[patch 9/9] mm: keep page cache radix tree nodes in check

From: Johannes Weiner
Date: Fri Jan 10 2014 - 13:13:09 EST

Previously, page cache radix tree nodes were freed after reclaim
emptied out their page pointers. But now reclaim stores shadow
entries in their place, which are only reclaimed when the inodes
themselves are reclaimed. This is problematic for bigger files that
are still in use after they have a significant amount of their cache
reclaimed, without any of those pages actually refaulting. The shadow
entries will just sit there and waste memory. In the worst case, the
shadow entries will accumulate until the machine runs out of memory.

To get this under control, the VM will track radix tree nodes
exclusively containing shadow entries on a per-NUMA node list.
Per-NUMA rather than global because we expect the radix tree nodes
themselves to be allocated node-locally and we want to reduce
cross-node references of otherwise independent cache workloads. A
simple shrinker will then reclaim these nodes on memory pressure.

A few things need to be stored in the radix tree node to implement the
shadow node LRU and allow tree deletions coming from the list:

1. There is no index available that would describe the reverse path
from the node up to the tree root, which is needed to perform a
deletion. To solve this, encode in each node its offset inside the
parent. This can be stored in the unused upper bits of the same
member that stores the node's height at no extra space cost.

2. The number of shadow entries needs to be counted in addition to the
regular entries, to quickly detect when the node is ready to go to
the shadow node LRU list. The current entry count is an unsigned
int but the maximum number of entries is 64, so a shadow counter
can easily be stored in the unused upper bits.

3. Tree modification needs tree lock and tree root, which are located
in the address space, so store an address_space backpointer in the
node. The parent pointer of the node is in a union with the 2-word
rcu_head, so the backpointer comes at no extra cost as well.

4. The node needs to be linked to an LRU list, which requires a list
head inside the node. This does increase the size of the node, but
it does not change the number of objects that fit into a slab page.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
include/linux/list_lru.h | 2 +
include/linux/mmzone.h | 1 +
include/linux/radix-tree.h | 32 +++++++++---
include/linux/swap.h | 1 +
lib/radix-tree.c | 36 ++++++++------
mm/filemap.c | 77 +++++++++++++++++++++++------
mm/list_lru.c | 8 +++
mm/truncate.c | 20 +++++++-
mm/vmstat.c | 1 +
mm/workingset.c | 121 +++++++++++++++++++++++++++++++++++++++++++++
10 files changed, 259 insertions(+), 40 deletions(-)

diff --git a/include/linux/list_lru.h b/include/linux/list_lru.h
index 3ce541753c88..b02fc233eadd 100644
--- a/include/linux/list_lru.h
+++ b/include/linux/list_lru.h
@@ -13,6 +13,8 @@
/* list_lru_walk_cb has to always return one of those */
enum lru_status {
LRU_REMOVED, /* item removed from list */
+ LRU_REMOVED_RETRY, /* item removed, but lock has been
+ dropped and reacquired */
LRU_ROTATE, /* item referenced, give another pass */
LRU_SKIP, /* item cannot be locked, skip */
LRU_RETRY, /* item not freeable. May drop the lock
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 118ba9f51e86..8cac5a7ef7a7 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -144,6 +144,7 @@ enum zone_stat_item {
#endif
WORKINGSET_REFAULT,
WORKINGSET_ACTIVATE,
+ WORKINGSET_NODERECLAIM,
NR_ANON_TRANSPARENT_HUGEPAGES,
NR_FREE_CMA_PAGES,
NR_VM_ZONE_STAT_ITEMS };
diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index 13636c40bc42..33170dbd9db4 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -72,21 +72,37 @@ static inline int radix_tree_is_indirect_ptr(void *ptr)
#define RADIX_TREE_TAG_LONGS \
((RADIX_TREE_MAP_SIZE + BITS_PER_LONG - 1) / BITS_PER_LONG)

+#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))
+
+/* Height component in node->path */
+#define RADIX_TREE_HEIGHT_SHIFT (RADIX_TREE_MAX_PATH + 1)
+#define RADIX_TREE_HEIGHT_MASK ((1UL << RADIX_TREE_HEIGHT_SHIFT) - 1)
+
+/* Internally used bits of node->count */
+#define RADIX_TREE_COUNT_SHIFT (RADIX_TREE_MAP_SHIFT + 1)
+#define RADIX_TREE_COUNT_MASK ((1UL << RADIX_TREE_COUNT_SHIFT) - 1)
+
struct radix_tree_node {
- unsigned int height; /* Height from the bottom */
+ unsigned int path; /* Offset in parent & height from the bottom */
unsigned int count;
union {
- struct radix_tree_node *parent; /* Used when ascending tree */
- struct rcu_head rcu_head; /* Used when freeing node */
+ struct {
+ /* Used when ascending tree */
+ struct radix_tree_node *parent;
+ /* For tree user */
+ void *private_data;
+ };
+ /* Used when freeing node */
+ struct rcu_head rcu_head;
};
+ /* For tree user */
+ struct list_head private_list;
void __rcu *slots[RADIX_TREE_MAP_SIZE];
unsigned long tags[RADIX_TREE_MAX_TAGS][RADIX_TREE_TAG_LONGS];
};

-#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))
-
/* root tags are stored in gfp_mask, shifted by __GFP_BITS_SHIFT */
struct radix_tree_root {
unsigned int height;
@@ -251,7 +267,7 @@ void *__radix_tree_lookup(struct radix_tree_root *root, unsigned long index,
struct radix_tree_node **nodep, void ***slotp);
void *radix_tree_lookup(struct radix_tree_root *, unsigned long);
void **radix_tree_lookup_slot(struct radix_tree_root *, unsigned long);
-bool __radix_tree_delete_node(struct radix_tree_root *root, unsigned long index,
+bool __radix_tree_delete_node(struct radix_tree_root *root,
struct radix_tree_node *node);
void *radix_tree_delete_item(struct radix_tree_root *, unsigned long, void *);
void *radix_tree_delete(struct radix_tree_root *, unsigned long);
diff --git a/include/linux/swap.h b/include/linux/swap.h
index b83cf61403ed..102e37bc82d5 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -264,6 +264,7 @@ struct swap_list_t {
void *workingset_eviction(struct address_space *mapping, struct page *page);
bool workingset_refault(void *shadow);
void workingset_activation(struct page *page);
+extern struct list_lru workingset_shadow_nodes;

/* linux/mm/page_alloc.c */
extern unsigned long totalram_pages;
diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index e601c56a43d0..0a0895371447 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -342,7 +342,8 @@ static int radix_tree_extend(struct radix_tree_root *root, unsigned long index)

/* Increase the height. */
newheight = root->height+1;
- node->height = newheight;
+ BUG_ON(newheight & ~RADIX_TREE_HEIGHT_MASK);
+ node->path = newheight;
node->count = 1;
node->parent = NULL;
slot = root->rnode;
@@ -400,11 +401,12 @@ int __radix_tree_create(struct radix_tree_root *root, unsigned long index,
/* Have to add a child node. */
if (!(slot = radix_tree_node_alloc(root)))
return -ENOMEM;
- slot->height = height;
+ slot->path = height;
slot->parent = node;
if (node) {
rcu_assign_pointer(node->slots[offset], slot);
node->count++;
+ slot->path |= offset << RADIX_TREE_HEIGHT_SHIFT;
} else
rcu_assign_pointer(root->rnode, ptr_to_indirect(slot));
}
@@ -496,7 +498,7 @@ void *__radix_tree_lookup(struct radix_tree_root *root, unsigned long index,
}
node = indirect_to_ptr(node);

- height = node->height;
+ height = node->path & RADIX_TREE_HEIGHT_MASK;
if (index > radix_tree_maxindex(height))
return NULL;

@@ -702,7 +704,7 @@ int radix_tree_tag_get(struct radix_tree_root *root,
return (index == 0);
node = indirect_to_ptr(node);

- height = node->height;
+ height = node->path & RADIX_TREE_HEIGHT_MASK;
if (index > radix_tree_maxindex(height))
return 0;

@@ -739,7 +741,7 @@ void **radix_tree_next_chunk(struct radix_tree_root *root,
{
unsigned shift, tag = flags & RADIX_TREE_ITER_TAG_MASK;
struct radix_tree_node *rnode, *node;
- unsigned long index, offset;
+ unsigned long index, offset, height;

if ((flags & RADIX_TREE_ITER_TAGGED) && !root_tag_get(root, tag))
return NULL;
@@ -770,7 +772,8 @@ void **radix_tree_next_chunk(struct radix_tree_root *root,
return NULL;

restart:
- shift = (rnode->height - 1) * RADIX_TREE_MAP_SHIFT;
+ height = rnode->path & RADIX_TREE_HEIGHT_MASK;
+ shift = (height - 1) * RADIX_TREE_MAP_SHIFT;
offset = index >> shift;

/* Index outside of the tree */
@@ -1140,7 +1143,7 @@ static unsigned long __locate(struct radix_tree_node *slot, void *item,
unsigned int shift, height;
unsigned long i;

- height = slot->height;
+ height = slot->path & RADIX_TREE_HEIGHT_MASK;
shift = (height-1) * RADIX_TREE_MAP_SHIFT;

for ( ; height > 1; height--) {
@@ -1203,7 +1206,8 @@ unsigned long radix_tree_locate_item(struct radix_tree_root *root, void *item)
}

node = indirect_to_ptr(node);
- max_index = radix_tree_maxindex(node->height);
+ max_index = radix_tree_maxindex(node->path &
+ RADIX_TREE_HEIGHT_MASK);
if (cur_index > max_index)
break;

@@ -1297,7 +1301,7 @@ static inline void radix_tree_shrink(struct radix_tree_root *root)
*
* Returns %true if @node was freed, %false otherwise.
*/
-bool __radix_tree_delete_node(struct radix_tree_root *root, unsigned long index,
+bool __radix_tree_delete_node(struct radix_tree_root *root,
struct radix_tree_node *node)
{
bool deleted = false;
@@ -1316,9 +1320,10 @@ bool __radix_tree_delete_node(struct radix_tree_root *root, unsigned long index,

parent = node->parent;
if (parent) {
- index >>= RADIX_TREE_MAP_SHIFT;
+ unsigned int offset;

- parent->slots[index & RADIX_TREE_MAP_MASK] = NULL;
+ offset = node->path >> RADIX_TREE_HEIGHT_SHIFT;
+ parent->slots[offset] = NULL;
parent->count--;
} else {
root_tag_clear_all(root);
@@ -1382,7 +1387,7 @@ void *radix_tree_delete_item(struct radix_tree_root *root,
node->slots[offset] = NULL;
node->count--;

- __radix_tree_delete_node(root, index, node);
+ __radix_tree_delete_node(root, node);

return entry;
}
@@ -1415,9 +1420,12 @@ int radix_tree_tagged(struct radix_tree_root *root, unsigned int tag)
EXPORT_SYMBOL(radix_tree_tagged);

static void
-radix_tree_node_ctor(void *node)
+radix_tree_node_ctor(void *arg)
{
- memset(node, 0, sizeof(struct radix_tree_node));
+ struct radix_tree_node *node = arg;
+
+ memset(node, 0, sizeof(*node));
+ INIT_LIST_HEAD(&node->private_list);
}

static __init unsigned long __maxindex(unsigned int height)
diff --git a/mm/filemap.c b/mm/filemap.c
index 65a374c0df4f..b93e223b59a9 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -110,11 +110,17 @@
static void page_cache_tree_delete(struct address_space *mapping,
struct page *page, void *shadow)
{
- if (shadow) {
- void **slot;
+ struct radix_tree_node *node;
+ unsigned long index;
+ unsigned int offset;
+ unsigned int tag;
+ void **slot;

- slot = radix_tree_lookup_slot(&mapping->page_tree, page->index);
- radix_tree_replace_slot(slot, shadow);
+ VM_BUG_ON(!PageLocked(page));
+
+ __radix_tree_lookup(&mapping->page_tree, page->index, &node, &slot);
+
+ if (shadow) {
mapping->nrshadows++;
/*
* Make sure the nrshadows update is committed before
@@ -123,9 +129,39 @@ static void page_cache_tree_delete(struct address_space *mapping,
* same time and miss a shadow entry.
*/
smp_wmb();
- } else
- radix_tree_delete(&mapping->page_tree, page->index);
+ }
mapping->nrpages--;
+
+ if (!node) {
+ /* Clear direct pointer tags in root node */
+ mapping->page_tree.gfp_mask &= __GFP_BITS_MASK;
+ radix_tree_replace_slot(slot, shadow);
+ return;
+ }
+
+ /* Clear tree tags for the removed page */
+ index = page->index;
+ offset = index & RADIX_TREE_MAP_MASK;
+ for (tag = 0; tag < RADIX_TREE_MAX_TAGS; tag++) {
+ if (test_bit(offset, node->tags[tag]))
+ radix_tree_tag_clear(&mapping->page_tree, index, tag);
+ }
+
+ /* Delete page, swap shadow entry */
+ radix_tree_replace_slot(slot, shadow);
+ node->count--;
+ if (shadow)
+ node->count += 1U << RADIX_TREE_COUNT_SHIFT;
+ else
+ if (__radix_tree_delete_node(&mapping->page_tree, node))
+ return;
+
+ /* Only shadow entries in there, keep track of this node */
+ if (!(node->count & RADIX_TREE_COUNT_MASK) &&
+ list_empty(&node->private_list)) {
+ node->private_data = mapping;
+ list_lru_add(&workingset_shadow_nodes, &node->private_list);
+ }
}

/*
@@ -471,27 +507,36 @@ EXPORT_SYMBOL_GPL(replace_page_cache_page);
static int page_cache_tree_insert(struct address_space *mapping,
struct page *page, void **shadowp)
{
+ struct radix_tree_node *node;
void **slot;
int error;

- slot = radix_tree_lookup_slot(&mapping->page_tree, page->index);
- if (slot) {
+ error = __radix_tree_create(&mapping->page_tree, page->index,
+ &node, &slot);
+ if (error)
+ return error;
+ if (*slot) {
void *p;

p = radix_tree_deref_slot_protected(slot, &mapping->tree_lock);
if (!radix_tree_exceptional_entry(p))
return -EEXIST;
- radix_tree_replace_slot(slot, page);
- mapping->nrshadows--;
- mapping->nrpages++;
if (shadowp)
*shadowp = p;
- return 0;
+ mapping->nrshadows--;
+ if (node)
+ node->count -= 1U << RADIX_TREE_COUNT_SHIFT;
}
- error = radix_tree_insert(&mapping->page_tree, page->index, page);
- if (!error)
- mapping->nrpages++;
- return error;
+ radix_tree_replace_slot(slot, page);
+ mapping->nrpages++;
+ if (node) {
+ node->count++;
+ /* Installed page, can't be shadow-only anymore */
+ if (!list_empty(&node->private_list))
+ list_lru_del(&workingset_shadow_nodes,
+ &node->private_list);
+ }
+ return 0;
}

static int __add_to_page_cache_locked(struct page *page,
diff --git a/mm/list_lru.c b/mm/list_lru.c
index 72f9decb0104..47a9faf4070b 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -88,10 +88,18 @@ restart:
ret = isolate(item, &nlru->lock, cb_arg);
switch (ret) {
case LRU_REMOVED:
+ case LRU_REMOVED_RETRY:
if (--nlru->nr_items == 0)
node_clear(nid, lru->active_nodes);
WARN_ON_ONCE(nlru->nr_items < 0);
isolated++;
+ /*
+ * If the lru lock has been dropped, our list
+ * traversal is now invalid and so we have to
+ * restart from scratch.
+ */
+ if (ret == LRU_REMOVED_RETRY)
+ goto restart;
break;
case LRU_ROTATE:
list_move_tail(item, &nlru->list);
diff --git a/mm/truncate.c b/mm/truncate.c
index 97606fa4c458..5c2615d7f4da 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -25,6 +25,9 @@
static void clear_exceptional_entry(struct address_space *mapping,
pgoff_t index, void *entry)
{
+ struct radix_tree_node *node;
+ void **slot;
+
/* Handled by shmem itself */
if (shmem_mapping(mapping))
return;
@@ -35,8 +38,21 @@ static void clear_exceptional_entry(struct address_space *mapping,
* without the tree itself locked. These unlocked entries
* need verification under the tree lock.
*/
- if (radix_tree_delete_item(&mapping->page_tree, index, entry) == entry)
- mapping->nrshadows--;
+ if (!__radix_tree_lookup(&mapping->page_tree, index, &node, &slot))
+ goto unlock;
+ if (*slot != entry)
+ goto unlock;
+ radix_tree_replace_slot(slot, NULL);
+ mapping->nrshadows--;
+ if (!node)
+ goto unlock;
+ node->count -= 1U << RADIX_TREE_COUNT_SHIFT;
+ /* No more shadow entries, stop tracking the node */
+ if (!(node->count >> RADIX_TREE_COUNT_SHIFT) &&
+ !list_empty(&node->private_list))
+ list_lru_del(&workingset_shadow_nodes, &node->private_list);
+ __radix_tree_delete_node(&mapping->page_tree, node);
+unlock:
spin_unlock_irq(&mapping->tree_lock);
}

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 3ac830d1b533..baa3ba586685 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -772,6 +772,7 @@ const char * const vmstat_text[] = {
#endif
"workingset_refault",
"workingset_activate",
+ "workingset_nodereclaim",
"nr_anon_transparent_hugepages",
"nr_free_cma",
"nr_dirty_threshold",
diff --git a/mm/workingset.c b/mm/workingset.c
index 8a6c7cff4923..7bb1a432c137 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -251,3 +251,124 @@ void workingset_activation(struct page *page)
{
atomic_long_inc(&page_zone(page)->inactive_age);
}
+
+/*
+ * Page cache radix tree nodes containing only shadow entries can grow
+ * excessively on certain workloads. That's why they are tracked on
+ * per-(NUMA)node lists and pushed back by a shrinker, but with a
+ * slightly higher threshold than regular shrinkers so we don't
+ * discard the entries too eagerly - after all, during light memory
+ * pressure is exactly when we need them.
+ */
+
+struct list_lru workingset_shadow_nodes;
+
+static unsigned long count_shadow_nodes(struct shrinker *shrinker,
+ struct shrink_control *sc)
+{
+ return list_lru_count_node(&workingset_shadow_nodes, sc->nid);
+}
+
+static enum lru_status shadow_lru_isolate(struct list_head *item,
+ spinlock_t *lru_lock,
+ void *arg)
+{
+ unsigned long *nr_reclaimed = arg;
+ struct address_space *mapping;
+ struct radix_tree_node *node;
+ unsigned int i;
+ int ret;
+
+ /*
+ * Page cache insertions and deletions synchroneously maintain
+ * the shadow node LRU under the mapping->tree_lock and the
+ * lru_lock. Because the page cache tree is emptied before
+ * the inode can be destroyed, holding the lru_lock pins any
+ * address_space that has radix tree nodes on the LRU.
+ *
+ * We can then safely transition to the mapping->tree_lock to
+ * pin only the address_space of the particular node we want
+ * to reclaim, take the node off-LRU, and drop the lru_lock.
+ */
+
+ node = container_of(item, struct radix_tree_node, private_list);
+ mapping = node->private_data;
+
+ /* Coming from the list, invert the lock order */
+ if (!spin_trylock_irq(&mapping->tree_lock)) {
+ spin_unlock(lru_lock);
+ ret = LRU_RETRY;
+ goto out;
+ }
+
+ list_del_init(item);
+ spin_unlock(lru_lock);
+
+ /*
+ * The nodes should only contain one or more shadow entries,
+ * no pages, so we expect to be able to remove them all and
+ * delete and free the empty node afterwards.
+ */
+
+ BUG_ON(!node->count);
+ BUG_ON(node->count & RADIX_TREE_COUNT_MASK);
+
+ for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) {
+ if (node->slots[i]) {
+ BUG_ON(!radix_tree_exceptional_entry(node->slots[i]));
+ node->slots[i] = NULL;
+ BUG_ON(node->count < (1U << RADIX_TREE_COUNT_SHIFT));
+ node->count -= 1U << RADIX_TREE_COUNT_SHIFT;
+ BUG_ON(!mapping->nrshadows);
+ mapping->nrshadows--;
+ }
+ }
+ BUG_ON(node->count);
+ inc_zone_state(page_zone(virt_to_page(node)), WORKINGSET_NODERECLAIM);
+ if (!__radix_tree_delete_node(&mapping->page_tree, node))
+ BUG();
+ (*nr_reclaimed)++;
+
+ spin_unlock_irq(&mapping->tree_lock);
+ ret = LRU_REMOVED_RETRY;
+out:
+ cond_resched();
+ spin_lock(lru_lock);
+ return ret;
+}
+
+static unsigned long scan_shadow_nodes(struct shrinker *shrinker,
+ struct shrink_control *sc)
+{
+ unsigned long nr_reclaimed = 0;
+
+ list_lru_walk_node(&workingset_shadow_nodes, sc->nid,
+ shadow_lru_isolate, &nr_reclaimed, &sc->nr_to_scan);
+
+ return nr_reclaimed;
+}
+
+static struct shrinker workingset_shadow_shrinker = {
+ .count_objects = count_shadow_nodes,
+ .scan_objects = scan_shadow_nodes,
+ .seeks = DEFAULT_SEEKS * 4,
+ .flags = SHRINKER_NUMA_AWARE,
+};
+
+static int __init workingset_init(void)
+{
+ int ret;
+
+ ret = list_lru_init(&workingset_shadow_nodes);
+ if (ret)
+ goto err;
+ ret = register_shrinker(&workingset_shadow_shrinker);
+ if (ret)
+ goto err_list_lru;
+ return 0;
+err_list_lru:
+ list_lru_destroy(&workingset_shadow_nodes);
+err:
+ return ret;
+}
+module_init(workingset_init);
--
1.8.4.2

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/