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    SubjectRe: [PATCH 3/4] memcg: hierarchical reclaim by CSS ID
    >  /*
    > - * Dance down the hierarchy if needed to reclaim memory. We remember the
    > - * last child we reclaimed from, so that we don't end up penalizing
    > - * one child extensively based on its position in the children list.
    > + * Visit the first child (need not be the first child as per the ordering
    > + * of the cgroup list, since we track last_scanned_child) of @mem and use
    > + * that to reclaim free pages from.
    > + */
    > +static struct mem_cgroup *
    > +mem_cgroup_select_victim(struct mem_cgroup *root_mem)
    > +{
    > + struct mem_cgroup *ret = NULL;
    > + struct cgroup_subsys_state *css;
    > + int nextid, found;
    > +
    > + if (!root_mem->use_hierarchy) {
    > + spin_lock(&root_mem->reclaim_param_lock);
    > + root_mem->scan_age++;
    > + spin_unlock(&root_mem->reclaim_param_lock);
    > + css_get(&root_mem->css);
    > + ret = root_mem;
    > + }
    > +
    > + while (!ret) {
    > + rcu_read_lock();
    > + nextid = root_mem->last_scanned_child + 1;
    > + css = css_get_next(&mem_cgroup_subsys, nextid, &root_mem->css,
    > + &found);
    > + if (css && css_is_populated(css) && css_tryget(css))

    I don't see why you need to check css_is_populated(css) ?

    > + ret = container_of(css, struct mem_cgroup, css);
    > +
    > + rcu_read_unlock();
    > + /* Updates scanning parameter */
    > + spin_lock(&root_mem->reclaim_param_lock);
    > + if (!css) {
    > + /* this means start scan from ID:1 */
    > + root_mem->last_scanned_child = 0;
    > + root_mem->scan_age++;
    > + } else
    > + root_mem->last_scanned_child = found;
    > + spin_unlock(&root_mem->reclaim_param_lock);
    > + }
    > +
    > + return ret;
    > +}
    > +


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