// SPDX-License-Identifier: GPL-2.0
/*
 * background writeback - scan btree for dirty data and write it to the backing
 * device
 *
 * Copyright 2010, 2011 Kent Overstreet <kent.overstreet@gmail.com>
 * Copyright 2012 Google, Inc.
 */

#include "bcache.h"
#include "btree.h"
#include "debug.h"
#include "writeback.h"

#include <linux/delay.h>
#include <linux/kthread.h>
#include <linux/sched/clock.h>
#include <trace/events/bcache.h>

static void update_gc_after_writeback(struct cache_set *c)
{
        if (c->gc_after_writeback != (BCH_ENABLE_AUTO_GC) ||
            c->gc_stats.in_use < BCH_AUTO_GC_DIRTY_THRESHOLD)
                return;

        c->gc_after_writeback |= BCH_DO_AUTO_GC;
}

/* Rate limiting */
static uint64_t __calc_target_rate(struct cached_dev *dc)
{
        struct cache_set *c = dc->disk.c;

        /*
         * This is the size of the cache, minus the amount used for
         * flash-only devices
         */
        uint64_t cache_sectors = c->nbuckets * c->cache->sb.bucket_size -
                                atomic_long_read(&c->flash_dev_dirty_sectors);

        /*
         * Unfortunately there is no control of global dirty data.  If the
         * user states that they want 10% dirty data in the cache, and has,
         * e.g., 5 backing volumes of equal size, we try and ensure each
         * backing volume uses about 2% of the cache for dirty data.
         */
        uint32_t bdev_share =
                div64_u64(bdev_nr_sectors(dc->bdev) << WRITEBACK_SHARE_SHIFT,
                                c->cached_dev_sectors);

        uint64_t cache_dirty_target =
                div_u64(cache_sectors * dc->writeback_percent, 100);

        /* Ensure each backing dev gets at least one dirty share */
        if (bdev_share < 1)
                bdev_share = 1;

        return (cache_dirty_target * bdev_share) >> WRITEBACK_SHARE_SHIFT;
}

static void __update_writeback_rate(struct cached_dev *dc)
{
        /*
         * PI controller:
         * Figures out the amount that should be written per second.
         *
         * First, the error (number of sectors that are dirty beyond our
         * target) is calculated.  The error is accumulated (numerically
         * integrated).
         *
         * Then, the proportional value and integral value are scaled
         * based on configured values.  These are stored as inverses to
         * avoid fixed point math and to make configuration easy-- e.g.
         * the default value of 40 for writeback_rate_p_term_inverse
         * attempts to write at a rate that would retire all the dirty
         * blocks in 40 seconds.
         *
         * The writeback_rate_i_inverse value of 10000 means that 1/10000th
         * of the error is accumulated in the integral term per second.
         * This acts as a slow, long-term average that is not subject to
         * variations in usage like the p term.
         */
        int64_t target = __calc_target_rate(dc);
        int64_t dirty = bcache_dev_sectors_dirty(&dc->disk);
        int64_t error = dirty - target;
        int64_t proportional_scaled =
                div_s64(error, dc->writeback_rate_p_term_inverse);
        int64_t integral_scaled;
        uint32_t new_rate;

        /*
         * We need to consider the number of dirty buckets as well
         * when calculating the proportional_scaled, Otherwise we might
         * have an unreasonable small writeback rate at a highly fragmented situation
         * when very few dirty sectors consumed a lot dirty buckets, the
         * worst case is when dirty buckets reached cutoff_writeback_sync and
         * dirty data is still not even reached to writeback percent, so the rate
         * still will be at the minimum value, which will cause the write
         * stuck at a non-writeback mode.
         */
        struct cache_set *c = dc->disk.c;

        int64_t dirty_buckets = c->nbuckets - c->avail_nbuckets;

        if (dc->writeback_consider_fragment &&
                c->gc_stats.in_use > BCH_WRITEBACK_FRAGMENT_THRESHOLD_LOW && dirty > 0) {
                int64_t fragment =
                        div_s64((dirty_buckets *  c->cache->sb.bucket_size), dirty);
                int64_t fp_term;
                int64_t fps;

                if (c->gc_stats.in_use <= BCH_WRITEBACK_FRAGMENT_THRESHOLD_MID) {
                        fp_term = (int64_t)dc->writeback_rate_fp_term_low *
                        (c->gc_stats.in_use - BCH_WRITEBACK_FRAGMENT_THRESHOLD_LOW);
                } else if (c->gc_stats.in_use <= BCH_WRITEBACK_FRAGMENT_THRESHOLD_HIGH) {
                        fp_term = (int64_t)dc->writeback_rate_fp_term_mid *
                        (c->gc_stats.in_use - BCH_WRITEBACK_FRAGMENT_THRESHOLD_MID);
                } else {
                        fp_term = (int64_t)dc->writeback_rate_fp_term_high *
                        (c->gc_stats.in_use - BCH_WRITEBACK_FRAGMENT_THRESHOLD_HIGH);
                }
                fps = div_s64(dirty, dirty_buckets) * fp_term;
                if (fragment > 3 && fps > proportional_scaled) {
                        /* Only overrite the p when fragment > 3 */
                        proportional_scaled = fps;
                }
        }

        if ((error < 0 && dc->writeback_rate_integral > 0) ||
            (error > 0 && time_before64(local_clock(),
                         dc->writeback_rate.next + NSEC_PER_MSEC))) {
                /*
                 * Only decrease the integral term if it's more than
                 * zero.  Only increase the integral term if the device
                 * is keeping up.  (Don't wind up the integral
                 * ineffectively in either case).
                 *
                 * It's necessary to scale this by
                 * writeback_rate_update_seconds to keep the integral
                 * term dimensioned properly.
                 */
                dc->writeback_rate_integral += error *
                        dc->writeback_rate_update_seconds;
        }

        integral_scaled = div_s64(dc->writeback_rate_integral,
                        dc->writeback_rate_i_term_inverse);

        new_rate = clamp_t(int32_t, (proportional_scaled + integral_scaled),
                        dc->writeback_rate_minimum, NSEC_PER_SEC);

        dc->writeback_rate_proportional = proportional_scaled;
        dc->writeback_rate_integral_scaled = integral_scaled;
        dc->writeback_rate_change = new_rate -
                        atomic_long_read(&dc->writeback_rate.rate);
        atomic_long_set(&dc->writeback_rate.rate, new_rate);
        dc->writeback_rate_target = target;
}

static bool idle_counter_exceeded(struct cache_set *c)
{
        int counter, dev_nr;

        /*
         * If c->idle_counter is overflow (idel for really long time),
         * reset as 0 and not set maximum rate this time for code
         * simplicity.
         */
        counter = atomic_inc_return(&c->idle_counter);
        if (counter <= 0) {
                atomic_set(&c->idle_counter, 0);
                return false;
        }

        dev_nr = atomic_read(&c->attached_dev_nr);
        if (dev_nr == 0)
                return false;

        /*
         * c->idle_counter is increased by writeback thread of all
         * attached backing devices, in order to represent a rough
         * time period, counter should be divided by dev_nr.
         * Otherwise the idle time cannot be larger with more backing
         * device attached.
         * The following calculation equals to checking
         *      (counter / dev_nr) < (dev_nr * 6)
         */
        if (counter < (dev_nr * dev_nr * 6))
                return false;

        return true;
}

/*
 * Idle_counter is increased every time when update_writeback_rate() is
 * called. If all backing devices attached to the same cache set have
 * identical dc->writeback_rate_update_seconds values, it is about 6
 * rounds of update_writeback_rate() on each backing device before
 * c->at_max_writeback_rate is set to 1, and then max wrteback rate set
 * to each dc->writeback_rate.rate.
 * In order to avoid extra locking cost for counting exact dirty cached
 * devices number, c->attached_dev_nr is used to calculate the idle
 * throushold. It might be bigger if not all cached device are in write-
 * back mode, but it still works well with limited extra rounds of
 * update_writeback_rate().
 */
static bool set_at_max_writeback_rate(struct cache_set *c,
                                       struct cached_dev *dc)
{
        /* Don't sst max writeback rate if it is disabled */
        if (!c->idle_max_writeback_rate_enabled)
                return false;

        /* Don't set max writeback rate if gc is running */
        if (!c->gc_mark_valid)
                return false;

        if (!idle_counter_exceeded(c))
                return false;

        if (atomic_read(&c->at_max_writeback_rate) != 1)
                atomic_set(&c->at_max_writeback_rate, 1);

        atomic_long_set(&dc->writeback_rate.rate, INT_MAX);

        /* keep writeback_rate_target as existing value */
        dc->writeback_rate_proportional = 0;
        dc->writeback_rate_integral_scaled = 0;
        dc->writeback_rate_change = 0;

        /*
         * In case new I/O arrives during before
         * set_at_max_writeback_rate() returns.
         */
        if (!idle_counter_exceeded(c) ||
            !atomic_read(&c->at_max_writeback_rate))
                return false;

        return true;
}

static void update_writeback_rate(struct work_struct *work)
{
        struct cached_dev *dc = container_of(to_delayed_work(work),
                                             struct cached_dev,
                                             writeback_rate_update);
        struct cache_set *c = dc->disk.c;

        /*
         * should check BCACHE_DEV_RATE_DW_RUNNING before calling
         * cancel_delayed_work_sync().
         */
        set_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
        /* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
        smp_mb__after_atomic();

        /*
         * CACHE_SET_IO_DISABLE might be set via sysfs interface,
         * check it here too.
         */
        if (!test_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags) ||
            test_bit(CACHE_SET_IO_DISABLE, &c->flags)) {
                clear_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
                /* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
                smp_mb__after_atomic();
                return;
        }

        /*
         * If the whole cache set is idle, set_at_max_writeback_rate()
         * will set writeback rate to a max number. Then it is
         * unncessary to update writeback rate for an idle cache set
         * in maximum writeback rate number(s).
         */
        if (atomic_read(&dc->has_dirty) && dc->writeback_percent &&
            !set_at_max_writeback_rate(c, dc)) {
                do {
                        if (!down_read_trylock((&dc->writeback_lock))) {
                                dc->rate_update_retry++;
                                if (dc->rate_update_retry <=
                                    BCH_WBRATE_UPDATE_MAX_SKIPS)
                                        break;
                                down_read(&dc->writeback_lock);
                                dc->rate_update_retry = 0;
                        }
                        __update_writeback_rate(dc);
                        update_gc_after_writeback(c);
                        up_read(&dc->writeback_lock);
                } while (0);
        }


        /*
         * CACHE_SET_IO_DISABLE might be set via sysfs interface,
         * check it here too.
         */
        if (test_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags) &&
            !test_bit(CACHE_SET_IO_DISABLE, &c->flags)) {
                schedule_delayed_work(&dc->writeback_rate_update,
                              dc->writeback_rate_update_seconds * HZ);
        }

        /*
         * should check BCACHE_DEV_RATE_DW_RUNNING before calling
         * cancel_delayed_work_sync().
         */
        clear_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
        /* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
        smp_mb__after_atomic();
}

static unsigned int writeback_delay(struct cached_dev *dc,
                                    unsigned int sectors)
{
        if (test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags) ||
            !dc->writeback_percent)
                return 0;

        return bch_next_delay(&dc->writeback_rate, sectors);
}

struct dirty_io {
        struct closure          cl;
        struct cached_dev       *dc;
        uint16_t                sequence;
        struct bio              bio;
};

static void dirty_init(struct keybuf_key *w)
{
        struct dirty_io *io = w->private;
        struct bio *bio = &io->bio;

        bio_init_inline(bio, NULL,
                 DIV_ROUND_UP(KEY_SIZE(&w->key), PAGE_SECTORS), 0);
        if (!io->dc->writeback_percent)
                bio->bi_ioprio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_IDLE, 0);

        bio->bi_iter.bi_size    = KEY_SIZE(&w->key) << 9;
        bio->bi_private         = w;
        bch_bio_map(bio, NULL);
}

static CLOSURE_CALLBACK(dirty_io_destructor)
{
        closure_type(io, struct dirty_io, cl);

        kfree(io);
}

static CLOSURE_CALLBACK(write_dirty_finish)
{
        closure_type(io, struct dirty_io, cl);
        struct keybuf_key *w = io->bio.bi_private;
        struct cached_dev *dc = io->dc;

        bio_free_pages(&io->bio);

        /* This is kind of a dumb way of signalling errors. */
        if (KEY_DIRTY(&w->key)) {
                int ret;
                unsigned int i;
                struct keylist keys;

                bch_keylist_init(&keys);

                bkey_copy(keys.top, &w->key);
                SET_KEY_DIRTY(keys.top, false);
                bch_keylist_push(&keys);

                for (i = 0; i < KEY_PTRS(&w->key); i++)
                        atomic_inc(&PTR_BUCKET(dc->disk.c, &w->key, i)->pin);

                ret = bch_btree_insert(dc->disk.c, &keys, NULL, &w->key);

                if (ret)
                        trace_bcache_writeback_collision(&w->key);

                atomic_long_inc(ret
                                ? &dc->disk.c->writeback_keys_failed
                                : &dc->disk.c->writeback_keys_done);
        }

        bch_keybuf_del(&dc->writeback_keys, w);
        up(&dc->in_flight);

        closure_return_with_destructor(cl, dirty_io_destructor);
}

static void dirty_endio(struct bio *bio)
{
        struct keybuf_key *w = bio->bi_private;
        struct dirty_io *io = w->private;

        if (bio->bi_status) {
                SET_KEY_DIRTY(&w->key, false);
                bch_count_backing_io_errors(io->dc, bio);
        }

        closure_put(&io->cl);
}

static CLOSURE_CALLBACK(write_dirty)
{
        closure_type(io, struct dirty_io, cl);
        struct keybuf_key *w = io->bio.bi_private;
        struct cached_dev *dc = io->dc;

        uint16_t next_sequence;

        if (atomic_read(&dc->writeback_sequence_next) != io->sequence) {
                /* Not our turn to write; wait for a write to complete */
                closure_wait(&dc->writeback_ordering_wait, cl);

                if (atomic_read(&dc->writeback_sequence_next) == io->sequence) {
                        /*
                         * Edge case-- it happened in indeterminate order
                         * relative to when we were added to wait list..
                         */
                        closure_wake_up(&dc->writeback_ordering_wait);
                }

                continue_at(cl, write_dirty, io->dc->writeback_write_wq);
                return;
        }

        next_sequence = io->sequence + 1;

        /*
         * IO errors are signalled using the dirty bit on the key.
         * If we failed to read, we should not attempt to write to the
         * backing device.  Instead, immediately go to write_dirty_finish
         * to clean up.
         */
        if (KEY_DIRTY(&w->key)) {
                dirty_init(w);
                io->bio.bi_opf = REQ_OP_WRITE;
                io->bio.bi_iter.bi_sector = KEY_START(&w->key);
                bio_set_dev(&io->bio, io->dc->bdev);
                io->bio.bi_end_io       = dirty_endio;

                /* I/O request sent to backing device */
                closure_bio_submit(io->dc->disk.c, &io->bio, cl);
        }

        atomic_set(&dc->writeback_sequence_next, next_sequence);
        closure_wake_up(&dc->writeback_ordering_wait);

        continue_at(cl, write_dirty_finish, io->dc->writeback_write_wq);
}

static void read_dirty_endio(struct bio *bio)
{
        struct keybuf_key *w = bio->bi_private;
        struct dirty_io *io = w->private;

        /* is_read = 1 */
        bch_count_io_errors(io->dc->disk.c->cache,
                            bio->bi_status, 1,
                            "reading dirty data from cache");

        dirty_endio(bio);
}

static CLOSURE_CALLBACK(read_dirty_submit)
{
        closure_type(io, struct dirty_io, cl);

        closure_bio_submit(io->dc->disk.c, &io->bio, cl);

        continue_at(cl, write_dirty, io->dc->writeback_write_wq);
}

static void read_dirty(struct cached_dev *dc)
{
        unsigned int delay = 0;
        struct keybuf_key *next, *keys[MAX_WRITEBACKS_IN_PASS], *w;
        size_t size;
        int nk, i;
        struct dirty_io *io;
        struct closure cl;
        uint16_t sequence = 0;

        BUG_ON(!llist_empty(&dc->writeback_ordering_wait.list));
        atomic_set(&dc->writeback_sequence_next, sequence);
        closure_init_stack(&cl);

        /*
         * XXX: if we error, background writeback just spins. Should use some
         * mempools.
         */

        next = bch_keybuf_next(&dc->writeback_keys);

        while (!kthread_should_stop() &&
               !test_bit(CACHE_SET_IO_DISABLE, &dc->disk.c->flags) &&
               next) {
                size = 0;
                nk = 0;

                do {
                        BUG_ON(ptr_stale(dc->disk.c, &next->key, 0));

                        /*
                         * Don't combine too many operations, even if they
                         * are all small.
                         */
                        if (nk >= MAX_WRITEBACKS_IN_PASS)
                                break;

                        /*
                         * If the current operation is very large, don't
                         * further combine operations.
                         */
                        if (size >= MAX_WRITESIZE_IN_PASS)
                                break;

                        /*
                         * Operations are only eligible to be combined
                         * if they are contiguous.
                         *
                         * TODO: add a heuristic willing to fire a
                         * certain amount of non-contiguous IO per pass,
                         * so that we can benefit from backing device
                         * command queueing.
                         */
                        if ((nk != 0) && bkey_cmp(&keys[nk-1]->key,
                                                &START_KEY(&next->key)))
                                break;

                        size += KEY_SIZE(&next->key);
                        keys[nk++] = next;
                } while ((next = bch_keybuf_next(&dc->writeback_keys)));

                /* Now we have gathered a set of 1..5 keys to write back. */
                for (i = 0; i < nk; i++) {
                        w = keys[i];

                        io = kzalloc(sizeof(*io) + sizeof(struct bio_vec) *
                                DIV_ROUND_UP(KEY_SIZE(&w->key), PAGE_SECTORS),
                                GFP_KERNEL);
                        if (!io)
                                goto err;

                        w->private      = io;
                        io->dc          = dc;
                        io->sequence    = sequence++;

                        dirty_init(w);
                        io->bio.bi_opf = REQ_OP_READ;
                        io->bio.bi_iter.bi_sector = PTR_OFFSET(&w->key, 0);
                        bio_set_dev(&io->bio, dc->disk.c->cache->bdev);
                        io->bio.bi_end_io       = read_dirty_endio;

                        if (bch_bio_alloc_pages(&io->bio, GFP_KERNEL))
                                goto err_free;

                        trace_bcache_writeback(&w->key);

                        down(&dc->in_flight);

                        /*
                         * We've acquired a semaphore for the maximum
                         * simultaneous number of writebacks; from here
                         * everything happens asynchronously.
                         */
                        closure_call(&io->cl, read_dirty_submit, NULL, &cl);
                }

                delay = writeback_delay(dc, size);

                while (!kthread_should_stop() &&
                       !test_bit(CACHE_SET_IO_DISABLE, &dc->disk.c->flags) &&
                       delay) {
                        schedule_timeout_interruptible(delay);
                        delay = writeback_delay(dc, 0);
                }
        }

        if (0) {
err_free:
                kfree(w->private);
err:
                bch_keybuf_del(&dc->writeback_keys, w);
        }

        /*
         * Wait for outstanding writeback IOs to finish (and keybuf slots to be
         * freed) before refilling again
         */
        closure_sync(&cl);
}

/* Scan for dirty data */

void bcache_dev_sectors_dirty_add(struct cache_set *c, unsigned int inode,
                                  uint64_t offset, int nr_sectors)
{
        struct bcache_device *d = c->devices[inode];
        unsigned int stripe_offset, sectors_dirty;
        int stripe;

        if (!d)
                return;

        stripe = offset_to_stripe(d, offset);
        if (stripe < 0)
                return;

        if (UUID_FLASH_ONLY(&c->uuids[inode]))
                atomic_long_add(nr_sectors, &c->flash_dev_dirty_sectors);

        stripe_offset = offset & (d->stripe_size - 1);

        while (nr_sectors) {
                int s = min_t(unsigned int, abs(nr_sectors),
                              d->stripe_size - stripe_offset);

                if (nr_sectors < 0)
                        s = -s;

                if (stripe >= d->nr_stripes)
                        return;

                sectors_dirty = atomic_add_return(s,
                                        d->stripe_sectors_dirty + stripe);
                if (sectors_dirty == d->stripe_size) {
                        if (!test_bit(stripe, d->full_dirty_stripes))
                                set_bit(stripe, d->full_dirty_stripes);
                } else {
                        if (test_bit(stripe, d->full_dirty_stripes))
                                clear_bit(stripe, d->full_dirty_stripes);
                }

                nr_sectors -= s;
                stripe_offset = 0;
                stripe++;
        }
}

static bool dirty_pred(struct keybuf *buf, struct bkey *k)
{
        struct cached_dev *dc = container_of(buf,
                                             struct cached_dev,
                                             writeback_keys);

        BUG_ON(KEY_INODE(k) != dc->disk.id);

        return KEY_DIRTY(k);
}

static void refill_full_stripes(struct cached_dev *dc)
{
        struct keybuf *buf = &dc->writeback_keys;
        unsigned int start_stripe, next_stripe;
        int stripe;
        bool wrapped = false;

        stripe = offset_to_stripe(&dc->disk, KEY_OFFSET(&buf->last_scanned));
        if (stripe < 0)
                stripe = 0;

        start_stripe = stripe;

        while (1) {
                stripe = find_next_bit(dc->disk.full_dirty_stripes,
                                       dc->disk.nr_stripes, stripe);

                if (stripe == dc->disk.nr_stripes)
                        goto next;

                next_stripe = find_next_zero_bit(dc->disk.full_dirty_stripes,
                                                 dc->disk.nr_stripes, stripe);

                buf->last_scanned = KEY(dc->disk.id,
                                        stripe * dc->disk.stripe_size, 0);

                bch_refill_keybuf(dc->disk.c, buf,
                                  &KEY(dc->disk.id,
                                       next_stripe * dc->disk.stripe_size, 0),
                                  dirty_pred);

                if (array_freelist_empty(&buf->freelist))
                        return;

                stripe = next_stripe;
next:
                if (wrapped && stripe > start_stripe)
                        return;

                if (stripe == dc->disk.nr_stripes) {
                        stripe = 0;
                        wrapped = true;
                }
        }
}

/*
 * Returns true if we scanned the entire disk
 */
static bool refill_dirty(struct cached_dev *dc)
{
        struct keybuf *buf = &dc->writeback_keys;
        struct bkey start = KEY(dc->disk.id, 0, 0);
        struct bkey end = KEY(dc->disk.id, MAX_KEY_OFFSET, 0);
        struct bkey start_pos;

        /*
         * make sure keybuf pos is inside the range for this disk - at bringup
         * we might not be attached yet so this disk's inode nr isn't
         * initialized then
         */
        if (bkey_cmp(&buf->last_scanned, &start) < 0 ||
            bkey_cmp(&buf->last_scanned, &end) > 0)
                buf->last_scanned = start;

        if (dc->partial_stripes_expensive) {
                refill_full_stripes(dc);
                if (array_freelist_empty(&buf->freelist))
                        return false;
        }

        start_pos = buf->last_scanned;
        bch_refill_keybuf(dc->disk.c, buf, &end, dirty_pred);

        if (bkey_cmp(&buf->last_scanned, &end) < 0)
                return false;

        /*
         * If we get to the end start scanning again from the beginning, and
         * only scan up to where we initially started scanning from:
         */
        buf->last_scanned = start;
        bch_refill_keybuf(dc->disk.c, buf, &start_pos, dirty_pred);

        return bkey_cmp(&buf->last_scanned, &start_pos) >= 0;
}

static int bch_writeback_thread(void *arg)
{
        struct cached_dev *dc = arg;
        struct cache_set *c = dc->disk.c;
        bool searched_full_index;

        bch_ratelimit_reset(&dc->writeback_rate);

        while (!kthread_should_stop() &&
               !test_bit(CACHE_SET_IO_DISABLE, &c->flags)) {
                down_write(&dc->writeback_lock);
                set_current_state(TASK_INTERRUPTIBLE);
                /*
                 * If the bache device is detaching, skip here and continue
                 * to perform writeback. Otherwise, if no dirty data on cache,
                 * or there is dirty data on cache but writeback is disabled,
                 * the writeback thread should sleep here and wait for others
                 * to wake up it.
                 */
                if (!test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags) &&
                    (!atomic_read(&dc->has_dirty) || !dc->writeback_running)) {
                        up_write(&dc->writeback_lock);

                        if (kthread_should_stop() ||
                            test_bit(CACHE_SET_IO_DISABLE, &c->flags)) {
                                set_current_state(TASK_RUNNING);
                                break;
                        }

                        schedule();
                        continue;
                }
                set_current_state(TASK_RUNNING);

                searched_full_index = refill_dirty(dc);

                if (searched_full_index &&
                    RB_EMPTY_ROOT(&dc->writeback_keys.keys)) {
                        atomic_set(&dc->has_dirty, 0);
                        SET_BDEV_STATE(&dc->sb, BDEV_STATE_CLEAN);
                        bch_write_bdev_super(dc, NULL);
                        /*
                         * If bcache device is detaching via sysfs interface,
                         * writeback thread should stop after there is no dirty
                         * data on cache. BCACHE_DEV_DETACHING flag is set in
                         * bch_cached_dev_detach().
                         */
                        if (test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags)) {
                                struct closure cl;

                                closure_init_stack(&cl);
                                memset(&dc->sb.set_uuid, 0, 16);
                                SET_BDEV_STATE(&dc->sb, BDEV_STATE_NONE);

                                bch_write_bdev_super(dc, &cl);
                                closure_sync(&cl);

                                up_write(&dc->writeback_lock);
                                break;
                        }

                        /*
                         * When dirty data rate is high (e.g. 50%+), there might
                         * be heavy buckets fragmentation after writeback
                         * finished, which hurts following write performance.
                         * If users really care about write performance they
                         * may set BCH_ENABLE_AUTO_GC via sysfs, then when
                         * BCH_DO_AUTO_GC is set, garbage collection thread
                         * will be wake up here. After moving gc, the shrunk
                         * btree may be helpful for following write requests.
                         */
                        if (c->gc_after_writeback ==
                            (BCH_ENABLE_AUTO_GC|BCH_DO_AUTO_GC)) {
                                c->gc_after_writeback &= ~BCH_DO_AUTO_GC;
                                force_wake_up_gc(c);
                        }
                }

                up_write(&dc->writeback_lock);

                read_dirty(dc);

                if (searched_full_index) {
                        unsigned int delay = dc->writeback_delay * HZ;

                        while (delay &&
                               !kthread_should_stop() &&
                               !test_bit(CACHE_SET_IO_DISABLE, &c->flags) &&
                               !test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags))
                                delay = schedule_timeout_interruptible(delay);

                        bch_ratelimit_reset(&dc->writeback_rate);
                }
        }

        if (dc->writeback_write_wq)
                destroy_workqueue(dc->writeback_write_wq);

        cached_dev_put(dc);
        wait_for_kthread_stop();

        return 0;
}

/* Init */
#define INIT_KEYS_EACH_TIME     500000

struct sectors_dirty_init {
        struct btree_op op;
        unsigned int    inode;
        size_t          count;
};

static int sectors_dirty_init_fn(struct btree_op *_op, struct btree *b,
                                 struct bkey *k)
{
        struct sectors_dirty_init *op = container_of(_op,
                                                struct sectors_dirty_init, op);
        if (KEY_INODE(k) > op->inode)
                return MAP_DONE;

        if (KEY_DIRTY(k))
                bcache_dev_sectors_dirty_add(b->c, KEY_INODE(k),
                                             KEY_START(k), KEY_SIZE(k));

        op->count++;
        if (!(op->count % INIT_KEYS_EACH_TIME))
                cond_resched();

        return MAP_CONTINUE;
}

static int bch_root_node_dirty_init(struct cache_set *c,
                                     struct bcache_device *d,
                                     struct bkey *k)
{
        struct sectors_dirty_init op;
        int ret;

        bch_btree_op_init(&op.op, -1);
        op.inode = d->id;
        op.count = 0;

        ret = bcache_btree(map_keys_recurse,
                           k,
                           c->root,
                           &op.op,
                           &KEY(op.inode, 0, 0),
                           sectors_dirty_init_fn,
                           0);
        if (ret < 0)
                pr_warn("sectors dirty init failed, ret=%d!\n", ret);

        /*
         * The op may be added to cache_set's btree_cache_wait
         * in mca_cannibalize(), must ensure it is removed from
         * the list and release btree_cache_alloc_lock before
         * free op memory.
         * Otherwise, the btree_cache_wait will be damaged.
         */
        bch_cannibalize_unlock(c);
        finish_wait(&c->btree_cache_wait, &(&op.op)->wait);

        return ret;
}

static int bch_dirty_init_thread(void *arg)
{
        struct dirty_init_thrd_info *info = arg;
        struct bch_dirty_init_state *state = info->state;
        struct cache_set *c = state->c;
        struct btree_iter_stack iter;
        struct bkey *k, *p;
        int cur_idx, prev_idx, skip_nr;

        k = p = NULL;
        prev_idx = 0;

        bch_btree_iter_stack_init(&c->root->keys, &iter, NULL);
        k = bch_btree_iter_next_filter(&iter.iter, &c->root->keys, bch_ptr_bad);
        BUG_ON(!k);

        p = k;

        while (k) {
                spin_lock(&state->idx_lock);
                cur_idx = state->key_idx;
                state->key_idx++;
                spin_unlock(&state->idx_lock);

                skip_nr = cur_idx - prev_idx;

                while (skip_nr) {
                        k = bch_btree_iter_next_filter(&iter.iter,
                                                       &c->root->keys,
                                                       bch_ptr_bad);
                        if (k)
                                p = k;
                        else {
                                atomic_set(&state->enough, 1);
                                /* Update state->enough earlier */
                                smp_mb__after_atomic();
                                goto out;
                        }
                        skip_nr--;
                }

                if (p) {
                        if (bch_root_node_dirty_init(c, state->d, p) < 0)
                                goto out;
                }

                p = NULL;
                prev_idx = cur_idx;
        }

out:
        /* In order to wake up state->wait in time */
        smp_mb__before_atomic();
        if (atomic_dec_and_test(&state->started))
                wake_up(&state->wait);

        return 0;
}

static int bch_btre_dirty_init_thread_nr(void)
{
        int n = num_online_cpus()/2;

        if (n == 0)
                n = 1;
        else if (n > BCH_DIRTY_INIT_THRD_MAX)
                n = BCH_DIRTY_INIT_THRD_MAX;

        return n;
}

void bch_sectors_dirty_init(struct bcache_device *d)
{
        int i;
        struct btree *b = NULL;
        struct bkey *k = NULL;
        struct btree_iter_stack iter;
        struct sectors_dirty_init op;
        struct cache_set *c = d->c;
        struct bch_dirty_init_state state;

retry_lock:
        b = c->root;
        rw_lock(0, b, b->level);
        if (b != c->root) {
                rw_unlock(0, b);
                goto retry_lock;
        }

        /* Just count root keys if no leaf node */
        if (c->root->level == 0) {
                bch_btree_op_init(&op.op, -1);
                op.inode = d->id;
                op.count = 0;

                for_each_key_filter(&c->root->keys,
                                    k, &iter, bch_ptr_invalid) {
                        if (KEY_INODE(k) != op.inode)
                                continue;
                        sectors_dirty_init_fn(&op.op, c->root, k);
                }

                rw_unlock(0, b);
                return;
        }

        memset(&state, 0, sizeof(struct bch_dirty_init_state));
        state.c = c;
        state.d = d;
        state.total_threads = bch_btre_dirty_init_thread_nr();
        state.key_idx = 0;
        spin_lock_init(&state.idx_lock);
        atomic_set(&state.started, 0);
        atomic_set(&state.enough, 0);
        init_waitqueue_head(&state.wait);

        for (i = 0; i < state.total_threads; i++) {
                /* Fetch latest state.enough earlier */
                smp_mb__before_atomic();
                if (atomic_read(&state.enough))
                        break;

                atomic_inc(&state.started);
                state.infos[i].state = &state;
                state.infos[i].thread =
                        kthread_run(bch_dirty_init_thread, &state.infos[i],
                                    "bch_dirtcnt[%d]", i);
                if (IS_ERR(state.infos[i].thread)) {
                        pr_err("fails to run thread bch_dirty_init[%d]\n", i);
                        atomic_dec(&state.started);
                        for (--i; i >= 0; i--)
                                kthread_stop(state.infos[i].thread);
                        goto out;
                }
        }

out:
        /* Must wait for all threads to stop. */
        wait_event(state.wait, atomic_read(&state.started) == 0);
        rw_unlock(0, b);
}

void bch_cached_dev_writeback_init(struct cached_dev *dc)
{
        sema_init(&dc->in_flight, 64);
        init_rwsem(&dc->writeback_lock);
        bch_keybuf_init(&dc->writeback_keys);

        dc->writeback_metadata          = true;
        dc->writeback_running           = false;
        dc->writeback_consider_fragment = true;
        dc->writeback_percent           = 10;
        dc->writeback_delay             = 30;
        atomic_long_set(&dc->writeback_rate.rate, 1024);
        dc->writeback_rate_minimum      = 8;

        dc->writeback_rate_update_seconds = WRITEBACK_RATE_UPDATE_SECS_DEFAULT;
        dc->writeback_rate_p_term_inverse = 40;
        dc->writeback_rate_fp_term_low = 1;
        dc->writeback_rate_fp_term_mid = 10;
        dc->writeback_rate_fp_term_high = 1000;
        dc->writeback_rate_i_term_inverse = 10000;

        /* For dc->writeback_lock contention in update_writeback_rate() */
        dc->rate_update_retry = 0;

        WARN_ON(test_and_clear_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags));
        INIT_DELAYED_WORK(&dc->writeback_rate_update, update_writeback_rate);
}

int bch_cached_dev_writeback_start(struct cached_dev *dc)
{
        dc->writeback_write_wq = alloc_workqueue("bcache_writeback_wq",
                                                WQ_MEM_RECLAIM | WQ_PERCPU, 0);
        if (!dc->writeback_write_wq)
                return -ENOMEM;

        cached_dev_get(dc);
        dc->writeback_thread = kthread_create(bch_writeback_thread, dc,
                                              "bcache_writeback");
        if (IS_ERR(dc->writeback_thread)) {
                cached_dev_put(dc);
                destroy_workqueue(dc->writeback_write_wq);
                return PTR_ERR(dc->writeback_thread);
        }
        dc->writeback_running = true;

        WARN_ON(test_and_set_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags));
        schedule_delayed_work(&dc->writeback_rate_update,
                              dc->writeback_rate_update_seconds * HZ);

        bch_writeback_queue(dc);

        return 0;
}