radix-tree: radix_tree_range_tag_if_tagged() can set incorrect tags

Commit ebf8aa44 ("radix-tree:
omplement function radix_tree_range_tag_if_tagged") does not safely
set tags on on intermediate tree nodes. The code walks down the tree
setting tags before it has fully resolved the path to the leaf under
the assumption there will be a leaf slot with the tag set in the
range it is searching.

Unfortunately, this is not a valid assumption - we can abort after
setting a tag on an intermediate node if we overrun the number of
tags we are allowed to set in a batch, or stop scanning because we
we have passed the last scan index before we reach a leaf slot with
the tag we are searching for set.

As a result, we can leave the function with tags set on intemediate
nodes which can be tripped over later by tag-based lookups. The
result of these stale tags is that lookup may end prematurely or
livelock because the lookup cannot make progress.

The fix for the problem involves reocrding the traversal path we
take to the leaf nodes, and only propagating the tags back up the
tree once the tag is set in the leaf node slot. We are already
recording the path for efficient traversal, so there is no
additional overhead to do the intermediately node tag setting in
this manner.

This fixes a radix tree lookup livelock triggered by the new
writeback sync livelock avoidance code introduced in commit
f446daae ("mm: implement writeback
livelock avoidance using page tagging").
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Acked-by: Jan Kara <jack@suse.cz>

lib/radix-tree.c

@@ -625,6 +625,13 @@ EXPORT_SYMBOL(radix_tree_tag_get);
  * also settag. The function stops either after tagging nr_to_tag items or
  * after reaching last_index.
  *
+ * The tags must be set from the leaf level only and propagated back up the
+ * path to the root. We must do this so that we resolve the full path before
+ * setting any tags on intermediate nodes. If we set tags as we descend, then
+ * we can get to the leaf node and find that the index that has the iftag
+ * set is outside the range we are scanning. This reults in dangling tags and
+ * can lead to problems with later tag operations (e.g. livelocks on lookups).
+ *
  * The function returns number of leaves where the tag was set and sets
  * *first_indexp to the first unscanned index.
  */
@@ -633,9 +640,13 @@ unsigned long radix_tree_range_tag_if_tagged(struct radix_tree_root *root,
 		unsigned long nr_to_tag,
 		unsigned int iftag, unsigned int settag)
 {
-	unsigned int height = root->height, shift;
-	unsigned long tagged = 0, index = *first_indexp;
-	struct radix_tree_node *open_slots[height], *slot;
+	unsigned int height = root->height;
+	struct radix_tree_path path[height];
+	struct radix_tree_path *pathp = path;
+	struct radix_tree_node *slot;
+	unsigned int shift;
+	unsigned long tagged = 0;
+	unsigned long index = *first_indexp;
 
 	last_index = min(last_index, radix_tree_maxindex(height));
 	if (index > last_index)
@@ -655,6 +666,13 @@ unsigned long radix_tree_range_tag_if_tagged(struct radix_tree_root *root,
 	shift = (height - 1) * RADIX_TREE_MAP_SHIFT;
 	slot = radix_tree_indirect_to_ptr(root->rnode);
 
+	/*
+	 * we fill the path from (root->height - 2) to 0, leaving the index at
+	 * (root->height - 1) as a terminator. Zero the node in the terminator
+	 * so that we can use this to end walk loops back up the path.
+	 */
+	path[height - 1].node = NULL;
+
 	for (;;) {
 		int offset;
 
@@ -663,17 +681,30 @@ unsigned long radix_tree_range_tag_if_tagged(struct radix_tree_root *root,
 			goto next;
 		if (!tag_get(slot, iftag, offset))
 			goto next;
+		if (height > 1) {
+			/* Go down one level */
+			height--;
+			shift -= RADIX_TREE_MAP_SHIFT;
+			path[height - 1].node = slot;
+			path[height - 1].offset = offset;
+			slot = slot->slots[offset];
+			continue;
+		}
+
+		/* tag the leaf */
+		tagged++;
 		tag_set(slot, settag, offset);
-		if (height == 1) {
-			tagged++;
-			goto next;
+
+		/* walk back up the path tagging interior nodes */
+		pathp = &path[0];
+		while (pathp->node) {
+			/* stop if we find a node with the tag already set */
+			if (tag_get(pathp->node, settag, pathp->offset))
+				break;
+			tag_set(pathp->node, settag, pathp->offset);
+			pathp++;
 		}
-		/* Go down one level */
-		height--;
-		shift -= RADIX_TREE_MAP_SHIFT;
-		open_slots[height] = slot;
-		slot = slot->slots[offset];
-		continue;
+
 next:
 		/* Go to next item at level determined by 'shift' */
 		index = ((index >> shift) + 1) << shift;
@@ -687,7 +718,7 @@ unsigned long radix_tree_range_tag_if_tagged(struct radix_tree_root *root,
 			 * last_index is guaranteed to be in the tree, what
 			 * we do below cannot wander astray.
 			 */
-			slot = open_slots[height];
+			slot = path[height - 1].node;
 			height++;
 			shift += RADIX_TREE_MAP_SHIFT;
 		}