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

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Message-ID: <48a24cb3-16dd-4fb9-9e52-ed82a68041e8@redhat.com>
Date: Mon, 21 Apr 2025 20:39:04 -0400
From: Waiman Long <llong@...hat.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>, Gregory Price <gourry@...rry.net>
Cc: Waiman Long <llong@...hat.com>, 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 4/21/25 8:10 PM, Shakeel Butt wrote:
> On Mon, Apr 21, 2025 at 07:58:44PM -0400, Gregory Price wrote:
>> On Mon, Apr 21, 2025 at 04:15:49PM -0700, Shakeel Butt wrote:
>>> On Mon, Apr 21, 2025 at 06:59:20PM -0400, Gregory Price wrote:
>>>> On Mon, Apr 21, 2025 at 10:39:58AM -0700, Shakeel Butt wrote:
>>>>> On Sat, Apr 19, 2025 at 08:14:29PM -0400, Waiman Long wrote:
>>>>>> 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.
>>>>> Stability in what sense? effective_mems will not get freed under us
>>>>> here or is there a chance for corrupted read here? node_isset() and
>>>>> nodes_empty() seems atomic. What's the worst that can happen without
>>>>> callback_lock?
>>>> Fairly sure nodes_empty is not atomic, it's a bitmap search.
>>> For bitmaps smaller than 64 bits, it seems atomic and MAX_NUMNODES seems
>>> smaller than 64 in all the archs.
>> Unfortunately, it's config-defined on (NODES_SHIFT) and the max is 1024.
>>
>> Is there an argument here for ignoring v1 and just doing the bit-check
>> without the lock?  Is there an easy ifdef way for us to just return true
>> if it's v1?
>>
> It is !(cgroup_subsys_on_dfl(cpuset_cgrp_subsys)) and I see cpuset_v2()
> and is_in_v2_mode() in kernel/cgroup/cpuset.c.

The is_in_v2_mode() function covers cpuset2 and cpuset1 with 
cpuset_v2_mode mount option, while the cpuset_v2() will only be true for 
cpuset2 and allowing compiling code out in case CPUSETS_V1 isn't set.

Cheers,
Longman

>