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Commit f2e5ff85 authored by Andrea Arcangeli's avatar Andrea Arcangeli Committed by Linus Torvalds
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ksm: don't fail stable tree lookups if walking over stale stable_nodes 



The stable_nodes can become stale at any time if the underlying pages gets
freed.  The stable_node gets collected and removed from the stable rbtree
if that is detected during the rbtree lookups.

Don't fail the lookup if running into stale stable_nodes, just restart the
lookup after collecting the stale stable_nodes.  Otherwise the CPU spent
in the preparation stage is wasted and the lookup must be repeated at the
next loop potentially failing a second time in a second stale stable_node.

If we don't prune aggressively we delay the merging of the unstable node
candidates and at the same time we delay the freeing of the stale
stable_nodes.  Keeping stale stable_nodes around wastes memory and it
can't provide any benefit.

Signed-off-by: default avatarAndrea Arcangeli <aarcange@redhat.com>
Acked-by: default avatarHugh Dickins <hughd@google.com>
Cc: Petr Holasek <pholasek@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent ad12695f

mm/ksm.c

+27 −5
Original line number Diff line number Diff line
@@ -1177,8 +1177,18 @@ 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);

		if (!tree_page)

			return NULL;

		if (!tree_page) {

			/*

			 * If we walked over a stale stable_node,

			 * get_ksm_page() will call rb_erase() and it

			 * may rebalance the tree from under us. So

			 * restart the search from scratch. Returning

			 * NULL would be safe too, but we'd generate

			 * false negative insertions just because some

			 * stable_node was stale.

			 */

			goto again;

		}



		ret = memcmp_pages(page, tree_page);

		put_page(tree_page);
@@ -1254,12 +1264,14 @@ static struct stable_node *stable_tree_insert(struct page *kpage) 
	unsigned long kpfn;

	struct rb_root *root;

	struct rb_node **new;

	struct rb_node *parent = NULL;

	struct rb_node *parent;

	struct stable_node *stable_node;



	kpfn = page_to_pfn(kpage);

	nid = get_kpfn_nid(kpfn);

	root = root_stable_tree + nid;

again:

	parent = NULL;

	new = &root->rb_node;



	while (*new) {
@@ -1269,8 +1281,18 @@ 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);

		if (!tree_page)

			return NULL;

		if (!tree_page) {

			/*

			 * If we walked over a stale stable_node,

			 * get_ksm_page() will call rb_erase() and it

			 * may rebalance the tree from under us. So

			 * restart the search from scratch. Returning

			 * NULL would be safe too, but we'd generate

			 * false negative insertions just because some

			 * stable_node was stale.

			 */

			goto again;

		}



		ret = memcmp_pages(kpage, tree_page);

		put_page(tree_page);