// 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.
   *
   *

   *		Refaulting inactive 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.
   *

   * That means if inactive cache is refaulting with a suitable refault
   * distance, we assume the cache workingset is transitioning and put
   * pressure on the current active list.
   *

   * 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.
   *

   *		Refaulting active pages
   *
   * If on the other hand the refaulting pages have recently been
   * deactivated, it means that the active list is no longer protecting
   * actively used cache from reclaim. The cache is NOT transitioning to
   * a different workingset; the existing workingset is thrashing in the
   * space allocated to the page 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

   * 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	((BITS_PER_LONG - BITS_PER_XA_VALUE) +	\

  			 1 + 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 xarray

   * 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,
  			 bool workingset)

  {

  	eviction >>= bucket_order;

  	eviction &= EVICTION_MASK;

  	eviction = (eviction << MEM_CGROUP_ID_SHIFT) | memcgid;

  	eviction = (eviction << NODES_SHIFT) | pgdat->node_id;

  	eviction = (eviction << 1) | workingset;

  	return xa_mk_value(eviction);

  }

  static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat,

  			  unsigned long *evictionp, bool *workingsetp)

  {

  	unsigned long entry = xa_to_value(shadow);

  	int memcgid, nid;

  	bool workingset;

  	workingset = entry & 1;
  	entry >>= 1;

  	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;

  	*workingsetp = workingset;

  }
  
  /**
   * workingset_eviction - note the eviction of a page from memory

   * @page: the page being evicted
   *

   * Returns a shadow entry to be stored in @page->mapping->i_pages in place

   * of the evicted @page so that a later refault can be detected.
   */

  void *workingset_eviction(struct page *page)

  {

  	struct pglist_data *pgdat = page_pgdat(page);

  	struct mem_cgroup *memcg = page_memcg(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, PageWorkingset(page));

  }
  
  /**
   * workingset_refault - evaluate the refault of a previously evicted page

   * @page: the freshly allocated replacement 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.

   */

  void workingset_refault(struct page *page, void *shadow)

  {
  	unsigned long refault_distance;

  	struct pglist_data *pgdat;

  	unsigned long active_file;
  	struct mem_cgroup *memcg;

  	unsigned long eviction;

  	struct lruvec *lruvec;

  	unsigned long refault;

  	bool workingset;

  	int memcgid;

  	unpack_shadow(shadow, &memcgid, &pgdat, &eviction, &workingset);

  	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)
  		goto out;

  	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);

  
  	/*

  	 * Calculate the refault distance

  	 *

  	 * 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 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);

  	/*
  	 * Compare the distance to the existing workingset size. We
  	 * don't act on pages that couldn't stay resident even if all
  	 * the memory was available to the page cache.
  	 */
  	if (refault_distance > active_file)
  		goto out;
  
  	SetPageActive(page);
  	atomic_long_inc(&lruvec->inactive_age);
  	inc_lruvec_state(lruvec, WORKINGSET_ACTIVATE);
  
  	/* Page was active prior to eviction */
  	if (workingset) {
  		SetPageWorkingset(page);
  		inc_lruvec_state(lruvec, WORKINGSET_RESTORE);

  	}

  out:

  	rcu_read_unlock();

  }
  
  /**
   * 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 xa_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 the i_pages lock.

  	 */

  	VM_WARN_ON_ONCE(!irqs_disabled());  /* For __inc_lruvec_page_state */

  	if (node->count && node->count == node->nr_values) {

  		if (list_empty(&node->private_list)) {

  			list_lru_add(&shadow_nodes, &node->private_list);

  			__inc_lruvec_slab_state(node, WORKINGSET_NODES);

  		}

  	} else {

  		if (!list_empty(&node->private_list)) {

  			list_lru_del(&shadow_nodes, &node->private_list);

  			__dec_lruvec_slab_state(node, WORKINGSET_NODES);

  		}

  	}
  }

  
  static unsigned long count_shadow_nodes(struct shrinker *shrinker,
  					struct shrink_control *sc)
  {

  	unsigned long max_nodes;

  	unsigned long nodes;

  	unsigned long pages;

  	nodes = list_lru_shrink_count(&shadow_nodes, sc);

  	/*

  	 * Approximate a reasonable limit for the 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 xa_nodes per page and 64 slots

  	 * each, this will reclaim shadow entries when they consume

  	 * ~1.8% of available memory:

  	 *

  	 * PAGE_SIZE / xa_nodes / node_entries * 8 / PAGE_SIZE

  	 */

  #ifdef CONFIG_MEMCG

  	if (sc->memcg) {

  		struct lruvec *lruvec;

  		int i;

  		lruvec = mem_cgroup_lruvec(NODE_DATA(sc->nid), sc->memcg);

  		for (pages = 0, i = 0; i < NR_LRU_LISTS; i++)

  			pages += lruvec_page_state_local(lruvec,
  							 NR_LRU_BASE + i);
  		pages += lruvec_page_state_local(lruvec, NR_SLAB_RECLAIMABLE);
  		pages += lruvec_page_state_local(lruvec, NR_SLAB_UNRECLAIMABLE);

  	} else
  #endif
  		pages = node_present_pages(sc->nid);

  	max_nodes = pages >> (XA_CHUNK_SHIFT - 3);

  	if (!nodes)
  		return SHRINK_EMPTY;

  	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) __must_hold(lru_lock)

  {

  	struct xa_node *node = container_of(item, struct xa_node, private_list);
  	XA_STATE(xas, node->array, 0);

  	struct address_space *mapping;

  	int ret;
  
  	/*
  	 * Page cache insertions and deletions synchroneously maintain

  	 * the shadow node LRU under the i_pages 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 nodes on the LRU.

  	 *

  	 * We can then safely transition to the i_pages 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.
  	 */

  	mapping = container_of(node->array, struct address_space, i_pages);

  
  	/* Coming from the list, invert the lock order */

  	if (!xa_trylock(&mapping->i_pages)) {

  		spin_unlock_irq(lru_lock);

  		ret = LRU_RETRY;
  		goto out;
  	}

  	list_lru_isolate(lru, item);

  	__dec_lruvec_slab_state(node, WORKINGSET_NODES);

  	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->nr_values))

  		goto out_invalid;

  	if (WARN_ON_ONCE(node->count != node->nr_values))

  		goto out_invalid;

  	mapping->nrexceptional -= node->nr_values;
  	xas.xa_node = xa_parent_locked(&mapping->i_pages, node);
  	xas.xa_offset = node->offset;
  	xas.xa_shift = node->shift + XA_CHUNK_SHIFT;
  	xas_set_update(&xas, workingset_update_node);
  	/*
  	 * We could store a shadow entry here which was the minimum of the
  	 * shadow entries we were tracking ...
  	 */
  	xas_store(&xas, NULL);

  	__inc_lruvec_slab_state(node, WORKINGSET_NODERECLAIM);

  out_invalid:

  	xa_unlock_irq(&mapping->i_pages);

  	ret = LRU_REMOVED_RETRY;
  out:

  	cond_resched();

  	spin_lock_irq(lru_lock);

  	return ret;
  }
  
  static unsigned long scan_shadow_nodes(struct shrinker *shrinker,
  				       struct shrink_control *sc)
  {

  	/* list_lru lock nests inside the IRQ-safe i_pages lock */

  	return list_lru_shrink_walk_irq(&shadow_nodes, sc, shadow_lru_isolate,
  					NULL);

  }
  
  static struct shrinker workingset_shadow_shrinker = {
  	.count_objects = count_shadow_nodes,
  	.scan_objects = scan_shadow_nodes,

  	.seeks = 0, /* ->count reports only fully expendable nodes */

  	.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE,

  };
  
  /*
   * Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe

   * i_pages 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
  ",

  	       timestamp_bits, max_order, bucket_order);

  	ret = prealloc_shrinker(&workingset_shadow_shrinker);

  	if (ret)
  		goto err;

  	ret = __list_lru_init(&shadow_nodes, true, &shadow_nodes_key,
  			      &workingset_shadow_shrinker);

  	if (ret)
  		goto err_list_lru;

  	register_shrinker_prepared(&workingset_shadow_shrinker);

  	return 0;
  err_list_lru:

  	free_prealloced_shrinker(&workingset_shadow_shrinker);

  err:
  	return ret;
  }
  module_init(workingset_init);