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    SubjectRe: [PATCH 3/4] memcg: hierarchical reclaim by CSS ID
    >>>>> +	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) ?
    >>>>
    >>> Main reason is for sanity. I don't like to hold css->refcnt of not populated css.
    >> I think this is a rare case. It's just a very short period when a cgroup is
    >> being created but not yet fully created.
    >>
    >>> Second reason is for avoinding unnecessary calls to try_to_free_pages(),
    >>> it's heavy. I should also add mem->res.usage == 0 case for skipping but not yet.
    >>>
    >> And if mem->res.usage == 0 is checked, css_is_popuated() is just redundant.
    >>
    > Hmm ? Can I check mem->res.usage before css_tryget() ?
    >

    I think you can. If css != NULL, css is valid (otherwise how can we access css->flags
    in css_tryget), so mem is valid. Correct me if I'm wrong. :)




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