[RFC][PATCH] memcg: fix a race when setting memcg.swappiness

Li Zefan
2009-01-14T03:25:12Z

(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 <lizf@cn.fujitsu.com>
---
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