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

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().

Link: https://lore.kernel.org/20260610232048.62930-1-shakeel.butt@linux.dev
Fixes: 5035ebc644ae ("mm: shrinkers: introduce debugfs interface for memory shrinkers")
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
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>
Reviewed-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Qi Zheng <qi.zheng@linux.dev>
Tested-by: Zenghui Yu (Huawei) <zenghui.yu@linux.dev>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Dave Chinner <david@fromorbit.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
index affa6443730213a31afda3a6af7f0f3b61f75d15..cda4e86428c8f3b4b3f286f55fda24a6f9bb8b33 100644 (file)
--- 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;
 }