mm: memcontrol: remove hierarchy restrictions for swappiness and oom_control

Per-memcg swappiness and oom killing can currently not be tweaked on a
memcg that is part of a hierarchy, but not the root of that hierarchy.
Users have complained that they can't configure this when they turned on
hierarchy mode.  In fact, with hierarchy mode becoming the default, this
restriction disables the tunables entirely.

But there is no good reason for this restriction.  The settings for
swappiness and OOM killing are taken from whatever memcg whose limit
triggered reclaim and OOM invocation, regardless of its position in the
hierarchy tree.

Allow setting swappiness on any group.  The knob on the root memcg
already reads the global VM swappiness, make it writable as well.

Allow disabling the OOM killer on any non-root memcg.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Tejun Heo <tj@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/cgroups/memory.txt

@@ -535,17 +535,15 @@ Note:
 
 5.3 swappiness
 
-Similar to /proc/sys/vm/swappiness, but affecting a hierarchy of groups only.
+Similar to /proc/sys/vm/swappiness, but only affecting reclaim that is
+triggered by this cgroup's hard limit.  The tunable in the root cgroup
+corresponds to the global swappiness setting.
+
 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).
-- a cgroup which uses hierarchy and it has other cgroup(s) below it.
-- a cgroup which uses hierarchy and not the root of hierarchy.
-
 5.4 failcnt
 
 A memory cgroup provides memory.failcnt and memory.memsw.failcnt files.
@@ -754,7 +752,6 @@ You can disable the OOM-killer by writing "1" to memory.oom_control file, as:
 
 	#echo 1 > memory.oom_control
 
-This operation is only allowed to the top cgroup of a sub-hierarchy.
 If OOM-killer is disabled, tasks under cgroup will hang/sleep
 in memory cgroup's OOM-waitqueue when they request accountable memory.
 

mm/memcontrol.c

@@ -5444,22 +5444,14 @@ static int mem_cgroup_swappiness_write(struct cgroup_subsys_state *css,
 				       struct cftype *cft, u64 val)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
-	struct mem_cgroup *parent = mem_cgroup_from_css(css_parent(&memcg->css));
 
-	if (val > 100 || !parent)
+	if (val > 100)
 		return -EINVAL;
 
-	mutex_lock(&memcg_create_mutex);
-
-	/* If under hierarchy, only empty-root can set this value */
-	if ((parent->use_hierarchy) || memcg_has_children(memcg)) {
-		mutex_unlock(&memcg_create_mutex);
-		return -EINVAL;
-	}
-
-	memcg->swappiness = val;
-
-	mutex_unlock(&memcg_create_mutex);
+	if (css_parent(css))
+		memcg->swappiness = val;
+	else
+		vm_swappiness = val;
 
 	return 0;
 }
@@ -5791,22 +5783,15 @@ static int mem_cgroup_oom_control_write(struct cgroup_subsys_state *css,
 	struct cftype *cft, u64 val)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
-	struct mem_cgroup *parent = mem_cgroup_from_css(css_parent(&memcg->css));
 
 	/* cannot set to root cgroup and only 0 and 1 are allowed */
-	if (!parent || !((val == 0) || (val == 1)))
+	if (!css_parent(css) || !((val == 0) || (val == 1)))
 		return -EINVAL;
 
-	mutex_lock(&memcg_create_mutex);
-	/* oom-kill-disable is a flag for subhierarchy. */
-	if ((parent->use_hierarchy) || memcg_has_children(memcg)) {
-		mutex_unlock(&memcg_create_mutex);
-		return -EINVAL;
-	}
 	memcg->oom_kill_disable = val;
 	if (!val)
 		memcg_oom_recover(memcg);
-	mutex_unlock(&memcg_create_mutex);
+
 	return 0;
 }