Release 4.11 mm/swap_slots.c

/*
 * 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);
}

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

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/* 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();
	}
}

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

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

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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;
}

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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;
}

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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);
	}
}

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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);
}

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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;
}

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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;
}

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/* 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;
}

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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;
}

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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;
}

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#endif /* CONFIG_SWAP */
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