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bh=2SvRs21mGVxCJSkrSt9EaDORfooGulFSzr+iBZ0ww28=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=Lxbh7HPQ7YCt/Q+bwgr4myrYnjUXT7Vo/3fMu/0XWpHdBnxLX3Dc45N+I1GmceUWp 7/7xSwT4hduI8gyQ4Y9aQ6+/VOpTM6qwz+mePKALM6yp31yBR3a7EFOjr1ojweFHOR NfrkaKUvcLGw/Quz1KS1SlLE8yDMtoGstP4K/kKcalN9kKHzOyMmjaaTiHl2IS6Avj GtA0r49DlModp3BIHRRYqyK+r0uDE9Ri9UL8KO+nROhXXi+Sv34OkStKqPC+PD6r4w G/Xc9Sl4IzGHM4RZ91PEYA65f4eJX+mIl2V16z7lpDcMhFMcEOeieXhtSEKmgVuWQE GVdaq8jEQ4Hvg== From: SeongJae Park To: Shakeel Butt Cc: SeongJae Park , Andrew Morton , Dave Chinner , Roman Gushchin , 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() Date: Wed, 10 Jun 2026 17:22:35 -0700 Message-ID: <20260611002236.69207-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260610232048.62930-1-shakeel.butt@linux.dev> References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 62AAE2000B X-Stat-Signature: i5snc9k5rxhe7er9gf4fmsi3iw1o15mq X-Rspamd-Server: rspam03 X-Rspam-User: X-HE-Tag: 1781137372-950993 X-HE-Meta: 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 xNTUcOoI tHoRl4qUgAG4ne9cGbD0FW+K08ZB3YU1QTarSRYPCuYK03oFwaO7ap6/qoFo2o0Ll7JoJuzjMSpba6jo9ZT6YeKbduCUmEun+T1pTdhlciZUO6YRBLyPsMZzkNZ888PbMXkom/YhFo6QKbEW31ITF38HdWo9sEagUQZ9jH/FWe4K2+Z+hU+5ywcMavHgKCPv6+cbM0EGv6/8HDx9MO5z5pqXDi5m8K67nmeyoEbTDJ2FTsedgdSnCk3suVxPTILfSVA/oYuDdjXJUSkeB4d53JxSKfTvGglBn+MD3Vhnihs1ZrS2nkuw789nACA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, 10 Jun 2026 16:20:48 -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(). 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 > Closes: https://lore.kernel.org/all/c052a064-cddb-494f-a0d8-f8a10b4b1c4d@linux.dev/ > Suggested-by: Nhat Pham > Signed-off-by: Shakeel Butt Reviewed-by: SeongJae Park Thanks, SJ [...]