bcachefs: Use btree_ptr_v2.mem_ptr to avoid hash table lookup

Nice performance optimization

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

fs/bcachefs/btree_cache.c

@@ -596,12 +596,13 @@ static noinline struct btree *bch2_btree_node_fill(struct bch_fs *c,
 	struct btree_cache *bc = &c->btree_cache;
 	struct btree *b;
 
+	BUG_ON(level + 1 >= BTREE_MAX_DEPTH);
 	/*
 	 * Parent node must be locked, else we could read in a btree node that's
 	 * been freed:
 	 */
-	BUG_ON(!btree_node_locked(iter, level + 1));
-	BUG_ON(level >= BTREE_MAX_DEPTH);
+	if (!bch2_btree_node_relock(iter, level + 1))
+		return ERR_PTR(-EINTR);
 
 	b = bch2_btree_node_mem_alloc(c);
 	if (IS_ERR(b))
@@ -624,13 +625,9 @@ static noinline struct btree *bch2_btree_node_fill(struct bch_fs *c,
 	}
 
 	/*
-	 * If the btree node wasn't cached, we can't drop our lock on
-	 * the parent until after it's added to the cache - because
-	 * otherwise we could race with a btree_split() freeing the node
-	 * we're trying to lock.
+	 * Unlock before doing IO:
 	 *
-	 * But the deadlock described below doesn't exist in this case,
-	 * so it's safe to not drop the parent lock until here:
+	 * XXX: ideally should be dropping all btree node locks here
 	 */
 	if (btree_node_read_locked(iter, level + 1))
 		btree_node_unlock(iter, level + 1);
@@ -667,16 +664,11 @@ struct btree *bch2_btree_node_get(struct bch_fs *c, struct btree_iter *iter,
 	struct btree *b;
 	struct bset_tree *t;
 
-	/*
-	 * XXX: locking optimization
-	 *
-	 * we can make the locking looser here - caller can drop lock on parent
-	 * node before locking child node (and potentially blocking): we just
-	 * have to have bch2_btree_node_fill() call relock on the parent and
-	 * return -EINTR if that fails
-	 */
-	EBUG_ON(!btree_node_locked(iter, level + 1));
 	EBUG_ON(level >= BTREE_MAX_DEPTH);
+
+	b = btree_node_mem_ptr(k);
+	if (b)
+		goto lock_node;
 retry:
 	b = btree_cache_find(bc, k);
 	if (unlikely(!b)) {
@@ -694,6 +686,7 @@ struct btree *bch2_btree_node_get(struct bch_fs *c, struct btree_iter *iter,
 		if (IS_ERR(b))
 			return b;
 	} else {
+lock_node:
 		/*
 		 * There's a potential deadlock with splits and insertions into
 		 * interior nodes we have to avoid:
@@ -740,6 +733,7 @@ struct btree *bch2_btree_node_get(struct bch_fs *c, struct btree_iter *iter,
 		}
 	}
 
+	/* XXX: waiting on IO with btree locks held: */
 	wait_on_bit_io(&b->flags, BTREE_NODE_read_in_flight,
 		       TASK_UNINTERRUPTIBLE);
 

fs/bcachefs/btree_cache.h

@@ -47,6 +47,13 @@ static inline u64 btree_ptr_hash_val(const struct bkey_i *k)
 	}
 }
 
+static inline struct btree *btree_node_mem_ptr(const struct bkey_i *k)
+{
+	return k->k.type == KEY_TYPE_btree_ptr_v2
+		? (void *)(unsigned long)bkey_i_to_btree_ptr_v2_c(k)->v.mem_ptr
+		: NULL;
+}
+
 /* is btree node in hash table? */
 static inline bool btree_node_hashed(struct btree *b)
 {

fs/bcachefs/btree_io.c

@@ -1647,6 +1647,7 @@ void __bch2_btree_node_write(struct bch_fs *c, struct btree *b,
 
 	b->written += sectors_to_write;
 
+	/* XXX: submitting IO with btree locks held: */
 	bch2_submit_wbio_replicas(&wbio->wbio, c, BCH_DATA_BTREE, &k.key);
 	return;
 err:

fs/bcachefs/btree_iter.c

@@ -912,6 +912,27 @@ static void btree_iter_prefetch(struct btree_iter *iter)
 		btree_node_unlock(iter, iter->level);
 }
 
+static noinline void btree_node_mem_ptr_set(struct btree_iter *iter,
+					    unsigned plevel, struct btree *b)
+{
+	struct btree_iter_level *l = &iter->l[plevel];
+	bool locked = btree_node_locked(iter, plevel);
+	struct bkey_packed *k;
+	struct bch_btree_ptr_v2 *bp;
+
+	if (!bch2_btree_node_relock(iter, plevel))
+		return;
+
+	k = bch2_btree_node_iter_peek_all(&l->iter, l->b);
+	BUG_ON(k->type != KEY_TYPE_btree_ptr_v2);
+
+	bp = (void *) bkeyp_val(&l->b->format, k);
+	bp->mem_ptr = (unsigned long)b;
+
+	if (!locked)
+		btree_node_unlock(iter, plevel);
+}
+
 static __always_inline int btree_iter_down(struct btree_iter *iter)
 {
 	struct bch_fs *c = iter->trans->c;
@@ -933,6 +954,10 @@ static __always_inline int btree_iter_down(struct btree_iter *iter)
 	mark_btree_node_locked(iter, level, lock_type);
 	btree_iter_node_set(iter, b);
 
+	if (tmp.k.k.type == KEY_TYPE_btree_ptr_v2 &&
+	    unlikely(b != btree_node_mem_ptr(&tmp.k)))
+		btree_node_mem_ptr_set(iter, level + 1, b);
+
 	if (iter->flags & BTREE_ITER_PREFETCH)
 		btree_iter_prefetch(iter);