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    SubjectRe: how to make memory.memsw.failcnt is nonzero
    On Fri 06-01-12 17:47:10, Peng Haitao wrote:
    >
    > Michal Hocko said the following on 2012-1-4 0:04:
    > >> # echo 15M > memory.memsw.limit_in_bytes
    > >> # dd if=/dev/zero of=/tmp/temp_file count=20 bs=1M
    > >> Killed
    > >> # grep "failcnt" /var/log/messages | tail -2
    > >> Dec 28 17:08:45 K-test kernel: memory: usage 10240kB, limit 10240kB, failcnt 86
    > >> Dec 28 17:08:45 K-test kernel: memory+swap: usage 10240kB, limit 15360kB, failcnt 0
    > >> # cat memory.memsw.failcnt
    > >> 0
    > >>
    > >> The limit is 15M, but memory+swap usage also is 10M.
    > >> I think memory+swap usage should be 15M and memsw.failcnt should be nonzero.
    > >>
    > > So there is almost 10M of page cache that we can simply reclaim. If we
    > > use 40M limit then we are OK. So this looks like the small limit somehow
    > > tricks our math in the reclaim path and we think there is nothing to
    > > reclaim.
    > > I will look into this.
    >
    > Thanks for you reply.
    > If there is something wrong, I think the bug will be in mem_cgroup_do_charge()
    > of mm/memcontrol.c
    >
    > 2210 ret = res_counter_charge(&memcg->res, csize, &fail_res);
    > 2211
    > 2212 if (likely(!ret)) {
    > 2213 if (!do_swap_account)
    > 2214 return CHARGE_OK;
    > 2215 ret = res_counter_charge(&memcg->memsw, csize, &fail_res);
    > 2216 if (likely(!ret))
    > 2217 return CHARGE_OK;
    > 2218
    > 2219 res_counter_uncharge(&memcg->res, csize);
    > 2220 mem_over_limit = mem_cgroup_from_res_counter(fail_res, memsw);
    > 2221 flags |= MEM_CGROUP_RECLAIM_NOSWAP;
    > 2222 } else
    > 2223 mem_over_limit = mem_cgroup_from_res_counter(fail_res, res);
    >
    > When hit memory.limit_in_bytes, res_counter_charge() will return -ENOMEM,
    > this will execute line 2222: } else.
    > But I think when hit memory.limit_in_bytes, the function should determine further
    > to memory.memsw.limit_in_bytes.
    > This think is OK?

    I don't think so. We have an invariant (hard limit is "stronger" than
    memsw limit) memory.limit_in_bytes <= memory.memsw.limit_in_bytes so
    when we hit the hard limit we do not have to consider memsw because
    resource counter:
    a) we already have to do reclaim for hard limit
    b) we check whether we might swap out later on in
    mem_cgroup_hierarchical_reclaim (root_memcg->memsw_is_minimum) so we
    will not end up swapping just to make hard limit ok and go over memsw
    limit.

    Please also note that we will retry charging after reclaim if there is a
    chance to meet the limit.
    Makes sense?
    --
    Michal Hocko
    SUSE Labs
    SUSE LINUX s.r.o.
    Lihovarska 1060/12
    190 00 Praha 9
    Czech Republic


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