bcachefs: btree node prefetching in check_topology

btree_and_journal_iter is old code that we want to get rid of, but we're
not ready to yet.

lack of btree node prefetching is, it turns out, a real performance
issue for fsck on spinning rust, so - add it.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_cache.c

@@ -711,6 +711,9 @@ static noinline struct btree *bch2_btree_node_fill(struct btree_trans *trans,
 	b = bch2_btree_node_mem_alloc(trans, level != 0);
 
 	if (bch2_err_matches(PTR_ERR_OR_ZERO(b), ENOMEM)) {
+		if (!path)
+			return b;
+
 		trans->memory_allocation_failure = true;
 		trace_and_count(c, trans_restart_memory_allocation_failure, trans, _THIS_IP_, path);
 		return ERR_PTR(btree_trans_restart(trans, BCH_ERR_transaction_restart_fill_mem_alloc_fail));
@@ -760,7 +763,8 @@ static noinline struct btree *bch2_btree_node_fill(struct btree_trans *trans,
 	}
 
 	if (!six_relock_type(&b->c.lock, lock_type, seq)) {
-		if (path)
+		BUG_ON(!path);
+
 		trace_and_count(c, trans_restart_relock_after_fill, trans, _THIS_IP_, path);
 		return ERR_PTR(btree_trans_restart(trans, BCH_ERR_transaction_restart_relock_after_fill));
 	}
@@ -1096,7 +1100,7 @@ int bch2_btree_node_prefetch(struct btree_trans *trans,
 	struct btree_cache *bc = &c->btree_cache;
 	struct btree *b;
 
-	BUG_ON(trans && !btree_node_locked(path, level + 1));
+	BUG_ON(path && !btree_node_locked(path, level + 1));
 	BUG_ON(level >= BTREE_MAX_DEPTH);
 
 	b = btree_cache_find(bc, k);

fs/bcachefs/btree_gc.c

@@ -390,6 +390,7 @@ static int bch2_btree_repair_topology_recurse(struct btree_trans *trans, struct
 	bch2_bkey_buf_init(&prev_k);
 	bch2_bkey_buf_init(&cur_k);
 	bch2_btree_and_journal_iter_init_node_iter(trans, &iter, b);
+	iter.prefetch = true;
 
 	while ((k = bch2_btree_and_journal_iter_peek(&iter)).k) {
 		BUG_ON(bpos_lt(k.k->p, b->data->min_key));
@@ -479,6 +480,7 @@ static int bch2_btree_repair_topology_recurse(struct btree_trans *trans, struct
 
 	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) {
 		bch2_bkey_buf_reassemble(&cur_k, c, k);
@@ -964,6 +966,7 @@ static int bch2_gc_btree_init_recurse(struct btree_trans *trans, struct btree *b
 	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;

fs/bcachefs/btree_journal_iter.c

 // SPDX-License-Identifier: GPL-2.0
 
 #include "bcachefs.h"
+#include "bkey_buf.h"
 #include "bset.h"
+#include "btree_cache.h"
 #include "btree_journal_iter.h"
 #include "journal_io.h"
 
@@ -334,9 +336,38 @@ void bch2_btree_and_journal_iter_advance(struct btree_and_journal_iter *iter)
 		iter->pos = bpos_successor(iter->pos);
 }
 
+static void btree_and_journal_iter_prefetch(struct btree_and_journal_iter *_iter)
+{
+	struct btree_and_journal_iter iter = *_iter;
+	struct bch_fs *c = iter.trans->c;
+	unsigned level = iter.journal.level;
+	struct bkey_buf tmp;
+	unsigned nr = test_bit(BCH_FS_started, &c->flags)
+		? (level > 1 ? 0 :  2)
+		: (level > 1 ? 1 : 16);
+
+	iter.prefetch = false;
+	bch2_bkey_buf_init(&tmp);
+
+	while (nr--) {
+		bch2_btree_and_journal_iter_advance(&iter);
+		struct bkey_s_c k = bch2_btree_and_journal_iter_peek(&iter);
+		if (!k.k)
+			break;
+
+		bch2_bkey_buf_reassemble(&tmp, c, k);
+		bch2_btree_node_prefetch(iter.trans, NULL, tmp.k, iter.journal.btree_id, level - 1);
+	}
+
+	bch2_bkey_buf_exit(&tmp, c);
+}
+
 struct bkey_s_c bch2_btree_and_journal_iter_peek(struct btree_and_journal_iter *iter)
 {
 	struct bkey_s_c btree_k, journal_k, ret;
+
+	if (iter->prefetch && iter->journal.level)
+		btree_and_journal_iter_prefetch(iter);
 again:
 	if (iter->at_end)
 		return bkey_s_c_null;

fs/bcachefs/btree_journal_iter.h

@@ -23,6 +23,7 @@ struct btree_and_journal_iter {
 	struct journal_iter	journal;
 	struct bpos		pos;
 	bool			at_end;
+	bool			prefetch;
 };
 
 struct bkey_i *bch2_journal_keys_peek_upto(struct bch_fs *, enum btree_id,