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

[Zenghui Yu]

On 6/11/26 7:20 AM, 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@xxxxxxxxx>
> Closes: https://lore.kernel.org/all/c052a064-cddb-494f-a0d8-f8a10b4b1c4d@xxxxxxxxx/
> Suggested-by: Nhat Pham <nphamcs@xxxxxxxxx>
> Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> ---
>  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;
>  }

Tested-by: Zenghui Yu (Huawei) <zenghui.yu@xxxxxxxxx>

Thanks for the fix!

Zenghui