/* * Memory merging support. * * This code enables dynamic sharing of identical pages found in different * memory areas, even if they are not shared by fork() * * Copyright (C) 2008-2009 Red Hat, Inc. * Authors: * Izik Eidus * Andrea Arcangeli * Chris Wright * Hugh Dickins * * This work is licensed under the terms of the GNU GPL, version 2. */ #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include /* * A few notes about the KSM scanning process, * to make it easier to understand the data structures below: * * In order to reduce excessive scanning, KSM sorts the memory pages by their * contents into a data structure that holds pointers to the pages' locations. * * Since the contents of the pages may change at any moment, KSM cannot just * insert the pages into a normal sorted tree and expect it to find anything. * Therefore KSM uses two data structures - the stable and the unstable tree. * * The stable tree holds pointers to all the merged pages (ksm pages), sorted * by their contents. Because each such page is write-protected, searching on * this tree is fully assured to be working (except when pages are unmapped), * and therefore this tree is called the stable tree. * * In addition to the stable tree, KSM uses a second data structure called the * unstable tree: this tree holds pointers to pages which have been found to * be "unchanged for a period of time". The unstable tree sorts these pages * by their contents, but since they are not write-protected, KSM cannot rely * upon the unstable tree to work correctly - the unstable tree is liable to * be corrupted as its contents are modified, and so it is called unstable. * * KSM solves this problem by several techniques: * * 1) The unstable tree is flushed every time KSM completes scanning all * memory areas, and then the tree is rebuilt again from the beginning. * 2) KSM will only insert into the unstable tree, pages whose hash value * has not changed since the previous scan of all memory areas. * 3) The unstable tree is a RedBlack Tree - so its balancing is based on the * colors of the nodes and not on their contents, assuring that even when * the tree gets "corrupted" it won't get out of balance, so scanning time * remains the same (also, searching and inserting nodes in an rbtree uses * the same algorithm, so we have no overhead when we flush and rebuild). * 4) KSM never flushes the stable tree, which means that even if it were to * take 10 attempts to find a page in the unstable tree, once it is found, * it is secured in the stable tree. (When we scan a new page, we first * compare it against the stable tree, and then against the unstable tree.) */ /** * struct mm_slot - ksm information per mm that is being scanned * @link: link to the mm_slots hash list * @mm_list: link into the mm_slots list, rooted in ksm_mm_head * @rmap_list: head for this mm_slot's list of rmap_items * @mm: the mm that this information is valid for */ struct mm_slot { struct hlist_node link; struct list_head mm_list; struct list_head rmap_list; struct mm_struct *mm; }; /** * struct ksm_scan - cursor for scanning * @mm_slot: the current mm_slot we are scanning * @address: the next address inside that to be scanned * @rmap_item: the current rmap that we are scanning inside the rmap_list * @seqnr: count of completed full scans (needed when removing unstable node) * * There is only the one ksm_scan instance of this cursor structure. */ struct ksm_scan { struct mm_slot *mm_slot; unsigned long address; struct rmap_item *rmap_item; unsigned long seqnr; }; /** * struct rmap_item - reverse mapping item for virtual addresses * @link: link into mm_slot's rmap_list (rmap_list is per mm) * @mm: the memory structure this rmap_item is pointing into * @address: the virtual address this rmap_item tracks (+ flags in low bits) * @oldchecksum: previous checksum of the page at that virtual address * @node: rb_node of this rmap_item in either unstable or stable tree * @next: next rmap_item hanging off the same node of the stable tree * @prev: previous rmap_item hanging off the same node of the stable tree */ struct rmap_item { struct list_head link; struct mm_struct *mm; unsigned long address; /* + low bits used for flags below */ union { unsigned int oldchecksum; /* when unstable */ struct rmap_item *next; /* when stable */ }; union { struct rb_node node; /* when tree node */ struct rmap_item *prev; /* in stable list */ }; }; #define SEQNR_MASK 0x0ff /* low bits of unstable tree seqnr */ #define NODE_FLAG 0x100 /* is a node of unstable or stable tree */ #define STABLE_FLAG 0x200 /* is a node or list item of stable tree */ /* The stable and unstable tree heads */ static struct rb_root root_stable_tree = RB_ROOT; static struct rb_root root_unstable_tree = RB_ROOT; #define MM_SLOTS_HASH_HEADS 1024 static struct hlist_head *mm_slots_hash; static struct mm_slot ksm_mm_head = { .mm_list = LIST_HEAD_INIT(ksm_mm_head.mm_list), }; static struct ksm_scan ksm_scan = { .mm_slot = &ksm_mm_head, }; static struct kmem_cache *rmap_item_cache; static struct kmem_cache *mm_slot_cache; /* The number of nodes in the stable tree */ static unsigned long ksm_pages_shared; /* The number of page slots additionally sharing those nodes */ static unsigned long ksm_pages_sharing; /* The number of nodes in the unstable tree */ static unsigned long ksm_pages_unshared; /* The number of rmap_items in use: to calculate pages_volatile */ static unsigned long ksm_rmap_items; /* Limit on the number of unswappable pages used */ static unsigned long ksm_max_kernel_pages; /* Number of pages ksmd should scan in one batch */ static unsigned int ksm_thread_pages_to_scan = 100; /* Milliseconds ksmd should sleep between batches */ static unsigned int ksm_thread_sleep_millisecs = 20; #define KSM_RUN_STOP 0 #define KSM_RUN_MERGE 1 #define KSM_RUN_UNMERGE 2 static unsigned int ksm_run = KSM_RUN_STOP; static DECLARE_WAIT_QUEUE_HEAD(ksm_thread_wait); static DEFINE_MUTEX(ksm_thread_mutex); static DEFINE_SPINLOCK(ksm_mmlist_lock); #define KSM_KMEM_CACHE(__struct, __flags) kmem_cache_create("ksm_"#__struct,\ sizeof(struct __struct), __alignof__(struct __struct),\ (__flags), NULL) static void __init ksm_init_max_kernel_pages(void) { ksm_max_kernel_pages = nr_free_buffer_pages() / 4; } static int __init ksm_slab_init(void) { rmap_item_cache = KSM_KMEM_CACHE(rmap_item, 0); if (!rmap_item_cache) goto out; mm_slot_cache = KSM_KMEM_CACHE(mm_slot, 0); if (!mm_slot_cache) goto out_free; return 0; out_free: kmem_cache_destroy(rmap_item_cache); out: return -ENOMEM; } static void __init ksm_slab_free(void) { kmem_cache_destroy(mm_slot_cache); kmem_cache_destroy(rmap_item_cache); mm_slot_cache = NULL; } static inline struct rmap_item *alloc_rmap_item(void) { struct rmap_item *rmap_item; rmap_item = kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL); if (rmap_item) ksm_rmap_items++; return rmap_item; } static inline void free_rmap_item(struct rmap_item *rmap_item) { ksm_rmap_items--; rmap_item->mm = NULL; /* debug safety */ kmem_cache_free(rmap_item_cache, rmap_item); } static inline struct mm_slot *alloc_mm_slot(void) { if (!mm_slot_cache) /* initialization failed */ return NULL; return kmem_cache_zalloc(mm_slot_cache, GFP_KERNEL); } static inline void free_mm_slot(struct mm_slot *mm_slot) { kmem_cache_free(mm_slot_cache, mm_slot); } static int __init mm_slots_hash_init(void) { mm_slots_hash = kzalloc(MM_SLOTS_HASH_HEADS * sizeof(struct hlist_head), GFP_KERNEL); if (!mm_slots_hash) return -ENOMEM; return 0; } static void __init mm_slots_hash_free(void) { kfree(mm_slots_hash); } static struct mm_slot *get_mm_slot(struct mm_struct *mm) { struct mm_slot *mm_slot; struct hlist_head *bucket; struct hlist_node *node; bucket = &mm_slots_hash[((unsigned long)mm / sizeof(struct mm_struct)) % MM_SLOTS_HASH_HEADS]; hlist_for_each_entry(mm_slot, node, bucket, link) { if (mm == mm_slot->mm) return mm_slot; } return NULL; } static void insert_to_mm_slots_hash(struct mm_struct *mm, struct mm_slot *mm_slot) { struct hlist_head *bucket; bucket = &mm_slots_hash[((unsigned long)mm / sizeof(struct mm_struct)) % MM_SLOTS_HASH_HEADS]; mm_slot->mm = mm; INIT_LIST_HEAD(&mm_slot->rmap_list); hlist_add_head(&mm_slot->link, bucket); } static inline int in_stable_tree(struct rmap_item *rmap_item) { return rmap_item->address & STABLE_FLAG; } /* * ksmd, and unmerge_and_remove_all_rmap_items(), must not touch an mm's * page tables after it has passed through ksm_exit() - which, if necessary, * takes mmap_sem briefly to serialize against them. ksm_exit() does not set * a special flag: they can just back out as soon as mm_users goes to zero. * ksm_test_exit() is used throughout to make this test for exit: in some * places for correctness, in some places just to avoid unnecessary work. */ static inline bool ksm_test_exit(struct mm_struct *mm) { return atomic_read(&mm->mm_users) == 0; } /* * We use break_ksm to break COW on a ksm page: it's a stripped down * * if (get_user_pages(current, mm, addr, 1, 1, 1, &page, NULL) == 1) * put_page(page); * * but taking great care only to touch a ksm page, in a VM_MERGEABLE vma, * in case the application has unmapped and remapped mm,addr meanwhile. * Could a ksm page appear anywhere else? Actually yes, in a VM_PFNMAP * mmap of /dev/mem or /dev/kmem, where we would not want to touch it. */ static int break_ksm(struct vm_area_struct *vma, unsigned long addr) { struct page *page; int ret = 0; do { cond_resched(); page = follow_page(vma, addr, FOLL_GET); if (!page) break; if (PageKsm(page)) ret = handle_mm_fault(vma->vm_mm, vma, addr, FAULT_FLAG_WRITE); else ret = VM_FAULT_WRITE; put_page(page); } while (!(ret & (VM_FAULT_WRITE | VM_FAULT_SIGBUS | VM_FAULT_OOM))); /* * We must loop because handle_mm_fault() may back out if there's * any difficulty e.g. if pte accessed bit gets updated concurrently. * * VM_FAULT_WRITE is what we have been hoping for: it indicates that * COW has been broken, even if the vma does not permit VM_WRITE; * but note that a concurrent fault might break PageKsm for us. * * VM_FAULT_SIGBUS could occur if we race with truncation of the * backing file, which also invalidates anonymous pages: that's * okay, that truncation will have unmapped the PageKsm for us. * * VM_FAULT_OOM: at the time of writing (late July 2009), setting * aside mem_cgroup limits, VM_FAULT_OOM would only be set if the * current task has TIF_MEMDIE set, and will be OOM killed on return * to user; and ksmd, having no mm, would never be chosen for that. * * But if the mm is in a limited mem_cgroup, then the fault may fail * with VM_FAULT_OOM even if the current task is not TIF_MEMDIE; and * even ksmd can fail in this way - though it's usually breaking ksm * just to undo a merge it made a moment before, so unlikely to oom. * * That's a pity: we might therefore have more kernel pages allocated * than we're counting as nodes in the stable tree; but ksm_do_scan * will retry to break_cow on each pass, so should recover the page * in due course. The important thing is to not let VM_MERGEABLE * be cleared while any such pages might remain in the area. */ return (ret & VM_FAULT_OOM) ? -ENOMEM : 0; } static void break_cow(struct mm_struct *mm, unsigned long addr) { struct vm_area_struct *vma; down_read(&mm->mmap_sem); if (ksm_test_exit(mm)) goto out; vma = find_vma(mm, addr); if (!vma || vma->vm_start > addr) goto out; if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma) goto out; break_ksm(vma, addr); out: up_read(&mm->mmap_sem); } static struct page *get_mergeable_page(struct rmap_item *rmap_item) { struct mm_struct *mm = rmap_item->mm; unsigned long addr = rmap_item->address; struct vm_area_struct *vma; struct page *page; down_read(&mm->mmap_sem); if (ksm_test_exit(mm)) goto out; vma = find_vma(mm, addr); if (!vma || vma->vm_start > addr) goto out; if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma) goto out; page = follow_page(vma, addr, FOLL_GET); if (!page) goto out; if (PageAnon(page)) { flush_anon_page(vma, page, addr); flush_dcache_page(page); } else { put_page(page); out: page = NULL; } up_read(&mm->mmap_sem); return page; } /* * get_ksm_page: checks if the page at the virtual address in rmap_item * is still PageKsm, in which case we can trust the content of the page, * and it returns the gotten page; but NULL if the page has been zapped. */ static struct page *get_ksm_page(struct rmap_item *rmap_item) { struct page *page; page = get_mergeable_page(rmap_item); if (page && !PageKsm(page)) { put_page(page); page = NULL; } return page; } /* * Removing rmap_item from stable or unstable tree. * This function will clean the information from the stable/unstable tree. */ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item) { if (in_stable_tree(rmap_item)) { struct rmap_item *next_item = rmap_item->next; if (rmap_item->address & NODE_FLAG) { if (next_item) { rb_replace_node(&rmap_item->node, &next_item->node, &root_stable_tree); next_item->address |= NODE_FLAG; ksm_pages_sharing--; } else { rb_erase(&rmap_item->node, &root_stable_tree); ksm_pages_shared--; } } else { struct rmap_item *prev_item = rmap_item->prev; BUG_ON(prev_item->next != rmap_item); prev_item->next = next_item; if (next_item) { BUG_ON(next_item->prev != rmap_item); next_item->prev = rmap_item->prev; } ksm_pages_sharing--; } rmap_item->next = NULL; } else if (rmap_item->address & NODE_FLAG) { unsigned char age; /* * Usually ksmd can and must skip the rb_erase, because * root_unstable_tree was already reset to RB_ROOT. * But be careful when an mm is exiting: do the rb_erase * if this rmap_item was inserted by this scan, rather * than left over from before. */ age = (unsigned char)(ksm_scan.seqnr - rmap_item->address); BUG_ON(age > 1); if (!age) rb_erase(&rmap_item->node, &root_unstable_tree); ksm_pages_unshared--; } rmap_item->address &= PAGE_MASK; cond_resched(); /* we're called from many long loops */ } static void remove_trailing_rmap_items(struct mm_slot *mm_slot, struct list_head *cur) { struct rmap_item *rmap_item; while (cur != &mm_slot->rmap_list) { rmap_item = list_entry(cur, struct rmap_item, link); cur = cur->next; remove_rmap_item_from_tree(rmap_item); list_del(&rmap_item->link); free_rmap_item(rmap_item); } } /* * Though it's very tempting to unmerge in_stable_tree(rmap_item)s rather * than check every pte of a given vma, the locking doesn't quite work for * that - an rmap_item is assigned to the stable tree after inserting ksm * page and upping mmap_sem. Nor does it fit with the way we skip dup'ing * rmap_items from parent to child at fork time (so as not to waste time * if exit comes before the next scan reaches it). * * Similarly, although we'd like to remove rmap_items (so updating counts * and freeing memory) when unmerging an area, it's easier to leave that * to the next pass of ksmd - consider, for example, how ksmd might be * in cmp_and_merge_page on one of the rmap_items we would be removing. */ static int unmerge_ksm_pages(struct vm_area_struct *vma, unsigned long start, unsigned long end) { unsigned long addr; int err = 0; for (addr = start; addr < end && !err; addr += PAGE_SIZE) { if (ksm_test_exit(vma->vm_mm)) break; if (signal_pending(current)) err = -ERESTARTSYS; else err = break_ksm(vma, addr); } return err; } #ifdef CONFIG_SYSFS /* * Only called through the sysfs control interface: */ static int unmerge_and_remove_all_rmap_items(void) { struct mm_slot *mm_slot; struct mm_struct *mm; struct vm_area_struct *vma; int err = 0; spin_lock(&ksm_mmlist_lock); ksm_scan.mm_slot = list_entry(ksm_mm_head.mm_list.next, struct mm_slot, mm_list); spin_unlock(&ksm_mmlist_lock); for (mm_slot = ksm_scan.mm_slot; mm_slot != &ksm_mm_head; mm_slot = ksm_scan.mm_slot) { mm = mm_slot->mm; down_read(&mm->mmap_sem); for (vma = mm->mmap; vma; vma = vma->vm_next) { if (ksm_test_exit(mm)) break; if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma) continue; err = unmerge_ksm_pages(vma, vma->vm_start, vma->vm_end); if (err) goto error; } remove_trailing_rmap_items(mm_slot, mm_slot->rmap_list.next); spin_lock(&ksm_mmlist_lock); ksm_scan.mm_slot = list_entry(mm_slot->mm_list.next, struct mm_slot, mm_list); if (ksm_test_exit(mm)) { hlist_del(&mm_slot->link); list_del(&mm_slot->mm_list); spin_unlock(&ksm_mmlist_lock); free_mm_slot(mm_slot); clear_bit(MMF_VM_MERGEABLE, &mm->flags); up_read(&mm->mmap_sem); mmdrop(mm); } else { spin_unlock(&ksm_mmlist_lock); up_read(&mm->mmap_sem); } } ksm_scan.seqnr = 0; return 0; error: up_read(&mm->mmap_sem); spin_lock(&ksm_mmlist_lock); ksm_scan.mm_slot = &ksm_mm_head; spin_unlock(&ksm_mmlist_lock); return err; } #endif /* CONFIG_SYSFS */ static u32 calc_checksum(struct page *page) { u32 checksum; void *addr = kmap_atomic(page, KM_USER0); checksum = jhash2(addr, PAGE_SIZE / 4, 17); kunmap_atomic(addr, KM_USER0); return checksum; } static int memcmp_pages(struct page *page1, struct page *page2) { char *addr1, *addr2; int ret; addr1 = kmap_atomic(page1, KM_USER0); addr2 = kmap_atomic(page2, KM_USER1); ret = memcmp(addr1, addr2, PAGE_SIZE); kunmap_atomic(addr2, KM_USER1); kunmap_atomic(addr1, KM_USER0); return ret; } static inline int pages_identical(struct page *page1, struct page *page2) { return !memcmp_pages(page1, page2); } static int write_protect_page(struct vm_area_struct *vma, struct page *page, pte_t *orig_pte) { struct mm_struct *mm = vma->vm_mm; unsigned long addr; pte_t *ptep; spinlock_t *ptl; int swapped; int err = -EFAULT; addr = page_address_in_vma(page, vma); if (addr == -EFAULT) goto out; ptep = page_check_address(page, mm, addr, &ptl, 0); if (!ptep) goto out; if (pte_write(*ptep)) { pte_t entry; swapped = PageSwapCache(page); flush_cache_page(vma, addr, page_to_pfn(page)); /* * Ok this is tricky, when get_user_pages_fast() run it doesnt * take any lock, therefore the check that we are going to make * with the pagecount against the mapcount is racey and * O_DIRECT can happen right after the check. * So we clear the pte and flush the tlb before the check * this assure us that no O_DIRECT can happen after the check * or in the middle of the check. */ entry = ptep_clear_flush(vma, addr, ptep); /* * Check that no O_DIRECT or similar I/O is in progress on the * page */ if ((page_mapcount(page) + 2 + swapped) != page_count(page)) { set_pte_at_notify(mm, addr, ptep, entry); goto out_unlock; } entry = pte_wrprotect(entry); set_pte_at_notify(mm, addr, ptep, entry); } *orig_pte = *ptep; err = 0; out_unlock: pte_unmap_unlock(ptep, ptl); out: return err; } /** * replace_page - replace page in vma by new ksm page * @vma: vma that holds the pte pointing to oldpage * @oldpage: the page we are replacing by newpage * @newpage: the ksm page we replace oldpage by * @orig_pte: the original value of the pte * * Returns 0 on success, -EFAULT on failure. */ static int replace_page(struct vm_area_struct *vma, struct page *oldpage, struct page *newpage, pte_t orig_pte) { struct mm_struct *mm = vma->vm_mm; pgd_t *pgd; pud_t *pud; pmd_t *pmd; pte_t *ptep; spinlock_t *ptl; unsigned long addr; pgprot_t prot; int err = -EFAULT; prot = vm_get_page_prot(vma->vm_flags & ~VM_WRITE); addr = page_address_in_vma(oldpage, vma); if (addr == -EFAULT) goto out; pgd = pgd_offset(mm, addr); if (!pgd_present(*pgd)) goto out; pud = pud_offset(pgd, addr); if (!pud_present(*pud)) goto out; pmd = pmd_offset(pud, addr); if (!pmd_present(*pmd)) goto out; ptep = pte_offset_map_lock(mm, pmd, addr, &ptl); if (!pte_same(*ptep, orig_pte)) { pte_unmap_unlock(ptep, ptl); goto out; } get_page(newpage); page_add_ksm_rmap(newpage); flush_cache_page(vma, addr, pte_pfn(*ptep)); ptep_clear_flush(vma, addr, ptep); set_pte_at_notify(mm, addr, ptep, mk_pte(newpage, prot)); page_remove_rmap(oldpage); put_page(oldpage); pte_unmap_unlock(ptep, ptl); err = 0; out: return err; } /* * try_to_merge_one_page - take two pages and merge them into one * @vma: the vma that hold the pte pointing into oldpage * @oldpage: the page that we want to replace with newpage * @newpage: the page that we want to map instead of oldpage * * Note: * oldpage should be a PageAnon page, while newpage should be a PageKsm page, * or a newly allocated kernel page which page_add_ksm_rmap will make PageKsm. * * This function returns 0 if the pages were merged, -EFAULT otherwise. */ static int try_to_merge_one_page(struct vm_area_struct *vma, struct page *oldpage, struct page *newpage) { pte_t orig_pte = __pte(0); int err = -EFAULT; if (!(vma->vm_flags & VM_MERGEABLE)) goto out; if (!PageAnon(oldpage)) goto out; get_page(newpage); get_page(oldpage); /* * We need the page lock to read a stable PageSwapCache in * write_protect_page(). We use trylock_page() instead of * lock_page() because we don't want to wait here - we * prefer to continue scanning and merging different pages, * then come back to this page when it is unlocked. */ if (!trylock_page(oldpage)) goto out_putpage; /* * If this anonymous page is mapped only here, its pte may need * to be write-protected. If it's mapped elsewhere, all of its * ptes are necessarily already write-protected. But in either * case, we need to lock and check page_count is not raised. */ if (write_protect_page(vma, oldpage, &orig_pte)) { unlock_page(oldpage); goto out_putpage; } unlock_page(oldpage); if (pages_identical(oldpage, newpage)) err = replace_page(vma, oldpage, newpage, orig_pte); out_putpage: put_page(oldpage); put_page(newpage); out: return err; } /* * try_to_merge_with_ksm_page - like try_to_merge_two_pages, * but no new kernel page is allocated: kpage must already be a ksm page. */ static int try_to_merge_with_ksm_page(struct mm_struct *mm1, unsigned long addr1, struct page *page1, struct page *kpage) { struct vm_area_struct *vma; int err = -EFAULT; down_read(&mm1->mmap_sem); if (ksm_test_exit(mm1)) goto out; vma = find_vma(mm1, addr1); if (!vma || vma->vm_start > addr1) goto out; err = try_to_merge_one_page(vma, page1, kpage); out: up_read(&mm1->mmap_sem); return err; } /* * try_to_merge_two_pages - take two identical pages and prepare them * to be merged into one page. * * This function returns 0 if we successfully mapped two identical pages * into one page, -EFAULT otherwise. * * Note that this function allocates a new kernel page: if one of the pages * is already a ksm page, try_to_merge_with_ksm_page should be used. */ static int try_to_merge_two_pages(struct mm_struct *mm1, unsigned long addr1, struct page *page1, struct mm_struct *mm2, unsigned long addr2, struct page *page2) { struct vm_area_struct *vma; struct page *kpage; int err = -EFAULT; /* * The number of nodes in the stable tree * is the number of kernel pages that we hold. */ if (ksm_max_kernel_pages && ksm_max_kernel_pages <= ksm_pages_shared) return err; kpage = alloc_page(GFP_HIGHUSER); if (!kpage) return err; down_read(&mm1->mmap_sem); if (ksm_test_exit(mm1)) { up_read(&mm1->mmap_sem); goto out; } vma = find_vma(mm1, addr1); if (!vma || vma->vm_start > addr1) { up_read(&mm1->mmap_sem); goto out; } copy_user_highpage(kpage, page1, addr1, vma); err = try_to_merge_one_page(vma, page1, kpage); up_read(&mm1->mmap_sem); if (!err) { err = try_to_merge_with_ksm_page(mm2, addr2, page2, kpage); /* * If that fails, we have a ksm page with only one pte * pointing to it: so break it. */ if (err) break_cow(mm1, addr1); } out: put_page(kpage); return err; } /* * stable_tree_search - search page inside the stable tree * @page: the page that we are searching identical pages to. * @page2: pointer into identical page that we are holding inside the stable * tree that we have found. * @rmap_item: the reverse mapping item * * This function checks if there is a page inside the stable tree * with identical content to the page that we are scanning right now. * * This function return rmap_item pointer to the identical item if found, * NULL otherwise. */ static struct rmap_item *stable_tree_search(struct page *page, struct page **page2, struct rmap_item *rmap_item) { struct rb_node *node = root_stable_tree.rb_node; while (node) { struct rmap_item *tree_rmap_item, *next_rmap_item; int ret; tree_rmap_item = rb_entry(node, struct rmap_item, node); while (tree_rmap_item) { BUG_ON(!in_stable_tree(tree_rmap_item)); cond_resched(); page2[0] = get_ksm_page(tree_rmap_item); if (page2[0]) break; next_rmap_item = tree_rmap_item->next; remove_rmap_item_from_tree(tree_rmap_item); tree_rmap_item = next_rmap_item; } if (!tree_rmap_item) return NULL; ret = memcmp_pages(page, page2[0]); if (ret < 0) { put_page(page2[0]); node = node->rb_left; } else if (ret > 0) { put_page(page2[0]); node = node->rb_right; } else { return tree_rmap_item; } } return NULL; } /* * stable_tree_insert - insert rmap_item pointing to new ksm page * into the stable tree. * * @page: the page that we are searching identical page to inside the stable * tree. * @rmap_item: pointer to the reverse mapping item. * * This function returns rmap_item if success, NULL otherwise. */ static struct rmap_item *stable_tree_insert(struct page *page, struct rmap_item *rmap_item) { struct rb_node **new = &root_stable_tree.rb_node; struct rb_node *parent = NULL; while (*new) { struct rmap_item *tree_rmap_item, *next_rmap_item; struct page *tree_page; int ret; tree_rmap_item = rb_entry(*new, struct rmap_item, node); while (tree_rmap_item) { BUG_ON(!in_stable_tree(tree_rmap_item)); cond_resched(); tree_page = get_ksm_page(tree_rmap_item); if (tree_page) break; next_rmap_item = tree_rmap_item->next; remove_rmap_item_from_tree(tree_rmap_item); tree_rmap_item = next_rmap_item; } if (!tree_rmap_item) return NULL; ret = memcmp_pages(page, tree_page); put_page(tree_page); parent = *new; if (ret < 0) new = &parent->rb_left; else if (ret > 0) new = &parent->rb_right; else { /* * It is not a bug that stable_tree_search() didn't * find this node: because at that time our page was * not yet write-protected, so may have changed since. */ return NULL; } } rmap_item->address |= NODE_FLAG | STABLE_FLAG; rmap_item->next = NULL; rb_link_node(&rmap_item->node, parent, new); rb_insert_color(&rmap_item->node, &root_stable_tree); ksm_pages_shared++; return rmap_item; } /* * unstable_tree_search_insert - search and insert items into the unstable tree. * * @page: the page that we are going to search for identical page or to insert * into the unstable tree * @page2: pointer into identical page that was found inside the unstable tree * @rmap_item: the reverse mapping item of page * * This function searches for a page in the unstable tree identical to the * page currently being scanned; and if no identical page is found in the * tree, we insert rmap_item as a new object into the unstable tree. * * This function returns pointer to rmap_item found to be identical * to the currently scanned page, NULL otherwise. * * This function does both searching and inserting, because they share * the same walking algorithm in an rbtree. */ static struct rmap_item *unstable_tree_search_insert(struct page *page, struct page **page2, struct rmap_item *rmap_item) { struct rb_node **new = &root_unstable_tree.rb_node; struct rb_node *parent = NULL; while (*new) { struct rmap_item *tree_rmap_item; int ret; tree_rmap_item = rb_entry(*new, struct rmap_item, node); page2[0] = get_mergeable_page(tree_rmap_item); if (!page2[0]) return NULL; /* * Don't substitute an unswappable ksm page * just for one good swappable forked page. */ if (page == page2[0]) { put_page(page2[0]); return NULL; } ret = memcmp_pages(page, page2[0]); parent = *new; if (ret < 0) { put_page(page2[0]); new = &parent->rb_left; } else if (ret > 0) { put_page(page2[0]); new = &parent->rb_right; } else { return tree_rmap_item; } } rmap_item->address |= NODE_FLAG; rmap_item->address |= (ksm_scan.seqnr & SEQNR_MASK); rb_link_node(&rmap_item->node, parent, new); rb_insert_color(&rmap_item->node, &root_unstable_tree); ksm_pages_unshared++; return NULL; } /* * stable_tree_append - add another rmap_item to the linked list of * rmap_items hanging off a given node of the stable tree, all sharing * the same ksm page. */ static void stable_tree_append(struct rmap_item *rmap_item, struct rmap_item *tree_rmap_item) { rmap_item->next = tree_rmap_item->next; rmap_item->prev = tree_rmap_item; if (tree_rmap_item->next) tree_rmap_item->next->prev = rmap_item; tree_rmap_item->next = rmap_item; rmap_item->address |= STABLE_FLAG; ksm_pages_sharing++; } /* * cmp_and_merge_page - first see if page can be merged into the stable tree; * if not, compare checksum to previous and if it's the same, see if page can * be inserted into the unstable tree, or merged with a page already there and * both transferred to the stable tree. * * @page: the page that we are searching identical page to. * @rmap_item: the reverse mapping into the virtual address of this page */ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item) { struct page *page2[1]; struct rmap_item *tree_rmap_item; unsigned int checksum; int err; if (in_stable_tree(rmap_item)) remove_rmap_item_from_tree(rmap_item); /* We first start with searching the page inside the stable tree */ tree_rmap_item = stable_tree_search(page, page2, rmap_item); if (tree_rmap_item) { if (page == page2[0]) /* forked */ err = 0; else err = try_to_merge_with_ksm_page(rmap_item->mm, rmap_item->address, page, page2[0]); put_page(page2[0]); if (!err) { /* * The page was successfully merged: * add its rmap_item to the stable tree. */ stable_tree_append(rmap_item, tree_rmap_item); } return; } /* * A ksm page might have got here by fork, but its other * references have already been removed from the stable tree. * Or it might be left over from a break_ksm which failed * when the mem_cgroup had reached its limit: try again now. */ if (PageKsm(page)) break_cow(rmap_item->mm, rmap_item->address); /* * In case the hash value of the page was changed from the last time we * have calculated it, this page to be changed frequely, therefore we * don't want to insert it to the unstable tree, and we don't want to * waste our time to search if there is something identical to it there. */ checksum = calc_checksum(page); if (rmap_item->oldchecksum != checksum) { rmap_item->oldchecksum = checksum; return; } tree_rmap_item = unstable_tree_search_insert(page, page2, rmap_item); if (tree_rmap_item) { err = try_to_merge_two_pages(rmap_item->mm, rmap_item->address, page, tree_rmap_item->mm, tree_rmap_item->address, page2[0]); /* * As soon as we merge this page, we want to remove the * rmap_item of the page we have merged with from the unstable * tree, and insert it instead as new node in the stable tree. */ if (!err) { rb_erase(&tree_rmap_item->node, &root_unstable_tree); tree_rmap_item->address &= ~NODE_FLAG; ksm_pages_unshared--; /* * If we fail to insert the page into the stable tree, * we will have 2 virtual addresses that are pointing * to a ksm page left outside the stable tree, * in which case we need to break_cow on both. */ if (stable_tree_insert(page2[0], tree_rmap_item)) stable_tree_append(rmap_item, tree_rmap_item); else { break_cow(tree_rmap_item->mm, tree_rmap_item->address); break_cow(rmap_item->mm, rmap_item->address); } } put_page(page2[0]); } } static struct rmap_item *get_next_rmap_item(struct mm_slot *mm_slot, struct list_head *cur, unsigned long addr) { struct rmap_item *rmap_item; while (cur != &mm_slot->rmap_list) { rmap_item = list_entry(cur, struct rmap_item, link); if ((rmap_item->address & PAGE_MASK) == addr) { if (!in_stable_tree(rmap_item)) remove_rmap_item_from_tree(rmap_item); return rmap_item; } if (rmap_item->address > addr) break; cur = cur->next; remove_rmap_item_from_tree(rmap_item); list_del(&rmap_item->link); free_rmap_item(rmap_item); } rmap_item = alloc_rmap_item(); if (rmap_item) { /* It has already been zeroed */ rmap_item->mm = mm_slot->mm; rmap_item->address = addr; list_add_tail(&rmap_item->link, cur); } return rmap_item; } static struct rmap_item *scan_get_next_rmap_item(struct page **page) { struct mm_struct *mm; struct mm_slot *slot; struct vm_area_struct *vma; struct rmap_item *rmap_item; if (list_empty(&ksm_mm_head.mm_list)) return NULL; slot = ksm_scan.mm_slot; if (slot == &ksm_mm_head) { root_unstable_tree = RB_ROOT; spin_lock(&ksm_mmlist_lock); slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list); ksm_scan.mm_slot = slot; spin_unlock(&ksm_mmlist_lock); next_mm: ksm_scan.address = 0; ksm_scan.rmap_item = list_entry(&slot->rmap_list, struct rmap_item, link); } mm = slot->mm; down_read(&mm->mmap_sem); if (ksm_test_exit(mm)) vma = NULL; else vma = find_vma(mm, ksm_scan.address); for (; vma; vma = vma->vm_next) { if (!(vma->vm_flags & VM_MERGEABLE)) continue; if (ksm_scan.address < vma->vm_start) ksm_scan.address = vma->vm_start; if (!vma->anon_vma) ksm_scan.address = vma->vm_end; while (ksm_scan.address < vma->vm_end) { if (ksm_test_exit(mm)) break; *page = follow_page(vma, ksm_scan.address, FOLL_GET); if (*page && PageAnon(*page)) { flush_anon_page(vma, *page, ksm_scan.address); flush_dcache_page(*page); rmap_item = get_next_rmap_item(slot, ksm_scan.rmap_item->link.next, ksm_scan.address); if (rmap_item) { ksm_scan.rmap_item = rmap_item; ksm_scan.address += PAGE_SIZE; } else put_page(*page); up_read(&mm->mmap_sem); return rmap_item; } if (*page) put_page(*page); ksm_scan.address += PAGE_SIZE; cond_resched(); } } if (ksm_test_exit(mm)) { ksm_scan.address = 0; ksm_scan.rmap_item = list_entry(&slot->rmap_list, struct rmap_item, link); } /* * Nuke all the rmap_items that are above this current rmap: * because there were no VM_MERGEABLE vmas with such addresses. */ remove_trailing_rmap_items(slot, ksm_scan.rmap_item->link.next); spin_lock(&ksm_mmlist_lock); ksm_scan.mm_slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list); if (ksm_scan.address == 0) { /* * We've completed a full scan of all vmas, holding mmap_sem * throughout, and found no VM_MERGEABLE: so do the same as * __ksm_exit does to remove this mm from all our lists now. * This applies either when cleaning up after __ksm_exit * (but beware: we can reach here even before __ksm_exit), * or when all VM_MERGEABLE areas have been unmapped (and * mmap_sem then protects against race with MADV_MERGEABLE). */ hlist_del(&slot->link); list_del(&slot->mm_list); spin_unlock(&ksm_mmlist_lock); free_mm_slot(slot); clear_bit(MMF_VM_MERGEABLE, &mm->flags); up_read(&mm->mmap_sem); mmdrop(mm); } else { spin_unlock(&ksm_mmlist_lock); up_read(&mm->mmap_sem); } /* Repeat until we've completed scanning the whole list */ slot = ksm_scan.mm_slot; if (slot != &ksm_mm_head) goto next_mm; ksm_scan.seqnr++; return NULL; } /** * ksm_do_scan - the ksm scanner main worker function. * @scan_npages - number of pages we want to scan before we return. */ static void ksm_do_scan(unsigned int scan_npages) { struct rmap_item *rmap_item; struct page *page; while (scan_npages--) { cond_resched(); rmap_item = scan_get_next_rmap_item(&page); if (!rmap_item) return; if (!PageKsm(page) || !in_stable_tree(rmap_item)) cmp_and_merge_page(page, rmap_item); else if (page_mapcount(page) == 1) { /* * Replace now-unshared ksm page by ordinary page. */ break_cow(rmap_item->mm, rmap_item->address); remove_rmap_item_from_tree(rmap_item); rmap_item->oldchecksum = calc_checksum(page); } put_page(page); } } static int ksmd_should_run(void) { return (ksm_run & KSM_RUN_MERGE) && !list_empty(&ksm_mm_head.mm_list); } static int ksm_scan_thread(void *nothing) { set_user_nice(current, 5); while (!kthread_should_stop()) { mutex_lock(&ksm_thread_mutex); if (ksmd_should_run()) ksm_do_scan(ksm_thread_pages_to_scan); mutex_unlock(&ksm_thread_mutex); if (ksmd_should_run()) { schedule_timeout_interruptible( msecs_to_jiffies(ksm_thread_sleep_millisecs)); } else { wait_event_interruptible(ksm_thread_wait, ksmd_should_run() || kthread_should_stop()); } } return 0; } int ksm_madvise(struct vm_area_struct *vma, unsigned long start, unsigned long end, int advice, unsigned long *vm_flags) { struct mm_struct *mm = vma->vm_mm; int err; switch (advice) { case MADV_MERGEABLE: /* * Be somewhat over-protective for now! */ if (*vm_flags & (VM_MERGEABLE | VM_SHARED | VM_MAYSHARE | VM_PFNMAP | VM_IO | VM_DONTEXPAND | VM_RESERVED | VM_HUGETLB | VM_INSERTPAGE | VM_MIXEDMAP | VM_SAO)) return 0; /* just ignore the advice */ if (!test_bit(MMF_VM_MERGEABLE, &mm->flags)) { err = __ksm_enter(mm); if (err) return err; } *vm_flags |= VM_MERGEABLE; break; case MADV_UNMERGEABLE: if (!(*vm_flags & VM_MERGEABLE)) return 0; /* just ignore the advice */ if (vma->anon_vma) { err = unmerge_ksm_pages(vma, start, end); if (err) return err; } *vm_flags &= ~VM_MERGEABLE; break; } return 0; } int __ksm_enter(struct mm_struct *mm) { struct mm_slot *mm_slot; int needs_wakeup; mm_slot = alloc_mm_slot(); if (!mm_slot) return -ENOMEM; /* Check ksm_run too? Would need tighter locking */ needs_wakeup = list_empty(&ksm_mm_head.mm_list); spin_lock(&ksm_mmlist_lock); insert_to_mm_slots_hash(mm, mm_slot); /* * Insert just behind the scanning cursor, to let the area settle * down a little; when fork is followed by immediate exec, we don't * want ksmd to waste time setting up and tearing down an rmap_list. */ list_add_tail(&mm_slot->mm_list, &ksm_scan.mm_slot->mm_list); spin_unlock(&ksm_mmlist_lock); set_bit(MMF_VM_MERGEABLE, &mm->flags); atomic_inc(&mm->mm_count); if (needs_wakeup) wake_up_interruptible(&ksm_thread_wait); return 0; } void __ksm_exit(struct mm_struct *mm) { struct mm_slot *mm_slot; int easy_to_free = 0; /* * This process is exiting: if it's straightforward (as is the * case when ksmd was never running), free mm_slot immediately. * But if it's at the cursor or has rmap_items linked to it, use * mmap_sem to synchronize with any break_cows before pagetables * are freed, and leave the mm_slot on the list for ksmd to free. * Beware: ksm may already have noticed it exiting and freed the slot. */ spin_lock(&ksm_mmlist_lock); mm_slot = get_mm_slot(mm); if (mm_slot && ksm_scan.mm_slot != mm_slot) { if (list_empty(&mm_slot->rmap_list)) { hlist_del(&mm_slot->link); list_del(&mm_slot->mm_list); easy_to_free = 1; } else { list_move(&mm_slot->mm_list, &ksm_scan.mm_slot->mm_list); } } spin_unlock(&ksm_mmlist_lock); if (easy_to_free) { free_mm_slot(mm_slot); clear_bit(MMF_VM_MERGEABLE, &mm->flags); mmdrop(mm); } else if (mm_slot) { down_write(&mm->mmap_sem); up_write(&mm->mmap_sem); } } #ifdef CONFIG_SYSFS /* * This all compiles without CONFIG_SYSFS, but is a waste of space. */ #define KSM_ATTR_RO(_name) \ static struct kobj_attribute _name##_attr = __ATTR_RO(_name) #define KSM_ATTR(_name) \ static struct kobj_attribute _name##_attr = \ __ATTR(_name, 0644, _name##_show, _name##_store) static ssize_t sleep_millisecs_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf) { return sprintf(buf, "%u\n", ksm_thread_sleep_millisecs); } static ssize_t sleep_millisecs_store(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t count) { unsigned long msecs; int err; err = strict_strtoul(buf, 10, &msecs); if (err || msecs > UINT_MAX) return -EINVAL; ksm_thread_sleep_millisecs = msecs; return count; } KSM_ATTR(sleep_millisecs); static ssize_t pages_to_scan_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf) { return sprintf(buf, "%u\n", ksm_thread_pages_to_scan); } static ssize_t pages_to_scan_store(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t count) { int err; unsigned long nr_pages; err = strict_strtoul(buf, 10, &nr_pages); if (err || nr_pages > UINT_MAX) return -EINVAL; ksm_thread_pages_to_scan = nr_pages; return count; } KSM_ATTR(pages_to_scan); static ssize_t run_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf) { return sprintf(buf, "%u\n", ksm_run); } static ssize_t run_store(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t count) { int err; unsigned long flags; err = strict_strtoul(buf, 10, &flags); if (err || flags > UINT_MAX) return -EINVAL; if (flags > KSM_RUN_UNMERGE) return -EINVAL; /* * KSM_RUN_MERGE sets ksmd running, and 0 stops it running. * KSM_RUN_UNMERGE stops it running and unmerges all rmap_items, * breaking COW to free the unswappable pages_shared (but leaves * mm_slots on the list for when ksmd may be set running again). */ mutex_lock(&ksm_thread_mutex); if (ksm_run != flags) { ksm_run = flags; if (flags & KSM_RUN_UNMERGE) { current->flags |= PF_OOM_ORIGIN; err = unmerge_and_remove_all_rmap_items(); current->flags &= ~PF_OOM_ORIGIN; if (err) { ksm_run = KSM_RUN_STOP; count = err; } } } mutex_unlock(&ksm_thread_mutex); if (flags & KSM_RUN_MERGE) wake_up_interruptible(&ksm_thread_wait); return count; } KSM_ATTR(run); static ssize_t max_kernel_pages_store(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t count) { int err; unsigned long nr_pages; err = strict_strtoul(buf, 10, &nr_pages); if (err) return -EINVAL; ksm_max_kernel_pages = nr_pages; return count; } static ssize_t max_kernel_pages_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf) { return sprintf(buf, "%lu\n", ksm_max_kernel_pages); } KSM_ATTR(max_kernel_pages); static ssize_t pages_shared_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf) { return sprintf(buf, "%lu\n", ksm_pages_shared); } KSM_ATTR_RO(pages_shared); static ssize_t pages_sharing_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf) { return sprintf(buf, "%lu\n", ksm_pages_sharing); } KSM_ATTR_RO(pages_sharing); static ssize_t pages_unshared_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf) { return sprintf(buf, "%lu\n", ksm_pages_unshared); } KSM_ATTR_RO(pages_unshared); static ssize_t pages_volatile_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf) { long ksm_pages_volatile; ksm_pages_volatile = ksm_rmap_items - ksm_pages_shared - ksm_pages_sharing - ksm_pages_unshared; /* * It was not worth any locking to calculate that statistic, * but it might therefore sometimes be negative: conceal that. */ if (ksm_pages_volatile < 0) ksm_pages_volatile = 0; return sprintf(buf, "%ld\n", ksm_pages_volatile); } KSM_ATTR_RO(pages_volatile); static ssize_t full_scans_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf) { return sprintf(buf, "%lu\n", ksm_scan.seqnr); } KSM_ATTR_RO(full_scans); static struct attribute *ksm_attrs[] = { &sleep_millisecs_attr.attr, &pages_to_scan_attr.attr, &run_attr.attr, &max_kernel_pages_attr.attr, &pages_shared_attr.attr, &pages_sharing_attr.attr, &pages_unshared_attr.attr, &pages_volatile_attr.attr, &full_scans_attr.attr, NULL, }; static struct attribute_group ksm_attr_group = { .attrs = ksm_attrs, .name = "ksm", }; #endif /* CONFIG_SYSFS */ static int __init ksm_init(void) { struct task_struct *ksm_thread; int err; ksm_init_max_kernel_pages(); err = ksm_slab_init(); if (err) goto out; err = mm_slots_hash_init(); if (err) goto out_free1; ksm_thread = kthread_run(ksm_scan_thread, NULL, "ksmd"); if (IS_ERR(ksm_thread)) { printk(KERN_ERR "ksm: creating kthread failed\n"); err = PTR_ERR(ksm_thread); goto out_free2; } #ifdef CONFIG_SYSFS err = sysfs_create_group(mm_kobj, &ksm_attr_group); if (err) { printk(KERN_ERR "ksm: register sysfs failed\n"); kthread_stop(ksm_thread); goto out_free2; } #endif /* CONFIG_SYSFS */ return 0; out_free2: mm_slots_hash_free(); out_free1: ksm_slab_free(); out: return err; } module_init(ksm_init)