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b=nfJm8OHhyNANdak70pijBgAfPa88EnbFGsqwMrmWGsIIpiZ9oe1IOmuKMfkZFtHIY l+EmlByl7y51Sjmkwawrghx/6hUDawrTZFLoQlCNN0lJU1ULlDME2LDFmpn+aBvtfG Mhi3qqSbWZXaybzsgN3VrlHVk6NeaEAfVxWXc0LRpcfE+77roaidWe151Bc1UoYOpp yJQjnoTQeWNQetvIJKRQvREGjwuYukX0EL8jxW9WdsuJGpMtyNnaV4643eo8tgitM2 Tb4ftJXCww3eNUNKUyoh6Yt1ieWGe6+17QiBr8kpNPFnGOUg4x+BsX7p40GVd1EWk7 kFZ4s4lbqwHtw== Date: Mon, 9 Nov 2026 13:13:46 +0000 From: Harry Yoo To: Uladzislau Rezki Cc: 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-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 29E1D180007 X-Spam-Status: No, score=-15.8 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 autolearn=ham autolearn_force=no version=4.0.1 X-Stat-Signature: kepop9fm1qfowoeqi6phc9z3paxy3ppw X-Rspam-User: X-HE-Tag: 1794230030-389108 X-HE-Meta: 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 zrXslVVS Hsmveb8MbdF8wgBRrp4dr4XTRufL74xwxaQCqbpHDPqPZC6tARQp5kbd5uAvfgiQnz6MEN9rZAiMBEcZ175N8Stn+GAJmI8Sr/zKTeJ9gsxexGnyUt4NqnZ45+05REYRr70WV24d77kRlTYgNl/ZvonEkUKDLydTF4HnCgmf5pnpXkLMvE6iRlAJXS/aZuQucbMJzkuDOu1Rbzpwo+V/g/s2q/bc0yWN8IY8GUl1lUmwLeE7d4VZyDu2rFfCH6TcFGlFC/z1RJrjGFJDl68rMpviJFQ== 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 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?) -- Cheers, Harry / Hyeonggon