linux-kernel - Re: how to make memory.memsw.failcnt is nonzero

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Message-ID: <20120106101219.GB10292@tiehlicka.suse.cz>
Date:	Fri, 6 Jan 2012 11:12:19 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Peng Haitao <penght@...fujitsu.com>
Cc:	cgroups@...r.kernel.org, kamezawa.hiroyu@...fujitsu.com,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: 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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