bcache: Explicitly track btree node's parent

This is prep work for the reworked btree insertion code.

The way we set b->parent is ugly and hacky... the problem is, when
btree_split() or garbage collection splits or rewrites a btree node, the
parent changes for all its (potentially already cached) children.

I may change this later and add some code to look through the btree node
cache and find all our cached child nodes and change the parent pointer
then...
Signed-off-by: Kent Overstreet <kmo@daterainc.com>

drivers/md/bcache/btree.c

@@ -884,6 +884,7 @@ static struct btree *mca_alloc(struct cache_set *c, struct bkey *k,
 
 	lock_set_subclass(&b->lock.dep_map, level + 1, _THIS_IP_);
 	b->level	= level;
+	b->parent	= (void *) ~0UL;
 
 	mca_reinit(b);
 
@@ -1898,7 +1899,7 @@ bool bch_btree_insert_check_key(struct btree *b, struct btree_op *op,
 
 static int btree_split(struct btree *b, struct btree_op *op)
 {
-	bool split, root = b == b->c->root;
+	bool split;
 	struct btree *n1, *n2 = NULL, *n3 = NULL;
 	uint64_t start_time = local_clock();
 
@@ -1920,7 +1921,7 @@ static int btree_split(struct btree *b, struct btree_op *op)
 		if (IS_ERR(n2))
 			goto err_free1;
 
-		if (root) {
+		if (!b->parent) {
 			n3 = bch_btree_node_alloc(b->c, b->level + 1, &op->cl);
 			if (IS_ERR(n3))
 				goto err_free2;
@@ -1928,7 +1929,8 @@ static int btree_split(struct btree *b, struct btree_op *op)
 
 		bch_btree_insert_keys(n1, op);
 
-		/* Has to be a linear search because we don't have an auxiliary
+		/*
+		 * Has to be a linear search because we don't have an auxiliary
 		 * search tree yet
 		 */
 
@@ -1960,6 +1962,8 @@ static int btree_split(struct btree *b, struct btree_op *op)
 	bch_btree_node_write(n1, &op->cl);
 
 	if (n3) {
+		/* Depth increases, make a new root */
+
 		bkey_copy_key(&n3->key, &MAX_KEY);
 		bch_btree_insert_keys(n3, op);
 		bch_btree_node_write(n3, &op->cl);
@@ -1967,7 +1971,9 @@ static int btree_split(struct btree *b, struct btree_op *op)
 		closure_sync(&op->cl);
 		bch_btree_set_root(n3);
 		rw_unlock(true, n3);
-	} else if (root) {
+	} else if (!b->parent) {
+		/* Root filled up but didn't need to be split */
+
 		op->keys.top = op->keys.bottom;
 		closure_sync(&op->cl);
 		bch_btree_set_root(n1);

drivers/md/bcache/btree.h

@@ -125,6 +125,7 @@ struct btree {
 	unsigned long		seq;
 	struct rw_semaphore	lock;
 	struct cache_set	*c;
+	struct btree		*parent;
 
 	unsigned long		flags;
 	uint16_t		written;	/* would be nice to kill */
@@ -327,12 +328,13 @@ static inline void rw_unlock(bool w, struct btree *b)
 ({									\
 	int _r, l = (b)->level - 1;					\
 	bool _w = l <= (op)->lock;					\
-	struct btree *_b = bch_btree_node_get((b)->c, key, l, op);	\
-	if (!IS_ERR(_b)) {						\
-		_r = bch_btree_ ## fn(_b, op, ##__VA_ARGS__);		\
-		rw_unlock(_w, _b);					\
+	struct btree *_child = bch_btree_node_get((b)->c, key, l, op);	\
+	if (!IS_ERR(_child)) {						\
+		_child->parent = (b);					\
+		_r = bch_btree_ ## fn(_child, op, ##__VA_ARGS__);	\
+		rw_unlock(_w, _child);					\
 	} else								\
-		_r = PTR_ERR(_b);					\
+		_r = PTR_ERR(_child);					\
 	_r;								\
 })
 
@@ -350,8 +352,10 @@ static inline void rw_unlock(bool w, struct btree *b)
 		bool _w = insert_lock(op, _b);				\
 		rw_lock(_w, _b, _b->level);				\
 		if (_b == (c)->root &&					\
-		    _w == insert_lock(op, _b))				\
+		    _w == insert_lock(op, _b)) {			\
+			_b->parent = NULL;				\
 			_r = bch_btree_ ## fn(_b, op, ##__VA_ARGS__);	\
+		}							\
 		rw_unlock(_w, _b);					\
 		bch_cannibalize_unlock(c, &(op)->cl);			\
 	} while (_r == -EINTR);						\