linux-kernel - [QUESTION] What memcg lifetime is required by list_lru

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Message-ID: <CAH5fLghFWi=xbTgaG7oFNJo_7B7zoMRLCzeJLXd_U5ODVGaAUA@mail.gmail.com>
Date: Wed, 27 Nov 2024 22:04:51 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Dave Chinner <david@...morbit.com>, Johannes Weiner <hannes@...xchg.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Nhat Pham <nphamcs@...il.com>
Cc: Qi Zheng <zhengqi.arch@...edance.com>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Muchun Song <muchun.song@...ux.dev>, Linux Memory Management List <linux-mm@...ck.org>, 
	Michal Hocko <mhocko@...nel.org>, Shakeel Butt <shakeel.butt@...ux.dev>, cgroups@...r.kernel.org, 
	open list <linux-kernel@...r.kernel.org>
Subject: [QUESTION] What memcg lifetime is required by list_lru_add?

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