[PATCH] mm: memcg: stop reclaim when a limit update is superseded

[PATCH] mm: memcg: stop reclaim when a limit update is superseded

[Guopeng Zhang]

From: Guopeng Zhang <zhangguopeng@kylinos.cn>

kernfs serializes file operations only per open file, so separate open
files can update the same memory.high or memory.max file concurrently.
Both handlers store the new limit before synchronous reclaim, but
continue to use the writer's local target in the reclaim loop. If another
writer raises or removes the limit, the first writer can continue
reclaiming toward a stale target.

For memory.max, this can leave the writer looping indefinitely once
reclaim retries are exhausted. The OOM path sees sufficient margin under
the current limit and returns true without killing, while the writer
still compares usage against its stale target and records another OOM
event.

Check the current limit at the start of each reclaim iteration and stop
if it no longer matches the writer's target.

Fixes: 8c8c383c04f6 ("mm: memcontrol: try harder to set a new memory.high")
Fixes: b6e6edcfa405 ("mm: memcontrol: reclaim and OOM kill when shrinking memory.max below usage")
Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
---
Reproducer:

Populate a cgroup with anonymous memory and disable swapping. Lower
memory.max from one open file, then restore it to "max" through another
open file after the new limit becomes visible.

Without the patch, the first writer remains blocked and repeatedly
increments the OOM event counter. With the patch, it returns normally.

 mm/memcontrol.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8319ad8c5c23..638bdc766616 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4798,6 +4798,9 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,
 		unsigned long nr_pages = page_counter_read(&memcg->memory);
 		unsigned long reclaimed;
 
+		if (high != READ_ONCE(memcg->memory.high))
+			break;
+
 		if (nr_pages <= high)
 			break;
 
@@ -4853,6 +4856,9 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
 	for (;;) {
 		unsigned long nr_pages = page_counter_read(&memcg->memory);
 
+		if (max != READ_ONCE(memcg->memory.max))
+			break;
+
 		if (nr_pages <= max)
 			break;
 
-- 
2.25.1

Re: [PATCH] mm: memcg: stop reclaim when a limit update is superseded

[Tao Cui]

在 2026/7/24 10:18, Guopeng Zhang 写道:
> From: Guopeng Zhang <zhangguopeng@kylinos.cn>
> 
> kernfs serializes file operations only per open file, so separate open
> files can update the same memory.high or memory.max file concurrently.
> Both handlers store the new limit before synchronous reclaim, but
> continue to use the writer's local target in the reclaim loop. If another
> writer raises or removes the limit, the first writer can continue
> reclaiming toward a stale target.
> 
> For memory.max, this can leave the writer looping indefinitely once
> reclaim retries are exhausted. The OOM path sees sufficient margin under
> the current limit and returns true without killing, while the writer
> still compares usage against its stale target and records another OOM
> event.
> 
> Check the current limit at the start of each reclaim iteration and stop
> if it no longer matches the writer's target.
> 

Fix looks correct to me.

Acked-by: Tao Cui <cuitao@kylinos.cn>

Nit: the message lumps both paths together, but only memory.max loops
indefinitely. memory.high has no OOM path, so it just spins
MAX_RECLAIM_RETRIES times and breaks on its own. Worth a line to avoid
conflating the severity.

> Fixes: 8c8c383c04f6 ("mm: memcontrol: try harder to set a new memory.high")
> Fixes: b6e6edcfa405 ("mm: memcontrol: reclaim and OOM kill when shrinking memory.max below usage")
> Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
> ---
> Reproducer:
> 
> Populate a cgroup with anonymous memory and disable swapping. Lower
> memory.max from one open file, then restore it to "max" through another
> open file after the new limit becomes visible.
> 
> Without the patch, the first writer remains blocked and repeatedly
> increments the OOM event counter. With the patch, it returns normally.
> 
>  mm/memcontrol.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8319ad8c5c23..638bdc766616 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4798,6 +4798,9 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,
>  		unsigned long nr_pages = page_counter_read(&memcg->memory);
>  		unsigned long reclaimed;
>  
> +		if (high != READ_ONCE(memcg->memory.high))
> +			break;
> +
>  		if (nr_pages <= high)
>  			break;
>  
> @@ -4853,6 +4856,9 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
>  	for (;;) {
>  		unsigned long nr_pages = page_counter_read(&memcg->memory);
>  
> +		if (max != READ_ONCE(memcg->memory.max))
> +			break;
> +
>  		if (nr_pages <= max)
>  			break;
>  

Re: [PATCH] mm: memcg: stop reclaim when a limit update is superseded

[Guopeng Zhang]

在 2026/7/24 11:32, Tao Cui 写道:
> 
> 
> 在 2026/7/24 10:18, Guopeng Zhang 写道:
>> From: Guopeng Zhang <zhangguopeng@kylinos.cn>
>>
>> kernfs serializes file operations only per open file, so separate open
>> files can update the same memory.high or memory.max file concurrently.
>> Both handlers store the new limit before synchronous reclaim, but
>> continue to use the writer's local target in the reclaim loop. If another
>> writer raises or removes the limit, the first writer can continue
>> reclaiming toward a stale target.
>>
>> For memory.max, this can leave the writer looping indefinitely once
>> reclaim retries are exhausted. The OOM path sees sufficient margin under
>> the current limit and returns true without killing, while the writer
>> still compares usage against its stale target and records another OOM
>> event.
>>
>> Check the current limit at the start of each reclaim iteration and stop
>> if it no longer matches the writer's target.
>>
> 
> Fix looks correct to me.
> 
> Acked-by: Tao Cui <cuitao@kylinos.cn>
> 
> Nit: the message lumps both paths together, but only memory.max loops
> indefinitely. memory.high has no OOM path, so it just spins
> MAX_RECLAIM_RETRIES times and breaks on its own. Worth a line to avoid
> conflating the severity.
> 
Hi,

Thanks for the review and Ack.

The message separates the two cases: the first paragraph describes the
stale-target reclaim behavior common to both, while the "For memory.max"
paragraph describes the OOM-based indefinite loop.

One detail is that memory.high is not limited to MAX_RECLAIM_RETRIES
iterations. The retry counter is decremented only when reclaim makes no
progress:

        if (!reclaimed && !nr_retries--)
                break;

If reclaim continues to make progress while pages are refaulted,
nr_retries is not decremented and the loop can still fail to converge.

Thanks,
Guopeng
>> Fixes: 8c8c383c04f6 ("mm: memcontrol: try harder to set a new memory.high")
>> Fixes: b6e6edcfa405 ("mm: memcontrol: reclaim and OOM kill when shrinking memory.max below usage")
>> Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
>> ---
>> Reproducer:
>>
>> Populate a cgroup with anonymous memory and disable swapping. Lower
>> memory.max from one open file, then restore it to "max" through another
>> open file after the new limit becomes visible.
>>
>> Without the patch, the first writer remains blocked and repeatedly
>> increments the OOM event counter. With the patch, it returns normally.
>>
>>  mm/memcontrol.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 8319ad8c5c23..638bdc766616 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -4798,6 +4798,9 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,
>>  		unsigned long nr_pages = page_counter_read(&memcg->memory);
>>  		unsigned long reclaimed;
>>  
>> +		if (high != READ_ONCE(memcg->memory.high))
>> +			break;
>> +
>>  		if (nr_pages <= high)
>>  			break;
>>  
>> @@ -4853,6 +4856,9 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
>>  	for (;;) {
>>  		unsigned long nr_pages = page_counter_read(&memcg->memory);
>>  
>> +		if (max != READ_ONCE(memcg->memory.max))
>> +			break;
>> +
>>  		if (nr_pages <= max)
>>  			break;
>>  
>