linux-kernel - Re: [PATH v2] cgroup: add cgroup_favordynmods= command-line option

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Date:   Wed, 6 Sep 2023 08:59:10 -0400
From:   Waiman Long <longman@...hat.com>
To:     Kamalesh Babulal <kamalesh.babulal@...cle.com>,
        Luiz Capitulino <luizcap@...zon.com>, tj@...nel.org,
        lizefan.x@...edance.com, hannes@...xchg.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     lcapitulino@...il.com
Subject: Re: [PATH v2] cgroup: add cgroup_favordynmods= command-line option

On 9/6/23 02:58, Kamalesh Babulal wrote:
> On 9/6/23 06:27, Luiz Capitulino wrote:
> [...]
>> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
>> index 1fb7f562289d..2b7d74304606 100644
>> --- a/kernel/cgroup/cgroup.c
>> +++ b/kernel/cgroup/cgroup.c
>> @@ -207,6 +207,8 @@ static u16 have_exit_callback __read_mostly;
>>   static u16 have_release_callback __read_mostly;
>>   static u16 have_canfork_callback __read_mostly;
>>   
>> +static bool have_favordynmods __ro_after_init = IS_ENABLED(CONFIG_CGROUP_FAVOR_DYNMODS);
>> +
>>   /* cgroup namespace for init task */
>>   struct cgroup_namespace init_cgroup_ns = {
>>   	.ns.count	= REFCOUNT_INIT(2),
>> @@ -2243,9 +2245,9 @@ static int cgroup_init_fs_context(struct fs_context *fc)
>>   	fc->user_ns = get_user_ns(ctx->ns->user_ns);
>>   	fc->global = true;
>>   
>> -#ifdef CONFIG_CGROUP_FAVOR_DYNMODS
>> -	ctx->flags |= CGRP_ROOT_FAVOR_DYNMODS;
>> -#endif
>> +	if (have_favordynmods)
>> +		ctx->flags |= CGRP_ROOT_FAVOR_DYNMODS;
>> +
>>   	return 0;
>>   }
>>   
>> @@ -6764,6 +6766,12 @@ static int __init enable_cgroup_debug(char *str)
>>   }
>>   __setup("cgroup_debug", enable_cgroup_debug);
>>   
>> +static int __init cgroup_favordynmods_setup(char *str)
>> +{
>> +	return (kstrtobool(str, &have_favordynmods) == 0);
>> +}
>> +__setup("cgroup_favordynmods=", cgroup_favordynmods_setup);
>> +
>>   /**
>>    * css_tryget_online_from_dir - get corresponding css from a cgroup dentry
>>    * @dentry: directory dentry of interest
> Consider a case where the kernel is compiled with
> CONFIG_CGROUP_FAVOR_DYNMODS=n and kernel command line is passed with
> cgroup_favordynmods=true, this would set the have_favordynmods to true.
> In cgroup_favordynmods_setup(), should it return 0 with a pr_warn(),
> when CONFIG_CGROUP_FAVOR_DYNMODS=n in the above case, or is this
> expected behavior?

According to the documentation of __setup:

/*
  * NOTE: __setup functions return values:
  * @fn returns 1 (or non-zero) if the option argument is "handled"
  * and returns 0 if the option argument is "not handled".
  */

So the return value should tell whether the input parameter is a 
recognizable true or false value, not whether it is true or false. 
kstrtobool returns 0 if it is a recognizable T/F value or -EINVAL 
otherwise. So the check is correct. I did double check that before I 
ack'ed the patch.

Cheers,
Longman