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

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Message-ID: <babdca88-1461-4d47-989a-c7a011ddc2bd@redhat.com>
Date: Sat, 19 Apr 2025 20:14:29 -0400
From: Waiman Long <llong@...hat.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>, Gregory Price <gourry@...rry.net>
Cc: linux-mm@...ck.org, cgroups@...r.kernel.org,
 linux-kernel@...r.kernel.org, kernel-team@...a.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 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;
>> +}