// 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->i_pages 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 the i_pages 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;

        nodes = list_lru_shrink_count(&shadow_nodes, sc);

        /*
         * 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)
                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)
{
        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 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 radix tree 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.
         */

        node = container_of(item, struct radix_tree_node, private_list);
        mapping = container_of(node->root, 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);
        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->i_pages, node,
                                 workingset_lookup_update(mapping));

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 = DEFAULT_SEEKS,
        .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\n",
               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);