From 4146d2d673e8d6abf9b30a5b5dd8cd95f29632eb Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Fri, 22 Feb 2013 16:35:11 -0800 Subject: [PATCH] ksm: make !merge_across_nodes migration safe The new KSM NUMA merge_across_nodes knob introduces a problem, when it's set to non-default 0: if a KSM page is migrated to a different NUMA node, how do we migrate its stable node to the right tree? And what if that collides with an existing stable node? ksm_migrate_page() can do no more than it's already doing, updating stable_node->kpfn: the stable tree itself cannot be manipulated without holding ksm_thread_mutex. So accept that a stable tree may temporarily indicate a page belonging to the wrong NUMA node, leave updating until the next pass of ksmd, just be careful not to merge other pages on to a misplaced page. Note nid of holding tree in stable_node, and recognize that it will not always match nid of kpfn. A misplaced KSM page is discovered, either when ksm_do_scan() next comes around to one of its rmap_items (we now have to go to cmp_and_merge_page even on pages in a stable tree), or when stable_tree_search() arrives at a matching node for another page, and this node page is found misplaced. In each case, move the misplaced stable_node to a list of migrate_nodes (and use the address of migrate_nodes as magic by which to identify them): we don't need them in a tree. If stable_tree_search() finds no match for a page, but it's currently exiled to this list, then slot its stable_node right there into the tree, bringing all of its mappings with it; otherwise they get migrated one by one to the original page of the colliding node. stable_tree_search() is now modelled more like stable_tree_insert(), in order to handle these insertions of migrated nodes. remove_node_from_stable_tree(), remove_all_stable_nodes() and ksm_check_stable_tree() have to handle the migrate_nodes list as well as the stable tree itself. Less obviously, we do need to prune the list of stale entries from time to time (scan_get_next_rmap_item() does it once each full scan): whereas stale nodes in the stable tree get naturally pruned as searches try to brush past them, these migrate_nodes may get forgotten and accumulate. Signed-off-by: Hugh Dickins Cc: Rik van Riel Cc: Petr Holasek Cc: Andrea Arcangeli Cc: Izik Eidus Cc: Gerald Schaefer Cc: KOSAKI Motohiro Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/ksm.c | 164 +++++++++++++++++++++++++++++++++++++++++++++---------- 1 file changed, 134 insertions(+), 30 deletions(-) diff --git a/mm/ksm.c b/mm/ksm.c index df0529926703..91f07d0e9ce7 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -122,13 +122,25 @@ struct ksm_scan { /** * struct stable_node - node of the stable rbtree * @node: rb node of this ksm page in the stable tree + * @head: (overlaying parent) &migrate_nodes indicates temporarily on that list + * @list: linked into migrate_nodes, pending placement in the proper node tree * @hlist: hlist head of rmap_items using this ksm page - * @kpfn: page frame number of this ksm page + * @kpfn: page frame number of this ksm page (perhaps temporarily on wrong nid) + * @nid: NUMA node id of stable tree in which linked (may not match kpfn) */ struct stable_node { - struct rb_node node; + union { + struct rb_node node; /* when node of stable tree */ + struct { /* when listed for migration */ + struct list_head *head; + struct list_head list; + }; + }; struct hlist_head hlist; unsigned long kpfn; +#ifdef CONFIG_NUMA + int nid; +#endif }; /** @@ -169,6 +181,9 @@ struct rmap_item { static struct rb_root root_unstable_tree[MAX_NUMNODES]; static struct rb_root root_stable_tree[MAX_NUMNODES]; +/* Recently migrated nodes of stable tree, pending proper placement */ +static LIST_HEAD(migrate_nodes); + #define MM_SLOTS_HASH_BITS 10 static DEFINE_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS); @@ -311,11 +326,6 @@ static void insert_to_mm_slots_hash(struct mm_struct *mm, hash_add(mm_slots_hash, &mm_slot->link, (unsigned long)mm); } -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, @@ -476,7 +486,6 @@ static void remove_node_from_stable_tree(struct stable_node *stable_node) { struct rmap_item *rmap_item; struct hlist_node *hlist; - int nid; hlist_for_each_entry(rmap_item, hlist, &stable_node->hlist, hlist) { if (rmap_item->hlist.next) @@ -488,8 +497,11 @@ static void remove_node_from_stable_tree(struct stable_node *stable_node) cond_resched(); } - nid = get_kpfn_nid(stable_node->kpfn); - rb_erase(&stable_node->node, &root_stable_tree[nid]); + if (stable_node->head == &migrate_nodes) + list_del(&stable_node->list); + else + rb_erase(&stable_node->node, + &root_stable_tree[NUMA(stable_node->nid)]); free_stable_node(stable_node); } @@ -712,6 +724,7 @@ static int remove_stable_node(struct stable_node *stable_node) static int remove_all_stable_nodes(void) { struct stable_node *stable_node; + struct list_head *this, *next; int nid; int err = 0; @@ -726,6 +739,12 @@ static int remove_all_stable_nodes(void) cond_resched(); } } + list_for_each_safe(this, next, &migrate_nodes) { + stable_node = list_entry(this, struct stable_node, list); + if (remove_stable_node(stable_node)) + err = -EBUSY; + cond_resched(); + } return err; } @@ -1113,25 +1132,30 @@ static struct page *try_to_merge_two_pages(struct rmap_item *rmap_item, */ static struct page *stable_tree_search(struct page *page) { - struct rb_node *node; - struct stable_node *stable_node; int nid; + struct rb_node **new; + struct rb_node *parent; + struct stable_node *stable_node; + struct stable_node *page_node; - stable_node = page_stable_node(page); - if (stable_node) { /* ksm page forked */ + page_node = page_stable_node(page); + if (page_node && page_node->head != &migrate_nodes) { + /* ksm page forked */ get_page(page); return page; } nid = get_kpfn_nid(page_to_pfn(page)); - node = root_stable_tree[nid].rb_node; +again: + new = &root_stable_tree[nid].rb_node; + parent = NULL; - while (node) { + while (*new) { struct page *tree_page; int ret; cond_resched(); - stable_node = rb_entry(node, struct stable_node, node); + stable_node = rb_entry(*new, struct stable_node, node); tree_page = get_ksm_page(stable_node, false); if (!tree_page) return NULL; @@ -1139,10 +1163,11 @@ static struct page *stable_tree_search(struct page *page) ret = memcmp_pages(page, tree_page); put_page(tree_page); + parent = *new; if (ret < 0) - node = node->rb_left; + new = &parent->rb_left; else if (ret > 0) - node = node->rb_right; + new = &parent->rb_right; else { /* * Lock and unlock the stable_node's page (which @@ -1152,13 +1177,49 @@ static struct page *stable_tree_search(struct page *page) * than kpage, but that involves more changes. */ tree_page = get_ksm_page(stable_node, true); - if (tree_page) + if (tree_page) { unlock_page(tree_page); - return tree_page; + if (get_kpfn_nid(stable_node->kpfn) != + NUMA(stable_node->nid)) { + put_page(tree_page); + goto replace; + } + return tree_page; + } + /* + * There is now a place for page_node, but the tree may + * have been rebalanced, so re-evaluate parent and new. + */ + if (page_node) + goto again; + return NULL; } } - return NULL; + if (!page_node) + return NULL; + + list_del(&page_node->list); + DO_NUMA(page_node->nid = nid); + rb_link_node(&page_node->node, parent, new); + rb_insert_color(&page_node->node, &root_stable_tree[nid]); + get_page(page); + return page; + +replace: + if (page_node) { + list_del(&page_node->list); + DO_NUMA(page_node->nid = nid); + rb_replace_node(&stable_node->node, + &page_node->node, &root_stable_tree[nid]); + get_page(page); + } else { + rb_erase(&stable_node->node, &root_stable_tree[nid]); + page = NULL; + } + stable_node->head = &migrate_nodes; + list_add(&stable_node->list, stable_node->head); + return page; } /* @@ -1215,6 +1276,7 @@ static struct stable_node *stable_tree_insert(struct page *kpage) INIT_HLIST_HEAD(&stable_node->hlist); stable_node->kpfn = kpfn; set_page_stable_node(kpage, stable_node); + DO_NUMA(stable_node->nid = nid); rb_link_node(&stable_node->node, parent, new); rb_insert_color(&stable_node->node, &root_stable_tree[nid]); @@ -1311,11 +1373,6 @@ struct rmap_item *unstable_tree_search_insert(struct rmap_item *rmap_item, static void stable_tree_append(struct rmap_item *rmap_item, struct stable_node *stable_node) { - /* - * Usually rmap_item->nid is already set correctly, - * but it may be wrong after switching merge_across_nodes. - */ - DO_NUMA(rmap_item->nid = get_kpfn_nid(stable_node->kpfn)); rmap_item->head = stable_node; rmap_item->address |= STABLE_FLAG; hlist_add_head(&rmap_item->hlist, &stable_node->hlist); @@ -1344,10 +1401,29 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item) unsigned int checksum; int err; - remove_rmap_item_from_tree(rmap_item); + stable_node = page_stable_node(page); + if (stable_node) { + if (stable_node->head != &migrate_nodes && + get_kpfn_nid(stable_node->kpfn) != NUMA(stable_node->nid)) { + rb_erase(&stable_node->node, + &root_stable_tree[NUMA(stable_node->nid)]); + stable_node->head = &migrate_nodes; + list_add(&stable_node->list, stable_node->head); + } + if (stable_node->head != &migrate_nodes && + rmap_item->head == stable_node) + return; + } /* We first start with searching the page inside the stable tree */ kpage = stable_tree_search(page); + if (kpage == page && rmap_item->head == stable_node) { + put_page(kpage); + return; + } + + remove_rmap_item_from_tree(rmap_item); + if (kpage) { err = try_to_merge_with_ksm_page(rmap_item, page, kpage); if (!err) { @@ -1464,6 +1540,27 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page) */ lru_add_drain_all(); + /* + * Whereas stale stable_nodes on the stable_tree itself + * get pruned in the regular course of stable_tree_search(), + * those moved out to the migrate_nodes list can accumulate: + * so prune them once before each full scan. + */ + if (!ksm_merge_across_nodes) { + struct stable_node *stable_node; + struct list_head *this, *next; + struct page *page; + + list_for_each_safe(this, next, &migrate_nodes) { + stable_node = list_entry(this, + struct stable_node, list); + page = get_ksm_page(stable_node, false); + if (page) + put_page(page); + cond_resched(); + } + } + for (nid = 0; nid < nr_node_ids; nid++) root_unstable_tree[nid] = RB_ROOT; @@ -1586,8 +1683,7 @@ static void ksm_do_scan(unsigned int scan_npages) 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); + cmp_and_merge_page(page, rmap_item); put_page(page); } } @@ -1964,6 +2060,7 @@ static void ksm_check_stable_tree(unsigned long start_pfn, unsigned long end_pfn) { struct stable_node *stable_node; + struct list_head *this, *next; struct rb_node *node; int nid; @@ -1984,6 +2081,13 @@ static void ksm_check_stable_tree(unsigned long start_pfn, cond_resched(); } } + list_for_each_safe(this, next, &migrate_nodes) { + stable_node = list_entry(this, struct stable_node, list); + if (stable_node->kpfn >= start_pfn && + stable_node->kpfn < end_pfn) + remove_node_from_stable_tree(stable_node); + cond_resched(); + } } static int ksm_memory_callback(struct notifier_block *self, -- 2.20.1