bcachefs: Add an option to control btree node prefetching

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

fs/bcachefs/btree_iter.c

@@ -897,7 +897,8 @@ static noinline int btree_node_iter_and_journal_peek(struct btree_trans *trans,
 
 	bch2_bkey_buf_reassemble(out, c, k);
 
-	if (flags & BTREE_ITER_PREFETCH)
+	if ((flags & BTREE_ITER_PREFETCH) &&
+	    c->opts.btree_node_prefetch)
 		ret = btree_path_prefetch_j(trans, path, &jiter);
 
 	bch2_btree_and_journal_iter_exit(&jiter);
@@ -929,7 +930,8 @@ static __always_inline int btree_path_down(struct btree_trans *trans,
 		bch2_bkey_buf_unpack(&tmp, c, l->b,
 				 bch2_btree_node_iter_peek(&l->iter, l->b));
 
-		if (flags & BTREE_ITER_PREFETCH) {
+		if ((flags & BTREE_ITER_PREFETCH) &&
+		    c->opts.btree_node_prefetch) {
 			ret = btree_path_prefetch(trans, path);
 			if (ret)
 				goto err;

fs/bcachefs/opts.h

@@ -458,7 +458,13 @@ enum fsck_err_opts {
 	  OPT_UINT(0, BCH_REPLICAS_MAX),				\
 	  BCH2_NO_SB_OPT,		1,				\
 	  "n",		"Data written to this device will be considered\n"\
-			"to have already been replicated n times")
+			"to have already been replicated n times")	\
+	x(btree_node_prefetch,		u8,				\
+	  OPT_FS|OPT_MOUNT|OPT_RUNTIME,					\
+	  OPT_BOOL(),							\
+	  BCH2_NO_SB_OPT,		true,				\
+	  NULL,		"BTREE_ITER_PREFETCH casuse btree nodes to be\n"\
+	  " prefetched sequentially")
 
 struct bch_opts {
 #define x(_name, _bits, ...)	unsigned _name##_defined:1;