bcachefs: Fix a KMSAN splat in btree_update_nodes_written()

We may sometimes read from uninitialized memory; we know, and that's ok.

We check if a btree node has been reused before waiting on any
outstanding IO.

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

diff --git a/fs/bcachefs/btree_update_interior.c b/fs/bcachefs/btree_update_interior.c
index d3e0cf01ba37ad1c564e6922d0b2faee05766d53..67f1e32028353d5477ffe24d1813b0ff3ef6e5e2 100644 (file)
--- a/fs/bcachefs/btree_update_interior.c
+++ b/fs/bcachefs/btree_update_interior.c
@@ -649,6 +649,14 @@ static int btree_update_nodes_written_trans(struct btree_trans *trans,
        return 0;
 }
 
+/* If the node has been reused, we might be reading uninitialized memory - that's fine: */
+static noinline __no_kmsan_checks bool btree_node_seq_matches(struct btree *b, __le64 seq)
+{
+       struct btree_node *b_data = READ_ONCE(b->data);
+
+       return (b_data ? b_data->keys.seq : 0) == seq;
+}
+
 static void btree_update_nodes_written(struct btree_update *as)
 {
        struct bch_fs *c = as->c;
@@ -677,17 +685,9 @@ static void btree_update_nodes_written(struct btree_update *as)
         * on disk:
         */
        for (i = 0; i < as->nr_old_nodes; i++) {
-               __le64 seq;
-
                b = as->old_nodes[i];
 
-               bch2_trans_begin(trans);
-               btree_node_lock_nopath_nofail(trans, &b->c, SIX_LOCK_read);
-               seq = b->data ? b->data->keys.seq : 0;
-               six_unlock_read(&b->c.lock);
-               bch2_trans_unlock_long(trans);
-
-               if (seq == as->old_nodes_seq[i])
+               if (btree_node_seq_matches(b, as->old_nodes_seq[i]))
                        wait_on_bit_io(&b->flags, BTREE_NODE_write_in_flight_inner,
                                       TASK_UNINTERRUPTIBLE);
        }