mm: ksm: do not block on page lock when searching stable tree

ksmd needs to search the stable tree to look for the suitable KSM page,
but the KSM page might be locked for a while due to i.e.  KSM page rmap
walk.  Basically it is not a big deal since commit 2c653d0 ("ksm:
introduce ksm_max_page_sharing per page deduplication limit"), since
max_page_sharing limits the number of shared KSM pages.

But it still sounds not worth waiting for the lock, the page can be
skip, then try to merge it in the next scan to avoid potential stall if
its content is still intact.

Introduce trylock mode to get_ksm_page() to not block on page lock, like
what try_to_merge_one_page() does.  And, define three possible
operations (nolock, lock and trylock) as enum type to avoid stacking up
bools and make the code more readable.

Return -EBUSY if trylock fails, since NULL means not find suitable KSM
page, which is a valid case.

With the default max_page_sharing setting (256), there is almost no
observed change comparing lock vs trylock.

However, with ksm02 of LTP, the reduced ksmd full scan time can be
observed, which has set max_page_sharing to 786432.  With lock version,
ksmd may tak 10s - 11s to run two full scans, with trylock version ksmd
may take 8s - 11s to run two full scans.  And, the number of
pages_sharing and pages_to_scan keep same.  Basically, this change has
no harm.

[[email protected]: fix BUG_ON()]
  Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Yang Shi <[email protected]>
Signed-off-by: Hugh Dickins <[email protected]>
Suggested-by: John Hubbard <[email protected]>
Reviewed-by: Kirill Tkhai <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/ksm.c

@@ -667,6 +667,12 @@ static void remove_node_from_stable_tree(struct stable_node *stable_node)
 	free_stable_node(stable_node);
 }
 
+enum get_ksm_page_flags {
+	GET_KSM_PAGE_NOLOCK,
+	GET_KSM_PAGE_LOCK,
+	GET_KSM_PAGE_TRYLOCK
+};
+
 /*
  * get_ksm_page: checks if the page indicated by the stable node
  * is still its ksm page, despite having held no reference to it.
@@ -686,7 +692,8 @@ static void remove_node_from_stable_tree(struct stable_node *stable_node)
  * a page to put something that might look like our key in page->mapping.
  * is on its way to being freed; but it is an anomaly to bear in mind.
  */
-static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it)
+static struct page *get_ksm_page(struct stable_node *stable_node,
+				 enum get_ksm_page_flags flags)
 {
 	struct page *page;
 	void *expected_mapping;
@@ -729,8 +736,15 @@ static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it)
 		goto stale;
 	}
 
-	if (lock_it) {
+	if (flags == GET_KSM_PAGE_TRYLOCK) {
+		if (!trylock_page(page)) {
+			put_page(page);
+			return ERR_PTR(-EBUSY);
+		}
+	} else if (flags == GET_KSM_PAGE_LOCK)
 		lock_page(page);
+
+	if (flags != GET_KSM_PAGE_NOLOCK) {
 		if (READ_ONCE(page->mapping) != expected_mapping) {
 			unlock_page(page);
 			put_page(page);
@@ -764,7 +778,7 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
 		struct page *page;
 
 		stable_node = rmap_item->head;
-		page = get_ksm_page(stable_node, true);
+		page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK);
 		if (!page)
 			goto out;
 
@@ -864,7 +878,7 @@ static int remove_stable_node(struct stable_node *stable_node)
 	struct page *page;
 	int err;
 
-	page = get_ksm_page(stable_node, true);
+	page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK);
 	if (!page) {
 		/*
 		 * get_ksm_page did remove_node_from_stable_tree itself.
@@ -1386,7 +1400,7 @@ static struct page *stable_node_dup(struct stable_node **_stable_node_dup,
 		 * stable_node parameter itself will be freed from
 		 * under us if it returns NULL.
 		 */
-		_tree_page = get_ksm_page(dup, false);
+		_tree_page = get_ksm_page(dup, GET_KSM_PAGE_NOLOCK);
 		if (!_tree_page)
 			continue;
 		nr += 1;
@@ -1509,7 +1523,7 @@ static struct page *__stable_node_chain(struct stable_node **_stable_node_dup,
 	if (!is_stable_node_chain(stable_node)) {
 		if (is_page_sharing_candidate(stable_node)) {
 			*_stable_node_dup = stable_node;
-			return get_ksm_page(stable_node, false);
+			return get_ksm_page(stable_node, GET_KSM_PAGE_NOLOCK);
 		}
 		/*
 		 * _stable_node_dup set to NULL means the stable_node
@@ -1614,7 +1628,8 @@ static struct page *stable_tree_search(struct page *page)
 			 * wrprotected at all times. Any will work
 			 * fine to continue the walk.
 			 */
-			tree_page = get_ksm_page(stable_node_any, false);
+			tree_page = get_ksm_page(stable_node_any,
+						 GET_KSM_PAGE_NOLOCK);
 		}
 		VM_BUG_ON(!stable_node_dup ^ !!stable_node_any);
 		if (!tree_page) {
@@ -1674,7 +1689,12 @@ 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_dup, true);
+			tree_page = get_ksm_page(stable_node_dup,
+						 GET_KSM_PAGE_TRYLOCK);
+
+			if (PTR_ERR(tree_page) == -EBUSY)
+				return ERR_PTR(-EBUSY);
+
 			if (unlikely(!tree_page))
 				/*
 				 * The tree may have been rebalanced,
@@ -1843,7 +1863,8 @@ static struct stable_node *stable_tree_insert(struct page *kpage)
 			 * wrprotected at all times. Any will work
 			 * fine to continue the walk.
 			 */
-			tree_page = get_ksm_page(stable_node_any, false);
+			tree_page = get_ksm_page(stable_node_any,
+						 GET_KSM_PAGE_NOLOCK);
 		}
 		VM_BUG_ON(!stable_node_dup ^ !!stable_node_any);
 		if (!tree_page) {
@@ -2069,6 +2090,9 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
 	remove_rmap_item_from_tree(rmap_item);
 
 	if (kpage) {
+		if (PTR_ERR(kpage) == -EBUSY)
+			return;
+
 		err = try_to_merge_with_ksm_page(rmap_item, page, kpage);
 		if (!err) {
 			/*
@@ -2243,7 +2267,8 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page)
 
 			list_for_each_entry_safe(stable_node, next,
 						 &migrate_nodes, list) {
-				page = get_ksm_page(stable_node, false);
+				page = get_ksm_page(stable_node,
+						    GET_KSM_PAGE_NOLOCK);
 				if (page)
 					put_page(page);
 				cond_resched();