Michal Hocko said the following on 2012-1-4 0:04:

quoted

# 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?

-- 
Best Regards,
Peng