/*
 * Manage cache of swap slots to be used for and returned from
 * swap.
 *
 * Copyright(c) 2016 Intel Corporation.
 *
 * Author: Tim Chen <tim.c.chen@linux.intel.com>
 *
 * We allocate the swap slots from the global pool and put
 * it into local per cpu caches.  This has the advantage
 * of no needing to acquire the swap_info lock every time
 * we need a new slot.
 *
 * There is also opportunity to simply return the slot
 * to local caches without needing to acquire swap_info
 * lock.  We do not reuse the returned slots directly but
 * move them back to the global pool in a batch.  This
 * allows the slots to coaellesce and reduce fragmentation.
 *
 * The swap entry allocated is marked with SWAP_HAS_CACHE
 * flag in map_count that prevents it from being allocated
 * again from the global pool.
 *
 * The swap slots cache is protected by a mutex instead of
 * a spin lock as when we search for slots with scan_swap_map,
 * we can possibly sleep.
 */

#include <linux/swap_slots.h>
#include <linux/cpu.h>
#include <linux/cpumask.h>
#include <linux/vmalloc.h>
#include <linux/mutex.h>

#ifdef CONFIG_SWAP

static DEFINE_PER_CPU(struct swap_slots_cache, swp_slots);
static bool     swap_slot_cache_active;
bool    swap_slot_cache_enabled;
static bool     swap_slot_cache_initialized;
DEFINE_MUTEX(swap_slots_cache_mutex);
/* Serialize swap slots cache enable/disable operations */
DEFINE_MUTEX(swap_slots_cache_enable_mutex);

static void __drain_swap_slots_cache(unsigned int type);
static void deactivate_swap_slots_cache(void);
static void reactivate_swap_slots_cache(void);

#define use_swap_slot_cache (swap_slot_cache_active && \
                swap_slot_cache_enabled && swap_slot_cache_initialized)
#define SLOTS_CACHE 0x1
#define SLOTS_CACHE_RET 0x2

static void deactivate_swap_slots_cache(void)
{
        mutex_lock(&swap_slots_cache_mutex);
        swap_slot_cache_active = false;
        __drain_swap_slots_cache(SLOTS_CACHE|SLOTS_CACHE_RET);
        mutex_unlock(&swap_slots_cache_mutex);
}

static void reactivate_swap_slots_cache(void)
{
        mutex_lock(&swap_slots_cache_mutex);
        swap_slot_cache_active = true;
        mutex_unlock(&swap_slots_cache_mutex);
}

/* Must not be called with cpu hot plug lock */
void disable_swap_slots_cache_lock(void)
{
        mutex_lock(&swap_slots_cache_enable_mutex);
        swap_slot_cache_enabled = false;
        if (swap_slot_cache_initialized) {
                /* serialize with cpu hotplug operations */
                get_online_cpus();
                __drain_swap_slots_cache(SLOTS_CACHE|SLOTS_CACHE_RET);
                put_online_cpus();
        }
}

static void __reenable_swap_slots_cache(void)
{
        swap_slot_cache_enabled = has_usable_swap();
}

void reenable_swap_slots_cache_unlock(void)
{
        __reenable_swap_slots_cache();
        mutex_unlock(&swap_slots_cache_enable_mutex);
}

static bool check_cache_active(void)
{
        long pages;

        if (!swap_slot_cache_enabled || !swap_slot_cache_initialized)
                return false;

        pages = get_nr_swap_pages();
        if (!swap_slot_cache_active) {
                if (pages > num_online_cpus() *
                    THRESHOLD_ACTIVATE_SWAP_SLOTS_CACHE)
                        reactivate_swap_slots_cache();
                goto out;
        }

        /* if global pool of slot caches too low, deactivate cache */
        if (pages < num_online_cpus() * THRESHOLD_DEACTIVATE_SWAP_SLOTS_CACHE)
                deactivate_swap_slots_cache();
out:
        return swap_slot_cache_active;
}

static int alloc_swap_slot_cache(unsigned int cpu)
{
        struct swap_slots_cache *cache;
        swp_entry_t *slots, *slots_ret;

        /*
         * Do allocation outside swap_slots_cache_mutex
         * as vzalloc could trigger reclaim and get_swap_page,
         * which can lock swap_slots_cache_mutex.
         */
        slots = vzalloc(sizeof(swp_entry_t) * SWAP_SLOTS_CACHE_SIZE);
        if (!slots)
                return -ENOMEM;

        slots_ret = vzalloc(sizeof(swp_entry_t) * SWAP_SLOTS_CACHE_SIZE);
        if (!slots_ret) {
                vfree(slots);
                return -ENOMEM;
        }

        mutex_lock(&swap_slots_cache_mutex);
        cache = &per_cpu(swp_slots, cpu);
        if (cache->slots || cache->slots_ret)
                /* cache already allocated */
                goto out;
        if (!cache->lock_initialized) {
                mutex_init(&cache->alloc_lock);
                spin_lock_init(&cache->free_lock);
                cache->lock_initialized = true;
        }
        cache->nr = 0;
        cache->cur = 0;
        cache->n_ret = 0;
        cache->slots = slots;
        slots = NULL;
        cache->slots_ret = slots_ret;
        slots_ret = NULL;
out:
        mutex_unlock(&swap_slots_cache_mutex);
        if (slots)
                vfree(slots);
        if (slots_ret)
                vfree(slots_ret);
        return 0;
}

static void drain_slots_cache_cpu(unsigned int cpu, unsigned int type,
                                  bool free_slots)
{
        struct swap_slots_cache *cache;
        swp_entry_t *slots = NULL;

        cache = &per_cpu(swp_slots, cpu);
        if ((type & SLOTS_CACHE) && cache->slots) {
                mutex_lock(&cache->alloc_lock);
                swapcache_free_entries(cache->slots + cache->cur, cache->nr);
                cache->cur = 0;
                cache->nr = 0;
                if (free_slots && cache->slots) {
                        vfree(cache->slots);
                        cache->slots = NULL;
                }
                mutex_unlock(&cache->alloc_lock);
        }
        if ((type & SLOTS_CACHE_RET) && cache->slots_ret) {
                spin_lock_irq(&cache->free_lock);
                swapcache_free_entries(cache->slots_ret, cache->n_ret);
                cache->n_ret = 0;
                if (free_slots && cache->slots_ret) {
                        slots = cache->slots_ret;
                        cache->slots_ret = NULL;
                }
                spin_unlock_irq(&cache->free_lock);
                if (slots)
                        vfree(slots);
        }
}

static void __drain_swap_slots_cache(unsigned int type)
{
        unsigned int cpu;

        /*
         * This function is called during
         *      1) swapoff, when we have to make sure no
         *         left over slots are in cache when we remove
         *         a swap device;
         *      2) disabling of swap slot cache, when we run low
         *         on swap slots when allocating memory and need
         *         to return swap slots to global pool.
         *
         * We cannot acquire cpu hot plug lock here as
         * this function can be invoked in the cpu
         * hot plug path:
         * cpu_up -> lock cpu_hotplug -> cpu hotplug state callback
         *   -> memory allocation -> direct reclaim -> get_swap_page
         *   -> drain_swap_slots_cache
         *
         * Hence the loop over current online cpu below could miss cpu that
         * is being brought online but not yet marked as online.
         * That is okay as we do not schedule and run anything on a
         * cpu before it has been marked online. Hence, we will not
         * fill any swap slots in slots cache of such cpu.
         * There are no slots on such cpu that need to be drained.
         */
        for_each_online_cpu(cpu)
                drain_slots_cache_cpu(cpu, type, false);
}

static int free_slot_cache(unsigned int cpu)
{
        mutex_lock(&swap_slots_cache_mutex);
        drain_slots_cache_cpu(cpu, SLOTS_CACHE | SLOTS_CACHE_RET, true);
        mutex_unlock(&swap_slots_cache_mutex);
        return 0;
}

int enable_swap_slots_cache(void)
{
        int ret = 0;

        mutex_lock(&swap_slots_cache_enable_mutex);
        if (swap_slot_cache_initialized) {
                __reenable_swap_slots_cache();
                goto out_unlock;
        }

        ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "swap_slots_cache",
                                alloc_swap_slot_cache, free_slot_cache);
        if (ret < 0)
                goto out_unlock;
        swap_slot_cache_initialized = true;
        __reenable_swap_slots_cache();
out_unlock:
        mutex_unlock(&swap_slots_cache_enable_mutex);
        return 0;
}

/* called with swap slot cache's alloc lock held */
static int refill_swap_slots_cache(struct swap_slots_cache *cache)
{
        if (!use_swap_slot_cache || cache->nr)
                return 0;

        cache->cur = 0;
        if (swap_slot_cache_active)
                cache->nr = get_swap_pages(SWAP_SLOTS_CACHE_SIZE, cache->slots);

        return cache->nr;
}

int free_swap_slot(swp_entry_t entry)
{
        struct swap_slots_cache *cache;

        cache = &get_cpu_var(swp_slots);
        if (use_swap_slot_cache && cache->slots_ret) {
                spin_lock_irq(&cache->free_lock);
                /* Swap slots cache may be deactivated before acquiring lock */
                if (!use_swap_slot_cache) {
                        spin_unlock_irq(&cache->free_lock);
                        goto direct_free;
                }
                if (cache->n_ret >= SWAP_SLOTS_CACHE_SIZE) {
                        /*
                         * Return slots to global pool.
                         * The current swap_map value is SWAP_HAS_CACHE.
                         * Set it to 0 to indicate it is available for
                         * allocation in global pool
                         */
                        swapcache_free_entries(cache->slots_ret, cache->n_ret);
                        cache->n_ret = 0;
                }
                cache->slots_ret[cache->n_ret++] = entry;
                spin_unlock_irq(&cache->free_lock);
        } else {
direct_free:
                swapcache_free_entries(&entry, 1);
        }
        put_cpu_var(swp_slots);

        return 0;
}

swp_entry_t get_swap_page(void)
{
        swp_entry_t entry, *pentry;
        struct swap_slots_cache *cache;

        /*
         * Preemption is allowed here, because we may sleep
         * in refill_swap_slots_cache().  But it is safe, because
         * accesses to the per-CPU data structure are protected by the
         * mutex cache->alloc_lock.
         *
         * The alloc path here does not touch cache->slots_ret
         * so cache->free_lock is not taken.
         */
        cache = raw_cpu_ptr(&swp_slots);

        entry.val = 0;
        if (check_cache_active()) {
                mutex_lock(&cache->alloc_lock);
                if (cache->slots) {
repeat:
                        if (cache->nr) {
                                pentry = &cache->slots[cache->cur++];
                                entry = *pentry;
                                pentry->val = 0;
                                cache->nr--;
                        } else {
                                if (refill_swap_slots_cache(cache))
                                        goto repeat;
                        }
                }
                mutex_unlock(&cache->alloc_lock);
                if (entry.val)
                        return entry;
        }

        get_swap_pages(1, &entry);

        return entry;
}

#endif /* CONFIG_SWAP */