Re: [PATCH v5 13/16] mm: memcontrol: reuse memory cgroup ID for kmem ID

[Muchun Song]

On Thu, Jan 6, 2022 at 1:03 AM Michal Koutný <mkoutny@xxxxxxxx> wrote:
>
> On Mon, Dec 20, 2021 at 04:56:46PM +0800, Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> > There are two idrs being used by memory cgroup, one is for kmem ID,
> > another is for memory cgroup ID. The maximum ID of both is 64Ki.
> > Both of them can limit the total number of memory cgroups.
> > Actually, we can reuse memory cgroup ID for kmem ID to simplify the
> > code.
>
> An interesting improvement.
>
> I'm a bit dense -- what's the purpose the MEM_CGROUP_ID_DIFF offset?

Hi Michal and Mika,

MEM_CGROUP_ID_DIFF is introduced to be consistent with before
that kmem ID starts with -1 and has no holes. Actually, it can be dropped
and make memcg->kmemcg_id equal to memcg->id.id directly.

> Couldn't this deduplication be extended to only use mem_cgroup.id.id
> instead of mem_cgroup.kmemcg_id? (With a boolean telling whether kmem
> accounting is active.)
>

Not easy to completely remove memcg->kmemcg_id since this filed
will be used to sync list_lru reparenting which will change
memcg->kmemcg_id to its parent's kmem ID (more details refers to
memcg_drain_all_list_lrus() in patch 10 of this series).

Thanks.