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SubjectRe: [PATCH] mm/memcg: bail out early when !memcg in mem_cgroup_lruvec
On Fri, 27 Nov 2020 11:08:35 +0800 Alex Shi <alex.shi@linux.alibaba.com> wrote:

> Sometime, we use NULL memcg in mem_cgroup_lruvec(memcg, pgdat)
> so we could get out early in the situation to avoid useless checking.
>
> Also warning if both parameter are NULL.

Why do you think a warning is needed here?

> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -613,14 +613,13 @@ static inline struct lruvec *mem_cgroup_lruvec(struct mem_cgroup *memcg,
> struct mem_cgroup_per_node *mz;
> struct lruvec *lruvec;
>
> - if (mem_cgroup_disabled()) {
> + VM_WARN_ON_ONCE(!memcg && !pgdat);
> +
> + if (mem_cgroup_disabled() || !memcg) {
> lruvec = &pgdat->__lruvec;
> goto out;
> }
>
> - if (!memcg)
> - memcg = root_mem_cgroup;
> -

This change isn't obviously equivalent, is it?

> mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
> lruvec = &mz->lruvec;
> out:

And the resulting code is awkward:

if (mem_cgroup_disabled() || !memcg) {
lruvec = &pgdat->__lruvec;
goto out;
}

mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
lruvec = &mz->lruvec;
out:


could be

if (mem_cgroup_disabled() || !memcg) {
lruvec = &pgdat->__lruvec;
} else {
mem_cgroup_per_node mz;

mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
lruvec = &mz->lruvec;
}

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