mergeable_page(struct rmap_item *rmap_item)
362 struct mm_struct *mm = rmap_item->mm;
363 unsigned long addr = rmap_item->address;
364 struct vm_area_struct *vma;
367 down_read(&mm->mmap_sem);
368 vma = find_vma(mm, addr);
369 if (!vma || vma->vm_start > addr)
371 if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
374 page = follow_page(vma, addr, FOLL_GET);
377 if (PageAnon(page)) {
378 flush_anon_page(vma, page, addr);
379 flush_dcache_page(page);
384 up_read(&mm->mmap_sem);
389 * get_ksm_page: checks if the page at the virtual address in rmap_item
390 * is still PageKsm, in which case we can trust the content of the page,
391 * and it returns the gotten page; but NULL if the page has been zapped.
393 static struct page *get_ksm_page(struct rmap_item *rmap_item)
397 page = get_mergeable_page(rmap_item);
398 if (page && !PageKsm(page)) {
406 * Removing rmap_item from stable or unstable tree.
407 * This function will clean the information from the stable/unstable tree.
409 static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
411 if (in_stable_tree(rmap_item)) {
412 struct rmap_item *next_item = rmap_item->next;
414 if (rmap_item->address & NODE_FLAG) {
416 rb_replace_node(&rmap_item->node,
419 next_item->address |= NODE_FLAG;
422 rb_erase(&rmap_item->node, &root_stable_tree);
426 struct rmap_item *prev_item = rmap_item->prev;
428 BUG_ON(prev_item->next != rmap_item);
429 prev_item->next = next_item;
431 BUG_ON(next_item->prev != rmap_item);
432 next_item->prev = rmap_item->prev;
437 rmap_item->next = NULL;
439 } else if (rmap_item->address & NODE_FLAG) {
442 * ksm_thread can and must skip the rb_erase, because
443 * root_unstable_tree was already reset to RB_ROOT.
444 * But __ksm_exit has to be careful: do the rb_erase
445 * if it's interrupting a scan, and this rmap_item was
446 * inserted by this scan rather than left from before.
448 * Because of the case in which remove_mm_from_lists
449 * increments seqnr before removing rmaps, unstable_nr
450 * may even be 2 behind seqnr, but should never be
451 * further behind. Yes, I did have trouble with this!
453 age = (unsigned char)(ksm_scan.seqnr - rmap_item->address);
456 rb_erase(&rmap_item->node, &root_unstable_tree);
457 ksm_pages_unshared--;
460 rmap_item->address &= PAGE_MASK;
462 cond_resched(); /* we're called from many long loops */
465 static void remove_trailing_rmap_items(struct mm_slot *mm_slot,
466 struct list_head *cur)
468 struct rmap_item *rmap_item;
470 while (cur != &mm_slot->rmap_list) {
471 rmap_item = list_entry(cur, struct rmap_item, link);
473 remove_rmap_item_from_tree(rmap_item);
474 list_del(&rmap_item->link);
475 free_rmap_item(rmap_item);
480 * Though it's very tempting to unmerge in_stable_tree(rmap_item)s rather
481 * than check every pte of a given vma, the locking doesn't quite work for
482 * that - an rmap_item is assigned to the stable tree after inserting ksm
483 * page and upping mmap_sem. Nor does it fit with the way we skip dup'ing
484 * rmap_items from parent to child at fork time (so as not to waste time
485 * if exit comes before the next scan reaches it).
487 * Similarly, although we'd like to remove rmap_items (so updating counts
488 * and freeing memory) when unmerging an area, it's easier to leave that
489 * to the next pass of ksmd - consider, for example, how ksmd might be
490 * in cmp_and_merge_page on one of the rmap_items we would be removing.
492 static int unmerge_ksm_pages(struct vm_area_struct *vma,
493 unsigned long start, unsigned long end)
498 for (addr = start; addr < end && !err; addr += PAGE_SIZE) {
499 if (signal_pending(current))
502 err = break_ksm(vma, addr);
507 static int unmerge_and_remove_all_rmap_items(void)
509 struct mm_slot *mm_slot;
510 struct mm_struct *mm;
511 struct vm_area_struct *vma;
514 spin_lock(&ksm_mmlist_lock);
515 mm_slot = list_entry(ksm_mm_head.mm_list.next,
516 struct mm_slot, mm_list);
517 spin_unlock(&ksm_mmlist_lock);
519 while (mm_slot != &ksm_mm_head) {
521 down_read(&mm->mmap_sem);
522 for (vma = mm->mmap; vma; vma = vma->vm_next) {
523 if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
525 err = unmerge_ksm_pages(vma,
526 vma->vm_start, vma->vm_end);
528 up_read(&mm->mmap_sem);
532 remove_trailing_rmap_items(mm_slot, mm_slot->rmap_list.next);
533 up_read(&mm->mmap_sem);
535 spin_lock(&ksm_mmlist_lock);
536 mm_slot = list_entry(mm_slot->mm_list.next,
537 struct mm_slot, mm_list);
538 spin_unlock(&ksm_mmlist_lock);
543 spin_lock(&ksm_mmlist_lock);
544 ksm_scan.mm_slot = &ksm_mm_head;
545 spin_unlock(&ksm_mmlist_lock);
549 static void remove_mm_from_lists(struct mm_struct *mm)
551 struct mm_slot *mm_slot;
553 spin_lock(&ksm_mmlist_lock);
554 mm_slot = get_mm_slot(mm);
557 * This mm_slot is always at the scanning cursor when we're
558 * called from scan_get_next_rmap_item; but it's a special
559 * case when we're called from __ksm_exit.
561 if (ksm_scan.mm_slot == mm_slot) {
562 ksm_scan.mm_slot = list_entry(
563 mm_slot->mm_list.next, struct mm_slot, mm_list);
564 ksm_scan.address = 0;
565 ksm_scan.rmap_item = list_entry(
566 &ksm_scan.mm_slot->rmap_list, struct rmap_item, link);
567 if (ksm_scan.mm_slot == &ksm_mm_head)
571 hlist_del(&mm_slot->link);
572 list_del(&mm_slot->mm_list);
573 spin_unlock(&ksm_mmlist_lock);
575 remove_trailing_rmap_items(mm_slot, mm_slot->rmap_list.next);
576 free_mm_slot(mm_slot);
577 clear_bit(MMF_VM_MERGEABLE, &mm->flags);
580 static u32 calc_checksum(struct page *page)
583 void *addr = kmap_atomic(page, KM_USER0);
584 checksum = jhash2(addr, PAGE_SIZE / 4, 17);
585 kunmap_atomic(addr, KM_USER0);
589 static int memcmp_pages(struct page *page1, struct page *page2)
594 addr1 = kmap_atomic(page1, KM_USER0);
595 addr2 = kmap_atomic(page2, KM_USER1);
596 ret = memcmp(addr1, addr2, PAGE_SIZE);
597 kunmap_atomic(addr2, KM_USER1);
598 kunmap_atomic(addr1, KM_USER0);
602 static inline int pages_identical(struct page *page1, struct page *page2)
604 return !memcmp_pages(page1, page2);
607 static int write_protect_page(struct vm_area_struct *vma, struct page *page,
610 struct mm_struct *mm = vma->vm_mm;
617 addr = page_address_in_vma(page, vma);
621 ptep = page_check_address(page, mm, addr, &ptl, 0);
625 if (pte_write(*ptep)) {
628 swapped = PageSwapCache(page);
629 flush_cache_page(vma, addr, page_to_pfn(page));
631 * Ok this is tricky, when get_user_pages_fast() run it doesnt
632 * take any lock, therefore the check that we are going to make
633 * with the pagecount against the mapcount is racey and
634 * O_DIRECT can happen right after the check.
635 * So we clear the pte and flush the tlb before the check
636 * this assure us that no O_DIRECT can happen after the check
637 * or in the middle of the check.
639 entry = ptep_clear_flush(vma, addr, ptep);
641 * Check that no O_DIRECT or similar I/O is in progress on the
644 if ((page_mapcount(page) + 2 + swapped) != page_count(page)) {
645 set_pte_at_notify(mm, addr, ptep, entry);
648 entry = pte_wrprotect(entry);
649 set_pte_at_notify(mm, addr, ptep, entry);
655 pte_unmap_unlock(ptep, ptl);
661 * replace_page - replace page in vma by new ksm page
662 * @vma: vma that holds the pte pointing to oldpage
663 * @oldpage: the page we are replacing by newpage
664 * @newpage: the ksm page we replace oldpage by
665 * @orig_pte: the original value of the pte
667 * Returns 0 on success, -EFAULT on failure.
669 static int replace_page(struct vm_area_struct *vma, struct page *oldpage,
670 struct page *newpage, pte_t orig_pte)
672 struct mm_struct *mm = vma->vm_mm;
682 prot = vm_get_page_prot(vma->vm_flags & ~VM_WRITE);
684 addr = page_address_in_vma(oldpage, vma);
688 pgd = pgd_offset(mm, addr);
689 if (!pgd_present(*pgd))
692 pud = pud_offset(pgd, addr);
693 if (!pud_present(*pud))
696 pmd = pmd_offset(pud, addr);
697 if (!pmd_present(*pmd))
700 ptep = pte_offset_map_lock(mm, pmd, addr, &ptl);
701 if (!pte_same(*ptep, orig_pte)) {
702 pte_unmap_unlock(ptep, ptl);
707 page_add_ksm_rmap(newpage);
709 flush_cache_page(vma, addr, pte_pfn(*ptep));
710 ptep_clear_flush(vma, addr, ptep);
711 set_pte_at_notify(mm, addr, ptep, mk_pte(newpage, prot));
713 page_remove_rmap(oldpage);
716 pte_unmap_unlock(ptep, ptl);
723 * try_to_merge_one_page - take two pages and merge them into one
724 * @vma: the vma that hold the pte pointing into oldpage
725 * @oldpage: the page that we want to replace with newpage
726 * @newpage: the page that we want to map instead of oldpage
729 * oldpage should be a PageAnon page, while newpage should be a PageKsm page,
730 * or a newly allocated kernel page which page_add_ksm_rmap will make PageKsm.
732 * This function returns 0 if the pages were merged, -EFAULT otherwise.
734 static int try_to_merge_one_page(struct vm_area_struct *vma,
735 struct page *oldpage,
736 struct page *newpage)
738 pte_t orig_pte = __pte(0);
741 if (!(vma->vm_flags & VM_MERGEABLE))
744 if (!PageAnon(oldpage))
751 * We need the page lock to read a stable PageSwapCache in
752 * write_protect_page(). We use trylock_page() instead of
753 * lock_page() because we don't want to wait here - we
754 * prefer to continue scanning and merging different pages,
755 * then come back to this page when it is unlocked.
757 if (!trylock_page(oldpage))
760 * If this anonymous page is mapped only here, its pte may need
761 * to be write-protected. If it's mapped elsewhere, all of its
762 * ptes are necessarily already write-protected. But in either
763 * case, we need to lock and check page_count is not raised.
765 if (write_protect_page(vma, oldpage, &orig_pte)) {
766 unlock_page(oldpage);
769 unlock_page(oldpage);
771 if (pages_identical(oldpage, newpage))
772 err = replace_page(vma, oldpage, newpage, orig_pte);
782 * try_to_merge_with_ksm_page - like try_to_merge_two_pages,
783 * but no new kernel page is allocated: kpage must already be a ksm page.
785 static int try_to_merge_with_ksm_page(struct mm_struct *mm1,
790 struct vm_area_struct *vma;
793 down_read(&mm1->mmap_sem);
794 vma = find_vma(mm1, addr1);
795 if (!vma || vma->vm_start > addr1)
798 err = try_to_merge_one_page(vma, page1, kpage);
800 up_read(&mm1->mmap_sem);
805 * try_to_merge_two_pages - take two identical pages and prepare them
806 * to be merged into one page.
808 * This function returns 0 if we successfully mapped two identical pages
809 * into one page, -EFAULT otherwise.
811 * Note that this function allocates a new kernel page: if one of the pages
812 * is already a ksm page, try_to_merge_with_ksm_page should be used.
814 static int try_to_merge_two_pages(struct mm_struct *mm1, unsigned long addr1,
815 struct page *page1, struct mm_struct *mm2,
816 unsigned long addr2, struct page *page2)
818 struct vm_area_struct *vma;
823 * The number of nodes in the stable tree
824 * is the number of kernel pages that we hold.
826 if (ksm_max_kernel_pages &&
827 ksm_max_kernel_pages <= ksm_pages_shared)
830 kpage = alloc_page(GFP_HIGHUSER);
834 down_read(&mm1->mmap_sem);
835 vma = find_vma(mm1, addr1);
836 if (!vma || vma->vm_start > addr1) {
837 up_read(&mm1->mmap_sem);
841 copy_user_highpage(kpage, page1, addr1, vma);
842 err = try_to_merge_one_page(vma, page1, kpage);
843 up_read(&mm1->mmap_sem);
846 err = try_to_merge_with_ksm_page(mm2, addr2, page2, kpage);
848 * If that fails, we have a ksm page with only one pte
849 * pointing to it: so break it.
852 break_cow(mm1, addr1);
860 * stable_tree_search - search page inside the stable tree
861 * @page: the page that we are searching identical pages to.
862 * @page2: pointer into identical page that we are holding inside the stable
863 * tree that we have found.
864 * @rmap_item: the reverse mapping item
866 * This function checks if there is a page inside the stable tree
867 * with identical content to the page that we are scanning right now.
869 * This function return rmap_item pointer to the identical item if found,
872 static struct rmap_item *stable_tree_search(struct page *page,
874 struct rmap_item *rmap_item)
876 struct rb_node *node = root_stable_tree.rb_node;
879 struct rmap_item *tree_rmap_item, *next_rmap_item;
882 tree_rmap_item = rb_entry(node, struct rmap_item, node);
883 while (tree_rmap_item) {
884 BUG_ON(!in_stable_tree(tree_rmap_item));
886 page2[0] = get_ksm_page(tree_rmap_item);
889 next_rmap_item = tree_rmap_item->next;
890 remove_rmap_item_from_tree(tree_rmap_item);
891 tree_rmap_item = next_rmap_item;
896 ret = memcmp_pages(page, page2[0]);
900 node = node->rb_left;
901 } else if (ret > 0) {
903 node = node->rb_right;
905 return tree_rmap_item;
913 * stable_tree_insert - insert rmap_item pointing to new ksm page
914 * into the stable tree.
916 * @page: the page that we are searching identical page to inside the stable
918 * @rmap_item: pointer to the reverse mapping item.
920 * This function returns rmap_item if success, NULL otherwise.
922 static struct rmap_item *stable_tree_insert(struct page *page,
923 struct rmap_item *rmap_item)
925 struct rb_node **new = &root_stable_tree.rb_node;
926 struct rb_node *parent = NULL;
929 struct rmap_item *tree_rmap_item, *next_rmap_item;
930 struct page *tree_page;
933 tree_rmap_item = rb_entry(*new, struct rmap_item, node);
934 while (tree_rmap_item) {
935 BUG_ON(!in_stable_tree(tree_rmap_item));
937 tree_page = get_ksm_page(tree_rmap_item);
940 next_rmap_item = tree_rmap_item->next;
941 remove_rmap_item_from_tree(tree_rmap_item);
942 tree_rmap_item = next_rmap_item;
947 ret = memcmp_pages(page, tree_page);
952 new = &parent->rb_left;
954 new = &parent->rb_right;
957 * It is not a bug that stable_tree_search() didn't
958 * find this node: because at that time our page was
959 * not yet write-protected, so may have changed since.
965 rmap_item->address |= NODE_FLAG | STABLE_FLAG;
966 rmap_item->next = NULL;
967 rb_link_node(&rmap_item->node, parent, new);
968 rb_insert_color(&rmap_item->node, &root_stable_tree);
975 * unstable_tree_search_insert - search and insert items into the unstable tree.
977 * @page: the page that we are going to search for identical page or to insert
978 * into the unstable tree
979 * @page2: pointer into identical page that was found inside the unstable tree
980 * @rmap_item: the reverse mapping item of page
982 * This function searches for a page in the unstable tree identical to the
983 * page currently being scanned; and if no identical page is found in the
984 * tree, we insert rmap_item as a new object into the unstable tree.
986 * This function returns pointer to rmap_item found to be identical
987 * to the currently scanned page, NULL otherwise.
989 * This function does both searching and inserting, because they share
990 * the same walking algorithm in an rbtree.
992 static struct rmap_item *unstable_tree_search_insert(struct page *page,
994 struct rmap_item *rmap_item)
996 struct rb_node **new = &root_unstable_tree.rb_node;
997 struct rb_node *parent = NULL;
1000 struct rmap_item *tree_rmap_item;
1003 tree_rmap_item = rb_entry(*new, struct rmap_item, node);
1004 page2[0] = get_mergeable_page(tree_rmap_item);
1009 * Don't substitute an unswappable ksm page
1010 * just for one good swappable forked page.
1012 if (page == page2[0]) {
1017 ret = memcmp_pages(page, page2[0]);
1022 new = &parent->rb_left;
1023 } else if (ret > 0) {
1025 new = &parent->rb_right;
1027 return tree_rmap_item;
1031 rmap_item->address |= NODE_FLAG;
1032 rmap_item->address |= (ksm_scan.seqnr & SEQNR_MASK);
1033 rb_link_node(&rmap_item->node, parent, new);
1034 rb_insert_color(&rmap_item->node, &root_unstable_tree);
1036 ksm_pages_unshared++;
1041 * stable_tree_append - add another rmap_item to the linked list of
1042 * rmap_items hanging off a given node of the stable tree, all sharing
1043 * the same ksm page.
1045 static void stable_tree_append(struct rmap_item *rmap_item,
1046 struct rmap_item *tree_rmap_item)
1048 rmap_item->next = tree_rmap_item->next;
1049 rmap_item->prev = tree_rmap_item;
1051 if (tree_rmap_item->next)
1052 tree_rmap_item->next->prev = rmap_item;
1054 tree_rmap_item->next = rmap_item;
1055 rmap_item->address |= STABLE_FLAG;
1057 ksm_pages_sharing++;
1061 * cmp_and_merge_page - first see if page can be merged into the stable tree;
1062 * if not, compare checksum to previous and if it's the same, see if page can
1063 * be inserted into the unstable tree, or merged with a page already there and
1064 * both transferred to the stable tree.
1066 * @page: the page that we are searching identical page to.
1067 * @rmap_item: the reverse mapping into the virtual address of this page
1069 static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
1071 struct page *page2[1];
1072 struct rmap_item *tree_rmap_item;
1073 unsigned int checksum;
1076 if (in_stable_tree(rmap_item))
1077 remove_rmap_item_from_tree(rmap_item);
1079 /* We first start with searching the page inside the stable tree */
1080 tree_rmap_item = stable_tree_search(page, page2, rmap_item);
1081 if (tree_rmap_item) {
1082 if (page == page2[0]) /* forked */
1085 err = try_to_merge_with_ksm_page(rmap_item->mm,
1092 * The page was successfully merged:
1093 * add its rmap_item to the stable tree.
1095 stable_tree_append(rmap_item, tree_rmap_item);
1101 * A ksm page might have got here by fork, but its other
1102 * references have already been removed from the stable tree.
1103 * Or it might be left over from a break_ksm which failed
1104 * when the mem_cgroup had reached its limit: try again now.
1107 break_cow(rmap_item->mm, rmap_item->address);
1110 * In case the hash value of the page was changed from the last time we
1111 * have calculated it, this page to be changed frequely, therefore we
1112 * don't want to insert it to the unstable tree, and we don't want to
1113 * waste our time to search if there is something identical to it there.
1115 checksum = calc_checksum(page);
1116 if (rmap_item->oldchecksum != checksum) {
1117 rmap_item->oldchecksum = checksum;
1121 tree_rmap_item = unstable_tree_search_insert(page, page2, rmap_item);
1122 if (tree_rmap_item) {
1123 err = try_to_merge_two_pages(rmap_item->mm,
1124 rmap_item->address, page,
1126 tree_rmap_item->address, page2[0]);
1128 * As soon as we merge this page, we want to remove the
1129 * rmap_item of the page we have merged with from the unstable
1130 * tree, and insert it instead as new node in the stable tree.
1133 rb_erase(&tree_rmap_item->node, &root_unstable_tree);
1134 tree_rmap_item->address &= ~NODE_FLAG;
1135 ksm_pages_unshared--;
1138 * If we fail to insert the page into the stable tree,
1139 * we will have 2 virtual addresses that are pointing
1140 * to a ksm page left outside the stable tree,
1141 * in which case we need to break_cow on both.
1143 if (stable_tree_insert(page2[0], tree_rmap_item))
1144 stable_tree_append(rmap_item, tree_rmap_item);
1146 break_cow(tree_rmap_item->mm,
1147 tree_rmap_item->address);
1148 break_cow(rmap_item->mm, rmap_item->address);
1156 static struct rmap_item *get_next_rmap_item(struct mm_slot *mm_slot,
1157 struct list_head *cur,
1160 struct rmap_item *rmap_item;
1162 while (cur != &mm_slot->rmap_list) {
1163 rmap_item = list_entry(cur, struct rmap_item, link);
1164 if ((rmap_item->address & PAGE_MASK) == addr) {
1165 if (!in_stable_tree(rmap_item))
1166 remove_rmap_item_from_tree(rmap_item);
1169 if (rmap_item->address > addr)
1172 remove_rmap_item_from_tree(rmap_item);
1173 list_del(&rmap_item->link);
1174 free_rmap_item(rmap_item);
1177 rmap_item = alloc_rmap_item();
1179 /* It has already been zeroed */
1180 rmap_item->mm = mm_slot->mm;
1181 rmap_item->address = addr;
1182 list_add_tail(&rmap_item->link, cur);
1187 static struct rmap_item *scan_get_next_rmap_item(struct page **page)
1189 struct mm_struct *mm;
1190 struct mm_slot *slot;
1191 struct vm_area_struct *vma;
1192 struct rmap_item *rmap_item;
1194 if (list_empty(&ksm_mm_head.mm_list))
1197 slot = ksm_scan.mm_slot;
1198 if (slot == &ksm_mm_head) {
1199 root_unstable_tree = RB_ROOT;
1201 spin_lock(&ksm_mmlist_lock);
1202 slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list);
1203 ksm_scan.mm_slot = slot;
1204 spin_unlock(&ksm_mmlist_lock);
1206 ksm_scan.address = 0;
1207 ksm_scan.rmap_item = list_entry(&slot->rmap_list,
1208 struct rmap_item, link);
1212 down_read(&mm->mmap_sem);
1213 for (vma = find_vma(mm, ksm_scan.address); vma; vma = vma->vm_next) {
1214 if (!(vma->vm_flags & VM_MERGEABLE))
1216 if (ksm_scan.address < vma->vm_start)
1217 ksm_scan.address = vma->vm_start;
1219 ksm_scan.address = vma->vm_end;
1221 while (ksm_scan.address < vma->vm_end) {
1222 *page = follow_page(vma, ksm_scan.address, FOLL_GET);
1223 if (*page && PageAnon(*page)) {
1224 flush_anon_page(vma, *page, ksm_scan.address);
1225 flush_dcache_page(*page);
1226 rmap_item = get_next_rmap_item(slot,
1227 ksm_scan.rmap_item->link.next,
1230 ksm_scan.rmap_item = rmap_item;
1231 ksm_scan.address += PAGE_SIZE;
1234 up_read(&mm->mmap_sem);
1239 ksm_scan.address += PAGE_SIZE;
1244 if (!ksm_scan.address) {
1246 * We've completed a full scan of all vmas, holding mmap_sem
1247 * throughout, and found no VM_MERGEABLE: so do the same as
1248 * __ksm_exit does to remove this mm from all our lists now.
1250 remove_mm_from_lists(mm);
1251 up_read(&mm->mmap_sem);
1252 slot = ksm_scan.mm_slot;
1253 if (slot != &ksm_mm_head)
1259 * Nuke all the rmap_items that are above this current rmap:
1260 * because there were no VM_MERGEABLE vmas with such addresses.
1262 remove_trailing_rmap_items(slot, ksm_scan.rmap_item->link.next);
1263 up_read(&mm->mmap_sem);
1265 spin_lock(&ksm_mmlist_lock);
1266 slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list);
1267 ksm_scan.mm_slot = slot;
1268 spin_unlock(&ksm_mmlist_lock);
1270 /* Repeat until we've completed scanning the whole list */
1271 if (slot != &ksm_mm_head)
1275 * Bump seqnr here rather than at top, so that __ksm_exit
1276 * can skip rb_erase on unstable tree until we run again.
1283 * ksm_do_scan - the ksm scanner main worker function.
1284 * @scan_npages - number of pages we want to scan before we return.
1286 static void ksm_do_scan(unsigned int scan_npages)
1288 struct rmap_item *rmap_item;
1291 while (scan_npages--) {
1293 rmap_item = scan_get_next_rmap_item(&page);
1296 if (!PageKsm(page) || !in_stable_tree(rmap_item))
1297 cmp_and_merge_page(page, rmap_item);
1298 else if (page_mapcount(page) == 1) {
1300 * Replace now-unshared ksm page by ordinary page.
1302 break_cow(rmap_item->mm, rmap_item->address);
1303 remove_rmap_item_from_tree(rmap_item);
1304 rmap_item->oldchecksum = calc_checksum(page);
1310 static int ksmd_should_run(void)
1312 return (ksm_run & KSM_RUN_MERGE) && !list_empty(&ksm_mm_head.mm_list);
1315 static int ksm_scan_thread(void *nothing)
1317 set_user_nice(current, 5);
1319 while (!kthread_should_stop()) {
1320 mutex_lock(&ksm_thread_mutex);
1321 if (ksmd_should_run())
1322 ksm_do_scan(ksm_thread_pages_to_scan);
1323 mutex_unlock(&ksm_thread_mutex);
1325 if (ksmd_should_run()) {
1326 schedule_timeout_interruptible(
1327 msecs_to_jiffies(ksm_thread_sleep_millisecs));
1329 wait_event_interruptible(ksm_thread_wait,
1330 ksmd_should_run() || kthread_should_stop());
1336 int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
1337 unsigned long end, int advice, unsigned long *vm_flags)
1339 struct mm_struct *mm = vma->vm_mm;
1343 case MADV_MERGEABLE:
1345 * Be somewhat over-protective for now!
1347 if (*vm_flags & (VM_MERGEABLE | VM_SHARED | VM_MAYSHARE |
1348 VM_PFNMAP | VM_IO | VM_DONTEXPAND |
1349 VM_RESERVED | VM_HUGETLB | VM_INSERTPAGE |
1350 VM_MIXEDMAP | VM_SAO))
1351 return 0; /* just ignore the advice */
1353 if (!test_bit(MMF_VM_MERGEABLE, &mm->flags)) {
1354 err = __ksm_enter(mm);
1359 *vm_flags |= VM_MERGEABLE;
1362 case MADV_UNMERGEABLE:
1363 if (!(*vm_flags & VM_MERGEABLE))
1364 return 0; /* just ignore the advice */
1366 if (vma->anon_vma) {
1367 err = unmerge_ksm_pages(vma, start, end);
1372 *vm_flags &= ~VM_MERGEABLE;
1379 int __ksm_enter(struct mm_struct *mm)
1381 struct mm_slot *mm_slot;
1384 mm_slot = alloc_mm_slot();
1388 /* Check ksm_run too? Would need tighter locking */
1389 needs_wakeup = list_empty(&ksm_mm_head.mm_list);
1391 spin_lock(&ksm_mmlist_lock);
1392 insert_to_mm_slots_hash(mm, mm_slot);
1394 * Insert just behind the scanning cursor, to let the area settle
1395 * down a little; when fork is followed by immediate exec, we don't
1396 * want ksmd to waste time setting up and tearing down an rmap_list.
1398 list_add_tail(&mm_slot->mm_list, &ksm_scan.mm_slot->mm_list);
1399 spin_unlock(&ksm_mmlist_lock);
1401 set_bit(MMF_VM_MERGEABLE, &mm->flags);
1404 wake_up_interruptible(&ksm_thread_wait);
1409 void __ksm_exit(struct mm_struct *mm)
1412 * This process is exiting: doesn't hold and doesn't need mmap_sem;
1413 * but we do need to exclude ksmd and other exiters while we modify
1414 * the various lists and trees.
1416 mutex_lock(&ksm_thread_mutex);
1417 remove_mm_from_lists(mm);
1418 mutex_unlock(&ksm_thread_mutex);
1421 #define KSM_ATTR_RO(_name) \
1422 static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
1423 #define KSM_ATTR(_name) \
1424 static struct kobj_attribute _name##_attr = \
1425 __ATTR(_name, 0644, _name##_show, _name##_store)
1427 static ssize_t sleep_millisecs_show(struct kobject *kobj,
1428 struct kobj_attribute *attr, char *buf)
1430 return sprintf(buf, "%u\n", ksm_thread_sleep_millisecs);
1433 static ssize_t sleep_millisecs_store(struct kobject *kobj,
1434 struct kobj_attribute *attr,
1435 const char *buf, size_t count)
1437 unsigned long msecs;
1440 err = strict_strtoul(buf, 10, &msecs);
1441 if (err || msecs > UINT_MAX)
1444 ksm_thread_sleep_millisecs = msecs;
1448 KSM_ATTR(sleep_millisecs);
1450 static ssize_t pages_to_scan_show(struct kobject *kobj,
1451 struct kobj_attribute *attr, char *buf)
1453 return sprintf(buf, "%u\n", ksm_thread_pages_to_scan);
1456 static ssize_t pages_to_scan_store(struct kobject *kobj,
1457 struct kobj_attribute *attr,
1458 const char *buf, size_t count)
1461 unsigned long nr_pages;
1463 err = strict_strtoul(buf, 10, &nr_pages);
1464 if (err || nr_pages > UINT_MAX)
1467 ksm_thread_pages_to_scan = nr_pages;
1471 KSM_ATTR(pages_to_scan);
1473 static ssize_t run_show(struct kobject *kobj, struct kobj_attribute *attr,
1476 return sprintf(buf, "%u\n", ksm_run);
1479 static ssize_t run_store(struct kobject *kobj, struct kobj_attribute *attr,
1480 const char *buf, size_t count)
1483 unsigned long flags;
1485 err = strict_strtoul(buf, 10, &flags);
1486 if (err || flags > UINT_MAX)
1488 if (flags > KSM_RUN_UNMERGE)
1492 * KSM_RUN_MERGE sets ksmd running, and 0 stops it running.
1493 * KSM_RUN_UNMERGE stops it running and unmerges all rmap_items,
1494 * breaking COW to free the unswappable pages_shared (but leaves
1495 * mm_slots on the list for when ksmd may be set running again).
1498 mutex_lock(&ksm_thread_mutex);
1499 if (ksm_run != flags) {
1501 if (flags & KSM_RUN_UNMERGE) {
1502 err = unmerge_and_remove_all_rmap_items();
1504 ksm_run = KSM_RUN_STOP;
1509 mutex_unlock(&ksm_thread_mutex);
1511 if (flags & KSM_RUN_MERGE)
1512 wake_up_interruptible(&ksm_thread_wait);
1518 static ssize_t max_kernel_pages_store(struct kobject *kobj,
1519 struct kobj_attribute *attr,
1520 const char *buf, size_t count)
1523 unsigned long nr_pages;
1525 err = strict_strtoul(buf, 10, &nr_pages);
1529 ksm_max_kernel_pages = nr_pages;
1534 static ssize_t max_kernel_pages_show(struct kobject *kobj,
1535 struct kobj_attribute *attr, char *buf)
1537 return sprintf(buf, "%lu\n", ksm_max_kernel_pages);
1539 KSM_ATTR(max_kernel_pages);
1541 static ssize_t pages_shared_show(struct kobject *kobj,
1542 struct kobj_attribute *attr, char *buf)
1544 return sprintf(buf, "%lu\n", ksm_pages_shared);
1546 KSM_ATTR_RO(pages_shared);
1548 static ssize_t pages_sharing_show(struct kobject *kobj,
1549 struct kobj_attribute *attr, char *buf)
1551 return sprintf(buf, "%lu\n", ksm_pages_sharing);
1553 KSM_ATTR_RO(pages_sharing);
1555 static ssize_t pages_unshared_show(struct kobject *kobj,
1556 struct kobj_attribute *attr, char *buf)
1558 return sprintf(buf, "%lu\n", ksm_pages_unshared);
1560 KSM_ATTR_RO(pages_unshared);
1562 static ssize_t pages_volatile_show(struct kobject *kobj,
1563 struct kobj_attribute *attr, char *buf)
1565 long ksm_pages_volatile;
1567 ksm_pages_volatile = ksm_rmap_items - ksm_pages_shared
1568 - ksm_pages_sharing - ksm_pages_unshared;
1570 * It was not worth any locking to calculate that statistic,
1571 * but it might therefore sometimes be negative: conceal that.
1573 if (ksm_pages_volatile < 0)
1574 ksm_pages_volatile = 0;
1575 return sprintf(buf, "%ld\n", ksm_pages_volatile);
1577 KSM_ATTR_RO(pages_volatile);
1579 static ssize_t full_scans_show(struct kobject *kobj,
1580 struct kobj_attribute *attr, char *buf)
1582 return sprintf(buf, "%lu\n", ksm_scan.seqnr);
1584 KSM_ATTR_RO(full_scans);
1586 static struct attribute *ksm_attrs[] = {
1587 &sleep_millisecs_attr.attr,
1588 &pages_to_scan_attr.attr,
1590 &max_kernel_pages_attr.attr,
1591 &pages_shared_attr.attr,
1592 &pages_sharing_attr.attr,
1593 &pages_unshared_attr.attr,
1594 &pages_volatile_attr.attr,
1595 &full_scans_attr.attr,
1599 static struct attribute_group ksm_attr_group = {
1604 static int __init ksm_init(void)
1606 struct task_struct *ksm_thread;
1609 err = ksm_slab_init();
1613 err = mm_slots_hash_init();
1617 ksm_thread = kthread_run(ksm_scan_thread, NULL, "ksmd");
1618 if (IS_ERR(ksm_thread)) {
1619 printk(KERN_ERR "ksm: creating kthread failed\n");
1620 err = PTR_ERR(ksm_thread);
1624 err = sysfs_create_group(mm_kobj, &ksm_attr_group);
1626 printk(KERN_ERR "ksm: register sysfs failed\n");
1633 kthread_stop(ksm_thread);
1635 mm_slots_hash_free();
1641 module_init(ksm_init)