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: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
mm/ksm.c | 32 +++++++++++++++++++++++++++-----
1 file changed, 27 insertions(+), 5 deletions(-)
diff --git a/mm/ksm.c b/mm/ksm.c
index e87dec7..9f182f9 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1177,8 +1177,18 @@ again:
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);
--
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