[PATCH] ksm: fix losing visibility of part of rmap_item->next list

[Andrea Arcangeli <aarcange@redhat.com>]

From: Andrea Arcangeli <aarcange@redhat.com>

The tree_item->rmap_item is the head of the list and as such it must
not be overwritten except in the case that the element we removed
(rmap_item) was the previous head of the list, in which case it would
also have rmap_item->prev set to null.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---

diff --git a/mm/ksm.c b/mm/ksm.c
index 74d921b..6d8dfee 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -397,10 +397,10 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
 				free_tree_item(tree_item);
 				nnodes_stable_tree--;
 			} else if (!rmap_item->prev) {
+				BUG_ON(tree_item->rmap_item != rmap_item);
 				tree_item->rmap_item = rmap_item->next;
-			} else {
-				tree_item->rmap_item = rmap_item->prev;
-			}
+			} else
+				BUG_ON(tree_item->rmap_item == rmap_item);
 		} else {
 			/*
 			 * We dont rb_erase(&tree_item->node) here, beacuse

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