radix_tree: take radix_tree_path off stack

Down, down in the deepest depths of GFP_NOIO page reclaim, we have
shrink_page_list() calling __remove_mapping() calling __delete_from_
swap_cache() or __delete_from_page_cache().

You would not expect those to need much stack, but in fact they call
radix_tree_delete(): which declares a 192-byte radix_tree_path array on
its stack (to record the node,offsets it visits when descending, in case
it needs to ascend to update them).  And if any tag is still set [1],
that calls radix_tree_tag_clear(), which declares a further such
192-byte radix_tree_path array on the stack.  (At least we have
interrupts disabled here, so won't then be pushing registers too.)

That was probably a good choice when most users were 32-bit (array of
half the size), and adding fields to radix_tree_node would have bloated
it unnecessarily.  But nowadays many are 64-bit, and each
radix_tree_node contains a struct rcu_head, which is only used when
freeing; whereas the radix_tree_path info is only used for updating the
tree (deleting, clearing tags or setting tags if tagged) when a lock
must be held, of no interest when accessing the tree locklessly.

So add a parent pointer to the radix_tree_node, in union with the
rcu_head, and remove all uses of the radix_tree_path.  There would be
space in that union to save the offset when descending as before (we can
argue that a lock must already be held to exclude other users), but
recalculating it when ascending is both easy (a constant shift and a
constant mask) and uncommon, so it seems better just to do that.

Two little optimizations: no need to decrement height when descending,
adjusting shift is enough; and once radix_tree_tag_if_tagged() has set
tag on a node and its ancestors, it need not ascend from that node
again.

perf on the radix tree test harness reports radix_tree_insert() as 2%
slower (now having to set parent), but radix_tree_delete() 24% faster.
Surely that's an exaggeration from rtth's artificially low map shift 3,
but forcing it back to 6 still rates radix_tree_delete() 8% faster.

[1] Can a pagecache tag (dirty, writeback or towrite) actually still be
set at the time of radix_tree_delete()? Perhaps not if the filesystem is
well-behaved.  But although I've not tracked any stack overflow down to
this cause, I have observed a curious case in which a dirty tag is set
and left set on tmpfs: page migration's migrate_page_copy() happens to
use __set_page_dirty_nobuffers() to set PageDirty on the newpage, and
that sets PAGECACHE_TAG_DIRTY as a side-effect - harmless to a
filesystem which doesn't use tags, except for this stack depth issue.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Nai Xia <nai.xia@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/radix-tree.c

@@ -48,16 +48,14 @@
 struct radix_tree_node {
 	unsigned int	height;		/* Height from the bottom */
 	unsigned int	count;
-	struct rcu_head	rcu_head;
+	union {
+		struct radix_tree_node *parent;	/* Used when ascending tree */
+		struct rcu_head	rcu_head;	/* Used when freeing node */
+	};
 	void __rcu	*slots[RADIX_TREE_MAP_SIZE];
 	unsigned long	tags[RADIX_TREE_MAX_TAGS][RADIX_TREE_TAG_LONGS];
 };
 
-struct radix_tree_path {
-	struct radix_tree_node *node;
-	int offset;
-};
-
 #define RADIX_TREE_INDEX_BITS  (8 /* CHAR_BIT */ * sizeof(unsigned long))
 #define RADIX_TREE_MAX_PATH (DIV_ROUND_UP(RADIX_TREE_INDEX_BITS, \
 					  RADIX_TREE_MAP_SHIFT))
@@ -256,6 +254,7 @@ static inline unsigned long radix_tree_maxindex(unsigned int height)
 static int radix_tree_extend(struct radix_tree_root *root, unsigned long index)
 {
 	struct radix_tree_node *node;
+	struct radix_tree_node *slot;
 	unsigned int height;
 	int tag;
 
@@ -274,18 +273,23 @@ static int radix_tree_extend(struct radix_tree_root *root, unsigned long index)
 		if (!(node = radix_tree_node_alloc(root)))
 			return -ENOMEM;
 
-		/* Increase the height.  */
-		node->slots[0] = indirect_to_ptr(root->rnode);
-
 		/* Propagate the aggregated tag info into the new root */
 		for (tag = 0; tag < RADIX_TREE_MAX_TAGS; tag++) {
 			if (root_tag_get(root, tag))
 				tag_set(node, tag, 0);
 		}
 
+		/* Increase the height.  */
 		newheight = root->height+1;
 		node->height = newheight;
 		node->count = 1;
+		node->parent = NULL;
+		slot = root->rnode;
+		if (newheight > 1) {
+			slot = indirect_to_ptr(slot);
+			slot->parent = node;
+		}
+		node->slots[0] = slot;
 		node = ptr_to_indirect(node);
 		rcu_assign_pointer(root->rnode, node);
 		root->height = newheight;
@@ -331,6 +335,7 @@ int radix_tree_insert(struct radix_tree_root *root,
 			if (!(slot = radix_tree_node_alloc(root)))
 				return -ENOMEM;
 			slot->height = height;
+			slot->parent = node;
 			if (node) {
 				rcu_assign_pointer(node->slots[offset], slot);
 				node->count++;
@@ -504,47 +509,41 @@ EXPORT_SYMBOL(radix_tree_tag_set);
 void *radix_tree_tag_clear(struct radix_tree_root *root,
 			unsigned long index, unsigned int tag)
 {
-	/*
-	 * The radix tree path needs to be one longer than the maximum path
-	 * since the "list" is null terminated.
-	 */
-	struct radix_tree_path path[RADIX_TREE_MAX_PATH + 1], *pathp = path;
+	struct radix_tree_node *node = NULL;
 	struct radix_tree_node *slot = NULL;
 	unsigned int height, shift;
+	int uninitialized_var(offset);
 
 	height = root->height;
 	if (index > radix_tree_maxindex(height))
 		goto out;
 
-	shift = (height - 1) * RADIX_TREE_MAP_SHIFT;
-	pathp->node = NULL;
+	shift = height * RADIX_TREE_MAP_SHIFT;
 	slot = indirect_to_ptr(root->rnode);
 
-	while (height > 0) {
-		int offset;
-
+	while (shift) {
 		if (slot == NULL)
 			goto out;
 
+		shift -= RADIX_TREE_MAP_SHIFT;
 		offset = (index >> shift) & RADIX_TREE_MAP_MASK;
-		pathp[1].offset = offset;
-		pathp[1].node = slot;
+		node = slot;
 		slot = slot->slots[offset];
-		pathp++;
-		shift -= RADIX_TREE_MAP_SHIFT;
-		height--;
 	}
 
 	if (slot == NULL)
 		goto out;
 
-	while (pathp->node) {
-		if (!tag_get(pathp->node, tag, pathp->offset))
+	while (node) {
+		if (!tag_get(node, tag, offset))
 			goto out;
-		tag_clear(pathp->node, tag, pathp->offset);
-		if (any_tag_set(pathp->node, tag))
+		tag_clear(node, tag, offset);
+		if (any_tag_set(node, tag))
 			goto out;
-		pathp--;
+
+		index >>= RADIX_TREE_MAP_SHIFT;
+		offset = index & RADIX_TREE_MAP_MASK;
+		node = node->parent;
 	}
 
 	/* clear the root's tag bit */
@@ -646,8 +645,7 @@ unsigned long radix_tree_range_tag_if_tagged(struct radix_tree_root *root,
 		unsigned int iftag, unsigned int settag)
 {
 	unsigned int height = root->height;
-	struct radix_tree_path path[height];
-	struct radix_tree_path *pathp = path;
+	struct radix_tree_node *node = NULL;
 	struct radix_tree_node *slot;
 	unsigned int shift;
 	unsigned long tagged = 0;
@@ -671,14 +669,8 @@ unsigned long radix_tree_range_tag_if_tagged(struct radix_tree_root *root,
 	shift = (height - 1) * RADIX_TREE_MAP_SHIFT;
 	slot = 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 (;;) {
+		unsigned long upindex;
 		int offset;
 
 		offset = (index >> shift) & RADIX_TREE_MAP_MASK;
@@ -686,12 +678,10 @@ 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) {
+		if (shift) {
 			/* Go down one level */
-			height--;
 			shift -= RADIX_TREE_MAP_SHIFT;
-			path[height - 1].node = slot;
-			path[height - 1].offset = offset;
+			node = slot;
 			slot = slot->slots[offset];
 			continue;
 		}
@@ -701,15 +691,27 @@ unsigned long radix_tree_range_tag_if_tagged(struct radix_tree_root *root,
 		tag_set(slot, settag, offset);
 
 		/* walk back up the path tagging interior nodes */
-		pathp = &path[0];
-		while (pathp->node) {
+		upindex = index;
+		while (node) {
+			upindex >>= RADIX_TREE_MAP_SHIFT;
+			offset = upindex & RADIX_TREE_MAP_MASK;
+
 			/* stop if we find a node with the tag already set */
-			if (tag_get(pathp->node, settag, pathp->offset))
+			if (tag_get(node, settag, offset))
 				break;
-			tag_set(pathp->node, settag, pathp->offset);
-			pathp++;
+			tag_set(node, settag, offset);
+			node = node->parent;
 		}
 
+		/*
+		 * Small optimization: now clear that node pointer.
+		 * Since all of this slot's ancestors now have the tag set
+		 * from setting it above, we have no further need to walk
+		 * back up the tree setting tags, until we update slot to
+		 * point to another radix_tree_node.
+		 */
+		node = NULL;
+
 next:
 		/* Go to next item at level determined by 'shift' */
 		index = ((index >> shift) + 1) << shift;
@@ -724,8 +726,7 @@ next:
 			 * last_index is guaranteed to be in the tree, what
 			 * we do below cannot wander astray.
 			 */
-			slot = path[height - 1].node;
-			height++;
+			slot = slot->parent;
 			shift += RADIX_TREE_MAP_SHIFT;
 		}
 	}
@@ -1299,7 +1300,7 @@ static inline void radix_tree_shrink(struct radix_tree_root *root)
 	/* try to shrink tree height */
 	while (root->height > 0) {
 		struct radix_tree_node *to_free = root->rnode;
-		void *newptr;
+		struct radix_tree_node *slot;
 
 		BUG_ON(!radix_tree_is_indirect_ptr(to_free));
 		to_free = indirect_to_ptr(to_free);
@@ -1320,10 +1321,12 @@ static inline void radix_tree_shrink(struct radix_tree_root *root)
 		 * (to_free->slots[0]), it will be safe to dereference the new
 		 * one (root->rnode) as far as dependent read barriers go.
 		 */
-		newptr = to_free->slots[0];
-		if (root->height > 1)
-			newptr = ptr_to_indirect(newptr);
-		root->rnode = newptr;
+		slot = to_free->slots[0];
+		if (root->height > 1) {
+			slot->parent = NULL;
+			slot = ptr_to_indirect(slot);
+		}
+		root->rnode = slot;
 		root->height--;
 
 		/*
@@ -1363,16 +1366,12 @@ static inline void radix_tree_shrink(struct radix_tree_root *root)
  */
 void *radix_tree_delete(struct radix_tree_root *root, unsigned long index)
 {
-	/*
-	 * The radix tree path needs to be one longer than the maximum path
-	 * since the "list" is null terminated.
-	 */
-	struct radix_tree_path path[RADIX_TREE_MAX_PATH + 1], *pathp = path;
+	struct radix_tree_node *node = NULL;
 	struct radix_tree_node *slot = NULL;
 	struct radix_tree_node *to_free;
 	unsigned int height, shift;
 	int tag;
-	int offset;
+	int uninitialized_var(offset);
 
 	height = root->height;
 	if (index > radix_tree_maxindex(height))
@@ -1385,39 +1384,35 @@ void *radix_tree_delete(struct radix_tree_root *root, unsigned long index)
 		goto out;
 	}
 	slot = indirect_to_ptr(slot);
-
-	shift = (height - 1) * RADIX_TREE_MAP_SHIFT;
-	pathp->node = NULL;
+	shift = height * RADIX_TREE_MAP_SHIFT;
 
 	do {
 		if (slot == NULL)
 			goto out;
 
-		pathp++;
+		shift -= RADIX_TREE_MAP_SHIFT;
 		offset = (index >> shift) & RADIX_TREE_MAP_MASK;
-		pathp->offset = offset;
-		pathp->node = slot;
+		node = slot;
 		slot = slot->slots[offset];
-		shift -= RADIX_TREE_MAP_SHIFT;
-		height--;
-	} while (height > 0);
+	} while (shift);
 
 	if (slot == NULL)
 		goto out;
 
 	/*
-	 * Clear all tags associated with the just-deleted item
+	 * Clear all tags associated with the item to be deleted.
+	 * This way of doing it would be inefficient, but seldom is any set.
 	 */
 	for (tag = 0; tag < RADIX_TREE_MAX_TAGS; tag++) {
-		if (tag_get(pathp->node, tag, pathp->offset))
+		if (tag_get(node, tag, offset))
 			radix_tree_tag_clear(root, index, tag);
 	}
 
 	to_free = NULL;
 	/* Now free the nodes we do not need anymore */
-	while (pathp->node) {
-		pathp->node->slots[pathp->offset] = NULL;
-		pathp->node->count--;
+	while (node) {
+		node->slots[offset] = NULL;
+		node->count--;
 		/*
 		 * Queue the node for deferred freeing after the
 		 * last reference to it disappears (set NULL, above).
@@ -1425,17 +1420,20 @@ void *radix_tree_delete(struct radix_tree_root *root, unsigned long index)
 		if (to_free)
 			radix_tree_node_free(to_free);
 
-		if (pathp->node->count) {
-			if (pathp->node == indirect_to_ptr(root->rnode))
+		if (node->count) {
+			if (node == indirect_to_ptr(root->rnode))
 				radix_tree_shrink(root);
 			goto out;
 		}
 
 		/* Node with zero slots in use so free it */
-		to_free = pathp->node;
-		pathp--;
+		to_free = node;
 
+		index >>= RADIX_TREE_MAP_SHIFT;
+		offset = index & RADIX_TREE_MAP_MASK;
+		node = node->parent;
 	}
+
 	root_tag_clear_all(root);
 	root->height = 0;
 	root->rnode = NULL;