On Mon 13-01-14 17:54:04, Hugh Dickins wrote:
> It is surprising that the mem_cgroup iterator can return memcgs which
> have not yet been fully initialized. By accident (or trial and error?)
> this appears not to present an actual problem; but it may be better to
> prevent such surprises, by skipping memcgs not yet online.
My understanding was that !online cgroups are not visible for the
iterator. it is css_online that has to be called before they are made
visible.
Tejun?
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> ---
> Decide for yourself whether to take this or not. I spent quite a while
> digging into a mysterious "trying to register non-static key" issue from
> lockdep, which originated from the iterator returning a vmalloc'ed memcg
> a moment before the res_counter_init()s had done their spin_lock_init()s.
> But the backtrace was an odd one of our own mis-devising, not a charge or
> reclaim or stats trace, so probably it's never been a problem for vanilla.
>
> mm/memcontrol.c | 6 ++----
> 1 file changed, 2 insertions(+), 4 deletions(-)
>
> --- mmotm/mm/memcontrol.c 2014-01-10 18:25:02.236448954 -0800
> +++ linux/mm/memcontrol.c 2014-01-12 22:21:10.700570471 -0800
> @@ -1119,10 +1119,8 @@ skip_node:
> * protected by css_get and the tree walk is rcu safe.
> */
> if (next_css) {
> - struct mem_cgroup *mem = mem_cgroup_from_css(next_css);
> -
> - if (css_tryget(&mem->css))
> - return mem;
> + if ((next_css->flags & CSS_ONLINE) && css_tryget(next_css))
> + return mem_cgroup_from_css(next_css);
> else {
> prev_css = next_css;
> goto skip_node;
--
Michal Hocko
SUSE Labs
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