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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b="O3/snmKj"; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C09AE606A32D; Wed, 4 Nov 2026 08:19:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=O3/snmKj Received: by smtp.kernel.org (Postfix) with ESMTPSA id 79D9A1F0093A; Wed, 4 Nov 2026 08:19:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793780354; bh=Xc7V+QdXVVO/sqjZeREmJmMcVmU9QYnc9ONRFJK1OpE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; 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-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list 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 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));