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