1 files changed, 304 insertions, 1122 deletions

diff --git a/fs/bcachefs/btree_gc.c b/fs/bcachefs/btree_gc.c
index 791470b0c654..dd1d9b74076e 100644
--- a/fs/bcachefs/btree_gc.c
+++ b/fs/bcachefs/btree_gc.c
@@ -20,6 +20,7 @@
 #include "buckets.h"
 #include "clock.h"
 #include "debug.h"
+#include "disk_accounting.h"
 #include "ec.h"
 #include "error.h"
 #include "extents.h"
@@ -28,6 +29,7 @@
 #include "move.h"
 #include "recovery_passes.h"
 #include "reflink.h"
+#include "recovery.h"
 #include "replicas.h"
 #include "super-io.h"
 #include "trace.h"
@@ -44,6 +46,22 @@
 #define DROP_PREV_NODE		11
 #define DID_FILL_FROM_SCAN	12
 
+static const char * const bch2_gc_phase_strs[] = {
+#define x(n)	#n,
+	GC_PHASES()
+#undef x
+	NULL
+};
+
+void bch2_gc_pos_to_text(struct printbuf *out, struct gc_pos *p)
+{
+	prt_str(out, bch2_gc_phase_strs[p->phase]);
+	prt_char(out, ' ');
+	bch2_btree_id_level_to_text(out, p->btree, p->level);
+	prt_char(out, ' ');
+	bch2_bpos_to_text(out, p->pos);
+}
+
 static struct bkey_s unsafe_bkey_s_c_to_s(struct bkey_s_c k)
 {
 	return (struct bkey_s) {{{
@@ -52,12 +70,6 @@ static struct bkey_s unsafe_bkey_s_c_to_s(struct bkey_s_c k)
 	}}};
 }
 
-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();
@@ -69,7 +81,7 @@ static inline void __gc_pos_set(struct bch_fs *c, struct gc_pos new_pos)
 
 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);
+	BUG_ON(gc_pos_cmp(new_pos, c->gc_pos) < 0);
 	__gc_pos_set(c, new_pos);
 }
 
@@ -97,35 +109,6 @@ static void btree_ptr_to_v2(struct btree *b, struct bkey_i_btree_ptr_v2 *dst)
 	}
 }
 
-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;
@@ -200,7 +183,7 @@ static int set_node_max(struct bch_fs *c, struct btree *b, struct bpos new_max)
 	bch2_btree_node_drop_keys_outside_node(b);
 
 	mutex_lock(&c->btree_cache.lock);
-	bch2_btree_node_hash_remove(&c->btree_cache, b);
+	__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);
@@ -209,10 +192,11 @@ static int set_node_max(struct bch_fs *c, struct btree *b, struct bpos new_max)
 	return 0;
 }
 
-static int btree_check_node_boundaries(struct bch_fs *c, struct btree *b,
+static int btree_check_node_boundaries(struct btree_trans *trans, struct btree *b,
 				       struct btree *prev, struct btree *cur,
 				       struct bpos *pulled_from_scan)
 {
+	struct bch_fs *c = trans->c;
 	struct bpos expected_start = !prev
 		? b->data->min_key
 		: bpos_successor(prev->key.k.p);
@@ -226,8 +210,9 @@ static int btree_check_node_boundaries(struct bch_fs *c, struct btree *b,
 	if (bpos_eq(expected_start, cur->data->min_key))
 		return 0;
 
-	prt_printf(&buf, "  at btree %s level %u:\n  parent: ",
-		   bch2_btree_id_str(b->c.btree_id), b->c.level);
+	prt_printf(&buf, "  at ");
+	bch2_btree_id_level_to_text(&buf, b->c.btree_id, b->c.level);
+	prt_printf(&buf, ":\n  parent: ");
 	bch2_bkey_val_to_text(&buf, c, bkey_i_to_s_c(&b->key));
 
 	if (prev) {
@@ -250,29 +235,29 @@ static int btree_check_node_boundaries(struct bch_fs *c, struct btree *b,
 			*pulled_from_scan = cur->data->min_key;
 			ret = DID_FILL_FROM_SCAN;
 		} else {
-			if (mustfix_fsck_err(c, btree_node_topology_bad_min_key,
+			if (mustfix_fsck_err(trans, btree_node_topology_bad_min_key,
 					     "btree node with incorrect min_key%s", buf.buf))
 				ret = set_node_min(c, cur, expected_start);
 		}
 	} else {									/* overlap */
 		if (prev && BTREE_NODE_SEQ(cur->data) > BTREE_NODE_SEQ(prev->data)) {	/* cur overwrites prev */
 			if (bpos_ge(prev->data->min_key, cur->data->min_key)) {		/* fully? */
-				if (mustfix_fsck_err(c, btree_node_topology_overwritten_by_next_node,
+				if (mustfix_fsck_err(trans, btree_node_topology_overwritten_by_next_node,
 						     "btree node overwritten by next node%s", buf.buf))
 					ret = DROP_PREV_NODE;
 			} else {
-				if (mustfix_fsck_err(c, btree_node_topology_bad_max_key,
+				if (mustfix_fsck_err(trans, btree_node_topology_bad_max_key,
 						     "btree node with incorrect max_key%s", buf.buf))
 					ret = set_node_max(c, prev,
 							   bpos_predecessor(cur->data->min_key));
 			}
 		} else {
 			if (bpos_ge(expected_start, cur->data->max_key)) {		/* fully? */
-				if (mustfix_fsck_err(c, btree_node_topology_overwritten_by_prev_node,
+				if (mustfix_fsck_err(trans, btree_node_topology_overwritten_by_prev_node,
 						     "btree node overwritten by prev node%s", buf.buf))
 					ret = DROP_THIS_NODE;
 			} else {
-				if (mustfix_fsck_err(c, btree_node_topology_bad_min_key,
+				if (mustfix_fsck_err(trans, btree_node_topology_bad_min_key,
 						     "btree node with incorrect min_key%s", buf.buf))
 					ret = set_node_min(c, cur, expected_start);
 			}
@@ -284,23 +269,25 @@ fsck_err:
 	return ret;
 }
 
-static int btree_repair_node_end(struct bch_fs *c, struct btree *b,
+static int btree_repair_node_end(struct btree_trans *trans, struct btree *b,
 				 struct btree *child, struct bpos *pulled_from_scan)
 {
+	struct bch_fs *c = trans->c;
 	struct printbuf buf = PRINTBUF;
 	int ret = 0;
 
 	if (bpos_eq(child->key.k.p, b->key.k.p))
 		return 0;
 
-	prt_printf(&buf, "at btree %s level %u:\n  parent: ",
-		   bch2_btree_id_str(b->c.btree_id), b->c.level);
+	prt_printf(&buf, "  at ");
+	bch2_btree_id_level_to_text(&buf, b->c.btree_id, b->c.level);
+	prt_printf(&buf, ":\n  parent: ");
 	bch2_bkey_val_to_text(&buf, c, bkey_i_to_s_c(&b->key));
 
 	prt_str(&buf, "\n  child: ");
 	bch2_bkey_val_to_text(&buf, c, bkey_i_to_s_c(&child->key));
 
-	if (mustfix_fsck_err(c, btree_node_topology_bad_max_key,
+	if (mustfix_fsck_err(trans, btree_node_topology_bad_max_key,
 			     "btree node with incorrect max_key%s", buf.buf)) {
 		if (b->c.level == 1 &&
 		    bpos_lt(*pulled_from_scan, b->key.k.p)) {
@@ -357,14 +344,14 @@ again:
 		ret = PTR_ERR_OR_ZERO(cur);
 
 		printbuf_reset(&buf);
+		bch2_btree_id_level_to_text(&buf, b->c.btree_id, b->c.level - 1);
+		prt_char(&buf, ' ');
 		bch2_bkey_val_to_text(&buf, c, bkey_i_to_s_c(cur_k.k));
 
-		if (mustfix_fsck_err_on(bch2_err_matches(ret, EIO), c,
-				btree_node_unreadable,
-				"Topology repair: unreadable btree node at btree %s level %u:\n"
+		if (mustfix_fsck_err_on(bch2_err_matches(ret, EIO),
+				trans, btree_node_read_error,
+				"Topology repair: unreadable btree node at\n"
 				"  %s",
-				bch2_btree_id_str(b->c.btree_id),
-				b->c.level - 1,
 				buf.buf)) {
 			bch2_btree_node_evict(trans, cur_k.k);
 			cur = NULL;
@@ -373,11 +360,9 @@ again:
 			if (ret)
 				break;
 
-			if (!btree_id_is_alloc(b->c.btree_id)) {
-				ret = bch2_run_explicit_recovery_pass(c, BCH_RECOVERY_PASS_scan_for_btree_nodes);
-				if (ret)
-					break;
-			}
+			ret = bch2_btree_lost_data(c, b->c.btree_id);
+			if (ret)
+				break;
 			continue;
 		}
 
@@ -386,7 +371,7 @@ again:
 			break;
 
 		if (bch2_btree_node_is_stale(c, cur)) {
-			bch_info(c, "btree node %s older than nodes found by scanning", buf.buf);
+			bch_info(c, "btree node older than nodes found by scanning\n  %s", buf.buf);
 			six_unlock_read(&cur->c.lock);
 			bch2_btree_node_evict(trans, cur_k.k);
 			ret = bch2_journal_key_delete(c, b->c.btree_id,
@@ -397,7 +382,7 @@ again:
 			continue;
 		}
 
-		ret = btree_check_node_boundaries(c, b, prev, cur, pulled_from_scan);
+		ret = btree_check_node_boundaries(trans, b, prev, cur, pulled_from_scan);
 		if (ret == DID_FILL_FROM_SCAN) {
 			new_pass = true;
 			ret = 0;
@@ -438,7 +423,7 @@ again:
 
 	if (!ret && !IS_ERR_OR_NULL(prev)) {
 		BUG_ON(cur);
-		ret = btree_repair_node_end(c, b, prev, pulled_from_scan);
+		ret = btree_repair_node_end(trans, b, prev, pulled_from_scan);
 		if (ret == DID_FILL_FROM_SCAN) {
 			new_pass = true;
 			ret = 0;
@@ -494,14 +479,13 @@ again:
 	}
 
 	printbuf_reset(&buf);
+	bch2_btree_id_level_to_text(&buf, b->c.btree_id, b->c.level);
+	prt_newline(&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))
+	if (mustfix_fsck_err_on(!have_child,
+			trans, btree_node_topology_interior_node_empty,
+			"empty interior btree node at %s", buf.buf))
 		ret = DROP_THIS_NODE;
 err:
 fsck_err:
@@ -527,28 +511,34 @@ int bch2_check_topology(struct bch_fs *c)
 {
 	struct btree_trans *trans = bch2_trans_get(c);
 	struct bpos pulled_from_scan = POS_MIN;
+	struct printbuf buf = PRINTBUF;
 	int ret = 0;
 
+	bch2_trans_srcu_unlock(trans);
+
 	for (unsigned i = 0; i < btree_id_nr_alive(c) && !ret; i++) {
 		struct btree_root *r = bch2_btree_id_root(c, i);
 		bool reconstructed_root = false;
 
+		printbuf_reset(&buf);
+		bch2_btree_id_to_text(&buf, i);
+
 		if (r->error) {
-			ret = bch2_run_explicit_recovery_pass(c, BCH_RECOVERY_PASS_scan_for_btree_nodes);
+			ret = bch2_btree_lost_data(c, i);
 			if (ret)
 				break;
 reconstruct_root:
-			bch_info(c, "btree root %s unreadable, must recover from scan", bch2_btree_id_str(i));
+			bch_info(c, "btree root %s unreadable, must recover from scan", buf.buf);
 
 			r->alive = false;
 			r->error = 0;
 
 			if (!bch2_btree_has_scanned_nodes(c, i)) {
-				mustfix_fsck_err(c, btree_root_unreadable_and_scan_found_nothing,
-						 "no nodes found for btree %s, continue?", bch2_btree_id_str(i));
-				bch2_btree_root_alloc_fake(c, i, 0);
+				mustfix_fsck_err(trans, btree_root_unreadable_and_scan_found_nothing,
+						 "no nodes found for btree %s, continue?", buf.buf);
+				bch2_btree_root_alloc_fake_trans(trans, i, 0);
 			} else {
-				bch2_btree_root_alloc_fake(c, i, 1);
+				bch2_btree_root_alloc_fake_trans(trans, i, 1);
 				bch2_shoot_down_journal_keys(c, i, 1, BTREE_MAX_DEPTH, POS_MIN, SPOS_MAX);
 				ret = bch2_get_scanned_nodes(c, i, 0, POS_MIN, SPOS_MAX);
 				if (ret)
@@ -565,9 +555,8 @@ reconstruct_root:
 		six_unlock_read(&b->c.lock);
 
 		if (ret == DROP_THIS_NODE) {
-			bch2_btree_node_hash_remove(&c->btree_cache, b);
 			mutex_lock(&c->btree_cache.lock);
-			list_move(&b->list, &c->btree_cache.freeable);
+			bch2_btree_node_hash_remove(&c->btree_cache, b);
 			mutex_unlock(&c->btree_cache.lock);
 
 			r->b = NULL;
@@ -575,519 +564,161 @@ reconstruct_root:
 			if (!reconstructed_root)
 				goto reconstruct_root;
 
-			bch_err(c, "empty btree root %s", bch2_btree_id_str(i));
-			bch2_btree_root_alloc_fake(c, i, 0);
+			bch_err(c, "empty btree root %s", buf.buf);
+			bch2_btree_root_alloc_fake_trans(trans, i, 0);
 			r->alive = false;
 			ret = 0;
 		}
 	}
 fsck_err:
+	printbuf_exit(&buf);
 	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)
+/* marking of btree keys/nodes: */
+
+static int bch2_gc_mark_key(struct btree_trans *trans, enum btree_id btree_id,
+			    unsigned level, struct btree **prev,
+			    struct btree_iter *iter, struct bkey_s_c k,
+			    bool initial)
 {
 	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, p, entry_c);
-
-		if (fsck_err_on(!g->gen_valid,
-				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_type_str(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 (fsck_err_on(gen_cmp(p.ptr.gen, g->gen) > 0,
-				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_type_str(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 (fsck_err_on(gen_cmp(g->gen, p.ptr.gen) > BUCKET_GC_GEN_MAX,
-				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_type_str(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 (fsck_err_on(!p.ptr.cached && gen_cmp(p.ptr.gen, g->gen) < 0,
-				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_type_str(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) !=
-				bucket_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_type_str(g->data_type),
-				bch2_data_type_str(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) {
-		if (is_root) {
-			bch_err(c, "cannot update btree roots yet");
-			ret = -EINVAL;
-			goto err;
-		}
-
-		struct bkey_i *new = kmalloc(bkey_bytes(k->k), GFP_KERNEL);
-		if (!new) {
-			ret = -BCH_ERR_ENOMEM_gc_repair_key;
-			bch_err_msg(c, ret, "allocating new key");
-			goto err;
-		}
 
-		bkey_reassemble(new, *k);
+	if (iter) {
+		struct btree_path *path = btree_iter_path(trans, iter);
+		struct btree *b = path_l(path)->b;
 
-		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:
-			 */
-			struct bkey_ptrs 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 {
-			struct bkey_ptrs ptrs;
-			union bch_extent_entry *entry;
-restart_drop_ptrs:
-			ptrs = bch2_bkey_ptrs(bkey_i_to_s(new));
-			bkey_for_each_ptr_decode(bkey_i_to_s(new).k, ptrs, p, entry) {
-				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(bkey_i_to_s_c(new), p, entry);
-
-				if ((p.ptr.cached &&
-				     (!g->gen_valid || gen_cmp(p.ptr.gen, g->gen) > 0)) ||
-				    (!p.ptr.cached &&
-				     gen_cmp(p.ptr.gen, g->gen) < 0) ||
-				    gen_cmp(g->gen, p.ptr.gen) > BUCKET_GC_GEN_MAX ||
-				    (g->data_type &&
-				     g->data_type != data_type)) {
-					bch2_bkey_drop_ptr(bkey_i_to_s(new), &entry->ptr);
-					goto restart_drop_ptrs;
-				}
-			}
-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;
-					}
-				}
-			}
-		}
-
-		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);
-		}
-
-		ret = bch2_journal_key_insert_take(c, btree_id, level, new);
-		if (ret) {
-			kfree(new);
-			goto err;
+		if (*prev != b) {
+			int ret = bch2_btree_node_check_topology(trans, b);
+			if (ret)
+				return ret;
 		}
-
-		*k = bkey_i_to_s_c(new);
+		*prev = b;
 	}
-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 };
 	struct printbuf buf = PRINTBUF;
 	int ret = 0;
 
-	deleted.p = k->k->p;
+	deleted.p = k.k->p;
 
 	if (initial) {
 		BUG_ON(bch2_journal_seq_verify &&
-		       k->k->version.lo > atomic64_read(&c->journal.seq));
-
-		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);
+		       k.k->bversion.lo > atomic64_read(&c->journal.seq));
+
+		if (fsck_err_on(btree_id != BTREE_ID_accounting &&
+				k.k->bversion.lo > atomic64_read(&c->key_version),
+				trans, bkey_version_in_future,
+				"key version number higher than recorded %llu\n  %s",
+				atomic64_read(&c->key_version),
+				(bch2_bkey_val_to_text(&buf, c, k), buf.buf)))
+			atomic64_set(&c->key_version, k.k->bversion.lo);
 	}
 
-	ret = bch2_check_fix_ptrs(trans, btree_id, level, is_root, k);
-	if (ret)
-		goto err;
-
-	if (mustfix_fsck_err_on(level && !bch2_dev_btree_bitmap_marked(c, *k),
-				c, btree_bitmap_not_marked,
+	if (mustfix_fsck_err_on(level && !bch2_dev_btree_bitmap_marked(c, k),
+				trans, btree_bitmap_not_marked,
 				"btree ptr not marked in member info btree allocated bitmap\n  %s",
-				(bch2_bkey_val_to_text(&buf, c, *k),
+				(printbuf_reset(&buf),
+				 bch2_bkey_val_to_text(&buf, c, k),
 				 buf.buf))) {
 		mutex_lock(&c->sb_lock);
-		bch2_dev_btree_bitmap_mark(c, *k);
+		bch2_dev_btree_bitmap_mark(c, k);
 		bch2_write_super(c);
 		mutex_unlock(&c->sb_lock);
 	}
 
-	ret = commit_do(trans, NULL, NULL, 0,
-			bch2_key_trigger(trans, btree_id, level, old,
-					 unsafe_bkey_s_c_to_s(*k), BTREE_TRIGGER_GC));
-fsck_err:
-err:
-	printbuf_exit(&buf);
-	bch_err_fn(c, ret);
-	return ret;
-}
-
-static int btree_gc_mark_node(struct btree_trans *trans, struct btree *b, bool initial)
-{
-	struct btree_node_iter iter;
-	struct bkey unpacked;
-	struct bkey_s_c k;
-	int ret = 0;
-
-	ret = bch2_btree_node_check_topology(trans, b);
-	if (ret)
-		return ret;
-
-	if (!btree_node_type_needs_gc(btree_node_type(b)))
-		return 0;
-
-	bch2_btree_node_iter_init_from_start(&iter, b);
-
-	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)
-			return ret;
-
-		bch2_btree_node_iter_advance(&iter, b);
-	}
-
-	return 0;
-}
-
-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);
+	/*
+	 * We require a commit before key_trigger() because
+	 * key_trigger(BTREE_TRIGGER_GC) is not idempotant; we'll calculate the
+	 * wrong result if we run it multiple times.
+	 */
+	unsigned flags = !iter ? BTREE_TRIGGER_is_root : 0;
 
+	ret = bch2_key_trigger(trans, btree_id, level, old, unsafe_bkey_s_c_to_s(k),
+			       BTREE_TRIGGER_check_repair|flags);
 	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);
+		goto out;
 
-		ret = bch2_gc_mark_key(trans, b->c.btree_id, b->c.level + 1,
-				       true, &k, initial);
+	if (trans->nr_updates) {
+		ret = bch2_trans_commit(trans, NULL, NULL, 0) ?:
+			-BCH_ERR_transaction_restart_nested;
+		goto out;
 	}
-	gc_pos_set(c, gc_pos_btree_root(b->c.btree_id));
-	mutex_unlock(&c->btree_root_lock);
 
+	ret = bch2_key_trigger(trans, btree_id, level, old, unsafe_bkey_s_c_to_s(k),
+			       BTREE_TRIGGER_gc|BTREE_TRIGGER_insert|flags);
+out:
+fsck_err:
+	printbuf_exit(&buf);
+	bch_err_fn(c, ret);
 	return ret;
 }
 
-static int bch2_gc_btree_init_recurse(struct btree_trans *trans, struct btree *b,
-				      unsigned target_depth)
+static int bch2_gc_btree(struct btree_trans *trans, enum btree_id btree, bool initial)
 {
 	struct bch_fs *c = trans->c;
-	struct btree_and_journal_iter iter;
-	struct bkey_s_c k;
-	struct bkey_buf cur;
-	struct printbuf buf = PRINTBUF;
+	unsigned target_depth = btree_node_type_has_triggers(__btree_node_type(0, btree)) ? 0 : 1;
 	int ret = 0;
 
-	ret = bch2_btree_node_check_topology(trans, b);
-	if (ret)
-		return ret;
+	/* We need to make sure every leaf node is readable before going RW */
+	if (initial)
+		target_depth = 0;
 
-	bch2_btree_and_journal_iter_init_node_iter(trans, &iter, b);
-	bch2_bkey_buf_init(&cur);
+	for (unsigned level = target_depth; level < BTREE_MAX_DEPTH; level++) {
+		struct btree *prev = NULL;
+		struct btree_iter iter;
+		bch2_trans_node_iter_init(trans, &iter, btree, POS_MIN, 0, level,
+					  BTREE_ITER_prefetch);
 
-	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);
+		ret = for_each_btree_key_continue(trans, iter, 0, k, ({
+			gc_pos_set(c, gc_pos_btree(btree, level, k.k->p));
+			bch2_gc_mark_key(trans, btree, level, &prev, &iter, k, initial);
+		}));
 		if (ret)
-			goto fsck_err;
-
-		bch2_btree_and_journal_iter_advance(&iter);
+			goto err;
 	}
 
-	if (b->c.level > target_depth) {
-		bch2_btree_and_journal_iter_exit(&iter);
-		bch2_btree_and_journal_iter_init_node_iter(trans, &iter, b);
-		iter.prefetch = true;
-
-		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 (bch2_err_matches(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);
+	/* root */
+	do {
+retry_root:
+		bch2_trans_begin(trans);
+
+		struct btree_iter iter;
+		bch2_trans_node_iter_init(trans, &iter, btree, POS_MIN,
+					  0, bch2_btree_id_root(c, btree)->b->c.level, 0);
+		struct btree *b = bch2_btree_iter_peek_node(&iter);
+		ret = PTR_ERR_OR_ZERO(b);
+		if (ret)
+			goto err_root;
 
-			if (ret)
-				break;
+		if (b != btree_node_root(c, b)) {
+			bch2_trans_iter_exit(trans, &iter);
+			goto retry_root;
 		}
-	}
-fsck_err:
-	bch2_bkey_buf_exit(&cur, 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;
 
-	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) {
+		gc_pos_set(c, gc_pos_btree(btree, b->c.level + 1, SPOS_MAX));
 		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);
-
+		ret = bch2_gc_mark_key(trans, btree, b->c.level + 1, NULL, NULL, k, initial);
+err_root:
+		bch2_trans_iter_exit(trans, &iter);
+	} while (bch2_err_matches(ret, BCH_ERR_transaction_restart));
+err:
 	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);
+	return cmp_int(gc_btree_order(l), gc_btree_order(r));
 }
 
-static int bch2_gc_btrees(struct bch_fs *c, bool initial, bool metadata_only)
+static int bch2_gc_btrees(struct bch_fs *c)
 {
 	struct btree_trans *trans = bch2_trans_get(c);
 	enum btree_id ids[BTREE_ID_NR];
+	struct printbuf buf = PRINTBUF;
 	unsigned i;
 	int ret = 0;
 
@@ -1095,257 +726,52 @@ static int bch2_gc_btrees(struct bch_fs *c, bool initial, bool metadata_only)
 		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 = 0; i < btree_id_nr_alive(c) && !ret; i++) {
+		unsigned btree = i < BTREE_ID_NR ? ids[i] : i;
 
-	for (i = BTREE_ID_NR; i < btree_id_nr_alive(c) && !ret; i++) {
-		if (!bch2_btree_id_root(c, i)->alive)
+		if (IS_ERR_OR_NULL(bch2_btree_id_root(c, btree)->b))
 			continue;
 
-		ret = initial
-			? bch2_gc_btree_init(trans, i, metadata_only)
-			: bch2_gc_btree(trans, i, initial, metadata_only);
+		ret = bch2_gc_btree(trans, btree, true);
 	}
 
+	printbuf_exit(&buf);
 	bch2_trans_put(trans);
 	bch_err_fn(c, ret);
 	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)
+static int bch2_mark_superblocks(struct bch_fs *c)
 {
-	u64 b = sector_to_bucket(ca, start);
+	gc_pos_set(c, gc_phase(GC_PHASE_sb));
 
-	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);
+	return bch2_trans_mark_dev_sbs_flags(c, BTREE_TRIGGER_gc);
 }
 
-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)
-{
-	mutex_lock(&c->sb_lock);
-	gc_pos_set(c, gc_phase(GC_PHASE_SB));
-
-	for_each_online_member(c, ca)
-		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)
 {
+	bch2_accounting_gc_free(c);
+
 	genradix_free(&c->reflink_gc_table);
 	genradix_free(&c->gc_stripes);
 
-	for_each_member_device(c, ca) {
-		kvfree(rcu_dereference_protected(ca->buckets_gc, 1));
-		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;
-	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, ca->dev_idx, ##__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(c, ca) {
-		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_type_str(i));
-			copy_dev_field(dev_usage_sectors_wrong,
-				       d[i].sectors,	"%s sectors", bch2_data_type_str(i));
-			copy_dev_field(dev_usage_fragmented_wrong,
-				       d[i].fragmented,	"%s fragmented", bch2_data_type_str(i));
-		}
-	}
-
-	{
-		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,
-			      b.hidden,		"hidden");
-		copy_fs_field(fs_usage_btree_wrong,
-			      b.btree,		"btree");
-
-		if (!metadata_only) {
-			copy_fs_field(fs_usage_data_wrong,
-				      b.data,	"data");
-			copy_fs_field(fs_usage_cached_wrong,
-				      b.cached,	"cached");
-			copy_fs_field(fs_usage_reserved_wrong,
-				      b.reserved,	"reserved");
-			copy_fs_field(fs_usage_nr_inodes_wrong,
-				      b.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_v1 *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);
-	bch_err_fn(c, ret);
-
-	percpu_up_write(&c->mark_lock);
-	printbuf_exit(&buf);
-	return ret;
+	for_each_member_device(c, ca)
+		genradix_free(&ca->buckets_gc);
 }
 
 static int bch2_gc_start(struct bch_fs *c)
 {
-	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(c, ca) {
-		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;
+		int ret = bch2_dev_usage_init(ca, true);
+		if (ret) {
+			bch2_dev_put(ca);
+			return ret;
 		}
-
-		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)
-{
-	for_each_member_device(c, ca) {
-		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)
@@ -1354,6 +780,7 @@ static inline bool bch2_alloc_v4_cmp(struct bch_alloc_v4 l,
 		l.oldest_gen != r.oldest_gen		||
 		l.data_type != r.data_type		||
 		l.dirty_sectors	!= r.dirty_sectors	||
+		l.stripe_sectors != r.stripe_sectors	||
 		l.cached_sectors != r.cached_sectors	 ||
 		l.stripe_redundancy != r.stripe_redundancy ||
 		l.stripe != r.stripe;
@@ -1361,58 +788,55 @@ static inline bool bch2_alloc_v4_cmp(struct bch_alloc_v4 l,
 
 static int bch2_alloc_write_key(struct btree_trans *trans,
 				struct btree_iter *iter,
-				struct bkey_s_c k,
-				bool metadata_only)
+				struct bch_dev *ca,
+				struct bkey_s_c k)
 {
 	struct bch_fs *c = trans->c;
-	struct bch_dev *ca = bch_dev_bkey_exists(c, iter->pos.inode);
-	struct bucket old_gc, gc, *b;
 	struct bkey_i_alloc_v4 *a;
-	struct bch_alloc_v4 old_convert, new;
+	struct bch_alloc_v4 old_gc, gc, old_convert, new;
 	const struct bch_alloc_v4 *old;
 	int ret;
 
+	if (!bucket_valid(ca, k.k->p.offset))
+		return 0;
+
 	old = bch2_alloc_to_v4(k, &old_convert);
-	new = *old;
+	gc = new = *old;
+
+	__bucket_m_to_alloc(&gc, *gc_bucket(ca, iter->pos.offset));
 
-	percpu_down_read(&c->mark_lock);
-	b = gc_bucket(ca, iter->pos.offset);
-	old_gc = *b;
+	old_gc = gc;
 
 	if ((old->data_type == BCH_DATA_sb ||
 	     old->data_type == BCH_DATA_journal) &&
 	    !bch2_dev_is_online(ca)) {
-		b->data_type = old->data_type;
-		b->dirty_sectors = old->dirty_sectors;
+		gc.data_type = old->data_type;
+		gc.dirty_sectors = old->dirty_sectors;
 	}
 
 	/*
-	 * b->data_type doesn't yet include need_discard & need_gc_gen states -
+	 * gc.data_type doesn't yet include need_discard & need_gc_gen states -
 	 * fix that here:
 	 */
-	b->data_type = __alloc_data_type(b->dirty_sectors,
-					 b->cached_sectors,
-					 b->stripe,
-					 *old,
-					 b->data_type);
-	gc = *b;
-
+	alloc_data_type_set(&gc, gc.data_type);
 	if (gc.data_type != old_gc.data_type ||
-	    gc.dirty_sectors != old_gc.dirty_sectors)
-		bch2_dev_usage_update_m(c, ca, &old_gc, &gc);
-	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;
+	    gc.dirty_sectors != old_gc.dirty_sectors) {
+		ret = bch2_alloc_key_to_dev_counters(trans, ca, &old_gc, &gc, BTREE_TRIGGER_gc);
+		if (ret)
+			return ret;
 
-	if (gen_after(old->gen, gc.gen))
-		return 0;
+		/*
+		 * Ugly: alloc_key_to_dev_counters(..., BTREE_TRIGGER_gc) is not
+		 * safe w.r.t. transaction restarts, so fixup the gc_bucket so
+		 * we don't run it twice:
+		 */
+		struct bucket *gc_m = gc_bucket(ca, iter->pos.offset);
+		gc_m->data_type = gc.data_type;
+		gc_m->dirty_sectors = gc.dirty_sectors;
+	}
 
-	if (fsck_err_on(new.data_type != gc.data_type, c,
-			alloc_key_data_type_wrong,
+	if (fsck_err_on(new.data_type != gc.data_type,
+			trans, 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,
@@ -1422,25 +846,22 @@ static int bch2_alloc_write_key(struct btree_trans *trans,
 		new.data_type = gc.data_type;
 
 #define copy_bucket_field(_errtype, _f)					\
-	if (fsck_err_on(new._f != gc._f, c, _errtype,			\
+	if (fsck_err_on(new._f != gc._f,				\
+			trans, _errtype,				\
 			"bucket %llu:%llu gen %u data type %s has wrong " #_f	\
-			": got %u, should be %u",			\
+			": got %llu, should be %llu",			\
 			iter->pos.inode, iter->pos.offset,		\
 			gc.gen,						\
 			bch2_data_type_str(gc.data_type),		\
-			new._f, gc._f))					\
+			(u64) new._f, (u64) 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);
+	copy_bucket_field(alloc_key_gen_wrong,			gen);
+	copy_bucket_field(alloc_key_dirty_sectors_wrong,	dirty_sectors);
+	copy_bucket_field(alloc_key_stripe_sectors_wrong,	stripe_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))
@@ -1454,31 +875,31 @@ static int bch2_alloc_write_key(struct btree_trans *trans,
 	a->v = new;
 
 	/*
-	 * The trigger normally makes sure this is set, but we're not running
+	 * The trigger normally makes sure these are 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);
+	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)
+static int bch2_gc_alloc_done(struct bch_fs *c)
 {
 	int ret = 0;
 
 	for_each_member_device(c, ca) {
 		ret = bch2_trans_run(c,
-			for_each_btree_key_upto_commit(trans, iter, BTREE_ID_alloc,
+			for_each_btree_key_max_commit(trans, iter, BTREE_ID_alloc,
 					POS(ca->dev_idx, ca->mi.first_bucket),
 					POS(ca->dev_idx, ca->mi.nbuckets - 1),
-					BTREE_ITER_SLOTS|BTREE_ITER_PREFETCH, k,
-					NULL, NULL, BCH_TRANS_COMMIT_lazy_rw,
-				bch2_alloc_write_key(trans, &iter, k, metadata_only)));
+					BTREE_ITER_slots|BTREE_ITER_prefetch, k,
+					NULL, NULL, BCH_TRANS_COMMIT_no_enospc,
+				bch2_alloc_write_key(trans, &iter, ca, k)));
 		if (ret) {
-			percpu_ref_put(&ca->ref);
+			bch2_dev_put(ca);
 			break;
 		}
 	}
@@ -1487,180 +908,23 @@ static int bch2_gc_alloc_done(struct bch_fs *c, bool metadata_only)
 	return ret;
 }
 
-static int bch2_gc_alloc_start(struct bch_fs *c, bool metadata_only)
+static int bch2_gc_alloc_start(struct bch_fs *c)
 {
-	for_each_member_device(c, ca) {
-		struct bucket_array *buckets = kvmalloc(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]");
-			return -BCH_ERR_ENOMEM_gc_alloc_start;
-		}
-
-		buckets->first_bucket	= ca->mi.first_bucket;
-		buckets->nbuckets	= ca->mi.nbuckets;
-		rcu_assign_pointer(ca->buckets_gc, buckets);
-	}
-
-	int ret = bch2_trans_run(c,
-		for_each_btree_key(trans, iter, BTREE_ID_alloc, POS_MIN,
-					 BTREE_ITER_PREFETCH, k, ({
-			struct bch_dev *ca = bch_dev_bkey_exists(c, k.k->p.inode);
-			struct bucket *g = gc_bucket(ca, k.k->p.offset);
-
-			struct bch_alloc_v4 a_convert;
-			const struct bch_alloc_v4 *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;
-		})));
-	bch_err_fn(c, ret);
-	return ret;
-}
+	int ret = 0;
 
-static void bch2_gc_alloc_reset(struct bch_fs *c, bool metadata_only)
-{
 	for_each_member_device(c, ca) {
-		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;
+		ret = genradix_prealloc(&ca->buckets_gc, ca->mi.nbuckets, GFP_KERNEL);
+		if (ret) {
+			bch2_dev_put(ca);
+			ret = -BCH_ERR_ENOMEM_gc_alloc_start;
+			break;
 		}
 	}
-}
-
-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_noupdate(trans, k);
-		ret = PTR_ERR_OR_ZERO(new);
-		if (ret)
-			goto out;
-
-		if (!r->refcount)
-			new->k.type = KEY_TYPE_deleted;
-		else
-			*bkey_refcount(bkey_i_to_s(new)) = cpu_to_le64(r->refcount);
-		ret = bch2_trans_update(trans, iter, new, 0);
-	}
-out:
-fsck_err:
-	printbuf_exit(&buf);
-	return ret;
-}
-
-static int bch2_gc_reflink_done(struct bch_fs *c, bool metadata_only)
-{
-	size_t idx = 0;
-
-	if (metadata_only)
-		return 0;
-
-	int ret = bch2_trans_run(c,
-		for_each_btree_key_commit(trans, iter,
-				BTREE_ID_reflink, POS_MIN,
-				BTREE_ITER_PREFETCH, k,
-				NULL, NULL, BCH_TRANS_COMMIT_no_enospc,
-			bch2_gc_write_reflink_key(trans, &iter, k, &idx)));
-	c->reflink_gc_nr = 0;
-	return ret;
-}
-
-static int bch2_gc_reflink_start(struct bch_fs *c,
-				 bool metadata_only)
-{
-
-	if (metadata_only)
-		return 0;
-
-	c->reflink_gc_nr = 0;
-
-	int ret = bch2_trans_run(c,
-		for_each_btree_key(trans, iter, BTREE_ID_reflink, POS_MIN,
-				   BTREE_ITER_PREFETCH, k, ({
-			const __le64 *refcount = bkey_refcount_c(k);
-
-			if (!refcount)
-				continue;
-
-			struct reflink_gc *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;
-			0;
-		})));
 
 	bch_err_fn(c, ret);
 	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)
@@ -1693,7 +957,8 @@ static int bch2_gc_write_stripes_key(struct btree_trans *trans,
 	if (bad)
 		bch2_bkey_val_to_text(&buf, c, k);
 
-	if (fsck_err_on(bad, c, stripe_sector_count_wrong,
+	if (fsck_err_on(bad,
+			trans, stripe_sector_count_wrong,
 			"%s", buf.buf)) {
 		struct bkey_i_stripe *new;
 
@@ -1714,30 +979,20 @@ fsck_err:
 	return ret;
 }
 
-static int bch2_gc_stripes_done(struct bch_fs *c, bool metadata_only)
+static int bch2_gc_stripes_done(struct bch_fs *c)
 {
-	if (metadata_only)
-		return 0;
-
 	return bch2_trans_run(c,
 		for_each_btree_key_commit(trans, iter,
 				BTREE_ID_stripes, POS_MIN,
-				BTREE_ITER_PREFETCH, k,
+				BTREE_ITER_prefetch, k,
 				NULL, NULL, BCH_TRANS_COMMIT_no_enospc,
 			bch2_gc_write_stripes_key(trans, &iter, k)));
 }
 
-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:
+ * bch2_check_allocations - 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
  *
@@ -1756,9 +1011,8 @@ static void bch2_gc_stripes_reset(struct bch_fs *c, bool metadata_only)
  *    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)
+int bch2_check_allocations(struct bch_fs *c)
 {
-	unsigned iter = 0;
 	int ret;
 
 	lockdep_assert_held(&c->state_lock);
@@ -1767,67 +1021,34 @@ int bch2_gc(struct bch_fs *c, bool initial, bool metadata_only)
 
 	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);
+	ret   = bch2_gc_accounting_start(c) ?:
+		bch2_gc_start(c) ?:
+		bch2_gc_alloc_start(c) ?:
+		bch2_gc_reflink_start(c);
 	if (ret)
 		goto out;
-again:
-	gc_pos_set(c, gc_phase(GC_PHASE_START));
 
-	bch2_mark_superblocks(c);
+	gc_pos_set(c, gc_phase(GC_PHASE_start));
 
-	ret = bch2_gc_btrees(c, initial, metadata_only);
+	ret = bch2_mark_superblocks(c);
+	bch_err_msg(c, ret, "marking superblocks");
+	if (ret)
+		goto out;
 
+	ret = bch2_gc_btrees(c);
 	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;
-	}
+	ret   = bch2_gc_alloc_done(c) ?:
+		bch2_gc_accounting_done(c) ?:
+		bch2_gc_stripes_done(c) ?:
+		bch2_gc_reflink_done(c);
 out:
-	if (!ret) {
-		bch2_journal_block(&c->journal);
-
-		ret   = bch2_gc_alloc_done(c, metadata_only) ?:
-			bch2_gc_done(c, initial, metadata_only) ?:
-			bch2_gc_stripes_done(c, metadata_only) ?:
-			bch2_gc_reflink_done(c, 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));
+	__gc_pos_set(c, gc_phase(GC_PHASE_not_running));
 
 	bch2_gc_free(c);
 	percpu_up_write(&c->mark_lock);
@@ -1852,24 +1073,31 @@ static int gc_btree_gens_key(struct btree_trans *trans,
 	struct bkey_i *u;
 	int ret;
 
-	percpu_down_read(&c->mark_lock);
+	if (unlikely(test_bit(BCH_FS_going_ro, &c->flags)))
+		return -EROFS;
+
+	rcu_read_lock();
 	bkey_for_each_ptr(ptrs, ptr) {
-		struct bch_dev *ca = bch_dev_bkey_exists(c, ptr->dev);
+		struct bch_dev *ca = bch2_dev_rcu(c, ptr->dev);
+		if (!ca)
+			continue;
 
-		if (ptr_stale(ca, ptr) > 16) {
-			percpu_up_read(&c->mark_lock);
+		if (dev_ptr_stale(ca, ptr) > 16) {
+			rcu_read_unlock();
 			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)];
+		struct bch_dev *ca = bch2_dev_rcu(c, ptr->dev);
+		if (!ca)
+			continue;
 
+		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);
+	rcu_read_unlock();
 	return 0;
 update:
 	u = bch2_bkey_make_mut(trans, iter, &k, 0);
@@ -1881,10 +1109,9 @@ update:
 	return 0;
 }
 
-static int bch2_alloc_write_oldest_gen(struct btree_trans *trans, struct btree_iter *iter,
-				       struct bkey_s_c k)
+static int bch2_alloc_write_oldest_gen(struct btree_trans *trans, struct bch_dev *ca,
+				       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;
@@ -1899,7 +1126,6 @@ static int bch2_alloc_write_oldest_gen(struct btree_trans *trans, struct btree_i
 		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);
 }
@@ -1909,16 +1135,20 @@ int bch2_gc_gens(struct bch_fs *c)
 	u64 b, start_time = local_clock();
 	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);
+
+	/*
+	 * We have to use trylock here. Otherwise, we would
+	 * introduce a deadlock in the RO path - we take the
+	 * state lock at the start of going RO.
+	 */
+	if (!down_read_trylock(&c->state_lock)) {
+		mutex_unlock(&c->gc_gens_lock);
+		return 0;
+	}
 
 	for_each_member_device(c, ca) {
 		struct bucket_gens *gens = bucket_gens(ca);
@@ -1927,7 +1157,7 @@ int bch2_gc_gens(struct bch_fs *c)
 
 		ca->oldest_gen = kvmalloc(gens->nbuckets, GFP_KERNEL);
 		if (!ca->oldest_gen) {
-			percpu_ref_put(&ca->ref);
+			bch2_dev_put(ca);
 			ret = -BCH_ERR_ENOMEM_gc_gens;
 			goto err;
 		}
@@ -1945,7 +1175,7 @@ int bch2_gc_gens(struct bch_fs *c)
 			ret = bch2_trans_run(c,
 				for_each_btree_key_commit(trans, iter, i,
 						POS_MIN,
-						BTREE_ITER_PREFETCH|BTREE_ITER_ALL_SNAPSHOTS,
+						BTREE_ITER_prefetch|BTREE_ITER_all_snapshots,
 						k,
 						NULL, NULL,
 						BCH_TRANS_COMMIT_no_enospc,
@@ -1954,14 +1184,23 @@ int bch2_gc_gens(struct bch_fs *c)
 				goto err;
 		}
 
+	struct bch_dev *ca = NULL;
 	ret = bch2_trans_run(c,
 		for_each_btree_key_commit(trans, iter, BTREE_ID_alloc,
 				POS_MIN,
-				BTREE_ITER_PREFETCH,
+				BTREE_ITER_prefetch,
 				k,
 				NULL, NULL,
-				BCH_TRANS_COMMIT_no_enospc,
-			bch2_alloc_write_oldest_gen(trans, &iter, k)));
+				BCH_TRANS_COMMIT_no_enospc, ({
+			ca = bch2_dev_iterate(c, ca, k.k->p.inode);
+			if (!ca) {
+				bch2_btree_iter_set_pos(&iter, POS(k.k->p.inode + 1, 0));
+				continue;
+			}
+			bch2_alloc_write_oldest_gen(trans, ca, &iter, k);
+		})));
+	bch2_dev_put(ca);
+
 	if (ret)
 		goto err;
 
@@ -1978,94 +1217,37 @@ err:
 		ca->oldest_gen = NULL;
 	}
 
-	up_read(&c->gc_lock);
+	up_read(&c->state_lock);
 	mutex_unlock(&c->gc_gens_lock);
 	if (!bch2_err_matches(ret, EROFS))
 		bch_err_fn(c, ret);
 	return ret;
 }
 
-static int bch2_gc_thread(void *arg)
+static void bch2_gc_gens_work(struct work_struct *work)
 {
-	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);
-
-	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
-		bch2_gc_gens(c);
-#endif
-		debug_check_no_locks_held();
-	}
-
-	return 0;
+	struct bch_fs *c = container_of(work, struct bch_fs, gc_gens_work);
+	bch2_gc_gens(c);
+	bch2_write_ref_put(c, BCH_WRITE_REF_gc_gens);
 }
 
-void bch2_gc_thread_stop(struct bch_fs *c)
+void bch2_gc_gens_async(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);
-	}
+	if (bch2_write_ref_tryget(c, BCH_WRITE_REF_gc_gens) &&
+	    !queue_work(c->write_ref_wq, &c->gc_gens_work))
+		bch2_write_ref_put(c, BCH_WRITE_REF_gc_gens);
 }
 
-int bch2_gc_thread_start(struct bch_fs *c)
+void bch2_fs_btree_gc_exit(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);
-	}
+int bch2_fs_btree_gc_init(struct bch_fs *c)
+{
+	seqcount_init(&c->gc_pos_lock);
+	INIT_WORK(&c->gc_gens_work, bch2_gc_gens_work);
 
-	get_task_struct(p);
-	c->gc_thread = p;
-	wake_up_process(p);
+	init_rwsem(&c->gc_lock);
+	mutex_init(&c->gc_gens_lock);
 	return 0;
 }