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b=O3/snmKj+Oq77n3PttxztnuIcIFYN0EpcanrM6a2w6qax1v2/dRhnuLA3toFV34K/ RNs5POBA8fBvDsg3p8XPozQad0dKkQz2UHF8RzJkXPzm6e71H/u8U+nNZtcu1dm7tw IU9xCcMQfRmxLcMHfUWkNMYcWggc+r5lj1zuBRMnIPDLukGt5tuu4+SqzzGVBxggNz SbFXmnKaF1qJeIA2lSQcrg01vtQ8uuE3mi9ypgm+zxJUWP7O5WGxetbbhSbGaj/YiA dYbkX1+We1DsmnHCKotFbMMOMMByTSVOlLGzxVykQGkoKp71qJsIS8Q5I/NyKf7Ojl cLFxy1Ooy022w== Message-ID: Date: Wed, 4 Nov 2026 09:19:10 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] mm/slab: don't queue the page cache work under krcp->lock Content-Language: en-US To: Harry Yoo , Zhan Xusheng Cc: urezki@gmail.com, paulmck@kernel.org, qiang.zhang@linux.dev, akpm@linux-foundation.org, hao.li@linux.dev, cl@gentwo.org, rientjes@google.com, roman.gushchin@linux.dev, linux-mm@kvack.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org, zhanxusheng@xiaomi.com References: <20261028134647.3176080-1-zhanxusheng1024@gmail.com> <20261028134647.3176080-2-zhanxusheng1024@gmail.com> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: y4knmipyxotep36tq1zrx7efq8yfpqiy X-Rspamd-Server: rspam01 X-Rspam-User: X-Spam-Status: No, score=-17.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Queue-Id: D4CAF180004 X-HE-Tag: 1793780355-188576 X-HE-Meta: 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 /P7xFdqw kJbMinecSZi9G9v2zAfLcBnD7r1ALl3wbxGS+/Mu66jmx9rv4tzvY6Y1Tmw1pynv9ySn9tmrkJaLI3mmck0V5a7dCjGpSneaCxVWsxylhcJLsi6F/1qaT31g/RqkIJ6DSQtirEi1A3+ZQBpnlwMDWUQDjR24rPVpjE6gJIKMrL5opPJVN7kxROmW+EV3qgCLl536QHiuOPPp49KLSr91w5Ug4IWSNR8ac5XUeRNo/noFCmg6DkJJ0ExLddtZzNgy31JAyaLheqnxjy8VrjsJJH7SzmWb7ByCjoY59zHRTyYOh3dbgBXwwQJAnsgftmUHhMPeqQ898rnVYIe6UP7uNJpQSUE6nuHoYfQW6OZH7JH060xcKk0JNSupBHhb9+LafcD1QepzEPybk/QmfYf0kucaXp2QWo7DS3xM5 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 11/2/26 15:35, Harry Yoo wrote: > Here I added bit more context and adjusted comment. End result: > > mm/slab: don't queue the page cache work under krcp->lock > > Commit 56292e8609e3 ("rcu/tree: Defer kvfree_rcu() allocation to a > clean context") deferred page allocations to a workqueue. To avoid > deadlocks with the scheduler, it used hrtimer to queue the work instead > of queueing it directly. > > Commit d0bfa8b3c411 ("kvfree_rcu: Release a page cache under memory > pressure") later added a backoff mechanism under memory pressure, but > did not consider that queue_delayed_work() with delay = 0 immediately > queues the work and introduced the deadlock commit 56292e8609e3 tried to > avoid. > > run_page_cache_worker() runs under krcp->lock, from kvfree_call_rcu() > callers that may hold more, such as set_cpus_allowed_force() under > pi_lock, so it kicks the refill work from an hrtimer. Under memory > pressure it instead queues the work with a delay of > rcutree.rcu_delay_page_cache_fill_msec, which may be 0, and a zero delay > queues it at once, taking pool->lock under krcp->lock. > > With the delay set to 0, calling kvfree_rcu() on vmalloc memory under > pi_lock after a drop_caches makes lockdep report: > > Chain exists of: > &pool->lock --> &p->pi_lock --> krc.lock > > Use the hrtimer when the delay is 0. > > Fixes: d0bfa8b3c411 ("kvfree_rcu: Release a page cache under memory pressure") > Cc: stable@vger.kernel.org > Signed-off-by: Zhan Xusheng > Link: https://patch.msgid.link/20261028134647.3176080-2-zhanxusheng1024@gmail.com > [ harry@kernel.org: add more context to changelog and adjust comment ] > Signed-off-by: Harry Yoo (Meta) Reviewed-by: Vlastimil Babka (SUSE) > --- > mm/slab_common.c | 10 +++++++++- > 1 file changed, 9 insertions(+), 1 deletion(-) > > diff --git a/mm/slab_common.c b/mm/slab_common.c > index 41dea01a8c1f..051ca63c0b67 100644 > --- a/mm/slab_common.c > +++ b/mm/slab_common.c > @@ -1987,7 +1987,15 @@ run_page_cache_worker(struct kfree_rcu_cpu *krcp) > > if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING && > !atomic_xchg(&krcp->work_in_progress, 1)) { > - if (atomic_read(&krcp->backoff_page_cache_fill)) { > + /* > + * Use the hrtimer to avoid a deadlock with the scheduler > + * by waking up the worker directly. queue_delayed_work() > + * uses a timer to queue the work, so it's fine. > + * But with delay = 0 it queues the work immediately. > + * Fall back to the hrtimer in that case. > + */ > + if (atomic_read(&krcp->backoff_page_cache_fill) && > + rcu_delay_page_cache_fill_msec) { > queue_delayed_work(rcu_reclaim_wq, > &krcp->page_cache_work, > msecs_to_jiffies(rcu_delay_page_cache_fill_msec));