text: // SPDX-License-Identifier: GPL-2.0
  /*
   * Workingset detection
   *
   * Copyright (C) 2013 Red Hat, Inc., Johannes Weiner
   */
  
  #include <linux/memcontrol.h>
  #include <linux/writeback.h>
  #include <linux/shmem_fs.h>
  #include <linux/pagemap.h>
  #include <linux/atomic.h>
  #include <linux/module.h>
  #include <linux/swap.h>
  #include <linux/dax.h>
  #include <linux/fs.h>
  #include <linux/mm.h>
  
  /*
   *		Double CLOCK lists
   *
   * Per node, two clock lists are maintained for file pages: the
   * inactive and the active list.  Freshly faulted pages start out at
   * the head of the inactive list and page reclaim scans pages from the
   * tail.  Pages that are accessed multiple times on the inactive list
   * are promoted to the active list, to protect them from reclaim,
   * whereas active pages are demoted to the inactive list when the
   * active list grows too big.
   *
   *   fault ------------------------+
   *                                 |
   *              +--------------+   |            +-------------+
   *   reclaim <- |   inactive   | <-+-- demotion |    active   | <--+
   *              +--------------+                +-------------+    |
   *                     |                                           |
   *                     +-------------- promotion ------------------+
   *
   *
   *		Access frequency and refault distance
   *
   * A workload is thrashing when its pages are frequently used but they
   * are evicted from the inactive list every time before another access
   * would have promoted them to the active list.
   *
   * In cases where the average access distance between thrashing pages
   * is bigger than the size of memory there is nothing that can be
   * done - the thrashing set could never fit into memory under any
   * circumstance.
   *
   * However, the average access distance could be bigger than the
   * inactive list, yet smaller than the size of memory.  In this case,
   * the set could fit into memory if it weren't for the currently
   * active pages - which may be used more, hopefully less frequently:
   *
   *      +-memory available to cache-+
   *      |                           |
   *      +-inactive------+-active----+
   *  a b | c d e f g h i | J K L M N |
   *      +---------------+-----------+
   *
   * It is prohibitively expensive to accurately track access frequency
   * of pages.  But a reasonable approximation can be made to measure
   * thrashing on the inactive list, after which refaulting pages can be
   * activated optimistically to compete with the existing active pages.
   *
   * Approximating inactive page access frequency - Observations:
   *
   * 1. When a page is accessed for the first time, it is added to the
   *    head of the inactive list, slides every existing inactive page
   *    towards the tail by one slot, and pushes the current tail page
   *    out of memory.
   *
   * 2. When a page is accessed for the second time, it is promoted to
   *    the active list, shrinking the inactive list by one slot.  This
   *    also slides all inactive pages that were faulted into the cache
   *    more recently than the activated page towards the tail of the
   *    inactive list.
   *
   * Thus:
   *
   * 1. The sum of evictions and activations between any two points in
   *    time indicate the minimum number of inactive pages accessed in
   *    between.
   *
   * 2. Moving one inactive page N page slots towards the tail of the
   *    list requires at least N inactive page accesses.
   *
   * Combining these:
   *
   * 1. When a page is finally evicted from memory, the number of
   *    inactive pages accessed while the page was in cache is at least
   *    the number of page slots on the inactive list.
   *
   * 2. In addition, measuring the sum of evictions and activations (E)
   *    at the time of a page's eviction, and comparing it to another
   *    reading (R) at the time the page faults back into memory tells
   *    the minimum number of accesses while the page was not cached.
   *    This is called the refault distance.
   *
   * Because the first access of the page was the fault and the second
   * access the refault, we combine the in-cache distance with the
   * out-of-cache distance to get the complete minimum access distance
   * of this page:
   *
   *      NR_inactive + (R - E)
   *
   * And knowing the minimum access distance of a page, we can easily
   * tell if the page would be able to stay in cache assuming all page
   * slots in the cache were available:
   *
   *   NR_inactive + (R - E) <= NR_inactive + NR_active
   *
   * which can be further simplified to
   *
   *   (R - E) <= NR_active
   *
   * Put into words, the refault distance (out-of-cache) can be seen as
   * a deficit in inactive list space (in-cache).  If the inactive list
   * had (R - E) more page slots, the page would not have been evicted
   * in between accesses, but activated instead.  And on a full system,
   * the only thing eating into inactive list space is active pages.
   *
   *
   *		Activating refaulting pages
   *
   * All that is known about the active list is that the pages have been
   * accessed more than once in the past.  This means that at any given
   * time there is actually a good chance that pages on the active list
   * are no longer in active use.
   *
   * So when a refault distance of (R - E) is observed and there are at
   * least (R - E) active pages, the refaulting page is activated
   * optimistically in the hope that (R - E) active pages are actually
   * used less frequently than the refaulting page - or even not used at
   * all anymore.
   *
   * If this is wrong and demotion kicks in, the pages which are truly
   * used more frequently will be reactivated while the less frequently
   * used once will be evicted from memory.
   *
   * But if this is right, the stale pages will be pushed out of memory
   * and the used pages get to stay in cache.
   *
   *
   *		Implementation
   *
   * For each node's file LRU lists, a counter for inactive evictions
   * and activations is maintained (node->inactive_age).
   *
   * On eviction, a snapshot of this counter (along with some bits to
   * identify the node) is stored in the now empty page cache radix tree
   * slot of the evicted page.  This is called a shadow entry.
   *
   * On cache misses for which there are shadow entries, an eligible
   * refault distance will immediately activate the refaulting page.
   */
  
  #define EVICTION_SHIFT	(RADIX_TREE_EXCEPTIONAL_ENTRY + \
  			 NODES_SHIFT +	\
  			 MEM_CGROUP_ID_SHIFT)
  #define EVICTION_MASK	(~0UL >> EVICTION_SHIFT)
  
  /*
   * Eviction timestamps need to be able to cover the full range of
   * actionable refaults. However, bits are tight in the radix tree
   * entry, and after storing the identifier for the lruvec there might
   * not be enough left to represent every single actionable refault. In
   * that case, we have to sacrifice granularity for distance, and group
   * evictions into coarser buckets by shaving off lower timestamp bits.
   */
  static unsigned int bucket_order __read_mostly;
  
  static void *pack_shadow(int memcgid, pg_data_t *pgdat, unsigned long eviction)
  {
  	eviction >>= bucket_order;
  	eviction = (eviction << MEM_CGROUP_ID_SHIFT) | memcgid;
  	eviction = (eviction << NODES_SHIFT) | pgdat->node_id;
  	eviction = (eviction << RADIX_TREE_EXCEPTIONAL_SHIFT);
  
  	return (void *)(eviction | RADIX_TREE_EXCEPTIONAL_ENTRY);
  }
  
  static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat,
  			  unsigned long *evictionp)
  {
  	unsigned long entry = (unsigned long)shadow;
  	int memcgid, nid;
  
  	entry >>= RADIX_TREE_EXCEPTIONAL_SHIFT;
  	nid = entry & ((1UL << NODES_SHIFT) - 1);
  	entry >>= NODES_SHIFT;
  	memcgid = entry & ((1UL << MEM_CGROUP_ID_SHIFT) - 1);
  	entry >>= MEM_CGROUP_ID_SHIFT;
  
  	*memcgidp = memcgid;
  	*pgdat = NODE_DATA(nid);
  	*evictionp = entry << bucket_order;
  }
  
  /**
   * workingset_eviction - note the eviction of a page from memory
   * @mapping: address space the page was backing
   * @page: the page being evicted
   *
   * Returns a shadow entry to be stored in @mapping->page_tree in place
   * of the evicted @page so that a later refault can be detected.
   */
  void *workingset_eviction(struct address_space *mapping, struct page *page)
  {
  	struct mem_cgroup *memcg = page_memcg(page);
  	struct pglist_data *pgdat = page_pgdat(page);
  	int memcgid = mem_cgroup_id(memcg);
  	unsigned long eviction;
  	struct lruvec *lruvec;
  
  	/* Page is fully exclusive and pins page->mem_cgroup */
  	VM_BUG_ON_PAGE(PageLRU(page), page);
  	VM_BUG_ON_PAGE(page_count(page), page);
  	VM_BUG_ON_PAGE(!PageLocked(page), page);
  
  	lruvec = mem_cgroup_lruvec(pgdat, memcg);
  	eviction = atomic_long_inc_return(&lruvec->inactive_age);
  	return pack_shadow(memcgid, pgdat, eviction);
  }
  
  /**
   * workingset_refault - evaluate the refault of a previously evicted page
   * @shadow: shadow entry of the evicted page
   *
   * Calculates and evaluates the refault distance of the previously
   * evicted page in the context of the node it was allocated in.
   *
   * Returns %true if the page should be activated, %false otherwise.
   */
  bool workingset_refault(void *shadow)
  {
  	unsigned long refault_distance;
  	unsigned long active_file;
  	struct mem_cgroup *memcg;
  	unsigned long eviction;
  	struct lruvec *lruvec;
  	unsigned long refault;
  	struct pglist_data *pgdat;
  	int memcgid;
  
  	unpack_shadow(shadow, &memcgid, &pgdat, &eviction);
  
  	rcu_read_lock();
  	/*
  	 * Look up the memcg associated with the stored ID. It might
  	 * have been deleted since the page's eviction.
  	 *
  	 * Note that in rare events the ID could have been recycled
  	 * for a new cgroup that refaults a shared page. This is
  	 * impossible to tell from the available data. However, this
  	 * should be a rare and limited disturbance, and activations
  	 * are always speculative anyway. Ultimately, it's the aging
  	 * algorithm's job to shake out the minimum access frequency
  	 * for the active cache.
  	 *
  	 * XXX: On !CONFIG_MEMCG, this will always return NULL; it
  	 * would be better if the root_mem_cgroup existed in all
  	 * configurations instead.
  	 */
  	memcg = mem_cgroup_from_id(memcgid);
  	if (!mem_cgroup_disabled() && !memcg) {
  		rcu_read_unlock();
  		return false;
  	}
  	lruvec = mem_cgroup_lruvec(pgdat, memcg);
  	refault = atomic_long_read(&lruvec->inactive_age);
  	active_file = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES);
  
  	/*
  	 * The unsigned subtraction here gives an accurate distance
  	 * across inactive_age overflows in most cases.
  	 *
  	 * There is a special case: usually, shadow entries have a
  	 * short lifetime and are either refaulted or reclaimed along
  	 * with the inode before they get too old.  But it is not
  	 * impossible for the inactive_age to lap a shadow entry in
  	 * the field, which can then can result in a false small
  	 * refault distance, leading to a false activation should this
  	 * old entry actually refault again.  However, earlier kernels
  	 * used to deactivate unconditionally with *every* reclaim
  	 * invocation for the longest time, so the occasional
  	 * inappropriate activation leading to pressure on the active
  	 * list is not a problem.
  	 */
  	refault_distance = (refault - eviction) & EVICTION_MASK;
  
  	inc_lruvec_state(lruvec, WORKINGSET_REFAULT);
  
  	if (refault_distance <= active_file) {
  		inc_lruvec_state(lruvec, WORKINGSET_ACTIVATE);
  		rcu_read_unlock();
  		return true;
  	}
  	rcu_read_unlock();
  	return false;
  }
  
  /**
   * workingset_activation - note a page activation
   * @page: page that is being activated
   */
  void workingset_activation(struct page *page)
  {
  	struct mem_cgroup *memcg;
  	struct lruvec *lruvec;
  
  	rcu_read_lock();
  	/*
  	 * Filter non-memcg pages here, e.g. unmap can call
  	 * mark_page_accessed() on VDSO pages.
  	 *
  	 * XXX: See workingset_refault() - this should return
  	 * root_mem_cgroup even for !CONFIG_MEMCG.
  	 */
  	memcg = page_memcg_rcu(page);
  	if (!mem_cgroup_disabled() && !memcg)
  		goto out;
  	lruvec = mem_cgroup_lruvec(page_pgdat(page), memcg);
  	atomic_long_inc(&lruvec->inactive_age);
  out:
  	rcu_read_unlock();
  }
  
  /*
   * Shadow entries reflect the share of the working set that does not
   * fit into memory, so their number depends on the access pattern of
   * the workload.  In most cases, they will refault or get reclaimed
   * along with the inode, but a (malicious) workload that streams
   * through files with a total size several times that of available
   * memory, while preventing the inodes from being reclaimed, can
   * create excessive amounts of shadow nodes.  To keep a lid on this,
   * track shadow nodes and reclaim them when they grow way past the
   * point where they would still be useful.
   */
  
  static struct list_lru shadow_nodes;
  
  void workingset_update_node(struct radix_tree_node *node)
  {
  	/*
  	 * Track non-empty nodes that contain only shadow entries;
  	 * unlink those that contain pages or are being freed.
  	 *
  	 * Avoid acquiring the list_lru lock when the nodes are
  	 * already where they should be. The list_empty() test is safe
  	 * as node->private_list is protected by &mapping->tree_lock.
  	 */
  	if (node->count && node->count == node->exceptional) {
  		if (list_empty(&node->private_list))
  			list_lru_add(&shadow_nodes, &node->private_list);
  	} else {
  		if (!list_empty(&node->private_list))
  			list_lru_del(&shadow_nodes, &node->private_list);
  	}
  }
  
  static unsigned long count_shadow_nodes(struct shrinker *shrinker,
  					struct shrink_control *sc)
  {
  	unsigned long max_nodes;
  	unsigned long nodes;
  	unsigned long cache;
  
  	/* list_lru lock nests inside IRQ-safe mapping->tree_lock */
  	local_irq_disable();
  	nodes = list_lru_shrink_count(&shadow_nodes, sc);
  	local_irq_enable();
  
  	/*
  	 * Approximate a reasonable limit for the radix tree nodes
  	 * containing shadow entries. We don't need to keep more
  	 * shadow entries than possible pages on the active list,
  	 * since refault distances bigger than that are dismissed.
  	 *
  	 * The size of the active list converges toward 100% of
  	 * overall page cache as memory grows, with only a tiny
  	 * inactive list. Assume the total cache size for that.
  	 *
  	 * Nodes might be sparsely populated, with only one shadow
  	 * entry in the extreme case. Obviously, we cannot keep one
  	 * node for every eligible shadow entry, so compromise on a
  	 * worst-case density of 1/8th. Below that, not all eligible
  	 * refaults can be detected anymore.
  	 *
  	 * On 64-bit with 7 radix_tree_nodes per page and 64 slots
  	 * each, this will reclaim shadow entries when they consume
  	 * ~1.8% of available memory:
  	 *
  	 * PAGE_SIZE / radix_tree_nodes / node_entries * 8 / PAGE_SIZE
  	 */
  	if (sc->memcg) {
  		cache = mem_cgroup_node_nr_lru_pages(sc->memcg, sc->nid,
  						     LRU_ALL_FILE);
  	} else {
  		cache = node_page_state(NODE_DATA(sc->nid), NR_ACTIVE_FILE) +
  			node_page_state(NODE_DATA(sc->nid), NR_INACTIVE_FILE);
  	}
  	max_nodes = cache >> (RADIX_TREE_MAP_SHIFT - 3);
  
  	if (nodes <= max_nodes)
  		return 0;
  	return nodes - max_nodes;
  }
  
  static enum lru_status shadow_lru_isolate(struct list_head *item,
  					  struct list_lru_one *lru,
  					  spinlock_t *lru_lock,
  					  void *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 = container_of(node->root, struct address_space, page_tree);
  
  	/* Coming from the list, invert the lock order */
  	if (!spin_trylock(&mapping->tree_lock)) {
  		spin_unlock(lru_lock);
  		ret = LRU_RETRY;
  		goto out;
  	}
  
  	list_lru_isolate(lru, 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.
  	 */
  	if (WARN_ON_ONCE(!node->exceptional))
  		goto out_invalid;
  	if (WARN_ON_ONCE(node->count != node->exceptional))
  		goto out_invalid;
  	for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) {
  		if (node->slots[i]) {
  			if (WARN_ON_ONCE(!radix_tree_exceptional_entry(node->slots[i])))
  				goto out_invalid;
  			if (WARN_ON_ONCE(!node->exceptional))
  				goto out_invalid;
  			if (WARN_ON_ONCE(!mapping->nrexceptional))
  				goto out_invalid;
  			node->slots[i] = NULL;
  			node->exceptional--;
  			node->count--;
  			mapping->nrexceptional--;
  		}
  	}
  	if (WARN_ON_ONCE(node->exceptional))
  		goto out_invalid;
  	inc_lruvec_page_state(virt_to_page(node), WORKINGSET_NODERECLAIM);
  	__radix_tree_delete_node(&mapping->page_tree, node,
  				 workingset_lookup_update(mapping));
  
  out_invalid:
  	spin_unlock(&mapping->tree_lock);
  	ret = LRU_REMOVED_RETRY;
  out:
  	local_irq_enable();
  	cond_resched();
  	local_irq_disable();
  	spin_lock(lru_lock);
  	return ret;
  }
  
  static unsigned long scan_shadow_nodes(struct shrinker *shrinker,
  				       struct shrink_control *sc)
  {
  	unsigned long ret;
  
  	/* list_lru lock nests inside IRQ-safe mapping->tree_lock */
  	local_irq_disable();
  	ret = list_lru_shrink_walk(&shadow_nodes, sc, shadow_lru_isolate, NULL);
  	local_irq_enable();
  	return ret;
  }
  
  static struct shrinker workingset_shadow_shrinker = {
  	.count_objects = count_shadow_nodes,
  	.scan_objects = scan_shadow_nodes,
  	.seeks = DEFAULT_SEEKS,
  	.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE,
  };
  
  /*
   * Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe
   * mapping->tree_lock.
   */
  static struct lock_class_key shadow_nodes_key;
  
  static int __init workingset_init(void)
  {
  	unsigned int timestamp_bits;
  	unsigned int max_order;
  	int ret;
  
  	BUILD_BUG_ON(BITS_PER_LONG < EVICTION_SHIFT);
  	/*
  	 * Calculate the eviction bucket size to cover the longest
  	 * actionable refault distance, which is currently half of
  	 * memory (totalram_pages/2). However, memory hotplug may add
  	 * some more pages at runtime, so keep working with up to
  	 * double the initial memory by using totalram_pages as-is.
  	 */
  	timestamp_bits = BITS_PER_LONG - EVICTION_SHIFT;
  	max_order = fls_long(totalram_pages - 1);
  	if (max_order > timestamp_bits)
  		bucket_order = max_order - timestamp_bits;
  	pr_info("workingset: timestamp_bits=%d max_order=%d bucket_order=%u\n",
  	       timestamp_bits, max_order, bucket_order);
  
  	ret = __list_lru_init(&shadow_nodes, true, &shadow_nodes_key);
  	if (ret)
  		goto err;
  	ret = register_shrinker(&workingset_shadow_shrinker);
  	if (ret)
  		goto err_list_lru;
  	return 0;
  err_list_lru:
  	list_lru_destroy(&shadow_nodes);
  err:
  	return ret;
  }
  module_init(workingset_init);
  