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b=P2pAqV9zNsybjktSvwbLvyRzqDcg64Whh6aePSx2mVgbHQscgHImYvCPhQUwvV9L9 jHffcv+bzo2ODhAbTKUXJJemL5ZepdW0ViWlAK2Cw/bazdz4P+GBz/9zWpF67vFr6S YHjTC6xuz6j474PcFeHwSo9v5ITmlej+z3MYz+KsHBaTfSKp08Sv/jjSQTb7zAYC7p NSSZNXZMn88nz25aNc454UX8QnvyTF9CS1h3VHdGRCVRw0YjhxRt7DXxGhLxeOdvcD hgKacUH+e6i1aboK6FOLu4DVGGfaPjAvMeiNmQFpY9Zmy6pQxJgibd0uG4ijlU3HxC bbZHkWG1e0BMA== From: SeongJae Park To: SeongJae Park Cc: Shakeel Butt , 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 17:26:47 -0700 Message-ID: <20260611002651.69383-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260611002236.69207-1-sj@kernel.org> References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam10 X-Rspam-User: X-Stat-Signature: a4d1n5uijz1mjk3krc9z8ddsbbqt1w57 X-Rspamd-Queue-Id: E9D6620003 X-HE-Tag: 1781137621-373798 X-HE-Meta: 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 N5Rtm6Ce YySXDrVdzMULgZ3HLabah10851/o4IuE9hhHlkCKcnuu/P00fdoWBboy0DupB/dnucb1wZOnekVHRX8XkceE+DE3mLOXJ/noebYkSGYjp8S5D/t6EB7js9jzxEEufoF/ley6epB6Ooswng6ZoT841IOtMYPVWb4qxygLyrDUEVJ/Tbyhl12iuopKC8YFFPawy6Q9iJ48cF95+FtvWK7QmzxT+nAY3VBFPfgOgt29SnZgifjOD23I4+s67PS5DDSjL7dGdfo7eTbfYSl8akaSmoOIakC5Z11cqqLLEEgOmCK1bw6wnj5IQoEArTg== 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 17:22:51 -0700 SeongJae Park wrote: > 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") 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 > > 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 [...]