ksm: introduce ksm_max_page_sharing per page deduplication limit

Without a max deduplication limit for each KSM page, the list of the
rmap_items associated to each stable_node can grow infinitely large.

During the rmap walk each entry can take up to ~10usec to process
because of IPIs for the TLB flushing (both for the primary MMU and the
secondary MMUs with the MMU notifier).  With only 16GB of address space
shared in the same KSM page, that would amount to dozens of seconds of
kernel runtime.

A ~256 max deduplication factor will reduce the latencies of the rmap
walks on KSM pages to order of a few msec.  Just doing the
cond_resched() during the rmap walks is not enough, the list size must
have a limit too, otherwise the caller could get blocked in (schedule
friendly) kernel computations for seconds, unexpectedly.

There's room for optimization to significantly reduce the IPI delivery
cost during the page_referenced(), but at least for page_migration in
the KSM case (used by hard NUMA bindings, compaction and NUMA balancing)
it may be inevitable to send lots of IPIs if each rmap_item->mm is
active on a different CPU and there are lots of CPUs.  Even if we ignore
the IPI delivery cost, we've still to walk the whole KSM rmap list, so
we can't allow millions or billions (ulimited) number of entries in the
KSM stable_node rmap_item lists.

The limit is enforced efficiently by adding a second dimension to the
stable rbtree.  So there are three types of stable_nodes: the regular
ones (identical as before, living in the first flat dimension of the
stable rbtree), the "chains" and the "dups".

Every "chain" and all "dups" linked into a "chain" enforce the invariant
that they represent the same write protected memory content, even if
each "dup" will be pointed by a different KSM page copy of that content.
This way the stable rbtree lookup computational complexity is unaffected
if compared to an unlimited max_sharing_limit.  It is still enforced
that there cannot be KSM page content duplicates in the stable rbtree
itself.

Adding the second dimension to the stable rbtree only after the
max_page_sharing limit hits, provides for a zero memory footprint
increase on 64bit archs.  The memory overhead of the per-KSM page
stable_tree and per virtual mapping rmap_item is unchanged.  Only after
the max_page_sharing limit hits, we need to allocate a stable_tree
"chain" and rb_replace() the "regular" stable_node with the newly
allocated stable_node "chain".  After that we simply add the "regular"
stable_node to the chain as a stable_node "dup" by linking hlist_dup in
the stable_node_chain->hlist.  This way the "regular" (flat) stable_node
is converted to a stable_node "dup" living in the second dimension of
the stable rbtree.

During stable rbtree lookups the stable_node "chain" is identified as
stable_node->rmap_hlist_len == STABLE_NODE_CHAIN (aka
is_stable_node_chain()).

When dropping stable_nodes, the stable_node "dup" is identified as
stable_node->head == STABLE_NODE_DUP_HEAD (aka is_stable_node_dup()).

The STABLE_NODE_DUP_HEAD must be an unique valid pointer never used
elsewhere in any stable_node->head/node to avoid a clashes with the
stable_node->node.rb_parent_color pointer, and different from
&migrate_nodes.  So the second field of &migrate_nodes is picked and
verified as always safe with a BUILD_BUG_ON in case the list_head
implementation changes in the future.

The STABLE_NODE_DUP is picked as a random negative value in
stable_node->rmap_hlist_len.  rmap_hlist_len cannot become negative when
it's a "regular" stable_node or a stable_node "dup".

The stable_node_chain->nid is irrelevant.  The stable_node_chain->kpfn
is aliased in a union with a time field used to rate limit the
stable_node_chain->hlist prunes.

The garbage collection of the stable_node_chain happens lazily during
stable rbtree lookups (as for all other kind of stable_nodes), or while
disabling KSM with "echo 2 >/sys/kernel/mm/ksm/run" while collecting the
entire stable rbtree.

While the "regular" stable_nodes and the stable_node "dups" must wait
for their underlying tree_page to be freed before they can be freed
themselves, the stable_node "chains" can be freed immediately if the
stable_node->hlist turns empty.  This is because the "chains" are never
pointed by any page->mapping and they're effectively stable rbtree KSM
self contained metadata.

[akpm@linux-foundation.org: fix non-NUMA build]
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Tested-by: Petr Holasek <pholasek@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Evgheni Dereveanchin <ederevea@redhat.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Gavin Guo <gavin.guo@canonical.com>
Cc: Jay Vosburgh <jay.vosburgh@canonical.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/vm/ksm.txt

@@ -98,6 +98,50 @@ use_zero_pages   - specifies whether empty pages (i.e. allocated pages
                    it is only effective for pages merged after the change.
                    Default: 0 (normal KSM behaviour as in earlier releases)
 
+max_page_sharing - Maximum sharing allowed for each KSM page. This
+                   enforces a deduplication limit to avoid the virtual
+                   memory rmap lists to grow too large. The minimum
+                   value is 2 as a newly created KSM page will have at
+                   least two sharers. The rmap walk has O(N)
+                   complexity where N is the number of rmap_items
+                   (i.e. virtual mappings) that are sharing the page,
+                   which is in turn capped by max_page_sharing. So
+                   this effectively spread the the linear O(N)
+                   computational complexity from rmap walk context
+                   over different KSM pages. The ksmd walk over the
+                   stable_node "chains" is also O(N), but N is the
+                   number of stable_node "dups", not the number of
+                   rmap_items, so it has not a significant impact on
+                   ksmd performance. In practice the best stable_node
+                   "dup" candidate will be kept and found at the head
+                   of the "dups" list. The higher this value the
+                   faster KSM will merge the memory (because there
+                   will be fewer stable_node dups queued into the
+                   stable_node chain->hlist to check for pruning) and
+                   the higher the deduplication factor will be, but
+                   the slowest the worst case rmap walk could be for
+                   any given KSM page. Slowing down the rmap_walk
+                   means there will be higher latency for certain
+                   virtual memory operations happening during
+                   swapping, compaction, NUMA balancing and page
+                   migration, in turn decreasing responsiveness for
+                   the caller of those virtual memory operations. The
+                   scheduler latency of other tasks not involved with
+                   the VM operations doing the rmap walk is not
+                   affected by this parameter as the rmap walks are
+                   always schedule friendly themselves.
+
+stable_node_chains_prune_millisecs - How frequently to walk the whole
+                   list of stable_node "dups" linked in the
+                   stable_node "chains" in order to prune stale
+                   stable_nodes. Smaller milllisecs values will free
+                   up the KSM metadata with lower latency, but they
+                   will make ksmd use more CPU during the scan. This
+                   only applies to the stable_node chains so it's a
+                   noop if not a single KSM page hit the
+                   max_page_sharing yet (there would be no stable_node
+                   chains in such case).
+
 The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/:
 
 pages_shared     - how many shared pages are being used
@@ -106,10 +150,29 @@ pages_unshared   - how many pages unique but repeatedly checked for merging
 pages_volatile   - how many pages changing too fast to be placed in a tree
 full_scans       - how many times all mergeable areas have been scanned
 
+stable_node_chains - number of stable node chains allocated, this is
+		     effectively the number of KSM pages that hit the
+		     max_page_sharing limit
+stable_node_dups   - number of stable node dups queued into the
+		     stable_node chains
+
 A high ratio of pages_sharing to pages_shared indicates good sharing, but
 a high ratio of pages_unshared to pages_sharing indicates wasted effort.
 pages_volatile embraces several different kinds of activity, but a high
 proportion there would also indicate poor use of madvise MADV_MERGEABLE.
 
+The maximum possible page_sharing/page_shared ratio is limited by the
+max_page_sharing tunable. To increase the ratio max_page_sharing must
+be increased accordingly.
+
+The stable_node_dups/stable_node_chains ratio is also affected by the
+max_page_sharing tunable, and an high ratio may indicate fragmentation
+in the stable_node dups, which could be solved by introducing
+fragmentation algorithms in ksmd which would refile rmap_items from
+one stable_node dup to another stable_node dup, in order to freeup
+stable_node "dups" with few rmap_items in them, but that may increase
+the ksmd CPU usage and possibly slowdown the readonly computations on
+the KSM pages of the applications.
+
 Izik Eidus,
 Hugh Dickins, 17 Nov 2009

mm/ksm.c

@@ -128,9 +128,12 @@ 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
+ * @hlist_dup: linked into the stable_node->hlist with a stable_node chain
  * @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 (perhaps temporarily on wrong nid)
+ * @chain_prune_time: time of the last full garbage collection
+ * @rmap_hlist_len: number of rmap_item entries in hlist or STABLE_NODE_CHAIN
  * @nid: NUMA node id of stable tree in which linked (may not match kpfn)
  */
 struct stable_node {
@@ -138,11 +141,24 @@ struct stable_node {
 		struct rb_node node;	/* when node of stable tree */
 		struct {		/* when listed for migration */
 			struct list_head *head;
+			struct {
+				struct hlist_node hlist_dup;
 				struct list_head list;
 			};
 		};
+	};
 	struct hlist_head hlist;
+	union {
 		unsigned long kpfn;
+		unsigned long chain_prune_time;
+	};
+	/*
+	 * STABLE_NODE_CHAIN can be any negative number in
+	 * rmap_hlist_len negative range, but better not -1 to be able
+	 * to reliably detect underflows.
+	 */
+#define STABLE_NODE_CHAIN -1024
+	int rmap_hlist_len;
 #ifdef CONFIG_NUMA
 	int nid;
 #endif
@@ -192,6 +208,7 @@ static struct rb_root *root_unstable_tree = one_unstable_tree;
 
 /* Recently migrated nodes of stable tree, pending proper placement */
 static LIST_HEAD(migrate_nodes);
+#define STABLE_NODE_DUP_HEAD ((struct list_head *)&migrate_nodes.prev)
 
 #define MM_SLOTS_HASH_BITS 10
 static DEFINE_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);
@@ -219,6 +236,18 @@ static unsigned long ksm_pages_unshared;
 /* The number of rmap_items in use: to calculate pages_volatile */
 static unsigned long ksm_rmap_items;
 
+/* The number of stable_node chains */
+static unsigned long ksm_stable_node_chains;
+
+/* The number of stable_node dups linked to the stable_node chains */
+static unsigned long ksm_stable_node_dups;
+
+/* Delay in pruning stale stable_node_dups in the stable_node_chains */
+static int ksm_stable_node_chains_prune_millisecs = 2000;
+
+/* Maximum number of page slots sharing a stable node */
+static int ksm_max_page_sharing = 256;
+
 /* Number of pages ksmd should scan in one batch */
 static unsigned int ksm_thread_pages_to_scan = 100;
 
@@ -287,6 +316,44 @@ static void __init ksm_slab_free(void)
 	mm_slot_cache = NULL;
 }
 
+static __always_inline bool is_stable_node_chain(struct stable_node *chain)
+{
+	return chain->rmap_hlist_len == STABLE_NODE_CHAIN;
+}
+
+static __always_inline bool is_stable_node_dup(struct stable_node *dup)
+{
+	return dup->head == STABLE_NODE_DUP_HEAD;
+}
+
+static inline void stable_node_chain_add_dup(struct stable_node *dup,
+					     struct stable_node *chain)
+{
+	VM_BUG_ON(is_stable_node_dup(dup));
+	dup->head = STABLE_NODE_DUP_HEAD;
+	VM_BUG_ON(!is_stable_node_chain(chain));
+	hlist_add_head(&dup->hlist_dup, &chain->hlist);
+	ksm_stable_node_dups++;
+}
+
+static inline void __stable_node_dup_del(struct stable_node *dup)
+{
+	hlist_del(&dup->hlist_dup);
+	ksm_stable_node_dups--;
+}
+
+static inline void stable_node_dup_del(struct stable_node *dup)
+{
+	VM_BUG_ON(is_stable_node_chain(dup));
+	if (is_stable_node_dup(dup))
+		__stable_node_dup_del(dup);
+	else
+		rb_erase(&dup->node, root_stable_tree + NUMA(dup->nid));
+#ifdef CONFIG_DEBUG_VM
+	dup->head = NULL;
+#endif
+}
+
 static inline struct rmap_item *alloc_rmap_item(void)
 {
 	struct rmap_item *rmap_item;
@@ -317,6 +384,8 @@ static inline struct stable_node *alloc_stable_node(void)
 
 static inline void free_stable_node(struct stable_node *stable_node)
 {
+	VM_BUG_ON(stable_node->rmap_hlist_len &&
+		  !is_stable_node_chain(stable_node));
 	kmem_cache_free(stable_node_cache, stable_node);
 }
 
@@ -498,25 +567,82 @@ static inline int get_kpfn_nid(unsigned long kpfn)
 	return ksm_merge_across_nodes ? 0 : NUMA(pfn_to_nid(kpfn));
 }
 
+static struct stable_node *alloc_stable_node_chain(struct stable_node *dup,
+						   struct rb_root *root)
+{
+	struct stable_node *chain = alloc_stable_node();
+	VM_BUG_ON(is_stable_node_chain(dup));
+	if (likely(chain)) {
+		INIT_HLIST_HEAD(&chain->hlist);
+		chain->chain_prune_time = jiffies;
+		chain->rmap_hlist_len = STABLE_NODE_CHAIN;
+#if defined (CONFIG_DEBUG_VM) && defined(CONFIG_NUMA)
+		chain->nid = -1; /* debug */
+#endif
+		ksm_stable_node_chains++;
+
+		/*
+		 * Put the stable node chain in the first dimension of
+		 * the stable tree and at the same time remove the old
+		 * stable node.
+		 */
+		rb_replace_node(&dup->node, &chain->node, root);
+
+		/*
+		 * Move the old stable node to the second dimension
+		 * queued in the hlist_dup. The invariant is that all
+		 * dup stable_nodes in the chain->hlist point to pages
+		 * that are wrprotected and have the exact same
+		 * content.
+		 */
+		stable_node_chain_add_dup(dup, chain);
+	}
+	return chain;
+}
+
+static inline void free_stable_node_chain(struct stable_node *chain,
+					  struct rb_root *root)
+{
+	rb_erase(&chain->node, root);
+	free_stable_node(chain);
+	ksm_stable_node_chains--;
+}
+
 static void remove_node_from_stable_tree(struct stable_node *stable_node)
 {
 	struct rmap_item *rmap_item;
 
+	/* check it's not STABLE_NODE_CHAIN or negative */
+	BUG_ON(stable_node->rmap_hlist_len < 0);
+
 	hlist_for_each_entry(rmap_item, &stable_node->hlist, hlist) {
 		if (rmap_item->hlist.next)
 			ksm_pages_sharing--;
 		else
 			ksm_pages_shared--;
+		VM_BUG_ON(stable_node->rmap_hlist_len <= 0);
+		stable_node->rmap_hlist_len--;
 		put_anon_vma(rmap_item->anon_vma);
 		rmap_item->address &= PAGE_MASK;
 		cond_resched();
 	}
 
+	/*
+	 * We need the second aligned pointer of the migrate_nodes
+	 * list_head to stay clear from the rb_parent_color union
+	 * (aligned and different than any node) and also different
+	 * from &migrate_nodes. This will verify that future list.h changes
+	 * don't break STABLE_NODE_DUP_HEAD.
+	 */
+#if GCC_VERSION >= 40903 /* only recent gcc can handle it */
+	BUILD_BUG_ON(STABLE_NODE_DUP_HEAD <= &migrate_nodes);
+	BUILD_BUG_ON(STABLE_NODE_DUP_HEAD >= &migrate_nodes + 1);
+#endif
+
 	if (stable_node->head == &migrate_nodes)
 		list_del(&stable_node->list);
 	else
-		rb_erase(&stable_node->node,
-			 root_stable_tree + NUMA(stable_node->nid));
+		stable_node_dup_del(stable_node);
 	free_stable_node(stable_node);
 }
 
@@ -635,6 +761,8 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
 			ksm_pages_sharing--;
 		else
 			ksm_pages_shared--;
+		VM_BUG_ON(stable_node->rmap_hlist_len <= 0);
+		stable_node->rmap_hlist_len--;
 
 		put_anon_vma(rmap_item->anon_vma);
 		rmap_item->address &= PAGE_MASK;
@@ -743,6 +871,31 @@ static int remove_stable_node(struct stable_node *stable_node)
 	return err;
 }
 
+static int remove_stable_node_chain(struct stable_node *stable_node,
+				    struct rb_root *root)
+{
+	struct stable_node *dup;
+	struct hlist_node *hlist_safe;
+
+	if (!is_stable_node_chain(stable_node)) {
+		VM_BUG_ON(is_stable_node_dup(stable_node));
+		if (remove_stable_node(stable_node))
+			return true;
+		else
+			return false;
+	}
+
+	hlist_for_each_entry_safe(dup, hlist_safe,
+				  &stable_node->hlist, hlist_dup) {
+		VM_BUG_ON(!is_stable_node_dup(dup));
+		if (remove_stable_node(dup))
+			return true;
+	}
+	BUG_ON(!hlist_empty(&stable_node->hlist));
+	free_stable_node_chain(stable_node, root);
+	return false;
+}
+
 static int remove_all_stable_nodes(void)
 {
 	struct stable_node *stable_node, *next;
@@ -753,7 +906,8 @@ static int remove_all_stable_nodes(void)
 		while (root_stable_tree[nid].rb_node) {
 			stable_node = rb_entry(root_stable_tree[nid].rb_node,
 						struct stable_node, node);
-			if (remove_stable_node(stable_node)) {
+			if (remove_stable_node_chain(stable_node,
+						     root_stable_tree + nid)) {
 				err = -EBUSY;
 				break;	/* proceed to next nid */
 			}
@@ -1138,6 +1292,163 @@ static struct page *try_to_merge_two_pages(struct rmap_item *rmap_item,
 	return err ? NULL : page;
 }
 
+static __always_inline
+bool __is_page_sharing_candidate(struct stable_node *stable_node, int offset)
+{
+	VM_BUG_ON(stable_node->rmap_hlist_len < 0);
+	/*
+	 * Check that at least one mapping still exists, otherwise
+	 * there's no much point to merge and share with this
+	 * stable_node, as the underlying tree_page of the other
+	 * sharer is going to be freed soon.
+	 */
+	return stable_node->rmap_hlist_len &&
+		stable_node->rmap_hlist_len + offset < ksm_max_page_sharing;
+}
+
+static __always_inline
+bool is_page_sharing_candidate(struct stable_node *stable_node)
+{
+	return __is_page_sharing_candidate(stable_node, 0);
+}
+
+static struct stable_node *stable_node_dup(struct stable_node *stable_node,
+					   struct page **tree_page,
+					   struct rb_root *root,
+					   bool prune_stale_stable_nodes)
+{
+	struct stable_node *dup, *found = NULL;
+	struct hlist_node *hlist_safe;
+	struct page *_tree_page;
+	int nr = 0;
+	int found_rmap_hlist_len;
+
+	if (!prune_stale_stable_nodes ||
+	    time_before(jiffies, stable_node->chain_prune_time +
+			msecs_to_jiffies(
+				ksm_stable_node_chains_prune_millisecs)))
+		prune_stale_stable_nodes = false;
+	else
+		stable_node->chain_prune_time = jiffies;
+
+	hlist_for_each_entry_safe(dup, hlist_safe,
+				  &stable_node->hlist, hlist_dup) {
+		cond_resched();
+		/*
+		 * We must walk all stable_node_dup to prune the stale
+		 * stable nodes during lookup.
+		 *
+		 * get_ksm_page can drop the nodes from the
+		 * stable_node->hlist if they point to freed pages
+		 * (that's why we do a _safe walk). The "dup"
+		 * stable_node parameter itself will be freed from
+		 * under us if it returns NULL.
+		 */
+		_tree_page = get_ksm_page(dup, false);
+		if (!_tree_page)
+			continue;
+		nr += 1;
+		if (is_page_sharing_candidate(dup)) {
+			if (!found ||
+			    dup->rmap_hlist_len > found_rmap_hlist_len) {
+				if (found)
+					put_page(*tree_page);
+				found = dup;
+				found_rmap_hlist_len = found->rmap_hlist_len;
+				*tree_page = _tree_page;
+
+				if (!prune_stale_stable_nodes)
+					break;
+				/* skip put_page */
+				continue;
+			}
+		}
+		put_page(_tree_page);
+	}
+
+	/*
+	 * nr is relevant only if prune_stale_stable_nodes is true,
+	 * otherwise we may break the loop at nr == 1 even if there
+	 * are multiple entries.
+	 */
+	if (prune_stale_stable_nodes && found) {
+		if (nr == 1) {
+			/*
+			 * If there's not just one entry it would
+			 * corrupt memory, better BUG_ON. In KSM
+			 * context with no lock held it's not even
+			 * fatal.
+			 */
+			BUG_ON(stable_node->hlist.first->next);
+
+			/*
+			 * There's just one entry and it is below the
+			 * deduplication limit so drop the chain.
+			 */
+			rb_replace_node(&stable_node->node, &found->node,
+					root);
+			free_stable_node(stable_node);
+			ksm_stable_node_chains--;
+			ksm_stable_node_dups--;
+		} else if (__is_page_sharing_candidate(found, 1)) {
+			/*
+			 * Refile our candidate at the head
+			 * after the prune if our candidate
+			 * can accept one more future sharing
+			 * in addition to the one underway.
+			 */
+			hlist_del(&found->hlist_dup);
+			hlist_add_head(&found->hlist_dup,
+				       &stable_node->hlist);
+		}
+	}
+
+	return found;
+}
+
+static struct stable_node *stable_node_dup_any(struct stable_node *stable_node,
+					       struct rb_root *root)
+{
+	if (!is_stable_node_chain(stable_node))
+		return stable_node;
+	if (hlist_empty(&stable_node->hlist)) {
+		free_stable_node_chain(stable_node, root);
+		return NULL;
+	}
+	return hlist_entry(stable_node->hlist.first,
+			   typeof(*stable_node), hlist_dup);
+}
+
+static struct stable_node *__stable_node_chain(struct stable_node *stable_node,
+					       struct page **tree_page,
+					       struct rb_root *root,
+					       bool prune_stale_stable_nodes)
+{
+	if (!is_stable_node_chain(stable_node)) {
+		if (is_page_sharing_candidate(stable_node)) {
+			*tree_page = get_ksm_page(stable_node, false);
+			return stable_node;
+		}
+		return NULL;
+	}
+	return stable_node_dup(stable_node, tree_page, root,
+			       prune_stale_stable_nodes);
+}
+
+static __always_inline struct stable_node *chain_prune(struct stable_node *s_n,
+						       struct page **t_p,
+						       struct rb_root *root)
+{
+	return __stable_node_chain(s_n, t_p, root, true);
+}
+
+static __always_inline struct stable_node *chain(struct stable_node *s_n,
+						 struct page **t_p,
+						 struct rb_root *root)
+{
+	return __stable_node_chain(s_n, t_p, root, false);
+}
+
 /*
  * stable_tree_search - search for page inside the stable tree
  *
@@ -1153,7 +1464,7 @@ static struct page *stable_tree_search(struct page *page)
 	struct rb_root *root;
 	struct rb_node **new;
 	struct rb_node *parent;
-	struct stable_node *stable_node;
+	struct stable_node *stable_node, *stable_node_dup, *stable_node_any;
 	struct stable_node *page_node;
 
 	page_node = page_stable_node(page);
@@ -1175,7 +1486,32 @@ static struct page *stable_tree_search(struct page *page)
 
 		cond_resched();
 		stable_node = rb_entry(*new, struct stable_node, node);
-		tree_page = get_ksm_page(stable_node, false);
+		stable_node_any = NULL;
+		stable_node_dup = chain_prune(stable_node, &tree_page, root);
+		if (!stable_node_dup) {
+			/*
+			 * Either all stable_node dups were full in
+			 * this stable_node chain, or this chain was
+			 * empty and should be rb_erased.
+			 */
+			stable_node_any = stable_node_dup_any(stable_node,
+							      root);
+			if (!stable_node_any) {
+				/* rb_erase just run */
+				goto again;
+			}
+			/*
+			 * Take any of the stable_node dups page of
+			 * this stable_node chain to let the tree walk
+			 * continue. All KSM pages belonging to the
+			 * stable_node dups in a stable_node chain
+			 * have the same content and they're
+			 * wrprotected at all times. Any will work
+			 * fine to continue the walk.
+			 */
+			tree_page = get_ksm_page(stable_node_any, false);
+		}
+		VM_BUG_ON(!stable_node_dup ^ !!stable_node_any);
 		if (!tree_page) {
 			/*
 			 * If we walked over a stale stable_node,
@@ -1198,6 +1534,34 @@ static struct page *stable_tree_search(struct page *page)
 		else if (ret > 0)
 			new = &parent->rb_right;
 		else {
+			if (page_node) {
+				VM_BUG_ON(page_node->head != &migrate_nodes);
+				/*
+				 * Test if the migrated page should be merged
+				 * into a stable node dup. If the mapcount is
+				 * 1 we can migrate it with another KSM page
+				 * without adding it to the chain.
+				 */
+				if (page_mapcount(page) > 1)
+					goto chain_append;
+			}
+
+			if (!stable_node_dup) {
+				/*
+				 * If the stable_node is a chain and
+				 * we got a payload match in memcmp
+				 * but we cannot merge the scanned
+				 * page in any of the existing
+				 * stable_node dups because they're
+				 * all full, we need to wait the
+				 * scanned page to find itself a match
+				 * in the unstable tree to create a
+				 * brand new KSM page to add later to
+				 * the dups of this stable_node.
+				 */
+				return NULL;
+			}
+
 			/*
 			 * Lock and unlock the stable_node's page (which
 			 * might already have been migrated) so that page
@@ -1205,24 +1569,22 @@ static struct page *stable_tree_search(struct page *page)
 			 * It would be more elegant to return stable_node
 			 * than kpage, but that involves more changes.
 			 */
-			tree_page = get_ksm_page(stable_node, true);
-			if (tree_page) {
+			tree_page = get_ksm_page(stable_node_dup, true);
+			if (unlikely(!tree_page))
+				/*
+				 * The tree may have been rebalanced,
+				 * so re-evaluate parent and new.
+				 */
+				goto again;
 			unlock_page(tree_page);
-				if (get_kpfn_nid(stable_node->kpfn) !=
-						NUMA(stable_node->nid)) {
+
+			if (get_kpfn_nid(stable_node_dup->kpfn) !=
+			    NUMA(stable_node_dup->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;
-		}
 	}
 
 	if (!page_node)
@@ -1232,22 +1594,72 @@ static struct page *stable_tree_search(struct page *page)
 	DO_NUMA(page_node->nid = nid);
 	rb_link_node(&page_node->node, parent, new);
 	rb_insert_color(&page_node->node, root);
+out:
+	if (is_page_sharing_candidate(page_node)) {
 		get_page(page);
 		return page;
+	} else
+		return NULL;
 
 replace:
+	if (stable_node_dup == stable_node) {
+		/* there is no chain */
 		if (page_node) {
+			VM_BUG_ON(page_node->head != &migrate_nodes);
 			list_del(&page_node->list);
 			DO_NUMA(page_node->nid = nid);
-		rb_replace_node(&stable_node->node, &page_node->node, root);
+			rb_replace_node(&stable_node->node, &page_node->node,
+					root);
+			if (is_page_sharing_candidate(page_node))
 				get_page(page);
+			else
+				page = NULL;
 		} else {
 			rb_erase(&stable_node->node, root);
 			page = NULL;
 		}
-	stable_node->head = &migrate_nodes;
-	list_add(&stable_node->list, stable_node->head);
+	} else {
+		VM_BUG_ON(!is_stable_node_chain(stable_node));
+		__stable_node_dup_del(stable_node_dup);
+		if (page_node) {
+			VM_BUG_ON(page_node->head != &migrate_nodes);
+			list_del(&page_node->list);
+			DO_NUMA(page_node->nid = nid);
+			stable_node_chain_add_dup(page_node, stable_node);
+			if (is_page_sharing_candidate(page_node))
+				get_page(page);
+			else
+				page = NULL;
+		} else {
+			page = NULL;
+		}
+	}
+	stable_node_dup->head = &migrate_nodes;
+	list_add(&stable_node_dup->list, stable_node_dup->head);
 	return page;
+
+chain_append:
+	/* stable_node_dup could be null if it reached the limit */
+	if (!stable_node_dup)
+		stable_node_dup = stable_node_any;
+	if (stable_node_dup == stable_node) {
+		/* chain is missing so create it */
+		stable_node = alloc_stable_node_chain(stable_node_dup,
+						      root);
+		if (!stable_node)
+			return NULL;
+	}
+	/*
+	 * Add this stable_node dup that was
+	 * migrated to the stable_node chain
+	 * of the current nid for this page
+	 * content.
+	 */
+	VM_BUG_ON(page_node->head != &migrate_nodes);
+	list_del(&page_node->list);
+	DO_NUMA(page_node->nid = nid);
+	stable_node_chain_add_dup(page_node, stable_node);
+	goto out;
 }
 
 /*
@@ -1264,7 +1676,8 @@ static struct stable_node *stable_tree_insert(struct page *kpage)
 	struct rb_root *root;
 	struct rb_node **new;
 	struct rb_node *parent;
-	struct stable_node *stable_node;
+	struct stable_node *stable_node, *stable_node_dup, *stable_node_any;
+	bool need_chain = false;
 
 	kpfn = page_to_pfn(kpage);
 	nid = get_kpfn_nid(kpfn);
@@ -1279,7 +1692,32 @@ static struct stable_node *stable_tree_insert(struct page *kpage)
 
 		cond_resched();
 		stable_node = rb_entry(*new, struct stable_node, node);
-		tree_page = get_ksm_page(stable_node, false);
+		stable_node_any = NULL;
+		stable_node_dup = chain(stable_node, &tree_page, root);
+		if (!stable_node_dup) {
+			/*
+			 * Either all stable_node dups were full in
+			 * this stable_node chain, or this chain was
+			 * empty and should be rb_erased.
+			 */
+			stable_node_any = stable_node_dup_any(stable_node,
+							      root);
+			if (!stable_node_any) {
+				/* rb_erase just run */
+				goto again;
+			}
+			/*
+			 * Take any of the stable_node dups page of
+			 * this stable_node chain to let the tree walk
+			 * continue. All KSM pages belonging to the
+			 * stable_node dups in a stable_node chain
+			 * have the same content and they're
+			 * wrprotected at all times. Any will work
+			 * fine to continue the walk.
+			 */
+			tree_page = get_ksm_page(stable_node_any, false);
+		}
+		VM_BUG_ON(!stable_node_dup ^ !!stable_node_any);
 		if (!tree_page) {
 			/*
 			 * If we walked over a stale stable_node,
@@ -1302,27 +1740,37 @@ static struct stable_node *stable_tree_insert(struct page *kpage)
 		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;
+			need_chain = true;
+			break;
 		}
 	}
 
-	stable_node = alloc_stable_node();
-	if (!stable_node)
+	stable_node_dup = alloc_stable_node();
+	if (!stable_node_dup)
 		return NULL;
 
-	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);
+	INIT_HLIST_HEAD(&stable_node_dup->hlist);
+	stable_node_dup->kpfn = kpfn;
+	set_page_stable_node(kpage, stable_node_dup);
+	stable_node_dup->rmap_hlist_len = 0;
+	DO_NUMA(stable_node_dup->nid = nid);
+	if (!need_chain) {
+		rb_link_node(&stable_node_dup->node, parent, new);
+		rb_insert_color(&stable_node_dup->node, root);
+	} else {
+		if (!is_stable_node_chain(stable_node)) {
+			struct stable_node *orig = stable_node;
+			/* chain is missing so create it */
+			stable_node = alloc_stable_node_chain(orig, root);
+			if (!stable_node) {
+				free_stable_node(stable_node_dup);
+				return NULL;
+			}
+		}
+		stable_node_chain_add_dup(stable_node_dup, stable_node);
+	}
 
-	return stable_node;
+	return stable_node_dup;
 }
 
 /*
@@ -1412,8 +1860,27 @@ struct rmap_item *unstable_tree_search_insert(struct rmap_item *rmap_item,
  * the same ksm page.
  */
 static void stable_tree_append(struct rmap_item *rmap_item,
-			       struct stable_node *stable_node)
+			       struct stable_node *stable_node,
+			       bool max_page_sharing_bypass)
 {
+	/*
+	 * rmap won't find this mapping if we don't insert the
+	 * rmap_item in the right stable_node
+	 * duplicate. page_migration could break later if rmap breaks,
+	 * so we can as well crash here. We really need to check for
+	 * rmap_hlist_len == STABLE_NODE_CHAIN, but we can as well check
+	 * for other negative values as an undeflow if detected here
+	 * for the first time (and not when decreasing rmap_hlist_len)
+	 * would be sign of memory corruption in the stable_node.
+	 */
+	BUG_ON(stable_node->rmap_hlist_len < 0);
+
+	stable_node->rmap_hlist_len++;
+	if (!max_page_sharing_bypass)
+		/* possibly non fatal but unexpected overflow, only warn */
+		WARN_ON_ONCE(stable_node->rmap_hlist_len >
+			     ksm_max_page_sharing);
+
 	rmap_item->head = stable_node;
 	rmap_item->address |= STABLE_FLAG;
 	hlist_add_head(&rmap_item->hlist, &stable_node->hlist);
@@ -1441,19 +1908,26 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
 	struct page *kpage;
 	unsigned int checksum;
 	int err;
+	bool max_page_sharing_bypass = false;
 
 	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));
+		    get_kpfn_nid(READ_ONCE(stable_node->kpfn)) !=
+		    NUMA(stable_node->nid)) {
+			stable_node_dup_del(stable_node);
 			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;
+		/*
+		 * If it's a KSM fork, allow it to go over the sharing limit
+		 * without warnings.
+		 */
+		if (!is_page_sharing_candidate(stable_node))
+			max_page_sharing_bypass = true;
 	}
 
 	/* We first start with searching the page inside the stable tree */
@@ -1473,7 +1947,8 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
 			 * add its rmap_item to the stable tree.
 			 */
 			lock_page(kpage);
-			stable_tree_append(rmap_item, page_stable_node(kpage));
+			stable_tree_append(rmap_item, page_stable_node(kpage),
+					   max_page_sharing_bypass);
 			unlock_page(kpage);
 		}
 		put_page(kpage);
@@ -1523,8 +1998,10 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
 			lock_page(kpage);
 			stable_node = stable_tree_insert(kpage);
 			if (stable_node) {
-				stable_tree_append(tree_rmap_item, stable_node);
-				stable_tree_append(rmap_item, stable_node);
+				stable_tree_append(tree_rmap_item, stable_node,
+						   false);
+				stable_tree_append(rmap_item, stable_node,
+						   false);
 			}
 			unlock_page(kpage);
 
@@ -2028,6 +2505,48 @@ static void wait_while_offlining(void)
 	}
 }
 
+static bool stable_node_dup_remove_range(struct stable_node *stable_node,
+					 unsigned long start_pfn,
+					 unsigned long end_pfn)
+{
+	if (stable_node->kpfn >= start_pfn &&
+	    stable_node->kpfn < end_pfn) {
+		/*
+		 * Don't get_ksm_page, page has already gone:
+		 * which is why we keep kpfn instead of page*
+		 */
+		remove_node_from_stable_tree(stable_node);
+		return true;
+	}
+	return false;
+}
+
+static bool stable_node_chain_remove_range(struct stable_node *stable_node,
+					   unsigned long start_pfn,
+					   unsigned long end_pfn,
+					   struct rb_root *root)
+{
+	struct stable_node *dup;
+	struct hlist_node *hlist_safe;
+
+	if (!is_stable_node_chain(stable_node)) {
+		VM_BUG_ON(is_stable_node_dup(stable_node));
+		return stable_node_dup_remove_range(stable_node, start_pfn,
+						    end_pfn);
+	}
+
+	hlist_for_each_entry_safe(dup, hlist_safe,
+				  &stable_node->hlist, hlist_dup) {
+		VM_BUG_ON(!is_stable_node_dup(dup));
+		stable_node_dup_remove_range(dup, start_pfn, end_pfn);
+	}
+	if (hlist_empty(&stable_node->hlist)) {
+		free_stable_node_chain(stable_node, root);
+		return true; /* notify caller that tree was rebalanced */
+	} else
+		return false;
+}
+
 static void ksm_check_stable_tree(unsigned long start_pfn,
 				  unsigned long end_pfn)
 {
@@ -2039,15 +2558,12 @@ static void ksm_check_stable_tree(unsigned long start_pfn,
 		node = rb_first(root_stable_tree + nid);
 		while (node) {
 			stable_node = rb_entry(node, struct stable_node, node);
-			if (stable_node->kpfn >= start_pfn &&
-			    stable_node->kpfn < end_pfn) {
-				/*
-				 * Don't get_ksm_page, page has already gone:
-				 * which is why we keep kpfn instead of page*
-				 */
-				remove_node_from_stable_tree(stable_node);
+			if (stable_node_chain_remove_range(stable_node,
+							   start_pfn, end_pfn,
+							   root_stable_tree +
+							   nid))
 				node = rb_first(root_stable_tree + nid);
-			} else
+			else
 				node = rb_next(node);
 			cond_resched();
 		}
@@ -2293,6 +2809,47 @@ static ssize_t use_zero_pages_store(struct kobject *kobj,
 }
 KSM_ATTR(use_zero_pages);
 
+static ssize_t max_page_sharing_show(struct kobject *kobj,
+				     struct kobj_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%u\n", ksm_max_page_sharing);
+}
+
+static ssize_t max_page_sharing_store(struct kobject *kobj,
+				      struct kobj_attribute *attr,
+				      const char *buf, size_t count)
+{
+	int err;
+	int knob;
+
+	err = kstrtoint(buf, 10, &knob);
+	if (err)
+		return err;
+	/*
+	 * When a KSM page is created it is shared by 2 mappings. This
+	 * being a signed comparison, it implicitly verifies it's not
+	 * negative.
+	 */
+	if (knob < 2)
+		return -EINVAL;
+
+	if (READ_ONCE(ksm_max_page_sharing) == knob)
+		return count;
+
+	mutex_lock(&ksm_thread_mutex);
+	wait_while_offlining();
+	if (ksm_max_page_sharing != knob) {
+		if (ksm_pages_shared || remove_all_stable_nodes())
+			err = -EBUSY;
+		else
+			ksm_max_page_sharing = knob;
+	}
+	mutex_unlock(&ksm_thread_mutex);
+
+	return err ? err : count;
+}
+KSM_ATTR(max_page_sharing);
+
 static ssize_t pages_shared_show(struct kobject *kobj,
 				 struct kobj_attribute *attr, char *buf)
 {
@@ -2331,6 +2888,46 @@ static ssize_t pages_volatile_show(struct kobject *kobj,
 }
 KSM_ATTR_RO(pages_volatile);
 
+static ssize_t stable_node_dups_show(struct kobject *kobj,
+				     struct kobj_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%lu\n", ksm_stable_node_dups);
+}
+KSM_ATTR_RO(stable_node_dups);
+
+static ssize_t stable_node_chains_show(struct kobject *kobj,
+				       struct kobj_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%lu\n", ksm_stable_node_chains);
+}
+KSM_ATTR_RO(stable_node_chains);
+
+static ssize_t
+stable_node_chains_prune_millisecs_show(struct kobject *kobj,
+					struct kobj_attribute *attr,
+					char *buf)
+{
+	return sprintf(buf, "%u\n", ksm_stable_node_chains_prune_millisecs);
+}
+
+static ssize_t
+stable_node_chains_prune_millisecs_store(struct kobject *kobj,
+					 struct kobj_attribute *attr,
+					 const char *buf, size_t count)
+{
+	unsigned long msecs;
+	int err;
+
+	err = kstrtoul(buf, 10, &msecs);
+	if (err || msecs > UINT_MAX)
+		return -EINVAL;
+
+	ksm_stable_node_chains_prune_millisecs = msecs;
+
+	return count;
+}
+KSM_ATTR(stable_node_chains_prune_millisecs);
+
 static ssize_t full_scans_show(struct kobject *kobj,
 			       struct kobj_attribute *attr, char *buf)
 {
@@ -2350,6 +2947,10 @@ static struct attribute *ksm_attrs[] = {
 #ifdef CONFIG_NUMA
 	&merge_across_nodes_attr.attr,
 #endif
+	&max_page_sharing_attr.attr,
+	&stable_node_chains_attr.attr,
+	&stable_node_dups_attr.attr,
+	&stable_node_chains_prune_millisecs_attr.attr,
 	&use_zero_pages_attr.attr,
 	NULL,
 };