bcachefs: backpointer_get_key() doesn't pull in btree node

We may not need to pull in a btree node when walking backpointers -
don't do so unnecessarily when using backpointer_get_key().

It'll still fall back to backpointer_get_node() in a few situations,
including btree roots (where an iterator can't point at just the key),
and races due to the interior update path not having deleted a
backpointer to an old node yet.

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

diff --git a/fs/bcachefs/backpointers.c b/fs/bcachefs/backpointers.c
index 1d30066e63dc3f90ce82d71694a44a4f612e4c25..3aff2b24de4aa79aca68466607102bcb285de382 100644 (file)
--- a/fs/bcachefs/backpointers.c
+++ b/fs/bcachefs/backpointers.c
@@ -244,27 +244,31 @@ struct bkey_s_c bch2_backpointer_get_key(struct btree_trans *trans,
        if (unlikely(bp.v->btree_id >= btree_id_nr_alive(c)))
                return bkey_s_c_null;
 
-       if (likely(!bp.v->level)) {
-               bch2_trans_node_iter_init(trans, iter,
-                                         bp.v->btree_id,
-                                         bp.v->pos,
-                                         0, 0,
-                                         iter_flags);
-               struct bkey_s_c k = bch2_btree_iter_peek_slot(iter);
-               if (bkey_err(k)) {
-                       bch2_trans_iter_exit(trans, iter);
-                       return k;
-               }
+       bch2_trans_node_iter_init(trans, iter,
+                                 bp.v->btree_id,
+                                 bp.v->pos,
+                                 0,
+                                 bp.v->level,
+                                 iter_flags);
+       struct bkey_s_c k = bch2_btree_iter_peek_slot(iter);
+       if (bkey_err(k)) {
+               bch2_trans_iter_exit(trans, iter);
+               return k;
+       }
 
-               if (k.k &&
-                   extent_matches_bp(c, bp.v->btree_id, bp.v->level, k, bp))
-                       return k;
+       if (k.k &&
+           extent_matches_bp(c, bp.v->btree_id, bp.v->level, k, bp))
+               return k;
 
-               bch2_trans_iter_exit(trans, iter);
+       bch2_trans_iter_exit(trans, iter);
+
+       if (!bp.v->level) {
                int ret = backpointer_target_not_found(trans, bp, k, last_flushed);
                return ret ? bkey_s_c_err(ret) : bkey_s_c_null;
        } else {
                struct btree *b = bch2_backpointer_get_node(trans, bp, iter, last_flushed);
+               if (b == ERR_PTR(-BCH_ERR_backpointer_to_overwritten_btree_node))
+                       return bkey_s_c_null;
                if (IS_ERR_OR_NULL(b))
                        return ((struct bkey_s_c) { .k = ERR_CAST(b) });