bcachefs: Change btree split threshold to be in u64s

This fixes a bug with very small btree nodes where splitting would end
up with one of the new nodes empty.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_cache.h

@@ -75,7 +75,7 @@ static inline unsigned btree_blocks(struct bch_fs *c)
 	return c->opts.btree_node_size >> c->block_bits;
 }
 
-#define BTREE_SPLIT_THRESHOLD(c)		(btree_blocks(c) * 3 / 4)
+#define BTREE_SPLIT_THRESHOLD(c)		(btree_max_u64s(c) * 3 / 4)
 
 #define BTREE_FOREGROUND_MERGE_THRESHOLD(c)	(btree_max_u64s(c) * 1 / 3)
 #define BTREE_FOREGROUND_MERGE_HYSTERESIS(c)			\

fs/bcachefs/btree_update_interior.c

@@ -1385,7 +1385,7 @@ static void btree_split(struct btree_update *as, struct btree *b,
 	if (keys)
 		btree_split_insert_keys(as, n1, iter, keys);
 
-	if (vstruct_blocks(n1->data, c->block_bits) > BTREE_SPLIT_THRESHOLD(c)) {
+	if (bset_u64s(&n1->set[0]) > BTREE_SPLIT_THRESHOLD(c)) {
 		trace_btree_split(c, b);
 
 		n2 = __btree_split_node(as, n1, iter);