linux-kernel - Re: [PATCH -next v2 2/2] memcg: remove mem_cgroup

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Message-ID: <48f734fa-531a-4b3f-9c96-02dd342d41d2@huaweicloud.com>
Date: Wed, 10 Dec 2025 16:42:09 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Michal Hocko <mhocko@...e.com>
Cc: hannes@...xchg.org, roman.gushchin@...ux.dev, shakeel.butt@...ux.dev,
 muchun.song@...ux.dev, akpm@...ux-foundation.org, axelrasmussen@...gle.com,
 yuanchu@...gle.com, weixugc@...gle.com, david@...nel.org,
 zhengqi.arch@...edance.com, lorenzo.stoakes@...cle.com,
 cgroups@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 lujialin4@...wei.com
Subject: Re: [PATCH -next v2 2/2] memcg: remove mem_cgroup_size()



On 2025/12/10 16:05, Michal Hocko wrote:
> On Wed 10-12-25 07:11:42, Chen Ridong wrote:
>> From: Chen Ridong <chenridong@...wei.com>
>>
>> The mem_cgroup_size helper is used only in apply_proportional_protection
>> to read the current memory usage. Its semantics are unclear and
>> inconsistent with other sites, which directly call page_counter_read for
>> the same purpose.
>>
>> Remove this helper and replace its usage with page_counter_read for
>> clarity. Additionally, rename the local variable 'cgroup_size' to 'usage'
>> to better reflect its meaning.
>>
>> This change is safe because page_counter_read() is only called when memcg
>> is enabled in the apply_proportional_protection.
>>
>> No functional changes intended.
> 
> I would prefer to keep the code as is. 
> 

I find the mem_cgroup_size() function name misleading—it suggests counting the number of memory
cgroups, but it actually returns the current memory usage.

When looking for a clearer alternative, I found mem_cgroup_usage(), which is only called by v1. This
raised the question of whether mem_cgroup_size() is truly necessary. Moreover, I noticed other code
locations simply call page_counter_read() directly to obtain current usage.

> Btw.
> [...]
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 670fe9fae5ba..fe48d0376e7c 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -2451,6 +2451,7 @@ static inline void calculate_pressure_balance(struct scan_control *sc,
>>  static unsigned long apply_proportional_protection(struct mem_cgroup *memcg,
>>  		struct scan_control *sc, unsigned long scan)
>>  {
>> +#ifdef CONFIG_MEMCG
>>  	unsigned long min, low;
>>  
>>  	mem_cgroup_protection(sc->target_mem_cgroup, memcg, &min, &low);
> [...]
>> @@ -2508,6 +2509,7 @@ static unsigned long apply_proportional_protection(struct mem_cgroup *memcg,
>>  		 */
>>  		scan = max(scan, SWAP_CLUSTER_MAX);
>>  	}
>> +#endif
>>  	return scan;
>>  }
> 
> This returns a random garbage for !CONFIG_MEMCG, doesn't it?
> 

-- 
Best regards,
Ridong