[PATCH 4/4] ksm: fix potential missing rmap_item for stable_node

[Miaohe Lin <linmiaohe@huawei.com>]

When remove rmap_item from stable tree, STABLE_FLAG of rmap_item is cleared
with head reserved. So the following scenario might happen:
For ksm page with rmap_item1:
cmp_and_merge_page
  stable_node->head = &migrate_nodes;
  remove_rmap_item_from_tree, but head still equal to stable_node;
  try_to_merge_with_ksm_page failed;
  return;
For the same ksm page with rmap_item2, stable node migration succeed this
time. The stable_node->head does not equal to migrate_nodes now.
For ksm page with rmap_item1 again:
cmp_and_merge_page
 stable_node->head != &migrate_nodes && rmap_item->head == stable_node
 return;
We would miss the rmap_item for stable_node and might result in failed
rmap_walk_ksm(). Fix this by set rmap_item->head to NULL when rmap_item
is removed from stable tree.

Fixes: 4146d2d673e8 ("ksm: make !merge_across_nodes migration safe")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 mm/ksm.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/ksm.c b/mm/ksm.c
index f9ca6d94bf7f..8609c67f04a2 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -791,6 +791,7 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
 		stable_node->rmap_hlist_len--;
 
 		put_anon_vma(rmap_item->anon_vma);
+		rmap_item->head = NULL;
 		rmap_item->address &= PAGE_MASK;
 
 	} else if (rmap_item->address & UNSTABLE_FLAG) {
-- 
2.19.1