cgroup: reset css on destruction

An associated css can be around for quite a while after a cgroup
directory has been removed. In general, it makes sense to reset it to
defaults so as not to worry about any remnants. For instance, memory
cgroup needs to reset memory.low, otherwise pages charged to a dead
cgroup might never get reclaimed. There's ->css_reset callback, which
would fit perfectly for the purpose. Currently, it's only called when a
subsystem is disabled in the unified hierarchy and there are other
subsystems dependant on it. Let's call it on css destruction as well.

Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index fcfad82149b17ab800480f3ccfec38024175c01a..46529502e9d5ad427a92f0da24ebf98dd3805852 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4787,6 +4787,9 @@ static void offline_css(struct cgroup_subsys_state *css)
        if (!(css->flags & CSS_ONLINE))
                return;
 
+       if (ss->css_reset)
+               ss->css_reset(css);
+
        if (ss->css_offline)
                ss->css_offline(css);