From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail203.messagelabs.com (mail203.messagelabs.com [216.82.254.243]) by kanga.kvack.org (Postfix) with SMTP id E97C26B004F for ; Tue, 13 Jan 2009 22:25:12 -0500 (EST) Message-ID: <496D5AE2.2020403@cn.fujitsu.com> Date: Wed, 14 Jan 2009 11:24:18 +0800 From: Li Zefan MIME-Version: 1.0 Subject: [RFC][PATCH] memcg: fix a race when setting memcg.swappiness Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: KAMEZAWA Hiroyuki , Balbir Singh , Paul Menage Cc: Daisuke Nishimura , Linux Containers , "linux-mm@kvack.org" List-ID: (suppose: memcg->use_hierarchy == 0 and memcg->swappiness == 60) echo 10 > /memcg/0/swappiness | mem_cgroup_swappiness_write() | ... | echo 1 > /memcg/0/use_hierarchy | mkdir /mnt/0/1 | sub_memcg->swappiness = 60; memcg->swappiness = 10; | In the above scenario, we end up having 2 different swappiness values in a single hierarchy. Note we can't use hierarchy_lock here, because it doesn't protect the create() method. Though IMO use cgroup_lock() in simple write functions is OK, Paul would like to avoid it. And he sugguested use a counter to count the number of children instead of check cgrp->children list: ================= create() does: lock memcg_parent memcg->swappiness = memcg->parent->swappiness; memcg_parent->child_count++; unlock memcg_parent and write() does: lock memcg if (!memcg->child_count) { memcg->swappiness = swappiness; } else { report error; } unlock memcg destroy() does: lock memcg_parent memcg_parent->child_count--; unlock memcg_parent ================= And there is a suble differnce with checking cgrp->children, that a cgroup is removed from parent's list in cgroup_rmdir(), while memcg->child_count is decremented in cgroup_diput(). Signed-off-by: Li Zefan --- mm/memcontrol.c | 10 +++++++++- 1 files changed, 9 insertions(+), 1 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index e2996b8..0274223 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -1971,6 +1971,7 @@ static int mem_cgroup_swappiness_write(struct cgroup *cgrp, struct cftype *cft, { struct mem_cgroup *memcg = mem_cgroup_from_cont(cgrp); struct mem_cgroup *parent; + if (val > 100) return -EINVAL; @@ -1978,15 +1979,22 @@ static int mem_cgroup_swappiness_write(struct cgroup *cgrp, struct cftype *cft, return -EINVAL; parent = mem_cgroup_from_cont(cgrp->parent); + + cgroup_lock(); + /* If under hierarchy, only empty-root can set this value */ if ((parent->use_hierarchy) || - (memcg->use_hierarchy && !list_empty(&cgrp->children))) + (memcg->use_hierarchy && !list_empty(&cgrp->children))) { + cgroup_unlock(); return -EINVAL; + } spin_lock(&memcg->reclaim_param_lock); memcg->swappiness = val; spin_unlock(&memcg->reclaim_param_lock); + cgroup_unlock(); + return 0; } -- 1.5.4.rc3 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org