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

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Message-ID: <20251210163634.GB643576@cmpxchg.org>
Date: Wed, 10 Dec 2025 11:36:34 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: Chen Ridong <chenridong@...weicloud.com>
Cc: mhocko@...nel.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 Wed, Dec 10, 2025 at 07:11:42AM +0000, 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.

+1

I don't think the helper adds much.

> --- 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);
> @@ -2485,7 +2486,7 @@ static unsigned long apply_proportional_protection(struct mem_cgroup *memcg,
>  		 * again by how much of the total memory used is under
>  		 * hard protection.
>  		 */
> -		unsigned long cgroup_size = mem_cgroup_size(memcg);
> +		unsigned long usage = page_counter_read(&memcg->memory);
>  		unsigned long protection;
>  
>  		/* memory.low scaling, make sure we retry before OOM */
> @@ -2497,9 +2498,9 @@ static unsigned long apply_proportional_protection(struct mem_cgroup *memcg,
>  		}
>  
>  		/* Avoid TOCTOU with earlier protection check */
> -		cgroup_size = max(cgroup_size, protection);
> +		usage = max(usage, protection);
>  
> -		scan -= scan * protection / (cgroup_size + 1);
> +		scan -= scan * protection / (usage + 1);
>  
>  		/*
>  		 * Minimally target SWAP_CLUSTER_MAX pages to keep
> @@ -2508,6 +2509,7 @@ static unsigned long apply_proportional_protection(struct mem_cgroup *memcg,
>  		 */
>  		scan = max(scan, SWAP_CLUSTER_MAX);
>  	}
> +#endif

To avoid the ifdef, how about making it

	bool mem_cgroup_protection(root, memcg, &min, &low, &usage)

and branch the scaling on that return value. The compiler should be
able to eliminate the entire branch in the !CONFIG_MEMCG case. And it
keeps a cleaner split between memcg logic and reclaim logic.