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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1781236830; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Kb9XhzHn/hQKeTGOyjGfHnMrWyxQdJZuAbxjI5R5xbo=; b=G5EcDdQpHo2eHjWBTqxZMmRBT/I9pw3ptbS2GYZEoYTNAWQ0Ke/jktVzTYdCrIR7U34HR8 XyQmjwvUdpJiCuRK1yO8YElKSgXnzACGMW3lCEN37Wm8+vZow58TLloRZsQv6AztvdLkLa FHar+0HFVajewrsqhstI0C1m6FxM6m4= From: Roman Gushchin To: Shakeel Butt Cc: Andrew Morton , Dave Chinner , Muchun Song , Qi Zheng , Meta kernel team , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Zenghui Yu , Nhat Pham Subject: Re: [PATCH] mm/shrinker: do not hold RCU lock in shrinker_debugfs_count_show() In-Reply-To: <20260610232048.62930-1-shakeel.butt@linux.dev> (Shakeel Butt's message of "Wed, 10 Jun 2026 16:20:48 -0700") References: <20260610232048.62930-1-shakeel.butt@linux.dev> Date: Thu, 11 Jun 2026 21:00:24 -0700 Message-ID: <87cxxwz9uv.fsf@linux.dev> MIME-Version: 1.0 Content-Type: text/plain X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: AE9F740012 X-Stat-Signature: gayq9m6htjuhr5da4mtbhfwn9xxxo1kp X-HE-Tag: 1781236832-169768 X-HE-Meta: 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 j7OX7B4V nWBJPUxjnJW9MDeX2A/ToNJvXZ56bEP8DSW7xGLlDFzzF1phpfjbpnOnzXmu2+8tDAIQwtRGkbtqiZTCssKoSkjYUNZRoBU87hxKE4zwcbIpVySqdrMNbDfwVtyX6r1zds6OjkEWM2weyy94YdhVObVBGwjWBtdC07y53KH49d9TokU3Of/+vder1iYdPN6CJuQO5e1kQFLoc2KMakE6ylrsSTplewpDPlShTNyahsHeKyIJSEvzvSezw1vuXoZgwIdMPCAitWa+370agd36CaGsyWG7tDgP3vZ3fVb5i3c0Z2/M= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Shakeel Butt 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 > Closes: https://lore.kernel.org/all/c052a064-cddb-494f-a0d8-f8a10b4b1c4d@linux.dev/ > Suggested-by: Nhat Pham > Signed-off-by: Shakeel Butt Reviewed-by: Roman Gushchin