The patch titled
Subject: ksm: don't fail stable tree lookups if walking over stale stable_nodes
has been added to the -mm tree. Its filename is
ksm-dont-fail-stable-tree-lookups-if-walking-over-stale-stable_nodes.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/ksm-dont-fail-stable-tree-lookups-if-walking-over-stale-stable_nodes.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/ksm-dont-fail-stable-tree-lookups-if-walking-over-stale-stable_nodes.patch
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------------------------------------------------------
From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Subject: 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 tree lookups.
Don't fail the lookup if running into stale stable_nodes, just restart the
lookup after collecting the stale entries. 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 entry.
This also will contribute to pruning the stable tree and releasing the
stable_node memory more efficiently.
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Petr Holasek <pholasek@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/ksm.c | 30 +++++++++++++++++++++++++++---
1 file changed, 27 insertions(+), 3 deletions(-)
diff -puN mm/ksm.c~ksm-dont-fail-stable-tree-lookups-if-walking-over-stale-stable_nodes mm/ksm.c
--- a/mm/ksm.c~ksm-dont-fail-stable-tree-lookups-if-walking-over-stale-stable_nodes
+++ a/mm/ksm.c
@@ -1225,7 +1225,18 @@ again:
stable_node = rb_entry(*new, struct stable_node, node);
tree_page = get_ksm_page(stable_node, false);
if (!tree_page)
- return NULL;
+ /*
+ * 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 which would waste CPU
+ * by doing the preparation work twice at the
+ * next KSM pass.
+ */
+ goto again;
ret = memcmp_pages(page, tree_page);
put_page(tree_page);
@@ -1301,12 +1312,14 @@ static struct stable_node *stable_tree_i
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) {
@@ -1317,7 +1330,18 @@ static struct stable_node *stable_tree_i
stable_node = rb_entry(*new, struct stable_node, node);
tree_page = get_ksm_page(stable_node, false);
if (!tree_page)
- return NULL;
+ /*
+ * 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 which would waste CPU
+ * by doing the preparation work twice at the
+ * next KSM pass.
+ */
+ goto again;
ret = memcmp_pages(kpage, tree_page);
put_page(tree_page);
_
Patches currently in -mm which might be from aarcange@xxxxxxxxxx are
ksm-fix-rmap_item-anon_vma-memory-corruption-and-vma-user-after-free.patch
ksm-add-cond_resched-to-the-rmap_walks.patch
ksm-dont-fail-stable-tree-lookups-if-walking-over-stale-stable_nodes.patch
ksm-use-the-helper-method-to-do-the-hlist_empty-check.patch
ksm-use-find_mergeable_vma-in-try_to_merge_with_ksm_page.patch
ksm-unstable_tree_search_insert-error-checking-cleanup.patch
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