From: Chen Ridong <chenridong@huawei.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.

Signed-off-by: Chen Ridong <chenridong@huawei.com>
---
 include/linux/memcontrol.h | 7 -------
 mm/memcontrol.c            | 5 -----
 mm/vmscan.c                | 8 +++++---
 3 files changed, 5 insertions(+), 15 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 6a48398a1f4e..bedeb606c691 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -919,8 +919,6 @@ static inline void mem_cgroup_handle_over_high(gfp_t gfp_mask)
 
 unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg);
 
-unsigned long mem_cgroup_size(struct mem_cgroup *memcg);
-
 void mem_cgroup_print_oom_context(struct mem_cgroup *memcg,
 				struct task_struct *p);
 
@@ -1328,11 +1326,6 @@ static inline unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg)
 	return 0;
 }
 
-static inline unsigned long mem_cgroup_size(struct mem_cgroup *memcg)
-{
-	return 0;
-}
-
 static inline void
 mem_cgroup_print_oom_context(struct mem_cgroup *memcg, struct task_struct *p)
 {
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index dbe7d8f93072..659ce171b1b3 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1621,11 +1621,6 @@ unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg)
 	return max;
 }
 
-unsigned long mem_cgroup_size(struct mem_cgroup *memcg)
-{
-	return page_counter_read(&memcg->memory);
-}
-
 void __memcg_memory_event(struct mem_cgroup *memcg,
 			  enum memcg_memory_event event, bool allow_spinning)
 {
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);
@@ -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
 	return scan;
 }
 
-- 
2.34.1

On Wed, Dec 10, 2025 at 07:11:42AM +0000, Chen Ridong wrote:
> From: Chen Ridong <chenridong@huawei.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.

On 2025/12/11 0:36, Johannes Weiner wrote:
> On Wed, Dec 10, 2025 at 07:11:42AM +0000, Chen Ridong wrote:
>> From: Chen Ridong <chenridong@huawei.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.

Much better, will update.

-- 
Best regards,
Ridong

On Wed 10-12-25 07:11:42, Chen Ridong wrote:
> From: Chen Ridong <chenridong@huawei.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. 

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?

-- 
Michal Hocko
SUSE Labs

On 2025/12/10 16:05, Michal Hocko wrote:
> On Wed 10-12-25 07:11:42, Chen Ridong wrote:
>> From: Chen Ridong <chenridong@huawei.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

On 2025/12/10 16:05, Michal Hocko wrote:
> On Wed 10-12-25 07:11:42, Chen Ridong wrote:
>> From: Chen Ridong <chenridong@huawei.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. 
> 
> 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?
> 

This returns what was passed as input. This means the scan behavior remains unchanged when memcg is
disabled. When memcg is enabled, the scan amount may be proportionally scaled.

-- 
Best regards,
Ridong

On Wed 10-12-25 16:31:37, Chen Ridong wrote:
> 
> 
> On 2025/12/10 16:05, Michal Hocko wrote:
[...]
> >> 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?
> > 
> 
> This returns what was passed as input. This means the scan behavior remains unchanged when memcg is
> disabled. When memcg is enabled, the scan amount may be proportionally scaled.

Right you are. My bad. Sorry for the confusion.
-- 
Michal Hocko
SUSE Labs