Re: [PATCH v2 1/4] mm: memcontrol: drop kmemcg_id and use the memcg ID for list_lru indexing

[Qinyun Tan <qinyuntan@linux.alibaba.com>]

Hi, Gregory

On 9/23/26 11:45 AM, Gregory Price wrote:
> On Mon, Sep 07, 2026 at 07:01:08PM +0800, Qinyun Tan wrote:
>> kmemcg_id is a copy of the memcg ID assigned in memcg_online_kmem(),
>> and is only used as the list_lru xarray index.  With
>> cgroup.memory=nokmem the assignment never happens, so every memcg
>> resolves to the per-node lists.  The next patch needs the index to
>> work under nokmem as well, so drop the copy and use the memcg ID.
>>
>> The ID works just as well as the copy did: root and NULL still
>> return -1 and use the per-node lists, and the ID is only released
>> after the list_lru reparenting, so a stale or recycled ID can never
>> reach a live list_lru entry.
>>
>> The early return of memcg_offline_kmem() under nokmem is dropped as
>> well, so the reparenting also covers lrus that stay memcg aware
>> without kmem accounting.
>>
>> Signed-off-by: Qinyun Tan <qinyuntan@linux.alibaba.com>
> 
> This breaks an assert in memcg_struct_check - i think you want to drop
> this line as well
> 
> ---
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5d7a26c91610..fcba9eb55659 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5886,8 +5886,6 @@ static void __init memcg_struct_check(void)
>  				      private_id_objcg);
>  	CACHELINE_ASSERT_GROUP_MEMBER(struct mem_cgroup, memcg_read_mostly,
>  				      private_id);
> -	CACHELINE_ASSERT_GROUP_MEMBER(struct mem_cgroup, memcg_read_mostly,
> -				      kmemcg_id);
>  	CACHELINE_ASSERT_GROUP_MEMBER(struct mem_cgroup, memcg_read_mostly,
>  				      oom_group);
>  

Good catch, thanks!  I missed the layout assertion when removing the
field.

Andrew spotted the same issue in v3 as well.  I will fold the fix in
when I send out the next revision.

Thanks for taking a look at the series!
 
Qinyun Tan