bcachefs: Bring back BTREE_ITER_CACHED_NOFILL

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_iter.c

@@ -1581,7 +1581,8 @@ struct bkey_s_c bch2_btree_path_peek_slot(struct btree_path *path, struct bkey *
 		EBUG_ON(ck &&
 			(path->btree_id != ck->key.btree_id ||
 			 !bkey_eq(path->pos, ck->key.pos)));
-		EBUG_ON(!ck || !ck->valid);
+		if (!ck || !ck->valid)
+			return bkey_s_c_null;
 
 		*u = ck->k->k;
 		k = bkey_i_to_s_c(ck->k);
@@ -1860,7 +1861,8 @@ struct bkey_s_c btree_trans_peek_key_cache(struct btree_iter *iter, struct bpos
 	if (!iter->key_cache_path)
 		iter->key_cache_path = bch2_path_get(trans, iter->btree_id, pos,
 						     iter->flags & BTREE_ITER_INTENT, 0,
-						     iter->flags|BTREE_ITER_CACHED);
+						     iter->flags|BTREE_ITER_CACHED|
+						     BTREE_ITER_CACHED_NOFILL);
 
 	iter->key_cache_path = bch2_btree_path_set_pos(trans, iter->key_cache_path, pos,
 					iter->flags & BTREE_ITER_INTENT);

fs/bcachefs/btree_key_cache.c

@@ -487,7 +487,7 @@ bch2_btree_path_traverse_cached_slowpath(struct btree_trans *trans, struct btree
 	path->l[0].lock_seq	= ck->c.lock.state.seq;
 	path->l[0].b		= (void *) ck;
 fill:
-	if (!ck->valid) {
+	if (!ck->valid && !(flags & BTREE_ITER_CACHED_NOFILL)) {
 		/*
 		 * Using the underscore version because we haven't set
 		 * path->uptodate yet:
@@ -508,7 +508,6 @@ bch2_btree_path_traverse_cached_slowpath(struct btree_trans *trans, struct btree
 		set_bit(BKEY_CACHED_ACCESSED, &ck->flags);
 
 	path->uptodate = BTREE_ITER_UPTODATE;
-	BUG_ON(!ck->valid);
 	BUG_ON(btree_node_locked_type(path, 0) != btree_lock_want(path, 0));
 
 	return ret;

fs/bcachefs/btree_types.h

@@ -207,6 +207,7 @@ struct btree_node_iter {
 #define BTREE_ITER_ALL_SNAPSHOTS	(1 << 11)
 #define BTREE_ITER_FILTER_SNAPSHOTS	(1 << 12)
 #define BTREE_ITER_NOPRESERVE		(1 << 13)
+#define BTREE_ITER_CACHED_NOFILL	(1 << 14)
 
 enum btree_path_uptodate {
 	BTREE_ITER_UPTODATE		= 0,