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Fri, 06 Nov 2026 11:32:57 +0000 X-Mizu-Trace-ID: 33a186fbf79cc617 X-Migadu-Flow: FLOW_OUT Date: Fri, 6 Nov 2026 19:32:46 +0800 From: Hao Li To: Harry Yoo Cc: Zhan Xusheng , vbabka@kernel.org, urezki@gmail.com, paulmck@kernel.org, qiang.zhang@linux.dev, akpm@linux-foundation.org, 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-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: meo13apyoyj1ufxn6xsmo7ejhtmy6mgi X-Rspamd-Queue-Id: C425140002 X-Rspam-User: X-Rspamd-Server: rspam08 X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-HE-Tag: 1793964780-212177 X-HE-Meta: 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 eevx51oY Kl2P3QbO8cjLEZbyhcW4yy54yOBPMO6OJ5JHGn43yYRWlP9AR9i46hwuu23CA9zLHcReySBSCdMoDMWcD9Jm4cQ+bXQfxBueEww2a2ZVOYOtf/rRxOPdm2H56Wxulc7LvvN6TZ+uI1yWGxb9rYm6QP5upfha7NvtjgtrxxP0nla2YzkcqTfH+XC3aBMOerJYlrJ/ql0At2eZvxJO6XJKzSjTZXPNufmzK5kVwrxQFX/LuIkkom3T6O3CPpjGzg2Q3ugvL/adH5SSWh0DhbfbXealYIMMVHYz3oxidgwJ7/sjnVfk2Ij7lypoypO67sRf2XCWQL6S7uq3nx8cNVm1X/pJmmmFEY+44vyTB1b0IcdLx/cLjX21g2NtbI3tH5jOcv7E/Xd3j6Fh2FO2wPPXBf0NkjSoT257xay3OkKhg5Zdh9dk= 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 Mon, Nov 02, 2026 at 02:35:40PM +0000, 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) > --- > 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)); > -- > 2.54.0 Just a thought: once this fix is applied, could we simplify the code further by always queuing the work via hrtimer? This would make the logic more understandable. For example: diff --git a/mm/slab_common.c b/mm/slab_common.c index ef5ff0a20249..e5213f952e16 100644 --- a/mm/slab_common.c +++ b/mm/slab_common.c @@ -1480,7 +1480,7 @@ struct kfree_rcu_cpu { struct delayed_work monitor_work; bool initialized; - struct delayed_work page_cache_work; + struct work_struct page_cache_work; atomic_t backoff_page_cache_fill; atomic_t work_in_progress; struct hrtimer hrtimer; @@ -1878,7 +1878,7 @@ static void fill_page_cache_func(struct work_struct *work) struct kvfree_rcu_bulk_data *bnode; struct kfree_rcu_cpu *krcp = container_of(work, struct kfree_rcu_cpu, - page_cache_work.work); + page_cache_work); unsigned long flags; int nr_pages; bool pushed; @@ -1974,34 +1974,27 @@ schedule_page_work_fn(struct hrtimer *t) struct kfree_rcu_cpu *krcp = container_of(t, struct kfree_rcu_cpu, hrtimer); - queue_delayed_work(system_highpri_wq, &krcp->page_cache_work, 0); + queue_work(system_highpri_wq, &krcp->page_cache_work); return HRTIMER_NORESTART; } static void run_page_cache_worker(struct kfree_rcu_cpu *krcp) { + ktime_t delay = 0; + // If cache disabled, bail out. if (!rcu_min_cached_objs) return; if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING && !atomic_xchg(&krcp->work_in_progress, 1)) { - /* - * 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)); - } else { - hrtimer_setup(&krcp->hrtimer, schedule_page_work_fn, CLOCK_MONOTONIC, - HRTIMER_MODE_REL); - hrtimer_start(&krcp->hrtimer, 0, HRTIMER_MODE_REL); - } + if (atomic_read(&krcp->backoff_page_cache_fill)) + delay = ms_to_ktime(rcu_delay_page_cache_fill_msec); + + hrtimer_setup(&krcp->hrtimer, schedule_page_work_fn, CLOCK_MONOTONIC, + HRTIMER_MODE_REL); + hrtimer_start(&krcp->hrtimer, delay, HRTIMER_MODE_REL); } } @@ -2295,7 +2288,7 @@ void __init kvfree_rcu_init(void) INIT_LIST_HEAD(&krcp->bulk_head[i]); INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor); - INIT_DELAYED_WORK(&krcp->page_cache_work, fill_page_cache_func); + INIT_WORK(&krcp->page_cache_work, fill_page_cache_func); krcp->initialized = true; }