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Fri, 06 Nov 2026 01:35:15 -0800 (PST) Received: from milan ([2001:9b1:d5a0:a500::24b]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-c4d35670169sm65381266b.42.2026.11.06.01.35.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 06 Nov 2026 01:35:15 -0800 (PST) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Fri, 6 Nov 2026 10:35:13 +0100 To: Zhan Xusheng Cc: vbabka@kernel.org, harry@kernel.org, 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 Subject: Re: [PATCH 2/2] mm/slab: don't queue the page cache work under krcp->lock Message-ID: References: <20261028134647.3176080-1-zhanxusheng1024@gmail.com> <20261028134647.3176080-2-zhanxusheng1024@gmail.com> X-Mailing-List: rcu@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261028134647.3176080-2-zhanxusheng1024@gmail.com> On Wed, Oct 28, 2026 at 09:46:47PM +0800, Zhan Xusheng wrote: > From: Zhan Xusheng > > 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") > Signed-off-by: Zhan Xusheng > --- > mm/slab_common.c | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/mm/slab_common.c b/mm/slab_common.c > index 3729aa7a6156..ef5ff0a20249 100644 > --- a/mm/slab_common.c > +++ b/mm/slab_common.c > @@ -1987,7 +1987,13 @@ 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)) { > + /* > + * A zero delay would queue the work right here, under > + * krcp->lock and possibly the caller's pi_lock, so let the > + * hrtimer do it. > + */ > + 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)); > -- > 2.43.0 > > Can we just clamp this to 1 - 100 seconds interval? diff --git a/mm/slab_common.c b/mm/slab_common.c index 7223a7596dab..2c9236236ac9 100644 --- a/mm/slab_common.c +++ b/mm/slab_common.c @@ -2249,12 +2249,12 @@ void __init kvfree_rcu_init(void) WQ_UNBOUND | WQ_MEM_RECLAIM, 0); WARN_ON(!rcu_reclaim_wq); - /* Clamp it to [0:100] seconds interval. */ - if (rcu_delay_page_cache_fill_msec < 0 || + /* Clamp it to [1:100] seconds interval. */ + if (rcu_delay_page_cache_fill_msec <= 0 || rcu_delay_page_cache_fill_msec > 100 * MSEC_PER_SEC) { rcu_delay_page_cache_fill_msec = - clamp(rcu_delay_page_cache_fill_msec, 0, + clamp(rcu_delay_page_cache_fill_msec, 1, (int) (100 * MSEC_PER_SEC)); pr_info("Adjusting rcutree.rcu_delay_page_cache_fill_msec to %d ms.\n", so we forbid zero as value to use? -- Uladzislau Rezki