[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