memcg: oom: fix totalpages calculation for memory.swappiness==0

memcg: oom: fix totalpages calculation for memory.swappiness==0

oom_badness() takes a totalpages argument which says how many pages are
available and it uses it as a base for the score calculation.  The value
is calculated by mem_cgroup_get_limit which considers both limit and
total_swap_pages (resp.  memsw portion of it).

This is usually correct but since fe35004f ("mm: avoid swapping out with
swappiness==0") we do not swap when swappiness is 0 which means that we
cannot really use up all the totalpages pages.  This in turn confuses oom
score calculation if the memcg limit is much smaller than the available
swap because the used memory (capped by the limit) is negligible comparing
to totalpages so the resulting score is too small if adj!=0 (typically
task with CAP_SYS_ADMIN or non zero oom_score_adj).  A wrong process might
be selected as result.

The problem can be worked around by checking mem_cgroup_swappiness==0 and
not considering swap at all in such a case.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>