[QUESTION] What memcg lifetime is required by list_lru_add?

[Alice Ryhl <aliceryhl@google.com>]

Dear SHRINKER and MEMCG experts,

When using list_lru_add() and list_lru_del(), it seems to be required
that you pass the same value of nid and memcg to both calls, since
list_lru_del() might otherwise try to delete it from the wrong list /
delete it while holding the wrong spinlock. I'm trying to understand
the implications of this requirement on the lifetime of the memcg.

Now, looking at list_lru_add_obj() I noticed that it uses rcu locking
to keep the memcg object alive for the duration of list_lru_add().
That rcu locking is used here seems to imply that without it, the
memcg could be deallocated during the list_lru_add() call, which is of
course bad. But rcu is not enough on its own to keep the memcg alive
all the way until the list_lru_del_obj() call, so how does it ensure
that the memcg stays valid for that long? And if there is a mechanism
to keep the memcg alive for the entire duration between add and del,
why is rcu locking needed? I don't see any refcounts being taken on
the memcg.

Is it because the memcg could be replaced by another memcg that has
the same value of memcg_kmem_id(memcg)?

tl;dr: what does list_lru_add actually require from the memcg
pointer's lifetime?

Alice