dm cache policy mq: reduce memory requirements

[deliverable/linux.git] / drivers / md / dm-cache-policy-mq.c

/*
 * Copyright (C) 2012 Red Hat. All rights reserved.
 *
 * This file is released under the GPL.
 */

#include "dm-cache-policy.h"
#include "dm.h"

#include <linux/hash.h>
#include <linux/module.h>
#include <linux/mutex.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>

#define DM_MSG_PREFIX "cache-policy-mq"

static struct kmem_cache *mq_entry_cache;

/*----------------------------------------------------------------*/

static unsigned next_power(unsigned n, unsigned min)
{
        return roundup_pow_of_two(max(n, min));
}

/*----------------------------------------------------------------*/

/*
 * Large, sequential ios are probably better left on the origin device since
 * spindles tend to have good bandwidth.
 *
 * The io_tracker tries to spot when the io is in one of these sequential
 * modes.
 *
 * Two thresholds to switch between random and sequential io mode are defaulting
 * as follows and can be adjusted via the constructor and message interfaces.
 */
#define RANDOM_THRESHOLD_DEFAULT 4
#define SEQUENTIAL_THRESHOLD_DEFAULT 512

enum io_pattern {
        PATTERN_SEQUENTIAL,
        PATTERN_RANDOM
};

struct io_tracker {
        enum io_pattern pattern;

        unsigned nr_seq_samples;
        unsigned nr_rand_samples;
        unsigned thresholds[2];

        dm_oblock_t last_end_oblock;
};

static void iot_init(struct io_tracker *t,
                     int sequential_threshold, int random_threshold)
{
        t->pattern = PATTERN_RANDOM;
        t->nr_seq_samples = 0;
        t->nr_rand_samples = 0;
        t->last_end_oblock = 0;
        t->thresholds[PATTERN_RANDOM] = random_threshold;
        t->thresholds[PATTERN_SEQUENTIAL] = sequential_threshold;
}

static enum io_pattern iot_pattern(struct io_tracker *t)
{
        return t->pattern;
}

static void iot_update_stats(struct io_tracker *t, struct bio *bio)
{
        if (bio->bi_sector == from_oblock(t->last_end_oblock) + 1)
                t->nr_seq_samples++;
        else {
                /*
                 * Just one non-sequential IO is enough to reset the
                 * counters.
                 */
                if (t->nr_seq_samples) {
                        t->nr_seq_samples = 0;
                        t->nr_rand_samples = 0;
                }

                t->nr_rand_samples++;
        }

        t->last_end_oblock = to_oblock(bio->bi_sector + bio_sectors(bio) - 1);
}

static void iot_check_for_pattern_switch(struct io_tracker *t)
{
        switch (t->pattern) {
        case PATTERN_SEQUENTIAL:
                if (t->nr_rand_samples >= t->thresholds[PATTERN_RANDOM]) {
                        t->pattern = PATTERN_RANDOM;
                        t->nr_seq_samples = t->nr_rand_samples = 0;
                }
                break;

        case PATTERN_RANDOM:
                if (t->nr_seq_samples >= t->thresholds[PATTERN_SEQUENTIAL]) {
                        t->pattern = PATTERN_SEQUENTIAL;
                        t->nr_seq_samples = t->nr_rand_samples = 0;
                }
                break;
        }
}

static void iot_examine_bio(struct io_tracker *t, struct bio *bio)
{
        iot_update_stats(t, bio);
        iot_check_for_pattern_switch(t);
}

/*----------------------------------------------------------------*/


/*
 * This queue is divided up into different levels.  Allowing us to push
 * entries to the back of any of the levels.  Think of it as a partially
 * sorted queue.
 */
#define NR_QUEUE_LEVELS 16u

struct queue {
        struct list_head qs[NR_QUEUE_LEVELS];
};

static void queue_init(struct queue *q)
{
        unsigned i;

        for (i = 0; i < NR_QUEUE_LEVELS; i++)
                INIT_LIST_HEAD(q->qs + i);
}

/*
 * Checks to see if the queue is empty.
 * FIXME: reduce cpu usage.
 */
static bool queue_empty(struct queue *q)
{
        unsigned i;

        for (i = 0; i < NR_QUEUE_LEVELS; i++)
                if (!list_empty(q->qs + i))
                        return false;

        return true;
}

/*
 * Insert an entry to the back of the given level.
 */
static void queue_push(struct queue *q, unsigned level, struct list_head *elt)
{
        list_add_tail(elt, q->qs + level);
}

static void queue_remove(struct list_head *elt)
{
        list_del(elt);
}

/*
 * Shifts all regions down one level.  This has no effect on the order of
 * the queue.
 */
static void queue_shift_down(struct queue *q)
{
        unsigned level;

        for (level = 1; level < NR_QUEUE_LEVELS; level++)
                list_splice_init(q->qs + level, q->qs + level - 1);
}

/*
 * Gives us the oldest entry of the lowest popoulated level.  If the first
 * level is emptied then we shift down one level.
 */
static struct list_head *queue_pop(struct queue *q)
{
        unsigned level;
        struct list_head *r;

        for (level = 0; level < NR_QUEUE_LEVELS; level++)
                if (!list_empty(q->qs + level)) {
                        r = q->qs[level].next;
                        list_del(r);

                        /* have we just emptied the bottom level? */
                        if (level == 0 && list_empty(q->qs))
                                queue_shift_down(q);

                        return r;
                }

        return NULL;
}

static struct list_head *list_pop(struct list_head *lh)
{
        struct list_head *r = lh->next;

        BUG_ON(!r);
        list_del_init(r);

        return r;
}

/*----------------------------------------------------------------*/

/*
 * Describes a cache entry.  Used in both the cache and the pre_cache.
 */
struct entry {
        struct hlist_node hlist;
        struct list_head list;
        dm_oblock_t oblock;

        /*
         * FIXME: pack these better
         */
        bool dirty:1;
        unsigned hit_count;
        unsigned generation;
        unsigned tick;
};

/*
 * Rather than storing the cblock in an entry, we allocate all entries in
 * an array, and infer the cblock from the entry position.
 *
 * Free entries are linked together into a list.
 */
struct entry_pool {
        struct entry *entries, *entries_end;
        struct list_head free;
        unsigned nr_allocated;
};

static int epool_init(struct entry_pool *ep, unsigned nr_entries)
{
        unsigned i;

        ep->entries = vzalloc(sizeof(struct entry) * nr_entries);
        if (!ep->entries)
                return -ENOMEM;

        ep->entries_end = ep->entries + nr_entries;

        INIT_LIST_HEAD(&ep->free);
        for (i = 0; i < nr_entries; i++)
                list_add(&ep->entries[i].list, &ep->free);

        ep->nr_allocated = 0;

        return 0;
}

static void epool_exit(struct entry_pool *ep)
{
        vfree(ep->entries);
}

static struct entry *alloc_entry(struct entry_pool *ep)
{
        struct entry *e;

        if (list_empty(&ep->free))
                return NULL;

        e = list_entry(list_pop(&ep->free), struct entry, list);
        INIT_LIST_HEAD(&e->list);
        INIT_HLIST_NODE(&e->hlist);
        ep->nr_allocated++;

        return e;
}

/*
 * This assumes the cblock hasn't already been allocated.
 */
static struct entry *alloc_particular_entry(struct entry_pool *ep, dm_cblock_t cblock)
{
        struct entry *e = ep->entries + from_cblock(cblock);
        list_del(&e->list);

        INIT_LIST_HEAD(&e->list);
        INIT_HLIST_NODE(&e->hlist);
        ep->nr_allocated++;

        return e;
}

static void free_entry(struct entry_pool *ep, struct entry *e)
{
        BUG_ON(!ep->nr_allocated);
        ep->nr_allocated--;
        INIT_HLIST_NODE(&e->hlist);
        list_add(&e->list, &ep->free);
}

static bool epool_empty(struct entry_pool *ep)
{
        return list_empty(&ep->free);
}

static bool in_pool(struct entry_pool *ep, struct entry *e)
{
        return e >= ep->entries && e < ep->entries_end;
}

static dm_cblock_t infer_cblock(struct entry_pool *ep, struct entry *e)
{
        return to_cblock(e - ep->entries);
}

/*----------------------------------------------------------------*/

struct mq_policy {
        struct dm_cache_policy policy;

        /* protects everything */
        struct mutex lock;
        dm_cblock_t cache_size;
        struct io_tracker tracker;

        /*
         * Entries come from two pools, one of pre-cache entries, and one
         * for the cache proper.
         */
        struct entry_pool pre_cache_pool;
        struct entry_pool cache_pool;

        /*
         * We maintain three queues of entries.  The cache proper,
         * consisting of a clean and dirty queue, contains the currently
         * active mappings.  Whereas the pre_cache tracks blocks that
         * are being hit frequently and potential candidates for promotion
         * to the cache.
         */
        struct queue pre_cache;
        struct queue cache_clean;
        struct queue cache_dirty;

        /*
         * Keeps track of time, incremented by the core.  We use this to
         * avoid attributing multiple hits within the same tick.
         *
         * Access to tick_protected should be done with the spin lock held.
         * It's copied to tick at the start of the map function (within the
         * mutex).
         */
        spinlock_t tick_lock;
        unsigned tick_protected;
        unsigned tick;

        /*
         * A count of the number of times the map function has been called
         * and found an entry in the pre_cache or cache.  Currently used to
         * calculate the generation.
         */
        unsigned hit_count;

        /*
         * A generation is a longish period that is used to trigger some
         * book keeping effects.  eg, decrementing hit counts on entries.
         * This is needed to allow the cache to evolve as io patterns
         * change.
         */
        unsigned generation;
        unsigned generation_period; /* in lookups (will probably change) */

        /*
         * Entries in the pre_cache whose hit count passes the promotion
         * threshold move to the cache proper.  Working out the correct
         * value for the promotion_threshold is crucial to this policy.
         */
        unsigned promote_threshold;

        /*
         * The hash table allows us to quickly find an entry by origin
         * block.  Both pre_cache and cache entries are in here.
         */
        unsigned nr_buckets;
        dm_block_t hash_bits;
        struct hlist_head *table;
};

/*----------------------------------------------------------------*/

/*
 * Simple hash table implementation.  Should replace with the standard hash
 * table that's making its way upstream.
 */
static void hash_insert(struct mq_policy *mq, struct entry *e)
{
        unsigned h = hash_64(from_oblock(e->oblock), mq->hash_bits);

        hlist_add_head(&e->hlist, mq->table + h);
}

static struct entry *hash_lookup(struct mq_policy *mq, dm_oblock_t oblock)
{
        unsigned h = hash_64(from_oblock(oblock), mq->hash_bits);
        struct hlist_head *bucket = mq->table + h;
        struct entry *e;

        hlist_for_each_entry(e, bucket, hlist)
                if (e->oblock == oblock) {
                        hlist_del(&e->hlist);
                        hlist_add_head(&e->hlist, bucket);
                        return e;
                }

        return NULL;
}

static void hash_remove(struct entry *e)
{
        hlist_del(&e->hlist);
}

/*----------------------------------------------------------------*/

static bool any_free_cblocks(struct mq_policy *mq)
{
        return !epool_empty(&mq->cache_pool);
}

static bool any_clean_cblocks(struct mq_policy *mq)
{
        return !queue_empty(&mq->cache_clean);
}

/*----------------------------------------------------------------*/

/*
 * Now we get to the meat of the policy.  This section deals with deciding
 * when to to add entries to the pre_cache and cache, and move between
 * them.
 */

/*
 * The queue level is based on the log2 of the hit count.
 */
static unsigned queue_level(struct entry *e)
{
        return min((unsigned) ilog2(e->hit_count), NR_QUEUE_LEVELS - 1u);
}

static bool in_cache(struct mq_policy *mq, struct entry *e)
{
        return in_pool(&mq->cache_pool, e);
}

/*
 * Inserts the entry into the pre_cache or the cache.  Ensures the cache
 * block is marked as allocated if necc.  Inserts into the hash table.
 * Sets the tick which records when the entry was last moved about.
 */
static void push(struct mq_policy *mq, struct entry *e)
{
        e->tick = mq->tick;
        hash_insert(mq, e);

        if (in_cache(mq, e))
                queue_push(e->dirty ? &mq->cache_dirty : &mq->cache_clean,
                           queue_level(e), &e->list);
        else
                queue_push(&mq->pre_cache, queue_level(e), &e->list);
}

/*
 * Removes an entry from pre_cache or cache.  Removes from the hash table.
 */
static void del(struct mq_policy *mq, struct entry *e)
{
        queue_remove(&e->list);
        hash_remove(e);
}

/*
 * Like del, except it removes the first entry in the queue (ie. the least
 * recently used).
 */
static struct entry *pop(struct mq_policy *mq, struct queue *q)
{
        struct entry *e;
        struct list_head *h = queue_pop(q);

        if (!h)
                return NULL;

        e = container_of(h, struct entry, list);
        hash_remove(e);

        return e;
}

/*
 * Has this entry already been updated?
 */
static bool updated_this_tick(struct mq_policy *mq, struct entry *e)
{
        return mq->tick == e->tick;
}

/*
 * The promotion threshold is adjusted every generation.  As are the counts
 * of the entries.
 *
 * At the moment the threshold is taken by averaging the hit counts of some
 * of the entries in the cache (the first 20 entries across all levels in
 * ascending order, giving preference to the clean entries at each level).
 *
 * We can be much cleverer than this though.  For example, each promotion
 * could bump up the threshold helping to prevent churn.  Much more to do
 * here.
 */

#define MAX_TO_AVERAGE 20

static void check_generation(struct mq_policy *mq)
{
        unsigned total = 0, nr = 0, count = 0, level;
        struct list_head *head;
        struct entry *e;

        if ((mq->hit_count >= mq->generation_period) && (epool_empty(&mq->cache_pool))) {
                mq->hit_count = 0;
                mq->generation++;

                for (level = 0; level < NR_QUEUE_LEVELS && count < MAX_TO_AVERAGE; level++) {
                        head = mq->cache_clean.qs + level;
                        list_for_each_entry(e, head, list) {
                                nr++;
                                total += e->hit_count;

                                if (++count >= MAX_TO_AVERAGE)
                                        break;
                        }

                        head = mq->cache_dirty.qs + level;
                        list_for_each_entry(e, head, list) {
                                nr++;
                                total += e->hit_count;

                                if (++count >= MAX_TO_AVERAGE)
                                        break;
                        }
                }

                mq->promote_threshold = nr ? total / nr : 1;
                if (mq->promote_threshold * nr < total)
                        mq->promote_threshold++;
        }
}

/*
 * Whenever we use an entry we bump up it's hit counter, and push it to the
 * back to it's current level.
 */
static void requeue_and_update_tick(struct mq_policy *mq, struct entry *e)
{
        if (updated_this_tick(mq, e))
                return;

        e->hit_count++;
        mq->hit_count++;
        check_generation(mq);

        /* generation adjustment, to stop the counts increasing forever. */
        /* FIXME: divide? */
        /* e->hit_count -= min(e->hit_count - 1, mq->generation - e->generation); */
        e->generation = mq->generation;

        del(mq, e);
        push(mq, e);
}

/*
 * Demote the least recently used entry from the cache to the pre_cache.
 * Returns the new cache entry to use, and the old origin block it was
 * mapped to.
 *
 * We drop the hit count on the demoted entry back to 1 to stop it bouncing
 * straight back into the cache if it's subsequently hit.  There are
 * various options here, and more experimentation would be good:
 *
 * - just forget about the demoted entry completely (ie. don't insert it
     into the pre_cache).
 * - divide the hit count rather that setting to some hard coded value.
 * - set the hit count to a hard coded value other than 1, eg, is it better
 *   if it goes in at level 2?
 */
static int demote_cblock(struct mq_policy *mq, dm_oblock_t *oblock)
{
        struct entry *demoted = pop(mq, &mq->cache_clean);

        if (!demoted)
                /*
                 * We could get a block from mq->cache_dirty, but that
                 * would add extra latency to the triggering bio as it
                 * waits for the writeback.  Better to not promote this
                 * time and hope there's a clean block next time this block
                 * is hit.
                 */
                return -ENOSPC;

        *oblock = demoted->oblock;
        free_entry(&mq->cache_pool, demoted);

        /*
         * We used to put the demoted block into the pre-cache, but I think
         * it's simpler to just let it work it's way up from zero again.
         * Stops blocks flickering in and out of the cache.
         */

        return 0;
}

/*
 * We modify the basic promotion_threshold depending on the specific io.
 *
 * If the origin block has been discarded then there's no cost to copy it
 * to the cache.
 *
 * We bias towards reads, since they can be demoted at no cost if they
 * haven't been dirtied.
 */
#define DISCARDED_PROMOTE_THRESHOLD 1
#define READ_PROMOTE_THRESHOLD 4
#define WRITE_PROMOTE_THRESHOLD 8

static unsigned adjusted_promote_threshold(struct mq_policy *mq,
                                           bool discarded_oblock, int data_dir)
{
        if (data_dir == READ)
                return mq->promote_threshold + READ_PROMOTE_THRESHOLD;

        if (discarded_oblock && (any_free_cblocks(mq) || any_clean_cblocks(mq))) {
                /*
                 * We don't need to do any copying at all, so give this a
                 * very low threshold.
                 */
                return DISCARDED_PROMOTE_THRESHOLD;
        }

        return mq->promote_threshold + WRITE_PROMOTE_THRESHOLD;
}

static bool should_promote(struct mq_policy *mq, struct entry *e,
                           bool discarded_oblock, int data_dir)
{
        return e->hit_count >=
                adjusted_promote_threshold(mq, discarded_oblock, data_dir);
}

static int cache_entry_found(struct mq_policy *mq,
                             struct entry *e,
                             struct policy_result *result)
{
        requeue_and_update_tick(mq, e);

        if (in_cache(mq, e)) {
                result->op = POLICY_HIT;
                result->cblock = infer_cblock(&mq->cache_pool, e);
        }

        return 0;
}

/*
 * Moves an entry from the pre_cache to the cache.  The main work is
 * finding which cache block to use.
 */
static int pre_cache_to_cache(struct mq_policy *mq, struct entry *e,
                              struct policy_result *result)
{
        int r;
        struct entry *new_e;

        /* Ensure there's a free cblock in the cache */
        if (epool_empty(&mq->cache_pool)) {
                result->op = POLICY_REPLACE;
                r = demote_cblock(mq, &result->old_oblock);
                if (r) {
                        result->op = POLICY_MISS;
                        return 0;
                }
        } else
                result->op = POLICY_NEW;

        new_e = alloc_entry(&mq->cache_pool);
        BUG_ON(!new_e);

        new_e->oblock = e->oblock;
        new_e->dirty = false;
        new_e->hit_count = e->hit_count;
        new_e->generation = e->generation;
        new_e->tick = e->tick;

        del(mq, e);
        free_entry(&mq->pre_cache_pool, e);
        push(mq, new_e);

        result->cblock = infer_cblock(&mq->cache_pool, new_e);

        return 0;
}

static int pre_cache_entry_found(struct mq_policy *mq, struct entry *e,
                                 bool can_migrate, bool discarded_oblock,
                                 int data_dir, struct policy_result *result)
{
        int r = 0;
        bool updated = updated_this_tick(mq, e);

        requeue_and_update_tick(mq, e);

        if ((!discarded_oblock && updated) ||
            !should_promote(mq, e, discarded_oblock, data_dir))
                result->op = POLICY_MISS;
        else if (!can_migrate)
                r = -EWOULDBLOCK;
        else
                r = pre_cache_to_cache(mq, e, result);

        return r;
}

static void insert_in_pre_cache(struct mq_policy *mq,
                                dm_oblock_t oblock)
{
        struct entry *e = alloc_entry(&mq->pre_cache_pool);

        if (!e)
                /*
                 * There's no spare entry structure, so we grab the least
                 * used one from the pre_cache.
                 */
                e = pop(mq, &mq->pre_cache);

        if (unlikely(!e)) {
                DMWARN("couldn't pop from pre cache");
                return;
        }

        e->dirty = false;
        e->oblock = oblock;
        e->hit_count = 1;
        e->generation = mq->generation;
        push(mq, e);
}

static void insert_in_cache(struct mq_policy *mq, dm_oblock_t oblock,
                            struct policy_result *result)
{
        int r;
        struct entry *e;

        if (epool_empty(&mq->cache_pool)) {
                result->op = POLICY_REPLACE;
                r = demote_cblock(mq, &result->old_oblock);
                if (unlikely(r)) {
                        result->op = POLICY_MISS;
                        insert_in_pre_cache(mq, oblock);
                        return;
                }

                /*
                 * This will always succeed, since we've just demoted.
                 */
                e = alloc_entry(&mq->cache_pool);
                BUG_ON(!e);

        } else {
                e = alloc_entry(&mq->cache_pool);
                result->op = POLICY_NEW;
        }

        e->oblock = oblock;
        e->dirty = false;
        e->hit_count = 1;
        e->generation = mq->generation;
        push(mq, e);

        result->cblock = infer_cblock(&mq->cache_pool, e);
}

static int no_entry_found(struct mq_policy *mq, dm_oblock_t oblock,
                          bool can_migrate, bool discarded_oblock,
                          int data_dir, struct policy_result *result)
{
        if (adjusted_promote_threshold(mq, discarded_oblock, data_dir) == 1) {
                if (can_migrate)
                        insert_in_cache(mq, oblock, result);
                else
                        return -EWOULDBLOCK;
        } else {
                insert_in_pre_cache(mq, oblock);
                result->op = POLICY_MISS;
        }

        return 0;
}

/*
 * Looks the oblock up in the hash table, then decides whether to put in
 * pre_cache, or cache etc.
 */
static int map(struct mq_policy *mq, dm_oblock_t oblock,
               bool can_migrate, bool discarded_oblock,
               int data_dir, struct policy_result *result)
{
        int r = 0;
        struct entry *e = hash_lookup(mq, oblock);

        if (e && in_cache(mq, e))
                r = cache_entry_found(mq, e, result);

        else if (iot_pattern(&mq->tracker) == PATTERN_SEQUENTIAL)
                result->op = POLICY_MISS;

        else if (e)
                r = pre_cache_entry_found(mq, e, can_migrate, discarded_oblock,
                                          data_dir, result);

        else
                r = no_entry_found(mq, oblock, can_migrate, discarded_oblock,
                                   data_dir, result);

        if (r == -EWOULDBLOCK)
                result->op = POLICY_MISS;

        return r;
}

/*----------------------------------------------------------------*/

/*
 * Public interface, via the policy struct.  See dm-cache-policy.h for a
 * description of these.
 */

static struct mq_policy *to_mq_policy(struct dm_cache_policy *p)
{
        return container_of(p, struct mq_policy, policy);
}

static void mq_destroy(struct dm_cache_policy *p)
{
        struct mq_policy *mq = to_mq_policy(p);

        kfree(mq->table);
        epool_exit(&mq->cache_pool);
        epool_exit(&mq->pre_cache_pool);
        kfree(mq);
}

static void copy_tick(struct mq_policy *mq)
{
        unsigned long flags;

        spin_lock_irqsave(&mq->tick_lock, flags);
        mq->tick = mq->tick_protected;
        spin_unlock_irqrestore(&mq->tick_lock, flags);
}

static int mq_map(struct dm_cache_policy *p, dm_oblock_t oblock,
                  bool can_block, bool can_migrate, bool discarded_oblock,
                  struct bio *bio, struct policy_result *result)
{
        int r;
        struct mq_policy *mq = to_mq_policy(p);

        result->op = POLICY_MISS;

        if (can_block)
                mutex_lock(&mq->lock);
        else if (!mutex_trylock(&mq->lock))
                return -EWOULDBLOCK;

        copy_tick(mq);

        iot_examine_bio(&mq->tracker, bio);
        r = map(mq, oblock, can_migrate, discarded_oblock,
                bio_data_dir(bio), result);

        mutex_unlock(&mq->lock);

        return r;
}

static int mq_lookup(struct dm_cache_policy *p, dm_oblock_t oblock, dm_cblock_t *cblock)
{
        int r;
        struct mq_policy *mq = to_mq_policy(p);
        struct entry *e;

        if (!mutex_trylock(&mq->lock))
                return -EWOULDBLOCK;

        e = hash_lookup(mq, oblock);
        if (e && in_cache(mq, e)) {
                *cblock = infer_cblock(&mq->cache_pool, e);
                r = 0;
        } else
                r = -ENOENT;

        mutex_unlock(&mq->lock);

        return r;
}

static void __mq_set_clear_dirty(struct mq_policy *mq, dm_oblock_t oblock, bool set)
{
        struct entry *e;

        e = hash_lookup(mq, oblock);
        BUG_ON(!e || !in_cache(mq, e));

        del(mq, e);
        e->dirty = set;
        push(mq, e);
}

static void mq_set_dirty(struct dm_cache_policy *p, dm_oblock_t oblock)
{
        struct mq_policy *mq = to_mq_policy(p);

        mutex_lock(&mq->lock);
        __mq_set_clear_dirty(mq, oblock, true);
        mutex_unlock(&mq->lock);
}

static void mq_clear_dirty(struct dm_cache_policy *p, dm_oblock_t oblock)
{
        struct mq_policy *mq = to_mq_policy(p);

        mutex_lock(&mq->lock);
        __mq_set_clear_dirty(mq, oblock, false);
        mutex_unlock(&mq->lock);
}

static int mq_load_mapping(struct dm_cache_policy *p,
                           dm_oblock_t oblock, dm_cblock_t cblock,
                           uint32_t hint, bool hint_valid)
{
        struct mq_policy *mq = to_mq_policy(p);
        struct entry *e;

        e = alloc_particular_entry(&mq->cache_pool, cblock);
        e->oblock = oblock;
        e->dirty = false;       /* this gets corrected in a minute */
        e->hit_count = hint_valid ? hint : 1;
        e->generation = mq->generation;
        push(mq, e);

        return 0;
}

static int mq_save_hints(struct mq_policy *mq, struct queue *q,
                         policy_walk_fn fn, void *context)
{
        int r;
        unsigned level;
        struct entry *e;

        for (level = 0; level < NR_QUEUE_LEVELS; level++)
                list_for_each_entry(e, q->qs + level, list) {
                        r = fn(context, infer_cblock(&mq->cache_pool, e),
                               e->oblock, e->hit_count);
                        if (r)
                                return r;
                }

        return 0;
}

static int mq_walk_mappings(struct dm_cache_policy *p, policy_walk_fn fn,
                            void *context)
{
        struct mq_policy *mq = to_mq_policy(p);
        int r = 0;

        mutex_lock(&mq->lock);

        r = mq_save_hints(mq, &mq->cache_clean, fn, context);
        if (!r)
                r = mq_save_hints(mq, &mq->cache_dirty, fn, context);

        mutex_unlock(&mq->lock);

        return r;
}

static void __remove_mapping(struct mq_policy *mq, dm_oblock_t oblock)
{
        struct entry *e;

        e = hash_lookup(mq, oblock);
        BUG_ON(!e || !in_cache(mq, e));

        del(mq, e);
        free_entry(&mq->cache_pool, e);
}

static void mq_remove_mapping(struct dm_cache_policy *p, dm_oblock_t oblock)
{
        struct mq_policy *mq = to_mq_policy(p);

        mutex_lock(&mq->lock);
        __remove_mapping(mq, oblock);
        mutex_unlock(&mq->lock);
}

static int __mq_writeback_work(struct mq_policy *mq, dm_oblock_t *oblock,
                              dm_cblock_t *cblock)
{
        struct entry *e = pop(mq, &mq->cache_dirty);

        if (!e)
                return -ENODATA;

        *oblock = e->oblock;
        *cblock = infer_cblock(&mq->cache_pool, e);
        e->dirty = false;
        push(mq, e);

        return 0;
}

static int mq_writeback_work(struct dm_cache_policy *p, dm_oblock_t *oblock,
                             dm_cblock_t *cblock)
{
        int r;
        struct mq_policy *mq = to_mq_policy(p);

        mutex_lock(&mq->lock);
        r = __mq_writeback_work(mq, oblock, cblock);
        mutex_unlock(&mq->lock);

        return r;
}

static void __force_mapping(struct mq_policy *mq,
                            dm_oblock_t current_oblock, dm_oblock_t new_oblock)
{
        struct entry *e = hash_lookup(mq, current_oblock);

        if (e && in_cache(mq, e)) {
                del(mq, e);
                e->oblock = new_oblock;
                e->dirty = true;
                push(mq, e);
        }
}

static void mq_force_mapping(struct dm_cache_policy *p,
                             dm_oblock_t current_oblock, dm_oblock_t new_oblock)
{
        struct mq_policy *mq = to_mq_policy(p);

        mutex_lock(&mq->lock);
        __force_mapping(mq, current_oblock, new_oblock);
        mutex_unlock(&mq->lock);
}

static dm_cblock_t mq_residency(struct dm_cache_policy *p)
{
        dm_cblock_t r;
        struct mq_policy *mq = to_mq_policy(p);

        mutex_lock(&mq->lock);
        r = to_cblock(mq->cache_pool.nr_allocated);
        mutex_unlock(&mq->lock);

        return r;
}

static void mq_tick(struct dm_cache_policy *p)
{
        struct mq_policy *mq = to_mq_policy(p);
        unsigned long flags;

        spin_lock_irqsave(&mq->tick_lock, flags);
        mq->tick_protected++;
        spin_unlock_irqrestore(&mq->tick_lock, flags);
}

static int mq_set_config_value(struct dm_cache_policy *p,
                               const char *key, const char *value)
{
        struct mq_policy *mq = to_mq_policy(p);
        enum io_pattern pattern;
        unsigned long tmp;

        if (!strcasecmp(key, "random_threshold"))
                pattern = PATTERN_RANDOM;
        else if (!strcasecmp(key, "sequential_threshold"))
                pattern = PATTERN_SEQUENTIAL;
        else
                return -EINVAL;

        if (kstrtoul(value, 10, &tmp))
                return -EINVAL;

        mq->tracker.thresholds[pattern] = tmp;

        return 0;
}

static int mq_emit_config_values(struct dm_cache_policy *p, char *result, unsigned maxlen)
{
        ssize_t sz = 0;
        struct mq_policy *mq = to_mq_policy(p);

        DMEMIT("4 random_threshold %u sequential_threshold %u",
               mq->tracker.thresholds[PATTERN_RANDOM],
               mq->tracker.thresholds[PATTERN_SEQUENTIAL]);

        return 0;
}

/* Init the policy plugin interface function pointers. */
static void init_policy_functions(struct mq_policy *mq)
{
        mq->policy.destroy = mq_destroy;
        mq->policy.map = mq_map;
        mq->policy.lookup = mq_lookup;
        mq->policy.set_dirty = mq_set_dirty;
        mq->policy.clear_dirty = mq_clear_dirty;
        mq->policy.load_mapping = mq_load_mapping;
        mq->policy.walk_mappings = mq_walk_mappings;
        mq->policy.remove_mapping = mq_remove_mapping;
        mq->policy.writeback_work = mq_writeback_work;
        mq->policy.force_mapping = mq_force_mapping;
        mq->policy.residency = mq_residency;
        mq->policy.tick = mq_tick;
        mq->policy.emit_config_values = mq_emit_config_values;
        mq->policy.set_config_value = mq_set_config_value;
}

static struct dm_cache_policy *mq_create(dm_cblock_t cache_size,
                                         sector_t origin_size,
                                         sector_t cache_block_size)
{
        struct mq_policy *mq = kzalloc(sizeof(*mq), GFP_KERNEL);

        if (!mq)
                return NULL;

        init_policy_functions(mq);
        iot_init(&mq->tracker, SEQUENTIAL_THRESHOLD_DEFAULT, RANDOM_THRESHOLD_DEFAULT);
        mq->cache_size = cache_size;

        if (epool_init(&mq->pre_cache_pool, from_cblock(cache_size))) {
                DMERR("couldn't initialize pool of pre-cache entries");
                goto bad_pre_cache_init;
        }

        if (epool_init(&mq->cache_pool, from_cblock(cache_size))) {
                DMERR("couldn't initialize pool of cache entries");
                goto bad_cache_init;
        }

        mq->tick_protected = 0;
        mq->tick = 0;
        mq->hit_count = 0;
        mq->generation = 0;
        mq->promote_threshold = 0;
        mutex_init(&mq->lock);
        spin_lock_init(&mq->tick_lock);

        queue_init(&mq->pre_cache);
        queue_init(&mq->cache_clean);
        queue_init(&mq->cache_dirty);

        mq->generation_period = max((unsigned) from_cblock(cache_size), 1024U);

        mq->nr_buckets = next_power(from_cblock(cache_size) / 2, 16);
        mq->hash_bits = ffs(mq->nr_buckets) - 1;
        mq->table = kzalloc(sizeof(*mq->table) * mq->nr_buckets, GFP_KERNEL);
        if (!mq->table)
                goto bad_alloc_table;

        return &mq->policy;

bad_alloc_table:
        epool_exit(&mq->cache_pool);
bad_cache_init:
        epool_exit(&mq->pre_cache_pool);
bad_pre_cache_init:
        kfree(mq);

        return NULL;
}

/*----------------------------------------------------------------*/

static struct dm_cache_policy_type mq_policy_type = {
        .name = "mq",
        .version = {1, 1, 0},
        .hint_size = 4,
        .owner = THIS_MODULE,
        .create = mq_create
};

static struct dm_cache_policy_type default_policy_type = {
        .name = "default",
        .version = {1, 1, 0},
        .hint_size = 4,
        .owner = THIS_MODULE,
        .create = mq_create
};

static int __init mq_init(void)
{
        int r;

        mq_entry_cache = kmem_cache_create("dm_mq_policy_cache_entry",
                                           sizeof(struct entry),
                                           __alignof__(struct entry),
                                           0, NULL);
        if (!mq_entry_cache)
                goto bad;

        r = dm_cache_policy_register(&mq_policy_type);
        if (r) {
                DMERR("register failed %d", r);
                goto bad_register_mq;
        }

        r = dm_cache_policy_register(&default_policy_type);
        if (!r) {
                DMINFO("version %u.%u.%u loaded",
                       mq_policy_type.version[0],
                       mq_policy_type.version[1],
                       mq_policy_type.version[2]);
                return 0;
        }

        DMERR("register failed (as default) %d", r);

        dm_cache_policy_unregister(&mq_policy_type);
bad_register_mq:
        kmem_cache_destroy(mq_entry_cache);
bad:
        return -ENOMEM;
}

static void __exit mq_exit(void)
{
        dm_cache_policy_unregister(&mq_policy_type);
        dm_cache_policy_unregister(&default_policy_type);

        kmem_cache_destroy(mq_entry_cache);
}

module_init(mq_init);
module_exit(mq_exit);

MODULE_AUTHOR("Joe Thornber <dm-devel@redhat.com>");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("mq cache policy");

MODULE_ALIAS("dm-cache-default");