[PATCH] radix_tree: take radix_tree_path off stack

From: Hugh Dickins
Date: Mon Dec 19 2011 - 01:41:58 EST

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@xxxxxxxxxx>
----

lib/radix-tree.c | 148 +++++++++++++++++++++------------------------
1 file changed, 70 insertions(+), 78 deletions(-)

--- 3.2-rc6/lib/radix-tree.c 2011-11-07 19:24:53.342418579 -0800
+++ linux/lib/radix-tree.c 2011-12-16 20:40:26.152758485 -0800
@@ -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_m
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 radi
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_
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_ta
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_ta
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_ta
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 +691,21 @@ unsigned long radix_tree_range_tag_if_ta
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;
}

+ /* optimization: no need to walk up from this node again */
+ node = NULL;
+
next:
/* Go to next item at level determined by 'shift' */
index = ((index >> shift) + 1) << shift;
@@ -724,8 +720,7 @@ 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 +1294,7 @@ static inline void radix_tree_shrink(str
/* 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 +1315,12 @@ static inline void radix_tree_shrink(str
* (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 +1360,12 @@ static inline void radix_tree_shrink(str
*/
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 +1378,35 @@ void *radix_tree_delete(struct radix_tre
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 +1414,20 @@ void *radix_tree_delete(struct radix_tre
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;
--
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