[PATCH 2/6] mm: workingset: turn shadow node shrinker bugs into warnings

[Johannes Weiner <hannes@cmpxchg.org>]

When the shadow page shrinker tries to reclaim a radix tree node but
finds it in an unexpected state - it should contain no pages, and
non-zero shadow entries - there is no need to kill the executing task
or even the entire system. Warn about the invalid state, then leave
that tree node be. Simply don't put it back on the shadow LRU for
future reclaim and move on.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/workingset.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/mm/workingset.c b/mm/workingset.c
index 617475f529f4..3cfc61d84a52 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -418,23 +418,27 @@ static enum lru_status shadow_lru_isolate(struct list_head *item,
 	 * no pages, so we expect to be able to remove them all and
 	 * delete and free the empty node afterwards.
 	 */
-	BUG_ON(!workingset_node_shadows(node));
-	BUG_ON(workingset_node_pages(node));
-
+	if (WARN_ON_ONCE(!workingset_node_shadows(node)))
+		goto out_invalid;
+	if (WARN_ON_ONCE(workingset_node_pages(node)))
+		goto out_invalid;
 	for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) {
 		if (node->slots[i]) {
-			BUG_ON(!radix_tree_exceptional_entry(node->slots[i]));
+			if (WARN_ON_ONCE(!radix_tree_exceptional_entry(node->slots[i])))
+				goto out_invalid;
+			if (WARN_ON_ONCE(!mapping->nrexceptional))
+				goto out_invalid;
 			node->slots[i] = NULL;
 			workingset_node_shadows_dec(node);
-			BUG_ON(!mapping->nrexceptional);
 			mapping->nrexceptional--;
 		}
 	}
-	BUG_ON(workingset_node_shadows(node));
+	if (WARN_ON_ONCE(workingset_node_shadows(node)))
+		goto out_invalid;
 	inc_node_state(page_pgdat(virt_to_page(node)), WORKINGSET_NODERECLAIM);
-	if (!__radix_tree_delete_node(&mapping->page_tree, node))
-		BUG();
+	__radix_tree_delete_node(&mapping->page_tree, node);
 
+out_invalid:
 	spin_unlock(&mapping->tree_lock);
 	ret = LRU_REMOVED_RETRY;
 out:
-- 
2.10.1