bcachefs: Add an option for btree node mem ptr optimization

bch2_btree_node_ptr_v2 has a field for stashing a pointer to the in
memory btree node; this is safe because we clear this field when reading
in nodes from disk and we never free in memory btree nodes - but, we
have bug reports that indicate something might be faulty with this
optimization, so let's add an option for it.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/btree_cache.c

@@ -767,9 +767,11 @@ struct btree *bch2_btree_node_get(struct btree_trans *trans, struct btree_iter *
 
 	EBUG_ON(level >= BTREE_MAX_DEPTH);
 
-	b = btree_node_mem_ptr(k);
-	if (b)
-		goto lock_node;
+	if (c->opts.btree_node_mem_ptr_optimization) {
+		b = btree_node_mem_ptr(k);
+		if (b)
+			goto lock_node;
+	}
 retry:
 	b = btree_cache_find(bc, k);
 	if (unlikely(!b)) {
@@ -902,9 +904,11 @@ struct btree *bch2_btree_node_get_noiter(struct bch_fs *c,
 
 	EBUG_ON(level >= BTREE_MAX_DEPTH);
 
-	b = btree_node_mem_ptr(k);
-	if (b)
-		goto lock_node;
+	if (c->opts.btree_node_mem_ptr_optimization) {
+		b = btree_node_mem_ptr(k);
+		if (b)
+			goto lock_node;
+	}
 retry:
 	b = btree_cache_find(bc, k);
 	if (unlikely(!b)) {

fs/bcachefs/opts.h

@@ -178,6 +178,11 @@ enum opt_type {
 	  OPT_BOOL(),							\
 	  BCH_SB_INODES_USE_KEY_CACHE,	true,				\
 	  NULL,		"Use the btree key cache for the inodes btree")	\
+	x(btree_node_mem_ptr_optimization, u8,				\
+	  OPT_MOUNT|OPT_RUNTIME,					\
+	  OPT_BOOL(),							\
+	  NO_SB_OPT,			true,				\
+	  NULL,		"Stash pointer to in memory btree node in btree ptr")\
 	x(gc_reserve_percent,		u8,				\
 	  OPT_FORMAT|OPT_MOUNT|OPT_RUNTIME,				\
 	  OPT_UINT(5, 21),						\