Re: [PATCH v3 2/2] vmscan,cgroup: apply mems_effective to reclaim

[Waiman Long <llong@redhat.com>]

On 4/19/25 2:48 PM, Shakeel Butt wrote:
> 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).

The callback_lock is required to ensure the stability of the 
effective_mems which may be in the process of being changed if not taken.

Cheers,
Longman

>
>> +	/* 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;
>> +}