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

[Muchun Song]

On Wed, Jan 19, 2022 at 5:33 PM Michal Koutný <mkoutny@xxxxxxxx> wrote:
>
> 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.

The synchronization is based on the lock of list_lru->lock.
memcg_list_lru_free() will help us do housekeeping.

>
> > 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's because we need to be reparenting.

>   - 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).

I agree it is nice to only allocate for current memcg, but
reparenting cannot handle failure of memory allocation.

>
> - 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 decommission the kmemcg_id completely).
>     (Not necessarily part of this series but independent approach.)
>

I have some questions about this thought.
We would attach more than one list_lrus to obj_cgroup,
right? How to arrange those list_lrus, array or linked-list?

Thanks.