Re: [PATCH v4 2/2] vmscan,cgroup: apply mems_effective to reclaim

[Gregory Price <gourry@gourry.net>]

On Mon, Apr 21, 2025 at 10:02:22PM -0400, Waiman Long wrote:
> > +bool cpuset_node_allowed(struct cgroup *cgroup, int nid)
> > +{
> > +	struct cgroup_subsys_state *css;
> > +	struct cpuset *cs;
> > +	bool allowed;
> > +
> > +	/*
> > +	 * In v1, mem_cgroup and cpuset are unlikely in the same hierarchy
> > +	 * and mems_allowed is likely to be empty even if we could get to it,
> > +	 * so return true to avoid taking a global lock on the empty check.
> > +	 */
> > +	if (!cpuset_v2())
> > +		return true;
> > +
> > +	css = cgroup_get_e_css(cgroup, &cpuset_cgrp_subsys);
> > +	if (!css)
> > +		return true;
> > +
> > +	cs = container_of(css, struct cpuset, css);
> > +	rcu_read_lock();
> 
> Sorry, I missed the fact that cgroup_get_e_css() will take a reference to
> the css and so it won't go away. In that case, rcu_read_lock() isn't really
> needed. However, I do want a comment to say that accessing effective_mems
> should normally requrie taking either a cpuset_mutex or callback_lock, but
> is skipped in this case to avoid taking a global lock in the reclaim path at
> the expense that the result may be inaccurate in some rare cases.
> 

I'll add a differential patch here.

~Gregory