On Fri, Jan 10, 2014 at 01:10:43PM -0500, Johannes Weiner wrote:
> 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:
Just a couple of things with the list_lru interfaces.
....
> @@ -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);
> + }
You can't do this list_empty(&node->private_list) check safely
externally to the list_lru code - only time that entry can be
checked safely is under the LRU list locks. This is the reason that
list_lru_add/list_lru_del return a boolean to indicate is the object
was added/removed from the list - they do this list_empty() check
internally. i.e. the correct, safe way to do conditionally update
state iff the object was added to the LRU is:
if (!(node->count & RADIX_TREE_COUNT_MASK)) {
if (list_lru_add(&workingset_shadow_nodes, &node->private_list))
node->private_data = mapping;
}
> + 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);
> + }
Same issue here:
if (node) {
node->count++;
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);
I think that we need to assert that the list lru lock is correctly
held here on return with LRU_REMOVED_RETRY. i.e.
case LRU_REMOVED_RETRY:
assert_spin_locked(&nlru->lock);
case LRU_REMOVED:
.....
> @@ -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);
Same issue with list_empty() check.
> +
> +/*
> + * 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;
list_lru_walk_node() returns the number of reclaimed objects (i.e.
the number of objects that returned LRU_REMOVED/LRU_REMOVED_RETRY
from the ->isolate callback). You don't need to count nr_reclaimed
yourself.
> +}
> +
> +static struct shrinker workingset_shadow_shrinker = {
> + .count_objects = count_shadow_nodes,
> + .scan_objects = scan_shadow_nodes,
> + .seeks = DEFAULT_SEEKS * 4,
> + .flags = SHRINKER_NUMA_AWARE,
> +};
Can you add a comment explaining how you calculated the .seeks
value? It's important to document the weighings/importance
we give to slab reclaim so we can determine if it's actually
acheiving the desired balance under different loads...
Cheers,
Dave.
--
Dave Chinner
da...@fromorbit.com
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