(2012/11/14 0:30), Michal Hocko wrote:
> mem_cgroup_iter curently relies on css->id when walking down a group
> hierarchy tree. This is really awkward because the tree walk depends on
> the groups creation ordering. The only guarantee is that a parent node
> is visited before its children.
> Example
> 1) mkdir -p a a/d a/b/c
> 2) mkdir -a a/b/c a/d
> Will create the same trees but the tree walks will be different:
> 1) a, d, b, c
> 2) a, b, c, d
>
> 574bd9f7 (cgroup: implement generic child / descendant walk macros) has
> introduced generic cgroup tree walkers which provide either pre-order
> or post-order tree walk. This patch converts css->id based iteration
> to pre-order tree walk to keep the semantic with the original iterator
> where parent is always visited before its subtree.
>
> cgroup_for_each_descendant_pre suggests using post_create and
> pre_destroy for proper synchronization with groups addidition resp.
> removal. This implementation doesn't use those because a new memory
> cgroup is fully initialized in mem_cgroup_create and css_tryget makes
> sure that the group is alive when we encounter it by iterator.
>
> If the reclaim cookie is used we need to store the last visited group
> into the iterator so we have to be careful that it doesn't disappear in
> the mean time. Elevated reference count on the memcg guarantees that
> the group will not vanish even though it has been already removed from
> the tree. In such a case css_tryget will fail and the iteration is
> retried (groups are linked with RCU safe lists so the forward progress
> is still possible). iter_lock will make sure that only one reclaimer
> will see the last_visited group and the reference count game around it.
>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
> ---
> mm/memcontrol.c | 64 ++++++++++++++++++++++++++++++++++++++++++-------------
> 1 file changed, 49 insertions(+), 15 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 0fe5177..5da1e58 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -142,8 +142,8 @@ struct mem_cgroup_stat_cpu {
> };
>
> struct mem_cgroup_reclaim_iter {
> - /* css_id of the last scanned hierarchy member */
> - int position;
> + /* last scanned hierarchy member with elevated ref count */
> + struct mem_cgroup *last_visited;
> /* scan generation, increased every round-trip */
> unsigned int generation;
> /* lock to protect the position and generation */
> @@ -1063,8 +1063,8 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> struct mem_cgroup *prev,
> struct mem_cgroup_reclaim_cookie *reclaim)
> {
> - struct mem_cgroup *memcg = NULL;
> - int id = 0;
> + struct mem_cgroup *memcg = NULL,
> + *last_visited = NULL;
>
> if (mem_cgroup_disabled())
> return NULL;
> @@ -1073,7 +1073,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> root = root_mem_cgroup;
>
> if (prev && !reclaim)
> - id = css_id(&prev->css);
> + last_visited = prev;
>
> if (prev && prev != root)
> css_put(&prev->css);
> @@ -1086,7 +1086,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>
> while (!memcg) {
> struct mem_cgroup_reclaim_iter *uninitialized_var(iter);
> - struct cgroup_subsys_state *css;
> + struct cgroup_subsys_state *css = NULL;
>
> if (reclaim) {
> int nid = zone_to_nid(reclaim->zone);
> @@ -1096,30 +1096,64 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> mz = mem_cgroup_zoneinfo(root, nid, zid);
> iter = &mz->reclaim_iter[reclaim->priority];
> spin_lock(&iter->iter_lock);
> + last_visited = iter->last_visited;
> if (prev && reclaim->generation != iter->generation) {
> + if (last_visited) {
> + mem_cgroup_put(last_visited);
> + iter->last_visited = NULL;
> + }
> spin_unlock(&iter->iter_lock);
> return NULL;
> }
> - id = iter->position;
> }
>
> rcu_read_lock();
> - css = css_get_next(&mem_cgroup_subsys, id + 1, &root->css, &id);
> - if (css) {
> - if (css == &root->css || css_tryget(css))
> - memcg = mem_cgroup_from_css(css);
> - } else
> - id = 0;
> - rcu_read_unlock();
> + /*
> + * Root is not visited by cgroup iterators so it needs a special
> + * treatment.
> + */
> + if (!last_visited) {
> + css = &root->css;
> + } else {
> + struct cgroup *next_cgroup;
> +
> + next_cgroup = cgroup_next_descendant_pre(
> + last_visited->css.cgroup,
> + root->css.cgroup);
Maybe I miss something but.... last_visited is holded by memcg's refcnt.
The cgroup pointed by css.cgroup is by cgroup's refcnt which can be freed
before memcg is freed and last_visited->css.cgroup is out of RCU cycle.
Is this safe ?
Thanks,
-Kame
> + if (next_cgroup)
> + css = cgroup_subsys_state(next_cgroup,
> + mem_cgroup_subsys_id);
> + }
> +
> + /*
> + * Even if we find a group we have to make sure it is alive.
> + * css && !memcg means that the groups should be skipped and
> + * we should continue the tree walk.
> + */
> + if (css == &root->css || (css && css_tryget(css)))
> + memcg = mem_cgroup_from_css(css);
>
> if (reclaim) {
> - iter->position = id;
> + struct mem_cgroup *curr = memcg;
> +
> + if (last_visited)
> + mem_cgroup_put(last_visited);
> +
> + if (css && !memcg)
> + curr = mem_cgroup_from_css(css);
> + if (curr)
> + mem_cgroup_get(curr);
> + iter->last_visited = curr;
> +
> if (!css)
> iter->generation++;
> else if (!prev && memcg)
> reclaim->generation = iter->generation;
> spin_unlock(&iter->iter_lock);
> + } else if (css && !memcg) {
> + last_visited = mem_cgroup_from_css(css);
> }
> + rcu_read_unlock();
>
> if (prev && !css)
> return NULL;
>
--
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