Re: [PATCH] mm/shrinker: do not hold RCU lock in shrinker_debugfs_count_show()

[Krzysztof Kozlowski <krzk@kernel.org>]

On 24/08/2026 15:42, Nick Huang wrote:
> On Wed, Jun 10, 2026 at 04:20:48PM -0700, Shakeel Butt wrote:
>> Reading the debugfs "count" file of a memcg-aware shrinker can sleep
>> inside an RCU read-side critical section:
>>
>>   BUG: sleeping function called from invalid context at kernel/cgroup/rstat.c:421
>>   RCU nest depth: 1, expected: 0
>>    css_rstat_flush
>>    mem_cgroup_flush_stats
>>    zswap_shrinker_count
>>    shrinker_debugfs_count_show
>>
>> shrinker_debugfs_count_show() invokes the ->count_objects() callback
>> under rcu_read_lock(). The zswap callback flushes memcg stats via
>> css_rstat_flush(), which may sleep, so it must not run under RCU.
>>
>> The RCU lock is not needed here. mem_cgroup_iter() takes RCU internally
>> and returns a memcg holding a css reference (dropped on the next
>> iteration or by mem_cgroup_iter_break()), so the memcg stays alive
>> without it. The shrinker is kept alive by the open debugfs file:
>> shrinker_free() removes the debugfs entries via
>> debugfs_remove_recursive(), which waits for in-flight readers to drain,
>> before call_rcu(..., shrinker_free_rcu_cb). The sibling "scan" handler
>> already invokes the sleeping ->scan_objects() callback with no RCU
>> section.
>>
>> Drop the rcu_read_lock()/rcu_read_unlock().
>>
>> Fixes: 5035ebc644ae ("mm: shrinkers: introduce debugfs interface for memory shrinkers")
>> Reported-by: Zenghui Yu <zenghui.yu@linux.dev>
>> Closes: https://lore.kernel.org/all/c052a064-cddb-494f-a0d8-f8a10b4b1c4d@linux.dev/
>> Suggested-by: Nhat Pham <nphamcs@gmail.com>
>> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
>> ---
>>  mm/shrinker_debug.c | 4 ----
>>  1 file changed, 4 deletions(-)
>>
>> diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
>> index affa64437302..cda4e86428c8 100644
>> --- a/mm/shrinker_debug.c
>> +++ b/mm/shrinker_debug.c
>> @@ -57,8 +57,6 @@ static int shrinker_debugfs_count_show(struct seq_file *m, void *v)
>>  	if (!count_per_node)
>>  		return -ENOMEM;
>>  
>> -	rcu_read_lock();
>> -
>>  	memcg_aware = shrinker->flags & SHRINKER_MEMCG_AWARE;
>>  
>>  	memcg = mem_cgroup_iter(NULL, NULL, NULL);
>> @@ -88,8 +86,6 @@ static int shrinker_debugfs_count_show(struct seq_file *m, void *v)
>>  		}
>>  	} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
>>  
>> -	rcu_read_unlock();
>> -
>>  	kfree(count_per_node);
>>  	return ret;
>>  }
>> -- 
>> 2.53.0-Meta
>>
> Reviewed-by: Nick Huang <sef1548@gmail.com>

Sorry, but what exactly did you review here? In the past you were
sending AI slop:
https://lore.kernel.org/all/aeNGbNyPxJssnkbO@lucifer/

Not even properly following the process.
https://lore.kernel.org/all/20260724143408.175870-1-sef1548@gmail.com/

https://lore.kernel.org/all/?q=f%3Asef1548%40gmail.com

Why suddenly reviewing MM patches?

Best regards,
Krzysztof