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

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

[Shakeel Butt]

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

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

[SeongJae Park]

On Wed, 10 Jun 2026 16:20:48 -0700 Shakeel Butt <shakeel.butt@linux.dev> 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().

All make sense to me, thank you for the nice description and the fix!

> 
> 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>

Reviewed-by: SeongJae Park <sj@kernel.org>


Thanks,
SJ

[...]

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

[SeongJae Park]

On Wed, 10 Jun 2026 17:22:51 -0700 SeongJae Park <sj@kernel.org> wrote:

> On Wed, 10 Jun 2026 16:20:48 -0700 Shakeel Butt <shakeel.butt@linux.dev> 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().
> 
> All make sense to me, thank you for the nice description and the fix!
> 
> > 
> > Fixes: 5035ebc644ae ("mm: shrinkers: introduce debugfs interface for memory shrinkers")

Forgot asking this, sorry.  Are you intentionally not adding Cc: stable@ here?
I think the user impact is arguably minor enough to not Cc-ing stable@, but
just thought it would be good to make the intention clear.

> > 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>
> 
> Reviewed-by: SeongJae Park <sj@kernel.org>


Thanks,
SJ

[...]

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

[Shakeel Butt]

On Wed, Jun 10, 2026 at 05:26:47PM -0700, SeongJae Park wrote:
> On Wed, 10 Jun 2026 17:22:51 -0700 SeongJae Park <sj@kernel.org> wrote:
> 
> > On Wed, 10 Jun 2026 16:20:48 -0700 Shakeel Butt <shakeel.butt@linux.dev> 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().
> > 
> > All make sense to me, thank you for the nice description and the fix!
> > 
> > > 
> > > Fixes: 5035ebc644ae ("mm: shrinkers: introduce debugfs interface for memory shrinkers")
> 
> Forgot asking this, sorry.  Are you intentionally not adding Cc: stable@ here?
> I think the user impact is arguably minor enough to not Cc-ing stable@, but
> just thought it would be good to make the intention clear.
> 

Haha I was just being lazy to think through if this should be CCed to stable or
not and letting others/reviewers do that for me :P

> > > 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>
> > 
> > Reviewed-by: SeongJae Park <sj@kernel.org>

Thanks.

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

[Qi Zheng]

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@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(-)
> 

LGTM, so:

Reviewed-by: Qi Zheng <qi.zheng@linux.dev>

Thanks!

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@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;
>  }

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

Thanks for the fix!

Zenghui

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

[Nhat Pham]

On Wed, Jun 10, 2026 at 4:21 PM Shakeel Butt <shakeel.butt@linux.dev> 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>

Too fast :)

Reviewed-by: Nhat Pham <nphamcs@gmail.com>

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

[Muchun Song]

> On Jun 11, 2026, at 07:20, Shakeel Butt <shakeel.butt@linux.dev> 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>

Acked-by: Muchun Song <muchun.song@linux.dev>

Thanks.

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

[Roman Gushchin]

Shakeel Butt <shakeel.butt@linux.dev> writes:

> 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>

Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>

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

[Nick Huang]

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>

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

[Krzysztof Kozlowski]

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

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

[Nick Huang]

On Tue, Aug 25, 2026 at 08:12:09AM +0200, Krzysztof Kozlowski wrote:
> 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

Hi Krzysztof

I saw this patch on the mailing list and was interested in the RCU issue, so I looked at the related code.
I checked that removing the outer RCU read-side critical section doesn't break the memcg lifetime, 
since mem_cgroup_iter() returns the memcg with a CSS reference held.
I should have explained what I checked instead of just adding a Reviewed-by.
Thanks for pointing that out.

Regards,
Nick Huang

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

[Krzysztof Kozlowski]

On 25/08/2026 15:54, Nick Huang wrote:
> On Tue, Aug 25, 2026 at 08:12:09AM +0200, Krzysztof Kozlowski wrote:
>> 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
> 
> Hi Krzysztof
> 
> I saw this patch on the mailing list and was interested in the RCU issue, so I looked at the related code.
> I checked that removing the outer RCU read-side critical section doesn't break the memcg lifetime, 
> since mem_cgroup_iter() returns the memcg with a CSS reference held.
> I should have explained what I checked instead of just adding a Reviewed-by.
> Thanks for pointing that out.

After all the maintainers already provided a review...

Best regards,
Krzysztof

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

[Nick Huang]

Krzysztof Kozlowski <krzk@kernel.org> 於 2026年8月25日週二 下午9:56寫道：
>
> On 25/08/2026 15:54, Nick Huang wrote:
> > On Tue, Aug 25, 2026 at 08:12:09AM +0200, Krzysztof Kozlowski wrote:
> >> 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
> >
> > Hi Krzysztof
> >
> > I saw this patch on the mailing list and was interested in the RCU issue, so I looked at the related code.
> > I checked that removing the outer RCU read-side critical section doesn't break the memcg lifetime,
> > since mem_cgroup_iter() returns the memcg with a CSS reference held.
> > I should have explained what I checked instead of just adding a Reviewed-by.
> > Thanks for pointing that out.
>
> After all the maintainers already provided a review...
>
> Best regards,
> Krzysztof
Hi Krzysztof
Sorry, I misread it and didn't realize that the maintainers had
already reviewed it.
Thanks for pointing that out, and thanks for taking the time to respond.
-- 
Regards,
Nick Huang