linux/fs/bcachefs/journal_reclaim.c

// SPDX-License-Identifier: GPL-2.0

#include "bcachefs.h"
#include "btree_key_cache.h"
#include "btree_update.h"
#include "buckets.h"
#include "errcode.h"
#include "error.h"
#include "journal.h"
#include "journal_io.h"
#include "journal_reclaim.h"
#include "replicas.h"
#include "sb-members.h"
#include "trace.h"

#include <linux/kthread.h>
#include <linux/sched/mm.h>

/* Free space calculations: */

static unsigned journal_space_from(struct journal_device *ja,
				   enum journal_space_from from)
{
	switch (from) {
	case journal_space_discarded:
		return ja->discard_idx;
	case journal_space_clean_ondisk:
		return ja->dirty_idx_ondisk;
	case journal_space_clean:
		return ja->dirty_idx;
	default:
		BUG();
	}
}

unsigned bch2_journal_dev_buckets_available(struct journal *j,
					    struct journal_device *ja,
					    enum journal_space_from from)
{
	unsigned available = (journal_space_from(ja, from) -
			      ja->cur_idx - 1 + ja->nr) % ja->nr;

	/*
	 * Don't use the last bucket unless writing the new last_seq
	 * will make another bucket available:
	 */
	if (available && ja->dirty_idx_ondisk == ja->dirty_idx)
		--available;

	return available;
}

static void journal_set_remaining(struct journal *j, unsigned u64s_remaining)
{
	union journal_preres_state old, new;
	u64 v = atomic64_read(&j->prereserved.counter);

	do {
		old.v = new.v = v;
		new.remaining = u64s_remaining;
	} while ((v = atomic64_cmpxchg(&j->prereserved.counter,
				       old.v, new.v)) != old.v);
}

static struct journal_space
journal_dev_space_available(struct journal *j, struct bch_dev *ca,
			    enum journal_space_from from)
{
	struct journal_device *ja = &ca->journal;
	unsigned sectors, buckets, unwritten;
	u64 seq;

	if (from == journal_space_total)
		return (struct journal_space) {
			.next_entry	= ca->mi.bucket_size,
			.total		= ca->mi.bucket_size * ja->nr,
		};

	buckets = bch2_journal_dev_buckets_available(j, ja, from);
	sectors = ja->sectors_free;

	/*
	 * We that we don't allocate the space for a journal entry
	 * until we write it out - thus, account for it here:
	 */
	for (seq = journal_last_unwritten_seq(j);
	     seq <= journal_cur_seq(j);
	     seq++) {
		unwritten = j->buf[seq & JOURNAL_BUF_MASK].sectors;

		if (!unwritten)
			continue;

		/* entry won't fit on this device, skip: */
		if (unwritten > ca->mi.bucket_size)
			continue;

		if (unwritten >= sectors) {
			if (!buckets) {
				sectors = 0;
				break;
			}

			buckets--;
			sectors = ca->mi.bucket_size;
		}

		sectors -= unwritten;
	}

	if (sectors < ca->mi.bucket_size && buckets) {
		buckets--;
		sectors = ca->mi.bucket_size;
	}

	return (struct journal_space) {
		.next_entry	= sectors,
		.total		= sectors + buckets * ca->mi.bucket_size,
	};
}

static struct journal_space __journal_space_available(struct journal *j, unsigned nr_devs_want,
			    enum journal_space_from from)
{
	struct bch_fs *c = container_of(j, struct bch_fs, journal);
	struct bch_dev *ca;
	unsigned i, pos, nr_devs = 0;
	struct journal_space space, dev_space[BCH_SB_MEMBERS_MAX];

	BUG_ON(nr_devs_want > ARRAY_SIZE(dev_space));

	rcu_read_lock();
	for_each_member_device_rcu(ca, c, i,
				   &c->rw_devs[BCH_DATA_journal]) {
		if (!ca->journal.nr)
			continue;

		space = journal_dev_space_available(j, ca, from);
		if (!space.next_entry)
			continue;

		for (pos = 0; pos < nr_devs; pos++)
			if (space.total > dev_space[pos].total)
				break;

		array_insert_item(dev_space, nr_devs, pos, space);
	}
	rcu_read_unlock();

	if (nr_devs < nr_devs_want)
		return (struct journal_space) { 0, 0 };

	/*
	 * We sorted largest to smallest, and we want the smallest out of the
	 * @nr_devs_want largest devices:
	 */
	return dev_space[nr_devs_want - 1];
}

void bch2_journal_space_available(struct journal *j)
{
	struct bch_fs *c = container_of(j, struct bch_fs, journal);
	struct bch_dev *ca;
	unsigned clean, clean_ondisk, total;
	s64 u64s_remaining = 0;
	unsigned max_entry_size	 = min(j->buf[0].buf_size >> 9,
				       j->buf[1].buf_size >> 9);
	unsigned i, nr_online = 0, nr_devs_want;
	bool can_discard = false;
	int ret = 0;

	lockdep_assert_held(&j->lock);

	rcu_read_lock();
	for_each_member_device_rcu(ca, c, i,
				   &c->rw_devs[BCH_DATA_journal]) {
		struct journal_device *ja = &ca->journal;

		if (!ja->nr)
			continue;

		while (ja->dirty_idx != ja->cur_idx &&
		       ja->bucket_seq[ja->dirty_idx] < journal_last_seq(j))
			ja->dirty_idx = (ja->dirty_idx + 1) % ja->nr;

		while (ja->dirty_idx_ondisk != ja->dirty_idx &&
		       ja->bucket_seq[ja->dirty_idx_ondisk] < j->last_seq_ondisk)
			ja->dirty_idx_ondisk = (ja->dirty_idx_ondisk + 1) % ja->nr;

		if (ja->discard_idx != ja->dirty_idx_ondisk)
			can_discard = true;

		max_entry_size = min_t(unsigned, max_entry_size, ca->mi.bucket_size);
		nr_online++;
	}
	rcu_read_unlock();

	j->can_discard = can_discard;

	if (nr_online < c->opts.metadata_replicas_required) {
		ret = JOURNAL_ERR_insufficient_devices;
		goto out;
	}

	nr_devs_want = min_t(unsigned, nr_online, c->opts.metadata_replicas);

	for (i = 0; i < journal_space_nr; i++)
		j->space[i] = __journal_space_available(j, nr_devs_want, i);

	clean_ondisk	= j->space[journal_space_clean_ondisk].total;
	clean		= j->space[journal_space_clean].total;
	total		= j->space[journal_space_total].total;

	if (!j->space[journal_space_discarded].next_entry)
		ret = JOURNAL_ERR_journal_full;

	if ((j->space[journal_space_clean_ondisk].next_entry <
	     j->space[journal_space_clean_ondisk].total) &&
	    (clean - clean_ondisk <= total / 8) &&
	    (clean_ondisk * 2 > clean))
		set_bit(JOURNAL_MAY_SKIP_FLUSH, &j->flags);
	else
		clear_bit(JOURNAL_MAY_SKIP_FLUSH, &j->flags);

	u64s_remaining  = (u64) clean << 6;
	u64s_remaining -= (u64) total << 3;
	u64s_remaining = max(0LL, u64s_remaining);
	u64s_remaining /= 4;
	u64s_remaining = min_t(u64, u64s_remaining, U32_MAX);
out:
	j->cur_entry_sectors	= !ret ? j->space[journal_space_discarded].next_entry : 0;
	j->cur_entry_error	= ret;
	journal_set_remaining(j, u64s_remaining);
	journal_set_watermark(j);

	if (!ret)
		journal_wake(j);
}

/* Discards - last part of journal reclaim: */

static bool should_discard_bucket(struct journal *j, struct journal_device *ja)
{
	bool ret;

	spin_lock(&j->lock);
	ret = ja->discard_idx != ja->dirty_idx_ondisk;
	spin_unlock(&j->lock);

	return ret;
}

/*
 * Advance ja->discard_idx as long as it points to buckets that are no longer
 * dirty, issuing discards if necessary:
 */
void bch2_journal_do_discards(struct journal *j)
{
	struct bch_fs *c = container_of(j, struct bch_fs, journal);
	struct bch_dev *ca;
	unsigned iter;

	mutex_lock(&j->discard_lock);

	for_each_rw_member(ca, c, iter) {
		struct journal_device *ja = &ca->journal;

		while (should_discard_bucket(j, ja)) {
			if (!c->opts.nochanges &&
			    ca->mi.discard &&
			    bdev_max_discard_sectors(ca->disk_sb.bdev))
				blkdev_issue_discard(ca->disk_sb.bdev,
					bucket_to_sector(ca,
						ja->buckets[ja->discard_idx]),
					ca->mi.bucket_size, GFP_NOFS);

			spin_lock(&j->lock);
			ja->discard_idx = (ja->discard_idx + 1) % ja->nr;

			bch2_journal_space_available(j);
			spin_unlock(&j->lock);
		}
	}

	mutex_unlock(&j->discard_lock);
}

/*
 * Journal entry pinning - machinery for holding a reference on a given journal
 * entry, holding it open to ensure it gets replayed during recovery:
 */

void bch2_journal_reclaim_fast(struct journal *j)
{
	bool popped = false;

	lockdep_assert_held(&j->lock);

	/*
	 * Unpin journal entries whose reference counts reached zero, meaning
	 * all btree nodes got written out
	 */
	while (!fifo_empty(&j->pin) &&
	       !atomic_read(&fifo_peek_front(&j->pin).count)) {
		j->pin.front++;
		popped = true;
	}

	if (popped)
		bch2_journal_space_available(j);
}

bool __bch2_journal_pin_put(struct journal *j, u64 seq)
{
	struct journal_entry_pin_list *pin_list = journal_seq_pin(j, seq);

	return atomic_dec_and_test(&pin_list->count);
}

void bch2_journal_pin_put(struct journal *j, u64 seq)
{
	if (__bch2_journal_pin_put(j, seq)) {
		spin_lock(&j->lock);
		bch2_journal_reclaim_fast(j);
		spin_unlock(&j->lock);
	}
}

static inline bool __journal_pin_drop(struct journal *j,
				      struct journal_entry_pin *pin)
{
	struct journal_entry_pin_list *pin_list;

	if (!journal_pin_active(pin))
		return false;

	if (j->flush_in_progress == pin)
		j->flush_in_progress_dropped = true;

	pin_list = journal_seq_pin(j, pin->seq);
	pin->seq = 0;
	list_del_init(&pin->list);

	/*
	 * Unpinning a journal entry may make journal_next_bucket() succeed, if
	 * writing a new last_seq will now make another bucket available:
	 */
	return atomic_dec_and_test(&pin_list->count) &&
		pin_list == &fifo_peek_front(&j->pin);
}

void bch2_journal_pin_drop(struct journal *j,
			   struct journal_entry_pin *pin)
{
	spin_lock(&j->lock);
	if (__journal_pin_drop(j, pin))
		bch2_journal_reclaim_fast(j);
	spin_unlock(&j->lock);
}

static enum journal_pin_type journal_pin_type(journal_pin_flush_fn fn)
{
	if (fn == bch2_btree_node_flush0 ||
	    fn == bch2_btree_node_flush1)
		return JOURNAL_PIN_btree;
	else if (fn == bch2_btree_key_cache_journal_flush)
		return JOURNAL_PIN_key_cache;
	else
		return JOURNAL_PIN_other;
}

void bch2_journal_pin_set(struct journal *j, u64 seq,
			  struct journal_entry_pin *pin,
			  journal_pin_flush_fn flush_fn)
{
	struct journal_entry_pin_list *pin_list;
	bool reclaim;

	spin_lock(&j->lock);

	if (seq < journal_last_seq(j)) {
		/*
		 * bch2_journal_pin_copy() raced with bch2_journal_pin_drop() on
		 * the src pin - with the pin dropped, the entry to pin might no
		 * longer to exist, but that means there's no longer anything to
		 * copy and we can bail out here:
		 */
		spin_unlock(&j->lock);
		return;
	}

	pin_list = journal_seq_pin(j, seq);

	reclaim = __journal_pin_drop(j, pin);

	atomic_inc(&pin_list->count);
	pin->seq	= seq;
	pin->flush	= flush_fn;

	if (flush_fn)
		list_add(&pin->list, &pin_list->list[journal_pin_type(flush_fn)]);
	else
		list_add(&pin->list, &pin_list->flushed);

	if (reclaim)
		bch2_journal_reclaim_fast(j);
	spin_unlock(&j->lock);

	/*
	 * If the journal is currently full,  we might want to call flush_fn
	 * immediately:
	 */
	journal_wake(j);
}

/**
 * bch2_journal_pin_flush: ensure journal pin callback is no longer running
 * @j:		journal object
 * @pin:	pin to flush
 */
void bch2_journal_pin_flush(struct journal *j, struct journal_entry_pin *pin)
{
	BUG_ON(journal_pin_active(pin));

	wait_event(j->pin_flush_wait, j->flush_in_progress != pin);
}

/*
 * Journal reclaim: flush references to open journal entries to reclaim space in
 * the journal
 *
 * May be done by the journal code in the background as needed to free up space
 * for more journal entries, or as part of doing a clean shutdown, or to migrate
 * data off of a specific device:
 */

static struct journal_entry_pin *
journal_get_next_pin(struct journal *j,
		     u64 seq_to_flush,
		     unsigned allowed_below_seq,
		     unsigned allowed_above_seq,
		     u64 *seq)
{
	struct journal_entry_pin_list *pin_list;
	struct journal_entry_pin *ret = NULL;
	unsigned i;

	fifo_for_each_entry_ptr(pin_list, &j->pin, *seq) {
		if (*seq > seq_to_flush && !allowed_above_seq)
			break;

		for (i = 0; i < JOURNAL_PIN_NR; i++)
			if ((((1U << i) & allowed_below_seq) && *seq <= seq_to_flush) ||
			    ((1U << i) & allowed_above_seq)) {
				ret = list_first_entry_or_null(&pin_list->list[i],
					struct journal_entry_pin, list);
				if (ret)
					return ret;
			}
	}

	return NULL;
}

/* returns true if we did work */
static size_t journal_flush_pins(struct journal *j,
				 u64 seq_to_flush,
				 unsigned allowed_below_seq,
				 unsigned allowed_above_seq,
				 unsigned min_any,
				 unsigned min_key_cache)
{
	struct journal_entry_pin *pin;
	size_t nr_flushed = 0;
	journal_pin_flush_fn flush_fn;
	u64 seq;
	int err;

	lockdep_assert_held(&j->reclaim_lock);

	while (1) {
		unsigned allowed_above = allowed_above_seq;
		unsigned allowed_below = allowed_below_seq;

		if (min_any) {
			allowed_above |= ~0;
			allowed_below |= ~0;
		}

		if (min_key_cache) {
			allowed_above |= 1U << JOURNAL_PIN_key_cache;
			allowed_below |= 1U << JOURNAL_PIN_key_cache;
		}

		cond_resched();

		j->last_flushed = jiffies;

		spin_lock(&j->lock);
		pin = journal_get_next_pin(j, seq_to_flush, allowed_below, allowed_above, &seq);
		if (pin) {
			BUG_ON(j->flush_in_progress);
			j->flush_in_progress = pin;
			j->flush_in_progress_dropped = false;
			flush_fn = pin->flush;
		}
		spin_unlock(&j->lock);

		if (!pin)
			break;

		if (min_key_cache && pin->flush == bch2_btree_key_cache_journal_flush)
			min_key_cache--;

		if (min_any)
			min_any--;

		err = flush_fn(j, pin, seq);

		spin_lock(&j->lock);
		/* Pin might have been dropped or rearmed: */
		if (likely(!err && !j->flush_in_progress_dropped))
			list_move(&pin->list, &journal_seq_pin(j, seq)->flushed);
		j->flush_in_progress = NULL;
		j->flush_in_progress_dropped = false;
		spin_unlock(&j->lock);

		wake_up(&j->pin_flush_wait);

		if (err)
			break;

		nr_flushed++;
	}

	return nr_flushed;
}

static u64 journal_seq_to_flush(struct journal *j)
{
	struct bch_fs *c = container_of(j, struct bch_fs, journal);
	struct bch_dev *ca;
	u64 seq_to_flush = 0;
	unsigned iter;

	spin_lock(&j->lock);

	for_each_rw_member(ca, c, iter) {
		struct journal_device *ja = &ca->journal;
		unsigned nr_buckets, bucket_to_flush;

		if (!ja->nr)
			continue;

		/* Try to keep the journal at most half full: */
		nr_buckets = ja->nr / 2;

		/* And include pre-reservations: */
		nr_buckets += DIV_ROUND_UP(j->prereserved.reserved,
					   (ca->mi.bucket_size << 6) -
					   journal_entry_overhead(j));

		nr_buckets = min(nr_buckets, ja->nr);

		bucket_to_flush = (ja->cur_idx + nr_buckets) % ja->nr;
		seq_to_flush = max(seq_to_flush,
				   ja->bucket_seq[bucket_to_flush]);
	}

	/* Also flush if the pin fifo is more than half full */
	seq_to_flush = max_t(s64, seq_to_flush,
			     (s64) journal_cur_seq(j) -
			     (j->pin.size >> 1));
	spin_unlock(&j->lock);

	return seq_to_flush;
}

/**
 * __bch2_journal_reclaim - free up journal buckets
 * @j:		journal object
 * @direct:	direct or background reclaim?
 * @kicked:	requested to run since we last ran?
 * Returns:	0 on success, or -EIO if the journal has been shutdown
 *
 * Background journal reclaim writes out btree nodes. It should be run
 * early enough so that we never completely run out of journal buckets.
 *
 * High watermarks for triggering background reclaim:
 * - FIFO has fewer than 512 entries left
 * - fewer than 25% journal buckets free
 *
 * Background reclaim runs until low watermarks are reached:
 * - FIFO has more than 1024 entries left
 * - more than 50% journal buckets free
 *
 * As long as a reclaim can complete in the time it takes to fill up
 * 512 journal entries or 25% of all journal buckets, then
 * journal_next_bucket() should not stall.
 */
static int __bch2_journal_reclaim(struct journal *j, bool direct, bool kicked)
{
	struct bch_fs *c = container_of(j, struct bch_fs, journal);
	bool kthread = (current->flags & PF_KTHREAD) != 0;
	u64 seq_to_flush;
	size_t min_nr, min_key_cache, nr_flushed;
	unsigned flags;
	int ret = 0;

	/*
	 * We can't invoke memory reclaim while holding the reclaim_lock -
	 * journal reclaim is required to make progress for memory reclaim
	 * (cleaning the caches), so we can't get stuck in memory reclaim while
	 * we're holding the reclaim lock:
	 */
	lockdep_assert_held(&j->reclaim_lock);
	flags = memalloc_noreclaim_save();

	do {
		if (kthread && kthread_should_stop())
			break;

		if (bch2_journal_error(j)) {
			ret = -EIO;
			break;
		}

		bch2_journal_do_discards(j);

		seq_to_flush = journal_seq_to_flush(j);
		min_nr = 0;

		/*
		 * If it's been longer than j->reclaim_delay_ms since we last flushed,
		 * make sure to flush at least one journal pin:
		 */
		if (time_after(jiffies, j->last_flushed +
			       msecs_to_jiffies(c->opts.journal_reclaim_delay)))
			min_nr = 1;

		if (j->prereserved.reserved * 4 > j->prereserved.remaining)
			min_nr = 1;

		if (fifo_free(&j->pin) <= 32)
			min_nr = 1;

		if (atomic_read(&c->btree_cache.dirty) * 2 > c->btree_cache.used)
			min_nr = 1;

		min_key_cache = min(bch2_nr_btree_keys_need_flush(c), (size_t) 128);

		trace_and_count(c, journal_reclaim_start, c,
				direct, kicked,
				min_nr, min_key_cache,
				j->prereserved.reserved,
				j->prereserved.remaining,
				atomic_read(&c->btree_cache.dirty),
				c->btree_cache.used,
				atomic_long_read(&c->btree_key_cache.nr_dirty),
				atomic_long_read(&c->btree_key_cache.nr_keys));

		nr_flushed = journal_flush_pins(j, seq_to_flush,
						~0, 0,
						min_nr, min_key_cache);

		if (direct)
			j->nr_direct_reclaim += nr_flushed;
		else
			j->nr_background_reclaim += nr_flushed;
		trace_and_count(c, journal_reclaim_finish, c, nr_flushed);

		if (nr_flushed)
			wake_up(&j->reclaim_wait);
	} while ((min_nr || min_key_cache) && nr_flushed && !direct);

	memalloc_noreclaim_restore(flags);

	return ret;
}

int bch2_journal_reclaim(struct journal *j)
{
	return __bch2_journal_reclaim(j, true, true);
}

static int bch2_journal_reclaim_thread(void *arg)
{
	struct journal *j = arg;
	struct bch_fs *c = container_of(j, struct bch_fs, journal);
	unsigned long delay, now;
	bool journal_empty;
	int ret = 0;

	set_freezable();

	j->last_flushed = jiffies;

	while (!ret && !kthread_should_stop()) {
		bool kicked = j->reclaim_kicked;

		j->reclaim_kicked = false;

		mutex_lock(&j->reclaim_lock);
		ret = __bch2_journal_reclaim(j, false, kicked);
		mutex_unlock(&j->reclaim_lock);

		now = jiffies;
		delay = msecs_to_jiffies(c->opts.journal_reclaim_delay);
		j->next_reclaim = j->last_flushed + delay;

		if (!time_in_range(j->next_reclaim, now, now + delay))
			j->next_reclaim = now + delay;

		while (1) {
			set_current_state(TASK_INTERRUPTIBLE|TASK_FREEZABLE);
			if (kthread_should_stop())
				break;
			if (j->reclaim_kicked)
				break;

			spin_lock(&j->lock);
			journal_empty = fifo_empty(&j->pin);
			spin_unlock(&j->lock);

			if (journal_empty)
				schedule();
			else if (time_after(j->next_reclaim, jiffies))
				schedule_timeout(j->next_reclaim - jiffies);
			else
				break;
		}
		__set_current_state(TASK_RUNNING);
	}

	return 0;
}

void bch2_journal_reclaim_stop(struct journal *j)
{
	struct task_struct *p = j->reclaim_thread;

	j->reclaim_thread = NULL;

	if (p) {
		kthread_stop(p);
		put_task_struct(p);
	}
}

int bch2_journal_reclaim_start(struct journal *j)
{
	struct bch_fs *c = container_of(j, struct bch_fs, journal);
	struct task_struct *p;
	int ret;

	if (j->reclaim_thread)
		return 0;

	p = kthread_create(bch2_journal_reclaim_thread, j,
			   "bch-reclaim/%s", c->name);
	ret = PTR_ERR_OR_ZERO(p);
	if (ret) {
		bch_err_msg(c, ret, "creating journal reclaim thread");
		return ret;
	}

	get_task_struct(p);
	j->reclaim_thread = p;
	wake_up_process(p);
	return 0;
}

static int journal_flush_done(struct journal *j, u64 seq_to_flush,
			      bool *did_work)
{
	int ret;

	ret = bch2_journal_error(j);
	if (ret)
		return ret;

	mutex_lock(&j->reclaim_lock);

	if (journal_flush_pins(j, seq_to_flush,
			       (1U << JOURNAL_PIN_key_cache)|
			       (1U << JOURNAL_PIN_other), 0, 0, 0) ||
	    journal_flush_pins(j, seq_to_flush,
			       (1U << JOURNAL_PIN_btree), 0, 0, 0))
		*did_work = true;

	spin_lock(&j->lock);
	/*
	 * If journal replay hasn't completed, the unreplayed journal entries
	 * hold refs on their corresponding sequence numbers
	 */
	ret = !test_bit(JOURNAL_REPLAY_DONE, &j->flags) ||
		journal_last_seq(j) > seq_to_flush ||
		!fifo_used(&j->pin);

	spin_unlock(&j->lock);
	mutex_unlock(&j->reclaim_lock);

	return ret;
}

bool bch2_journal_flush_pins(struct journal *j, u64 seq_to_flush)
{
	bool did_work = false;

	if (!test_bit(JOURNAL_STARTED, &j->flags))
		return false;

	closure_wait_event(&j->async_wait,
		journal_flush_done(j, seq_to_flush, &did_work));

	return did_work;
}

int bch2_journal_flush_device_pins(struct journal *j, int dev_idx)
{
	struct bch_fs *c = container_of(j, struct bch_fs, journal);
	struct journal_entry_pin_list *p;
	u64 iter, seq = 0;
	int ret = 0;

	spin_lock(&j->lock);
	fifo_for_each_entry_ptr(p, &j->pin, iter)
		if (dev_idx >= 0
		    ? bch2_dev_list_has_dev(p->devs, dev_idx)
		    : p->devs.nr < c->opts.metadata_replicas)
			seq = iter;
	spin_unlock(&j->lock);

	bch2_journal_flush_pins(j, seq);

	ret = bch2_journal_error(j);
	if (ret)
		return ret;

	mutex_lock(&c->replicas_gc_lock);
	bch2_replicas_gc_start(c, 1 << BCH_DATA_journal);

	/*
	 * Now that we've populated replicas_gc, write to the journal to mark
	 * active journal devices. This handles the case where the journal might
	 * be empty. Otherwise we could clear all journal replicas and
	 * temporarily put the fs into an unrecoverable state. Journal recovery
	 * expects to find devices marked for journal data on unclean mount.
	 */
	ret = bch2_journal_meta(&c->journal);
	if (ret)
		goto err;

	seq = 0;
	spin_lock(&j->lock);
	while (!ret) {
		struct bch_replicas_padded replicas;

		seq = max(seq, journal_last_seq(j));
		if (seq >= j->pin.back)
			break;
		bch2_devlist_to_replicas(&replicas.e, BCH_DATA_journal,
					 journal_seq_pin(j, seq)->devs);
		seq++;

		spin_unlock(&j->lock);
		ret = bch2_mark_replicas(c, &replicas.e);
		spin_lock(&j->lock);
	}
	spin_unlock(&j->lock);
err:
	ret = bch2_replicas_gc_end(c, ret);
	mutex_unlock(&c->replicas_gc_lock);

	return ret;
}