bcachefs: Fix for long running btree transactions & key cache

While a btree transaction is running, we hold a SRCU read lock on the
btree key cache that prevents btree key cache keys from being freed -
this is so that relock() operations won't access freed memory.

The downside of this is that long running btree transactions prevent
memory from being freed from the key cache. This adds a check in
bch2_trans_begin() - if the transaction has been running longer than 1
second, drop and retake the SRCU read lock and zero out pointers to
unlock key cache paths.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_iter.c

@@ -2756,6 +2756,20 @@ void *__bch2_trans_kmalloc(struct btree_trans *trans, size_t size)
 	return p;
 }
 
+static noinline void bch2_trans_reset_srcu_lock(struct btree_trans *trans)
+{
+	struct bch_fs *c = trans->c;
+	struct btree_path *path;
+
+	trans_for_each_path(trans, path)
+		if (path->cached && !btree_node_locked(path, 0))
+			path->l[0].b = ERR_PTR(-BCH_ERR_no_btree_node_srcu_reset);
+
+	srcu_read_unlock(&c->btree_trans_barrier, trans->srcu_idx);
+	trans->srcu_idx = srcu_read_lock(&c->btree_trans_barrier);
+	trans->srcu_lock_time	= jiffies;
+}
+
 /**
  * bch2_trans_begin() - reset a transaction after a interrupted attempt
  * @trans: transaction to reset
@@ -2811,6 +2825,9 @@ u32 bch2_trans_begin(struct btree_trans *trans)
 		bch2_trans_relock(trans);
 	}
 
+	if (unlikely(time_after(jiffies, trans->srcu_lock_time + HZ)))
+		bch2_trans_reset_srcu_lock(trans);
+
 	trans->last_restarted_ip = _RET_IP_;
 	if (trans->restarted)
 		bch2_btree_path_traverse_all(trans);
@@ -2897,6 +2914,7 @@ void __bch2_trans_init(struct btree_trans *trans, struct bch_fs *c, unsigned fn_
 		trans->nr_max_paths = s->nr_max_paths;
 
 	trans->srcu_idx = srcu_read_lock(&c->btree_trans_barrier);
+	trans->srcu_lock_time	= jiffies;
 
 	if (IS_ENABLED(CONFIG_BCACHEFS_DEBUG_TRANSACTIONS)) {
 		struct btree_trans *pos;

fs/bcachefs/btree_types.h

@@ -411,6 +411,7 @@ struct btree_trans {
 	enum bch_errcode	restarted:16;
 	u32			restart_count;
 	unsigned long		last_restarted_ip;
+	unsigned long		srcu_lock_time;
 
 	/*
 	 * For when bch2_trans_update notices we'll be splitting a compressed

fs/bcachefs/errcode.h

@@ -53,6 +53,7 @@
 	x(BCH_ERR_no_btree_node,	no_btree_node_down)			\
 	x(BCH_ERR_no_btree_node,	no_btree_node_init)			\
 	x(BCH_ERR_no_btree_node,	no_btree_node_cached)			\
+	x(BCH_ERR_no_btree_node,	no_btree_node_srcu_reset)		\
 	x(0,				btree_insert_fail)			\
 	x(BCH_ERR_btree_insert_fail,	btree_insert_btree_node_full)		\
 	x(BCH_ERR_btree_insert_fail,	btree_insert_need_mark_replicas)	\