linux-kernel - Re: [PATCH v3 2/2] vmscan,cgroup: apply mems

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Message-ID: <ro3uqeyri65voutamqttzipfk7yiya4zv5kdiudcmhacrm6tej@br7ebk2kanf4>
Date: Sat, 19 Apr 2025 11:48:45 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Gregory Price <gourry@...rry.net>
Cc: linux-mm@...ck.org, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org, kernel-team@...a.com, longman@...hat.com, hannes@...xchg.org, 
	mhocko@...nel.org, roman.gushchin@...ux.dev, muchun.song@...ux.dev, tj@...nel.org, 
	mkoutny@...e.com, akpm@...ux-foundation.org
Subject: Re: [PATCH v3 2/2] vmscan,cgroup: apply mems_effective to reclaim

On Sat, Apr 19, 2025 at 01:38:24AM -0400, Gregory Price wrote:
>  
> +bool cpuset_node_allowed(struct cgroup *cgroup, int nid)
> +{
> +	struct cgroup_subsys_state *css;
> +	unsigned long flags;
> +	struct cpuset *cs;
> +	bool allowed;
> +
> +	css = cgroup_get_e_css(cgroup, &cpuset_cgrp_subsys);
> +	if (!css)
> +		return true;
> +
> +	cs = container_of(css, struct cpuset, css);
> +	spin_lock_irqsave(&callback_lock, flags);

Do we really need callback_lock here? We are not modifying and I am
wondering if simple rcu read lock is enough here (similar to
update_nodemasks_hier() where parent's effective_mems is accessed within
rcu read lock).

> +	/* On v1 effective_mems may be empty, simply allow */
> +	allowed = node_isset(nid, cs->effective_mems) ||
> +		  nodes_empty(cs->effective_mems);
> +	spin_unlock_irqrestore(&callback_lock, flags);
> +	css_put(css);
> +	return allowed;
> +}