Re: [RFC] memcg usage_in_bytes does not account file mapped and slab memory

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Fri, 2 Mar 2012 20:27:53 +0400
Anton Vorontsov <anton.vorontsov@linaro.org> wrote:

> ... and thus is useless for low memory notifications.
> 
> Hi all!
> 
> While working on userspace low memory killer daemon (a supposed
> substitution for the kernel low memory killer, i.e.
> drivers/staging/android/lowmemorykiller.c), I noticed that current
> cgroups memory notifications aren't suitable for such a daemon.
> 
> Suppose we want to install a notification when free memory drops below
> 8 MB. Logically (taking memory hotplug aside), using current usage_in_bytes
> notifications we would install an event on 'total_ram - 8MB' threshold.
> 
> But as usage_in_bytes doesn't account file mapped memory and memory
> used by kernel slab, the formula won't work.
> 
> Currently I use the following patch that makes things going:
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 228d646..c8abdc5 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3812,6 +3812,9 @@ static inline u64 mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
>  
>         val = mem_cgroup_recursive_stat(memcg, MEM_CGROUP_STAT_CACHE);
>         val += mem_cgroup_recursive_stat(memcg, MEM_CGROUP_STAT_RSS);
> +       val += mem_cgroup_recursive_stat(memcg, MEM_CGROUP_STAT_FILE_MAPPED);
> +       val += global_page_state(NR_SLAB_RECLAIMABLE);
> +       val += global_page_state(NR_SLAB_UNRECLAIMABLE);
> 
> 
> But here are some questions:
> 
> 1. Is there any particular reason we don't currently account file mapped
>    memory in usage_in_bytes?
> 

CACHE includes all file caches. Why do you think FILE_MAPPED is not included in CACHE ?


>    To me, MEM_CGROUP_STAT_FILE_MAPPED hunk seems logical even if we
>    don't use it for lowmemory notifications.
> 
>    Plus, it seems that FILE_MAPPED _is_ accounted for the non-root
>    cgroups, so I guess it's clearly a bug for the root memcg?
> 
> 2. As for NR_SLAB_RECLAIMABLE and NR_SLAB_UNRECLAIMABLE, it seems that
>    these numbers are only applicable for the root memcg.
>    I'm not sure that usage_in_bytes semantics should actually account
>    these, but I tend to think that we should.
> 

Now, SLAB is not accounted by memcg at all.
See memifo if necessary.

> All in all, not accounting both 1. and 2. looks like bugs to me.
> 

It's spec. not bug. If you want to see slab status in memcg's file,
Please add kernel memory accounting feature. There has been already 2 proposals.
Check them and comment.

Thanks,
-Kame

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