Adding upstream version 6.7.7.upstream/6.7.7

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2146 insertions, 0 deletions

diff --git a/fs/bcachefs/btree_gc.c b/fs/bcachefs/btree_gc.c
new file mode 100644
index 0000000000..30ab78a245
--- /dev/null
+++ b/fs/bcachefs/btree_gc.c
@@ -0,0 +1,2146 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2010 Kent Overstreet <kent.overstreet@gmail.com>
+ * Copyright (C) 2014 Datera Inc.
+ */
+
+#include "bcachefs.h"
+#include "alloc_background.h"
+#include "alloc_foreground.h"
+#include "bkey_methods.h"
+#include "bkey_buf.h"
+#include "btree_journal_iter.h"
+#include "btree_key_cache.h"
+#include "btree_locking.h"
+#include "btree_update_interior.h"
+#include "btree_io.h"
+#include "btree_gc.h"
+#include "buckets.h"
+#include "clock.h"
+#include "debug.h"
+#include "ec.h"
+#include "error.h"
+#include "extents.h"
+#include "journal.h"
+#include "keylist.h"
+#include "move.h"
+#include "recovery.h"
+#include "reflink.h"
+#include "replicas.h"
+#include "super-io.h"
+#include "trace.h"
+
+#include <linux/slab.h>
+#include <linux/bitops.h>
+#include <linux/freezer.h>
+#include <linux/kthread.h>
+#include <linux/preempt.h>
+#include <linux/rcupdate.h>
+#include <linux/sched/task.h>
+
+#define DROP_THIS_NODE		10
+#define DROP_PREV_NODE		11
+
+static bool should_restart_for_topology_repair(struct bch_fs *c)
+{
+	return c->opts.fix_errors != FSCK_FIX_no &&
+		!(c->recovery_passes_complete & BIT_ULL(BCH_RECOVERY_PASS_check_topology));
+}
+
+static inline void __gc_pos_set(struct bch_fs *c, struct gc_pos new_pos)
+{
+	preempt_disable();
+	write_seqcount_begin(&c->gc_pos_lock);
+	c->gc_pos = new_pos;
+	write_seqcount_end(&c->gc_pos_lock);
+	preempt_enable();
+}
+
+static inline void gc_pos_set(struct bch_fs *c, struct gc_pos new_pos)
+{
+	BUG_ON(gc_pos_cmp(new_pos, c->gc_pos) <= 0);
+	__gc_pos_set(c, new_pos);
+}
+
+/*
+ * Missing: if an interior btree node is empty, we need to do something -
+ * perhaps just kill it
+ */
+static int bch2_gc_check_topology(struct bch_fs *c,
+				  struct btree *b,
+				  struct bkey_buf *prev,
+				  struct bkey_buf cur,
+				  bool is_last)
+{
+	struct bpos node_start	= b->data->min_key;
+	struct bpos node_end	= b->data->max_key;
+	struct bpos expected_start = bkey_deleted(&prev->k->k)
+		? node_start
+		: bpos_successor(prev->k->k.p);
+	struct printbuf buf1 = PRINTBUF, buf2 = PRINTBUF;
+	int ret = 0;
+
+	if (cur.k->k.type == KEY_TYPE_btree_ptr_v2) {
+		struct bkey_i_btree_ptr_v2 *bp = bkey_i_to_btree_ptr_v2(cur.k);
+
+		if (!bpos_eq(expected_start, bp->v.min_key)) {
+			bch2_topology_error(c);
+
+			if (bkey_deleted(&prev->k->k)) {
+				prt_printf(&buf1, "start of node: ");
+				bch2_bpos_to_text(&buf1, node_start);
+			} else {
+				bch2_bkey_val_to_text(&buf1, c, bkey_i_to_s_c(prev->k));
+			}
+			bch2_bkey_val_to_text(&buf2, c, bkey_i_to_s_c(cur.k));
+
+			if (__fsck_err(c,
+				       FSCK_CAN_FIX|
+				       FSCK_CAN_IGNORE|
+				       FSCK_NO_RATELIMIT,
+				       btree_node_topology_bad_min_key,
+				       "btree node with incorrect min_key at btree %s level %u:\n"
+				       "  prev %s\n"
+				       "  cur %s",
+				       bch2_btree_id_str(b->c.btree_id), b->c.level,
+				       buf1.buf, buf2.buf) && should_restart_for_topology_repair(c)) {
+				bch_info(c, "Halting mark and sweep to start topology repair pass");
+				ret = bch2_run_explicit_recovery_pass(c, BCH_RECOVERY_PASS_check_topology);
+				goto err;
+			} else {
+				set_bit(BCH_FS_INITIAL_GC_UNFIXED, &c->flags);
+			}
+		}
+	}
+
+	if (is_last && !bpos_eq(cur.k->k.p, node_end)) {
+		bch2_topology_error(c);
+
+		printbuf_reset(&buf1);
+		printbuf_reset(&buf2);
+
+		bch2_bkey_val_to_text(&buf1, c, bkey_i_to_s_c(cur.k));
+		bch2_bpos_to_text(&buf2, node_end);
+
+		if (__fsck_err(c, FSCK_CAN_FIX|FSCK_CAN_IGNORE|FSCK_NO_RATELIMIT,
+			  btree_node_topology_bad_max_key,
+			  "btree node with incorrect max_key at btree %s level %u:\n"
+			  "  %s\n"
+			  "  expected %s",
+			  bch2_btree_id_str(b->c.btree_id), b->c.level,
+			  buf1.buf, buf2.buf) &&
+		    should_restart_for_topology_repair(c)) {
+			bch_info(c, "Halting mark and sweep to start topology repair pass");
+			ret = bch2_run_explicit_recovery_pass(c, BCH_RECOVERY_PASS_check_topology);
+			goto err;
+		} else {
+			set_bit(BCH_FS_INITIAL_GC_UNFIXED, &c->flags);
+		}
+	}
+
+	bch2_bkey_buf_copy(prev, c, cur.k);
+err:
+fsck_err:
+	printbuf_exit(&buf2);
+	printbuf_exit(&buf1);
+	return ret;
+}
+
+static void btree_ptr_to_v2(struct btree *b, struct bkey_i_btree_ptr_v2 *dst)
+{
+	switch (b->key.k.type) {
+	case KEY_TYPE_btree_ptr: {
+		struct bkey_i_btree_ptr *src = bkey_i_to_btree_ptr(&b->key);
+
+		dst->k.p		= src->k.p;
+		dst->v.mem_ptr		= 0;
+		dst->v.seq		= b->data->keys.seq;
+		dst->v.sectors_written	= 0;
+		dst->v.flags		= 0;
+		dst->v.min_key		= b->data->min_key;
+		set_bkey_val_bytes(&dst->k, sizeof(dst->v) + bkey_val_bytes(&src->k));
+		memcpy(dst->v.start, src->v.start, bkey_val_bytes(&src->k));
+		break;
+	}
+	case KEY_TYPE_btree_ptr_v2:
+		bkey_copy(&dst->k_i, &b->key);
+		break;
+	default:
+		BUG();
+	}
+}
+
+static void bch2_btree_node_update_key_early(struct btree_trans *trans,
+					     enum btree_id btree, unsigned level,
+					     struct bkey_s_c old, struct bkey_i *new)
+{
+	struct bch_fs *c = trans->c;
+	struct btree *b;
+	struct bkey_buf tmp;
+	int ret;
+
+	bch2_bkey_buf_init(&tmp);
+	bch2_bkey_buf_reassemble(&tmp, c, old);
+
+	b = bch2_btree_node_get_noiter(trans, tmp.k, btree, level, true);
+	if (!IS_ERR_OR_NULL(b)) {
+		mutex_lock(&c->btree_cache.lock);
+
+		bch2_btree_node_hash_remove(&c->btree_cache, b);
+
+		bkey_copy(&b->key, new);
+		ret = __bch2_btree_node_hash_insert(&c->btree_cache, b);
+		BUG_ON(ret);
+
+		mutex_unlock(&c->btree_cache.lock);
+		six_unlock_read(&b->c.lock);
+	}
+
+	bch2_bkey_buf_exit(&tmp, c);
+}
+
+static int set_node_min(struct bch_fs *c, struct btree *b, struct bpos new_min)
+{
+	struct bkey_i_btree_ptr_v2 *new;
+	int ret;
+
+	new = kmalloc_array(BKEY_BTREE_PTR_U64s_MAX, sizeof(u64), GFP_KERNEL);
+	if (!new)
+		return -BCH_ERR_ENOMEM_gc_repair_key;
+
+	btree_ptr_to_v2(b, new);
+	b->data->min_key	= new_min;
+	new->v.min_key		= new_min;
+	SET_BTREE_PTR_RANGE_UPDATED(&new->v, true);
+
+	ret = bch2_journal_key_insert_take(c, b->c.btree_id, b->c.level + 1, &new->k_i);
+	if (ret) {
+		kfree(new);
+		return ret;
+	}
+
+	bch2_btree_node_drop_keys_outside_node(b);
+	bkey_copy(&b->key, &new->k_i);
+	return 0;
+}
+
+static int set_node_max(struct bch_fs *c, struct btree *b, struct bpos new_max)
+{
+	struct bkey_i_btree_ptr_v2 *new;
+	int ret;
+
+	ret = bch2_journal_key_delete(c, b->c.btree_id, b->c.level + 1, b->key.k.p);
+	if (ret)
+		return ret;
+
+	new = kmalloc_array(BKEY_BTREE_PTR_U64s_MAX, sizeof(u64), GFP_KERNEL);
+	if (!new)
+		return -BCH_ERR_ENOMEM_gc_repair_key;
+
+	btree_ptr_to_v2(b, new);
+	b->data->max_key	= new_max;
+	new->k.p		= new_max;
+	SET_BTREE_PTR_RANGE_UPDATED(&new->v, true);
+
+	ret = bch2_journal_key_insert_take(c, b->c.btree_id, b->c.level + 1, &new->k_i);
+	if (ret) {
+		kfree(new);
+		return ret;
+	}
+
+	bch2_btree_node_drop_keys_outside_node(b);
+
+	mutex_lock(&c->btree_cache.lock);
+	bch2_btree_node_hash_remove(&c->btree_cache, b);
+
+	bkey_copy(&b->key, &new->k_i);
+	ret = __bch2_btree_node_hash_insert(&c->btree_cache, b);
+	BUG_ON(ret);
+	mutex_unlock(&c->btree_cache.lock);
+	return 0;
+}
+
+static int btree_repair_node_boundaries(struct bch_fs *c, struct btree *b,
+					struct btree *prev, struct btree *cur)
+{
+	struct bpos expected_start = !prev
+		? b->data->min_key
+		: bpos_successor(prev->key.k.p);
+	struct printbuf buf1 = PRINTBUF, buf2 = PRINTBUF;
+	int ret = 0;
+
+	if (!prev) {
+		prt_printf(&buf1, "start of node: ");
+		bch2_bpos_to_text(&buf1, b->data->min_key);
+	} else {
+		bch2_bkey_val_to_text(&buf1, c, bkey_i_to_s_c(&prev->key));
+	}
+
+	bch2_bkey_val_to_text(&buf2, c, bkey_i_to_s_c(&cur->key));
+
+	if (prev &&
+	    bpos_gt(expected_start, cur->data->min_key) &&
+	    BTREE_NODE_SEQ(cur->data) > BTREE_NODE_SEQ(prev->data)) {
+		/* cur overwrites prev: */
+
+		if (mustfix_fsck_err_on(bpos_ge(prev->data->min_key,
+						cur->data->min_key), c,
+				btree_node_topology_overwritten_by_next_node,
+				"btree node overwritten by next node at btree %s level %u:\n"
+				"  node %s\n"
+				"  next %s",
+				bch2_btree_id_str(b->c.btree_id), b->c.level,
+				buf1.buf, buf2.buf)) {
+			ret = DROP_PREV_NODE;
+			goto out;
+		}
+
+		if (mustfix_fsck_err_on(!bpos_eq(prev->key.k.p,
+						 bpos_predecessor(cur->data->min_key)), c,
+				btree_node_topology_bad_max_key,
+				"btree node with incorrect max_key at btree %s level %u:\n"
+				"  node %s\n"
+				"  next %s",
+				bch2_btree_id_str(b->c.btree_id), b->c.level,
+				buf1.buf, buf2.buf))
+			ret = set_node_max(c, prev,
+					   bpos_predecessor(cur->data->min_key));
+	} else {
+		/* prev overwrites cur: */
+
+		if (mustfix_fsck_err_on(bpos_ge(expected_start,
+						cur->data->max_key), c,
+				btree_node_topology_overwritten_by_prev_node,
+				"btree node overwritten by prev node at btree %s level %u:\n"
+				"  prev %s\n"
+				"  node %s",
+				bch2_btree_id_str(b->c.btree_id), b->c.level,
+				buf1.buf, buf2.buf)) {
+			ret = DROP_THIS_NODE;
+			goto out;
+		}
+
+		if (mustfix_fsck_err_on(!bpos_eq(expected_start, cur->data->min_key), c,
+				btree_node_topology_bad_min_key,
+				"btree node with incorrect min_key at btree %s level %u:\n"
+				"  prev %s\n"
+				"  node %s",
+				bch2_btree_id_str(b->c.btree_id), b->c.level,
+				buf1.buf, buf2.buf))
+			ret = set_node_min(c, cur, expected_start);
+	}
+out:
+fsck_err:
+	printbuf_exit(&buf2);
+	printbuf_exit(&buf1);
+	return ret;
+}
+
+static int btree_repair_node_end(struct bch_fs *c, struct btree *b,
+				 struct btree *child)
+{
+	struct printbuf buf1 = PRINTBUF, buf2 = PRINTBUF;
+	int ret = 0;
+
+	bch2_bkey_val_to_text(&buf1, c, bkey_i_to_s_c(&child->key));
+	bch2_bpos_to_text(&buf2, b->key.k.p);
+
+	if (mustfix_fsck_err_on(!bpos_eq(child->key.k.p, b->key.k.p), c,
+				btree_node_topology_bad_max_key,
+			"btree node with incorrect max_key at btree %s level %u:\n"
+			"  %s\n"
+			"  expected %s",
+			bch2_btree_id_str(b->c.btree_id), b->c.level,
+			buf1.buf, buf2.buf)) {
+		ret = set_node_max(c, child, b->key.k.p);
+		if (ret)
+			goto err;
+	}
+err:
+fsck_err:
+	printbuf_exit(&buf2);
+	printbuf_exit(&buf1);
+	return ret;
+}
+
+static int bch2_btree_repair_topology_recurse(struct btree_trans *trans, struct btree *b)
+{
+	struct bch_fs *c = trans->c;
+	struct btree_and_journal_iter iter;
+	struct bkey_s_c k;
+	struct bkey_buf prev_k, cur_k;
+	struct btree *prev = NULL, *cur = NULL;
+	bool have_child, dropped_children = false;
+	struct printbuf buf = PRINTBUF;
+	int ret = 0;
+
+	if (!b->c.level)
+		return 0;
+again:
+	prev = NULL;
+	have_child = dropped_children = false;
+	bch2_bkey_buf_init(&prev_k);
+	bch2_bkey_buf_init(&cur_k);
+	bch2_btree_and_journal_iter_init_node_iter(&iter, c, b);
+
+	while ((k = bch2_btree_and_journal_iter_peek(&iter)).k) {
+		BUG_ON(bpos_lt(k.k->p, b->data->min_key));
+		BUG_ON(bpos_gt(k.k->p, b->data->max_key));
+
+		bch2_btree_and_journal_iter_advance(&iter);
+		bch2_bkey_buf_reassemble(&cur_k, c, k);
+
+		cur = bch2_btree_node_get_noiter(trans, cur_k.k,
+					b->c.btree_id, b->c.level - 1,
+					false);
+		ret = PTR_ERR_OR_ZERO(cur);
+
+		printbuf_reset(&buf);
+		bch2_bkey_val_to_text(&buf, c, bkey_i_to_s_c(cur_k.k));
+
+		if (mustfix_fsck_err_on(ret == -EIO, c,
+				btree_node_unreadable,
+				"Topology repair: unreadable btree node at btree %s level %u:\n"
+				"  %s",
+				bch2_btree_id_str(b->c.btree_id),
+				b->c.level - 1,
+				buf.buf)) {
+			bch2_btree_node_evict(trans, cur_k.k);
+			ret = bch2_journal_key_delete(c, b->c.btree_id,
+						      b->c.level, cur_k.k->k.p);
+			cur = NULL;
+			if (ret)
+				break;
+			continue;
+		}
+
+		if (ret) {
+			bch_err_msg(c, ret, "getting btree node");
+			break;
+		}
+
+		ret = btree_repair_node_boundaries(c, b, prev, cur);
+
+		if (ret == DROP_THIS_NODE) {
+			six_unlock_read(&cur->c.lock);
+			bch2_btree_node_evict(trans, cur_k.k);
+			ret = bch2_journal_key_delete(c, b->c.btree_id,
+						      b->c.level, cur_k.k->k.p);
+			cur = NULL;
+			if (ret)
+				break;
+			continue;
+		}
+
+		if (prev)
+			six_unlock_read(&prev->c.lock);
+		prev = NULL;
+
+		if (ret == DROP_PREV_NODE) {
+			bch2_btree_node_evict(trans, prev_k.k);
+			ret = bch2_journal_key_delete(c, b->c.btree_id,
+						      b->c.level, prev_k.k->k.p);
+			if (ret)
+				break;
+
+			bch2_btree_and_journal_iter_exit(&iter);
+			bch2_bkey_buf_exit(&prev_k, c);
+			bch2_bkey_buf_exit(&cur_k, c);
+			goto again;
+		} else if (ret)
+			break;
+
+		prev = cur;
+		cur = NULL;
+		bch2_bkey_buf_copy(&prev_k, c, cur_k.k);
+	}
+
+	if (!ret && !IS_ERR_OR_NULL(prev)) {
+		BUG_ON(cur);
+		ret = btree_repair_node_end(c, b, prev);
+	}
+
+	if (!IS_ERR_OR_NULL(prev))
+		six_unlock_read(&prev->c.lock);
+	prev = NULL;
+	if (!IS_ERR_OR_NULL(cur))
+		six_unlock_read(&cur->c.lock);
+	cur = NULL;
+
+	if (ret)
+		goto err;
+
+	bch2_btree_and_journal_iter_exit(&iter);
+	bch2_btree_and_journal_iter_init_node_iter(&iter, c, b);
+
+	while ((k = bch2_btree_and_journal_iter_peek(&iter)).k) {
+		bch2_bkey_buf_reassemble(&cur_k, c, k);
+		bch2_btree_and_journal_iter_advance(&iter);
+
+		cur = bch2_btree_node_get_noiter(trans, cur_k.k,
+					b->c.btree_id, b->c.level - 1,
+					false);
+		ret = PTR_ERR_OR_ZERO(cur);
+
+		if (ret) {
+			bch_err_msg(c, ret, "getting btree node");
+			goto err;
+		}
+
+		ret = bch2_btree_repair_topology_recurse(trans, cur);
+		six_unlock_read(&cur->c.lock);
+		cur = NULL;
+
+		if (ret == DROP_THIS_NODE) {
+			bch2_btree_node_evict(trans, cur_k.k);
+			ret = bch2_journal_key_delete(c, b->c.btree_id,
+						      b->c.level, cur_k.k->k.p);
+			dropped_children = true;
+		}
+
+		if (ret)
+			goto err;
+
+		have_child = true;
+	}
+
+	printbuf_reset(&buf);
+	bch2_bkey_val_to_text(&buf, c, bkey_i_to_s_c(&b->key));
+
+	if (mustfix_fsck_err_on(!have_child, c,
+			btree_node_topology_interior_node_empty,
+			"empty interior btree node at btree %s level %u\n"
+			"  %s",
+			bch2_btree_id_str(b->c.btree_id),
+			b->c.level, buf.buf))
+		ret = DROP_THIS_NODE;
+err:
+fsck_err:
+	if (!IS_ERR_OR_NULL(prev))
+		six_unlock_read(&prev->c.lock);
+	if (!IS_ERR_OR_NULL(cur))
+		six_unlock_read(&cur->c.lock);
+
+	bch2_btree_and_journal_iter_exit(&iter);
+	bch2_bkey_buf_exit(&prev_k, c);
+	bch2_bkey_buf_exit(&cur_k, c);
+
+	if (!ret && dropped_children)
+		goto again;
+
+	printbuf_exit(&buf);
+	return ret;
+}
+
+int bch2_check_topology(struct bch_fs *c)
+{
+	struct btree_trans *trans = bch2_trans_get(c);
+	struct btree *b;
+	unsigned i;
+	int ret = 0;
+
+	for (i = 0; i < btree_id_nr_alive(c) && !ret; i++) {
+		struct btree_root *r = bch2_btree_id_root(c, i);
+
+		if (!r->alive)
+			continue;
+
+		b = r->b;
+		if (btree_node_fake(b))
+			continue;
+
+		btree_node_lock_nopath_nofail(trans, &b->c, SIX_LOCK_read);
+		ret = bch2_btree_repair_topology_recurse(trans, b);
+		six_unlock_read(&b->c.lock);
+
+		if (ret == DROP_THIS_NODE) {
+			bch_err(c, "empty btree root - repair unimplemented");
+			ret = -BCH_ERR_fsck_repair_unimplemented;
+		}
+	}
+
+	bch2_trans_put(trans);
+
+	return ret;
+}
+
+static int bch2_check_fix_ptrs(struct btree_trans *trans, enum btree_id btree_id,
+			       unsigned level, bool is_root,
+			       struct bkey_s_c *k)
+{
+	struct bch_fs *c = trans->c;
+	struct bkey_ptrs_c ptrs_c = bch2_bkey_ptrs_c(*k);
+	const union bch_extent_entry *entry_c;
+	struct extent_ptr_decoded p = { 0 };
+	bool do_update = false;
+	struct printbuf buf = PRINTBUF;
+	int ret = 0;
+
+	/*
+	 * XXX
+	 * use check_bucket_ref here
+	 */
+	bkey_for_each_ptr_decode(k->k, ptrs_c, p, entry_c) {
+		struct bch_dev *ca = bch_dev_bkey_exists(c, p.ptr.dev);
+		struct bucket *g = PTR_GC_BUCKET(ca, &p.ptr);
+		enum bch_data_type data_type = bch2_bkey_ptr_data_type(*k, &entry_c->ptr);
+
+		if (!g->gen_valid &&
+		    (c->opts.reconstruct_alloc ||
+		     fsck_err(c, ptr_to_missing_alloc_key,
+			      "bucket %u:%zu data type %s ptr gen %u missing in alloc btree\n"
+			      "while marking %s",
+			      p.ptr.dev, PTR_BUCKET_NR(ca, &p.ptr),
+			      bch2_data_types[ptr_data_type(k->k, &p.ptr)],
+			      p.ptr.gen,
+			      (printbuf_reset(&buf),
+			       bch2_bkey_val_to_text(&buf, c, *k), buf.buf)))) {
+			if (!p.ptr.cached) {
+				g->gen_valid		= true;
+				g->gen			= p.ptr.gen;
+			} else {
+				do_update = true;
+			}
+		}
+
+		if (gen_cmp(p.ptr.gen, g->gen) > 0 &&
+		    (c->opts.reconstruct_alloc ||
+		     fsck_err(c, ptr_gen_newer_than_bucket_gen,
+			      "bucket %u:%zu data type %s ptr gen in the future: %u > %u\n"
+			      "while marking %s",
+			      p.ptr.dev, PTR_BUCKET_NR(ca, &p.ptr),
+			      bch2_data_types[ptr_data_type(k->k, &p.ptr)],
+			      p.ptr.gen, g->gen,
+			      (printbuf_reset(&buf),
+			       bch2_bkey_val_to_text(&buf, c, *k), buf.buf)))) {
+			if (!p.ptr.cached) {
+				g->gen_valid		= true;
+				g->gen			= p.ptr.gen;
+				g->data_type		= 0;
+				g->dirty_sectors	= 0;
+				g->cached_sectors	= 0;
+				set_bit(BCH_FS_NEED_ANOTHER_GC, &c->flags);
+			} else {
+				do_update = true;
+			}
+		}
+
+		if (gen_cmp(g->gen, p.ptr.gen) > BUCKET_GC_GEN_MAX &&
+		    (c->opts.reconstruct_alloc ||
+		     fsck_err(c, ptr_gen_newer_than_bucket_gen,
+			      "bucket %u:%zu gen %u data type %s: ptr gen %u too stale\n"
+			      "while marking %s",
+			      p.ptr.dev, PTR_BUCKET_NR(ca, &p.ptr), g->gen,
+			      bch2_data_types[ptr_data_type(k->k, &p.ptr)],
+			      p.ptr.gen,
+			      (printbuf_reset(&buf),
+			       bch2_bkey_val_to_text(&buf, c, *k), buf.buf))))
+			do_update = true;
+
+		if (!p.ptr.cached && gen_cmp(p.ptr.gen, g->gen) < 0 &&
+		    (c->opts.reconstruct_alloc ||
+		     fsck_err(c, stale_dirty_ptr,
+			      "bucket %u:%zu data type %s stale dirty ptr: %u < %u\n"
+			      "while marking %s",
+			      p.ptr.dev, PTR_BUCKET_NR(ca, &p.ptr),
+			      bch2_data_types[ptr_data_type(k->k, &p.ptr)],
+			      p.ptr.gen, g->gen,
+			      (printbuf_reset(&buf),
+			       bch2_bkey_val_to_text(&buf, c, *k), buf.buf))))
+			do_update = true;
+
+		if (data_type != BCH_DATA_btree && p.ptr.gen != g->gen)
+			continue;
+
+		if (fsck_err_on(bucket_data_type(g->data_type) &&
+				bucket_data_type(g->data_type) != data_type, c,
+				ptr_bucket_data_type_mismatch,
+				"bucket %u:%zu different types of data in same bucket: %s, %s\n"
+				"while marking %s",
+				p.ptr.dev, PTR_BUCKET_NR(ca, &p.ptr),
+				bch2_data_types[g->data_type],
+				bch2_data_types[data_type],
+				(printbuf_reset(&buf),
+				 bch2_bkey_val_to_text(&buf, c, *k), buf.buf))) {
+			if (data_type == BCH_DATA_btree) {
+				g->data_type	= data_type;
+				set_bit(BCH_FS_NEED_ANOTHER_GC, &c->flags);
+			} else {
+				do_update = true;
+			}
+		}
+
+		if (p.has_ec) {
+			struct gc_stripe *m = genradix_ptr(&c->gc_stripes, p.ec.idx);
+
+			if (fsck_err_on(!m || !m->alive, c,
+					ptr_to_missing_stripe,
+					"pointer to nonexistent stripe %llu\n"
+					"while marking %s",
+					(u64) p.ec.idx,
+					(printbuf_reset(&buf),
+					 bch2_bkey_val_to_text(&buf, c, *k), buf.buf)))
+				do_update = true;
+
+			if (fsck_err_on(m && m->alive && !bch2_ptr_matches_stripe_m(m, p), c,
+					ptr_to_incorrect_stripe,
+					"pointer does not match stripe %llu\n"
+					"while marking %s",
+					(u64) p.ec.idx,
+					(printbuf_reset(&buf),
+					 bch2_bkey_val_to_text(&buf, c, *k), buf.buf)))
+				do_update = true;
+		}
+	}
+
+	if (do_update) {
+		struct bkey_ptrs ptrs;
+		union bch_extent_entry *entry;
+		struct bch_extent_ptr *ptr;
+		struct bkey_i *new;
+
+		if (is_root) {
+			bch_err(c, "cannot update btree roots yet");
+			ret = -EINVAL;
+			goto err;
+		}
+
+		new = kmalloc(bkey_bytes(k->k), GFP_KERNEL);
+		if (!new) {
+			bch_err_msg(c, ret, "allocating new key");
+			ret = -BCH_ERR_ENOMEM_gc_repair_key;
+			goto err;
+		}
+
+		bkey_reassemble(new, *k);
+
+		if (level) {
+			/*
+			 * We don't want to drop btree node pointers - if the
+			 * btree node isn't there anymore, the read path will
+			 * sort it out:
+			 */
+			ptrs = bch2_bkey_ptrs(bkey_i_to_s(new));
+			bkey_for_each_ptr(ptrs, ptr) {
+				struct bch_dev *ca = bch_dev_bkey_exists(c, ptr->dev);
+				struct bucket *g = PTR_GC_BUCKET(ca, ptr);
+
+				ptr->gen = g->gen;
+			}
+		} else {
+			bch2_bkey_drop_ptrs(bkey_i_to_s(new), ptr, ({
+				struct bch_dev *ca = bch_dev_bkey_exists(c, ptr->dev);
+				struct bucket *g = PTR_GC_BUCKET(ca, ptr);
+				enum bch_data_type data_type = bch2_bkey_ptr_data_type(*k, ptr);
+
+				(ptr->cached &&
+				 (!g->gen_valid || gen_cmp(ptr->gen, g->gen) > 0)) ||
+				(!ptr->cached &&
+				 gen_cmp(ptr->gen, g->gen) < 0) ||
+				gen_cmp(g->gen, ptr->gen) > BUCKET_GC_GEN_MAX ||
+				(g->data_type &&
+				 g->data_type != data_type);
+			}));
+again:
+			ptrs = bch2_bkey_ptrs(bkey_i_to_s(new));
+			bkey_extent_entry_for_each(ptrs, entry) {
+				if (extent_entry_type(entry) == BCH_EXTENT_ENTRY_stripe_ptr) {
+					struct gc_stripe *m = genradix_ptr(&c->gc_stripes,
+									entry->stripe_ptr.idx);
+					union bch_extent_entry *next_ptr;
+
+					bkey_extent_entry_for_each_from(ptrs, next_ptr, entry)
+						if (extent_entry_type(next_ptr) == BCH_EXTENT_ENTRY_ptr)
+							goto found;
+					next_ptr = NULL;
+found:
+					if (!next_ptr) {
+						bch_err(c, "aieee, found stripe ptr with no data ptr");
+						continue;
+					}
+
+					if (!m || !m->alive ||
+					    !__bch2_ptr_matches_stripe(&m->ptrs[entry->stripe_ptr.block],
+								       &next_ptr->ptr,
+								       m->sectors)) {
+						bch2_bkey_extent_entry_drop(new, entry);
+						goto again;
+					}
+				}
+			}
+		}
+
+		ret = bch2_journal_key_insert_take(c, btree_id, level, new);
+		if (ret) {
+			kfree(new);
+			goto err;
+		}
+
+		if (level)
+			bch2_btree_node_update_key_early(trans, btree_id, level - 1, *k, new);
+
+		if (0) {
+			printbuf_reset(&buf);
+			bch2_bkey_val_to_text(&buf, c, *k);
+			bch_info(c, "updated %s", buf.buf);
+
+			printbuf_reset(&buf);
+			bch2_bkey_val_to_text(&buf, c, bkey_i_to_s_c(new));
+			bch_info(c, "new key %s", buf.buf);
+		}
+
+		*k = bkey_i_to_s_c(new);
+	}
+err:
+fsck_err:
+	printbuf_exit(&buf);
+	return ret;
+}
+
+/* marking of btree keys/nodes: */
+
+static int bch2_gc_mark_key(struct btree_trans *trans, enum btree_id btree_id,
+			    unsigned level, bool is_root,
+			    struct bkey_s_c *k,
+			    bool initial)
+{
+	struct bch_fs *c = trans->c;
+	struct bkey deleted = KEY(0, 0, 0);
+	struct bkey_s_c old = (struct bkey_s_c) { &deleted, NULL };
+	unsigned flags =
+		BTREE_TRIGGER_GC|
+		(initial ? BTREE_TRIGGER_NOATOMIC : 0);
+	int ret = 0;
+
+	deleted.p = k->k->p;
+
+	if (initial) {
+		BUG_ON(bch2_journal_seq_verify &&
+		       k->k->version.lo > atomic64_read(&c->journal.seq));
+
+		ret = bch2_check_fix_ptrs(trans, btree_id, level, is_root, k);
+		if (ret)
+			goto err;
+
+		if (fsck_err_on(k->k->version.lo > atomic64_read(&c->key_version), c,
+				bkey_version_in_future,
+				"key version number higher than recorded: %llu > %llu",
+				k->k->version.lo,
+				atomic64_read(&c->key_version)))
+			atomic64_set(&c->key_version, k->k->version.lo);
+	}
+
+	ret = commit_do(trans, NULL, NULL, 0,
+			bch2_mark_key(trans, btree_id, level, old, *k, flags));
+fsck_err:
+err:
+	if (ret)
+		bch_err_fn(c, ret);
+	return ret;
+}
+
+static int btree_gc_mark_node(struct btree_trans *trans, struct btree *b, bool initial)
+{
+	struct bch_fs *c = trans->c;
+	struct btree_node_iter iter;
+	struct bkey unpacked;
+	struct bkey_s_c k;
+	struct bkey_buf prev, cur;
+	int ret = 0;
+
+	if (!btree_node_type_needs_gc(btree_node_type(b)))
+		return 0;
+
+	bch2_btree_node_iter_init_from_start(&iter, b);
+	bch2_bkey_buf_init(&prev);
+	bch2_bkey_buf_init(&cur);
+	bkey_init(&prev.k->k);
+
+	while ((k = bch2_btree_node_iter_peek_unpack(&iter, b, &unpacked)).k) {
+		ret = bch2_gc_mark_key(trans, b->c.btree_id, b->c.level, false,
+				       &k, initial);
+		if (ret)
+			break;
+
+		bch2_btree_node_iter_advance(&iter, b);
+
+		if (b->c.level) {
+			bch2_bkey_buf_reassemble(&cur, c, k);
+
+			ret = bch2_gc_check_topology(c, b, &prev, cur,
+					bch2_btree_node_iter_end(&iter));
+			if (ret)
+				break;
+		}
+	}
+
+	bch2_bkey_buf_exit(&cur, c);
+	bch2_bkey_buf_exit(&prev, c);
+	return ret;
+}
+
+static int bch2_gc_btree(struct btree_trans *trans, enum btree_id btree_id,
+			 bool initial, bool metadata_only)
+{
+	struct bch_fs *c = trans->c;
+	struct btree_iter iter;
+	struct btree *b;
+	unsigned depth = metadata_only ? 1 : 0;
+	int ret = 0;
+
+	gc_pos_set(c, gc_pos_btree(btree_id, POS_MIN, 0));
+
+	__for_each_btree_node(trans, iter, btree_id, POS_MIN,
+			      0, depth, BTREE_ITER_PREFETCH, b, ret) {
+		bch2_verify_btree_nr_keys(b);
+
+		gc_pos_set(c, gc_pos_btree_node(b));
+
+		ret = btree_gc_mark_node(trans, b, initial);
+		if (ret)
+			break;
+	}
+	bch2_trans_iter_exit(trans, &iter);
+
+	if (ret)
+		return ret;
+
+	mutex_lock(&c->btree_root_lock);
+	b = bch2_btree_id_root(c, btree_id)->b;
+	if (!btree_node_fake(b)) {
+		struct bkey_s_c k = bkey_i_to_s_c(&b->key);
+
+		ret = bch2_gc_mark_key(trans, b->c.btree_id, b->c.level + 1,
+				       true, &k, initial);
+	}
+	gc_pos_set(c, gc_pos_btree_root(b->c.btree_id));
+	mutex_unlock(&c->btree_root_lock);
+
+	return ret;
+}
+
+static int bch2_gc_btree_init_recurse(struct btree_trans *trans, struct btree *b,
+				      unsigned target_depth)
+{
+	struct bch_fs *c = trans->c;
+	struct btree_and_journal_iter iter;
+	struct bkey_s_c k;
+	struct bkey_buf cur, prev;
+	struct printbuf buf = PRINTBUF;
+	int ret = 0;
+
+	bch2_btree_and_journal_iter_init_node_iter(&iter, c, b);
+	bch2_bkey_buf_init(&prev);
+	bch2_bkey_buf_init(&cur);
+	bkey_init(&prev.k->k);
+
+	while ((k = bch2_btree_and_journal_iter_peek(&iter)).k) {
+		BUG_ON(bpos_lt(k.k->p, b->data->min_key));
+		BUG_ON(bpos_gt(k.k->p, b->data->max_key));
+
+		ret = bch2_gc_mark_key(trans, b->c.btree_id, b->c.level,
+				       false, &k, true);
+		if (ret)
+			goto fsck_err;
+
+		if (b->c.level) {
+			bch2_bkey_buf_reassemble(&cur, c, k);
+			k = bkey_i_to_s_c(cur.k);
+
+			bch2_btree_and_journal_iter_advance(&iter);
+
+			ret = bch2_gc_check_topology(c, b,
+					&prev, cur,
+					!bch2_btree_and_journal_iter_peek(&iter).k);
+			if (ret)
+				goto fsck_err;
+		} else {
+			bch2_btree_and_journal_iter_advance(&iter);
+		}
+	}
+
+	if (b->c.level > target_depth) {
+		bch2_btree_and_journal_iter_exit(&iter);
+		bch2_btree_and_journal_iter_init_node_iter(&iter, c, b);
+
+		while ((k = bch2_btree_and_journal_iter_peek(&iter)).k) {
+			struct btree *child;
+
+			bch2_bkey_buf_reassemble(&cur, c, k);
+			bch2_btree_and_journal_iter_advance(&iter);
+
+			child = bch2_btree_node_get_noiter(trans, cur.k,
+						b->c.btree_id, b->c.level - 1,
+						false);
+			ret = PTR_ERR_OR_ZERO(child);
+
+			if (ret == -EIO) {
+				bch2_topology_error(c);
+
+				if (__fsck_err(c,
+					  FSCK_CAN_FIX|
+					  FSCK_CAN_IGNORE|
+					  FSCK_NO_RATELIMIT,
+					  btree_node_read_error,
+					  "Unreadable btree node at btree %s level %u:\n"
+					  "  %s",
+					  bch2_btree_id_str(b->c.btree_id),
+					  b->c.level - 1,
+					  (printbuf_reset(&buf),
+					   bch2_bkey_val_to_text(&buf, c, bkey_i_to_s_c(cur.k)), buf.buf)) &&
+				    should_restart_for_topology_repair(c)) {
+					bch_info(c, "Halting mark and sweep to start topology repair pass");
+					ret = bch2_run_explicit_recovery_pass(c, BCH_RECOVERY_PASS_check_topology);
+					goto fsck_err;
+				} else {
+					/* Continue marking when opted to not
+					 * fix the error: */
+					ret = 0;
+					set_bit(BCH_FS_INITIAL_GC_UNFIXED, &c->flags);
+					continue;
+				}
+			} else if (ret) {
+				bch_err_msg(c, ret, "getting btree node");
+				break;
+			}
+
+			ret = bch2_gc_btree_init_recurse(trans, child,
+							 target_depth);
+			six_unlock_read(&child->c.lock);
+
+			if (ret)
+				break;
+		}
+	}
+fsck_err:
+	bch2_bkey_buf_exit(&cur, c);
+	bch2_bkey_buf_exit(&prev, c);
+	bch2_btree_and_journal_iter_exit(&iter);
+	printbuf_exit(&buf);
+	return ret;
+}
+
+static int bch2_gc_btree_init(struct btree_trans *trans,
+			      enum btree_id btree_id,
+			      bool metadata_only)
+{
+	struct bch_fs *c = trans->c;
+	struct btree *b;
+	unsigned target_depth = metadata_only ? 1 : 0;
+	struct printbuf buf = PRINTBUF;
+	int ret = 0;
+
+	b = bch2_btree_id_root(c, btree_id)->b;
+
+	if (btree_node_fake(b))
+		return 0;
+
+	six_lock_read(&b->c.lock, NULL, NULL);
+	printbuf_reset(&buf);
+	bch2_bpos_to_text(&buf, b->data->min_key);
+	if (mustfix_fsck_err_on(!bpos_eq(b->data->min_key, POS_MIN), c,
+				btree_root_bad_min_key,
+			"btree root with incorrect min_key: %s", buf.buf)) {
+		bch_err(c, "repair unimplemented");
+		ret = -BCH_ERR_fsck_repair_unimplemented;
+		goto fsck_err;
+	}
+
+	printbuf_reset(&buf);
+	bch2_bpos_to_text(&buf, b->data->max_key);
+	if (mustfix_fsck_err_on(!bpos_eq(b->data->max_key, SPOS_MAX), c,
+				btree_root_bad_max_key,
+			"btree root with incorrect max_key: %s", buf.buf)) {
+		bch_err(c, "repair unimplemented");
+		ret = -BCH_ERR_fsck_repair_unimplemented;
+		goto fsck_err;
+	}
+
+	if (b->c.level >= target_depth)
+		ret = bch2_gc_btree_init_recurse(trans, b, target_depth);
+
+	if (!ret) {
+		struct bkey_s_c k = bkey_i_to_s_c(&b->key);
+
+		ret = bch2_gc_mark_key(trans, b->c.btree_id, b->c.level + 1, true,
+				       &k, true);
+	}
+fsck_err:
+	six_unlock_read(&b->c.lock);
+
+	if (ret < 0)
+		bch_err_fn(c, ret);
+	printbuf_exit(&buf);
+	return ret;
+}
+
+static inline int btree_id_gc_phase_cmp(enum btree_id l, enum btree_id r)
+{
+	return  (int) btree_id_to_gc_phase(l) -
+		(int) btree_id_to_gc_phase(r);
+}
+
+static int bch2_gc_btrees(struct bch_fs *c, bool initial, bool metadata_only)
+{
+	struct btree_trans *trans = bch2_trans_get(c);
+	enum btree_id ids[BTREE_ID_NR];
+	unsigned i;
+	int ret = 0;
+
+	for (i = 0; i < BTREE_ID_NR; i++)
+		ids[i] = i;
+	bubble_sort(ids, BTREE_ID_NR, btree_id_gc_phase_cmp);
+
+	for (i = 0; i < BTREE_ID_NR && !ret; i++)
+		ret = initial
+			? bch2_gc_btree_init(trans, ids[i], metadata_only)
+			: bch2_gc_btree(trans, ids[i], initial, metadata_only);
+
+	for (i = BTREE_ID_NR; i < btree_id_nr_alive(c) && !ret; i++) {
+		if (!bch2_btree_id_root(c, i)->alive)
+			continue;
+
+		ret = initial
+			? bch2_gc_btree_init(trans, i, metadata_only)
+			: bch2_gc_btree(trans, i, initial, metadata_only);
+	}
+
+	if (ret < 0)
+		bch_err_fn(c, ret);
+
+	bch2_trans_put(trans);
+	return ret;
+}
+
+static void mark_metadata_sectors(struct bch_fs *c, struct bch_dev *ca,
+				  u64 start, u64 end,
+				  enum bch_data_type type,
+				  unsigned flags)
+{
+	u64 b = sector_to_bucket(ca, start);
+
+	do {
+		unsigned sectors =
+			min_t(u64, bucket_to_sector(ca, b + 1), end) - start;
+
+		bch2_mark_metadata_bucket(c, ca, b, type, sectors,
+					  gc_phase(GC_PHASE_SB), flags);
+		b++;
+		start += sectors;
+	} while (start < end);
+}
+
+static void bch2_mark_dev_superblock(struct bch_fs *c, struct bch_dev *ca,
+				     unsigned flags)
+{
+	struct bch_sb_layout *layout = &ca->disk_sb.sb->layout;
+	unsigned i;
+	u64 b;
+
+	for (i = 0; i < layout->nr_superblocks; i++) {
+		u64 offset = le64_to_cpu(layout->sb_offset[i]);
+
+		if (offset == BCH_SB_SECTOR)
+			mark_metadata_sectors(c, ca, 0, BCH_SB_SECTOR,
+					      BCH_DATA_sb, flags);
+
+		mark_metadata_sectors(c, ca, offset,
+				      offset + (1 << layout->sb_max_size_bits),
+				      BCH_DATA_sb, flags);
+	}
+
+	for (i = 0; i < ca->journal.nr; i++) {
+		b = ca->journal.buckets[i];
+		bch2_mark_metadata_bucket(c, ca, b, BCH_DATA_journal,
+					  ca->mi.bucket_size,
+					  gc_phase(GC_PHASE_SB), flags);
+	}
+}
+
+static void bch2_mark_superblocks(struct bch_fs *c)
+{
+	struct bch_dev *ca;
+	unsigned i;
+
+	mutex_lock(&c->sb_lock);
+	gc_pos_set(c, gc_phase(GC_PHASE_SB));
+
+	for_each_online_member(ca, c, i)
+		bch2_mark_dev_superblock(c, ca, BTREE_TRIGGER_GC);
+	mutex_unlock(&c->sb_lock);
+}
+
+#if 0
+/* Also see bch2_pending_btree_node_free_insert_done() */
+static void bch2_mark_pending_btree_node_frees(struct bch_fs *c)
+{
+	struct btree_update *as;
+	struct pending_btree_node_free *d;
+
+	mutex_lock(&c->btree_interior_update_lock);
+	gc_pos_set(c, gc_phase(GC_PHASE_PENDING_DELETE));
+
+	for_each_pending_btree_node_free(c, as, d)
+		if (d->index_update_done)
+			bch2_mark_key(c, bkey_i_to_s_c(&d->key), BTREE_TRIGGER_GC);
+
+	mutex_unlock(&c->btree_interior_update_lock);
+}
+#endif
+
+static void bch2_gc_free(struct bch_fs *c)
+{
+	struct bch_dev *ca;
+	unsigned i;
+
+	genradix_free(&c->reflink_gc_table);
+	genradix_free(&c->gc_stripes);
+
+	for_each_member_device(ca, c, i) {
+		kvpfree(rcu_dereference_protected(ca->buckets_gc, 1),
+			sizeof(struct bucket_array) +
+			ca->mi.nbuckets * sizeof(struct bucket));
+		ca->buckets_gc = NULL;
+
+		free_percpu(ca->usage_gc);
+		ca->usage_gc = NULL;
+	}
+
+	free_percpu(c->usage_gc);
+	c->usage_gc = NULL;
+}
+
+static int bch2_gc_done(struct bch_fs *c,
+			bool initial, bool metadata_only)
+{
+	struct bch_dev *ca = NULL;
+	struct printbuf buf = PRINTBUF;
+	bool verify = !metadata_only &&
+		!c->opts.reconstruct_alloc &&
+		(!initial || (c->sb.compat & (1ULL << BCH_COMPAT_alloc_info)));
+	unsigned i, dev;
+	int ret = 0;
+
+	percpu_down_write(&c->mark_lock);
+
+#define copy_field(_err, _f, _msg, ...)					\
+	if (dst->_f != src->_f &&					\
+	    (!verify ||							\
+	     fsck_err(c, _err, _msg ": got %llu, should be %llu"	\
+		      , ##__VA_ARGS__, dst->_f, src->_f)))		\
+		dst->_f = src->_f
+#define copy_dev_field(_err, _f, _msg, ...)				\
+	copy_field(_err, _f, "dev %u has wrong " _msg, dev, ##__VA_ARGS__)
+#define copy_fs_field(_err, _f, _msg, ...)				\
+	copy_field(_err, _f, "fs has wrong " _msg, ##__VA_ARGS__)
+
+	for (i = 0; i < ARRAY_SIZE(c->usage); i++)
+		bch2_fs_usage_acc_to_base(c, i);
+
+	for_each_member_device(ca, c, dev) {
+		struct bch_dev_usage *dst = ca->usage_base;
+		struct bch_dev_usage *src = (void *)
+			bch2_acc_percpu_u64s((u64 __percpu *) ca->usage_gc,
+					     dev_usage_u64s());
+
+		for (i = 0; i < BCH_DATA_NR; i++) {
+			copy_dev_field(dev_usage_buckets_wrong,
+				       d[i].buckets,	"%s buckets", bch2_data_types[i]);
+			copy_dev_field(dev_usage_sectors_wrong,
+				       d[i].sectors,	"%s sectors", bch2_data_types[i]);
+			copy_dev_field(dev_usage_fragmented_wrong,
+				       d[i].fragmented,	"%s fragmented", bch2_data_types[i]);
+		}
+
+		copy_dev_field(dev_usage_buckets_ec_wrong,
+			       buckets_ec,		"buckets_ec");
+	}
+
+	{
+		unsigned nr = fs_usage_u64s(c);
+		struct bch_fs_usage *dst = c->usage_base;
+		struct bch_fs_usage *src = (void *)
+			bch2_acc_percpu_u64s((u64 __percpu *) c->usage_gc, nr);
+
+		copy_fs_field(fs_usage_hidden_wrong,
+			      hidden,		"hidden");
+		copy_fs_field(fs_usage_btree_wrong,
+			      btree,		"btree");
+
+		if (!metadata_only) {
+			copy_fs_field(fs_usage_data_wrong,
+				      data,	"data");
+			copy_fs_field(fs_usage_cached_wrong,
+				      cached,	"cached");
+			copy_fs_field(fs_usage_reserved_wrong,
+				      reserved,	"reserved");
+			copy_fs_field(fs_usage_nr_inodes_wrong,
+				      nr_inodes,"nr_inodes");
+
+			for (i = 0; i < BCH_REPLICAS_MAX; i++)
+				copy_fs_field(fs_usage_persistent_reserved_wrong,
+					      persistent_reserved[i],
+					      "persistent_reserved[%i]", i);
+		}
+
+		for (i = 0; i < c->replicas.nr; i++) {
+			struct bch_replicas_entry *e =
+				cpu_replicas_entry(&c->replicas, i);
+
+			if (metadata_only &&
+			    (e->data_type == BCH_DATA_user ||
+			     e->data_type == BCH_DATA_cached))
+				continue;
+
+			printbuf_reset(&buf);
+			bch2_replicas_entry_to_text(&buf, e);
+
+			copy_fs_field(fs_usage_replicas_wrong,
+				      replicas[i], "%s", buf.buf);
+		}
+	}
+
+#undef copy_fs_field
+#undef copy_dev_field
+#undef copy_stripe_field
+#undef copy_field
+fsck_err:
+	if (ca)
+		percpu_ref_put(&ca->ref);
+	if (ret)
+		bch_err_fn(c, ret);
+
+	percpu_up_write(&c->mark_lock);
+	printbuf_exit(&buf);
+	return ret;
+}
+
+static int bch2_gc_start(struct bch_fs *c)
+{
+	struct bch_dev *ca = NULL;
+	unsigned i;
+
+	BUG_ON(c->usage_gc);
+
+	c->usage_gc = __alloc_percpu_gfp(fs_usage_u64s(c) * sizeof(u64),
+					 sizeof(u64), GFP_KERNEL);
+	if (!c->usage_gc) {
+		bch_err(c, "error allocating c->usage_gc");
+		return -BCH_ERR_ENOMEM_gc_start;
+	}
+
+	for_each_member_device(ca, c, i) {
+		BUG_ON(ca->usage_gc);
+
+		ca->usage_gc = alloc_percpu(struct bch_dev_usage);
+		if (!ca->usage_gc) {
+			bch_err(c, "error allocating ca->usage_gc");
+			percpu_ref_put(&ca->ref);
+			return -BCH_ERR_ENOMEM_gc_start;
+		}
+
+		this_cpu_write(ca->usage_gc->d[BCH_DATA_free].buckets,
+			       ca->mi.nbuckets - ca->mi.first_bucket);
+	}
+
+	return 0;
+}
+
+static int bch2_gc_reset(struct bch_fs *c)
+{
+	struct bch_dev *ca;
+	unsigned i;
+
+	for_each_member_device(ca, c, i) {
+		free_percpu(ca->usage_gc);
+		ca->usage_gc = NULL;
+	}
+
+	free_percpu(c->usage_gc);
+	c->usage_gc = NULL;
+
+	return bch2_gc_start(c);
+}
+
+/* returns true if not equal */
+static inline bool bch2_alloc_v4_cmp(struct bch_alloc_v4 l,
+				     struct bch_alloc_v4 r)
+{
+	return  l.gen != r.gen				||
+		l.oldest_gen != r.oldest_gen		||
+		l.data_type != r.data_type		||
+		l.dirty_sectors	!= r.dirty_sectors	||
+		l.cached_sectors != r.cached_sectors	 ||
+		l.stripe_redundancy != r.stripe_redundancy ||
+		l.stripe != r.stripe;
+}
+
+static int bch2_alloc_write_key(struct btree_trans *trans,
+				struct btree_iter *iter,
+				struct bkey_s_c k,
+				bool metadata_only)
+{
+	struct bch_fs *c = trans->c;
+	struct bch_dev *ca = bch_dev_bkey_exists(c, iter->pos.inode);
+	struct bucket gc, *b;
+	struct bkey_i_alloc_v4 *a;
+	struct bch_alloc_v4 old_convert, new;
+	const struct bch_alloc_v4 *old;
+	enum bch_data_type type;
+	int ret;
+
+	if (bkey_ge(iter->pos, POS(ca->dev_idx, ca->mi.nbuckets)))
+		return 1;
+
+	old = bch2_alloc_to_v4(k, &old_convert);
+	new = *old;
+
+	percpu_down_read(&c->mark_lock);
+	b = gc_bucket(ca, iter->pos.offset);
+
+	/*
+	 * b->data_type doesn't yet include need_discard & need_gc_gen states -
+	 * fix that here:
+	 */
+	type = __alloc_data_type(b->dirty_sectors,
+				 b->cached_sectors,
+				 b->stripe,
+				 *old,
+				 b->data_type);
+	if (b->data_type != type) {
+		struct bch_dev_usage *u;
+
+		preempt_disable();
+		u = this_cpu_ptr(ca->usage_gc);
+		u->d[b->data_type].buckets--;
+		b->data_type = type;
+		u->d[b->data_type].buckets++;
+		preempt_enable();
+	}
+
+	gc = *b;
+	percpu_up_read(&c->mark_lock);
+
+	if (metadata_only &&
+	    gc.data_type != BCH_DATA_sb &&
+	    gc.data_type != BCH_DATA_journal &&
+	    gc.data_type != BCH_DATA_btree)
+		return 0;
+
+	if (gen_after(old->gen, gc.gen))
+		return 0;
+
+	if (c->opts.reconstruct_alloc ||
+	    fsck_err_on(new.data_type != gc.data_type, c,
+			alloc_key_data_type_wrong,
+			"bucket %llu:%llu gen %u has wrong data_type"
+			": got %s, should be %s",
+			iter->pos.inode, iter->pos.offset,
+			gc.gen,
+			bch2_data_types[new.data_type],
+			bch2_data_types[gc.data_type]))
+		new.data_type = gc.data_type;
+
+#define copy_bucket_field(_errtype, _f)					\
+	if (c->opts.reconstruct_alloc ||				\
+	    fsck_err_on(new._f != gc._f, c, _errtype,			\
+			"bucket %llu:%llu gen %u data type %s has wrong " #_f	\
+			": got %u, should be %u",			\
+			iter->pos.inode, iter->pos.offset,		\
+			gc.gen,						\
+			bch2_data_types[gc.data_type],			\
+			new._f, gc._f))					\
+		new._f = gc._f;						\
+
+	copy_bucket_field(alloc_key_gen_wrong,
+			  gen);
+	copy_bucket_field(alloc_key_dirty_sectors_wrong,
+			  dirty_sectors);
+	copy_bucket_field(alloc_key_cached_sectors_wrong,
+			  cached_sectors);
+	copy_bucket_field(alloc_key_stripe_wrong,
+			  stripe);
+	copy_bucket_field(alloc_key_stripe_redundancy_wrong,
+			  stripe_redundancy);
+#undef copy_bucket_field
+
+	if (!bch2_alloc_v4_cmp(*old, new))
+		return 0;
+
+	a = bch2_alloc_to_v4_mut(trans, k);
+	ret = PTR_ERR_OR_ZERO(a);
+	if (ret)
+		return ret;
+
+	a->v = new;
+
+	/*
+	 * The trigger normally makes sure this is set, but we're not running
+	 * triggers:
+	 */
+	if (a->v.data_type == BCH_DATA_cached && !a->v.io_time[READ])
+		a->v.io_time[READ] = max_t(u64, 1, atomic64_read(&c->io_clock[READ].now));
+
+	ret = bch2_trans_update(trans, iter, &a->k_i, BTREE_TRIGGER_NORUN);
+fsck_err:
+	return ret;
+}
+
+static int bch2_gc_alloc_done(struct bch_fs *c, bool metadata_only)
+{
+	struct btree_trans *trans = bch2_trans_get(c);
+	struct btree_iter iter;
+	struct bkey_s_c k;
+	struct bch_dev *ca;
+	unsigned i;
+	int ret = 0;
+
+	for_each_member_device(ca, c, i) {
+		ret = for_each_btree_key_commit(trans, iter, BTREE_ID_alloc,
+				POS(ca->dev_idx, ca->mi.first_bucket),
+				BTREE_ITER_SLOTS|BTREE_ITER_PREFETCH, k,
+				NULL, NULL, BTREE_INSERT_LAZY_RW,
+			bch2_alloc_write_key(trans, &iter, k, metadata_only));
+
+		if (ret < 0) {
+			bch_err_fn(c, ret);
+			percpu_ref_put(&ca->ref);
+			break;
+		}
+	}
+
+	bch2_trans_put(trans);
+	return ret < 0 ? ret : 0;
+}
+
+static int bch2_gc_alloc_start(struct bch_fs *c, bool metadata_only)
+{
+	struct bch_dev *ca;
+	struct btree_trans *trans = bch2_trans_get(c);
+	struct btree_iter iter;
+	struct bkey_s_c k;
+	struct bucket *g;
+	struct bch_alloc_v4 a_convert;
+	const struct bch_alloc_v4 *a;
+	unsigned i;
+	int ret;
+
+	for_each_member_device(ca, c, i) {
+		struct bucket_array *buckets = kvpmalloc(sizeof(struct bucket_array) +
+				ca->mi.nbuckets * sizeof(struct bucket),
+				GFP_KERNEL|__GFP_ZERO);
+		if (!buckets) {
+			percpu_ref_put(&ca->ref);
+			bch_err(c, "error allocating ca->buckets[gc]");
+			ret = -BCH_ERR_ENOMEM_gc_alloc_start;
+			goto err;
+		}
+
+		buckets->first_bucket	= ca->mi.first_bucket;
+		buckets->nbuckets	= ca->mi.nbuckets;
+		rcu_assign_pointer(ca->buckets_gc, buckets);
+	}
+
+	ret = for_each_btree_key2(trans, iter, BTREE_ID_alloc, POS_MIN,
+				  BTREE_ITER_PREFETCH, k, ({
+		ca = bch_dev_bkey_exists(c, k.k->p.inode);
+		g = gc_bucket(ca, k.k->p.offset);
+
+		a = bch2_alloc_to_v4(k, &a_convert);
+
+		g->gen_valid	= 1;
+		g->gen		= a->gen;
+
+		if (metadata_only &&
+		    (a->data_type == BCH_DATA_user ||
+		     a->data_type == BCH_DATA_cached ||
+		     a->data_type == BCH_DATA_parity)) {
+			g->data_type		= a->data_type;
+			g->dirty_sectors	= a->dirty_sectors;
+			g->cached_sectors	= a->cached_sectors;
+			g->stripe		= a->stripe;
+			g->stripe_redundancy	= a->stripe_redundancy;
+		}
+
+		0;
+	}));
+err:
+	bch2_trans_put(trans);
+	if (ret)
+		bch_err_fn(c, ret);
+	return ret;
+}
+
+static void bch2_gc_alloc_reset(struct bch_fs *c, bool metadata_only)
+{
+	struct bch_dev *ca;
+	unsigned i;
+
+	for_each_member_device(ca, c, i) {
+		struct bucket_array *buckets = gc_bucket_array(ca);
+		struct bucket *g;
+
+		for_each_bucket(g, buckets) {
+			if (metadata_only &&
+			    (g->data_type == BCH_DATA_user ||
+			     g->data_type == BCH_DATA_cached ||
+			     g->data_type == BCH_DATA_parity))
+				continue;
+			g->data_type = 0;
+			g->dirty_sectors = 0;
+			g->cached_sectors = 0;
+		}
+	}
+}
+
+static int bch2_gc_write_reflink_key(struct btree_trans *trans,
+				     struct btree_iter *iter,
+				     struct bkey_s_c k,
+				     size_t *idx)
+{
+	struct bch_fs *c = trans->c;
+	const __le64 *refcount = bkey_refcount_c(k);
+	struct printbuf buf = PRINTBUF;
+	struct reflink_gc *r;
+	int ret = 0;
+
+	if (!refcount)
+		return 0;
+
+	while ((r = genradix_ptr(&c->reflink_gc_table, *idx)) &&
+	       r->offset < k.k->p.offset)
+		++*idx;
+
+	if (!r ||
+	    r->offset != k.k->p.offset ||
+	    r->size != k.k->size) {
+		bch_err(c, "unexpected inconsistency walking reflink table at gc finish");
+		return -EINVAL;
+	}
+
+	if (fsck_err_on(r->refcount != le64_to_cpu(*refcount), c,
+			reflink_v_refcount_wrong,
+			"reflink key has wrong refcount:\n"
+			"  %s\n"
+			"  should be %u",
+			(bch2_bkey_val_to_text(&buf, c, k), buf.buf),
+			r->refcount)) {
+		struct bkey_i *new = bch2_bkey_make_mut(trans, iter, &k, 0);
+
+		ret = PTR_ERR_OR_ZERO(new);
+		if (ret)
+			return ret;
+
+		if (!r->refcount)
+			new->k.type = KEY_TYPE_deleted;
+		else
+			*bkey_refcount(new) = cpu_to_le64(r->refcount);
+	}
+fsck_err:
+	printbuf_exit(&buf);
+	return ret;
+}
+
+static int bch2_gc_reflink_done(struct bch_fs *c, bool metadata_only)
+{
+	struct btree_trans *trans;
+	struct btree_iter iter;
+	struct bkey_s_c k;
+	size_t idx = 0;
+	int ret = 0;
+
+	if (metadata_only)
+		return 0;
+
+	trans = bch2_trans_get(c);
+
+	ret = for_each_btree_key_commit(trans, iter,
+			BTREE_ID_reflink, POS_MIN,
+			BTREE_ITER_PREFETCH, k,
+			NULL, NULL, BTREE_INSERT_NOFAIL,
+		bch2_gc_write_reflink_key(trans, &iter, k, &idx));
+
+	c->reflink_gc_nr = 0;
+	bch2_trans_put(trans);
+	return ret;
+}
+
+static int bch2_gc_reflink_start(struct bch_fs *c,
+				 bool metadata_only)
+{
+	struct btree_trans *trans;
+	struct btree_iter iter;
+	struct bkey_s_c k;
+	struct reflink_gc *r;
+	int ret = 0;
+
+	if (metadata_only)
+		return 0;
+
+	trans = bch2_trans_get(c);
+	c->reflink_gc_nr = 0;
+
+	for_each_btree_key(trans, iter, BTREE_ID_reflink, POS_MIN,
+			   BTREE_ITER_PREFETCH, k, ret) {
+		const __le64 *refcount = bkey_refcount_c(k);
+
+		if (!refcount)
+			continue;
+
+		r = genradix_ptr_alloc(&c->reflink_gc_table, c->reflink_gc_nr++,
+				       GFP_KERNEL);
+		if (!r) {
+			ret = -BCH_ERR_ENOMEM_gc_reflink_start;
+			break;
+		}
+
+		r->offset	= k.k->p.offset;
+		r->size		= k.k->size;
+		r->refcount	= 0;
+	}
+	bch2_trans_iter_exit(trans, &iter);
+
+	bch2_trans_put(trans);
+	return ret;
+}
+
+static void bch2_gc_reflink_reset(struct bch_fs *c, bool metadata_only)
+{
+	struct genradix_iter iter;
+	struct reflink_gc *r;
+
+	genradix_for_each(&c->reflink_gc_table, iter, r)
+		r->refcount = 0;
+}
+
+static int bch2_gc_write_stripes_key(struct btree_trans *trans,
+				     struct btree_iter *iter,
+				     struct bkey_s_c k)
+{
+	struct bch_fs *c = trans->c;
+	struct printbuf buf = PRINTBUF;
+	const struct bch_stripe *s;
+	struct gc_stripe *m;
+	bool bad = false;
+	unsigned i;
+	int ret = 0;
+
+	if (k.k->type != KEY_TYPE_stripe)
+		return 0;
+
+	s = bkey_s_c_to_stripe(k).v;
+	m = genradix_ptr(&c->gc_stripes, k.k->p.offset);
+
+	for (i = 0; i < s->nr_blocks; i++) {
+		u32 old = stripe_blockcount_get(s, i);
+		u32 new = (m ? m->block_sectors[i] : 0);
+
+		if (old != new) {
+			prt_printf(&buf, "stripe block %u has wrong sector count: got %u, should be %u\n",
+				   i, old, new);
+			bad = true;
+		}
+	}
+
+	if (bad)
+		bch2_bkey_val_to_text(&buf, c, k);
+
+	if (fsck_err_on(bad, c, stripe_sector_count_wrong,
+			"%s", buf.buf)) {
+		struct bkey_i_stripe *new;
+
+		new = bch2_trans_kmalloc(trans, bkey_bytes(k.k));
+		ret = PTR_ERR_OR_ZERO(new);
+		if (ret)
+			return ret;
+
+		bkey_reassemble(&new->k_i, k);
+
+		for (i = 0; i < new->v.nr_blocks; i++)
+			stripe_blockcount_set(&new->v, i, m ? m->block_sectors[i] : 0);
+
+		ret = bch2_trans_update(trans, iter, &new->k_i, 0);
+	}
+fsck_err:
+	printbuf_exit(&buf);
+	return ret;
+}
+
+static int bch2_gc_stripes_done(struct bch_fs *c, bool metadata_only)
+{
+	struct btree_trans *trans;
+	struct btree_iter iter;
+	struct bkey_s_c k;
+	int ret = 0;
+
+	if (metadata_only)
+		return 0;
+
+	trans = bch2_trans_get(c);
+
+	ret = for_each_btree_key_commit(trans, iter,
+			BTREE_ID_stripes, POS_MIN,
+			BTREE_ITER_PREFETCH, k,
+			NULL, NULL, BTREE_INSERT_NOFAIL,
+		bch2_gc_write_stripes_key(trans, &iter, k));
+
+	bch2_trans_put(trans);
+	return ret;
+}
+
+static void bch2_gc_stripes_reset(struct bch_fs *c, bool metadata_only)
+{
+	genradix_free(&c->gc_stripes);
+}
+
+/**
+ * bch2_gc - walk _all_ references to buckets, and recompute them:
+ *
+ * @c:			filesystem object
+ * @initial:		are we in recovery?
+ * @metadata_only:	are we just checking metadata references, or everything?
+ *
+ * Returns: 0 on success, or standard errcode on failure
+ *
+ * Order matters here:
+ *  - Concurrent GC relies on the fact that we have a total ordering for
+ *    everything that GC walks - see  gc_will_visit_node(),
+ *    gc_will_visit_root()
+ *
+ *  - also, references move around in the course of index updates and
+ *    various other crap: everything needs to agree on the ordering
+ *    references are allowed to move around in - e.g., we're allowed to
+ *    start with a reference owned by an open_bucket (the allocator) and
+ *    move it to the btree, but not the reverse.
+ *
+ *    This is necessary to ensure that gc doesn't miss references that
+ *    move around - if references move backwards in the ordering GC
+ *    uses, GC could skip past them
+ */
+int bch2_gc(struct bch_fs *c, bool initial, bool metadata_only)
+{
+	unsigned iter = 0;
+	int ret;
+
+	lockdep_assert_held(&c->state_lock);
+
+	down_write(&c->gc_lock);
+
+	bch2_btree_interior_updates_flush(c);
+
+	ret   = bch2_gc_start(c) ?:
+		bch2_gc_alloc_start(c, metadata_only) ?:
+		bch2_gc_reflink_start(c, metadata_only);
+	if (ret)
+		goto out;
+again:
+	gc_pos_set(c, gc_phase(GC_PHASE_START));
+
+	bch2_mark_superblocks(c);
+
+	ret = bch2_gc_btrees(c, initial, metadata_only);
+
+	if (ret)
+		goto out;
+
+#if 0
+	bch2_mark_pending_btree_node_frees(c);
+#endif
+	c->gc_count++;
+
+	if (test_bit(BCH_FS_NEED_ANOTHER_GC, &c->flags) ||
+	    (!iter && bch2_test_restart_gc)) {
+		if (iter++ > 2) {
+			bch_info(c, "Unable to fix bucket gens, looping");
+			ret = -EINVAL;
+			goto out;
+		}
+
+		/*
+		 * XXX: make sure gens we fixed got saved
+		 */
+		bch_info(c, "Second GC pass needed, restarting:");
+		clear_bit(BCH_FS_NEED_ANOTHER_GC, &c->flags);
+		__gc_pos_set(c, gc_phase(GC_PHASE_NOT_RUNNING));
+
+		bch2_gc_stripes_reset(c, metadata_only);
+		bch2_gc_alloc_reset(c, metadata_only);
+		bch2_gc_reflink_reset(c, metadata_only);
+		ret = bch2_gc_reset(c);
+		if (ret)
+			goto out;
+
+		/* flush fsck errors, reset counters */
+		bch2_flush_fsck_errs(c);
+		goto again;
+	}
+out:
+	if (!ret) {
+		bch2_journal_block(&c->journal);
+
+		ret   = bch2_gc_stripes_done(c, metadata_only) ?:
+			bch2_gc_reflink_done(c, metadata_only) ?:
+			bch2_gc_alloc_done(c, metadata_only) ?:
+			bch2_gc_done(c, initial, metadata_only);
+
+		bch2_journal_unblock(&c->journal);
+	}
+
+	percpu_down_write(&c->mark_lock);
+	/* Indicates that gc is no longer in progress: */
+	__gc_pos_set(c, gc_phase(GC_PHASE_NOT_RUNNING));
+
+	bch2_gc_free(c);
+	percpu_up_write(&c->mark_lock);
+
+	up_write(&c->gc_lock);
+
+	/*
+	 * At startup, allocations can happen directly instead of via the
+	 * allocator thread - issue wakeup in case they blocked on gc_lock:
+	 */
+	closure_wake_up(&c->freelist_wait);
+
+	if (ret)
+		bch_err_fn(c, ret);
+	return ret;
+}
+
+static int gc_btree_gens_key(struct btree_trans *trans,
+			     struct btree_iter *iter,
+			     struct bkey_s_c k)
+{
+	struct bch_fs *c = trans->c;
+	struct bkey_ptrs_c ptrs = bch2_bkey_ptrs_c(k);
+	const struct bch_extent_ptr *ptr;
+	struct bkey_i *u;
+	int ret;
+
+	percpu_down_read(&c->mark_lock);
+	bkey_for_each_ptr(ptrs, ptr) {
+		struct bch_dev *ca = bch_dev_bkey_exists(c, ptr->dev);
+
+		if (ptr_stale(ca, ptr) > 16) {
+			percpu_up_read(&c->mark_lock);
+			goto update;
+		}
+	}
+
+	bkey_for_each_ptr(ptrs, ptr) {
+		struct bch_dev *ca = bch_dev_bkey_exists(c, ptr->dev);
+		u8 *gen = &ca->oldest_gen[PTR_BUCKET_NR(ca, ptr)];
+
+		if (gen_after(*gen, ptr->gen))
+			*gen = ptr->gen;
+	}
+	percpu_up_read(&c->mark_lock);
+	return 0;
+update:
+	u = bch2_bkey_make_mut(trans, iter, &k, 0);
+	ret = PTR_ERR_OR_ZERO(u);
+	if (ret)
+		return ret;
+
+	bch2_extent_normalize(c, bkey_i_to_s(u));
+	return 0;
+}
+
+static int bch2_alloc_write_oldest_gen(struct btree_trans *trans, struct btree_iter *iter,
+				       struct bkey_s_c k)
+{
+	struct bch_dev *ca = bch_dev_bkey_exists(trans->c, iter->pos.inode);
+	struct bch_alloc_v4 a_convert;
+	const struct bch_alloc_v4 *a = bch2_alloc_to_v4(k, &a_convert);
+	struct bkey_i_alloc_v4 *a_mut;
+	int ret;
+
+	if (a->oldest_gen == ca->oldest_gen[iter->pos.offset])
+		return 0;
+
+	a_mut = bch2_alloc_to_v4_mut(trans, k);
+	ret = PTR_ERR_OR_ZERO(a_mut);
+	if (ret)
+		return ret;
+
+	a_mut->v.oldest_gen = ca->oldest_gen[iter->pos.offset];
+	a_mut->v.data_type = alloc_data_type(a_mut->v, a_mut->v.data_type);
+
+	return bch2_trans_update(trans, iter, &a_mut->k_i, 0);
+}
+
+int bch2_gc_gens(struct bch_fs *c)
+{
+	struct btree_trans *trans;
+	struct btree_iter iter;
+	struct bkey_s_c k;
+	struct bch_dev *ca;
+	u64 b, start_time = local_clock();
+	unsigned i;
+	int ret;
+
+	/*
+	 * Ideally we would be using state_lock and not gc_lock here, but that
+	 * introduces a deadlock in the RO path - we currently take the state
+	 * lock at the start of going RO, thus the gc thread may get stuck:
+	 */
+	if (!mutex_trylock(&c->gc_gens_lock))
+		return 0;
+
+	trace_and_count(c, gc_gens_start, c);
+	down_read(&c->gc_lock);
+	trans = bch2_trans_get(c);
+
+	for_each_member_device(ca, c, i) {
+		struct bucket_gens *gens = bucket_gens(ca);
+
+		BUG_ON(ca->oldest_gen);
+
+		ca->oldest_gen = kvmalloc(gens->nbuckets, GFP_KERNEL);
+		if (!ca->oldest_gen) {
+			percpu_ref_put(&ca->ref);
+			ret = -BCH_ERR_ENOMEM_gc_gens;
+			goto err;
+		}
+
+		for (b = gens->first_bucket;
+		     b < gens->nbuckets; b++)
+			ca->oldest_gen[b] = gens->b[b];
+	}
+
+	for (i = 0; i < BTREE_ID_NR; i++)
+		if (btree_type_has_ptrs(i)) {
+			c->gc_gens_btree = i;
+			c->gc_gens_pos = POS_MIN;
+
+			ret = for_each_btree_key_commit(trans, iter, i,
+					POS_MIN,
+					BTREE_ITER_PREFETCH|BTREE_ITER_ALL_SNAPSHOTS,
+					k,
+					NULL, NULL,
+					BTREE_INSERT_NOFAIL,
+				gc_btree_gens_key(trans, &iter, k));
+			if (ret && !bch2_err_matches(ret, EROFS))
+				bch_err_fn(c, ret);
+			if (ret)
+				goto err;
+		}
+
+	ret = for_each_btree_key_commit(trans, iter, BTREE_ID_alloc,
+			POS_MIN,
+			BTREE_ITER_PREFETCH,
+			k,
+			NULL, NULL,
+			BTREE_INSERT_NOFAIL,
+		bch2_alloc_write_oldest_gen(trans, &iter, k));
+	if (ret && !bch2_err_matches(ret, EROFS))
+		bch_err_fn(c, ret);
+	if (ret)
+		goto err;
+
+	c->gc_gens_btree	= 0;
+	c->gc_gens_pos		= POS_MIN;
+
+	c->gc_count++;
+
+	bch2_time_stats_update(&c->times[BCH_TIME_btree_gc], start_time);
+	trace_and_count(c, gc_gens_end, c);
+err:
+	for_each_member_device(ca, c, i) {
+		kvfree(ca->oldest_gen);
+		ca->oldest_gen = NULL;
+	}
+
+	bch2_trans_put(trans);
+	up_read(&c->gc_lock);
+	mutex_unlock(&c->gc_gens_lock);
+	return ret;
+}
+
+static int bch2_gc_thread(void *arg)
+{
+	struct bch_fs *c = arg;
+	struct io_clock *clock = &c->io_clock[WRITE];
+	unsigned long last = atomic64_read(&clock->now);
+	unsigned last_kick = atomic_read(&c->kick_gc);
+	int ret;
+
+	set_freezable();
+
+	while (1) {
+		while (1) {
+			set_current_state(TASK_INTERRUPTIBLE);
+
+			if (kthread_should_stop()) {
+				__set_current_state(TASK_RUNNING);
+				return 0;
+			}
+
+			if (atomic_read(&c->kick_gc) != last_kick)
+				break;
+
+			if (c->btree_gc_periodic) {
+				unsigned long next = last + c->capacity / 16;
+
+				if (atomic64_read(&clock->now) >= next)
+					break;
+
+				bch2_io_clock_schedule_timeout(clock, next);
+			} else {
+				schedule();
+			}
+
+			try_to_freeze();
+		}
+		__set_current_state(TASK_RUNNING);
+
+		last = atomic64_read(&clock->now);
+		last_kick = atomic_read(&c->kick_gc);
+
+		/*
+		 * Full gc is currently incompatible with btree key cache:
+		 */
+#if 0
+		ret = bch2_gc(c, false, false);
+#else
+		ret = bch2_gc_gens(c);
+#endif
+		if (ret < 0)
+			bch_err_fn(c, ret);
+
+		debug_check_no_locks_held();
+	}
+
+	return 0;
+}
+
+void bch2_gc_thread_stop(struct bch_fs *c)
+{
+	struct task_struct *p;
+
+	p = c->gc_thread;
+	c->gc_thread = NULL;
+
+	if (p) {
+		kthread_stop(p);
+		put_task_struct(p);
+	}
+}
+
+int bch2_gc_thread_start(struct bch_fs *c)
+{
+	struct task_struct *p;
+
+	if (c->gc_thread)
+		return 0;
+
+	p = kthread_create(bch2_gc_thread, c, "bch-gc/%s", c->name);
+	if (IS_ERR(p)) {
+		bch_err_fn(c, PTR_ERR(p));
+		return PTR_ERR(p);
+	}
+
+	get_task_struct(p);
+	c->gc_thread = p;
+	wake_up_process(p);
+	return 0;
+}