Re: [PATCH v5 10/16] mm: list_lru: allocate list_lru_one only when needed

[Michal Koutný]

On Tue, Jan 18, 2022 at 08:05:44PM +0800, Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> I have thought about this. It's a little different to rely on objcg
> reparenting since the user can get memcg from objcg and
> then does not realize the memcg has reparented.

When you pointed that out, I'm now also wondering how
memcg_list_lru_alloc() would be synchronized against
reparenting/renumbering of kmemcg_ids. What I suspect is that newly
allocated mlru may be stored into the xarray with a stale kmemcg_id.

> Maybe holding css_set_lock can do that. I do not think this
> is a good choice.

I agree, it doesn't sound well.

> Do you have any thoughts about this?

Thoughts / questions of what I don't undestand well:
- Why do you allocate mlrus for all ancestors in memcg_list_lru_alloc()?
  - It'd be sufficient to allocate just for the current memcg.
  - Possibly allocate ancestors upon reparenting (to simplify the
    allocation from slab_pre_alloc_hook itself).
    
- What is the per-kmemcg_id lookup good for?
  - I observe most calls of list_lru_from_memcg_idx() come from callers
    that know memcg (or even objcg).
  - The non-specific use case seems list_lru_walk_node() working with
    per-node and not per-memcg projection.
    - Consequently that is only used over all nodes anyway
      (list_lru_walk().
  - The idea behind this question is -- attach the list_lrus to
    obj_cgroup (and decomission the kmemcg_id completely).
    (Not necessarily part of this series but independent approach.)

Thanks,
Michal