text: /*
   * 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 <trace/events/bcache.h>
  
  /* Rate limiting */
  
  static void __update_writeback_rate(struct cached_dev *dc)
  {
  	struct cache_set *c = dc->disk.c;
  	uint64_t cache_sectors = c->nbuckets * c->sb.bucket_size -
  				bcache_flash_devs_sectors_dirty(c);
  	uint64_t cache_dirty_target =
  		div_u64(cache_sectors * dc->writeback_percent, 100);
  
  	int64_t target = div64_u64(cache_dirty_target * bdev_sectors(dc->bdev),
  				   c->cached_dev_sectors);
  
  	/* PD controller */
  
  	int64_t dirty = bcache_dev_sectors_dirty(&dc->disk);
  	int64_t derivative = dirty - dc->disk.sectors_dirty_last;
  	int64_t proportional = dirty - target;
  	int64_t change;
  
  	dc->disk.sectors_dirty_last = dirty;
  
  	/* Scale to sectors per second */
  
  	proportional *= dc->writeback_rate_update_seconds;
  	proportional = div_s64(proportional, dc->writeback_rate_p_term_inverse);
  
  	derivative = div_s64(derivative, dc->writeback_rate_update_seconds);
  
  	derivative = ewma_add(dc->disk.sectors_dirty_derivative, derivative,
  			      (dc->writeback_rate_d_term /
  			       dc->writeback_rate_update_seconds) ?: 1, 0);
  
  	derivative *= dc->writeback_rate_d_term;
  	derivative = div_s64(derivative, dc->writeback_rate_p_term_inverse);
  
  	change = proportional + derivative;
  
  	/* Don't increase writeback rate if the device isn't keeping up */
  	if (change > 0 &&
  	    time_after64(local_clock(),
  			 dc->writeback_rate.next + NSEC_PER_MSEC))
  		change = 0;
  
  	dc->writeback_rate.rate =
  		clamp_t(int64_t, (int64_t) dc->writeback_rate.rate + change,
  			1, NSEC_PER_MSEC);
  
  	dc->writeback_rate_proportional = proportional;
  	dc->writeback_rate_derivative = derivative;
  	dc->writeback_rate_change = change;
  	dc->writeback_rate_target = target;
  }
  
  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);
  
  	down_read(&dc->writeback_lock);
  
  	if (atomic_read(&dc->has_dirty) &&
  	    dc->writeback_percent)
  		__update_writeback_rate(dc);
  
  	up_read(&dc->writeback_lock);
  
  	schedule_delayed_work(&dc->writeback_rate_update,
  			      dc->writeback_rate_update_seconds * HZ);
  }
  
  static unsigned writeback_delay(struct cached_dev *dc, unsigned 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;
  	struct bio		bio;
  };
  
  static void dirty_init(struct keybuf_key *w)
  {
  	struct dirty_io *io = w->private;
  	struct bio *bio = &io->bio;
  
  	bio_init(bio);
  	if (!io->dc->writeback_percent)
  		bio_set_prio(bio, IOPRIO_PRIO_VALUE(IOPRIO_CLASS_IDLE, 0));
  
  	bio->bi_iter.bi_size	= KEY_SIZE(&w->key) << 9;
  	bio->bi_max_vecs	= DIV_ROUND_UP(KEY_SIZE(&w->key), PAGE_SECTORS);
  	bio->bi_private		= w;
  	bio->bi_io_vec		= bio->bi_inline_vecs;
  	bch_bio_map(bio, NULL);
  }
  
  static void dirty_io_destructor(struct closure *cl)
  {
  	struct dirty_io *io = container_of(cl, struct dirty_io, cl);
  	kfree(io);
  }
  
  static void write_dirty_finish(struct closure *cl)
  {
  	struct dirty_io *io = container_of(cl, 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 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_error)
  		SET_KEY_DIRTY(&w->key, false);
  
  	closure_put(&io->cl);
  }
  
  static void write_dirty(struct closure *cl)
  {
  	struct dirty_io *io = container_of(cl, struct dirty_io, cl);
  	struct keybuf_key *w = io->bio.bi_private;
  
  	dirty_init(w);
  	bio_set_op_attrs(&io->bio, REQ_OP_WRITE, 0);
  	io->bio.bi_iter.bi_sector = KEY_START(&w->key);
  	io->bio.bi_bdev		= io->dc->bdev;
  	io->bio.bi_end_io	= dirty_endio;
  
  	closure_bio_submit(&io->bio, cl);
  
  	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;
  
  	bch_count_io_errors(PTR_CACHE(io->dc->disk.c, &w->key, 0),
  			    bio->bi_error, "reading dirty data from cache");
  
  	dirty_endio(bio);
  }
  
  static void read_dirty_submit(struct closure *cl)
  {
  	struct dirty_io *io = container_of(cl, struct dirty_io, cl);
  
  	closure_bio_submit(&io->bio, cl);
  
  	continue_at(cl, write_dirty, io->dc->writeback_write_wq);
  }
  
  static void read_dirty(struct cached_dev *dc)
  {
  	unsigned delay = 0;
  	struct keybuf_key *w;
  	struct dirty_io *io;
  	struct closure cl;
  
  	closure_init_stack(&cl);
  
  	/*
  	 * XXX: if we error, background writeback just spins. Should use some
  	 * mempools.
  	 */
  
  	while (!kthread_should_stop()) {
  
  		w = bch_keybuf_next(&dc->writeback_keys);
  		if (!w)
  			break;
  
  		BUG_ON(ptr_stale(dc->disk.c, &w->key, 0));
  
  		if (KEY_START(&w->key) != dc->last_read ||
  		    jiffies_to_msecs(delay) > 50)
  			while (!kthread_should_stop() && delay)
  				delay = schedule_timeout_interruptible(delay);
  
  		dc->last_read	= KEY_OFFSET(&w->key);
  
  		io = kzalloc(sizeof(struct dirty_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;
  
  		dirty_init(w);
  		bio_set_op_attrs(&io->bio, REQ_OP_READ, 0);
  		io->bio.bi_iter.bi_sector = PTR_OFFSET(&w->key, 0);
  		io->bio.bi_bdev		= PTR_CACHE(dc->disk.c,
  						    &w->key, 0)->bdev;
  		io->bio.bi_end_io	= read_dirty_endio;
  
  		if (bio_alloc_pages(&io->bio, GFP_KERNEL))
  			goto err_free;
  
  		trace_bcache_writeback(&w->key);
  
  		down(&dc->in_flight);
  		closure_call(&io->cl, read_dirty_submit, NULL, &cl);
  
  		delay = writeback_delay(dc, KEY_SIZE(&w->key));
  	}
  
  	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 inode,
  				  uint64_t offset, int nr_sectors)
  {
  	struct bcache_device *d = c->devices[inode];
  	unsigned stripe_offset, stripe, sectors_dirty;
  
  	if (!d)
  		return;
  
  	stripe = offset_to_stripe(d, offset);
  	stripe_offset = offset & (d->stripe_size - 1);
  
  	while (nr_sectors) {
  		int s = min_t(unsigned, 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)
  			set_bit(stripe, d->full_dirty_stripes);
  		else
  			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 start_stripe, stripe, next_stripe;
  	bool wrapped = false;
  
  	stripe = offset_to_stripe(&dc->disk, KEY_OFFSET(&buf->last_scanned));
  
  	if (stripe >= dc->disk.nr_stripes)
  		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;
  	bool searched_full_index;
  
  	while (!kthread_should_stop()) {
  		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()) {
  				set_current_state(TASK_RUNNING);
  				return 0;
  			}
  
  			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);
  			cached_dev_put(dc);
  			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)) {
  				up_write(&dc->writeback_lock);
  				break;
  			}
  		}
  
  		up_write(&dc->writeback_lock);
  
  		bch_ratelimit_reset(&dc->writeback_rate);
  		read_dirty(dc);
  
  		if (searched_full_index) {
  			unsigned delay = dc->writeback_delay * HZ;
  
  			while (delay &&
  			       !kthread_should_stop() &&
  			       !test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags))
  				delay = schedule_timeout_interruptible(delay);
  		}
  	}
  
  	return 0;
  }
  
  /* Init */
  
  struct sectors_dirty_init {
  	struct btree_op	op;
  	unsigned	inode;
  };
  
  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));
  
  	return MAP_CONTINUE;
  }
  
  void bch_sectors_dirty_init(struct bcache_device *d)
  {
  	struct sectors_dirty_init op;
  
  	bch_btree_op_init(&op.op, -1);
  	op.inode = d->id;
  
  	bch_btree_map_keys(&op.op, d->c, &KEY(op.inode, 0, 0),
  			   sectors_dirty_init_fn, 0);
  
  	d->sectors_dirty_last = bcache_dev_sectors_dirty(d);
  }
  
  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		= true;
  	dc->writeback_percent		= 10;
  	dc->writeback_delay		= 30;
  	dc->writeback_rate.rate		= 1024;
  
  	dc->writeback_rate_update_seconds = 5;
  	dc->writeback_rate_d_term	= 30;
  	dc->writeback_rate_p_term_inverse = 6000;
  
  	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, 0);
  	if (!dc->writeback_write_wq)
  		return -ENOMEM;
  
  	dc->writeback_thread = kthread_create(bch_writeback_thread, dc,
  					      "bcache_writeback");
  	if (IS_ERR(dc->writeback_thread))
  		return PTR_ERR(dc->writeback_thread);
  
  	schedule_delayed_work(&dc->writeback_rate_update,
  			      dc->writeback_rate_update_seconds * HZ);
  
  	bch_writeback_queue(dc);
  
  	return 0;
  }
  