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

commit 9a5a8f19b43430752067ecaee62fc59e11e88fa6 upstream.

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 fe35004fbf9e ("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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
index cc0ebc5241b39f2e9513d89a39d880771cab9c67..fd129f62aaa4c395ae5e2beda84fac3ce8d05d7a 100644 (file)
--- a/Documentation/cgroups/memory.txt
+++ b/Documentation/cgroups/memory.txt
@@ -440,6 +440,10 @@ Note:
 5.3 swappiness
 
 Similar to /proc/sys/vm/swappiness, but affecting a hierarchy of groups only.
+Please note that unlike the global swappiness, memcg knob set to 0
+really prevents from any swapping even if there is a swap storage
+available. This might lead to memcg OOM killer if there are no file
+pages to reclaim.
 
 Following cgroups' swappiness can't be changed.
 - root cgroup (uses /proc/sys/vm/swappiness).

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c8425b144b85823514207ab8949ed41e6f459685..d027a2489a9f1d01a49482a9d3904f3694a9558f 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1457,17 +1457,26 @@ static int mem_cgroup_count_children(struct mem_cgroup *memcg)
 u64 mem_cgroup_get_limit(struct mem_cgroup *memcg)
 {
        u64 limit;
-       u64 memsw;
 
        limit = res_counter_read_u64(&memcg->res, RES_LIMIT);
-       limit += total_swap_pages << PAGE_SHIFT;
 
-       memsw = res_counter_read_u64(&memcg->memsw, RES_LIMIT);
        /*
-        * If memsw is finite and limits the amount of swap space available
-        * to this memcg, return that limit.
+        * Do not consider swap space if we cannot swap due to swappiness
         */
-       return min(limit, memsw);
+       if (mem_cgroup_swappiness(memcg)) {
+               u64 memsw;
+
+               limit += total_swap_pages << PAGE_SHIFT;
+               memsw = res_counter_read_u64(&memcg->memsw, RES_LIMIT);
+
+               /*
+                * If memsw is finite and limits the amount of swap space
+                * available to this memcg, return that limit.
+                */
+               limit = min(limit, memsw);
+       }
+
+       return limit;
 }
 
 /*