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Mon, 09 Nov 2026 05:47:17 -0800 (PST) Received: from milan ([2001:9b1:d5a0:a500::24b]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-6b721eedd40sm1643775a12.32.2026.11.09.05.47.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 09 Nov 2026 05:47:16 -0800 (PST) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Mon, 9 Nov 2026 14:47:14 +0100 To: Harry Yoo Cc: Uladzislau Rezki , Zhan Xusheng , vbabka@kernel.org, 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: On Mon, Nov 09, 2026 at 01:13:46PM +0000, Harry Yoo wrote: > On Mon, Nov 09, 2026 at 12:48:31PM +0100, Uladzislau Rezki wrote: > > On Fri, Nov 06, 2026 at 12:21:28PM +0000, Harry Yoo wrote: > > > On Fri, Nov 06, 2026 at 10:35:13AM +0100, Uladzislau Rezki wrote: > > > > 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? > > > > > > You mean clamp to [1:100,000] milliseconds interval? > > > > > Yes. It is now in milliseconds. > > Thanks for clarifying. > > > So starting with 1 - means that it can be configured at least from 1 msec > > whereas 0 is forbidden. Also we can drop the extra "if condition" proposed > > in the patch as it never can be zero. > > But can we explain why forbiding 0 msec can be justified (when there's > an alternative approach to fix it?) > I mean it is better to fix the threshold instead of adding checks. -- Uladzislau Rezki